In a step-down chopper using PWM, Ton = 3 × 10^-3 s and Toff = 1 × 10^-3 s. What is the chopping frequency?

Introduction / Context:
Choppers convert a fixed DC input to a variable DC output by high-frequency switching. The chopping frequency is the reciprocal of the switching period, important for sizing filters and estimating ripple and switching losses.

Given Data / Assumptions:

• On-time Ton = 3 × 10^-3 s.
• Off-time Toff = 1 × 10^-3 s.
• PWM operation with period T = Ton + Toff.
• Units for the final numeric answer are in hertz (Hz) though the options are shown without units.

Concept / Approach:

Chopping frequency f is defined as f = 1 / T, where T is the total switching period. Duty ratio D = Ton / T is not directly needed here but could be used to confirm operating point and average output voltage for a step-down chopper: Vout ≈ D * Vin (ideal case).

Step-by-Step Solution:

Verification / Alternative check:

If the chopper turns on for 3 ms and off for 1 ms, four such milliseconds complete one full cycle, so there are 250 cycles in one second, confirming 250 Hz.

Why Other Options Are Wrong:

• 333.33: corresponds to 3 ms period, not 4 ms.
• 500 or 1000: would require 2 ms or 1 ms total period respectively, not given here.
• 125: would require 8 ms period, not applicable.

Common Pitfalls:

• Adding Ton and Toff incorrectly or confusing duty with frequency.
• Forgetting that frequency is the reciprocal of total period, not of Ton alone.

Final Answer:

250