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When does TO220F FET blow up?


Advanced Member level 5
Jun 13, 2021
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Hi Supposing you have a TO220F FET , which you wire to a gate drive and current source so it dissipates 3W
The TO220F is stood up in free air, 23degC. No heatsink, No copper pour round its leg solder points.
How long would you expect it to work before it went bang?

Tjmax is 150C
Rthermal is 62.5 C / W

So Tjunc = 23 + 62.5 x 3 = 210.5 C

So guessing wont take long.....depends on how much heat is being
removed by PCB layout thru leads, in your PCB case not much. Also
this assumes stagnant air velocity.

Regards, Dana.
Some experiences :

Section 8 shows some failure modes :

Regards, Dana.
--- Updated ---

Of course there is always considerations, in time, eg. thermal mass, that give rise to
thermal time constant :

Regards, Dana.
Last edited:

never done this before, still I try to find a physical/mathematical solution.

you have 3W of heat.
* One part of it is disspated as heat into the environment
* the other part is used to rise the temperature of the TO220.

For ease of math, let´s assume the dissipated heat into the environment is linear with temperature difference to ambient temperature.
And let´s just use the heat capacity of a (random) TO-220


Then the critical temperature of 150°C is after about 60 seconds.

Attached a ZIP file containing the EXCEL97 file. Should open with older and newer Excel versions.



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Last edited:
Thanks, anyway...the STF3LN80K5 blew up on me.....after an 8 hour day of running the 20W flyback in overload and short ccting it repeatedly, the STF3LN80K5 eventually blew up after being on with the flyback having 40W output (overload)...and had been on for 10mins that time. It was running on 90VAC and was in pretty deep continuous mode at this overload level. The gate series resistor was was the only value i had at the time....but Ciss is only 100pF for this FET anyway. The RMS FET current would have been around 700mA RMS...but the switching losses would have been appreciable since it was in deep CCM.

I had also been running it with 59W output from 265VAC.....for kind of 10 mins at a time, repeatedly, throughout the day...and it didnt blow up then...but it was still in DCM in that case.
(overload shutdown was ~60W at 265VAC and ~40W at 90VAC).

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