Types of Transformers

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Electronic Transformers

There are many types of transformers. What distinguishes an electronic transformer from

other types of transformers?

Electronic transformers are simply transformers used in electronic applications.

This is a very broad definition; consequently there are many types of electronic

transformers. Examples of types of electronic transformers include ( but not limited to )power, pulse, instrument, current, switching ( or switch mode ), inverting, signal, step-up,

step-down, impedance matching, high voltage and saturable. Some of the preceding types

can be divided into more sub-types. Types of switching transformers include ( but not

limited to ) flyback, feed forward converter ( also called buck ), and boost. Gatedrive transformers and trigger transformers are types of pulse transformers ( depending

on who you talk to). The feed forward type includes a push-pull center-tap and a

half bridge configuration. It becomes apparent from the preceding type designationsthat the type designation of an electronic transformer is determined by its intended

application. To learn more about a particular type, click on one of the available links for

electronic transformer types.

Electronic transformers may be further described by their basic structure and/or

construction style. Many current transformers are wound on toroidal cores; hence the

transformer is referred to as a toroidal current transformer. Many transformer coils are

wound on bobbins ( spools ) or tubes. The transformer core is inserted into and aroundthe coil. These transformers may be referred to as bobbin wound or tube wound

structures. There are many core shapes available; E, E-I, U, U-I, Pot, RM, PQ, EP, EFD,

and others.

Electronic transformers may be further described by the methods of mounting and

electrical terminations. Transformers mounted on printed circuit boards may be pin-

thru or surface mount. Transformer windings are terminated to bobbin pins or surfacemount pads. The pins or pads are then soldered to the printed circuit board. Some

transformers have lead wires. These wires are often referred to as flying leads.

Electronic transformers may be used to supply power, transmit signals, establish voltage

isolation between circuits, sense voltage and current levels, modify voltage and currentlevels, provide impedance matching, and filtering. Lightly loaded transformers may

perform some inductor-like functions, such as storing energy and limiting current flow.

Do electronic transformers have any characteristics common to all electronic

transformers? Not really. Most electronic transformers can easily be held in your hand,even in a childs hand, but there are some too large to hold. Due to ever-higher operating

frequencies, more electronic transformers are being made from ferrite core materials, but

some specialized applications use other core materials.


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Despite the many types of electronic transformers, their theory of operation does not

differ. Electrical functions are usually similar but design characteristics can differ incertain ways. Some examples are; unipolar versus bipolar core utilization, saturating or

not saturating, degree of energy storage, regulation, and transformer impedance.

Butler Winding can make ( and has made ) electronic transformers in a wide variety ofshapes and sizes. This includes; various standard types of core with bobbin structures (E, EP, EFD, PQ, POT, U and others ), toroids, and some custom designs. Our upper

limits are 40 pounds of weight and 2 kilowatts of power. We have experience with foil

windings, litz wire windings, and perfect layering. For toroids, we can ( and have done )sector winding, progressive winding, bank winding, and progressive bank winding.

Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes

two programmable automated machines and a taping machine for toroids. Butler windinghas vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure

quality, Butler Winding purchased two programmable automated testing machines. Most

of our production is 100% tested on these machines. For more information on Butler

Windings capabilities, click on our capabilities link.

Toroidal Transformer

Toroidal transformers are the high performers among transformers. They offer the

smallest size (by volume and weight), less leakage inductance, and lower electromagnetic

interference (EMI). Their windings cool better because of theproportionally larger surface area. A 360 degree

wound toroidal transformer has a high degree of

symmetry. Its geometry leads to near complete

magnetic field cancellation outside of its coil, hencethe toroidal transformer has less leakage inductanceand less EMI when compared against other

transformers of equal power rating. Toroidal

round core cross section are better performers than toroidal transformers with arectangular cross section. The cancellation is more complete for the round cross section.

The round cross section also gives a shorter turn length per unit of cross sectional area,

hence lower winding resistances. The toroidal transformer also has better winding towinding magnetic coupling because of its toroidal shape. The coupling is dependent on

the winding being wound a full 360 degrees around the core and wound directly over the

prior winding, hence sector wound windings do not couple as well and have higher

leakage inductance. As winding turns are positioned further away from the core lesscomplete coupling will occur; hence toroidal transformers with multi-layered windings

will exhibit more leakage inductance.

transformers with a

Toroidal transformers can be used in any electronic transformer application that can

accommodate its shape. Although usable, toroidal transformers are not always practicalfor some applications. Gapped toroidal transformers usually require that the gap be filled

with some type of insulating material to facilitate the winding process. This is an extra
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expense. Split core current transformers can be assembled directly on a conductor while

toroids must be passed over a disconnected end of the conductor. A toroid can be split intwo, but a suitable clamping mechanism (difficult and costly) is required. Some printed

circuit boards are space critical. Mounting a toroidal transformer flat on the board may

take up too much precious board area. Some applications also have restricted height so

the toroid cannot be mounted vertically.

Generally speaking toroidal transformers are more expensive than bobbin or tube wound

transformers. Sufficient winding wire must first be wound (loaded) onto the winding

shuttle, then wound onto the toroidal transformers core. After that, the best situation,from a cost perspective, is no insulation required over the winding and the next winding

uses the same wire size. If the wire is different, then the leftover wire must be removed

and the wire for the next winding must be loaded. However, if the winding must beinsulated, then if must either be insulated (taped) by hand or the toroidal transformer

must be removed and taken to a separate taping machine, then placed back on the toroid

winding machine after taping. The shuttle must then be loaded with the wire size and type

for the toroidal transformers next winding. A toroidal transformer with a single winding(auto-transformer, current transformer) wound on a coated core will probably be cost

competitive with an equivalent bobbin or tube wound transformer since the toroidal

transformer will not require a bobbin or tube. The cost differential will then depend on

the method and cost of mounting the transformers.

Toroidal transformer cores are available in many materials: silicon steel, nickel iron,

moly-permalloy powder, iron powdered, amorphous, ferrites, and others. Silicon steeland nickel iron are available as tape wound cores or laminated pieces. Non-magnetic

toroids are also available to make air core toroidal transformers.

Butler Winding manufactures toroidal transformers in a wide variety of materials andsizes. To ensure quality, Butler Winding purchased two programmable automated testingmachines. Most of our production is 100% tested on these machines. For more

information on Butler Windings capabilities, click on our capabilities link.

Toroidal Transformer - Ferrite Core

Transformers

As today's electronic designers pack more components into less space, there is increasing

demand for high performance components. Toroidal transformers with a ferrite core arein the high performance category. A 360 degree wound ferrite core toroid (and toroids in

general) has a high degree of symmetry. Its geometry leads to near complete magnetic

field cancellation outside of its coil, hence the toroidal transformer has less leakage

inductance and less EMI when compared against other coils of equal power rating.

Today's electronic devices are being operated at ever increasing high frequency. Ferrite

core toroidal transformer manufacturers have developed core materials that can operate
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above 1 megahertz at low gauss levels. Use of a ferrite core toroidal transformer

combines the performance features of the toroidal shape and the low loss feature of theferrite core material. . In high frequency applications a ferrite core toroidal transformer

can offer smaller size (by volume & weight) and lower losses. Core materials have been

developed for power applications and filtering applications. Some have a temperature

coefficient designed to offset capacitor temperature drift for tuned filter applications.Core material initial relative permeability can range from 750 to 15000. Materials for the

highest frequencies usually have lower permeability.

Toroidal transformers with ferrite cores are commercially available in a variety of sizesranging from one tenth to five and one half inches outside diameter. The weight ranges

from a fraction of an ounce to 1.763 pounds. Larger ones exist but they are specialty

items. Some larger specialty They can be purchased with or without an insulating

coating. Several voltage ratings are available.

Butler Winding makes toroidal transformers and ferrite core toroid coils in a wide variety

of materials and sizes. Butler Winding also does "bobbin wound" and "tube wound".Butler winding has a variety of winding machines, bobbin/tube and toroid. That includestwo programmable automated machines and a taping machine for toroids. Butler Winding

has vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure

quality, Butler Winding purchased two programmable automated testing machines. Mostof our production is 100% tested on these machines. For more information on Butler

Winding's capabilities, click on our "capabilities" link.

Toroidal Transformer - Tape Wound

TransformersTape wound core toroidal transformers are made by wrapping thin long strips of

magnetic material around a winding mandrel. Originally, tape wound core toroids weredeveloped to replace vacuum tubes. Vacuum tubes were fragile and required frequent

replacement. The tape wound core toroidal transformers were more reliable. Magnetic

coupling permitted the mixing of signals while maintaining electrical isolation betweencircuits. Tape wound core toroidal transformers also developed along another path. Early

toroidal transformers used thin ring shaped laminations stamped from electrical steel. The

steel from the center was waste material. A core was made by stacking these rings to the

desired height. The laminated stack reduced core eddy currents. Lower eddy currentsresult in lower core losses. The thinner the laminations, the lower the losses were, but the

more time it took to process and stack the laminations. Designers adapted the tape wound

core process to general-purpose transformers as well. Winding tape wound core toroidaltrsnaformers were much faster than stacking toroidal cores; hence use of thinner material

became more practical. The process was then adapted to rectangular cores known as C

cores.


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Today, tape wound core toroidal transformers can be made with strip as thin as

0.000125. They are available in alloys of silicon steel, nickel-iron, cobalt-iron, andamorphous metals. Some materials are processed to enhance square loop properties. With

appropriate gauss de-rating, the thin strip extends the useful frequency range up to 10 to

20 kilohertz depending on the type of material. Ferrite cores have lower core losses and

cost less per unit weight, but their saturation levels are much lower. Low weight andminimal space are desired features for aviation and aerospace applications. Consequently

tape wound core toroids are usually preferred over ferrites for these applications provided

the operating frequency is not too high.

Tape wound core toroids wound with nickel-iron alloys are particularly sensitive to shock

and vibration. These cores need to be place in a protective box with a damping medium

such as silicon oil. Silicon steel alloys are the least sensitive. Silicon steel is frequently

used without a protective box. It depends on the particular application.

Butler Winding produces tape wound core toroidal transformers in a wide variety of

materials and sizes. For more information on Butler Windings capabilities, click on ourcapabilities link.

Flyback Transformers - Kickback

Transformers

A simple and low cost power supply is bound to be quite popular. The single endedflyback circuit topology fits this description. The flyback transformer utilizes the

"flyback" action ( also known as "kickback" ) of an inductor or flyback transformer to

convert the input voltage and current to the desired output voltage and current. Figures1A and 1B show simple flyback transformer schematics for an inductor and a flyback

transformer. These schematics do not show any parasitic effects ( such as leakage

inductance and winding capacitance ). Modern flyback transformer and circuit designnow permit use in excess of 300 watts of power, but most applications are less than 50

watts.

By definition a transformer directly couples energy from one winding to another winding.

A flyback transformer does not act as a true transformer. A flyback transformer firststores energy received from the input power supply (charging portion of a cycle) and then

transfers energy (discharge portion of a cycle) to the output, usually a storage capacitor

with a load connected across its terminals. An application in which a complete dischargeis followed by a short period of inactivity (known as idle time) is defined to be operating

in a discontinuous mode. An application in which a partial discharge is followed by

charging is defined to be operating in the continuous mode. See figures 2A and 2B forillustration.

Gapped core structures increase the magnetizing force needed to reach saturation andlower the inductance of the flyback transformer (or inductor). Consequently, a gapped
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flyback transformer (or inductor) can handle higher peak current values, and thereby

storing more energy, most of which is stored in the magnetic field of the gap. For thesereasons almost all flyback transformers (or inductors) are gapped. The gap may be a

discrete physical gap, several smaller discrete physical gaps or a distributed gap.

Distributed gaps are inherently present in low permeability powdered cores. The bulk of

the stored energy is stored in the magnetic field of the gap(s). Most modern flybacktransformers are operated at high frequency hence gapped ferrite core materials are

typically used.

Butler winding can make (and has made) flyback transformers in a wide variety of shapesand sizes. This includes; various standard types of core with bobbin structures (E, EP,

EFD, EC, ETD, PQ, POT, U and others), toroids, and some custom designs. We have

experience with foil windings, litz wire windings, and perfect layering. For toroids, wecan (and have done) sector winding, progressive winding, bank winding, and progressive

bank winding. Butler winding has a variety of winding machines, bobbin/tube and toroid.

That includes two programmable automated machines and a taping machine for toroids.

To ensure quality, Butler Winding purchased two programmable automated testingmachines. Most of our production is 100% tested on

these machines. For more information on our capabilities, click on our "capabilities" link.

How does a flyback transformer ( or inductor ) work?

Flyback circuits repeat a cycle of two or three stages; a charging stage, a discharging

stage, and in some applications idle time following a complete discharge. Chargingcreates a magnetic field. Discharging action results from the collapse of the magnetic

field. The typical flyback transformer application is a unipolar application. The

magnetic field flux density varies up in down in value ( 0 or larger ) but keeps the same (hence unipolar ) direction.

Charging Stage: The flyback transformer ( or inductor ) draws current from the power

source. The current increases over time. The current flow creates a magnetic field fluxthat also increases over time. Energy is stored within the magnetic field. The associated

positive flux change over time induces a voltage in the flyback transformer ( or inductor )

which opposes the source voltage. Typically, a diode and a capacitor are series connected

across a flyback transformer winding ( or inductor ). A load resistor is then connectedacross the capacitor. The diode is oriented to block current flow from the flyback

transformer ( or source ) to the capacitor and the load resistor during the charging stage.

Controlling the charging time duration (known as duty cycle) in a cycle can control theamount of energy stored during each cycle. Stored energy value, E = ( I x I x L ) / 2,

where E is in joules, I = current in amps, L = inductance in Henries. Current is defined by

the differential equation V(t) = L x di/dt. Applying this equation to applications withconstant source voltage and constant inductance value one obtains the following

equation; I = Io + V x t / L , where I = currents in amps, Io = starting current in amps, V

= voltage in volts across the flyback transformer winding ( or inductor ), L = inductancein Henries, and t = elapsed time in seconds. Note that increasing L will decrease the


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current. Stored energy will consequently decrease because effects of the current squared

decrease will more than offset the effects of the inductance increase. Also be aware thatthe flyback transformer ( or inductor ) input voltage is less than the source voltage due to

switching and resistive voltage drops in the circuit.

Discharge Stage: The current ( which creates the magnetic field ) from the source is theninterrupted by opening a switch, thereby causing the magnetic field to collapse ordecrease, hence a reversal in the direction of the magnetic field flux change ( negative

flux change over time ). The negative flux change induces a voltage in the opposite

direction from that induced during the charging stage. The terms flyback or kickback

originate from the induced voltage reversal that occurs when the supply current isinterrupted. The reversed induced voltage(s) tries to create ( induce ) a current flow. The

open switch prevents current from flowing through the power supply. With the voltagereversed, the diode now permits current flow through it, hence current flows into the

capacitor and the load across the capacitor. If current can flow, then the resulting flow of

current is in the direction, which tries to maintain the existing magnetic field. The

induced current cannot maintain this field but does slow down the decline of the magneticfield. A slower decline translates to a lower induced flyback voltage. If current cannot

flow, the magnetic field will decline very rapidly and consequently create a much higherinduced voltage. In effect, the flyback action will create the necessary voltage needed to

discharge the energy stored in the flyback transformer or inductor. This principle, along

with controlling the duration of the charging stage, allows a flyback inductor to increaseor decrease the voltage without the use of a step-up or step-down turns ratio. In the

typical flyback circuit, the output capacitor clamps the flyback voltage to the capacitor

voltage plus the diode and resistive voltage drops. For a sufficiently large & fully charged

capacitor, the clamping capacitor voltage can be treated as a constant value. Theequations V(t) = L x di/dt, and I = Io + V x t / L can also be applied to the discharge

stage. Use the inductance value of the discharging winding and the time duration of thedischarging stage. The time will either be the cycle time minus the charging time ( no idletime ), or the time it takes to fully discharge the magnetic field thereby reaching zero

current. The cycle time equals the period which equals 1 / frequency.

Idle Stage: This stage occurs whenever the flyback transformer ( or inductor ) hascompletely discharged its stored energy. Input and output current ( of the transformer or

inductor ) is at zero value.

Other Principles of Operation

Equal Ampere-Turns Condition: A magnetic field is created by the current flow throughthe winding(s). The current creates a magnetizing force, H, and a magnetic field fluxdensity B. A core dependent correlation will exist between B and H. B is not usually

linear with H. By definition H is proportional to the product of the winding turns and the

current flowing through the winding, hence ampere-turns. In classical physics, the

magnetic field flux cannot instantaneously change value if the source of the field ( thecurrent flow ) is removed. When the source current is removed from the flyback

transformer ( or inductor ) the charging stage ends and the discharge stage begins. The


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value of the magnetic field will be the same for both stages at that point in time ( cannot

instantaneously change to another value ). The same magnetic core is used for bothstages, hence if the magnetic field is the same, then the magnetizing force, H, must be the

same. Consequently the ampere-turns at the end of the charging stage must equal the

ampere-turns at the start of the discharge stage. If there are multiple outputs then the total

amperes turns of all outputs at the start of the discharge stage must equal the ampere-turns at the end of the charging stage. The same condition applies at the start of the

charging stage. The total ampere-turns of all outputs at the start of the charging stage

must equal the ampere-turns at the end of the discharge stage. Note that there are zeroampere-turns at both the start and end of an idle stage when an idle stage exists.

Zero Average Voltage: During steady state operation, the average voltage across thecharging winding must equal the average voltage across the discharge winding, or

equivalently, the volt-seconds of the charging stage must equal the volt-seconds of the

discharge stage. If not, flux density increases over time and the core saturates. Assuming

a 1:1 turns ratio, then from V1 x t1 = V2 x t2 one can obtain t1 / t2 = V2 / V1 for bothcontinuous and discontinuous modes of operation. For continuous mode operation, t1 +

t2 = 1 / operating frequency.

Conservation of Energy: Power out cannot exceed power in. Sum up output power ( V x

I ) of each output at maximum steady state load plus allowances for parasitic outputpower losses ( diode and resistive losses ). Divide power in watts by operating frequency.

The result is the energy in Joules that must be discharged each cycle into the output

storage capacitor during steady state operation. It is also the amount of energy that mustbe added to the flyback transformer ( or inductor ) during the charging stage. The energy

being transferred equals ( Ipeak x Ipeak Imin. x Imin. ) x L /2. If operating in the

continuous mode, the stored energy will exceed the energy being transferred because thestarting level of stored energy is above zero ( Imin. > 0 ). The flyback transformer ( or

inductor ) must be designed to handle the peak stored energy, Ipeak x Ipeak x L / 2. Thepower source will have to supply the transferred energy plus the parasitic switching and

resistive losses of the charging circuit, plus some power allowance for transient

conditions. Take this value and divide by the power supply voltage. The result will be theaverage input current.

Need additional information about Flyback Transformers?

Contact Butler Winding. Ask for engineering assistance.
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Power Transformers - Switch Mode

What differentiates a power transformer and a switch mode power transformer from other

transformers? Power transformers (and inductors) are essentially A.C. (alternatingcurrent) devices. They cannot sustain transformer operation from a fixed D.C. (direct

current) voltage source. However they can sustain transformer operation in a transient

condition(s) that allows resetting or reversal of the transformers magnetic flux levels. AnA.C. voltage source keeps reversing the polarity of the voltage

being applied across the transformer. Consequently the magnetic

fields keeps reversing. Voltage reversal can also beaccomplished with a D.C. source such as a battery. The

connections between the D.C. source and the

transformers are repeatedly switched, thereby reversing the

voltage polarity across the transformer, hence reversing the magnetic field. Thetransformer can also be switched off from the D.C. source. In this case the magnetic field

simply collapses until it reaches its residual value (ideally equal to zero). This collapse

resets the transformers magnetic field. Switch mode power transformers (and supplies)

get their name from the switching action needed to sustain transformer operation. Bycontrolling the amount of on time and off time of the switches, one can also control

the amount of power delivered to the transformers load (or load circuit). The voltage canbe fed to the switch mode power transformer in voltage pulses. The pulse duration is a

portion of an overall cycle time. The cycle time is equal to the inverse of the operating

frequency. The terms duty cycle and pulse width modulation arise from the control

of the switching on time and off time.


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Switch mode power transformers are used extensively in electronic applications, usually

within a switch mode power supply. A switch mode power supply is usually poweredfrom a D.C. source, such as a battery. The switching mode power supply converts the

input D.C. source to one or more output D.C. sources. The power supplies are often

referred to as DC to DC converters. In similar fashion, the switch mode power

transformers are often referred to as DC to DC transformers (or DC-DCtransformers). A switch mode power transformer can have several secondary windings.

Consequently, the switch mode transformers permits multiple outputs which can be

electrically isolated from one another. Transformer action permits one to step up orstep down the voltage as needed via an appropriate turns ratio. Pulse width modulation

is used to provide voltage regulation.

Many electronic applications require some sort of power supply which converts powerfrom the conventional low frequency sinusoidal A.C. wall socket (for example, 115V 60

Hz) to the necessary voltage, current, and/or waveform required by the circuit. Typically

the circuits need a well-regulated D.C. voltage. Designers often choose either a rectifier

type circuit (to convert A.C. voltage to D.C. voltage), a switch mode power supply, orboth. For the both case, the A.C. voltage is first rectified to provide a D.C. voltage. The

D.C. voltage varies as the A.C. voltage varies, hence good voltage regulation cannot be

assured. One or more switching mode power supplies follow the rectifying circuitry. Theswitching mode power supplies provide a more tightly regulated output voltage. A.C.

rectification is not a necessity. Although tricky, it is possible, through switching actions,

to divide (chop) the A.C. waveform into a series of pulses, which are directly fed intothe switching mode power transformer. Pulse width modulation is used to control the

regulation.

Butler Winding can make (and has made) switching mode power transformers (and /or

inductors) for Buck, Flyback, and Boost applications (discussed below) in a wide varietyof shapes and sizes. This includes; various standard types of core with bobbin

structures (E, EP, EFD, PQ, POT, U and others), toroids, and some custom designs. Our

upper limits are 40 pounds of weight and 2 kilowatts of power. We have experience withfoil windings, litz wire windings, and perfect layering. For toroids, we can (and have

done) sector winding, progressive winding, bank winding, and progressive bank winding.

Butler winding has a variety of winding machines, bobbin/tube and toroid. That includestwo programmable automated machines and a taping machine for toroids. Butler Winding

has vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure

quality, Butler Winding purchased two programmable automated testing machines. Mostof our production is 100% tested on these machines. For more information on Butler


Switching Mode Power Transformers, Basic Application Circuits

The design of a switch mode power transformer will differ depending upon the type of

circuit used. There are many variations of switching mode power supplies, but they canbe narrowed down to three basic circuit configurations (each also has a mirrored

configuration); Buck, Boost, and Flyback. Be aware that the name for the Buck


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circuit varies from industry to industry and from person to person. It may also be referred

to as an inverter, D.C. converter, forward converter, feed forward, and others.There are also unipolar and bipolar (push-pull) versions. The basic Buck circuit is

illustrated in Figure 1A with an inductor and in Figure 1B with both a switch mode power

transformer and an inductor. A push-pull version is shown in Figure 4. The basic

Flyback circuit is illustrated in Figure 2A with an inductor and in Figure 2B with aswitch mdoe power transformer. The basic boost circuit is illustrated in Figure 3A with

an inductor, Figure 3B and 3C with a transformer and in Figure 5 with a push-pull

forward converter type of switch mode power transformer. The circuits shown in Figures1A, 2A, and 3A, which have no switch mode power transformers, are the simplest

circuits. They are useful for explaining the operating theory.

The Forward Converter (Buck) Circuit

The inductors in all of the buck circuits act as filtering elements to smooth out the ripple

and reduce peak currents. Since they must store energy for part of a cycle they usually

have a discrete air gap(s) or a distributed air gap in the magnetic core path.

The switch mode power transformer in the Buck Circuit of Figure 1B couples energy

from the input side (primary) to the output side (secondary). An ideal transformer doesnot store any energy and consequently does not provide any ripple filtering. The inductor

does the ripple filtering. Ideally, a Buck circuit transformer couples energy without

storing it (hence it meets the true definition of a transformer). The transformer does not

need to do any ripple filtering. The transformer should have minimal air gap. The ontime on the transistor (switch) controls how much energy is delivered to the capacitor

hence it regulates the output voltage. Note that for the inductor circuit of Figure 1, the

average capacitor voltage can never be more than the source voltage even for ideal circuit

components. Real life voltage drops (diode, transistor, winding resistance) ensure that theaverage output voltage will be less than the source voltage. The transformer in Figures1B remove this voltage limit and can also provide electrical isolation between input and

output.

The circuits of Figures 1A and 1B are unipolar applications of forward converters. Push-

pull versions, such as that shown in Figure 4, are bipolar applications. Unipolar andbipolar applications are explained further below. Click on the available link for more

information about push-pull switching mode power transformers.


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Inductive Flyback (Kickback) in Switch Mode Power Transformers

Unlike the Buck transformer; the flyback inductor, flyback transformer, boost inductor,and boost transformer intentionally store energy during the on time (charging portion)

of a cycle and then discharge energy during the off time portion. (Technically, since

they intentionally store energy, the switch mode flyback and boost power transformersare not true transformers.) They usually have a discrete air gap(s) or a distributed gap intheir cores magnetic path. The transistor is turned on and current flows into the inductor

or transformer (which has inductance). When the transistor is turned off, the input current

that formed and maintained the cores magnetic field become zero. The magnetic fieldcollapses causing a voltage reversal to occur in the inductor or transformer. The

collapsing magnetic field induces sufficiently high voltage (known as inductive kickback

voltage) to discharge energy into the capacitor connected to the inductor or to the switchmode power transformer secondary. Inductive discharge into the capacitor continues until

the magnetic field completely dissipates or power is restored to the input. Restoring the

power starts the inductive charging cycle again. The use of inductive kickback permit the

output voltages of the inductor circuits of Figures 2A and 3A to be either lower, equal, orgreater than the input source voltage. A transformer step up is not needed to achieve

voltages higher than the source voltage. Flyback transformers are usually preferred over

flyback inductors. The appropriate turns ratio can optimize current levels. Thetransformer can provide voltage isolation between input and output, and removes a

polarity restriction that comes with a flyback inductor design.

Boost Inductor Circuits

You might ask what distinguishes the boost inductor application from the flyback

inductor application. One characteristic is the polarity reversal of the output capacitor due

to the placement of the circuit components. Compare the circuits of Figures 2A and 3A.The diode in the flyback circuit, Figure 2A, completely blocks direct flow of current from

the input source to the capacitor regardless of the capacitors voltage value. The capacitor

can only be charged by the inductive kickback. The diode in the boost circuit, Figure 3A,

permits current flow from the input source to the capacitor without the use of inductivekickback if the capacitor voltage is sufficiently low. Consequently it both stores energy

and passes through energy during the charging portion of a cycle. Pass through

current flow stops whenever the capacitor voltage approaches the value of the source


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voltage minus the diode voltage drop. (Further increase requires the inductive kickback

voltage.) This may be a desirable feature for rapid power supply startup

Few designers are aware of the boost transformer circuit shown in Figure 3B because the

circuit is not very practical. With only half-wave rectification it is either a forward (Buck)

converter transformer application or a flyback transformer application depending onchoice of polarity. Full wave rectification, as shown, permits it to duplicate the boostinductor actions discussed in the prior paragraph; both storing energy and passing

through energy (by transformer coupling like a Buck transformer) during the charging

portion of a cycle if the secondary capacitor voltage is sufficiently low. It acts likes aflyback transformer during the discharging portion of the cycle. It is rarely used with the

full wave rectification as shown. It has seen some limited use as modified in the circuit

shown in Figure 3C. The transformer has two secondary windings. One is used as a

Forward (Buck) converter. The other is used as a flyback. It effectively divides the full-

wave rectification into two half-wave applications. A more common boost inductorapplication is shown in Figure 5. A boost inductor is used with a push-pull (Buck)

transformer. High power power supplies might use this type of circuit. In this

application both switches are not open at the same time. Both switches are closed tocharge the inductor, otherwise the switches are alternated on and off with one closed and

one open.


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Unipolar versus Bipolar

What is the difference? When a current flows through an inductor or a transformer amagnetic field is created in its core. The value of the magnetic field will be greater than

zero and it will have a direction associated with it. This direction is also referred to as the

polarity of the field. If the value of the current varies, then the value of the magnetic fieldwill vary accordingly, but the field polarity (direction) will remain the same as long as the

current direction does not reverse. When an inductor or transformer continually operates

with the same magnetic polarity it is a unipolar application. The circuits shown in Figures

1 through 3, including A thru C versions, are all unipolar applications. Applications were

the magnetic field polarity is continually reversing are bipolar applications. A.C.applications are bipolar applications. Push-Pull types of forward converters (Buck) are

bipolar applications. Push-pull transformers are often used in inverter circuits tocreate A.C. voltage from a D.C. source. A push-pull center-tap application is shown in

Figure 4. There are several types of push-pull applications. More information about

push-pull transformer applications is available on this website. Click on the available

link.

Electronic Transformer - Inverter

TransformerThe term "inverter" is associated with several different electronic applications.

In logic circuits "inverter" may be a logic inverter, the equivalent of a "Not" gate. Inanalogue signal processing an inverter can be a circuit which inverts the phase of the

signal being transmitted. In power conversion applications an inverter is an electronic

transformer which converts power from a Direct Current (D.C.) source into AlternatingCurrent (A.C.) power. Power conversion inverters can be divided into two sub-categories,


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voltage-fed inverters and current-fed inverters. Voltage-fed inverters are more common

than the current-fed inverters. The electronic transformers used in inverter circuits areoften called inverter transformers. Inverters produce A.C. power by switching the polarity

of the D.C. power source across the D.C. power sources load. The early inverters used

mechanical switches to do the switching. Vacuum tubes replaced mechanical switches in

low power applications. Eventually semiconductor based switches (diodes, transistors,F.E.T.s, S.C.R.s, etc.) replaced both mechanical and vacuum tube switches.

The schematic in Figure 1A illustrates a very simple inverter circuit. The circuit does not

have an inverter electronic transformer. The switches are alternated on and off (cycled),but are not on at the same time. The load will see alternating square wave pulses of

voltage equal to the source voltage minus the circuits resistive voltage drops. The pulse

voltage cannot be adjusted, but the average load voltage can be made less than the sourcevoltage by holding both switches open (off) at the same time.

The portion (ratio < 1) of time during a cycle that a switch is on is called the duty

cycle. The inverter schematic in Figure 1B utilizes a capacitor and another switch to

provide a lower load voltage. One switch controls the amount of charge delivered to thecapacitor hence it also controls the capacitor voltage. The set of two switches alternately

switches the polarity for the connection between the capacitor and the load. The load

voltage cannot exceed the input source voltage.

The inverter schematic of Figure 1C adds an electronic transformer inverter with two

secondary windings. The switching action sends alternating current through the inverter

transformers primary winding. This is referred to as push-pull action. The core hasbipolar utilization. Bipolar utilization is discussed further below. The inverter

transformers turns ratio can permit either higher or lower load voltage. The inverter

transformers output is an A.C. square wave. Output filter networks can be used to obtain

sine wave output. The inverter transformer can also provide electrical isolation betweenthe inverter transformers input and output sides. Full wave rectification can be applied to

the inverter transformers outputs to obtain a D.C. voltage of different value than that of

the input source. This is shown in the schematic of Figure 1D.

Compare the schematic of Figure 2A to the one in Figure 1D. Note in figure 2A thecenter-tap connections on the electronic transformer windings, a set of two switches

instead of a set of four switches on the input side, the two diodes on the secondary instead

of four, and the output filter inductor between the capacitor and load. The invertertransformer center-taps allow use of fewer switches and diodes. The inductors smooth out

the current surges from the rectification thereby maintaining tighter output voltage

regulation (less ripple voltage). The circuit in Figures 2A depicts a typical Push-PullForward Converter circuit. Be aware that the name for a Forward Converter circuit

(and transformer) varies from industry to industry and from person to person. It may also

be referred to as Buck, inverter, D.C. converter, feed forward, and others. Thereare also unipolar versions and there are bipolar versions that utilize saturable transformers

to trigger transistor switching.


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Since the connections of the electronic transformer "inverter" are alternated, the current

direction through the electronic transformer will also alternate. Consequently themagnetic field polarity of the inverter transformers core will alternate between positive

and negative flux directions. This is known as bipolar utilization of the inverter

transformers core. This is graphically illustrated in Figure 2B. The B-H curve shown

is also known as a hysteresis loop. The area inside the loop is related to the core loss. Athinner loop means less core loss. Also note the residual flux density point. In a Unipolar

application the flux density, B, would never return to zero value. It would stop at Br

when the current (hence also the magnetizing force, H) returns to zero. The appliedvoltage reversal (by switching action) ensures that the flux density returns to zero.

Bipolar utilization permits use of a smaller core than unipolar utilization because it

permits a larger change in the cores flux density. Fewer turns are needed to handle the

same amount of power. Compare Figure 2B to Figures 3C, 4C, and 5C.

Unipolar utilization occurs if the magnetic flux remains in one direction. The value may

vary up and down but does not cross zero value. A unipolar application is illustrated in

Figures 3A, 3B, and 3C. Some designers may refer to the transformer in Figure 3A as aninverter transformer, but it is not. It is serving as a pulse transformer with a resistive load.

If we assume it to be an ideal transformer, then there is no core loss, no leakage

inductance, does not store any energy, and the residual flux density is zero. Figure 3Bshows the expected output if a rectangular voltage pulse is placed across the transformer

(turn switch on, then off). The output will also be a rectangular pulse without any

distortion. There will be a change in amplitude because of the transformers turns ratio.The ideal transformers lack of stored energy eliminates the possibility of an inductive

kickback voltage spike. This circuit does not produce an A.C. output, hence no true

inverter action.

A non-ideal electronic transformer has finite inductance hence it stores some inductive

energy in its magnetic field. A lower inductance results in more stored energy. Consider

the non-ideal gapped transformer in the circuit shown in Figure 4A. The gap lowers theinductance of the transformer; consequently more current can flow when the switch is

closed (compared to no gap). When the switch is closed the transformer directly couples

power to the load plus it stores energy in its magnetic field. The field is created by themagnetizing current. The current flow due to the load does not contribute to the stored

energy. When the switch is opened the magnetic field collapses. The collapse creates an

inductive kickback voltage of reversed polarity. The induced secondary voltage causes


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current to flow through the load resistor in the reversed direction. (This is how a flyback

transformer functions.) The load sees alternating current although it usually has anasymmetrical waveform. One could claim that the circuits and transformer have inverter

action.

The energy stored in the electronic transformers magnetic field is dissipated as heatproduced by current flowing through the load resistor. Current of declining value willcontinue to flow until either all of the stored energy is dissipated or the switch is closed

again. If completely dissipated, then the output shown in Figure 4B and the generalized

hysteresis loop of Figure 4C apply. The transformer is said to be operating indiscontinuous mode. The load voltage and load current reach zero value, and the cores

flux density reaches its residual value. Note that the flux density averaged over time is

greater than zero. This holds for all unipolar applications. If the switch is closed againbefore all the energy is dissipated, then the output shown in Figure 5B and the

generalized hysteresis curve of Figure 5C applies. The transformer is said to be operating

in the continuous mode. The load voltage and load current remain above zero value, and

the flux density does not reach its residual value. The output waveform in Figure 5B ismore rectangular than that of Figure 4B.

The circuits in Figures 4A and 5A are not very practical inverter transformer circuits. To

be useful the transformer must store as much energy as it directly couples to its load.Consequently, the transformer will tend to be lightly loaded and designed to have

appreciable magnetizing current. Output filters would be required to produce a more

symmetrical output waveform. These circuits find little use as shown here. There are


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D.C. biased unipolar applications, which function as inverters. They are not discussed

here.

Saturable Transformers as Inverter Transformers

Figure 6A shows a Royer Inverter Circuit schematic that uses saturable transformers.The saturable transformer also functions as the inverter transformer. Figure 6B shows a

Jensen Circuit which uses a saturable transformer and a power transformer. The power

transformer functions as the inverter transformer. Both of these circuits make use ofpush-pull switching to achieve the inverter action. The feature of these two circuits is

the transistor switching action that is activated by a voltage spike created when thesaturable transformer enters saturation. An oscillation develops which maintains the

necessary switching action. The theory of operation is not discussed here. It may be

available on this website at some future date from the issue date of this website page.

Check the available links.

Buck Boost Transformer - Push Pull

Transformer

When it comes to power conversion, the buck boost or "push pull" transformerapplication is well known. The buck boost transformer configuration is widely used in

converting direct current (D.C.) voltage into another value of D.C. voltage, and in

inverters. Inverters convert direct current into alternating current (A.C.). The push pulltransformer is usually the preferred choice in high power switching transformer

applications exceeding one kilowatt. It is usually used in a circuit known as a "forward

converter" circuit. Be aware that the name for the "forward converter" circuit varies fromindustry to industry and from person to person. It may also be referred to as an "inverter",

"D.C. converter", "buck", "feed forward", and others. A basic "forward converter"

transformer circuit is illustrated in Figure 1A. It is not a push pull transformer

application. The output inductor reduces ripple voltage. Pulse width modulation is used to

control the value of the output voltage
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A center-tapped buck boost transformer application circuit is illustrated in Figure 2A.

Figure 2A only shows one output. Multiple voltage outputs are possible by using either atapped secondary winding or using multiple secondaries. Some other buck boost

transformer versions are discussed further below. They are illustrated in Figures 3, 4, 5,

and 6. (These include some push pull transformers without the center-taps.)

The core of the transformer in Figure 1A is operated in a unipolar fashion. Unipolaroperation is depicted graphically in Figure 1B. The core's magnetic "B-H" loop remains

in one quadrant of the "B-H" grid. A loop occurs once every cycle. The flux density "B"

and the magnetizing force "H" never cross zero hence always retain the same (or one)polarity. "H" does not have to return to zero value. The core in a push pull transformer

has bipolar operation. Both "B" and "H" cross zero value and reverse polarity. Bipolar

operation is depicted graphically in Figure 2B. Note that the "dB" value (change in B) inFigure 2B for the bipolar push pull transformer can be more than twice the "dB" value

shown in Figure 1B for the unipolar forward converter (assuming the same core

material). Push pull transformer (bipolar) operation permits one to handle the same

amount of power in a smaller package than for that of a unipolar operation. There aretradeoffs. The buck boost transformer operation requires more switching elements and its

control circuitry is more complicated. Consequently a push pull transformer application is

more expensive. The voltage pulses must be adequately controlled to avoid phenomenaknown as saturation walk. Center tapped push pull transformers have winding

capacitance issues at higher frequencies. Winding imbalances can contribute to saturation

walk.

Power ratings for push pull or buck boost transformer can vary from a fraction of a Watt

to Kilowatts. Megawatts is possible, but definitely beyond Butler Winding's capabilities.

Size correlates with power hence size (and weight) can vary from a fraction of a cubiccentimeter (several grams) to multiple cubic meters (thousands of kilograms). Buck boost

transformers can be wound on toroids, bobbins, and tubes. Core materials vary dependingon the application. Laminated or tape wound grain oriented silicon steel is common for

low frequency inverter buck boost transformers. Ferrite core materials are common for

high frequency switching push pull transformers. If minimal size is a requirement, nickel-iron alloys may be chosen for the 1 to 20 kilohertz range. Minimal energy storage is

desired so cores have minimal air gaps in their structure.

Butler Winding manufactures buck boost transformers in a wide variety of shapes and

sizes. This includes; various standard types of core with bobbin structures (E, EP, EFD,

PQ, POT, U and others), toroids, and some custom designs. Our upper limits are 40pounds of weight and 2 kilowatts of power. We have experience with foil windings, litz

wire windings, and perfect layering. For toroids, we can (and have done) sector winding,

progressive winding, bank winding, and progressive bank winding. Butler winding has avariety of winding machines, bobbin/tube and toroid. That includes two programmable

automated machines and a taping machine for toroids. Butler Winding has vacuum

chamber(s) for vacuum impregnation and can also encapsulate. To ensure quality, Butler

Winding purchased two programmable automated testing machines. Most of our
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production is 100% tested on these machines. For more information on Butler Winding's

capabilities, click on our "capabilities" link.

Push Pull - Buck Boost Transformer Rectification

The push pull / buck boost transformer in Figure 3 is the same as the push pull

transformer in Figure 2A except for secondary rectification. Figure 2A achieves full waverectification using a center-tap. It requires two diodes. Figure 3 achieves full wave

rectification with a full wave bridge. It requires four diodes. Four diodes result in morepower loss, but elimination of the center-tap simplifies transformer construction and

reduces winding capacitance. The primary and secondary winding halves as shown in

Figure 2A conduct current on alternate half cycles. Their maximum duty cycle is a 0.5ratio (or 50%). Figure 3 requires approximately half of the secondary turns of Figure 2A,

but its secondary winding may see a maximum duty cycle near 1 (or 100%), hence its

wire must handle twice the r.m.s. current value. Both transformers are about the same

size.


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Half Bridge Push-Pull Transformers

Compare figure 4 to figure 2A. Figure 4 is a half bridge push pull / buck boosttransformer application. This configuration eliminates the primary center-tap and reduces

primary winding capacitance. The two series connected capacitors shown in Figure 4

effectively cut the input voltage to the push pull transformer in half. Consequently, forthe same power rating, the push pull / buck boost transformer requires one quarter of the

total primary turns to support the halved voltage, but it must handle twice the amount of

input current. The primary winding may see a maximum current duty cycle near 1, henceits wire may see 4 times the r.m.s current value as wire used in the primary winding

halves of Figure 2A. Both transformers are about the same size. To achieve the sameoutput voltage, the number of secondary turns is about the same as that of figure 2A, but

the secondary over primary turns ratio is quadrupled because the primary turns of figure 4

are one quarter of that of figure 2A. The output of figure 4 is a full wave center-tap

configuration. Alternately, it could be a full wave bridge configuration withapproximately half the number of secondary turns.

Full Bridge Push Pull Transformers


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Compare figure 5 to figure 4. Figure 5 is a full bridge push pull / buck boost

transformer application. Like the half bridge configuration of figure 4, this configurationeliminates the primary center-tap, reduces primary winding capac-itance, & is about the

same size. The two series connected capacitors are replaced by two additional transistors

as shown in Figure 4. The voltage supplied to the input of the push pull transformer of

figure 5 is the same as that for figure 2A. For the same power rating and source voltage,the push pull transformer of figure 5 requires half the primary turns as that of figure 2A

and it must handle the same amount of input current. The primary winding of figure 5

may see a max current duty cycle near 1, hence its wire may see 2 times the r.m.s currentvalue as wire used in the primary winding halves of Figure 2A. For the same output

voltage, the number of secondary turns is about the same as that of figure 2A, but the

secondary over primary turns ratio is doubled because the primary turns (fig. 5) arehalved. The output of figure 5 is a full wave center-tap configuration. Alternately, it

could be a full wave bridge configuration with approximately half the number of

secondary turns.

The Boost Push Pull Transformer Application

The prior push pull transformer applications utilize an inductor in the output circuit to

reduce output voltage ripple. If there were more than one output, an inductor would beused with each output. An alternate would be to place one inductor in series with the

primary center-tap of a push-pull center-tap transformer. This circuit is illustrated in

Figure 6. To charge the inductor the two transistors are made to conduct at the same time.Charging current flow through both halves of the primary winding but in opposite

directions resulting in magnetic cancellation of each other hence the transformer

windings act as a short to ground. Opening one of the transistor switches results in

current flow in only one of the primary winding halves. Alternate opening of thetransistor switches results in a push-pull transformer action. Control circuitry is more

complex.


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Pulse Transformers

The magnetic flux in a typical A.C. transformer core alternates

between positive and negative values. The magnetic flux in the

typical pulse transformer does not. The typical pulse transformeroperates in an unipolar mode ( flux density may meet but does

not cross zero ).

A fixed D.C. current could be used to create a biasing D.C.

magnetic field in the transformer core, thereby forcing the field to

cross over the zero line. Pulse transformers usually (not always) operate at high

frequency necessitating use of low loss cores (usually ferrites). Figure 1A shows theelectrical schematic for a pulse transformer. Figure 1B shows an equivalent high

frequency circuit representation for a transformer which is applicable to pulse

transformers. The circuit treats parasitic elements, leakage inductances and winding

capacitance, as lumped circuit elements, but they are actually distributed elements. Pulsetransformers can be divided into two major types, power and signal.

An example of a power pulse transformer application would be precise control of aheating element from a fixed D.C. voltage source. The voltage may be stepped up or

down as needed by the pulse transformers turns ratio. The power to the pulse

transformer is turned on and off using a switch (or switching device) at an operatingfrequency and a pulse duration that delivers the required amount of power. Consequently,

the temperature is also controlled. The transformer provides electrical isolation between

the input and output. The transformers used in forward converter power supplies are

essentially power type pulse transformers. There exists high-power pulse transformerdesigns that have exceeded 500 kilowatts of power capacity.

The design of signal type of pulse transformer focuses on the delivery of a signal at the

output. The transformer delivers a pulse-like signal or a series of pulses. The turns ratio

of the pulse transformer can be used to adjust signal amplitude and provide impedance

matching between the source and load. Pulse transformers are often used in thetransmittal of digital data and in the gate drive circuitry of transistors, F.E.T.s, S.C.R.s,and etc. In the latter application, the pulse transformers may be referred to as gate


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transformers or gate drive transformers. Signal type of pulse transformers handle

relatively low levels of power. For digital data transmission, transformers are designed tominimized signal distortion. The transformers might be operated with a D.C. bias current.

Many signal type pulse transformers are also categorized as wideband transformers.

Signal type pulse transformers are frequently used in communication systems and digital

networks.

Pulse transformer designs vary widely in terms of power rating, inductance, voltage level

(low to high), operating frequency, size, impedance, bandwidth (frequency response),

packaging, winding capacitance, and other parameters. Designers try to minimize

parasitic elements such as leakage inductance and winding capacitance by using windingconfigurations which optimize the coupling between the windings.

Butler Winding can make (and has made) pulse transformers in a wide variety of shapes

and sizes. This includes; various standard types of core with bobbin structures ( E, EP,EFD, PQ, POT, U and others ), toroids, and some custom designs. Our upper limits are

40 pounds of weight and 2 kilowatts of power. We have experience with foil windings,litz wire windings, and perfect layering. For toroids, we can ( and have done ) sectorwinding, progressive winding, bank winding, and progressive bank winding. Butler

winding has a variety of winding machines, bobbin/tube and toroid. That includes two

programmable automated machines and a taping machine for toroids. Butler winding has

vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensurequality, Butler Winding purchased two programmable automated testing machines. Most

of our production is 100% tested on these machines. For more information on Butler


PULSE TRANSFORMER OPERATING PRINCIPLES

Pulse transformer designers usually seek to minimize voltage droop, rise time, and pulse

distortion. Droop is the decline of the output pulse voltage over the duration of one pulse.

It is cause by the magnetizing current increasing during the time duration of the pulse. To
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understand how voltage droop and pulse distortion occurs, one needs to understand the

magnetizing ( exciting, or no-load ) current effects, load current effects, and the effects ofleakage inductance and winding capacitance. The designer also needs to avoid core

saturation and therefore needs to understand the voltage-time product.

Magnetizing ( No-Load ) Current, its Effects, and Its Relation to Saturation

Consider the simple pulse transformer circuit of Figure 2A and its equivalent circuit ofFigure 2B. There is no source impedance, winding capacitances, or secondary leakage

inductance to worry about. With both switches open, there cannot be any primary or

secondary currents flowing. Now close the primary switch. Since the secondary load isnot connected, the pulse transformers primary winding acts like an inductor placed

across a voltage source. Primary current begins to flow. This is the magnetizing current (

no secondary current ) and is governed by the differential equation V(t) = L x d(I)/dt +Rp x I(t), with units of volts, henries, amps, and seconds. If the power supply has

constant voltage, Rp = zero, & L = Lkp+Lm is constant, the differential equation can besolved for I(t), I(t) = Io + V x t / ( Lkp+Lm ), where Io = the initial current which equalszero. Notice that the current increases at a linear rate over time and that the rate in

inversely proportional to the inductance. The current flows through Np turns creating Np

x I(t) amount of magnetizing force ( amp-turns ) which in turns creates a magnetic flux

density in the pulse transformer core. Eventually the increasing primary magnetizingcurrent would exceed the magnetic flux capacity of the pulse transformer core and will

saturate the core. Once saturation occurs the primary current rapidly increases towards

infinity ( in theory ). In a real circuit the primary winding resistance ( and sourceimpedance ) would limit the current. See Figure 3A for graphical illustration. For non-

zero Rp, I(t) = Io + ( V/Rp ) x ( 1 e to the ( -Rp x t / ( Lkp + Lm )) power ). The effect

of Rp is graphically illustrated in Figures 3B and 3C. Rp extends the time it takes for theunloaded transformer ( or an inductor ) to saturate. If Rp is sufficiently large, it prevents

the transformer ( or inductor ) from saturating altogether. Regardless of saturation, Rp

places an upper limit on the primary current value.


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Voltage Droop

For Rp = 0 the source voltage divides proportional across Lkp and Lm hence the voltage

across Lm = V x Lm / ( Lm+Lkp ) = Vm. The induced secondary voltage becomes equalto Ns x Vm / Np. For Rp > zero a voltage drop occurs across Rp. The value of this dropincreases in value as the primary current increases with time, hence Vm decrease over

time and consequently the secondary voltage declines over time. Thus Rp and

magnetizing current contribute to secondary voltage droop. Lkp does not contribute to the

droop in the no-load case but does contribute to a lower secondary starting voltage forboth the no load and under load cases. Droop is graphically illustrated in Figure 4B.

Compare it against the ideal pulse shown in Figure 4A.

Voltage-time product
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Pulse transformers, being typically unipolar (D.C.) applications, require the primaryswitch to be opened ( thereby removing the voltage source ) before saturation occurs,whereas A.C. applications reversed the applied voltage before saturation occurs. Unipolar

applications require that sufficient time be allowed to pass to re-set the core before

starting the next pulse. This time permits the magnetic field to collapse ( reset ). The field

does not completely collapse to zero value ( unless forced to zero, or lower ) because ofcore material remanence. A slight air gap may be used to bring remanence closer to zero

value. The gap lowers the pulse transformer inductance. The flux range between

remanence and the maximum flux is referred to as dB, the maximum change in fluxdensity during the pulse duration, dt. The dB of the typical pulse transformer is less than

half for that of an A.C. application because flux in A.C. applications can go from positive

Bmax to negative Bmax. Operating frequency and maximum expected temperature affectthe choice of maximum usable flux density value, Bmax. Saturation can be avoided by

applying the following equation; dB x Np x Ac x Sf = V x dt x 100000000, where dt is

the maximum time duration of the pulse, Ac is the cores cross-sectional area and Sf is

the core stacking factor ratio. Units are gausses, turns, square centimeters, volts and

seconds. Be aware that dt does not include reset time, tr. Maximum operating frequencyequals 1 / ( dt + tr ). The voltage-time product, V x dt is quite useful. The size and cost of

a pulse transformer is roughly proportional to this product.

Kickback Voltage

In the foregoing discussion the primary switch was opened thereby interrupting the

current flowing through the transformer primary. The resulting collapse in the magneticfield will induce a voltage reversal in the transformer windings. The more rapid the field

collapse is, the higher the induced voltage. The transformer will try to dissipate the

energy stored in its collapsing magnetic field. If the transformer was under load, the

induced voltage would cause current to flow into the load. In the no-load case of thisexample, the transformer does not have any readily available place to dissipate the

energy. The transformer will generate the voltage necessary to dissipate the stored

energy, hence a high voltage kickback ( or flyback or backswing ) voltage will occur inthe windings. In a real circuit the transformer will induce eddy currents in its core thereby

dissipating the energy as core loss. In a real circuit the high voltages can damage the

switching elements ( transistors, F.E.T.s, S.C.R.s, etc. ). Many designs include protectivecircuitry across the primary winding.

Secondary Load Current Effects and Rise Time

Consider again the simple pulse transformer circuit of Figure 2A and its equivalent

circuit of Figure 2B. Initally, with both switches open, there cannot be any primary or

secondary currents flowing. Close the secondary load switch and then close the primaryswitch. Current flows through the primary winding. The L x dI(t)/dt action induces a

voltage in the primary winding which opposes the source voltage. A voltage, Vsi, is also

induced in the secondary winding causing secondary current to flow. The ampere-turnscreated by the secondary current work against the induced voltage that opposes the

source voltage. Consequently, the source voltage supplies more current flow through the


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primary. Currents rapidly increase until either the secondary current or primary current

encounters a current limitation. Examples of such limits are the secondary load andwinding resistances limiting the secondary current or the source impedance and primary

winding resistance and primary leakage inductance limiting the primary current. Once a

limit is encountered, an equilibrium is quickly established except for the magnetizing

current. The primary current has two components; Irs, the load current transformed (reflected ) to the primary winding and Im, the magnetizing current. As in the no-load

case, the magnetizing current starts at zero and increases over time. The pulse

transformer must be switched off before saturation occurs.

In this example the load is resistive, there is no secondary leakage inductance, and thereis no secondary winding capacitance; hence a purely resistive load current is reflected to

the primary winding. The primary current is larger than it was in the no-load case,hence more voltage drop is expected across the primary winding resistance. Consequently

less voltage, Vm, is available across Lm which results in less induced voltage in the

secondary winding. Secondary current flow through the secondary winding resistance

causes another voltage drop hence lower transformer output voltage. Under load, both theprimary and secondary winding resistance contribute to a lower secondary voltage. The

secondary winding resistance does not contribute to pulse droop.

The reflected load current, Irs, does not flow thorughthe mutual inductance, Lm, but doeflow through the primary leakage inductance, Lkp. Lkp restricts the flow of the primary

current ( hence reflected load current also ). Consequently the reflected load current

cannot immediately reach its full value ( nor can the secondary current ). It is effectivelydelayed. Until the reflected load current reaches its full value, a larger voltage drop will

occur across Lkp then there was in the no-load case. This larger voltage diminishes in

value over time. Consequently Vm exhibits a time delay in reaching peak voltage value.

This delay is also seen in the secondary output voltage. This delay is known as rise time.Rise time is graphically illustrated in Figure 4B.

Effects of Winding Capacitance, Secondary Leakage Inductance, and Core loss

Now consider the equivalent pulse transformer circuit of Figure 5. The circuit has all the

components of the circuit in Figure 2B, but also has primary winding capacitance,secondary winding capacitance, core loss, and secondary leakage inductance. Start with

both switches open and no capacitive energy and no inductive energy. All currents are

initially zero. Close the secondary switch then close the primary switch. The primary

leakage inductance, Lkp, restricts the flow of primary current by opposing the sourcevoltage. The opposing voltage is generated by Lkp x d(I)/dt action. Current flow ( from

the source ) finds the uncharged winding capacitance, Cp to be a much easier path, hence

a relatively large amount of current flows into the winding capacitance. This largeamount of current could be called a surge current because it will diminish over time as

the capacitance is charged. The surge causes a relatively large voltage drop across the

primary winding resistance, Rp, thereby initially lowering the voltage available to Lkpand Lm. Over time, as the surge current diminishes, the voltage drop across Rp

diminishes, and the voltage across Lkp and Lm reaches full ( peak ) value. The surge


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effectively delays the peak voltage across Lm. This in turn delays peak secondary

voltage. The delay contributes to rise time, hence Cp contributes to rise time. Asdiscussed earlier, Lpk restricts flow of the reflected load current and consequently also

contributes to rise

time. A

similar consequence occurs with the secondary winding capacitance, Cs. Any current

supplied by induced secondary voltage must charge Cs as the secondary voltage tries to

rise to peak value. This delays the secondary in reaching peak voltage, hence Cs also

contributes to rise time.

Secondary leakage inductance, Lks, restricts secondary current flow just like Lkp

restricted primary current flow. Lks also delays the secondary peak output voltage, hence

it also contributes to rise time.

Core loss resistance, Rc, provides a relatively small current shunt path across Lm just like

the reflected secondary load current does. It has the same effect but the effect is much

smaller.

To summarize, Winding capacitances and leakage inductances act to increase rise time. (They also generate trailing edges which is discussed later. ) They may also contribute to

spurious oscillations. In a typical pulse transformer design, core loss does not have much

effect.

The Trailing Edge

For an ideal pulse transformer, once the primary switch is opened the secondary pulse

should immediately end. This does not happen. The pulse transformer tries to dissipatethe energy stored in Lm and in the parasitic components Cp, Cs, Lkp, and Lks. The

inductance will induce voltages as their magnetic fields collapse. The capacitor charge

will drain, but will not drain instantaneously. The capacitances may temporarily supplycurrent to the inductances. As a result, there is a sloped decline of the secondary output


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voltage after the primary switch is opened. This sloped decline is referred to as the

trailing edge. Some combinations of capactiance and inductance could producespurious oscillations ( known as ringing ). A trailing edge is graphically illustrated in

Figure 3B.

Pulse Distortion

Ideally the output pulse waveform should be identical in shape to the input pulse

waveform except for a desired amplitude change due to the step-up or step-downturns ratio. Any other deviation is considered to be distortion. Rise time, droop, trailing

edges, and spurious oscillations are all considered to be signal distortions.

Figure 3B illustrates all of these distortions.

Electronic Transformer - Trigger

TransformersThere are many types of eletronic transformers. What distinguishes a trigger transformer

from other types of electronic transformers? Basically, it is application! As the word

trigger implies, a trigger transformer is used in a circuit that initiates some sort ofaction or event. Once initiated, some applications may no longer require continued

presence of a voltage to complete the action or event. Other applications may need the

voltage but for a limited amount of time. Regardless, the application provides a voltage

pulse to the trigger transformers primary. The trigger transformers turns ratio steps upor steps down the secondary voltage as needed. The trigger transformers secondary then

supplies voltage or current to its load. The load is usually the gate of a semiconductorswitch such as a transistor, F.E.T., S.C.R., etc.. The trigger transformer also providesvoltage isolation between the primary side circuit and the secondary side circuit. Most

circuit designers would refer to the trigger transformer as a type of pulse transformer.

This website provides some explanation on pulse transformer operation. Click on the

Electronic Transformers button and then select Pulse Transformer.

One example of a trigger transformer application is the electronic flash in modern

cameras. A basic circuit is shown in Figure 1. A charging circuit takes energy from a

battery and charges two electrolytic capacitors ( approx. 300V ). The negative sides areboth connected to ground. One capacitor is much larger than the other is. It is connected

to the electrodes of a glass tube filled with xenon gas. This capacitor provides the energy

needed to produce the flash, but lacks sufficient voltage to initiate the flash. The primaryof the trigger transformer is attached to the positive side of the smaller capacitor through

a switch. The trigger transformer secondary is connected to a metal plate(s) or grid(s) that

partially surrounds the glass tube. The trigger transformer is designed to step up thevoltage to high voltage levels. When the switch is closed the trigger transformer places

high voltage across the plates. The high voltage ionizes the gas inside the tube. The gas

becomes conductive. The large capacitor discharges through the gas thereby producing a


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bright white flash. The capacitor rapidly discharges its energy and must be recharged to

produce another flash. The switch between the trigger transformer and the smallercapacitor is opened. A small drain resistor is placed across the high voltage plates to

discharge the voltage on the plates. In this example the trigger transformer aided the

initiation ( or triggering ) of the flash by delivering a stepped up voltage pulse. Figure 1

shows the trigger transformer windings grounded together. With proper circuit design thetrigger transformer could also provide voltage isolation.

In the preceding example, the trigger transformer ( which is a pulse electronic

transformer ) design does not saturate the core and usually employs unipolar coreutilization. There are trigger transformer applications that use bipolar core utilization

and/or intentionally saturates the core. Bipolar core utilization mean the magnetic flux

alternates between positive and negative directions. Unipolar means the flux directionremains either positive or negative. Two examples of this are found in the Royer

Inverter Circuit and the closely related Jensen Circuit. These are shown in Figure 2A

and 2B. Operating theory will not be discussed in detail here but is briefly summarized;

transformer saturation repeatedly occurs in alternating directions which in turn triggers (switches ) the transistors on and off in alternating fashion, thereby creating an A.C.

output voltage. The switching of the transistors forces the current direction to alternate

which then forces the alternating direction of core saturation. For more information aboutsaturable transformers click on the Electronic Transformers button, then select

Saturable Transformers.

Figure 3 is a unipolar application which shows how a trigger transformer can use coresaturation can to shorten the time duration of a pulse. The trigger transformer usually has

a high impedance load ( lightly loaded ) hence it acts much like a saturated inductor but

with voltage step up or step down capability and voltage isolation. The primary winding

of the trigger transformer has much higher impedance than the series resistor untilsaturation occurs. Before saturation most of the circuits voltage drop is across the trigger

transformers primary. The trigger transformers turns ratio can adjust the secondary

output voltage. There will be voltage droop. After saturation, most of the voltage drop isacross the resistor, the secondary output voltage is substantially reduced, and the time

duration of the output pulse has been reduced. The pulses time duration can be

calculated from the transformers volt-second product. This website provides someexplanation of the volt-second product. Click on the Electronic Transformers button

and then select Pulse Transformer.

Butler Winding can make ( and has made ) pulse and trigger transformers. There are a

wide variety of shapes and sizes available. This includes; various standard types of corewith bobbin structures ( E, EP, EFD, PQ, POT, U and others ), toroids, and some custom

designs. Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have

experience with foil windings, litz wire windings, and perfect layering. For toroids, wecan ( and have done ) sector winding, progressive winding, bank winding, and

progressive bank winding. Butler winding has a variety of winding machines,

bobbin/tube and toroid. That includes two programmable automated machines and ataping machine for toroids. Butler winding has vacuum chamber(s) for vacuum
http://www.butlerwinding.com/elelectronic-transformer/pulse_transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/pulse_transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/pulse_transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/pulse_transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/pulse_transformer/index.htmlhttp://www.butlerwinding.com/elelectronic-transformer/pulse_transformer/index.html


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impregnation and can also encapsulate. To ensure quality, Butler Winding purchased two

programmable automated testing machines. Most of our production is 100% tested onthese machines. For more information on Butler Windings capabilities, click on our

capabilities link.

Gate Drive Transformers - Electronic

TransformerThere are many types of transformers. What distinguishes a gate drive transformer fromother types of transformers? Basically, it is application! Modern day electronic circuits

utilize many gated semiconductor devices such as ordinary transistors, field effect

transistors, and S.C.R.s and others. Gate drive transformers are used in some of these

circuits. A signal must be supplied to ( or removed from ) the devices gate node toactivate ( or deactivate ) the device. When used, gate drive transformers are located


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within the circuitry driving the gate. Gate drive transformers are used to modify the

voltage level to the gate, provide impedance matching, and to provide voltage isolation.Gate drive transformer may be used to deliver voltage to the grids or plates of a vacuum

tube or flash tube.

Some gate drive transformers simply deliver a voltage pulse or a series of voltage pulsesto a semiconductor gate. A gate drive transformer functioning in this manner could alsobe called a pulse transformer. Most circuit designers would consider these gate drive

transformers to be a type of pulse transformer. If the gate drive transformers pulse

initiates some action or event, the gate

Documents

Types of Transformers