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?
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AT is kept constant and T_on is varied
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BT_on is kept constant and T is varied
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Cboth T and T_on are varied
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Deither T or T_on is varied
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E—
Answer
Correct Answer: T is kept constant and T_on is varied
Explanation
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:
Set T = constant (fixed f_s).Adjust T_on to achieve desired α (0–1).Output responds linearly (ideal case): V_avg ∝ α.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