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First Semester M.E. (PE) Degree Examination, March 2013 (2K8 Scheme) POWER ELECTRONICS PE 112 : Power Semiconductor Devices Time : 3 Hours

Max. Marks : 100

Instruction : Answer any five full questions. 1. a) Show that power loss in any semiconductor power device decreases with decrease in switching frequency.

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b) Discuss the merits, limitations and area of application of various power switching devices.

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2. a) What are differences between logic level transistors and power transistors ?

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b) Explain the concept of second breakdown in power transistor and how it is overcome in power MOSFET.

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c) Explain the following w.r.t. BJT

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i) Delay time ii) Storage time iii) Rise time. 3. a) What are the different types of diodes ? Explain the principle of operation and its characteristics of a Schottky diode.

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b) The 4 static switches in the ckt of fig. 3b are operated according to the timing diagram shown. The forward ON state voltage drop of each switch is 2.5 V. Each switch has a thermal resistance from junction to casing equal to 1.2° C/W. Each is mounted on a heat sink and the thermal resistance of device casing to heat sink surface is 0.8° C/W and that of heat sink surface to ambient is 0.5° C/W. The switching frequency is small and therefore the switching power loss can be neglected. The forward leakage current can also be neglected. P.T.O.

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a) Determine the peak and average power dissipation in each switch. b) Determine the junction temperature in the device and also that on the surface of the heat sink for an ambient temperature of 45°C. 10

Fig. 3 b

Timing diagram

4. a) Explain with relevant waveforms, the switching characteristics of a power MOSFET.

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b) With a neat sketch, explain the gate drive circuit employed for a power MOSFET.

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5. a) Explain with simple diagrams, why can thyristor be made to have larger current capabilities in the ON state than BJT. b) Explain the latch up process in IGBT.

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c) The thyristor in the switching ckt of fig. 5 c is in the ON state. Its holding current is 150 mA and latching current of 200 mA. An additional resistance R is introduced in the ckt by opening the switch S a/c this resistance. What is the lowest value of R that will cause the thyristor to turn OFF when S is opened. Determine the minimum duration of the gate pulse necessary to ensure turn ON. 10

fig. 5 c 6. a) Explain with the help of a basic structure, the principle of operation, switching characteristics and important rating of the SCR. 10 b) Bring out the differences between GTO, SIT and MCT. 7. Write short notes on :

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a) Design of heat sink for SCR. b) Reverse recovery transients of a power diode. c) Protection ckts. d) Emitter turn off thyristor. __________________