MOSFET getting too hot

Thread Starter

Jack J. He

Joined Apr 24, 2017
9
Hello,
I am working on a circuit for some heating elements and I am trying to control them in order to limit the amount of heat they produce. To do so I am using a bunch of N-Channel MOSFETs that I control using a teensy 3.5's PWM signal (488 Hz). The teensy cannot produce a high enough voltage to be able to run the nmos effectively and as such I am using a 3.3V to 5V level shifter to provide a high enough gate voltage to reduce Rds,on. I am powering the heaters with a 24V power supply and I am using a 5V DC-DC converter to provide power to the teensy and to provide a reference voltage to the level shifter. I have a 100 Ohm resistor from level shifter output to gate to limit current and a 10kOhm resistor from gate to source to allow the mosfet to discharge.

3.3V to 5V level shifter: http://www.mouser.com/ProductDetail/SparkFun-Electronics/BOB-12009/?qs=WyAARYrbSnb%2bGYLWggQnjQ==&gclid=Cj0KCQjwi97NBRD1ARIsAPXVWWBRT5mQV-07DzXEsrCvUHC7szT0kbg3AMQUTV_p-2UGTRVs6k0LoRkaAlPpEALw_wcB

MOSFET datasheet: http://www.vishay.com/docs/91300/91300.pdf

What I found is that the MOSFETs get really hot when I run them. I was expecting them to warm up but this exceeds what I calculated. At 5V, Rds,on is supposed to be 77mOhm and since I am passing at most 2 amps I should get at most a 19C temperature increase for a thermal resistance of 62C/W. I even added TO220 heatsinks to help them stay at room temperature. I am kind of stumped and not sure of what the problem is. The only solution I can currently think of is to add more MOSFETs to spread the current. Is possible that the level shifter is the problem and it is not sending a high enough voltage to gate? What is a good way to troubleshoot this?
 

AlbertHall

Joined Jun 4, 2014
11,960
The level shifter high voltage side output is pulled up only by a resistor (maybe 10k - the datasheet link on the mouser website is faulty). As you have a 10k to 0V this will seriously limit the gate voltage on your MOSFET gate. The level shifter will pull the output down close to 0V if your input goes that low so it may work correctly if you just remove your 10k to 0V.
 

Thread Starter

Jack J. He

Joined Apr 24, 2017
9
You are right, the high side output is pulled up by a 10kOhm resistor on the shifter. So removing the 10kOhm resistor will help increase the voltage? How can I provide a path for the gate to discharge in this case?
 

AlbertHall

Joined Jun 4, 2014
11,960
I just measured the drain to source voltage with the mosfet on and I found that it was about 1.2V. This time I was passing about 1A through it so this would give an Rds,on of 1.2Ohm which is way higher than it should be.
That's not too bad if it has only 2.5V on the gate!
 

Thread Starter

Jack J. He

Joined Apr 24, 2017
9
The level shifter high voltage side output is pulled up only by a resistor (maybe 10k - the datasheet link on the mouser website is faulty). As you have a 10k to 0V this will seriously limit the gate voltage on your MOSFET gate. The level shifter will pull the output down close to 0V if your input goes that low so it may work correctly if you just remove your 10k to 0V.
If I remove the 10kOhm resistor won't the MOSFET stay on?
 
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