DC SSR heat sink question (is it really necessary?)

[steve]

Ok, thanks. This is probably a dumb newbie question, but just so I'm clear what I'm calculating here, let me try an example and see if I'm right here (none of my designs have had parts other than voltage regs and in one design triacs which would get any significant heat, and in those cases I just generously applied heat sinks

Say I have a diode rated at 10A. It has forward voltage Vf=1V, RΘja=10K/W. I assume that we're just working on the power dissipated as heat, which would be calculated by the amps and the voltage drop (1V * 10A = 10W, ergo 100°K or 180°F above ambient temperature).

For the MOSFETs, I see RθJC=6.2°C/W and RθJA=62.5°C/W maximum. RDS(on) ranges from 0.088 to 0.14Ω so let's take the larger value, so if I run a load of say 5A on it, the dissipated power would be P=Id**2 * Rds(on) = 25 * 0.14 = 3.5W, so the RθJC=6.2°C/W gives a rise on the case of 21.7°C or 39°F, but the RθJA=62.5°C/W gives an answer of 218.75°C or 393°F above ambient temperature. I can see 3.5W heating up the part by 22°C, but I'm not sure why the RθJA is so much higher.


Datasheet from http://www.datasheetcatalog.org/data...es/ds28010.pdf and http://www.fairchildsemi.com/ds/FQ/FQPF13N06L.pdf

[P. Short]

Would you willingly touch a 7W light bulb (i.e. an incandescent night-light) that had been on for a while? I think not. And if the exterior is that hot, think about how much hotter the tiny heat source (filament or junction) must be. The 393°F number that you give doesn't seem unreasonable for the circumstances...

[steve]

Granted, but what am I misunderstanding about what RθJC and RθJA actually represent, then? If that's the amount the internal junction heats, and how much the part heats the outside air, I'd expect the junction temp to be a lot hotter than the outside, but the numbers seem to say the opposite, so I'm assuming that I'm misinterpreting this somehow.

[P. Short]

RθJA is a measure of the temperature difference between the ambient environment and the junction. This is usually relevant when you do not have any heatsink. The transistor is either in open air without any flow restrictions (in which case the air temperature would be the ambient temperature), or in an enclosure (in which case the temperature inside the enclosure is the ambient temperature, which is probably much higher than the room temperature).

RθJC is a measure of the temperature difference inside the component between some specified spot on the component case and the junction. You use this number when you can figure out what the temperature is at that specific spot on the case, either by measuring it or by calculating it based on the thermal resistance of a heatsink of some sort and the thermal resistance of the mounting scheme (including thermal goop, any insulator, etc). The temperature of that location on the package is probably a lot higher than the ambient (air) temperature.

[steve]

Okay, that makes sense now. Thanks.