# Thermal management of MOSFETS in TO-263 package

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.

• I would change that diode, somehow, with a MOSFET. – abdullah kahraman Jun 19 '12 at 10:00

Just a fan blowing over the PCB won't be sufficient; a TO-263 doesn't have good thermal contact with the air surrounding it. I would solder this

heatsink over the FET. Like I said in another answer earlier today an SMD loses much more heat through conduction than through convection, but the heatsink has better convection loss thanks to its larger surface. The heatsink mounts over the SMD, not on it.

Heat transfer will occur via the PCB's copper, so increase the TO-263's pad so that the heatsink shares it.

• I have also seen similar heatsinks that solder to the PCB but paste mount to the component allowing much greater conduction. – Kortuk Jun 19 '12 at 12:19
• @Kortuk - Yes, I thought about that too, but it looks as if they deliberately left rather much room between the two, maybe to increase cooling by having air flow under the heatsink as well. I don't know which will work better; this heat-conducting paste is normally applied only in a thin layer to make sure there's no low-conductive air between surfaces. I have no idea of thermal conductivity of several mm thick paste. – stevenvh Jun 19 '12 at 12:26
• Ohh, I realize this is not the case for this heatsink, I was noting there are many others in existence designed to touch with this general design. I bet this heatsink helps a lot with keeping the PCB well coupled. – Kortuk Jun 19 '12 at 12:28

If your board is double sided, you can punch through with an array of vias to the back side and greatly increase the surface area of the copper. I typically use an array of 12-20 vias directly under the tab with good results. According to the data you posted you will need additional cooling if you want to run this circuit at room temperature.

$$30°C + \frac{40°C}{W} \times \frac{22W}{8in^2 \; copper} = 140°C \text{ juction temperature - not cool!}$$

• Something is not right here. The final units are degC/in^2... – Frederick Jul 6 '15 at 21:31