I'm using SUM60N02 MOSFETs in TO-263 package for my car battery charger power unit. In datasheet they have specified thermal resistance:
- Junction-to-Ambient - 40 °C/W when mounted on 1" square PCB
- Junction-to-Case 1.25 °C/W
I have 8 MOSFETs laid out on ~6 square inch of copper:
I will be charging a 12V lead acid battery with a maximum 20A current. Input voltage for MOSFETs will be 16V, i don't really know what battery voltage will be during bulk charge, but i guess ~14.5V.
So, I have:
\$16V - 0.4V\$ (Schottky diode, on separate board)\$ - 14.5 = 1.1V\$
\$P = 1.1V \times 20A = 22W\$ of heat dissipated.
I'm going to use a fan for cooling.
What is the best strategy for thermal management? Should just put a fan over PCB, or should i put a heatsink on top of transistors and then a fan? Or should i mount a heatsink onto a copper? I'm lost because it seems TO-263 package is optimized to transfer heat to PCB and I have no experience with serious power dissipation.