Problem Driving a MOSFET


Joined Feb 8, 2018
From what I can find, the package-to-ambient thermal resistance for a DIP-8 is about 70°C/W. A temperature rise of 32°C means the power dissipated is about 0.46 W.

The driver Icc is 6 mA maximum with no load, so the power contribution is 72 mW with Vcc of 12 V. The IRED power with the measured Vf and 5 V through 330 ohms is about 15 mW. The total power dissipation of the driver is therefore about 87 mW and the expected package temperature rise should be about 6°C. Junction to ambient thermal resistance for the package is typically about 110°C/W, so even the die temperature rise should be no more than 10°C. 32°C package temperature rise is far too much (not destructive, but indicative of something being wrong).

I return to the FET temperature rise as an indicator that something is not right. If the gate is being driven with a static 12 volts, the ON resistance of the FET should be no more than about 5 milliohms, and that is allowing for the die being hotter than it should be. 4 amperes and 5 mΩ yield 80 mW. At that power the case temperature rise of the FET should be next to nothing. 10°C rise is far higher than expected, suggesting that either the temperature measurement is wrong or there is significant switching loss because there is some un-detected switching that should not be happening.

If an oscilloscope is not available, put a small capacitor, such as 10 nF, in series with your meter and measure the AC voltage between drain and source of the FET when the FET should be ON in steady-state. It will take a few seconds for meter reading to stabilize, but it should be zero. If it isn't, the FET is switching when no switching is expected. This method can also be used to check the output of the driver.

Thread Starter


Joined Jul 6, 2018
Hi I have tried the 0.1uF cap on pins 5 & 8. Unfortunately It does not make any difference to the Opto running hot, However I will keep it there as it is in the Datasheet, Thanks.
Thanks EBP for your help. I have checked the temperatures again as follows

State Regulator 555 Opto MOSFET
All power OFF 27 27 27 27
Power on FET Off 30 30 30 27
Power on FET On 35 34 52 33

I have also checked the AC voltage with a meter and 0.1uF cap as described and get 0.062v (Source and Drain on FET) also I get 0.056v (Gate on FET and Pin 8 on OPTO). Does this make any sense?
Thanks for all your help, Colin


Joined Jun 5, 2013
If it is getting hot with the latest schematic I suspect you damaged the chip with your previous wrong connections.


Thread Starter


Joined Jul 6, 2018
I have now tested with a new Optocoupler. Temperatures are all fine now, so I must have cooked the part in my earlier attempts. I would like to thank everyone who has helped me with this problem. The results are great.
All the best