# MEE10 Electric Power System Analysis- transistor power dissipations

Question 1: (5 marks)

The schematic diagram shown here is for a “buck” converter circuit, a type of DC-DC “switching” power conversion circuit: In this circuit, the transistor is either fully on or fully off; that is, driven between the extremes of saturation or cutoff. By avoiding the transistor’s “active” mode (where it would drop substantial voltage while conducting current), very low transistor power dissipations can be achieved. With little power wasted in the form of heat, “switching” power conversion circuits are typically very efficient.

Trace all current directions during both states of the transistor. Also, mark the inductor’s voltage polarity during both states of the transistor.

Question 2: (5 marks)

The schematic diagram shown here is for a “boost” converter circuit, a type of DC-DC “switching” power conversion circuit:

In this circuit, the transistor is either fully on or fully off; that is, driven between the extremes of saturation or cutoff. By avoiding the transistor’s “active” mode (where it would drop substantial voltage while conducting current), very low transistor power dissipations can be achieved. With
little power wasted in the form of heat, “switching” power conversion circuits are typically very efficient.

Trace all current directions during both states of the transistor. Also, mark the inductor’s voltage polarity during both states of the transistor.

Question 3: (5 marks)

Predict how the operation of this buck converter circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

• Drive circuit fails with a constant “low” (0 volts) output signal:
• Drive circuit fails with a constant “high” (V) output signal:
• Diode fails shorted:

For each of these conditions, explain why the resulting effects will occur.

Question 4: (5 marks)

So-called linear regulator circuits work by adjusting either a series resistance or a shunt resistance to maintain output voltage at some fractional value of input voltage: