Hi Everyone,

My UPS 600 VA with battery 12V 7 AH uses mosfets P55nf06. It works normally on mains voltage but when it switches from mains to battery power, a sparks is generated on heatsink to which two of the mosfets are added.

I changed the mosfets but the problem remains the same. On spark, the ups switches off for few seconds.

what could be the reason?

thanks

 

The mosfets should be isolated from the heatsink with mica washers and mouting kit screws.

Check the Tab doesn't make Continuity with the Heatsink.

 

The mosfets should be isolated from the heatsink with mica washers.

Transistor mounting screws also need isolation, and a thin layer of thermal grease applied to both sides of the mica washer.

 

do all type of mosfets requires mica washer?

 

If the Tab is isolated no, but it's goo practice to use them with silicon grease.