1) LINEAR POWER SUPPLYAs mentioned earlier that two main types of power

supply units are available, linear and switching

regulators. The linear power supply is also known as

the Series Control regulator or the Series Pass

regulator.

The circuit for a linear power supply is shown in

Figure 13. Transistors Q1 and Q2 form the Darlington

coupling that is shown in more detail in Figure 14. The

Darlington coupling transistor amplifies the current.

The total amplification of the Darlington is determined

by the multiplying the current at the base of the

transistor with the current amplification ratio. For

example, if one thousandth of one Ampere(1mA) is

applied to the base of transistor Q2 which has a

current amplification ratio of 100 and it is amplified to

100mA. It then becomes the input to the base of Q1

and is amplified to 10,000mA or equal to 10A.

Likewise, if 0.5mA is applied to the base of Q2, it will

be amplified to 5A at the emitter of Q1.

This is an amplification of 10,000 times (Q2 100 x

Q1 100).

7

2.LINEAR AND SWITCHING REGULATED POWER SUPPLY

AC Input Capacitor

Transistor

erroramplifier

Voltagereferrence

Sampling ResistorOutput voltage deviderfor error amplifier

Power sorce for error amplifier

sencing

sencing

Full-WaveRectifier

Output

Output

Fig.13 Linear Power Supply

Fig.14 Darlington Coupling Circuit

Emitter

Base

Base

Collector

Collector

Amplified Current Ratio(Hfe.)of Q1 is 100

EmitterAmplified Current Ratio(Hfe.)of Q1 is 100

The DC output of a power supply needs to be stable

despite the input voltage varying. Whatever the

change in the AC input, we define the secondary

voltage of the transformer so that it is within the range

that the transistor Q1 can handle in order to control

the output voltage. The transistor Q1 changes the

voltage by changing the current. This is best

understood by a simple look at Ohms Law which

states that voltage equals current(Amps) multiplied

by resistance. So, a Darlington transistor coupler

changes current flow and thus the voltage.

Now please take for granted that in order for this

circuit to work correctly the voltage coming from the

secondary of the transformer must be higher than the

output voltage. Then, if for example we require a 5V

DC supply we might need to define the secondary

output voltage from the transformer as 10V. The DC

input voltage 10V is then regulated by adjusting the

current to Q1 to produce the required 5V output (see

Figure 16a).

Figure 16b and 16c show the operation for increased

or reduced voltage input to Q1.

In Figure 16a, 16b and 16c, the voltage drop on the

Darlington coupling is 5V, 6V and 4V respectively.

Voltage drop is the voltage difference across Q1 or in

other words the difference between the output of the

secondary, 10V, and the output of the power supply,

5V. The power dissipation in Q1 is given by Watts =

Volts x I (current). The voltage drop from collector to

8

Transister

Reference Voltage

UnstabledPower Supply

Base

ErrorAmplifier Regulator

Fig.15 Linear Power Supply

5V

6V

4V

5V

0V

0V

0V

+5V(=11V– 6V)

+5V(=9V– 4V)

+5V(=10V– 5V)

11V

10V

9V

DCInput

DCInput

DCInput

20A

20A 20A

D7

D7

D7

–

+

+10V

+11V

+9V

Regulated

5VRegulated

5VRegulated

Out Put Voltage sensing

RL

RL

RL

Q2

A2

–

+A2

–

+A1

Q2

Q2

Q1

Q1

Q1

VR

VR

VR

Fig.16a Normal Condition

Fig.16b Input Voltage Increased by 10%

Fig.16c Input Voltage Dcreased by 10%

emitter of Q1 is multiplied by the output current of the

Q1; i.e. at 5V and 20A the power dissipation is 5V x

20A=100W. A greater voltage drop will mean a

greater dissipation and ultimately heat and loss of

energy so 16b is a better scenario than 16a. linear

power supplies reduce the voltage by simply burning

up the extra voltage.

This excess heat must be radiated, to ensure that

the unit does not get too hot and fail. A fan or heat

sink to radiate the heat is therefore necessary and

this makes the power supply large and heavy.

The error amplifier controls the output of the

Darlington coupling, Q1 and Q2 (see Figure 15a). On

the output voltage sensing, it monitors the output

regulation between the sampling resistors R6 and R7.

The error amplifier compares the actual output

sensing voltage with the zener diode reference

voltage. To allow the error amplifier to detect when

the actual output voltage has changed it acts as a

comparator, using as a voltage reference source the

zener diode D7. The D7 should generate a stabilized

voltage that is not affected by the temperature at

which the diode is running. It amplifies this and sends

it to the base of Q2. This controls the output voltage

to be the same voltage of the zener diode reference

voltage at the A1 minus input terminal from the

sensing. When both voltages become the same, the

output voltage is regulated. Therefore the output

voltage is protected against unwanted output voltage

changes caused by the load changing or input voltage

varying.

9

2) SWITCHING POWER SUPPLYBefore explaining the theory of operation of a

switching or switch mode power supply let us take a

brief look at the history.

NASA first developed switching power supplies in

the 1950's because they needed a small and

lightweight power supply for a rocket. Linear power

supplies had been available for more than thirty years

but were not suitable for use on the rocket because of

their size and weight.

This was in the era of the development of

electronics, when the IC, diode and transistor were

just mass-produced. Prior to this l inear power

supplies used valves, which were robust as far as

temperature and power ratings.

The Japanese power supply industry changed

valves to transistorized power supplies in the 1960's

despite the problem with the transistor requiring

efficient heat sink compared with the valves which

had not. Additionally the original Germanium PNP

transistors could not tolerate any over stressing unlike

the more robust valves. Despite the addition of the

heat sink, the change did however allow a reduction

in size of the linear units.

The first development of switch mode was to cut

down the power consumption of the transistor by

using as like relays to turn the power on and off. This

increased the efficiency from an average of 40% on

linear units to 60% on switching units and reduced the

amount of heat sink required by the power transistors.

There was still however the problem of the large and

heavy transformer using AC line frequency (50/60Hz) ;

the only way to miniaturize these was to increase the

switching frequency.

This was achieved by the development of the line-

switching transistor (a high voltage-switching

transistor). Mass production of this transistor was

started in the USA in the late 1960s. By rectifying the

line input and then increasing the switching frequency

on the transformer, size and weight could be reduced

significantly. In this was the efficiency of the power

supply was increased more than 70%.

The first stage however, used the 50/60 Hz

transformers and was known as a series switching

power supply (see Figure 17). This circuit is the

foundation for the present day switching technology.

The theory of the operational amplifier acting as an

error amplifier using the reference voltage is the same

as for the linear power supply, however the control is

different. When the error amplifier A1 judges the

output voltage to be changed it generates and sends

a signal to the pulse width modulator (PWM). The

signal modulated to the pulse width required, and

then through the driver (DRV) the switching cycle of

Q1 is varied until the output is re-stabilized. Q1 the

switching transistor is switched by the high frequency

oscillation (generally >20kHz) from the oscillator

(OSC) and controlled by the signal to the base of the

transistor from the operational amplifier (see Figure

18).

Figure 19a, 19b and 19c show how switching the

voltage supply to a lamp affects the output. Figure 19

shows the circuit used, consisting of a battery, a

switch and a lamp.

When the supply is always on, the light from the

10

F

AC Input

DRV PWM

OSC

Sensing

ReferenceVoltage

OP-AmpOscillator

Driver

Choke

–+

A1

C1

D3D4

T1

D1

VREF

D2

Q1

D5

D5

C2

L1

: DiodePWM : Pulse Width Modulator

Switching Transister

Switch

DC InputControlCircuit

Filter Load

Sensing

Transistor Q1

RL

Fig.17b Series Switching Power Supply Block Diagram

Q1

BaseControl Input ON/OFF

Control Input On

Control Input Off

Switch

Lamp

Bright Constant Light

Constant Light ButBrightness is cut

Constant LightBut Dim

SwitchBattery

(a)Switch Alwas on

(b)Voltage Swithed at 50%on, 50%off

(c)Voltage Swithed at 25%on, 75%off

0V

0V

0V

ON50%

ON25%

OFF75%

OFF75%

ON25%

OFF50%

OFF50%

ON50% ON

ON

Fig.18 Switching Transistor

Fig.19 Switching and the Effect on a Lamp

Fig.17a Series Switching Power Supply

lamp is bright and constant. When the supply is

switched on and off at a high frequency and a regular

cycle, with the on and off periods both being 50 % of

each cycle the light from the lamp is still constant but

the brightness is reduced, compared with Figure 19a.

In the same way, when the on period is 25% of the

cycle as shown in Figure 19c the light is constant but

very much dimmer.

The lamp filament does not respond to the high

frequency and averages the energy over the on/ off

periods giving a stable output. With the switching

power supply, changing the percentage of the on/off

periods of the cycle stabilizes the output voltage. This

is called the Duty Cycle.

This switched waveform or pulse waveform is on the

output side and needs to be smoothed by a filter

circuit to give a smooth supply to the load. Figure 20a

shows the regulated input to the switch element being

input to the filter circuit and the final smoothed output

from the filter circuit.

Figure 20b shows the switching wave form from the

switching element with a 33% duty cycle and shows

the relationship between the voltage of the switched

wave form and the output. 33% of the cycle is 'on' at

15VDC and 66% 'off' at 0VDC. The filter circuit like

the lamp filament averages as shown in Figure 20b.

These output of 5VDC are divided by 3 as the on

period.

The efficiency (output power divided by the input

power) of the series switching power supply as shown

in Figure 17 is fairly good, but the transformer as

mentioned earlier is large and heavy as it is still a 50-

60 Hz transformer.

Most of today's switching power supplies use the

input side switching technique (Line Switching) with

high voltage (greater than 800V) switching transistors.

11

Input Pulse Waveform Output

12V0V

15V0V

5V

0V

5V

0V

Filter

Filter

66%

15V

Duty Cycle ON ON ON

OFF OFF

(a)

(b)

Fig.20 Switching Waveform

Elements of the line switching power supply are the

same as those of the series switching power supply,

but a number of differences do exist. In the line

switching unit the switching transistor is in the input

side (before the transformer) whereas in the series

switching unit it is in the size of the transformers with

the line switching transformer being small and

lightweight in comparison to the series switching

transformer.

An addition to the circuit in the line-switching unit is

the Opt- coupler. This ensures the isolation between

the output and the input on the control signal line. And

this is necessary because the error amplifier, and the

associated control circuitry is secondary or output

related but the switching transistor to which the

control signal must be fed is, in the case of the line

switching power supply, primary or input related. One

of the main functions of the transformer is isolating

primary from secondary circuits. This isolation would

be made worthless if the control signal line does not

also have primary/ secondary isolation.

3) CONTROLLING SWITCHINGPOWER SUPPLY

There are typical four methods of control of the

switching power supply, these being:

1) Pulse Width Modulation

2) Frequency Modulation

3) Pulse Width and Frequency Modulation

4) Pulse Number Modulation

12

High Frequency TransformerHigh Speed Switching Diodes

FilterOp-Amp

VRef

PWM

OSC

High VoltageSwitching Transister

Opto-Coupler

AC Input

Driver

Primary Seconary

Isolation

Control Signal

Fig.21 Line Switching Power Supply

No Load

Light Load

Full Load

Fixed

Fig.22 Pulse Width Modulation Control Waveform

L1

Q1

Q1

C1

C1

C1

C1

D1VCC

VCC

VCC

VCC

D1

D1

D1

L1

L1

L1

+ –

+–

RL

RL

RL

RL

+

+

+

+

Load

Switching Transister

(a) The Filter Circuit

(b) The Equivalant Circuitof Figure (a)

(c) Switch Q1 on

(d) Switch Q1 off

1) Pulse Width Modulation

This method is the most established and is the most

commonly used. As shown by Figure 22 the

frequency of the cycle is fixed but the width of the

pulse is variable to maintain the required output

voltage with varying load conditions.

2) Frequency Modulation

As shown in Figure 23, the pulse width remains

constant but the frequency changes to maintain the

output voltage with varying load conditions.

3) Pulse Width and Frequency Modulation

This is a combination of methods 1) and 2), both the

pulse width and the frequency are changed to

maintain the output voltage with varying load.

4) Pulse Number Modulation

This method is for stabilized the output voltage by

that the high frequency pulse number signal, which is

oscillated by the regular frequency, is controlled by

the modulation signal.

4)SWITCHING POWER SUPPLYTOPOLOGIES

There are two-main types of switching power

supplies known as the Forward Converter Topology

and the Fly Back Converter Topology. The Forward

Converter transfers the energy to the secondary side

while the switching transistor in the primary side is

turned on. The Flyback Converter only transfers

energy to the secondary when the switching transistor

is turned off. Each topology has a number of

variations:

(i) Forward Converter Topology

(a) Single-Ended Forward Converter Topology

(b) Push-Pull Converter Topology

(c) Half-Bridge Converter Topology

(d) Full-Bridge Converter Topology

(e) Buck Switching Regulator Topology

(ii) Flyback Converter Topology

(a) Single-Ended Flyback Converter Topology

(b) Boost Switching Regulator Topology

(c) Buck Boost Switching Regulator Topology

Before considering these topologies let us, review

the functions of the coil and transformer. Figure 24

shows the switching inductive filter circuit that uses

the coil.

13

Fre

quen

cy to

Hig

h

No Load

Light Load

Full Load

Fig.23 Frequency Moduration Control Waveform

Fig.24 Inductive Filter Circuit

When the switch Q1 is on (closed) as shown in

Figure 24c, the current from Vcc in the input side

flows through the switch Q1 to the coil L1 and thence

to the load RL. An induced voltage is generated by

this as the coil L1 works to maintain the current flow in

the direction shown by the arrow in Figure 24c.

In Figure 24d the switch Q1 is on (closed) as shown

in Figure 24c to the off position. With the switch open

no current flows from Vcc, but with the energy

accumulated in the coil the current is trying to

maintain and fed to the load through the free-wheeling

diode D, generating polarity reversal voltage the so-

called "inductive kick" across the coil.

The coil works to maintain the previous condition of

the current, therefore when the switch is on and

current flows from Vcc to the coil generates induced

voltage trying to stop the current. Conversely, when

the switch is off the coil works to maintain the

previous condition (i.e. that current flows from Vcc). In

doing so, an inverse voltage is generated. When the

switch is again closed the cycle re-starts.

The transformer has the same characteristic, as the

coil L. Figure 25 is a transformer schematic diagram.

Figure 25a is when current is started to flow at

primary of the transformer and Figure 25b show when

current is stopped to flow at primary. Black dot on the

coil as shown in the figure 25 is represented the start

of the windings. When the current is flown, induced

voltage is generated in the direction towards the start

of the primary and secondary winding as shown in

Figure 25a. When the current is stopped to flow, this

is reverse voltage is induced in the direction away

from the start of windings as shown in Figure 25b.

(i) Forward Converter Topology

(a) Single-Ended Forward Converter Topology

The single-ended forward converter topology is a

switching method using one transistor as shown in

Figure 26. When the transistor Q1 is on, the energy is

transferred from the primary to the secondary through

the transformer. When the switching transistor Q1 is

on the current flows in the direction of the solid arrow

through the diode D1 and is supplied to the load RL.

When the transistor is off the energy accumulated in

the coil L1 flows through the free-wheeling diode D2

14

(a) Switch On

(b) Switch Off

Start of Winding Start of Winding

Start of Winding

Induced Voltage

Current

Current

Current

Current

Current

LoadLoad

LoadLoad

Fig.25

Primary Side

Current Secondary Side

Fig.26 Single Ended Forwared Converter Topology

as shown by the dotted line in Figure 26 supplying the

power to the load RL. Using this circuit medium

(<400W) output power levels can be obtained.

(b) Push-Pull Converter Topology

The push-pull converter topology uses two

transistors which are turned on and off in turn. When

either transistor Q1 or Q2 are turned on, the on state

transistor supplies energy to the secondary side via

the transformer T1. When both transistors are turned

off the high-speed rectifier diodes (D1 and D2) act as

a free-wheeling diode to release the energy from the

coil. This circuit is used when high (kW) output power

is required. Care must be taken in the design of this

circuit as cross current conduction may occur with

both transistors on at once.

(c) Half-Bridge Converter Topology

The operation of the half bridge is the same as the

push-pull except that only Vcc/2 is applied to the

primary winding of the transformer T1 and Vcc is

applied to the transistor that is off state. This circuit is

used mainly for 200V AC line input power supply.

(d) Full-Bridge Converter Topology

The full-bridge converter topology takes the

capacitors C1 and C2 from the previous topology, the

half bridge converter topology, and instead of them

uses transistors. It basically works the same as the

half bridge converter. This method is mainly used for

200VAC input and power supplies over 300 watts in

size.

(e) Buck Switching Regulator Topology

This topology was explained previously. Please

refer back to Fig. 17.

(ii) Flyback Converter Topology

(a) Single-Ended Flyback Converter Topology

The single-ended flyback converter is switching

15

Fig.27 Push-pull Converter Topology

Current flows whenQ1 is On and Q2 is Off

Current flows whenQ2 is On and Q1 is Off

Vcc

Vcc12

Fig.28 Half-Bridge Converter Topology

Current flows whenQ2 and Q3 are On

Current flows whenQ1 and Q4 are On

Fig.29 Full-Bridge Converter Topology

Currert flows whenQ1 is Off

Fig.30 Single-Ended Flyback Converter Topology

method using one transistor on the primary side and

works such that when Q1 switches off then the

accumulated energy in transformer is supplied to the

load. This topology has fewer components than other

methods, which means lower cost. The rectification

circuit on the secondary side is also very simple. It

however cannot handle high wattage.

(b) Boost Switching Regulator Topology

The boost switching regulator topology also uses

one transistor. When the transistor is on the energy is

charged to the coil L1, and L1 discharges this energy

to the output when the transistor is switched off. This

method is very efficient and is called a boost regulator

because the output voltage is higher than the input

Vcc.

(c) Boost Switching Regulator Topology

The theory of operation for the buck boost switching

topology is the same as the boost switching regulator

topology. The special feature of the buck boost

regulator is the polarity reversal of the input Vcc

voltage.

16

Q1 Off Current Flow

Q1 OnCurrent Flow

Fig.31 Boost Switching Regulator Topology

Fig.32 Chopper Buck Boost Method

Q1 OnCurrent Flow Q1 Off

Current Flow

Q1 ON

5) LINEAR REGULATORS VERSUSSWITCHING REGULATORS

This chapter has briefly covered the theory of linear

and switching regulator power supplies. There are a

number of contrasts between the two, the main ones

being efficiency, weight, size, ripple and noise.

(1) Weight, Size

(2) Efficiency

(3) Ripple and Noise

The switching power supply has the advantage over

the l inear power supply with size, weight and

efficiency but is weak compared to the linear on ripple

and noise.A switch made power supply is 1/3 to 1/5 of

the size and weight of a linear power supply and is

being reduced as technology advances. In addition

the efficiency of a switching power supply (80%) is

very much better than the linear (30 - 50%). Efficiency

is the output DC power divided by the input effective

power as shown below.

Efficiency

= (Output DC Power / Input Effective Power) x 100 %

Figure 33 shows a comparison of efficiencies for a

linear and a switching power supply.

The input effective power can not be found by

multiplying the input current and the input voltage

together, as the AC input current is not continuous

(see Figure 34). The input effective power should be

measured using a watt meter as shown in Figure 34,

however the voltmeter and current meter should be

disconnected before using the wattmeter otherwise

the effective power across the meter will be measured

in addition to the effective power across the power

supply input. The power factor of the power supply

can be found using the following formulas:

Power Factor = Input Effective Power/ Input VA

Input VA = Input AC Voltage x Input AC Current

17

Watt Meter

LoadSwitch ModePower Supply

OutputACInput

ACVoltMeter

AC CurrentMeter

AC Voltage Waveform(Voltage is ExistenceThroughout)

Effective Power

AC Current Waveform(Current is Only Existencein Section 4 )

No Current

Fig.34 The Effective Power

Radiated (Power Loss) 100W

(a) Linear Requlator

Efficiency 39%

166W 66W

Input 5V13A

Output

Efficiency 39%= X 100%66W166W

Radiated (Power Loss) 16W

(b) Switching Requlator

Efficiency 80%

82W 66W

Input 5V13A

Output

Efficiency 80%= X 100%66W82W

Fig.33 Comparison of Efficiencies

Ripple and noise are the weak point of switching

power supplies as they have a very much higher

ripple and noise content in the output than linear

power supplies. The ripple and noise consists of three

elements. The first, which is twice the frequency of

the input, is from the rectification of the AC. The

second is the switching frequency more than 20kHz.

The third is the switching spikes from the switching on

and off the switching transistors (see Figure 35).

Connecting capacitors across the load terminals with

very short leads can reduce the noise. Electrolytic

capacitors will reduce low frequency ripple and

spikes, ceramic capacitors will reduce medium

frequency ripple and spikes and film capacitors high

frequency ripple and spikes.

18

10ms(50Hz)

MaximumRappleVoltage

Spike

Fig.35 Ripple and Noise

1) What Nemic-Lambda requires of apower supply.

HIGH RELIABILITY AND SAFETY

As the heart of electronic equipment, a power supply

failure is fatal to the system. For this reason, power

supply designers aim for high reliability and longevity

in their designs. In addition, we have the goal of not

only meeting all safety organization standards, but

also ensuring higher reliability through FMEA. If a

problem with circuitry or mechanics is found counter

measures are established and you can be sure, the

aspect of safety takes a high priority.

HIGH EFFICIENCY

While saving energy is one of the goals of higher

efficiency. There are still other very important reasons

like excessive heat from loss, and smaller size and

weight can be achieved through higher efficiency. For

those reasons high efficiency is always pushed in

power supply design.

SMALL-SIZED and LIGHT-WEIGHT

As the size of all electronic equipment decreases,

naturally power supply size is demanded to decrease

also. Unfortunately, power supplies are said to be

behind in the race for compact size. However as

faster power devices (transistors, diodes, FET etc...)

grow faster and circuit design advances the switching

frequency will increases and power supplies will get

smaller.

3. When using the power supply

NOISE REDUCTION

Power supplies through their own operation create

noise. Some people might judge the quality of a

power supply based on whether or not it controls this

noise. Accordingly, we are creating power supplies

that conform to FCC, VDE, VCCI and other regulatory

agency's regulations and standards. At the same time

designers are combating noise from operation they

are also concerned with external noise immunity and

surge noise, not only to protect from damage but to

act as a filter for the customers equipment.

INRUSH CURRENT REDUCTION

When a power supply start up, the input capacitor

begins to charge and this causes an inrush current.

This current can have a harmful effect on both the

power supply and the equipment it supplies Therefore

there must be an inrush protection circuit to prevent

the surge of electricity.

COOLING CONCEPT

Even with the highest efficiency, there is still 10-20%

loss. This loss becomes heat, which can damage the

equipment and cause parts to fail early and the life

span of the power supply, and equipment life can be

shortened. Heat resistant parts can be used to sturdy

the power supply but still the customers need to

carefully choose the location and position where they

will place the power supply in order to increase

longevity and reliability.

POWER SUPPLY PROTECTION

There are protective functions in case, for example,

of a short in the load equipment there is Over Current

Protection. Likewise, in the case of abnormality with

the power supply there is Over Voltage Protection.

There are still other protection functions and some

power supplies even have self-analysis. Now you can

obtain a power supply with more protective features

than ever before.

2) INRUSH CURRENTSwitching power supplies and the electronic

equipment, they are ever becoming smaller. Yet,

when the power button is pushed a special care must

be taken. Even though electronics are getting smaller

and weigh less, when the power button is tuned on

the input filtering capacitors in power supplies draw a

surge current in order to charge themselves. This

surge is called an inrush current. If, for example, the

switch is not specified for a high enough current it

could be damaged by this inrush current.

In this case one might choose switch with a higher

current capacity, choose a power supply that is

designed to handle the inrush current or place some

sort of inrush current protection circuit in front of the

power supply. Be aware that if many power supplies

are used the inrush current will be the sum of their

individual inrush currents. A more detailed coverage

on how to deal with inrush currents will come in the

section on "How to use the power supply".

3) THERMAL DESIGN CONCEPTWe have come this far in our explanation of power

supplies but we might be able to simple describe the

power supply as, the heart that supplies energy to the

electronic equipment.

Both the linear power supply and the switching

power supply have their strengths and weaknesses

but its likely that the current trend towards smaller,

lighter and more energy conscious power supplies will

continue and that the switching power supply will

continue to be the most commonly required power

supply.

However, even with the high efficiency of a switching

power supply there is still loss and its resultant heat.

A problem cannot be avoided. Some people go as far

as to say those power supply design is "heat design".

To the customers any loss in a power supply is going

to mean heat and it is with that in mind that power

supplies should be used.

19