Switching regulators

Switching regulators
Linear Power Supplies
Switch Mode Power Supply(SMPS)
Complete Switched Mode Regulator
Step Down Or Buck Switching Regulator
Switching Boost Regulator
(Step Up Converter)
Switching Buck – Boost Regulator
Cuk Regulators
Comparison Of Switching Regulator
Advantage & Disadvantage Of SMP

1. Switching Regulators

2. INTRODUCTION Linear Power Supplies Switch Mode Power Supply(SMPS) Complete Switched Mode Regulator Step Down Or Buck Switching Regulator Switching Boost Regulator (Step Up Converter) Switching Buck – Boost Regulator Cuk Regulators Comparison Of Switching Regulator Advantage & Disadvantage Of SMPS

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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 V in as it is to the input of the filter and the filter input is disconnected from the dc input voltage V in when the switch is open circuited. operation

9. Filter input voltage = V in … when switch is on. Filter input voltage = 0 … when switch is off.

10. 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 , t on = On-time of the pulse wavefoam t off = Off-time of the pulse wavefoam

11. 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

12. Complete Switched Mode Regulator

13. The series pass transistor Q acts as a switch The resistor R 1 and R 1 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

14. 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.

15. If the output V 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 t on increase to increase the duty cycle and hence output voltage will increase. Thus regulation will be practically achieved.

16. Classification Of SMPS SMPS Non isolated converters Isolated converters Buck regulator Boost regulator Buck boost regulator Fly back converters Forward converter Full bridge converter Half bridge converter

17. Q 1 is a power transistor which is turned ON and OFF by the rectangular pulses applied at its base. D FW is a freewheeling diode, while L and C 2 form a low pass filter. V in is the unregulated power supply. Step Down Or Buck Switching Regulator

18. 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: V o = D x V in The duty cycle “D” can be varied between 0 and 1. Therefore average output voltage V will vary between 0 and V in. Average output voltage V o is less then or equal to V in this circuit is called as the “switching buck regulator”.

19. When Q 1 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 D FW is reversed biased remains off. When Q 1 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 = W ave forms And Operation Mode 1(when Q 1 is on) Mode 2(when Q 1 is off)

20. 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

21. The switching boost regulator also called as step up converter. Switching Boost Regulator

22. When Q 1 is on the points X and Y are circuited. So Q XY = 0. The voltage across indicator is V in Volts and the current through it increases linearly. The inductor L stores energy during mode 1. W ave forms And Operation Mode 1(when Q 1 is on)

23. As soon as Q 1 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 Q 1 is off)

24. 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.

25. 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

26. 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

27. When Q 1 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 Q 1 is turned off, the current through L is interrupted abruptly. A negative voltage is induced into L which will forward bias diode D 1. This mode comes to an end when the current through diode reduces to zero. W ave forms And Operation Mode 1(Q 1 is on) Mode 2(Q 1 is off D 1 is on )

28. When all the devices are in the off state, the capacitor C 2 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)

29. 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

30. 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

31. Initially when the input voltage V s is applied and the transistor T is in the off state, the capacitor C 1 charges through L 1 and D M to a voltage equal to V s . The circuit operation can be divided into two modes. W ave forms And Operation

32. At t = 0, transistor T is turned ON and it starts acting as a closed switch. So current through L 1 starts increasing. The voltage across C 1 gets applied across D m to reverse bias it, and turns it off. The inductor L 1 continues to store energy. Capacitor C 1 will discharge its energy through the circuit formed by C 1 , C 2 , load and L 2 . Mode 1 comes to end at t = t 1 . Mode 1 (o to t 1 )

33. At t = t 1 the transistor T is turned off. Capacitor C 1 is charged from the input supply and the energy stored in L 2 is transferred to the load. The diode D m and transistor T provide a synchronous switching action. C 1 act as a medium to transfer energy from source to load. Mode 2 (t 1 to t 2 )

34. 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 C 1 is high. This circuit requires an additional capacitor and an inductor. Advantages Of Cuk Regulators Disadvantages

35. Sr. No Parameter Buck Regulator Boost Regulator Buck 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 V = D x V in V = V in (1 – D) V = D X V in 1 - D Comparison Of Switching Regulators

36. 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

37. 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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