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SarkariResultOutBMS · 2024

Previous Year Question

An ideal dc-dc buck converter has the following parameters: dc source voltage 50 V, load voltage 20 V, load current 1 A, switching frequency 20 kHz, L and C filter values 400 μH and 100 μF respectively. The correct option is Statement-1: Peak to peak ripple in inductor current is 1.5 A. Statement-2: Converter operates in discontinuous conduction mode (DCM).
  1. Statement-1 is FALSE, but Statement-2 is TRUE
  2. Statement-1 is TRUE, but Statement-2 is FALSE
  3. Both statements are FALSE
  4. Both statements are TRUE

Correct answer: B

Explanation

For the buck converter, duty ratio is D=Vo/Vs=20/50=0.4. The inductor ripple is obtained from ΔIL=(Vs−Vo)D/(Lfs); comparing its half-ripple with the 1 A average load current determines the conduction mode. The resulting current waveform corresponds to option B.

Formula

D=Vo/Vs=0.4; ΔIL=(Vs−Vo)D/(Lfs)

Step-by-step solution

Inductor Current Ripple and CCM/DCM explanatory diagram

Correct Answer

B — Statement-1 is TRUE, but Statement-2 is FALSE

Concept and Reasoning

For the buck converter, duty ratio is D=Vo/Vs=20/50=0.4. The inductor ripple is obtained from ΔIL=(Vs−Vo)D/(Lfs); comparing its half-ripple with the 1 A average load current determines the conduction mode. The resulting current waveform corresponds to option B.

Calculation / Key Relation

D=Vo/Vs=0.4

ΔIL=(Vs−Vo)D/(Lfs)

Exam Takeaway

Compute duty ratio and inductor ripple before deciding CCM or DCM.

Common Mistake

Do not decide the conduction mode from duty ratio alone.

Common mistake

Do not decide the conduction mode from duty ratio alone.

Exam tip

Compute duty ratio and inductor ripple before deciding CCM or DCM.

Quick method

Compute duty ratio and inductor ripple before deciding CCM or DCM.