Dc Choppers
Upcoming SlideShare
Loading in...5
×
 

Like this? Share it with your network

Share

Dc Choppers

on

  • 38,459 views

 

Statistics

Views

Total Views
38,459
Views on SlideShare
37,938
Embed Views
521

Actions

Likes
14
Downloads
2,047
Comments
13

3 Embeds 521

http://www.ustudy.in 465
http://www.slideshare.net 46
http://ustudy.in 10

Accessibility

Upload Details

Uploaded via as Microsoft PowerPoint

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel

15 of 13 Post a comment

  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment
  • Power ElectronicsPower Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics
  • Power Electronics

Dc Choppers Presentation Transcript

  • 1. DC Choppers Power Electronics by Prof. M. Madhusudhan Rao 1 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 2. 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 2 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 3. 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 3 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 4. Principle Of Step-down Chopper C hopper i0 + V R V 0 − Power Electronics by Prof. M. Madhusudhan Rao 4 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 5. • 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 5 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 6. v0 V V dc t tON tOFF i0 V /R Idc t T Power Electronics by Prof. M. Madhusudhan Rao 6 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 7. 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 7 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 8. Average Output Voltage  tON  Vdc = V    tON + tOFF   tON  Vdc = V   = V .d  T   tON  but   = d = duty cycle  t  Power Electronics by Prof. M. Madhusudhan Rao 8 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 9. Average Output Current Vdc I dc = R V  tON  V I dc =  = d R T  R RMS value of output voltage tON 1 VO = ∫ v dt 2 o T 0 Power Electronics by Prof. M. Madhusudhan Rao 9 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 10. But during tON , vo = V Therefore RMS output voltage tON 1 VO = ∫ 2 V dt T 0 2 V tON VO = tON = .V T T VO = d .V Power Electronics by Prof. M. Madhusudhan Rao 10 10 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 11. Output power PO = VO I O VO But IO = R ∴ Output power 2 V PO = O R 2 dV PO = R Power Electronics by Prof. M. Madhusudhan Rao 11 11 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 12. 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 12 12 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 13. 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 13 13 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 14. 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 14 14 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 15. V 0 V tON tOFF t T V 0 V t tON tOFF Power Electronics by Prof. M. Madhusudhan Rao 15 15 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 16. 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 16 16 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 17. v0 V tON tOFF t T v0 V tO N tO FF t T Power Electronics by Prof. M. Madhusudhan Rao 17 17 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 18. Step-down Chopper With R-L Load C hopper i0 + R V V FW D L 0 E − Power Electronics by Prof. M. Madhusudhan Rao 18 18 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 19. • 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 19 19 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 20. • 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 20 20 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 21. v0 O u tp u t v o lta g e V tO N tO FF t T i0 O u tp u t Im ax c u rre n t C o n tin u o u s I m in c u rre n t t i0 O u tp u t c u rre n t D is c o n tin u o u s c u rre n t t Power Electronics by Prof. M. Madhusudhan Rao 21 21 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 22. Expressions For Load Current iO For Continuous Current Operation When Chopper Is ON (0 ≤ t ≤ tON) Power Electronics by Prof. M. Madhusudhan Rao 22 22 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 23. i0 + R V V 0 L E - Power Electronics by Prof. M. Madhusudhan Rao 23 23 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 24. 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 V −E I min IO ( S ) = +  R R LS  S +  S +  L L Power Electronics by Prof. M. Madhusudhan Rao 24 24 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 25. Principle Of Step-up Chopper I L D + + − L C O V V A O D C hopper − Power Electronics by Prof. M. Madhusudhan Rao 25 25 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 26. • 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 26 26 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 27. • 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 27 27 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 28. • 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 28 28 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 29. 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 29 29 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 30. 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 30 30 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 31. ∴ VItON = ( VO − V ) ItOFF V [ tON + tOFF ] VO = tOFF  T  VO = V    T − tON  Where T = Chopping period or period of switching. Power Electronics by Prof. M. Madhusudhan Rao 31 31 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 32. 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 32 32 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 33. 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 33 33 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 34. Class A Chopper i0 v0 + C hopper L O v0 V V A FW D D − i0 Power Electronics by Prof. M. Madhusudhan Rao 34 34 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 35. • 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 35 35 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 36. • 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 36 36 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 37. ig T h y r is to r g a te p u ls e t i0 O u tp u t c u rre n t CH O N t v0 F W D C o n d u c ts O u tp u t v o lta g e t tO N T Power Electronics by Prof. M. Madhusudhan Rao 37 37 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 38. Class B Chopper D i0 v0 + R V L v0 C hopper E − i0 − Power Electronics by Prof. M. Madhusudhan Rao 38 38 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 39. • 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 39 39 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 40. • 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 40 40 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 41. ig T h y r is to r g a te p u ls e t i0 tO FF tO N T t O u tp u t c u rre n t Im ax Im in D c o n d u c ts C h o p p e r c o n d u c ts v0 O u tp u t v o lta g e t Power Electronics by Prof. M. Madhusudhan Rao 41 41 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 42. Class C Chopper CH 1 D 1 i0 v0 + V R CH 2 D L v0 2 C hopper i0 E − Power Electronics by Prof. M. Madhusudhan Rao 42 42 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 43. • 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 43 43 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 44. • 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 Prof. M. Madhusudhan flows from load to source.44 Power Electronics by and power Rao 44 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 45. • 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 45 45 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 46. ig 1 G a te p u ls e of C H 1 t ig 2 G a te p u ls e of C H 2 t i0 O u tp u t c u rre n t t D 1 CH 1 D 2 CH 2 D 1 CH 1 D 2 CH 2 O N O N O N O N V 0 O u tp u t v o lta g e t Power Electronics by Prof. M. Madhusudhan Rao 46 46 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 47. Class D Chopper v0 CH 1 D 2 R i0 L E V + v0 − i0 D 1 CH 2 Power Electronics by Prof. M. Madhusudhan Rao 47 47 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 48. • 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. •Power Electronics by Prof. M. Madhusudhan RaoV . Output voltage vO = - 48 48 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 49. • 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 49 49 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 50. ig1 G a te p u ls e of C H 1 t ig2 G a te p u ls e of C H 2 t i0 O u tp u t c u rre n t t C H 1,C H 2 D 1 ,D 2 C o n d u c tin g O N v0 O u tp u t v o lta g e V A v era g e v0 t Power Electronics by Prof. M. Madhusudhan Rao 50 50 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 51. ig 1 G a te p u ls e of C H 1 t ig 2 G a te p u ls e of C H 2 t i0 O u tp u t c u rre n t CH 1 CH 2 t D 1, D 2 v0 O u tp u t v o lta g e V t A v era g e v0 Power Electronics by Prof. M. Madhusudhan Rao 51 51 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 52. Class E Chopper CH 1 D CH 3 D 1 3 i0 R L E V + − v0 CH 2 D CH 4 D 2 4 Power Electronics by Prof. M. Madhusudhan Rao 52 52 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 53. Four Quadrant Operation v0 CH 2 - D 4 C o n d u c ts CH 1 - CH 4 O N D 1 - D 4 C o n d u c ts CH 4 - D 2 C o n d u c ts i0 CH 3 - CH 2 O N D 2 - D 3 C o n d u c ts C H 2 - D 4 C o n d u c ts C H 4 - D 2 C o n d u c ts Power Electronics by Prof. M. Madhusudhan Rao 53 53 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 54. • 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 Kumar,= -V. MSRIT vO E&E Dept., Power Electronics by Prof. M. Madhusudhan Rao Prof. T.K. Anantha 54 54
  • 55. • 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 55 55 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 56. • 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 56 56 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 57. 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 57 57 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 58. • 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 58 58 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 59. i0 + C hopper L O v0 V FW D A D − Power Electronics by Prof. M. Madhusudhan Rao 59 59 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 60. Mode-1 Operation L S T 1 + + IL V C _ C iC L V O S A D L D 1 _ Power Electronics by Prof. M. Madhusudhan Rao 60 60 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 61. • 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 61 61 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 62. Capacitor Discharge Current C iC ( t ) = V sin ω t L 1 Where ω= LC & Capacitor Voltage VC ( t ) = V cos ω t Power Electronics by Prof. M. Madhusudhan Rao 62 62 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 63. Mode-2 Operation IL + L S _ IL V C C L V + T O S 2 A D _ Power Electronics by Prof. M. Madhusudhan Rao 63 63 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 64. • 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 64 64 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 65. 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 65 65 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 66. • 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 66 66 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 67. • 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 67 67 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 68. Mode-3 Operation IL + L S + IL V S _ C T 2 L V O S A FW D D _ Power Electronics by Prof. M. Madhusudhan Rao 68 68 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 69. • 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 69 69 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 70. The instantaneous capacitor voltage is LS VC ( t ) = VS + I L sin ω S t C Where 1 ωS = LS C Power Electronics by Prof. M. Madhusudhan Rao 70 70 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 71. Mode-4 Operation L S + + IL V C _ C L D 1 O V S A L D _ FW D Power Electronics by Prof. M. Madhusudhan Rao 71 71 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 72. • 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 72 72 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 73. Mode-5 Operation IL L • Both thyristors are off FW D O A and the load current D flows through the FWD. • This mode will end once thyristor T1 is fired. Power Electronics by Prof. M. Madhusudhan Rao 73 73 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 74. ic C a p a c ito r C u rr e n t IL 0 t Ip iT 1 Ip IL C u rre n t th ro u g h T 1 t 0 Power Electronics by Prof. M. Madhusudhan Rao 74 74 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 75. vT1 Vc V o l ta g e a c r o s s T 1 t 0 vo V s+ V c Vs O u tp u t V o lta g e t vc Vc t C a p a c ito r V o lta g e -V c tc td Power Electronics by Prof. M. Madhusudhan Rao 75 75 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 76. 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 76 76 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  • 77. • 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 77 77 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT