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Topic: My mosfet is heating, can't understand why (Read 11760 times) previous topic - next topic

MarkT

Quote from:  link=topic=124963.msg940899#msg940899 date=1349098312



MOSFETS only get hot when they're not completely switched on. When they're completely on or off their resistance is so low/high that the power dissipated by the MOSFET is negligible.



WHAT?

When a MOSFET is completely on its resistance will be <= Rds(on) [max]. (Provided Vds << Vth).  Its power dissipation is I-squared-R which can be _extremely_ large.  It all depends on the values of I and R.  30A and 0.01ohm gives 9W, 10A and 0.2ohm gives 50W, 4A and 0.006 ohm gives 96mW.   96mW needs no heatsink, 9W needs a big heatsink, 50W requires large heatsink and/or water- or forced-air-cooling.

You have to _calculate_ the on-state dissipation.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

fungus


Quote from:  link=topic=124963.msg940899#msg940899 date=1349098312



MOSFETS only get hot when they're not completely switched on. When they're completely on or off their resistance is so low/high that the power dissipated by the MOSFET is negligible.



WHAT?

When a MOSFET is completely on its resistance will be <= Rds(on) [max]. (Provided Vds << Vth).  Its power dissipation is I-squared-R which can be _extremely_ large.  It all depends on the values of I and R.  30A and 0.01ohm gives 9W, 10A and 0.2ohm gives 50W, 4A and 0.006 ohm gives 96mW.   96mW needs no heatsink, 9W needs a big heatsink, 50W requires large heatsink and/or water- or forced-air-cooling.

You have to _calculate_ the on-state dissipation.


Ok, maybe I shouldn't have said "only". OTOH 30A is a monstrous current.

Advanced Arduino

dhenry

Quote
OTOH 30A is a monstrous current.


For <$20, you get a ESC, built around a MEGA8, that switches that much current, FAST too.

MarkT

All the time people get MOSFET overheating issues here because they don't know things like :

1)  power = I-squared-R(on)   is the way to choose a MOSFET.
2)  Vthr is not Vgs(on)
3)  > 0.3W necessitates a heatsink
4)  > 5W needs quite a large heatsink or a fan
5)  An IRF520 is totally obsolete
6)  The max current rating is irrelevant unless you are using a water-cooled heatsink!

Its very common to see people believe a 10A MOSFET will take 5A continuous without any thermal issues(!)
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

dhenry

Quote
6)  The max current rating is irrelevant unless you are using a water-cooled heatsink!


Absolutely correct.

Those current ratings are pulsed, and with the case temperature (typically) at 25c.

Effectively, you need absolutely perfect case-to-ambient heat transfer for that rating to be relevant.

retrolefty


All the time people get MOSFET overheating issues here because they don't know things like :

1)  power = I-squared-R(on)   is the way to choose a MOSFET.
2)  Vthr is not Vgs(on)
3)  > 0.3W necessitates a heatsink
4)  > 5W needs quite a large heatsink or a fan
5)  An IRF520 is totally obsolete
6)  The max current rating is irrelevant unless you are using a water-cooled heatsink!

Its very common to see people believe a 10A MOSFET will take 5A continuous without any thermal issues(!)


I would add:

7) Gate drive current can be significant in high power (high gate capacitance) mosfets switching at high speed and often a purpose designed high current gate driver chip is called for.
8 ) Higher switching frequencies increases device dissapation as more transitions accure per unit of time and becomes a significant contributer to device losses.


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