Mounting TO-220 to the front or back of heatsink?

[trevortjes]

Hi all!

I was playing around in kicad (Please don't look at/pay attention to the rest of the components/board or the size of the heatsink, the board is not laid out yet and this is purely a theoretical question!) and found a heatsink 3D model for a TO-220 package. My first instinct was to mount it like this:




It seemed logical as it looks like it should go in there, while this might not even be the case. But then I wondered, when the heat spreads to the fins etc, won't you create a chamber of heat around the TO-220?

 So what about flipping the heatsink around so the fins radiate away from the TO-220?



This way it seems more practical as the heat is radiated away from the TO-220 instead of accumulating around it.

Yet in many datasheets I see TO-220s mounted to heatsinks according to the 1st picture. I know that picture 1 is normal for TO-220s that are supposed to be mounted flat on the board. But this is purely a question for upright mounted TO-220s and heatsinks.

So what is more proper? What is the theory?

[strawberry]

PCB airflow vents under heatsink could help

[Gyro]

If the heatsink is mounted with vanes off the edge of the board then cooling will be more efficient (assuming space below) but it will need decent mechanical mountings to avoid stressing the legs on the TO220.

[Phoenix]

It is designed to fit in the cutout. If you mount the fins away from the to220 there is a lot of wasted space in the cutout that could be extra fin surface area. A different heatsink would be better.

Those style heatsinks should have solder pins on the bottom for mechanical mounting to the pcb.

[magic]

So what is more proper? What is the theory?

I think the "inside" will be a bit hotter (hotter air, more infrared from the sink) but the ability of TO220 to dissipate heat is very limited anyway, so its temperature will still be determined mostly by heatsink temperature. The heatsink is not hotter than the transistor, it isn't adding heat to it. It just limits the ability of the package to shed heat by itself, which is low to begin with.

It could be an experiment. Put two identical transistors on opposite sides and see which gets hotter by comparing their base-emitter voltages, for example. And how much hotter.

[Benta]

Cooling-wise, the position of the TO-220 is totally marginal. The difference between inside or outside mounting won't be measurable. It's the heatsink dissipating the heat, not the transistor.

[Doctorandus_P]

Heatsink on the outside will expose it to more free air and therefore improve cooling a bit.

Another point is the flatness of the mounting surface between the heatsink and the TO220. If you use a surface that is not designed for it, it may be less flat and this hinders heat transfer between the TO220 and the heatsink.