(a)What are the 5 characteristics of an ‘ideal’ power semiconductor switch?

(b)The latching current of a thyristor circuit in Fig. Q1(c) is 50 mA. The duration ofthe firing pulse is 50 μs. Will the thyristor get fired?

(c) If the latching current in the circuit shown in Fig. Q1 (d) is 4 mA, obtain the minimum width of the gating pulse required to properly turn-on the thyristor.

2. (a)A thyristor is connected in series with a 50 Ω resistor to a 220 V sinusoidal supply.

(i) Sketch the waveform of the current through the resistor for one cycle if the firing angle is α.

(ii) If the thyristor is controlled to switch on at a firing angle of 40°, determine the average current in the resistor.

(b) The thyristor in (a) is upgraded to a fully controlled converter.

(i) Sketch the waveform of the current through the resistor for one cycle if the firing angle is α.

(ii) Determine the average resistor current when the firing angle is 20°.

(iii) If the average resistor current is 1 A, what is the firing angle?

3.(a) List the Six types of power converter circuits?

(b) State one of power electronics device which is suitable for high frequency applications.

(c) For the single phase thyristor rectifier circuit shown below, with a complex load and a freewheeling diode:

Describe, with the aid of a diagram, the output waveform and equivalent voltage for a thyristor firing angle of 90° .

4. Referring to the Fig. Q.4, a resistive load of 15 Ω is connected to a voltage source of 120V at 50 Hz through a back-to-back thyristor pair. The firing angles of thyristors T1and T2 are equal and at α =3π . Determine

(i) the rms output voltage Vo;

(ii) the input power factor;

(iii) the average current through each thyristor;

(iv) the rms current through each thyristor.

(v) the rating of the thyristors.

20 2 100 V 0.5 H Fig Q 1(c)