Previous Year Questions- Power Semiconductor Devices and Commutation Techniques - 2 | Power Electronics - Electrical Engineering (EE) PDF Download

Q21: A 1 : 1 Pulse Transformer (PT) is used to trigger the SCR in the below figure. The SCR is rated at 1.5 kV, 250 A with I_L = 250 mA,  I_H = 150 mA, and I_Gmax = 150 mA, I_Gmin = 100 mA. The SCR is connected to an inductive load, where L = 150 mH in series with a small resistance and the supply voltage is 200 V dc. The forward drops of all transistors/diodes and gate-cathode junction during ON state are 1.0 V
The resistance R should be     (2007)
(a) 4.7kΩ
(b) 470kΩ
(c) 47Ω
(d) 4.7Ω
Ans: (c)
Sol: When the pulses are applied to the base of the transistor. Transistor operates in ON state. So, the forward voltage drop in trnasistor V_CE = 1V.
D₁ is forward baised and voltage drop in diode V_D1 = 1V.
D₂ is reversed biasesd and acts as open circuit. Capacitor behaves as open circuit for dc voltage.
Forward voltage drop of gate cathode junction V_gk = 1V
Voltage drop across resistor R,
To ensure turn -ON of SCR

Q22: The circuit in the figure is a current commutated dc-dc chopper where, Th_M is the main SCR and  Th_AUX is the auxiliary SCR. The load current is constant at 10 A. Th_M is ON.  Th_AUX is trigged at t = 0.  Th_M is turned OFF between.       (2007)
(a)  0μs < t ≤ 25μs
(b) 25μs < t ≤ 50μs
(c) $50\mu s<t\le 75\mu s$50μs < t ≤ 75μs
(d) 75μs < t ≤ 100μs
Ans: (c)
Sol: At t = 0⁻, v_c = V_s, i_c = 0 and i_T1 = I₀
At t = 0, Th_aux is triggered, a resonant current i_c designs to flow from C through Th_aux, L and back to C.
This resonant current is given by
after half a cycle of As i_c tends to reverse, Th_aux is turned off.
When v_c = −V_s right hand plate has positive polarity, resonant current i_c now builds up through C, L, D and Th_m. As this current i_c grows opposite to forward thyristor current of  Th_m, net forward current i_m = I₀ − i_c begins to decrease. Finaly when i_c in the reversed direction attains the value  I₀, i_m is reduced to zero and Th_m is turned off.

Q23: In the circuit of adjacent figure the diode connects the ac source to a pure inductance L.
The diode conducts for      (2007)
(a) 90°
(b) 180°
(c) 270°
(d) 360°
Ans: (d)
Sol: Diode conducts for 360°.

Q24: An SCR having a turn ON times of 5 μsec, latching current of 50 mA and holding current of 40 mA is triggered by a short duration pulse and is used in the circuit shown in figure. The minimum pulse width required to turn the SCR ON will be      (2006)
(a) 251 μsec
(b) 150 μsec
(c) 100 μsec
(d) 5 μsec
Ans: (b)
Sol: Current through 5kΩ resistor,
Current through inductor,
Anode current,
I_a = i_R + i_L = 0.02+5(1−e^−40t)
Let minimum pulse width is T
To turn on i_a ≥ latching current  
⇒ 0.02 + 5(1 − e^−40t) = 50mA = 0.5
T = 150 μsec

Q25: A voltage commutation circuit is shown in figure. If the turn-off time of the SCR is 50 μsec and a safety margin of 2 is considered, then what will be the approximate minimum value of capacitor required for proper commutation ?     (2006)
(a) 2.88 μF
(b) 1.44 μF
(c) 0.91 μF
(d) 0.72 μF
Ans: (a)
Sol: In this type of commutation, athyristor carrying load current is commutated by transferring its load current to another incoming thyristor.
Firing of SCR Th₁ commutates Th₂ and subsequently, firing of SCR Th₂ would turn-off Th₁.
Circuit turn-off time t_c1 for Th₁
 t_c1 = R₁C ln 2
and circuit turn-off time t_c2 for Th₂
Safety margin = 2

Q26: An electronics switch S is required to block voltage of either polarity during its OFF state as shown in the figure (a). This switch is required to conduct in only one direction its ON state as shown in the figure (b)
Which of the following are valid realizations of the switch S?     (2005)
(a) Only P
(b) P and Q
(c) P and R
(d) R and S
Ans: (c)
Sol: 1: Switch s blocks voltage of both polarity, it means s can block forward as well as reverse voltage.
2: Current through s, flows in forward direction only.
Thyristor blocks voltage in either polarity until gate is triggered. Once the thyristor is triggered current flows in forward direction.
when the reverse voltage is applied across the device (2) and (4), current flows through diode indicated with D. So, these device do not satisfy the requirement.
Transistor blocks positive voltage and diode blocks, negative voltage. Once the basae signal is applied, the device conducts in forward direction.

Q27: The figure shows the voltage across a power semiconductor device and the current through the device during a switching transitions. Is the transition a turn ON transition or a turn OFF transition ? What is the energy lost during the transition ?      (2005)

(a) Turn ON, (VI/2) (t₁ + t₂)
(b) Turn OFF, VI (t₁+t₂)
(c) Turn ON, VI (t₁+t₂)
(d) Turn OFF, (VI/2) (t₁ + t₂)
Ans: (a)
Sol: During interval t₂, voltage starts decreasing and becomes zero and current starts increasing and becomes constant (I), so transition is turn on.
During t₁ interval:
Power loss = vi
E₁ = Energy loss = ∫vi dt = V∫i dt
V is constant during this period v = V
∫i dt represents area under i - t curve
During  t₂ interval:
Power loss = vi
E₂ = Energy loss = ∫vi dt = I ∫ v dt
i is constant during this period i = I  
∫v dt represents area under v - t curve
Total energy lost during the transition

Q28: The conduction loss versus device current characteristic of a power MOSFET is best approximated by      (2005)
(a) a parabola
(b) a straight line
(c) a rectangular hyperbola
(d) an exponentially decaying function
Ans: (a)
Sol: Let, I = device current
$RON=$R_ON = ON state resistance of power MOSFET
Conduction loss = P = I²R_ON
Therefore, condition losses versus device current characteristics can be best approximated by a parabola.

Q29: A MOSFET rated for 15 A, carries a periodic current as shown in figure. The ON state resistance of the MOSFET is 0.15 Ω. The average ON state loss in the MOSFET is      (2004)
(a) 33.8 W
(b) 15.0 W
(c) 7.5 W
(d) 3.8 W
Ans: (c)
Sol: Rated current during on state I = 10
ON state resistance R_ON = 0.15Ω
MOSFET is ON
MOSFET is OFF
Average ON state Loss,

Q30: A bipolar junction transistor (BJT) is used as a power control switch by biasing it in the cut-off region (OFF state) or in the saturation region (ON state). In the ON state, for the BJT     (2004)
(a) both the base-emitter and base-collector junctions are reverse biased
(b) the base-emitter junction is reverse biased, and the base-collector junction is forward biased
(c) the base-emitter junction is forward biased, and the base-collector junction is reverse biased
(d) both the base-emitter and base-collector junctions are forward biased
Ans: (d)
Sol: V_CB = V_CE − V_DE...(i)
Under saturated state, V_BES is greater than V_CES this means base -emitter junction (BES) is forward baised. Further eq, (i) shows that V_CB is negative under saturated conditions, therefore base-collector junction (CBJ) is also forward baised.

Q31: Figure shows a MOSFET with an integral body diode. It is employed as a power switching device in the ON and OFF states through appropriate control. The ON and OFF states of the switch are given on the  V_DS - I_S plane by      (2003)(a) (b) (c) (d) Ans: (b)
Sol: 

when reverse current flow through diode D.
So I_S < 0 and V_DS = 0
When MOSFET is in ON state
 I_S > 0 and V_DS = 0
When MOSFET is in OFF state
 I_S = 0 and V_DS > 0

Q32: Figure shows a thyristor with the standard terminations of anode (A), cathode (K), gate (G) and the different junctions named J₁, J₂ and  J₃. When the thyristor is turned on and conducting        (2003)
(a) J₁ and J₂ are forward biased and J₃ is reverse biased
(b) J₁ and J₃ are forward biased and J₂ is reverse biased
(c) J₁ is forward biased and J₂ and J₃ are reverse biased
(d) J₁, J₂ and J₃ are all forward biased
Ans: (d)

Q33: The main reason for connecting a pulse transformer at the output stage of a thyristor triggering circuit is to      (2001)
(a) amplifying the power of the triggering pulse
(b) provide electrical isolation
(c) reduce the turn on time of the thyristor
(d) avoid spurious triggering of the thyristor due to noise
Ans: (b)