Heat Sink for Mosfets question:

I have 3 mosfets:

a) 1 for line powered flyback convertor switching, Peak Drain Voltage is 500V b) 1 for high side of 12V DC motor control (for free wheeling) c) 1 for low side of 12V DC motor control

The circuit is such that when the power supply is on, battery charges and the motor is disabled and when the power supply is off, the motor may be running on battery.

So at any instant, either (a) is on or (b) and (c) are on.

- I would like to mount all 3 mosfets on a single heat sink, using mica insulators.. Is this ok?

- Somehow if I could mount all 3 on the metal case, it would be great, but there is an issue of safety .. Any advise on how to do this safely?

- This box is going to be used in a vehicle, so I plan to air-seal it for protection against moisture, water and dust [that's why avoiding separate heat sinks while using case as heat sink is better]. I assume that the heat dissipation out of the box will be a problem. Any ideas for this?


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Mica is fine though anodized aluminium is my choice. Need to know your power dissipation, extrusion type of heat sink, temperatures at which the semiconductors are getting to, to be able to give you a better answer. For extra reliable military equipment maximum acceptable temperatues in active devices is 105C wheras in commercial fields 200C. Your failure rate will go up in the latter case.

You need a heat sink with a low temperature resistance, but still that depends on what dissipation power you're talking about. If your total power dissipation is 100W or a 50C or less temperature rise then natural convection cooling is sufficient. Otherwise above those values you need fan / forced air cooling.

Get heat sinks with bilateral fins , add a big fan, transistors in casings with a below 10W/C temperature resistance.

After doing that do some approximations with a thermal model of the heat flow patterns of what the max temperature you are going to get . Don't forget to add the ambient temperature.

Come back here if you got problems on that .

"Go easy on the whisky"


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Most powe mosfets are plastic package with metal back, such as you onlu need a mica waster to isolate from the heatsink, you do not normally need the inserts and nylon bolts. It depends how how your mosfets get, if only "warm" you can leave them on the case, though remember that case metal is very thin and will create a hotspot where the mosfets are bolted on to the case. For highpower applications the heatsink needs to be 5-10mm thick where the mosfets bolt on. Cheaper heatsinks are normally 0.5mm-3mm thick, they are "ok" for most cases though if your switching some KW of power I would look for something "chuncky".

I normally run my mosfets connected to nothing or a very small clip on heatsink, if they run hot to the touch (as in burn your finger) then you need better cooling. Note that it could take several hours for a larger heatsink to rise in temp. You may want to invest in a digital thermal tester, then you can see how hot you are running.

Get mosfets with low RDS on, Mosfets ive tested here, then can vary from a couple of watts heat loss to a over 100watts loss. Invest in a good mosfets for the job and you probably won't even need a heatsink at all.

HTH Chris

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You used the term "temperature resistance" several times, but that's wrong, the correct term is thermal resistance. This makes more difference than you might imagine.

    - Win
Reply to
Winfield Hill

Perhaps a "heat spreader" between the mosfets and case would be good..maybe a fairly thick piece of aluminum plate.

Reply to
phatty mo

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