EE POWER ELECTRONICS

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1 EE POWER ELECTRONICS UNIT III - DC TO DC CONVERTER PART A 1.What is meant by time ratio or PWM control (duty cycle) of a DC chopper? (M/J16) The ratio of a period to the total time period is known as time control ratio (or) duty ratio. It is given by T on /T. 2. What is constant frequency control of chopper? (N/D12) The chopper frequency is kept constant. Hence total period T remains constant. T on and T off both are varied to vary the duty cycle 3.What are the applications of DC Chopper? (N/D14)(N/D15) (i) Electric locomotives (ii) Battery operated cars. (iii) Power supplies. 4.Distinguish between time ratio control and current limit control employed in a DC chopper. S.N Time ratio control o 1 Switch is controlled by varying time period 2 Pulse width modulation and frequency modulation are the types of TRC 3 PWM is the most widely used technique Current Limit Control Switch is controlled by varying amplitude limits Current is allowed to fluctuate between maximum and minimum value CLC is rarely used, because it requires sensors to sense current values. (N/D 14)(N/D15) 5. What are the circuit configuration used for SMPS? (M/J14) The circuit configuration used for SMPS are two types 1) Transformer less type 2) Transformer type 6. Define current limit control in dc to dc converter?(m/j15) The switch is controlled by the current amplitude limits. The current is allowed to fluctuate or change only between 2 values i.e. maximum current (I max) and minimum current (I min) 7. What is meant by duty cycle? (M/J12) (M/J16) Duty cycle is defined as the ratio of the on time of the chopper to the total time period of the chopper. It is denoted by α. 8. What are the two types of control strategies? (M/J13) (N/D16) a. Time ratio control b. b.current limit control c. 9. What are the advantages and disadvantages of cuk converter?(m/j 14) Cuk converter has the following advantages. Continuous input current. Continuous output current. Output voltage can be either greater or less than input voltage.the glaring drawback of this derived converter topology is that the polarity of the output is reversed. This is not acceptable for various reasons.

2 10. What is need for resonaont converter? (M/J 13) In normal pwm converter has more switching loss during turn ON and Turn OFF process. use resonant converter to minimize the switching losses, and provide better commutation during Turn ON and Turn OFF process 11. What is chopper? (N/D13) Chopper is DC to DC converter,it converts fixed dc to variable dc. 12. Explain the working of IV quadrant chopper.(n/d12) Four quadrant chopper has the capability to operate in all the four quadrants. Hence it is used in reversible dc drives. The braking is regenerative. Hence four quadrant chopper drives are highly efficient. Their dynamic response is also fast. 13. What are the different classification of chopper depending upon the direction of current and voltage. (N/D16) Different types of choppers Class A chopper Class B chopper Class C chopper Class D chopper Class E chopper

3 PART-B 1. Explain the working of boost converter with neat waveform also derive the expression of peak to peak voltage across the capacitor. (N/D12)/ (M/J12) (N/D16)

5 2. Explain the basic circuit and waveform and principle of operation of step up converter (M/J13) (N/D 13)(N/D14).(M/J15)

6 3. (i)explain the operation of class-c and class-d types of two quadrant chopper? (M/J14) 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. 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. Class D is a two quadrant chopper. When both CH1 and CH2 are triggered simultaneously, the output

7 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. 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. (ii)draw the power circuit diagram of Cuk regulator and explain its operation with equivalent circuit for different modes with necessary waveforms?(jun 2014)

9 4. (i)explain the Control strategies applied to dc chopper(may-june 2012)(DEC 2015)(ii)A step up chopper is used to deliver load voltage of 660V from 220V dc source if non conduction time of chopper is 100µs, Compute the pulse width. If pulse width is halved find new output voltage.(may- June 2012)

11 5. (i)explain the operation of working of buck and boost converter with neat waveform and necessary voltage equations(n/d15)(ii)explain the working of buck-boost converter for continuous current mode of operation with neat waveform also derive the expression of peak to peak voltage across the capacitor.(n/d15)(m/j16)

13 6. (i)distinguish between linear power supply and switched mode power supply(m/j 15) 7. (ii)explain the Operation of SMPS.(N/D14)

14 8.(iii).Write a short note on resonant switching (N/D13)

This voltage is comparable in magnitude to the operating voltages.

16 9. Explain the working principle of voltage commutated chopper showing the current and voltage waveform across each device. (N/D16) (M/J16) In a voltage commutated thyristor circuit a voltage source is impressed across the SCR to be turned off, mostly by an auxiliary SCR. This voltage is comparable in magnitude to the operating voltages. The current in the conducting SCR is immediately quenched, however the reversebiasing voltage must be maintained for a period greater than that required for the device to turnoff. With a large reverse voltage turning it off, the device offers the fastest turn- off time obtainable from that particular device. It is an exposition of hard turn-off where the reverse biasing stress is maximum.

17 Fig. illustrates voltage commutation. Th M is the main SCR and Th Aux is the Auxiliary. As a consequence of the previous cycle, Capacitor C is charged with the dot as positive. When the Main SCR is triggered, it carries the load current, which is held practically level by the large filter inductance, L F and the Free-wheeling diode. Additionally, the charged Capacitor swings half a cycle through Th M, L and D ending with a negative at the dot. The reverse voltage may be less than its positive value as some energy is lost in the various components in the path. The half cycle capacitor current adds to the load current and is taken by the Main SCR. With the negative at the dot C-Th Aux is enabled to commutate Th M. When Th Aux is triggered the negative charge of the capacitor is impressed onto Th M and it immediately turns off. The SCR does take the reverse recovery current in the process. Thereafter, the level load current charges the capacitor linearly to the supply voltage with the dot again as positive. The Load voltage peaks by the addition of the capacitor voltage to the supply when Th Aux is triggered. The voltage falls as the capacitor discharges both changes being linear because of the level load current. When the Capacitor voltage returns to zero, the load voltage equals supply voltage. The turn-off time offered by the commutation circuit to the SCR lasts till this stage starting from the triggering of Th Aux. Now the capacitor is progressively positively charged and the load voltage is equally diminished from the supply voltage. Th Aux is naturally commutated when the capacitor is fully charged and a small excess voltage switches on the free wheeling diode. With the positive at the dot the capacitor is again ready for the next cycle. Here Th Aux must be switched before Th M to charge C to desired polarity. Voltage commutation may be chosen for comparatively fast switching and it can be identified from the steep fall of the SCR current. There is no overlapping operation between the incoming and the outgoing devices and both currents fall and rise sharply. Stresses on all the three semiconductors can be expected to be high here.

Module 3. DC to DC Converters. Version 2 EE IIT, Kharagpur 1

Module 3. DC to DC Converters. Version 2 EE IIT, Kharagpur 1 Module 3 DC to DC Converters Version 2 EE IIT, Kharagpur 1 Lesson 2 Commutation of Thyristor-Based Circuits Part-II Version 2 EE IIT, Kharagpur 2 This lesson provides the reader the following: (i) (ii)