LOL…beast is right…omg.
I’ve seen that pack mentioned here but never read the posts. A verrrry interesting option for a power hungry setup that needs a lot of cooling.
Once you add on the cooling system it’s even more of a monster. With all the “free cooling” we can get when moving I’m wondering if it all ends up being worthwhile? I’m the esk8 noob though so I’m not familiar enough with the setups where using this pack would be a great fit.
Wow…I’d love to play with one or two of those things though. Just to see what can be done. Time and my wallet make that impossible though.
I used SUP57N20 soldered with it’s base to IHS of some CPU, then using standard thermal paste to the heatsink, temperature on the package was 100C I think.
If I would be to use modern CPU cooler with thermal pipes I could reach up to 350W.
Then you were way, wayyyyyy over that FET”s max junction temp.
The case temp is measured at the junction of the drain lead to the epoxy case, the base of the drain tab, or (the best way) on the middle of the FETs rear drain plate, using a small thermocouple. The thermal resistance of the epoxy case itself is incredibly high resulting in a large drop in temperature from the drain to the top of the epoxy case.
We can’t confuse a capability of a component with a rating. Sure, we can have TO-220 MOSFETs dissipating 350W, and they might even do that for a while, but it is far beyond the MOSFET’s ratings and the life of that MOSFET will be very short. I was taking those TO-264’s up to 700W, and some lasted a decent amount of time, but they all eventually blew when doing reliability testing at much above 150W using a 100mm sq. 0.17°C/W pin fin heat sink and high speed/pressure fan.
In my experience a good fan-cooled heat sink with a single TO-220 can get up to about 100W before you start approaching the max junction temp of the MOSFET. High performance cooling might get you a bit higher but you are bottlenecked by the TO-220’s high junction-case resistance and tiny contact patch.
Yeah resistance of the epoxy is high but there’s nothing cooling it from the top so measurement of 100C would result in actual temperature of the die being 110-120 the mosfet is rated for 175C
I’ve never heard of mosfets aging.
Thermal resistance (and the temp difference it causes) exists whether there is cooling or not.
It’s a function of the material itself. Cooling can help but it’s still there.
How are you determining that 20°C temp rise from the epoxy to the junction?
From the drain pin or rear of the FET (the datasheet doesn’t say) to the junction it is at least a 125°C difference at 250W and that is a MUCH lower thermal resistance path than from the epoxy top to the junction.
Check out thermal fatigue of semiconductors. There is failure from just running at high temperatures (but still below the junction rating) and there are additional failure modes from thermal cycling. Once mounted to a circuit board there can be other failures due to the different coefficients of thermal expansion between the FET and the circuit board. This is a big problem for leadless MOSFET packages where there is no “flex”.
Check out hot carrier injection, time dependent dielectric breakdown, bias temperature instability, and hotspotting/thermal runaway of switching FETs when biased in their linear region.
That last item is a particular problem when switching FETs are used as an electronic load. Even a lot of power engineers aren’t aware of this issue. Your FET has DC plot line in its SOA graph though, which is good. Just stay as far away from the DC line as you can, especially at higher voltages.
