Electronics > Projects, Designs, and Technical Stuff
Disapating 750W of MOSFET heat for under $100
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nAyPDJ:
Well, I've been stuck on this issue for quite a while. I'm trying to dissipate 750W (I'm running the FETs in linear mode).

Here are my current thoughts:

- 8x parallel MOSFETs (13N50H, TO-263 (1.14°C/W ea)
- 2x Chinese CPU fan (supposedly 0.18°C/W)

My idea was to place 4 MOSFETs per heatsink by bolting one of these to each heatsink:



which would have worked out (marginally) fine with +110°C @ 750W, but apparently I did my math wrong and it's actually +174°C @ 750W.

as an aside, here's the math:
--- Code: ---((1 ∕ ((0.18°C/W)^−1 × 2)) + (1 ∕ ((1.14°C/W)^−1 × 8))) × 750W
--- End code ---

Now, I can place two more MOSFETs on each heatsink, but even then I'm still at +138.75°C @ 750W.

So I clearly need a new package. The heatsinks have a reasonable thermal resistivity, but 1.14°C/W on the MOSFET is just crazy.

The TO-3P package is much more reasonable, with the KIA Semicon 8150A having a thermal resistivity of 0.38°C/W, which works out to +103°C  @ 750W with 8 of them, and a nice, cool, +91°C @ 750W with 12 of them.

However I don't see a good way to mount them. I can't screw them into the base of the heatsink because of the heat pipes. Thoughts on how to solve this problem? I was thinking of baking the heatsink in the oven @ 450°F & soldering the MOSFETs on it then… but that sounds like a bad idea.
wraper:
100W per mosfet in linear mode, good luck with that. Hot spots on the die guaranteed. Even if used as switchers, it would be highly questionable. Not to say that max power rating is given at 25oC case temperature. Look at figure 10 to cool down your enthusiasm. Also:
--- Quote ---Total power dissipation
derate above 25ºC 1.56 0.39 1.56 W/ºC
--- End quote ---
At your intended case temperature basically near zero power dissipation is allowed.
coromonadalix:
why not use the to-3p package instead ???

duak:
I wouldn't try to solder to the cooler or try to heat it in an oven.

The previous posts are giving good advice.  There are recent postings on this board (as well as many manufacturers websites) discussing the use of MOSFETs in linear mode.  Also, look up "Electro-Thermal Instability".  Also, you really have to discount the specs that manufacturers give until you can test your parts under your conditions.  There's usually some gotcha somewhere.

Anyway, you should be able to press the TO-3P transistors to the cooler with some sort of plate - something that uses the original CPU and cooler mounting hardware.  Each device may require an individual spring, washer and locating pin to balance the pressures.  Many power semiconductors are mounted on heat sinks with flat springs and if done properly, works well.

If the pad on the cooler is too small you might be able to use a heat spreader that accepts the TO-3P transistors that are held to it with screws.  The spreader is then held to the cooler as above.  Copper is expensive and hard to work.  Aluminum is cheaper but not as good.
nAyPDJ:

--- Quote from: coromonadalix on April 11, 2019, 03:38:04 am ---why not use the to-3p package instead ???

--- End quote ---

I want to, but I don't see a good way to mount it onto the heatsink. usually, a screw would be used, but putting a screw into a heat pipe will let the magic vapor out.


--- Quote from: wraper on April 11, 2019, 03:31:41 am ---100W per mosfet in linear mode, good luck with that. Hot spots on the die guaranteed. Even if used as switchers, it would be highly questionable.

--- End quote ---

Sounds like the kind of knowledge only gained through experience :).


--- Quote from: wraper on April 11, 2019, 03:31:41 am ---Not to say that max power rating is given at 25oC case temperature.

--- End quote ---

I was under the impression that power derating was only necessary when ambient is above 25°C, but I guess ambient as far as the chip is concerned is the package temperature.


--- Quote from: wraper on April 11, 2019, 03:31:41 am ---Look at figure 10 to cool down your enthusiasm.

--- End quote ---

Fig 10 in the 8150A DS is Rdson vs Junction Temp, which indicates an overall resistance of 250-300mΩ with 8 MOSFETS @ 125°C. Still sounds fairly reasonable.

Fig 10 in the 13N50H DS is max current vs case temp, which is also fine since I will only be putting 3.125A through each MOSFET in an 8x. I should have mentioned that I've decided to design for 25A--I don't want to be dealing with wires as thick as my fingers.



--- Quote from: wraper on April 11, 2019, 03:31:41 am ---Also:
--- Quote ---Total power dissipation
derate above 25ºC 1.56 0.39 1.56 W/ºC
--- End quote ---
At your intended case temperature basically zero power dissipation is allowed.

--- End quote ---

Oh, I see, that makes sense. However the TO-3P package would be fine in a 12-mosfet configuration:
--- Code: ---750W/12 − (333W−91°C×2.63(W/°C))=−31.17 W
--- End code ---
, or 31W of headroom each.


Thank you both for all the great advice :)
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