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Disapating 750W of MOSFET heat for under $100
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jonroger:
I expect that you need something about 10x bigger than this 70W load:

https://www.tindie.com/products/Kaktus/mightywatt-70w-electronic-load-for-arduino/?utm_source=hackaday&utm_medium=link&utm_campaign=fromstore#specs

Try to figure out an alternative way to do whatever it is you are trying to do.
max_torque:
Commercial high power DC loads share the power disipation across a LOT of pass elements. I have a Chroma 5kW Dc load that has 296 individual mosfets in it, across  8 parallel "cards" each with a massive high power fan blown heat sink.

If you want lots of power in a small package, then to get the necessary heat flux you'll need a high deltaT, and of course, to get a higher deltaT than the max junction temp of your power silicon you'll need to use sub zero forced cooling.  That might be as extreme as liquid nitrogen chilling if you are really pushing the envelope...


The advantage that dropping power across a dumb resistor is of course the fact that the resistor, being a dumb passive element, can run at very elevated temperatures without failure. 

For example, here is a 50 watt "resistor" that operates at something like 2,500 degrees centigrade




 :-DD
magic:

--- Quote from: nAyPDJ on April 11, 2019, 05:49:12 pm ---Nope, the plan was to mount the heatsink on top of the PCB, so that the heat is transferred through the plastic chassis. Unfortunately I did the math wrong before designing that board, and that package has too much thermal resistance.

--- End quote ---
Frankly, your math is still wrong. The numbers you use are junction-to-metal and metal-to-sink. Junction-to-plastic and plastic-to-sink would surely be worse. It's not completely useless - I have improved cooling of my motherboard's VRM by gluing heatsinks to the plastic tops of SMD FETs, but we are talking maybe 10W, not 750W.


--- Quote from: nAyPDJ on April 11, 2019, 05:49:12 pm ---You'd think so, but the exact opposite is true. MOSFETs rated for 200W can be found for about $1, but chassis mount resistors rated to 80W cost around $5.
--- End quote ---
FETs rated for 200W are rated so if you keep their metal plate at 25°C ;)


--- Quote from: nAyPDJ on April 11, 2019, 05:49:12 pm ---This is what really bugs me about cooler manufactures: they do a crap job of actually providing specs for their products. They'll sometimes publish a power rating--meaningless, since they never publish the max temperature. However, they also *never* publish any kind of thermal resistance numbers.
--- End quote ---
The only real source of information on computer coolers are reviews.
Also, thermal resistance may depend on operating point so it's hard to define, those things aren't simple slabs of metal as you probably already know. The heatpipes need to reach some temperature for optimum efficiency.
Typical operating temperatures of those coolers are tens of °C, at Tj=90~100°C most chips start to throttle clocks or shut down, they can't handle more than that.

By the way, Fairchild UniFET line seems to be usable for linear operation too, here's the SOA of their beefiest parts. Disclaimer: I've never used them, only considered building something similar and gave up because I only really needed <100W of power at the time, which makes things a lot easier.
EmmanuelFaure:
"Disapating 750W of MOSFET heat for under $100"

The Rth of a common CPU cooler, costing about $20, is about 0.5°C/W. Let's say you want a max delta T = 50K (Between junction and air), this leads to 100W dissipated for $20, 750W for $150. That doesn't take into account all the custom hardware to mount your FETs, the assembly work, etc.

For this range of dissipated power, there's no any heatsink cheaper per watt dissipated than CPU coolers, they're manufactured by millions. Any solution with industrial parts will cost more than that.
Mr.B:
This idea will not meet your $100 budget unfortunately.
This is what I did when I built my programmable DC load.
The FETs are IXYS IXTH30N50L2 (Single N Channel 600 V 215 mO 240 nC 400 W Power Mosfet - TO-247)
These FETs are expensive - ~NZD 17 each.
They are mounted with the case against the PCB and their thermal tabs against the CPU water cooler.
There are four FETs, two each side of the PCB and two CPU water coolers, one each side clamped together.
If the SOA in the datasheet is to be believed, I should be able to max it out at 200w per FET (Tc=75C).
However, I have not pushed it beyond 100w per FET, 400w for the whole module.
Pics below.
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