# Power Semiconductor Devices and Commutation Techniques

 Question 1
A resistor and a capacitor are connected in series to a 10 V dc supply through a switch. The switch is closed at $t=0$, and the capacitor voltage is found to cross 0 V at $t=0.4 \tau$, where $\tau$ is the circuit time constant. The absolute value of percentage change required in the initial capacitor voltage if the zero crossing has to happen at $t=0.2 \tau$ is _______ (rounded off to 2 decimal places).
 A 24.24 B 78.83 C 12.45 D 54.99
GATE EE 2020   Power Electronics
Question 1 Explanation:
If initial charge polarities on the capacitor is opposite to the supply voltage then only the capacitor voltage crosses the zero line.
\begin{aligned} V_{c}(t) \; \Rightarrow \;& \text{Final value} \\ &+ (\text{Initial value - Final value}) e^{-t/\tau }\\ 0&=10+(-V_{0}-10)e^{-0.4} \\ 10&=(V_{0}+10)e^{-0.4} \\ V_{0}&=4.918 V \\ \text{Now, } t&=0.2\tau \\ 0&=10+(-{V_{0}}'-10)e^{-0.2} \\ {V_{0}}'&=2.214 \\ \%\text{change in voltage} &= \frac{4.918-2.214}{4.918}\times 100 \%\\ &=54.99\% \end{aligned}
 Question 2
A non-ideal diode is biased with a voltage of -0.03 V, and a diode current of $I_1$ is measured. The thermal voltage is 26 mV and the ideality factor for the diode is 15/13. The voltage, in V, at which the measured current increases to 1.5$I_1$ is closest to:
 A -0.02 B -0.09 C -1.5 D -4.5
GATE EE 2020   Power Electronics
Question 2 Explanation:
\begin{aligned} I_{1}&=i_{0}\left [ e^{\frac{0.03}{15/13\times 26mV}}-1 \right ] \\ V_{D}&=\text{ -ve '1' can not be neglected}\\&\text{in diode current equation} \\ I_{1}&=I_{0}[e^{-30mV/30mV}-1] \\ &=I_{0}[e^{-1}-1] \\ &=-0.64\: I_{0} \\ 1.5I_{1}&=I_{0}[e^{V_{D}/30mV}-1] \\ -1.5\times 0.64 I_{0}&=I_{0}[e^{V_{D}/30mV}-1]\\-0.96&=e^{V_{D2}/30mV}-1\\1-0.96&=e^{V_{D2}/30mV}\\0.04&=e^{V_{D2}/30mV}\\ 30 mV \ln (0.04)&=V_{D}\\ V_{D}&=-0.09 V \end{aligned}
 Question 3
A single-phase inverter is fed from a 100 V dc source and is controlled using a quasisquare wave modulation scheme to produce an output waveform, $v(t)$. as shown. The angle $\sigma$ is adjusted to entirely eliminate the $3^{rd}$ harmonic component from the output voltage. Under this condition, for $v(t)$, the magnitude of the $5^{th}$ harmonic component as a percentage of the magnitude of the fundamental component is _______(rounded off to 2 decimal places).
 A 15 B 10 C 20 D 25
GATE EE 2020   Power Electronics
Question 3 Explanation:
Using result,
\begin{aligned} V_{n}&=\frac{4V_{s}}{n\pi }\cos n\sigma\ \text{For, } V_{3}=0\\ \cos 3\sigma &=0 \\ 3\sigma &=\frac{\pi }{2} \\ \sigma &=\frac{\pi }{6}\\ \text{Now, } \frac{V_{5}}{V_{1}}&=\frac{\cos 5\sigma }{5\cos\sigma }\\&=\frac{\cos 5\pi /6}{5\cos \pi /6}=-\frac{1}{5} \\ \% \left | \frac{V_{5}}{V_{1}} \right |&=\frac{1}{5}\times 100=20\% \end{aligned}
 Question 4
Four power semiconductor devices are shown in the figure along with their relevant terminals. The device(s) that can carry dc current continuously in the direction shown when gated appropriately is (are)
 A Triac only B Triac and MOSFET C Triac and GTO D Thyristor and Triac
GATE EE 2018   Power Electronics
 Question 5
For the power semiconductor devices IGBT, MOSFET, Diode and Thyristor, which one of the following statements is TRUE?
 A All of the four are majority carrier devices. B All the four are minority carrier devices C IGBT and MOSFET are majority carrier devices, whereas Diode and Thyristor are minority carrier devices. D MOSFET is majority carrier device, whereas IGBT, Diode, Thyristor are minority carrier devices.
GATE EE 2017-SET-1   Power Electronics
 Question 6
The voltage ($v_{s}$) across and the current ($i_{s}$) through a semiconductor switch during a turn-ON transition are shown in figure. The energy dissipated during the turn-ON transition, in mJ, is _______.
 A 25 B 50 C 75 D 100
GATE EE 2016-SET-1   Power Electronics
Question 6 Explanation:
\begin{aligned} \text{Energy}&=\int_{0}^{T_1}V\cdot i dt+\int_{0}^{T_2}V\cdot i dt\\ &=V\left [ \frac{1}{2}IT_1 \right ]+I\left [ \frac{1}{2}VT_2 \right ]\\ &=600\left [ \frac{150}{2}\times 1 \times 10^{-6} \right ]\\ &+100\left [ \frac{1}{2} \times 600 \times 1 \times 10^{-6} \right ]\\ \text{ Energy}&=75mJ \end{aligned}
 Question 7
A steady dc current of 100 A is flowing through a power module (S,D) as shown in Figure (a). The V-I characteristics of the IGBT (S) and the diode (D) are shown in Figures (b) and (c), respectively. The conduction power loss in the power module (S,D), in watts, is ________.
 A 100 B 135 C 240 D 170
GATE EE 2016-SET-1   Power Electronics
Question 7 Explanation:
No current flows through IGBT. So current flows only in diode. Equivalent circuit of diode is as shown below,

\begin{aligned} V&=IR+0.7\\ &=(100 \times 0.01)+0.7\\ V&=1.7V \end{aligned}
Power loss during conduction,
\begin{aligned} P&=V \times I\\ &= 1.7 \times 100=170W \end{aligned}
 Question 8
The circuit shown is meant to supply a resistive load $R_{L}$ from two separate DC voltage sources. The switches S1 and S2 are controlled so that only one of them is ON at any instant. S1 is turned on for 0.2 ms and S2 is turned on for 0.3 ms in a 0.5 ms switching cycle time period. Assuming continuous conduction of the inductor current and negligible ripple on the capacitor voltage, the output voltage $V_{0}$ (in Volt) across $R_{L}$ is ___________.
 A 3 B 7 C 9 D 11
GATE EE 2015-SET-1   Power Electronics
Question 8 Explanation:

$S_1$ is turn ON for 0.2 ms
$S_2$ is turn ON for 0.3 ms.
Switching cycle time period is 0.5 ms.

the output voltage $V_0$ across $R_L$ is

\begin{aligned} V_0&= \frac{1}{0.5 \times 10^{-3}}\left [ \int_{0}^{0.2}10dt +\int_{0.2}^{0.5}5 dt \right ]\\ &= \frac{1}{0.5 \times 10^{-3}} [10(0.2)+5(0.5-0.2)]\\ &=\frac{1}{0.5 \times 10^{-3}} [2 \times 10^{-3}+1.5 \times 10^{-3}] \\ &= \frac{3.5 \times 10^{-3}}{0.5 \times 10^{-3}}=7V \end{aligned}
 Question 9
Figure shows four electronic switches (i), (ii), (iii) and (iv). Which of the switches can block voltages of either polarity (applied between terminals 'a' and 'b') when the active device is in the OFF state ?
 A (i), (ii) and (iii) B (ii), (iii) and (iv) C (ii) and (iii) D (i) and (iv)
GATE EE 2014-SET-1   Power Electronics
Question 9 Explanation:

As given that the active device is in OFF state it means the device can block the voltage when 'a' is positive with respect to 'b' and when 'b' is positive with respect to 'a' then the diode is reverse baised and voltage is blocked.

When 'a' is positive with respect to 'b', as given the device is OFF so it will block the voltage.
When 'b' is positive with respect to 'a' it will block the voltage.
Where as the other two devices will conduct when 'b' is positive with respect to 'a'.
 Question 10
The typical ratio of latching current to holding current in a 20 A thyristor is
 A 5 B 2 C 1 D 0.5
GATE EE 2012   Power Electronics
Question 10 Explanation:
For medium power thyristor of rating 6 A to 60 A the ratio of the latching current to holding current is 1.5 to 2.
There are 10 questions to complete.