# When do MOSFET's generate the most heat?

Discussion in 'General Electronics Chat' started by StToonz, Dec 7, 2012.

1. ### StToonz Thread Starter New Member

Nov 30, 2012
14
0
Hey Guys,

I have an interesting question that i am having trouble researching online.

when do MOSFET's generate the most heat?

Thanks for you help in advance!

2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
727
When switching on or off, there are short periods where the channel resistance is low, but not fully to RDSon. The main factor is when conducting currents, P=I^2R, with R being RDSon, and I being the current through the MOSFET.

ErnieM likes this.
3. ### gerty AAC Fanatic!

Aug 30, 2007
1,234
342
I would say when they are being used in the linear region, partially on with a
voltage drop across them that causes the heat.

4. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,395
1,195
When its used the "most incorrectly"
When its being asked to conduct the largest amount of current.
I^2xR

5. ### GopherT AAC Fanatic!

Nov 23, 2012
8,025
6,786
I agree with Gerty. MOSFETs are best used when they are ON or OFF. No heat to dissipate in the off condition. Only the little ON resistance X current squared gives you the heat dissipated when completely on.

The real heat is generated when partially on, (Linear region). The voltage difference measured across the MOSFET D and S AND current flow through the MOSFET means power is put into the MOSFET. It has to dissipate that power somehow (heat, light or work). The MOSFET doesn't do work or generate light so it will only get hot.

StToonz likes this.
6. ### Papabravo Expert

Feb 24, 2006
11,703
2,464
Twinkle twinkle little star,
Power's equal I squared R.

That is a somewhat flippant way of saying:
Heat is proportional to power.
Power is dissipated when a current flows through a resistance.

Changing the resistance of the channel for the same current causes a linear increase in power.
Changing the current in the channel for the same resistance causes a quadratic increase in power.

7. ### StToonz Thread Starter New Member

Nov 30, 2012
14
0
wow thank you all for the quick response. this helped me out a bunch. and gave me somewhere to start answering this question.

I love this site more and more with each post.

Thanks

8. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,953
1,822
thatoneguy got it correct.

Back when I was designing switching power supplies we almost ignored the "on" state power. The major power loss in a switched device are the turn on (small) and turn off (huge) losses.

Of course, if you're running in a linear mode there are no switching losses, but lots of power being burned off.