Pulse-width modulation (PWM) strategy in choppers In PWM control of a step-down chopper, how are the switching period T and the on-time T_on typically managed to regulate the output?

Introduction / Context:
PWM is the most common control method for DC choppers and switch-mode supplies. It modulates the duty ratio to control the average output while operating at a fixed switching frequency to simplify filtering and EMI design.

Given Data / Assumptions:

• Step-down (buck) chopper topology.
• Objective: regulate average output voltage/current.
• Practical constraint: fixed switching frequency for magnetics and compliance.

Concept / Approach:

Holding T constant fixes the switching frequency f_s = 1/T. Varying T_on changes duty cycle α = T_on/T, thereby controlling the average output proportionally (ideally V_out = αV_in for resistive load).

Step-by-Step Solution:

Verification / Alternative check:

Many controllers use sawtooth carrier versus error-amplified signal; the comparator creates variable T_on pulses at fixed T.

Why Other Options Are Wrong:

• Varying T complicates magnetics and EMI; not standard in PWM.
• Varying both is unnecessary and adds complexity.
• “Either” is too broad; classic PWM uses fixed T, variable T_on.

Common Pitfalls:

Confusing PWM with frequency modulation (PFM), which varies T instead of duty at fixed T.

Final Answer:

T is kept constant and T_on is varied