why do mosfets burn when they are protected by fuses

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sdowney717

Joined Jul 18, 2012
711
Don't believe all the first page of a mosfet data sheet says. The numbers are just for marketing. That style of mosfet is only good for ~75 watts, no matter what the DS says. It is a limit set by the case of the mosfet. Read this for more eye opening - http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html
I have noticed that there is some variation in lead thickness between TO220 packages, the IRF1010E and IRF 3205 have thicker leads compared to FQP 50N06, I had to drill bigger holes for them to slide in on another inverter upgrade.
But yeah, the leads do not look like they can take that much current.

I mocked up fitting some TO-247 packed mosfets in the space made for TO-220 and they can fit with some mods.
The board holes need to be enlarged for the leads and the heat sink bolt holes for the clamps would need to be drilled higher up.

Maybe for the fun of it, I might get some IRFP3077, they have 200 amp continuous rating, pulse rating goes from 390 to 850, 200 watts versus 340 watts heat dissipation.
So they should be able to handle significant more current.

http://www.irf.com/product-info/datasheets/data/irfp3077pbf.pdf

IRF does list this info, so maybe these are more honest amp numbers.
Calculated continuous current based on maximum allowable junction
temperature. Bond wire current limit is 120A. Note that current
limitations arising from heating of the device leads may occur with
some lead mounting arrangements
 

shortbus

Joined Sep 30, 2009
10,045
Read the fine print in the 'notes' under the data also. Their idea of "pulse rating" is misleading too. Most say the pulse rating is for "400Hz at 2% duty cycle", that means it will handle the pulse amps at a 2% on time of 400Hz. If my math is correct, that means for 0.00005 second out of every 0.0025 second. Not so impressive when it reads that way.:)
 
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