Power Devices and Commutation Set-1
[1993 : 1 Mark] The thermal resistance between the body of a power semiconductor device and the ambient is expressed as
(a) Voltage across the device divided by current through the device.
(b) Average power dissipated in the device divided by the square of the rms current in the device
(c) Average power dissipated in the device divided by the temperature difference from body to ambient
(d) Temperature difference from body to ambient divided by average power dissipated in the device.
Answer : (d)
Explanation :- Thermal resistance (θ) is analogous to electrical resistance, and indicates the resistance offered by the semiconductor device to the flow of heat generated internally to the ambient. So, option (d) is the correct answer.
[1994 : 1 Mark] A triac can be triggered by a gate pulse of _____ polarity.
Ans. Both positive and negative
Solution :
It can be triggered by Both positive and negative polarity. So, ans is both positive and negative.
[1994 : 1 Mark] A switched mode power supply operating at 20 kHz to 100 kHz range uses as the main switching element.
(a) Thyristor
(b) MOSFET
(c) TRIAC
(d) UJT
Answer : (b)
Solution :
MOSFET is used for this frequency range.MOSFET is a switched mode power supply that operates deliberately at range of 20 kHz to 100 kHz as its main switching element. For regulating the DC voltage Mosfet is sued in SMPS.
So, option (b) is the correct answer.
[1995 : 1 Mark] Figure show two thyristors, each rated 500 A (continuous) sharing a load current. Current through thyristor y is 120 A. A current through thyristor x will be nearly _____ A
Solution :
Since both the thyristors are working in parallel, the voltage difference will be same.
120 x 0.05 = Ix x 0.06
I_x=\frac{120\times 5}{6}=100\;A[1996 : 1 Mark] The TRIAC can be used in
(a) Inverter
(b) Rectifier
(c) Multi-quadrant chopper
(d) AC voltage Regulator
Answer : (d)
Explanation :- We know that TRIAC is triggered into conduction in both direction by a gate signal like that of SCR. Basically, TRIAC is an semiconductor device which has three terminals that controls the flow of current. So, option (d) is the correct answer.
[1996 : 1 Mark] Which semiconductor power device out of the following is not a current triggered device ?
(a) Thyristor
(b) GTO
(c) TRIAC
(d) MOSFET
Answer : (d)
Explanation :- The correct answer is mOSFET because, in MOSFET, the voltage input to the gate controls the flow of current from source to drain. Therefore MOSFET is the voltage controlled device.
[1996 : 1 Mark] Which of the following does not cause permanent damage of an SCR ?
(A) High current
(b) High rate of rise of current
(c) High temperature rise
(d) High rate of rise of voltage
Answer : (d)
Explanation :- When there is an high rate rise of voltage , the SCR is turned on , though it does not damage the SCR, but can cause permanent damage to SCR.
So, option (d) is the correct answer.
[1998 : 1 Mark] The MOSFET switch in its ON state may be considered equivalent to
(a) Resistor
(b) Inductor
(c) Capacitor
(d) Battery
Answer : (a)
Solution :
During on state MOSFET works as resistor. Therefore, option (a) is the correct answer.
[1998 : 1 Mark] The uncontrolled electronic switch employed in power electronic converters is
(a) Thyristor
(b) Bipolar junction transistor
(c) Diode
(d) MOSFET
Answer : (c)
Explanation :-A diode is an uncontrollable and unidirectional switch hence, its operation is determined by the direction of the flow of current in a circuit.Therefore, diode is the correct option.
[1998 : 1 Mark] In a commutation circuit employeed to turn-off an SCR, satisfactory turn-off is obtained when
(a) Circuit turn-off time < device turn-off time
(b) Circuit turn-off > device turn-off time
(c) Circuit time constant > device turn-off time
(d) Circuit time constant < device turn-off time
Answer : (b)
Solution :
The circuit should not turn-off before making off the thyristor.Therefore, option (b) is the correct answer.