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Primary FET heatsink connected to earth in offline flyback?

Flyback

Well-Known Member
In an offline Flyback SMPS, using a plastic case TO220 FET such as FSCQ1565, do you agree, its better to connect the FET heatsink to primary ground rather than earth? (Wouldn’t common mode EMC be worse if it was connected to Earth?)

Also: Same question as above but for the secondary TO220 diode?

FSCQ1565 datasheet:
https://www.onsemi.com/pub/Collateral/FSCQ1565RT-D.pdf
 

unclejed613

Well-Known Member
Most Helpful Member
the heatsink on the primary side is usually connected to the "hot" ground, or sometimes just left floating. since many devices using an offline switcher are only using ungrounded power cords (blu-ray players, home theater all-in-ones, etc...), grounding that primary heatsink to the chassis could become a safety hazard if the FETs drain were to arc through to the heatsink (such as might happen with a lightning strike) it's ok to chassis ground the secondary side het sinks, but a bad idea for the primary side.
 

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