methods said:
Yea - Camlight has good advice - clearly a subject matter expert.
I want to put a "methods thread disclaimer" on it though.
It should be noted that Camlight is a Mosfet aficionado - borderline perfectionist
I believe that he would also not have us go over 75V with the 100V 4110 mosfets right?
Only if you want those MOSFETs to last for years without problems.
Otherwise, bringing the voltage up from there brings the expected life span down. There's always a trade-off, ya' just gotta pick which part of the trade-off is more important. I see a lot of FETs blowing up in controllers being used within their alleged ratings and it's because a lot of these basics aren't being paid attention to. For DIY'ers, and others who don't mind hacking their controllers if the FETs blow, by all means crank it up!
Your disclaimer is a good one and I never meant my post to be accepted as gospel for all FET use. Just for longest life possible. Each person can go as far from there as they want.
methods said:
His advice is sound and you should listen but that does not mean that if you do something janky it wont work.
I think we have an order of magnitude of overkill here so use what you've got laying around - it will work just fine.
Certainly not worth a tear-down.
-methods
Yup, it is conservative. Where I have the option, I like my stuff to never, ever need repair. But, where I have to, I'll go beyond the specs and cross my fingers. I've got an electronic load I built here that operates at 140C at 2400W, the max temp for good reliability. Do I ever operate it at 2400W? Nope.
Typically, it runs at 3200W or so and up to 4000W....over 225C for the MOSFETs. And not one MOSFET has blown yet. But, they will.
If you're modifying or repairing a marginal design, adding thermal compound to a Kapton/mica-only setup might give you enough of a thermal margin to keep the unit from blowing (or again). But, absolutely, most specs can be pushed. And some can be pushed very, very hard. Just have to expect that the component might fail at any time. I like to understand what a good design can do and then push from there. A lot of folks start assuming that a 100V FET has no chance of blowing at 100V and 175C temp and then wonder what the heck happened when it blew at 101V. Knowing that the FET is stressed pretty hard at 80% of its rating can give the user a better idea of the failure curve they're dealing with, and where the curve starts. This lets them exceed the specs with better expectations of might happen.
Some folks might not know that they can't just replace mica+grease, or a gap-pad, with dry Kapton and that it also needs grease to match the thermal specs of any greased setup they're replacing. Kapton doesn't have to be greased, but it sure works better when it does.
By all means, don't let any of this prevent you from going full-bore janky!
methods said:
Edit - there will be a trade off between effective thermal coupling and margin on isolation right?
The thinner you go with the Kapton the greater the possibility of catastrophic failure.
Kapton has a dielectric strength of over 2,000V/mil of thickness. No reason not to go as thin as possible...and lots of benefits. If you want to go thicker to prevent penetration by a burr or something else, it's much better IMHO to get rid of the burr since it's probably long enough to go through any thickness of Kapton you might use.
I should make clear that I never, ever want anyone to not try to push the ratings for anything they're using. I just want them to know what's going on and how to better understand how the system works to improve its reliability...so the system can be pushed even harder.
