Switching Regulators

INTRODUCTIONLinear Power SuppliesSwitch Mode Power Supply(SMPS)Complete Switched Mode RegulatorStep Down Or Buck Switching RegulatorSwitching Boost Regulator

(Step Up Converter)Switching Buck – Boost RegulatorCuk RegulatorsComparison Of Switching RegulatorAdvantage & Disadvantage Of SMPS

Linear Power Supplies• The series pass transistor is the controlling element

which appears in series with thee load. The voltage across this transistor is automatically adjust to obtain the regulated output voltage.

• In the linear regulator the series pass transistor is operated in its active region.

• Therefore the voltage across Vce in the on state is high. As it is carrying the load current, the power dissipation in the power transistor is very high.

• The output voltage of the linear regulator is adjusted automatically at the desired level by adjusting the voltage across the series pass transistor. This voltage is adjusted by feeding back a part of the output voltage.

• Due to high power dissipation taking place in the series pass transistor, the efficiency of linear regulator is low.

• The linear regulators are used in those applications where we need a very low ripple in the output voltage and where the voltage regulation should be vary low. The typical application are computers, video games, printers etc.

Advantages Of Linear Regulator• The ripple contents in the output voltage waveforms is

very low.• No RFI/EMI, as the power device is not operated as

switch.• High speed transistor need to be used.

Disadvantages Of Linear Regulator• The efficiency of linear regulators is very low of the

order of 40% to 50%. This is due to the operation of series pass power device in the active region.

• Large heat sink for the power transistor is required bulkly.

• A large bulkly 50Hz transformer is required.

Switch Mode Power Supply• The block diagram shows that the SMPS is also

basically a series regulator. Then how is it different from linear power supply?

• In SMPS, the series pass transistor does not operate in its active region. Instead it operates as a switch.

• The basic switch mode power supply consists of four components namely the unregulated dc voltage source Vin, an electronic switch S, a pulse generator and a filter.

• The pulse generator generates rectangular pulse which are applied to the control terminal of an electronic switch. This switch is turned on and off with the help of these rectangular pulses.

• The switch is an electronic switch which is typically a transistor or MOSFET. It is used in its saturation and cut-off regions and not in active region.

• When the switch is on, it connects the unregulated dc input Vin as it is to the input of the filter and the filter input is disconnected from the dc input voltage Vin when the switch is open circuited.

operation

• Filter input voltage = Vin …when switch is on.• Filter input voltage = 0 …when switch is off.

• Therefore at the input of the filter we get a rectangular waveform. The average value of this waveform can be adjusted by changing either the duty cycle or frequency of the rectangular pluses produced by the pulse generator. The duty cycle is defined as,

• Where , ton = On-time of the pulse wavefoam toff = Off-time of the pulse wavefoam

• Typically, the operating frequency of the switching regulator will be in the range of 10 to 50 KHz. That means the total time T is of the order of 100 µS to 20 µS.

• The filter then converts the rectangular waveform at its input into a smooth dc voltage by removing the ripple contents.

Or

Complete Switched Mode Regulator

• The series pass transistor Q acts as a switch • The resistor R1 and R1 from a resistive feedback

network. The feedback voltage is given by:

• This feed back voltage is applied at the non – inverting (+) termiinal of the error amplifier.

• The difference between feedback voltage and the reference voltage is called as “error”.

• The dc control voltage is compared with the triangular waveform generated by the triangular wave generator.

Operation

• The comparator output is a rectangular waveform. This waveform is used to turn on and off transistor.

• The rectangular wave foam at the emitter of Q is filtered by by the LC filter to produce a pure ripple free dc voltage at the regular output.

• The size of LC components is small.

• If the output V0 reduces below the regulated value, then the feedback voltage decreases. The output voltage of error amplifier will decrease.

• This will increase the on time of the pulse at the output of the comparator. Thus on time ton increase to increase the duty cycle and hence output voltage will increase. Thus regulation will be practically achieved.

Classification Of SMPSSMPS

Non isolatedconverters

Isolated converters

Buckregulator

Boostregulator Buck boost

regulator

Fly backconverters

Forwardconverter

Full bridgeconverter

Half bridgeconverter

• Q1 is a power transistor which is turned ON and OFF by the rectangular pulses applied at its base.

• DFW is a freewheeling diode, while L and C2 form a low pass filter. Vin is the unregulated power supply.

Step Down Or Buck Switching Regulator

• As discussed earlier, we can vary the average output voltage by changing either the duty cycle or frequency.

• The expression for average output voltage is terms of duty cycle is given by:Vo = D x Vin

• The duty cycle “D” can be varied between 0 and 1. Therefore average output voltage V0 will vary between 0 and Vin.

• Average output voltage Vo is less then or equal to Vin this circuit is called as the “switching buck regulator”.

• When Q1 is on the input dc voltage V gets connected at the input of the LC filter.

• Energy is given to the LC filter and the load during this mode of operation. The diode DFW is reversed biased remains off.

• When Q1 is turned off, there is a self induced voltage which appears across the inductance L.

• The output voltage can be varied by varying the duty cycle of the power transistor. The output voltage is V out = DV where D =

Waveforms And OperationMode 1(when Q1 is on)

Mode 2(when Q1 is off)

• It needs only one transistor.• It is a simple circuit.• The circuit efficiency is high.• The di/dt of the load current is limited by inductor L.

• Input current is discontinuous and smoothing input filter is generally required.

• It can procedure an output voltage of only polarity. Polarity reversa is not possible.

• The output current is unidirectional.• It needs a separate protection circuit against a

possible short circuit, across the diode path.

Advantages Of Buck Regulators

Disadvantages

• The switching boost regulator also called as step up converter.

Switching Boost Regulator

• When Q1 is on the points X and Y are circuited. So QXY = 0.

• The voltage across indicator is Vin Volts and the current through it increases linearly.

• The inductor L stores energy during mode 1.

Waveforms And OperationMode 1(when Q1 is on)

• As soon as Q1 is turned off, the current through L is suddenly interrupted.

• This will induce a negative voltage across it to forward bias diode D.

• The stored energy by the L in mode I is delivered to the load during this mode.

• It possible to obtain an output voltage which is higher than the supply voltage, therefore it is known as boost regulator.

Mode 2(when Q1 is off)

• The output voltage can be varied by varying the duty cycle of the output waveform.

• Where D is the duty cycle. This equation shows that the output voltage is higher then the input voltage for all the values of D.

• The boost regulator is used in those applications where output voltage greater than the input voltage is required.

• It can step up the output voltage without a transformer.• Its efficiency is high due to the use of single transistor.• The input current is continuous.

• A high peak current flows through the power transistor.• Output voltage is sensitive to changes in duty cycle D.

So it is difficult to stabilize the regulator.• A larger filter capacitor and a large inductor is required

to be used.

Advantages Of Boost Regulators

Disadvantages

• The buck-boost regulator is a non isolated type converter and it is also known as inverting regulator.

• The buck-boost regulator is a type of flyback converter whose operation is very similar to a boost regulator.

Switching Buck Boost Regulator

• When Q1 is turned on, the supply voltage V gets conneted across the inductance L. The inductance current starts increasing linearly. Diode is reversed biased, the inductance will store energy during this mode of operation.

• As soon as the transistor Q1 is turned off, the current through L is interrupted abruptly.

• A negative voltage is induced into L which will forward bias diode D1.

• This mode comes to an end when the current through diode reduces to zero.

Waveforms And OperationMode 1(Q1 is on)

Mode 2(Q1 is off D1 is on)

• When all the devices are in the off state, the capacitor C2 will discharge through the load in this mode of operation. This circuit is called as inverting as the output voltage is negative.

• This regulator is used in those applications where output voltage grater rhen the input voltage with a negative polarity.

• The waveforms are for the discontinuous condition. The conduction can be made continuous by increasing the duty cycle above 50%.

• But with a duty cycle value above 50% the stability and regulation problem will have to be faced.

• The buck boost regulator has all drawbacks of the boost regulator.

Mode 3(All devices off)

• This circuit produces a negative output voltage without transformer.

• Its efficiency is high.• The rate of change of fault current (di/dt) is limited to a

safe value by the inductor L.• Its very easy to implement the short circuited protection.

• Input current is discontinuous.• A high peak current flows through the transistor

Advantages Of Boost Regulators

Disadvantages

• This circuit is similar to the buck-boost regulator but it provides an output voltage which is less than or grater than the input voltage with a polarity reversal.

Cuk Regulator

• Initially when the input voltage Vs is applied and the transistor T is in the off state, the capacitor C1 charges through L1 and DM to a voltage equal to Vs .

• The circuit operation can be divided into two modes.

Waveforms And Operation

• At t = 0, transistor T is turned ON and it starts acting as a closed switch. So current through L1 starts increasing.

• The voltage across C1 gets applied across Dm to reverse bias it, and turns it off.

• The inductor L1 continues to store energy. Capacitor C1 will discharge its energy through the circuit formed by C1, C2, load and L2.

• Mode 1 comes to end at t = t1.

Mode 1 (o to t1)

• At t = t1 the transistor T is turned off. Capacitor C1 is charged from the input supply and the energy stored in L2 is transferred to the load.

• The diode Dm and transistor T provide a synchronous switching action.

• C1 act as a medium to transfer energy from source to load.

Mode 2 (t1 to t2)

• The cuk regulator operation is based on the transfer of capacitor energy. Hence the input current is continuous.

• This circuit has low switching losses.• It has a high efficiency.

• A high value peak current flows through the transistor.

• Ripple current of the capacitor C1 is high.• This circuit requires an additional capacitor and an

inductor.

Advantages Of Cuk Regulators

Disadvantages

Sr. No Parameter Buck

RegulatorBoost

RegulatorBuck Boost Regulator

1 Average Output Voltage

Less then or equal to input

voltage

Equal to or grater than input

voltage

Less than equal to or grater than input voltage

2 Position Of Transistor

In series with the load

In parallel with the load

In series with the load

3 Number Of Transistor One One One

4 Expression for Output Voltage V0 = D x Vin

V0 = Vin (1 – D)

V0 = D X Vin 1 - D

Comparison Of Switching Regulators

• Low power dissipation in the series pass transistor as it operates as a switch and not in the active region.

• High efficiency due to reduced power dissipation in the transistor.

• The smaller size of L and C at high operating frequencies and need of smaller heat sink for the series pass transistor.

• Higher power handling capacity.

Advantages Of SMPS

• Increased switching loss in the series pass transistor due to high frequency switching.

• Radio Frequency Interference to the neighbouring electronics circuits.

• There are no isolation between input and output.• The load requires separate protection circuitry.• The transient response is slow as compared to the

linear power supplies.• Ripple content is the output is higher than that for

linear power supply.• Load regulation is poor as compared to the linear

regulators.

Disadvantages Of SMPS

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