DHANALAKSHMI SRINIVASAN

INSTITUTE OF RESEARCH AND TECHNOLOGY

SIRUVACHUR-621113

DEPARTMENT OF EEE

16 MARKS QUESTIONS AND ANSWERS

SUBJECT CODE: EE2301

SUBJECT NAME: POWER ELECTRONICS

PART-B

UNIT-I

1.a) i)Draw the dynamic characteristics of SCR during turn-on and turn-off and explain.

(8 Marks)(May/June 2010)

(8 Marks) (Nov/Dec 2010)

(16 Marks)(Nov/Dec 2011)

Turn-on:

The turn-off time of SCR is defined as the time from initiation of gate drive to the time when anode

current reaches to its full value.

The turn-on time (ton) is sum of td, tr and tp.

Ton=td+tr+tp

Turn-off: Toff=trr+tgr

ii) Draw the simplified model of MOSFET to show the inter-electrode capacitance. Discuss the

importance of these capacitances. (8 Marks) (May/June 2010)

2.i) Describe the structure of an IGBT. (8 Marks) (May/June-2010)

IGBT is obtained by combining the properties of BJT and MOSFET.

It has three terminals- Gate, Collector, Emitter

Structure of IGBT is similar to MOSFET. The structure has additional P+ layer. This layer is collector

of IGBT.

ii) Calculate the switching losses of an IGBT for the following condition both for resistance and

clamped inductive load. Vcc=200V, Tf1= 0.5μs, Icm = 50A, trv= 0.5μs, f=20Hz, Tf2= 25μs.

(8 Marks) (May/June-2010)

3.a) Explain the operation of IGBT with the help of neat structural diagram and suitable

waveforms. (16 Marks)(Nov/Dec-2010), (16 Marks) (Apr/May-2011)

It combines both MOSFET and BJT. It has high input impedance like MOSFET and low on-state

power loss as BJT.

Static Characteristics:

b) Write short notes on: i) Snubber circuit for BJT (8 Marks) (Apr/May-2011) ii) Commutation circuit

of SCR (8 Marks) (Apr/May-2011)

The snubber circuit is mainly used for dv/dt protection.

It consists of series combination of resistance Rs and capacitance Cs in parallel with the device BJT.

ii) Commutation circuit of

SCR: SCR can be turned off by applying reverse bias to allow SCR to recover its forward blocking ability.

The commutation circuit consists of commutating capacitor C and auxiliary SCR T2.

6. a) Describe about any one driver circuit and snubber circuit for MOSFET and IGBT. (16

Marks)(May/June2010)

Drive circuit for MOSFET:

Snubber Circuit: The transient over voltages can switch on the devices. The MOSFET and IGBT can be protected against

transient voltage by RC network.

It is connected in parallel across the device.

7. a)i) Discuss the different modes of operation of thyristor with the help of its static VI

characteristics. (8 Marks)(May/June2012)

Static VI characteristics:

Modes of operation: Reverse blocking mode

Forward blocking mode

Forward conduction mode

ii) Explain why Triac is rarely operated in I quadrant with negative gate current and in II quadrant

with positive gate current. (8 Marks)(May/June-2012)

Mode II MT2 positive, negative gate current: The carrier flow from P2 to N2 is like a conventional gate current and right handside port P1N1P2N2

conducts like SCR.

Mode III MT2 negative, positive gate current: This regenerative process continues until the structure P2N1P1N4 completely turns on.

8) Explain how a power MOSFET is turned-on and turned-off.

(6 Marks)(Nov/Dec-2012)

Switching Characteristics: Turn-on:

Sum of delay time and rise time of device.

ton= tdn+tr Turn off process: Sum of turn off delay time and fall time.

toff = tdf+tf

UNIT-2

1. [a] Discuss the effect of source inductance of the performance of single phase full converter.

[May/Jun 2010][Apr/May 2011] [16 marks]

Inductance and resistance must present in the input source and time is required for current change to taken place.

Effect of Source Inductance of the Performance of Single Phase Full Converter

2. Draw the necessary circuit of single phase full converter with RL load, voltage and current

waveforms and explain its operation. [May/Jun 2010[16 marks]

Explain the operation of 1фfull bridge converter with RL load for continuous and discontinuous

load currents. [Nov/Dec 2011[10 marks]

Single phase full converter has four SCR’s.

Positive Half cycle: [α to π+ α] Negative Half cycle: [π+α to 2π+α]

Voltage and current waveforms for continuous and discontinuous load

3.[ii] Discuss the working of above converter in the converter mode with RLE load.

[Nov/Dec 2010] [6 marks]

Assume large value of ‘L’ to get continuous load current and ripple free.

It operates in converter mode when firing angle < 90o.

Power flows from source to load.

Positive half cycle [α to π+ α]: SCRs are triggered when Vs >E.

Current path is P – T1 –RLE – T2 – N.

Negative half cycle [π+ α to 2π+ α]: At ωt =π+α, T3, T4 conducts.

Current path is N – T3 –RLE – T4 – P

4. [i] A single phase semi converter is operated from 120V, 50HZ ac supply. The load current with

an average value Idc is continuous and ripple free firing angle α =π/6. Determine

1. Displacement factor

2. Harmonic factor of input current

3. Input power factor.

[Nov/Dec 2010] [10 marks]

[ii] Write a note on Battery charger [Nov/Dec 2010] [6 marks] The AC voltage is given to square wave converter. The square wave is given to saw tooth

generator.

Saw tooth wave form generated with battery voltage. As battery voltage is increased, firing angle is increased, output voltage is decreased.

Thus changing voltage for battery is reduced.

5. Explain the principle of operation of single phase dual converter with neat power circuit diagram.

[Apr/May 2011][16 marks] 1 ф full converter operates in two quadrants for inductive load. If two such 1 ф full converters are

connected back to back, then four quadrant operations are obtained. - Dual converter.

UNIT-3

1. Describe the expression for the output voltage of a step up chopper and explain its control strategies. [May/Jun 2010][8 marks]

2.Discuss the principle of operation of DC-DC step down chopper with suitable wave form Device an expression for its average DC output voltage. [Nov/Dec 2010][8 marks]

3.Describe the working principle of Boost converter with necessary circuit and wave forms.

[May/Jun 2012][16 marks]

Explain the various modes of operation of Boost DC-Dc counter with necessary waveforms

[Apr/May 2011][16 marks] The Boost regulator provides higher output voltage than the input voltage.

It is like step up chopper .The switch is fist turned on. It conducts from 0 to x T.

Hence current flows in inductance, it stores the energy during this period.

Output voltage and current is maintained by the filter capacitor.

The switch is turned off at αT. Hence inductance generates large voltage L.diL/dt .

This voltage forward biases D1, to maintain current. The current flow through load.

The output current is ripple free and continuous due to capacitor.

From αT to T, the inductance energy is transferred to the capacitance and load.

4. With a neat sketch and output voltage waveforms, Explain the working of full bridge SMPS.

[Nov/Dec 2011][16 marks]

Output voltage waveforms of full bridge SMPS.

UNIT-4

1. What is the need for controlling the output voltage of inverters? Classify the various techniques

adopted to vary inverter gain and brief on Sinusoidal PWM. (16 Marks) (May/June-2012) State the

different methods of voltage control in inverters. Describe any one of the PWM control in inverter.

(10 Marks) (May/June-2010)

The various methods of voltage control in inverters are

Multiple pulse width modulation

Sinusoidal pulse width modulation

Modified Sinusoidal pulse width modulation

2. Describe the working of a 1Φ full bridge inverter with relavant circuit and waveforms.

(8 Marks) (Nov/Dec-2010)

4. Explain the Harmonic reduction by transformer corner lines and stepped wave inverters.

(16 Marks) (Nov/Dec-2010)

Harmonic reduction by transformer connections:

Harmonic reduction by using stepped wave inverters:

5. Describe the operation of single phase auto sequential commutated current source inverter with

power circuit and waveforms. (16 Marks) (Apr/May-2011)

Mode 1:

T1 and T2 conducts

The current path is T1 – D1 – Load – D2 – T2

The capacitor C1 and C2 are charged to a voltage Vc = VL

This voltage VL is greater than RI.

Mode 2:

T3 and T4turns on. The capacitor C1 and C2 applies a reverse voltage to conducting SCRT1 and T2 and they turns off.

The current path is T3 – C1 – D1 – Load – D2 – C2 – T4

When capacitor voltage falls from VL , diode D3, D4 conducts. Mode 3:

T3 , T4 and diodes D1 ,D2 ,D3 ,D4 conducts, so the load is connected in parallel with both capacitors C1 and C2.

The load current falls to zero and reverse.

When load current equals to – I, D1 ,D2 reverse biases.

Mode 4:

T3 , T4 and diodes D3 ,D4 conducts.

The current path is T3 – D3 – Load – D4 – T4

The capacitor voltage is Vc = -VL

UNIT-5

1.i) Describe the operation of single phase full wave ac voltage controller with the help of voltage

and current waveform. Also derive the expression for RMS value of output voltage.

(16 Marks) (Nov/Dec-2010)

2.i) Describe three phase to three phase cycloconverter with relevant circuit arrangement using 18

thyristors. (8Marks) (Nov/Dec2010)

It has two groups of SCR’s per phase.

Each group consists of 3 thyristors, thus in total six thyristors are required per phase.

This means, 18 SCR’s are required for the whole circuit.

The firing schedules of SCR’s of phase groups Y and B are same as that of group R but lag by

120° and 240° respectively.

2. Explain the working of multistage sequential control of AC voltage controller.

(16 mark) (Apr/May 2011, May/June 2012)

Multiple bidirectional voltage controllers are employed in parallel. Each is fed from the multiple tap transformer. Depending upon the desired output voltage,

particulars controllers are triggered.

The transformer have ‘n’ taps. Each tap has the output voltage of Vs/n.

3. Matrix converters for power factor control

The matrix converter uses the matrix of switches so that any of the input phase voltage can be connected to any of the output load phase.

There is exactly one switch for each of the possible connections between supply and load.

Advantages:

Inherent bidirectional power flow control

Input output waveforms are sinusoidal

No dc-links is required and hence compact

Power factor is controlled by independent control of load current.

Disadvantages: The switches are not available for high powers.

Implementation is complex

The maximum voltage transfer ratio is limited

Switches requires protection and commutation circuits

Applications:

Matrix converters are used for low power 3φ AC controllers

Used for power factor control.