# Power Electronics Questions and Answers – SOAs

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “SOAs”.

1. For a transistor, the safe operating area (SOA) is a plot of
a) Ib versus Vce
b) Ib versus Ic
c) Ic versus Vce
d) Ic versus time

Explanation: For reliable operation the collector current & voltage must remain within the SOA curves.

2. The forward safe operating area (FSOA) pertains to the operation when
a) the device is fired at a 50% Duty cycle
b) the device is forward-biased
c) the device is operated on AC
d) the device is operated on DC

Explanation: The FSOA is for forward biased operations. The FSOA is plotted for AC as well as DC for different duty cycles. Hence, option (b) is the most appropriate choice.

3. The SOAs are plotted always on a _________ scale
a) time
b) frequency
c) logarithmic
d) polynomial

Explanation: The scale is always logarithmic, irrespective of the type of device.
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4. As the FSOA increases, the pulse width
a) decreases
b) increases
c) remains constant
d) vanishes

Explanation: On reduced pulse width values, the devices can operated on higher voltages & currents.

5. The SOAs provided by the manufacturers are for
a) single pulse operation & a particular temperature
b) multi pulse operation & all the temperature
c) all the conditions
d) a particular duty cycle operation

Explanation: The manufacturer specifies the SOAs only for single pulse DC operation & a particular temperature (usually 20Degree Centigrade Scale). For actual operations, The SOA’s have to be modified using the thermal impedance charts.

6. A device is operating at Ic = 4A & Vce = 50V. For the device to operate at Ic = 20A (Without damaging),
a) voltage should be increased
b) voltage should be reduced
c) voltage can be kept constant
d) current has to increased further

Explanation: For safe operation, the values should be within the limits. P = V.I – with increase in one of the values, another value should decrease.

7. For a BJT, find the maximum power dissipation when the device is safely operated at Vce = 90V and Ic = 0.5A
a) 40 Watts
b) 35 Watts
c) 45 Watts
d) 30 Watts

Explanation: P=90*0.5=45Watts.

8. The SOA for a MOSFET is plotted for
a) Id versus Vds
b) Ig versus Id
c) Ig versus Vds
d) Id versus Vgs

Explanation: It is a plot of drain current vs drain to source voltage.

9. The SOA for an IGBT is plotted for
a) Ic versus Vge
b) Ig versus Ic
c) Ig versus Vce
d) Ic versus Vce

Explanation: It is a plot of collector current vs collector to emitter voltage.

10. For MOSFET’s SOA, as the pulse width goes on increasing, the maximum voltage rating ____ & current rating ____
a) is constant, increases
b) increases, decreases
c) decreases, is constant
d) constant, decreases

Explanation: Refer MOSFET’s SOA.

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