# What's wrong with this MOSFET?



## Tesseract (Sep 27, 2008)

Of course it's too good to be true. The drain current rating is at a case temperature of 25C. Good luck maintaining that.

Also, practical/experienced engineers limit the maximum current through the leads on a TO-247 package to 80A. 160A _rms_??? Gimme a break!

IXYS makes fine products, but they are known to play fast and loose with their datasheets, especially their "advance" or preliminary specs.


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## tj4fa (May 25, 2008)

Tesseract said:


> IXYS makes fine products, but they are known to play fast and loose with their datasheets, especially their "advance" or preliminary specs.


From their data sheet (Disclaimer allows them to play fast and loose)

*ADVANCE TECHNICAL INFORMATION​*The product presented herein is under development. The Technical Specifications offered are derived
from a subjective evaluation of the design, based upon prior knowledge and experience, and constitute a
"considered reflection" of the anticipated result. IXYS reserves the right to change limits, test​conditions, and dimensions without notice.


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## Salty9 (Jul 13, 2009)

I saw Digi-Key had a small stock of these at $9.50 each. Are the first customers basically beta-testers?


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## blackpanther-st (Apr 4, 2009)

Tesseract said:


> Also, practical/experienced engineers limit the maximum current through the leads on a TO-247 package to 80A. 160A _rms_??? Gimme a break!



So would that make this a good contender for 80A usage with a little head room if current limiting isn't quite so snappy as it should be?


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## Tesseract (Sep 27, 2008)

tjfa: Sure, sure - they got the proper disclaimer, alright, but I still think it's _technically_ (ahem) dishonest to make wild spec claims anyway. IRF used to be really bad about this, too (and their high side driver ICs are real disasters - don't even get me started on that) but in general all of the manufacturers of pcb mounted power devices give them unrealistic ratings. Contrast that to a datasheet for a real power device in a real power package. You get current ratings for much more realistic temperatures of 100C (sometimes even 150C!) at the die and 70-90C at the device's heatsink interface. Those current ratings you can actually believe in and even still people like me will derate them somewhat.

Salty9: no, you probably won't be an inadvertent beta tester for IXYS if you buy these new fets. I was only saying that trying to put 160A rms through one of them will pretty much end in tears.

blackpanther-st: Huh? That was a very "lateral" interpretation of what I wrote. Rather than simply say no, I'll instead advise you to consider the load inductance and supply voltage and then figure out the rate of rise in current over time. If your current limiting is fast enough it will turn off the gates before exceeding the I^2t limit line then you are fine; if not, then you are toast (literally, not laterally)


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## Tesseract (Sep 27, 2008)

Just to expand on the whole amp rating for pcb mounted components....

Take a look at the actual dimensions of the lead wires (much less the bondwires!?!) on the TO-247 package: 1.27mm x 0.68mm. This gives you an area of around 0.86mm^2 (equiv. diameter of 0.1047cm) which, folks, is just a hair larger than 18ga. wire (whose free air current rating is 16A, btw).

Of course, a conductor will happily (or grudgingly?) carry current right up until the point when it melts. That can be estimated with an equation that relates the diameter of the conductor to it's bulk resistivity. For the copper terminals on a TO-247 part that point is around 86A:

Fusing Current (A) = 2530 * (diameter in cm)^1.5

The package will act as a heatsink for the wire, allowing slightly more than this to be carried, theoretically speaking, but that means the terminals are adding heat to the package, which is not what one would normally describe as "helpful".

Thus, this is why smart (or experienced) engineers will allow a MAXIMUM current of around 80A through a TO-247 part. Most won't even push their luck that much unless they are very confident about the load cycle ahead of time. I can't think of an application where one would be less confident of the load cycle than a motor controller for an EV....

Anyway, this little game between the semi manufacturers and engineers has been going on for decades and is unlikely to change anytime soon. It's rather like the stereo manufacturers of the 70's or ev controller manufacturers of today


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## TheSGC (Nov 15, 2007)

I think pumping over 50A in any TO-type casing that uses leads is a recipe for disaster for any period of time. The casings might be larger with more heat sinkable space, but they still use the same tiny leads.

It's kind of like putting a Soliton1 or a Zilla in a golf cart, but leaving the #2 gauge cable in and wondering why the cables explode when you floor it. The hardware is no good unless the supply lines can handle the juice, with the hardware being a massive fet and the supply lines being the little leads.


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