vvr problem

[FYL]

I just implementing VVR in my express, but can’t decide between heat sink out of chassis, and cooling direct on chassis.

If your build uses a standard aluminum chassis, direct mounting is OK (using suitable insulation, etc.). You may add a heatsink if you wish.

[Kagliostro]

Hi FYL

Mosfet to insulator to chassis, with the latter dissipating heat

thanks

my answer was because a 2mm thick ceramic insulator between mosfet and heat sink or chassis seems to me to be too thick and don't help the heat dissipation

isn't better a conductive paste and insulate the heat sink ?

Kagliostro

[FYL]

a 2mm thick ceramic insulator between mosfet and heat sink or chassis seems to me to be too thick and don't help the heat dissipation

The insulator acts as a coupler/spreader. Thick aluminum (or beryllium) oxide insulators are the norm when it comes to high voltage/hich frequency apps.

For instance the standard Aavid Thermalloy 4180, perfect for TO-247 components such as power mosfets, is app. 2mm thick.

isn't better a conductive paste and insulate the heat sink ?

This would be very dangerous. You should always insulate at the component level.