Enclosure temperature query

I have 2 Mosfets (TO-220) dissipating 1.5W each inside a black ABS enclosure roughly the size of a cigarette pack.

To lower the internal temperature (which was 100degC/212degF with TO-220 heatsinks), I have attached a piece of aluminium bar 70x20x3mm to each. The internal temp is now constant at 65C/149F at an ambient temp of 22C/72F. Maximum ambient would not be more than 40C/104F I assume.

Is this a reasonable/unreasonable operating temperature? Is it safe? Would this high temp affect the longevity of the electronic parts?

Will painting the aluminium black make any difference?

Cheers, Pete.

Yes, but will the black abs case last indefinitely? Depending upon the the manufacturer and quality, they may not be rated for continuous operation above 60 - 70C. Unless specified by the manufacturer I wouldn't be running a black abs case this size with the internal temp above 50C max at ambient 25C. Perhaps the OP can tell us the make and model of the abs enclosure.

Thanks Roger,

I follow all except your 2nd last sentance starting with "At 65C ...." do you mean At 90C, ...?

Also from my reading of the specs Rds(on) increases with temperature, so it is desirable to keep the temp as low as possible, AIUI.

Cheers, Pete.

yes and no

PCB is contoured for this box only, and there are more than just one.

Cheers, Pete.

I imagine you would get a better heat dissipation if you used a diecast box with a suitable insulator on the mosfets. There would be much more surface area exposed to the air than the bar enclosed in the ABS enclosure.

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Regards ......... Rheilly Phoull

enclosure

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I think you measured air temp inside the box. My guess is that the aluminium is pretty well at that same temperature because most of the temperature gradient is across the box walls.

If the aluminium is at 65C, at 1.5 watt, junction temperature will be only

2C higher, since thermal resistance case-sink for TO-220 about 1C per watt. Max junction rated temperature will be 175C, so you are 100C plus under the limit ! At 65C, only 25C higher, you still have plenty to spare. At this temperature, your MOSFETS will last indefinitely.

Roger Lascelles

You need to read up Motorola's AN1040.

Graham