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DC Choppers Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 1 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 2 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 3 Principle Of Step-down Chopper Chopper i0 V + V0 R Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 4 • 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 5 v0 V Vdc t tON tOFF i0 V/R Idc t T Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 6 Vdc Average value of output or load voltage. I dc Average value of output or load current. tON Time interval for which SCR conducts. tOFF Time interval for which SCR is OFF. T tON tOFF Period of switching or chopping period. 1 f Freq. of chopper switching or chopping freq. T Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 7 Average Output Voltage tON Vdc V tON tOFF tON Vdc V V .d T tON but t d duty cycle Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 8 Average Output Current Vdc I dc R V tON V I dc d R T R RMS value of output voltage 1 VO T tON v dt 2 o 0 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 9 But during tON , vo V Therefore RMS output voltage 1 VO T tON V dt 2 0 2 tON V VO tON .V T T VO d .V Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 10 10 Output power PO VO I O VO IO But R Output power 2 O V PO R 2 dV PO R Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 11 11 Effective input resistance of chopper V Ri I dc R Ri d The output voltage can be varied by varying the duty cycle. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 12 12 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 13 13 Pulse Width Modulation • tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant. • Output voltage is varied by varying the ON time tON Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 14 14 V0 V tON tOFF t T V0 V t tON tOFF Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 15 15 Variable Frequency Control • Chopping frequency ‘f’ is varied keeping either tON or tOFF 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 tOFF load current may become discontinuous Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 16 16 v0 V tON tOFF t T v0 V tON tOFF t T Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 17 17 Step-down Chopper With R-L Load Chopper i0 + R V FWD L E Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT V0 18 18 • 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’. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 19 19 • 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 Imax and Imin • When current becomes equal to Imax the chopper is turned-off and it is turned-on when current reduces to Imin. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 20 20 v0 Output voltage V tON i0 tOFF T t Imax Output current Imin Continuous current i0 t Output current Discontinuous current t Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 21 21 Expressions For Load Current iO For Continuous Current Operation When Chopper Is ON (0 t tON) Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 22 22 i0 + R V V0 L E - Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 23 23 diO V iO R L E dt Taking Laplace Transform V E RI O S L S .I O S iO 0 S S At t 0, initial current iO 0 I min I min V E IO S R R LS S S L L Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 24 24 Taking Inverse Laplace Transform R R t t V E L L iO t 1 e I min e R This expression is valid for 0 t tON , i.e., during the period chopper is ON. At the instant the chopper is turned off, load current is iO tON I max Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 25 25 When Chopper is OFF i0 R L E Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 26 26 When Chopper is OFF 0 t tOFF diO 0 RiO L E dt Talking Laplace transform E 0 RI O S L SI O S iO 0 S Redefining time origin we have at t 0, initial current iO 0 I Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT max 27 27 I max E IO S R R S LS S L L Taking Inverse Laplace Transform iO t I max e R t L E 1 e R Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT R t L 28 28 The expression is valid for 0 t tOFF , i.e., during the period chopper is OFF At the instant the chopper is turned ON or at the end of the off period, the load current is iO tOFF I min Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 29 29 To Find I max & I min From equation At R R t t V E L L iO t 1 e I min e R t tON dT , iO t I max I max V E 1 e R Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT dRT L I min e dRT L 30 30 From equation iO t I max e At R t L E 1 e R R t L t tOFF T tON , iO t I min t tOFF 1 d T Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 31 31 1 d RT E L I min I max e 1 e L R Substituting for I min in equation 1 d RT I max dRT dRT V E L L 1 e I e min R I max dRT V 1 e L RT R 1 e L we get, Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT E R 32 32 Substituting for I max in equation I min I max e 1 d RT L 1 d RT E 1 e L R we get, V e 1 E I min RT R R L e 1 I max I min is known as the steady state ripple. dRT L Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 33 33 Therefore peak-to-peak ripple current I I max I min Average output voltage Vdc d .V Average output current I max I min I dc approx 2 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 34 34 Assuming load current varies linearly from I min to I max instantaneous load current is given by iO I min I .t for 0 t tON dT dT I max I min iO I min t dT Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 35 35 RMS value of load current I O RMS I O RMS I O RMS 1 dT dT 1 dT dT 1 dT dT 2 i 0 dt 0 0 0 I max I min t I min dt dT 2 2 2 I max I min 2 2 I min I max I min t I min dt t dT dT Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 36 36 RMS value of output current 2 2 I max I min I O RMS I min I min I max I min 3 RMS chopper current I CH I CH 1 T dT 1 T dT 1 2 i dt 2 0 0 0 I max I min I min dT Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 2 t dt 37 37 2 I max I min d I min I min I max I min 3 2 I CH I CH d I O RMS Effective input resistance is V Ri IS Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 38 38 1 2 Where I S Average source current I S dI dc V Ri dI dc Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 39 39 Principle Of Step-up Chopper I L + D + C V Chopper L O A D VO Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 40 40 • 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, tON. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 41 41 • When the chopper is off, the inductor current I is forced to flow through the diode D and load for a period, tOFF. • The current tends to decrease resulting in reversing the polarity of induced EMF in L. • Therefore voltage across load is given by dI VO V L i.e., VO V dt Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 42 42 • 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 43 43 Expression For Output Voltage Assume the average inductor current to be I during ON and OFF time of Chopper. When Chopper is ON Voltage across inductor L V Therefore energy stored in inductor = V .I .tON Where tON ON period of chopper. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 44 44 When Chopper is OFF (energy is supplied by inductor to load) Voltage across L VO V Energy supplied by inductor L VO V ItOFF where tOFF OFF period of Chopper. Neglecting losses, energy stored in inductor L = energy supplied by inductor L Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 45 45 VItON VO V ItOFF VO V tON tOFF tOFF T VO V T tON Where T = Chopping period or period of switching. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 46 46 T tON tOFF 1 VO V tON 1 T 1 VO V 1 d tON Where d duty cyle T Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 47 47 For variation of duty cycle ' d ' in the range of 0 d 1 the output voltage VO will vary in the range V VO Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 48 48 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 49 49 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 50 50 V 460 V, Vdc = 350 V, f = 2 kHz Chopping period Output voltage 1 T f 1 T 0.5 m sec 3 2 10 tON Vdc V T Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 51 51 Conduction period of thyristor T Vdc tON V 3 0.5 10 350 tON 460 tON 0.38 msec Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 52 52 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 53 53 V 200 V , tON 200 s, Vdc 600V T Vdc V T tON T 600 200 6 T 200 10 Solving for T T 300 s Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 54 54 Chopping frequency 1 f T 1 f 3.33KHz 6 300 10 Pulse width is halved tON 200 10 2 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 6 100 s 55 55 Frequency is constant f 3.33KHz 1 T 300 s f T Output voltage = V T tON 300 106 200 300 Volts 6 300 100 10 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 56 56 Problem • A dc chopper has a resistive load of 20 and input voltage VS = 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 57 57 VS 220V , R 20, f 10 kHz tON d 0.80 T Vch = Voltage drop across chopper = 1.5 volts Average output voltage tON Vdc VS Vch T Vdc 0.80 220 1.5 174.8 Volts Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 58 58 Chopper ON time, tON dT Chopping period, 1 T f 1 3 T 0.1 10 secs 100 μsecs 3 10 10 Chopper ON time, tON dT tON 0.80 0.110 3 3 tON 0.08 10 80 μsecs Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 59 59 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 60 60 I dc 30 Amps, f 250 Hz, V 110 V , R 2 1 1 Chopping period, T 4 103 4 msecs f 250 Vdc I dc & Vdc dV R dV I dc R I dc R 30 2 d 0.545 V 110 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 61 61 Chopper ON period, 3 tON dT 0.545 4 10 2.18 msecs Chopper OFF period, tOFF T tON tOFF 4 103 2.18 103 tOFF 1.82 103 1.82 msec Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 62 62 • 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 63 63 Chopper V i0 + R v0 V 200 V , R 10, Chopper voltage drop Vch 2V d 0.60, f 1 kHz. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 64 64 Average output voltage Vdc d V Vch Vdc 0.60 200 2 118.8 Volts RMS value of output voltage VO d V Vch VO 0.6 200 2 153.37 Volts Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 65 65 Effective input resistance of chopper is V V Ri I S I dc Vdc 118.8 I dc 11.88 Amps R 10 V V 200 Ri 16.83 I S I dc 11.88 Output power is 1 PO T dT 2 0 v 1 dt R T 0 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT dT 0 V Vch R 2 dt 66 66 d V Vch PO R 2 0.6 200 2 2 PO 10 2352.24 watts Input power, 1 Pi T 1 PO T dT Vi dt O 0 V V Vch 0 R dt dT Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 67 67 dV V Vch PO R 0.6 200 200 2 PO 2376 watts 10 Chopper efficiency, PO 100 Pi 2352.24 100 99% 2376 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 68 68 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 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 69 69 L 5 H , R 10, f 1000 Hz, V 200 V , tON : tOFF 2 : 3 Chopping period, 1 1 T 1 msecs f 1000 tON 2 tOFF 3 tON 2 tOFF 3 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 70 70 T tON tOFF 2 T tOFF tOFF 3 5 T tOFF 3 tOFF 3 T 5 3 3 T 110 0.6 msec 5 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 71 71 tON T tOFF tON 1 0.6 103 0.4 msec Duty cycle, 3 tON 0.4 10 d 0.4 3 T 110 Maximum value of load current is given by V 1 e RT R 1 e L I max dRT L Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT E R 72 72 Since there is no voltage source in the load circuit, E = 0 I max I max dRT V 1 e L RT R 1 e L 3 0.4 10 1 10 5 200 1 e 101103 10 1 e 5 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 73 73 1 e 20 2103 1 e 8.0047A 0.8103 I max I max Minimum value of load current with E = 0 is given by V e 1 RT R e L 1 dRT L I min Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 74 74 200 e 1 I min 7.995 A 3 10110 10 e 5 1 Average load current I max I min I dc 2 8.0047 7.995 I dc 8 A 2 0.4101103 5 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 75 75 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 76 76 V = 200 V, R = 5 , L = 5 mH, f = 1kHz, d = 0.5, E = 0 Chopping period is 1 1 3 T 110 secs 3 f 110 Chopper i0 + R FWD L E Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT v0 77 77 Maximum value of load current is given by I max I max I max dRT V 1 e L E RT R R 1 e L 3 0.5 5 1 10 3 200 1 e 510 0 51103 5 1 e 5103 1 e 0.5 40 24.9 A 1 1 e Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 78 78 Minimum value of load current is given by I min I min I min dRT L V e 1 E RT R R L e 1 3 0.5 5 1 10 3 200 e 510 1 0 3 5110 5 e 5103 1 e0.5 1 40 1 15.1 A e 1 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 79 79 Average value of load current is I1 I 2 I dc 2 for linear variation of currents 24.9 15.1 I dc 20 A 2 RMS load current is given by 1 2 2 I max I min I O RMS I min I min I max I min 3 8080 Power Electronics by Prof. M. Madhusudhan Rao 2 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 24.9 15.1 2 15.1 15.1 24.9 15.1 3 2 I O RMS 1 2 96.04 I O RMS 228.01 147.98 20.2 A 3 RMS chopper current is given by I ch d I O RMS 0.5 20.2 14.28 A Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 81 81 1 2 Effective input resistance is V Ri IS I S = Average source current I S dI dc I S 0.5 20 10 A Therefore effective input resistance is V 200 Ri 20 IS 10 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 82 82 Classification Of Choppers • Choppers are classified as – – – – – Class A Chopper Class B Chopper Class C Chopper Class D Chopper Class E Chopper Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 83 83 Class A Chopper i0 + v0 Chopper V FWD L O A D v0 V Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT i0 84 84 • When chopper is ON, supply voltage V is connected across the load. • When chopper is OFF, vO = 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 . Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 85 85 • 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 86 86 ig Thyristor gate pulse t i0 Output current CH ON t FWD Conducts v0 Output voltage tON t T Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 87 87 Class B Chopper D i0 v0 + R L v0 V Chopper E Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT i0 88 88 • 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 89 89 • 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 90 90 ig Thyristor gate pulse t i0 tOFF tON T Output current Imax Imin v0 t D conducts Chopper conducts Output voltage t Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 91 91 Expression for Output Current Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 92 92 During the interval diode 'D' conducts voltage equation is given by LdiO V RiO E dt For the initial condition i.e., iO t I min at t 0 The solution of the above equation is obtained along similar lines as in step-down chopper with R-L load Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 93 93 V E iO t 1 e R At t tOFF R t L I min e R t L 0 t tOFF iO t I max R R t tOFF V E OFF L L I max 1 e I e min R During the interval chopper is ON voltage equation is given by LdiO 0 RiO E dt Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 94 94 Redefining the time origin, at t 0 iO t I max The solution for the stated initial condition is iO t I max e R t L At t tON I min I max e R t E L 1 e R iO t I min R tON L 0 t tON R tON E L 1 e R Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 95 95 Class C Chopper CH1 D1 i0 + v0 R V CH2 D2 L v0 Chopper E Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT i0 96 96 • Class C Chopper is a combination of Class A and Class B Choppers. • For first quadrant operation, CH1 is ON or D2 conducts. • For second quadrant operation, CH2 is ON or D1 conducts. • When CH1 is ON, the load current is positive. • The output voltage is equal to ‘V’ & the load receives power from the source. • When CH1 is turned OFF, energy stored in inductance L forces current to flow through the diode D2 and the output voltage is zero. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 97 97 • Current continues to flow in positive direction. • When CH2 is triggered, the voltage E forces current to flow in opposite direction through L and CH2 . • The output voltage is zero. • On turning OFF CH2 , the energy stored in the inductance drives current through diode D1 and the supply • Output voltage is V, the input current becomes negative and power flows from load to source. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 98 98 • Average output voltage is positive • Average output current can take both positive and negative values. • Choppers CH1 & CH2 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 99 99 ig1 Gate pulse of CH1 t ig2 Gate pulse of CH2 t i0 Output current t D1 CH1 ON D2 CH2 ON D1 CH1 ON V0 D2 CH2 ON Output voltage t Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 100 100 Class D Chopper v0 CH1 D2 R i0 L V + v0 D1 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT E i0 CH2 101 101 • Class D is a two quadrant chopper. • When both CH1 and CH2 are triggered simultaneously, the output voltage vO = V and output current flows through the load. • When CH1 and CH2 are turned OFF, the load current continues to flow in the same direction through load, D1 and D2 , due to the energy stored in the inductor L. • Output voltage vO = - V . Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 102 102 • Average load voltage is positive if chopper ON time is more than the OFF time • Average output voltage becomes negative if tON < tOFF . • Hence the direction of load current is always positive but load voltage can be positive or negative. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 103 103 ig1 Gate pulse of CH1 t ig2 Gate pulse of CH2 t i0 Output current v0 CH1,CH2 ON t D1,D2 Conducting Output voltage V Average v0 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT t 104 104 ig1 Gate pulse of CH1 t ig2 Gate pulse of CH2 t i0 Output current CH1 CH2 t D1, D2 v0 Output voltage V Average v0 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT t 105 105 Class E Chopper CH1 i0 V CH3 D1 R + CH2 L v0 D2 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT D3 E CH4 D4 106 106 Four Quadrant Operation v0 CH2 - D4 Conducts D1 - D4 Conducts CH1 - CH4 ON CH4 - D2 Conducts i0 CH3 - CH2 ON CH2 - D4 Conducts D2 - D3 Conducts CH4 - D2 Conducts Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 107 107 • Class E is a four quadrant chopper • When CH1 and CH4 are triggered, output current iO flows in positive direction through CH1 and CH4, and with output voltage vO = V. • This gives the first quadrant operation. • When both CH1 and CH4 are OFF, the energy stored in the inductor L drives iO through D2 and D3 in the same direction, but output voltage vO = -V. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 108 108 • Therefore the chopper operates in the fourth quadrant. • When CH2 and CH3 are triggered, the load current iO flows in opposite direction & output voltage vO = -V. • Since both iO and vO are negative, the chopper operates in third quadrant. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 109 109 • When both CH2 and CH3 are OFF, the load current iO continues to flow in the same direction D1 and D4 and the output voltage vO = V. • Therefore the chopper operates in second quadrant as vO is positive but iO is negative. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 110 110 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 111 111 • 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 112 112 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 113 113 i0 Chopper V FWD + L O A D v0 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 114 114 tON Average output voltage, Vdc V dV T Vdc dV Average output current, I dc R R The thyristor is commutated at the instant t tON V output current at the instant of commutation is R since V is the output voltage at that instant. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 115 115 Free wheeling diode (FWD) will never conduct in a resistive load. Average & RMS free wheeling diode currents are zero. VO RMS But 1 T tON v dt 2 0 0 vO V during tON Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 116 116 VO RMS VO RMS 1 T tON V dt 2 0 tON V T 2 VO RMS dV tON Where duty cycle, d T Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 117 117 RMS value of thyristor current = RMS value of load current VO RMS R dV R Average value of thyristor current = Average value of load current dV R Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 118 118 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 119 119 T1 LS + a iT1 IL + C _ b T2 FWD iC VS _ L D1 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT + L O A D vO _ 120 120 • To start the circuit, capacitor ‘C’ is initially charged with polarity (with plate ‘a’ positive) by triggering the thyristor T2. • Capacitor ‘C’ gets charged through VS, C, T2 and load. • As the charging current decays to zero thyristor T2 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 121 121 • For convenience the chopper operation is divided into five modes. – Mode-1 – Mode-2 – Mode-3 – Mode-4 – Mode-5 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 122 122 Mode-1 Operation T1 LS + IL + VC _C iC VS L D1 L O A D _ Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 123 123 • • • • Thyristor T1 is fired at t = 0. The supply voltage comes across the load. Load current IL flows through T1 and load. At the same time capacitor discharges through T1, D1, L1, & ‘C’ and the capacitor reverses its voltage. • This reverse voltage on capacitor is held constant by diode D1. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 124 124 Capacitor Discharge Current C iC t V sin t L 1 Where LC & Capacitor Voltage VC t V cos t Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 125 125 Mode-2 Operation IL + LS VC VS _ IL C + T2 L O A D _ Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 126 126 • Thyristor T2 is now fired to commutate thyristor T1. • When T2 is ON capacitor voltage reverse biases T1 and turns if off. • The capacitor discharges through the load from –V to 0. • Discharge time is known as circuit turn-off time. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 127 127 Circuit turn-off time is given by VC C tC IL Where I L is load current. t C depends on load current, it must be designed for the worst case condition which occur at the maximum value of load current and minimum value of capacitor voltage. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 128 128 • Capacitor recharges back to the supply voltage (with plate ‘a’ positive). • This time is called the recharging time and is given by VS C td IL • The total time required for the capacitor to discharge and recharge is called the commutation time and it is given by tr tC td Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 129 129 • At the end of Mode-2 capacitor has recharged to VS and the free wheeling diode starts conducting. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 130 130 Mode-3 Operation IL + LS VS IL + _C T2 VS FWD L O A D _ Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 131 131 • FWD starts conducting and the load current decays. • The energy stored in source inductance LS is transferred to capacitor. • Hence capacitor charges to a voltage higher than supply voltage, T2 naturally turns off. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 132 132 The instantaneous capacitor voltage is VC t VS I L LS sin S t C Where S 1 LS C Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 133 133 Mode-4 Operation LS + IL + VC _C L O A D D1 VS L _ Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT FWD 134 134 • 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 LS, VS, FWD, D1 and L. • When this current reduces to zero D1 will stop conducting and the capacitor voltage will be same as the supply voltage Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 135 135 Mode-5 Operation IL FWD L O A D • Both thyristors are off and the load current flows through the FWD. • This mode will end once thyristor T1 is fired. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 136 136 ic Capacitor Current IL 0 Ip iT1 IL t Ip Current through T 1 0 Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT t 137 137 v T1 Voltage across T1 Vc t 0 vo Vs+Vc Output Voltage Vs t vc Vc t Capacitor Voltage -Vc tc td Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 138 138 Disadvantages • A starting circuit is required and the starting circuit should be such that it triggers thyristor T2 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. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 139 139 • Chopper cannot be tested without connecting load. • Thyristor T1 has to carry load current as well as resonant current resulting in increasing its peak current rating. Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha Kumar, E&E Dept., MSRIT 140 140