# MOSFET Heat Sink selection

MOSFET heat sink calculation: I am designing H bridge to drive brushed DC motor . The MOSFET I'm using is IRFB4620PbF. I calculated the MOSFET losses which are 7.8W per MOSFET. I'm not sure if I am doing the calculations correctly for heat sink selection.

Tj = Pd (RthJC + RthCS + RthSA) + Tamb

Tj junction temperature : 150C Pd = dissipated power (RMS current through the part x voltage drop) + switching losses = 7.8W
RthJC = thermal resistance, junction to case (from the device data sheet) = 1.045C/W RthCS = thermal resistance, case to heat sink (from the device data sheet) = 0.5C/W RthSA = thermal resistance of the heatsink to air.

Tamb = ambient temperature immediately adjacent to the module in your application = 25C

Susbstituting the above values I get the RthSA as 14.48C/W.

Could you please let me know if the calculations are correct or am I missing something. Thank you for your help.

• Looks about right but add some safety margin. (Tamb is over 25C here today!) Having worked out 14C/W I would be looking for heatsinks 10C/W or even 5C/W or better if they meet your space, weight and cost budgets. It can be a cheap way to improve reliability and equipment life.
– user16324
Commented Jul 2, 2014 at 12:02
• @BrianDrummond 25C would be considered pleasantly cool where I live: It's around 36C here today :-) Commented Aug 1, 2014 at 15:35
• I might interject that all bets are off if the heatsink isn't exposed to free-flowing air - if it's surrounded by obstructions, it's not going to sink heat at all well. Commented Aug 31, 2014 at 16:08