POWER ELECTRONICS Mock Exam 1 Mock Exam 1

a) A step-up Chopper has input voltage of 220 V and output voltage of 660 V. if the non-conducting time of thyristor-chopper is $100 \mu s$, compute the pulse width of output voltage. In case pulse width is halved for constant frequency operation, determine the new output voltage. [8 marks]

b) With the aid of relevant diagram, explain the principles of operation of a step-up chopper. [7 marks]

c) Highlight the two basic reasons why an ordinary power supply needs regulation. Two power supplies A and B are available in the market. Power supply A has no-load and full-load voltages of 30V and 28V respectively whereas these values are 35V and 29V for power supply B. Which is better power supply? [5 marks]

a) Using relevant diagrams and Shockley diode equation, describe the $v-i$ characteristics of power diodes. [clearly identify the regions in your description] [7 marks]

b) The reverse recovery time of a diode is $t_{rr} = 3\mu s$ and the rate of fall of the diode current is $\frac{di}{dt} = 30A/\mu s$. Determine the storage charge $Q_{RR}$, and (b) the peak reverse current $I_{RR}$. [4 marks]

c) Highlight four major strength of silicon carbide, SiC, as compared with normal silicon, Si, when used as an electronic switch in power electronic circuits. [4 marks]

a) Power (brightness) of a 100 W, 110 V lamp is to be varied by controlling firing angle of SCR full-wave circuit; the r.m.s. value of a.c. voltage appearing across each SCR being 110 V. Find the r.m.s. voltage and current in the lamp at firing angle of $60^\circ$. [7 marks]

b) An SCR has a breakover voltage of 350 V, a trigger current of 10 mA and holding current of 10 mA. What do you infer from it? What will happen if the gate current is increased to 15mA? [4 marks]

c) Explain Forced Commutation method of switching OFF an SCR. [4 marks]