Previous Year Questions- Choppers - 2 | Power Electronics - Electrical Engineering (EE) PDF Download

Q16: For the switching converter shown in the following figure, assume steady-state operation. Also assume that the components are ideal, the inductor current is always positive and continuous and switching period is T_s. If the voltage V_L is as shown, the duty cycle of the switch S is ______.       (SET-2 (2015))
(a) 0
(b) 0.25
(c) 0.5
(d) 0.75
Ans: (d)
Sol: Hence, it is boost regular,

Q17: A buck converter feeding a variable resistive load is shown in the figure. The switching frequency of the switch S is 100 kHz and the duty ratio is 0.6. The output voltage V_o is 36 V. Assume that all the components are ideal, and that the output voltage is ripple-free. The value of R (in Ohm) that will make the inductor current (i_L) just continuous is _______.       (SET-2 (2015))
(a) 1240
(b) 2500
(c) 3695
(d) 1478
Ans: (b)
Sol: Condition for boundary of inductor current (i_L)

Q18: A self commutating switch SW, operated at duty cycle δ is used to control the load voltage as shown in the figure
Under steady state operating conditions, the average voltage across the inductor and the capacitor respectively, are      (SET-1 (2015))
(a)
(b)
(c)
(d)
Ans: (a)
Sol: The average voltage across inductor, V_L= 0 and hence the given circuit is a boost regular so average voltage across capacitor is equal to average voltage across the load so,

Q19: In the following chopper, the duty ratio of switch S is 0.4. If the inductor and capacitor are sufficiently large to ensure continuous inductor current and ripple free capacitor voltage, the charging current (in Ampere) of the 5 V battery, under steady-state, is_____.      (SET-1  (2015)
(a) 1
(b) 1.5
(c) 2.5
(d) 3
Ans: (a)
Sol: Step down chopper

Q20: A step-up chopper is used to feed a load at 400 V dc from a 250 V dc source. The inductor current is continuous. If the 'off' time of the switch is 20 μs, the switching frequency of the chopper in kHz is _____.        (SET-2 (2014))
(a) 16
(b) 31.2
(c) 38.25
(d) 45.75
Ans: (b)
Sol: Given step up chopper,

Q21: Figure (i) shows the circuit diagram of a chopper. The switch S in the circuit in figure (i) is switched such that the voltage v_D across the diode has the wave shape as shown in figure (ii). The capacitance C is large so that the voltage across it is constant. If switch S and the diode are ideal, the peak to peak ripple (in A) in the inductor current is ______         (SET-1 (2014))
(a) 1.5
(b) 2.5
(c) 3.5
(d) 4.5
Ans: (b)
Sol: The circuit shown is a Buck chopper
The peak to peak ripple in the inductor current is

Q22: In the figure shown below, the chopper feeds a resistive load from a battery source. MOSFET Q is switched at 250 kHz, with a duty ratio of 0.4. All elements of the circuit are assumed to be ideal.
The PEAK-TO-PEAK source current ripple in Amps is      (2013)
(a) 0.96
(b) 0.144
(c) 0.192
(d) 0.288
Ans: (c)
Sol: Here,
∴ ripple source current = (V/L)(D.T.)
= 0.192 A

Q23: In the figure shown below, the chopper feeds a resistive load from a battery source. MOSFET Q is switched at 250 kHz, with a duty ratio of 0.4. All elements of the circuit are assumed to be ideal.
The average source current in Amps in steady-state is       (2013)
(a) 3/2
(b) 5/3
(c) 5/2
(d) 15/4
Ans: (b)
Sol: As it is a boost converter, average current through capacitor is zero.
When Q is ON,  I_c = −I₀
I_c = I_s − I₀
Therefore, average through capacitor

Q24: In the circuit shown, an ideal switch S is operated at 100 kHz with a duty ratio of 50%. Given that Δi_c is 1.6 A peak-to-peak and I₀ is 5 A dc, the peak current in S is      (2012)
(a) 6.6 A
(b) 5.0 A
(c) 5.8 A
(d) 4.2 A
Ans: (c)
Sol: Peak current

Q25: In the chopper circuit shown, the main thyristor (T_M) is operated at a duty ratio of 0.8 which is much larger the commutation interval. If the maximum allowable reapplied dv/dt on T_M is 50 V/μs, what should be the theoretical minimum value of C₁ ? Assume current ripple through L₀ to be negligible.      (2009)

(a) 0.2 μF
(b) 0.02 μF
(c) 2 μF
(d) 20 μF
Ans: (a)

Q26: In the circuit shown in the figure, the switch is operated at a duty cycle of 0.5. A large capacitor is connected across the load. The inductor current is assumed to be continuous. The average voltage across the load and the average current through the diode will respectively be      (2008)
(a) 10V, 2A
(b) 10V, 8A
(c) 40V, 2A
(d) 40V, 8A
Ans: (c)
Sol: When, switch is ON, V₀ = 0 and i_D = 0
The circuit shown in the figure is step up chopper and for step up chopper, average output voltage,

Average current through diode

Q27: A voltage commutated chopper operating at 1 kHz is used to control the speed of dc as shown in figure. The load current is assumed to be constant at 10 A.
The average output voltage of the chopper will be      (2006)
(a) 70 V
(b) 47.5 V
(c) 35 V
(d) 0 V
Ans: (b)
Sol: 
Q28: A voltage commutated chopper operating at 1 kHz is used to control the speed of dc as shown in figure. The load current is assumed to be constant at 10 A.
The minimum time in μ sec for which the SCR M should be ON is      (2006)
(a) 280
(b) 140
(c) 70
(d) 0
Ans: (b)
Sol: Initially, main thyristor (m) and auxiliary thyristor (A) are off and capacitor is assumed charged to voltage V with upper plate positive. When 'm' is turned on at t=0, source voltage V is applied across load and load current I₀ begins to flow which is assumed to remain constant.
With 'm' ON at t = 0, anotheroscillatory circuit consisting of C, m, L and D is formed where the capacitor current is given by
When, ω₀t = π, i_c = 0
Between
 capacitor voltage charges from +V_s to−V_s sinusoidally and the lower plate becomes positive.
At ω₀t = π, i_c = 0, i_T1 = I₀ and V_c = −V_s
At t₁, thyristor A is turned on and capacitor voltage V applies a reverse voltage across thyristor in and SCR 'm' is turned off. The load current is now carried by C and SCR A.
So, minimum time for which SCR 'm' should be ON.

Q29: The given figure shows a step-down chopper switched at 1 kHz with a duty ratio D = 0.5. The peak-peak ripple in the load current is close to      (2005)
(a) 10A
(b) 0.5A
(c) 0.125A
(d) 0.25A
Ans: (c)
Sol: Here,

Q30: Figure shows a chopper. The device S₁ is the main switching device. S₂ is the auxiliary commutation device. S₁ is rated for 400 V, 60 A. S₂ is rated for 400 V, 30 A. The load current is 20 A. The main device operates with a duty ratio of 0.5. The peak current through S₁ is      (2004)
(a) 10A
(b) 20A
(c) 30A
(d) 40A
Ans: (d)
Sol: Initially main thyristor (S₁) and auxiliary thyristor (S₂) are off and capacitor is assumed chargeed to voltage V_s with upper plate positive. When S₁ is turned on, source voltage V_s is applied across load and load current I₀ begins to flow which is assumed to remain constant. With  S₁ ON another oscillatory circuit consisting of C,  S₁ and L is formed where the capacitor current is given by
So current through $𝑆1$S₁,

Q31: Figure shows a chopper operating from a 100 V dc input. The duty ratio of the main switch S is 0.8. The load is sufficiently inductive so that the load current is ripple free. The average current through the diode D under steady state is       (2004)
(a) 1.6 A
(b) 6.4 A
(c) 8.0 A
(d) 10.0 A
Ans: (a)
Sol: During the period T_ON chopper on and load voltage is equal to source voltage (V_s = 100V). During the interval T_OFF, chopper is off, load current (I₀) flows through the diode as result load voltage is zero during T_OFF.
Average load voltage,
Average load current,
As load current is ripple free, so average diode current,

Q32: A chopper is employed to charge a battery as shown in figure. The charging current is 5 A. The duty ratio is 0.2. The chopper output voltage is also shown in the figure. The peak to peak ripple current in the charging current is     (2003)
(a) 0.48 A
(b) 1.2 A
(c) 2.4 A
(d) 1 A
Ans: (a)
Sol: During T_on,
Voltage across inductor  
Current through inductor,
where ΔI = peak to peak ripple current equating A₁ and A
A₁ = A₂

Q33: In the chopper circuit shown in figure, the input dc voltage has a constant value V_s. The output voltage V_o is assumed ripple-free. The switch S is operated with a switching time period T and a duty ratio D. what is the value of D at the boundary of continuous and discontinuous conduction of the inductor current i_L?      (2002)

(a) D = 1 - (V_s/V_o)
(b) D = 2L/RT
(c) D = 1 - (2L/RT)
(d) D = RT/L
Ans: (c)
Sol: As the given chopper is buck chopper.
this is applicable only at the boundary of continuous and discontinuous

Q34: A step down chopper is operated in the continuous conduction mode in steady state with a constant duty ratio D. If V_o is the magnitude of the dc output voltage and if V_s is the magnitude of the dc input voltage, the ratio V_o/V_s is given by      (2002)
(a) D
(b) 1-D
(c) 1/(1-D)
(d) D/(1-D)
Ans: (a)
Sol: