Transformer and inductor saturation: why do MOSFETs blow for no reason?

You replaced the MOSFET and it blew again at switch-on. Often the culprit isn't the MOSFET but the transformer core: loaded with more flux than it can carry, the winding suddenly turns into a plain wire.

What does it do? A ferrite core can only carry energy up to a certain magnetic flux (B). Below that limit the winding resists fast changes in current and the current draws a clean ramp. At the limit (saturation) the core can't take more flux, the inductance collapses and almost nothing limits the current: it multiplies within a few hundred nanoseconds. If current limiting can't keep up, the MOSFET, Rs and sometimes the IC all go. Hot ferrite saturates earlier.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/3: Normal: current ramps

While the core isn't saturated, the winding resists fast changes in current: the current draws a clean, linear ramp. Flux (B) rises in step with current (H).

1. Gate

The longer the on-time, the more flux builds up in the core. The most common cause of saturation is a switch that stays on longer than it should.

On the scope

Multimeter≈12 V pulses

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

🔍 How to spot it on a board

•On the scope, the tip of the current ramp curling upwards (a 'ski-jump' shape)
•A MOSFET that blows again and again on the same board; in push-pull and half-bridge, always the switch on the same side
•A transformer or inductor that runs hot or hums; a cracked core or one that was taken apart before

🔧 How to measure it

•Scope the current-sense resistor: is the ramp straight, or does its tip curl?
•Are the frequency and duty on OUT normal? Measure the RT/CT parts.
•Measure the transformer's inductance with an LCR meter and compare with a good board; a shorted turn drops it sharply.
•In push-pull, compare the widths of the two gate pulses.

⚠ Common faults

•Low frequency (the RT/CT capacitor has drifted): the on-time lengthens and the core saturates.
•Imbalance in push-pull/half-bridge, or a bad series capacitor: the flux walks to one side and the same switch always blows.
•Wrong spare transformer, or a core whose air gap changed: low inductance, early saturation.
•Faulty current-sense resistor or CS filter: no protection when saturation hits.

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Go further

Component: transformer Component: inductor Concept: current sensing The forward converter How it works: welding machine