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Fault-finding a switch-mode output

Power, transformers and three-phase · Advanced · about 20 minutes · part of Electronics Technician

Symptom: the 5 V buck converter is putting out nearly 11 V. A regulator that goes high has lost its feedback. Measure FB (it should sit at 1.25 V), then, with the power off, the two feedback resistors.

Fault-finding a switch-mode output: the circuit on the simulator's bench

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How it works

The controller adjusts its duty until FB = 1.25 V. With R_top open, R_bot pulls FB to 0 V whatever the output does, so the duty winds up to its 92 % maximum: V_out ≈ 0.92 × 12 V less losses.

Repaired, V_out = 1.25 × (1 + 3k ÷ 1k) = 5.00 V.

Try it on the bench

  1. After the repair, change V_in to 9 V: the output stays at 5 V.

Parts on the bench

In the course

Electronics Technician, module “Power supplies”.

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