help choosing heatsinks for transistors

[riotubes]

Hi,

I'm ordering parts from mouser and digikey USA and need some help finding proper heatsinks for two transistor types

IRFI820GPBF-ND (digikey #) TO-220F

689-LND150N3-G (mouser #) TO-92

I can order from either site, and prefer copper if available...

Both parts are used in two places

dac output stage with about 30ma running through it

70V regulator board that feeds the dac stage

There's a zillion heatsinks on these sites, and I don't know where to begin. Any help is greatly appreciated!

[dmills]

Calculating heat sinks is fairly straightforward:

Given worst case voltage across and current through the part, calculate the amount of dissipation (so a transistor running 30mA at 70V across the part (Not 70V total rail!), will dissipate 70 * 30 = 2100mW = 2.1W.

Next figure out the maximum junction temperature you can live with (say 120 degrees C if unsure), and the maximum ambient air temperature around the heatsink (I use 40 degrees C for an external heatsink, or 60 for an internal one, probably both well conservative), so lets say this heatsink is inside the box:

Tj(max) -Tamb = 120 - 60 = 60 degrees C, and power is 2.1W, so the maximum total thermal resistance from the junction to the air of 60/2.1 = 29C/W.

Subtract the thermal resistance of the package internals (from the datasheet), and the insulating washer (if required) and you get a maximum value for the thermal resistance of the heatsink.

Pick one with a thermal resistance lower then the value calculated (Half the value calculated if you can make it fit is good for reliability).

Regards, Dan.

[riotubes]

Very educational feedback. Thank you Dan!