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Power Electronics by Prof. M. Madhusudhan Rao 1 1 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT DC Choppers

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Power Electronics by Prof. M. Madhusudhan Rao 2 2 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Introduction Chopper is a static device. A variable dc voltage is obtained from a constant dc voltage source. Also known as dc-to-dc converter. Widely used for motor control. Also used in regenerative braking. Thyristor converter offers greater efficiency, faster response, lower maintenance, smaller size and smooth control.

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Power Electronics by Prof. M. Madhusudhan Rao 3 3 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Choppers are of Two Types Step-down choppers. Step-up choppers. In step down chopper output voltage is less than input voltage. In step up chopper output voltage is more than input voltage.

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Power Electronics by Prof. M. Madhusudhan Rao 4 4 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Principle Of Step-down Chopper

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Power Electronics by Prof. M. Madhusudhan Rao 5 5 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT A step-down chopper with resistive load. The thyristor in the circuit acts as a switch. When thyristor is ON, supply voltage appears across the load When thyristor is OFF, the voltage across the load will be zero.

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Power Electronics by Prof. M. Madhusudhan Rao 6 6 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 7 7 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 8 8 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 9 9 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 10 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 11 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 12 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 13 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Methods Of Control The output dc voltage can be varied by the following methods. –Pulse width modulation control or constant frequency operation. –Variable frequency control.

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Power Electronics by Prof. M. Madhusudhan Rao 14 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Pulse Width Modulation t ON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant. Output voltage is varied by varying the ON time t ON

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Power Electronics by Prof. M. Madhusudhan Rao 15 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 16 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Variable Frequency Control Chopping frequency ‘f’ is varied keeping either t ON or t OFF constant. To obtain full output voltage range, frequency has to be varied over a wide range. This method produces harmonics in the output and for large t OFF load current may become discontinuous

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Power Electronics by Prof. M. Madhusudhan Rao 17 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 18 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Step-down Chopper With R-L Load

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Power Electronics by Prof. M. Madhusudhan Rao 19 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT When chopper is ON, supply is connected across load. Current flows from supply to load. When chopper is OFF, load current continues to flow in the same direction through FWD due to energy stored in inductor ‘L’.

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Power Electronics by Prof. M. Madhusudhan Rao 20 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Load current can be continuous or discontinuous depending on the values of ‘L’ and duty cycle ‘d’ For a continuous current operation, load current varies between two limits I max and I min When current becomes equal to I max the chopper is turned-off and it is turned-on when current reduces to I min.

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Power Electronics by Prof. M. Madhusudhan Rao 21 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 22 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Expressions For Load Current i O For Continuous Current Operation When Chopper Is ON (0 t t ON )

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Power Electronics by Prof. M. Madhusudhan Rao 23 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 24 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 25 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 26 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT When Chopper is OFF

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Power Electronics by Prof. M. Madhusudhan Rao 27 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 28 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 29 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 30 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 31 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 32 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 33 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 34 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 35 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 36 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 37 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 38 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 39 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 40 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Principle Of Step-up Chopper

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Power Electronics by Prof. M. Madhusudhan Rao 41 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Step-up chopper is used to obtain a load voltage higher than the input voltage V. The values of L and C are chosen depending upon the requirement of output voltage and current. When the chopper is ON, the inductor L is connected across the supply. The inductor current ‘I’ rises and the inductor stores energy during the ON time of the chopper, t ON.

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Power Electronics by Prof. M. Madhusudhan Rao 42 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT When the chopper is off, the inductor current I is forced to flow through the diode D and load for a period, t OFF. The current tends to decrease resulting in reversing the polarity of induced EMF in L. Therefore voltage across load is given by

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Power Electronics by Prof. M. Madhusudhan Rao 43 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT A large capacitor ‘C’ connected across the load, will provide a continuous output voltage. Diode D prevents any current flow from capacitor to the source. Step up choppers are used for regenerative braking of dc motors.

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Power Electronics by Prof. M. Madhusudhan Rao 44 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Expression For Output Voltage

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Power Electronics by Prof. M. Madhusudhan Rao 45 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 46 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 47 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 48 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 49 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Performance Parameters The thyristor requires a certain minimum time to turn ON and turn OFF. Duty cycle d can be varied only between a min. & max. value, limiting the min. and max. value of the output voltage. Ripple in the load current depends inversely on the chopping frequency, f. To reduce the load ripple current, frequency should be as high as possible.

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Power Electronics by Prof. M. Madhusudhan Rao 50 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Problem A Chopper circuit is operating on TRC at a frequency of 2 kHz on a 460 V supply. If the load voltage is 350 volts, calculate the conduction period of the thyristor in each cycle.

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Power Electronics by Prof. M. Madhusudhan Rao 51 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 52 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 53 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Problem Input to the step up chopper is 200 V. The output required is 600 V. If the conducting time of thyristor is 200 sec. Compute –Chopping frequency, –If the pulse width is halved for constant frequency of operation, find the new output voltage.

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Power Electronics by Prof. M. Madhusudhan Rao 54 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 55 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 56 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 57 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Problem A dc chopper has a resistive load of 20 and input voltage V S = 220V. When chopper is ON, its voltage drop is 1.5 volts and chopping frequency is 10 kHz. If the duty cycle is 80%, determine the average output voltage and the chopper on time.

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Power Electronics by Prof. M. Madhusudhan Rao 58 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 59 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 60 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Problem In a dc chopper, the average load current is 30 Amps, chopping frequency is 250 Hz, supply voltage is 110 volts. Calculate the ON and OFF periods of the chopper if the load resistance is 2 ohms.

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Power Electronics by Prof. M. Madhusudhan Rao 61 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 62 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 63 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT A dc chopper in figure has a resistive load of R = 10 and input voltage of V = 200 V. When chopper is ON, its voltage drop is 2 V and the chopping frequency is 1 kHz. If the duty cycle is 60%, determine –Average output voltage –RMS value of output voltage –Effective input resistance of chopper –Chopper efficiency.

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Power Electronics by Prof. M. Madhusudhan Rao 64 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 65 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 66 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 67 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 68 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 69 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Problem A chopper is supplying an inductive load with a free-wheeling diode. The load inductance is 5 H and resistance is 10 .. The input voltage to the chopper is 200 volts and the chopper is operating at a frequency of 1000 Hz. If the ON/OFF time ratio is 2:3. Calculate –Maximum and minimum values of load current in one cycle of chopper operation. –Average load current

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Power Electronics by Prof. M. Madhusudhan Rao 70 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 71 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 72 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 73 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 74 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 75 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 76 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Problem A chopper feeding on RL load is shown in figure, with V = 200 V, R = 5 , L = 5 mH, f = 1 kHz, d = 0.5 and E = 0 V. Calculate –Maximum and minimum values of load current. –Average value of load current. –RMS load current. –Effective input resistance as seen by source. –RMS chopper current.

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Power Electronics by Prof. M. Madhusudhan Rao 77 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 78 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 79 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 80 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 81 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 82 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 83 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Classification Of Choppers Choppers are classified as –Class A Chopper –Class B Chopper –Class C Chopper –Class D Chopper –Class E Chopper

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Power Electronics by Prof. M. Madhusudhan Rao 84 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class A Chopper

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Power Electronics by Prof. M. Madhusudhan Rao 85 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT When chopper is ON, supply voltage V is connected across the load. When chopper is OFF, v O = 0 and the load current continues to flow in the same direction through the FWD. The average values of output voltage and current are always positive. Class A Chopper is a first quadrant chopper.

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Power Electronics by Prof. M. Madhusudhan Rao 86 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class A Chopper is a step-down chopper in which power always flows form source to load. It is used to control the speed of dc motor. The output current equations obtained in step down chopper with R-L load can be used to study the performance of Class A Chopper.

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Power Electronics by Prof. M. Madhusudhan Rao 87 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 88 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class B Chopper

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Power Electronics by Prof. M. Madhusudhan Rao 89 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT When chopper is ON, E drives a current through L and R in a direction opposite to that shown in figure. During the ON period of the chopper, the inductance L stores energy. When Chopper is OFF, diode D conducts, and part of the energy stored in inductor L is returned to the supply.

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Power Electronics by Prof. M. Madhusudhan Rao 90 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Average output voltage is positive. Average output current is negative. Therefore Class B Chopper operates in second quadrant. In this chopper, power flows from load to source. Class B Chopper is used for regenerative braking of dc motor. Class B Chopper is a step-up chopper.

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Power Electronics by Prof. M. Madhusudhan Rao 91 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 92 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Expression for Output Current

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Power Electronics by Prof. M. Madhusudhan Rao 93 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 94 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 95 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 96 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class C Chopper

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Power Electronics by Prof. M. Madhusudhan Rao 97 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class C Chopper is a combination of Class A and Class B Choppers. For first quadrant operation, CH 1 is ON or D 2 conducts. For second quadrant operation, CH 2 is ON or D 1 conducts. When CH 1 is ON, the load current is positive. The output voltage is equal to ‘V’ & the load receives power from the source. When CH 1 is turned OFF, energy stored in inductance L forces current to flow through the diode D 2 and the output voltage is zero.

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Power Electronics by Prof. M. Madhusudhan Rao 98 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Current continues to flow in positive direction. When CH 2 is triggered, the voltage E forces current to flow in opposite direction through L and CH 2. The output voltage is zero. On turning OFF CH 2, the energy stored in the inductance drives current through diode D 1 and the supply Output voltage is V, the input current becomes negative and power flows from load to source.

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Power Electronics by Prof. M. Madhusudhan Rao 99 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Average output voltage is positive Average output current can take both positive and negative values. Choppers CH 1 & CH 2 should not be turned ON simultaneously as it would result in short circuiting the supply. Class C Chopper can be used both for dc motor control and regenerative braking of dc motor. Class C Chopper can be used as a step-up or step-down chopper.

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Power Electronics by Prof. M. Madhusudhan Rao 100 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 101 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class D Chopper

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Power Electronics by Prof. M. Madhusudhan Rao 102 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class D is a two quadrant chopper. When both CH 1 and CH 2 are triggered simultaneously, the output voltage v O = V and output current flows through the load. When CH 1 and CH 2 are turned OFF, the load current continues to flow in the same direction through load, D 1 and D 2, due to the energy stored in the inductor L. Output voltage v O = - V.

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Power Electronics by Prof. M. Madhusudhan Rao 103 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Average load voltage is positive if chopper ON time is more than the OFF time Average output voltage becomes negative if t ON < t OFF. Hence the direction of load current is always positive but load voltage can be positive or negative.

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Power Electronics by Prof. M. Madhusudhan Rao 104 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 105 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 106 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class E Chopper

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Power Electronics by Prof. M. Madhusudhan Rao 107 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Four Quadrant Operation

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Power Electronics by Prof. M. Madhusudhan Rao 108 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Class E is a four quadrant chopper When CH 1 and CH 4 are triggered, output current i O flows in positive direction through CH 1 and CH 4, and with output voltage v O = V. This gives the first quadrant operation. When both CH 1 and CH 4 are OFF, the energy stored in the inductor L drives i O through D 2 and D 3 in the same direction, but output voltage v O = -V.

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Power Electronics by Prof. M. Madhusudhan Rao 109 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Therefore the chopper operates in the fourth quadrant. When CH 2 and CH 3 are triggered, the load current i O flows in opposite direction & output voltage v O = -V. Since both i O and v O are negative, the chopper operates in third quadrant.

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Power Electronics by Prof. M. Madhusudhan Rao 110 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT When both CH 2 and CH 3 are OFF, the load current i O continues to flow in the same direction D 1 and D 4 and the output voltage v O = V. Therefore the chopper operates in second quadrant as v O is positive but i O is negative.

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Power Electronics by Prof. M. Madhusudhan Rao 111 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Effect Of Source & Load Inductance The source inductance should be as small as possible to limit the transient voltage. Also source inductance may cause commutation problem for the chopper. Usually an input filter is used to overcome the problem of source inductance.

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Power Electronics by Prof. M. Madhusudhan Rao 112 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT The load ripple current is inversely proportional to load inductance and chopping frequency. Peak load current depends on load inductance. To limit the load ripple current, a smoothing inductor is connected in series with the load.

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Power Electronics by Prof. M. Madhusudhan Rao 113 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Problem For the first quadrant chopper shown in figure, express the following variables as functions of V, R and duty cycle ‘d’ in case load is resistive. –Average output voltage and current –Output current at the instant of commutation –Average and RMS free wheeling diode current. –RMS value of output voltage –RMS and average thyristor currents.

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Power Electronics by Prof. M. Madhusudhan Rao 114 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 115 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 116 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 117 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 118 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 119 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Impulse Commutated Chopper Impulse commutated choppers are widely used in high power circuits where load fluctuation is not large. This chopper is also known as –Parallel capacitor turn-off chopper –Voltage commutated chopper –Classical chopper.

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Power Electronics by Prof. M. Madhusudhan Rao 120 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 121 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT To start the circuit, capacitor ‘C’ is initially charged with polarity (with plate ‘a’ positive) by triggering the thyristor T 2. Capacitor ‘C’ gets charged through V S, C, T 2 and load. As the charging current decays to zero thyristor T 2 will be turned-off. With capacitor charged with plate ‘a’ positive the circuit is ready for operation. Assume that the load current remains constant during the commutation process.

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Power Electronics by Prof. M. Madhusudhan Rao 122 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT For convenience the chopper operation is divided into five modes. –Mode-1 –Mode-2 –Mode-3 –Mode-4 –Mode-5

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Power Electronics by Prof. M. Madhusudhan Rao 123 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Mode-1 Operation

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Power Electronics by Prof. M. Madhusudhan Rao 124 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Thyristor T 1 is fired at t = 0. The supply voltage comes across the load. Load current I L flows through T 1 and load. At the same time capacitor discharges through T 1, D 1, L 1, & ‘C’ and the capacitor reverses its voltage. This reverse voltage on capacitor is held constant by diode D 1.

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Power Electronics by Prof. M. Madhusudhan Rao 125 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 126 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Mode-2 Operation

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Power Electronics by Prof. M. Madhusudhan Rao 127 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Thyristor T 2 is now fired to commutate thyristor T 1. When T 2 is ON capacitor voltage reverse biases T 1 and turns if off. The capacitor discharges through the load from –V to 0. Discharge time is known as circuit turn-off time.

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Power Electronics by Prof. M. Madhusudhan Rao 128 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 129 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Capacitor recharges back to the supply voltage (with plate ‘a’ positive). This time is called the recharging time and is given by The total time required for the capacitor to discharge and recharge is called the commutation time and it is given by

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Power Electronics by Prof. M. Madhusudhan Rao 130 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT At the end of Mode-2 capacitor has recharged to V S and the free wheeling diode starts conducting.

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Power Electronics by Prof. M. Madhusudhan Rao 131 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Mode-3 Operation

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Power Electronics by Prof. M. Madhusudhan Rao 132 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT FWD starts conducting and the load current decays. The energy stored in source inductance L S is transferred to capacitor. Hence capacitor charges to a voltage higher than supply voltage, T 2 naturally turns off.

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Power Electronics by Prof. M. Madhusudhan Rao 133 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 134 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Mode-4 Operation

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Power Electronics by Prof. M. Madhusudhan Rao 135 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Capacitor has been overcharged i.e. its voltage is above supply voltage. Capacitor starts discharging in reverse direction. Hence capacitor current becomes negative. The capacitor discharges through L S, V S, FWD, D 1 and L. When this current reduces to zero D 1 will stop conducting and the capacitor voltage will be same as the supply voltage

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Power Electronics by Prof. M. Madhusudhan Rao 136 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Mode-5 Operation Both thyristors are off and the load current flows through the FWD. This mode will end once thyristor T 1 is fired.

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Power Electronics by Prof. M. Madhusudhan Rao 137 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 138 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Power Electronics by Prof. M. Madhusudhan Rao 139 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Disadvantages A starting circuit is required and the starting circuit should be such that it triggers thyristor T 2 first. Load voltage jumps to almost twice the supply voltage when the commutation is initiated. The discharging and charging time of commutation capacitor are dependent on the load current and this limits high frequency operation, especially at low load current.

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Power Electronics by Prof. M. Madhusudhan Rao 140 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Chopper cannot be tested without connecting load. Thyristor T 1 has to carry load current as well as resonant current resulting in increasing its peak current rating.

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