Page 30 – DC-DC Converters GATE Questions & Numerical Problems
Subject: Power Electronics
Topic: Buck, Boost, Buck-Boost Converters
Level: GATE / PSU Practice
🔹 Conceptual Questions
Q1. In a Buck converter operating in CCM, if duty cycle D = 0.6 and input voltage is 100V, find output voltage.
Vo = D Vs = 0.6 × 100 = 60 V
Q2. A Boost converter has D = 0.4 and input voltage 50V. Find output voltage.
Vo = Vs / (1 − D) = 50 / 0.6 = 83.33 V
Q3. Buck-Boost converter with D = 0.5 and Vs = 40V. Find output voltage.
Vo = − (D / (1 − D)) Vs
= − (0.5 / 0.5) × 40 = −40 V
🔹 Numerical Problem 1 – Ripple Current
Given: Buck converter
Vs = 120V
D = 0.5
L = 2 mH
f = 10 kHz
Find: Inductor ripple current.
T = 1/f = 1/10000 = 0.0001 s
ΔI = Vs D (1 − D) T / L
ΔI = 120 × 0.5 × 0.5 × 0.0001 / 0.002
= 1.5 A
🔹 Numerical Problem 2 – Determine Mode (CCM/DCM)
If minimum current becomes zero → DCM
If minimum current > 0 → CCM
Boundary condition: Imin = 0
🔹 GATE Trick Questions
- As D → 1 in Boost → Vo increases sharply
- Buck converter never increases voltage
- Buck-Boost reverses polarity
- Volt-second balance always applies in steady state
Final Exam Formula Sheet
Buck → Vo = D Vs
Boost → Vo = Vs / (1 − D)
Buck-Boost → Vo = − (D / (1 − D)) Vs
Practice Daily – Shaktimatha Learning
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