EE6503 - 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 Ton/T.

2. What is constant frequency control of chopper? (N/D12) The chopper frequency is kept constant. Hence total period T remains constant. Ton and Toff 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.

(N/D 14)(N/D15) S.N

o Time ratio control Current Limit Control

1 Switch is controlled by varying

time period Switch is controlled by varying

amplitude limits 2 Pulse width modulation and

frequency modulation are the

types of TRC

Current is allowed to fluctuate

between maximum and

minimum value 3 PWM is the most widely used

technique CLC is rarely used, because it

requires sensors to sense current

values.

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.

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

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)

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

13)(N/D14).(M/J15)

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

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)

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)

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)

6. (i)Distinguish between linear power supply and switched mode power supply(M/J 15) 7. (ii)Explain the Operation of SMPS.(N/D14)

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

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 reverse-

biasing voltage must be maintained for a period greater than that required for the device to turn-

off. 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.

Fig. illustrates voltage commutation. ThM is the main SCR and ThAux 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, LF and the Free-wheeling diode. Additionally, the charged Capacitor swings

half a cycle through ThM, 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-ThAux is enabled to commutate ThM. When ThAux is triggered

the negative charge of the capacitor is impressed onto ThM 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

ThAux 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 ThAux. Now the capacitor is progressively positively

charged and the load voltage is equally diminished from the supply voltage. ThAux 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

ThAux must be switched before ThM 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.