PULSE TRANSFORMERPULSE TRANSFORMER

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A pulse transformer is a transformer that is optimised A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (i.e, pulses for transmitting rectangular electrical pulses (i.e, pulses with fast rise and fall time and constant amplitude).with fast rise and fall time and constant amplitude).

There are generally two type of pulse transformer. These There are generally two type of pulse transformer. These areare

1.signal pulse transformer1.signal pulse transformer

2.power pulse transformer2.power pulse transformer

Another type of special pulse transformer is there high Another type of special pulse transformer is there high voltage pulse transformer.voltage pulse transformer.

These different transformers are used These different transformers are used

in digital logic, telecommunication, to in digital logic, telecommunication, to

interface control circuit and power interface control circuit and power

circuit, pulsed power application. circuit, pulsed power application.

EQUIVALENT CIRCUIT OF PULSE TRANSFORMER

TERMS IN PULSE TERMS IN PULSE TRANSFORMERTRANSFORMER

1.Magnetizing ( No-Load ) Current, its Effects, and Its 1.Magnetizing ( No-Load ) Current, its Effects, and Its Relation to SaturationRelation to Saturation

2.Voltage Droop2.Voltage Droop

33.. Voltage-time product Voltage-time product

4. Kickback Voltage4. Kickback Voltage

5.Rise Time5.Rise Time

66. . The Trailing EdgeThe Trailing Edge

7. Pulse Distortion7. Pulse Distortion

1.Magnetizing ( No-Load ) Current, its 1.Magnetizing ( No-Load ) Current, its Effects, and Its Relation to SaturationEffects, and Its Relation to Saturation

Consider the simple pulse transformer circuit and it’s equivalent Consider the simple pulse transformer circuit and it’s equivalent circuit.circuit.

• Now close the primary switch. Since the secondary load is not Now close the primary switch. Since the secondary load is not

connected, the pulse transformer’s primary winding acts like an connected, the pulse transformer’s primary winding acts like an inductor placed across a voltage source. Primary current begins to inductor placed across a voltage source. Primary current begins to flow. This is the magnetizing current .flow. This is the magnetizing current .

• Rp = zero, & L = Lkp+Lm is constant then the current increases at Rp = zero, & L = Lkp+Lm is constant then the current increases at a linear rate over time and that the rate in inverselya linear rate over time and that the rate in inversely

proportional to the inductance . The increasing primary proportional to the inductance . The increasing primary magnetizing current would exceed the magnetic flux capacity of magnetizing current would exceed the magnetic flux capacity of the pulse transformer. Andthe pulse transformer. And

will saturate the core Oncewill saturate the core Once

saturation occurs the primary saturation occurs the primary

current rapidly increases current rapidly increases

towards infinity ( in theory ). towards infinity ( in theory ).

• In a real circuit the primary winding resistance ( and source In a real circuit the primary winding resistance ( and source impedance ) would limit the currentimpedance ) would limit the current. .

• For non-zero Rp, I(t) = Io + ( V/Rp ) x ( 1 – e to the ( -Rp x t / ( Lkp For non-zero Rp, I(t) = Io + ( V/Rp ) x ( 1 – e to the ( -Rp x t / ( Lkp + Lm )) power ) + Lm )) power )

• Rp extends the time it takes for the unloaded transformer ( or an Rp extends the time it takes for the unloaded transformer ( or an

inductor ) to saturateinductor ) to saturate

2.Voltage Droop2.Voltage Droop

• The voltage droop is the decline of output pulse over duration of The voltage droop is the decline of output pulse over duration of one pulse.one pulse.

• For Rp > zero a voltage drop occurs across Rp. The value of this For Rp > zero a voltage drop occurs across Rp. The value of this drop increases in value as the primary current increases with time, drop increases in value as the primary current increases with time, hence Vm decrease over time and consequently the secondary hence Vm decrease over time and consequently the secondary voltage declines over timevoltage declines over time. .

• Thus Rp and magnetizing current contribute to secondary voltage Thus Rp and magnetizing current contribute to secondary voltage droop droop

• Lkp does not contribute to the droop in the “no-load” case but Lkp does not contribute to the droop in the “no-load” case but does contribute to a lower secondary starting voltage for both the does contribute to a lower secondary starting voltage for both the

“no load” and “under load” cases.“no load” and “under load” cases.

3.3. Voltage-time product Voltage-time product• Unipolar applications require that sufficient time be allowed to Unipolar applications require that sufficient time be allowed to

pass to re-set the core before starting the next pulse.pass to re-set the core before starting the next pulse. This time This time permits the magnetic field to collapse .permits the magnetic field to collapse .

• The field does not completely collapse to zero value ( unless The field does not completely collapse to zero value ( unless forced to zero, or lower ) because of core material remanence. A forced to zero, or lower ) because of core material remanence. A slight air gap may be used to bring remanence closer to zero slight air gap may be used to bring remanence closer to zero value. The gap lowers the pulse transformer inductance. value. The gap lowers the pulse transformer inductance.

• Saturation can be avoided by applying the following equation; Saturation can be avoided by applying the following equation;

dB x Np x Ac x Sf = V x dt x 100000000dB x Np x Ac x Sf = V x dt x 100000000 dt=the maximum time duration of the pulsedt=the maximum time duration of the pulse

v=voltagev=voltage

Ac=the core’s cross-sectional area Ac=the core’s cross-sectional area

Sf=the core stacking factor ratio. Sf=the core stacking factor ratio.

Np=number of primary turnsNp=number of primary turns

dB=flux range between remanence and the maximum flux dB=flux range between remanence and the maximum flux

dt=maximum time duration of the pulse dt=maximum time duration of the pulse

4. Kickback Voltage4. Kickback Voltage

• If the primary switch was opened thereby interrupting the current If the primary switch was opened thereby interrupting the current flowing through the transformer primary. The resulting collapse in flowing through the transformer primary. The resulting collapse in the magnetic field will induce a voltage reversal in the transformer the magnetic field will induce a voltage reversal in the transformer

windingswindings ..

• The more rapid the field collapse is, the higher the induced The more rapid the field collapse is, the higher the induced

voltage.voltage. • In the no-load case transformer will generate the voltage In the no-load case transformer will generate the voltage

necessary to dissipate the stored energy, hence a high voltage necessary to dissipate the stored energy, hence a high voltage “kickback” ( or flyback or backswing ) voltage will occur in the “kickback” ( or flyback or backswing ) voltage will occur in the windings. windings.

• The transformer will induce eddy currents in its core thereby The transformer will induce eddy currents in its core thereby dissipating the energy as core loss dissipating the energy as core loss

5.Rise Time5.Rise Time• The primary leakage inductance, Lkp, restricts the flow of primary The primary leakage inductance, Lkp, restricts the flow of primary

current by opposing the source voltage .current by opposing the source voltage .• The opposing voltage is generated by Lkp x d(I)/dt action. Current The opposing voltage is generated by Lkp x d(I)/dt action. Current

flow ( from the source ) finds the uncharged winding capacitance, flow ( from the source ) finds the uncharged winding capacitance, Cp to be a much easier path, hence a relatively large amount of Cp to be a much easier path, hence a relatively large amount of

current flows into the winding capacitance.current flows into the winding capacitance. • The currentThe current will diminish over time as the capacitance is charged.will diminish over time as the capacitance is charged.• It causes a relatively large voltage drop across the primary It causes a relatively large voltage drop across the primary

winding resistance, Rp, thereby initially lowering the voltage winding resistance, Rp, thereby initially lowering the voltage available to Lkp and Lm available to Lkp and Lm

• It takes time to diminish voltage drop across Rp.It takes time to diminish voltage drop across Rp.• This delays the peak voltage across Lm. This in turn delays peak This delays the peak voltage across Lm. This in turn delays peak

secondary voltage. The delay contributes to rise time, hence Cp secondary voltage. The delay contributes to rise time, hence Cp contributes to rise time. contributes to rise time.

• Lpk restricts flow of the reflected load current and consequently Lpk restricts flow of the reflected load current and consequently also contributes to rise time. also contributes to rise time.

• Similar consequence occurs with the secondary winding Similar consequence occurs with the secondary winding capacitance, Cs and Secondary leakage inductance, Lks.capacitance, Cs and Secondary leakage inductance, Lks.

6.The Trailing Edge6.The Trailing Edge• The pulse transformer tries to dissipate the energy stored in Lm The pulse transformer tries to dissipate the energy stored in Lm

and in the parasitic components Cp, Cs, Lkp, and Lks. and in the parasitic components Cp, Cs, Lkp, and Lks. • The inductance will induce voltages as their magnetic fields The inductance will induce voltages as their magnetic fields

collapse. The capacitor charge will drain, but will not drain collapse. The capacitor charge will drain, but will not drain instantaneously. The capacitances may temporarily supply current instantaneously. The capacitances may temporarily supply current to the inductances .to the inductances .

• As a result, there is a sloped decline of the secondary output As a result, there is a sloped decline of the secondary output voltage after the primary switch is opened known as voltage after the primary switch is opened known as Trailing Trailing

EdgeEdge..

7.Pulse Distortion7.Pulse Distortion

• Rise time, droop, trailing edges, and spurious oscillations are all Rise time, droop, trailing edges, and spurious oscillations are all considered to be signal distortion.considered to be signal distortion.

Advantage Of Pulse TransformerAdvantage Of Pulse Transformer

1.It separates high voltage power circuit from low voltage 1.It separates high voltage power circuit from low voltage control circuit.control circuit.

2.These transformers are high frequency transformer thus size 2.These transformers are high frequency transformer thus size reduces.reduces.

3.These transformer can have more than one secondary side 3.These transformer can have more than one secondary side so we cant use more device at a time .so we cant use more device at a time .

This is a high voltage transformer of This is a high voltage transformer of

Pearson Electronics . It has voltage outputPearson Electronics . It has voltage output

Of 100kV-500kV.This transformer is keptOf 100kV-500kV.This transformer is kept

in high voltage insulating oilin high voltage insulating oil

UseUse

Pulse transformer-Pulse transformer- Signal pulse transformers are used in digital circuits, Signal pulse transformers are used in digital circuits,

telecommunication etc.telecommunication etc.

Power pulse transformers are used to isolate power circuit from Power pulse transformers are used to isolate power circuit from control circuit.control circuit.

High voltage pulse transformer-High voltage pulse transformer- These transformers are used in radar application, for pulsed These transformers are used in radar application, for pulsed

power application.power application.

CONCLUSIONCONCLUSION

The pulse transformer are efficient in isolation and The pulse transformer are efficient in isolation and protection of control circuit and widely used as it takes protection of control circuit and widely used as it takes small area due to small size. This prevents wastage of small area due to small size. This prevents wastage of space. One pulse transformer can have two secondary space. One pulse transformer can have two secondary sides this decreases the number of pulse transformer and sides this decreases the number of pulse transformer and also decreases cost and complexity of system. also decreases cost and complexity of system.

ReferenceReference

• Butler Winding Butler Winding • Pearson Electronics Pearson Electronics

• Wikipedia, the free encyclopediaWikipedia, the free encyclopedia • www.google.comwww.google.com

Any Question Any Question

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