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LM5121: High-Side FET overheating

Horst Theile
Prodigy 20 points
Part Number: LM5121

Tool/software:

We designed a 24V to 48V solution with LM5121. The output current can be approx. 0 to 1A. There are a few damaged units with all N-FETs damaged. Most obvious damage is visible on high-side FET (package damaged). With an IR camera I can see this FET heating up quite fast. This aligns with results from Webench. Webbench doesn't show a solution for low output current (e.g. 1A). Reason seems to be too high temperature of the high-side FET. When selecting higher output current, Webench shows lower temperature.

Please explain why the high-side FET heats up that much?