1. Power Electronics Unit IV Prof. V. N. Bhonge
Dept. of Electronics & Telecomm.
Shri Sant Gajanan Maharaj College of Engg,
            
            
            
            
Shegaon –

2. Unit V: Chopper and Cycloconverter
Basic principles of chopper, time ratio control and current limit control techniques, voltage commutated chopper ckt., Jones chopper, step-up chopper and AC chopper. Basic principle of cycloconverter, single phase to single phase cycloconverter.
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

3. Introduction Chopper is a static device.
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

4. APPLICATIONS Widely used for motor control.
SSGMCE
Shegaon
Widely used for motor control.
Also used in regenerative braking.
Switched-mode power supply (SMPS)
battery chargers
Prof. V. N. Bhonge
Dept. of E & T

5. Chopper
SSGMCE
Shegaon
Definition:
Converting the unregulated DC input to a controlled
DC output with a desired voltage level.
General block diagram:
Prof. V. N. Bhonge
Dept. of E & T

6. Types of Choppers are Two Types: 1) Step-down choppers.
SSGMCE
Shegaon
Two Types:
1) Step-down choppers.
2) 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.
Prof. V. N. Bhonge
Dept. of E & T

7. Principle Of Step-down Chopper
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

8. Waveforms:
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

9. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

10. Output voltage as function of duty cycle
Output voltage varies linearly with duty cycle.
It is possible to control output voltage from zero to Vi as duty cycle varies from zero to 1.
Dept. of E & T
Prof. V. N. Bhonge

11. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

12. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

13. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

14. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

15. Chopper Control Technique
SSGMCE
Shegaon
The operation of thyriststor can be controlled in the following two ways
1) Time Ratio Control (TRC).
2) Current Limit Control (CLC)
Prof. V. N. Bhonge
Dept. of E & T

16. Time Ratio Control (TRC).
SSGMCE
Shegaon
The output dc voltage can be varied by the following methods.
Types of Time Ratio Control (TRC).
1) Pulse width modulation control or
constant frequency operation.
2) Variable frequency control.
Prof. V. N. Bhonge
Dept. of E & T

17. 1) Constant frequency operation ( Pulse Width Modulation)
SSGMCE
Shegaon
tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant.
Output voltage is varied by varying the ON time tON
Prof. V. N. Bhonge
Dept. of E & T

18. 2) Variable Frequency Control
SSGMCE
Shegaon
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
Prof. V. N. Bhonge
Dept. of E & T

19. Methods for varing Average Output Voltage
Pulse-Width Modulation
Pulse-Frequency Modulation
Pulse width TON is varied while overall switching period is kept constant.
Pulse width TON is kept constant while the period (frequency) is varied.

20. Performance Parameters
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

21. Problem-1
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

22. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

23. Problem-2
SSGMCE
Shegaon
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 i) Chopping frequency, ii) If the pulse width is halved for constant frequency of operation, find the new output voltage.
Prof. V. N. Bhonge
Dept. of E & T

24. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

25. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

26. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

27. Problem-3
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

28. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

29. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

30. Problem-4
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

31. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

32. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

33. Problem-5
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
i) Average output voltage
ii) RMS value of output voltage
iii) Effective input resistance of chopper
iv) Chopper efficiency.
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

34. Solution:
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

35. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

36. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

37. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

38. Voltage Commuted chopper Henman’s Chopper(Oscillation Chopper)
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

39. Capacitor ‘C’ gets charged through VS, C, T2 and load.
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

40. For convenience the chopper operation is divided into five modes.
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

41. Mode-1 Operation Thyristor T1 is fired at t = 0.
SSGMCE
Shegaon
Thyristor T1 is fired at t = 0.
The supply voltage comes across the load.
Load current IL flows through T1 and load.
Prof. V. N. Bhonge
Dept. of E & T

42. This reverse voltage on capacitor is held constant by diode D1.
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.
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

43. Mode-2 Operation Thyristor T2 is now fired to commutate thyristor T1.
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

44. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

45. This time is called the recharging time and is given by
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
SSGMCE
Shegaon
At the end of Mode-2 capacitor has recharged to VS and the free wheeling diode starts conducting.
Prof. V. N. Bhonge
Dept. of E & T

46. Mode-3 Operation FWD starts conducting and the load current decays.
SSGMCE
Shegaon
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.
Prof. V. N. Bhonge
Dept. of E & T

47. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

48. Mode-4 Operation
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

49. Capacitor starts discharging in reverse direction.
SSGMCE
Shegaon
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
Prof. V. N. Bhonge
Dept. of E & T

50. Mode-5 Operation
SSGMCE
Shegaon
Both thyristors are off and the load current flows through the FWD.
This mode will end once thyristor T1 is fired.
Prof. V. N. Bhonge
Dept. of E & T

51. SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

52. Disadvantages
SSGMCE
Shegaon
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.
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.
Prof. V. N. Bhonge
Dept. of E & T

53. Basic Step-Up Chopper Circuit Prof. V. N. Bhonge Dept. of E & T
Operating function
Vo > Vs
The circuit using a thyristor, (S), diode, (D1), inductor, (L), and load, (R), is shown in Fig.
On State
Off state
Prof. V. N. Bhonge Dept. of E & T

54. Prof. V. N. Bhonge Dept. of E & T
On-State
Off-State
When S is on (D is off), capacitor energy supplies the load voltage.
Vo=Vc (if capacitor is charged)
During on-state of switch S, voltage across inductor instantly becomes equal to input supply voltage. Current through it increases gradually and stores energy in its magnetic field.
For very first time, when S is closed Vo=0, as capacitor is not charged.
When S is off (D is on), inductor voltage reverses its polarity and adds in input voltage to provide output voltage which is equal to:
V0=Vi+VL
During off state of S, capacitor charges and voltage at it gradually build up to Vi+VL
(This capacitor voltages serves as load voltage when next time S in on)
If S is off forever, inductor acts as short circuit. It does not develop any voltage and
Vo= Vi
Prof. V. N. Bhonge Dept. of E & T

55. Voltage and current waveforms for duty cycle 50%
d= 0.5 means Switch is on and off for equal time intervals.
Energy that inductor develops during on-state is completely dessipated during off-state.
If duty cycle increases above 0.5, inductor will not dessipate its energy completely in off-states. The remaining inductor voltage (due to left-over energy) adds up next time when switch is off and more increased voltage appears at output.
Prof. V. N. Bhonge Dept. of E & T

56. Prof. V. N. Bhonge Dept. of E & T
If duty cycle increases above 0.5, inductor will not dessipate its energy completely in off-states. The remaining inductor voltage (due to left-over energy) adds up next time when switch is off and more increased voltage appears at output.
Neglecting losses, energy transferred by inductance during TOFF must equal the energy gained by it during period TON
Final expression for output load voltage is:
Vo=Vi [1/(1-d)]
If switch is open (d=0), output voltage is equal to input voltage. As d increases, output voltage becomes larger than input voltage.
So output voltage is always higher than input voltage if switch is operated at an appropriately high frequency.
Prof. V. N. Bhonge Dept. of E & T

57. Prof. V. N. Bhonge Dept. of E & T
Example-1
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 i) Chopping frequency, ii) If the pulse width is halved for constant frequency of operation, find the new output voltage. solution
Prof. V. N. Bhonge Dept. of E & T

58. Prof. V. N. Bhonge Dept. of E & Ti)
ii)
Prof. V. N. Bhonge Dept. of E & T

59. Prof. V. N. Bhonge Dept. of E & T
Example-2
A step-up chopper supplies 150V, to a R=25Ω, L=200 µH
load from a 40V source, T=200µs. Calculate:
a) the value of D,
b) the minimum instantaneous load current, Imin,
c) the peak instantaneous load current, Imax,
d) the maximum peak-to-peak load ripple current,ΔImax
e) the average value of the load current, Idc,
Solution:
Vo = Vs ( ) , =40/(1-D)
D = 0.733, T=200µs
ton = DT (ton=146.7µs, toff= 53.3 µs)
b) From eq. (5.34): Imin = 7.83A
c) From eq. (5.35): Imax = 37.17A
d) From eq. (5.32): Δ I = 29.34A
e) Idc = Vs/R = 150/25 = 6A
Prof. V. N. Bhonge Dept. of E & T

60. Prof. V. N. Bhonge Dept. of E & T
Applications of DC/DC
converters
Prof. V. N. Bhonge Dept. of E & T

61. DC/DC converter in Drive systems
a) Step-down choppers applied in high performance
DC drive systems e.g. electric traction, electric vehicles, and machine tools.
The DC motors with their winding inductances and mechanical inertia act no filters resulting in high-quality armature currents.
The average output voltage of step-down choppers is a linear
function of the switching duty ratio.
Prof. V. N. Bhonge Dept. of E & T

62. Prof. V. N. Bhonge Dept. of E & T
Step-up choppers are used primarily in radar and ignition
systems. The DC choppers can be modified for two-quadrant and four- quadrant.
i. Two-quadrant choppers may be part of power supply (PS) system that contains battery packs and renewable DC sources as photovoltaic arrays, fuel cells, or wind turbines.
ii. Four-quadrant choppers are applied in drives in which
regenerative breaking of DC motors is desired e.g.
transportation systems with frequent stops. The DC choppers with inductive outputs serve as input to current driven inverters.
Prof. V. N. Bhonge Dept. of E & T

63. Cycloconverters
SSGMCE
Shegaon
Cycloconverters directly convert ac signals of one frequency (usually line frequency) to ac signals of variable frequency. These variable frequency ac signals can then be used to directly control the speed of ac motors. Thyristor-based cycloconverters are typically used in low speed, high power (multi-MW) applications for driving induction and wound field synchronous motors.
Prof. V. N. Bhonge
Dept. of E & T

64. Phase-Controlled Cycloconverters
SSGMCE
Shegaon
The basic principle of cycloconversion is illustrated by the single phase-to-single phase converter shown below.
Prof. V. N. Bhonge
Dept. of E & T

65. A positive center-tap thyristor converter is connected in anti-parallel with a negative converter of the same type. This allows current/voltage of either polarity to be controlled in the load. The waveforms are shown below.
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

66. An integral half-cycle output wave is created which has a fundamental frequency f0=(1/n) fi where n is the number of input half-cycles per half-cycle of the output. The thyristor firing angle can be set to control the fundamental component of the output signal. Step-up frequency conversion can be achieved by alternately switching high frequency switching devices (e.g. IGBTs, instead of thyristors) between positive and negative limits at high frequency to generate carrier-frequency modulated output.
SSGMCE
Shegaon
Prof. V. N. Bhonge
Dept. of E & T

67. A Cycloconverter Circuit (a)
Prof. V. N. Bhonge Dept. of E & T

68. A Cycloconverter Circuit (b)
Prof. V. N. Bhonge Dept. of E & T

69. SSGMCE
Shegaon
Thank You
Prof. V. N. Bhonge
Dept. of E & T