Mosfet getting hot (140° F)even when in spec

Thread Starter

Kuhny1

Joined Jul 2, 2015
20
How much current can the microprocessor pin source and sink? This current needs to charge and discharge a pretty big capacitance which will slow things down a bit.. Try to look at the waveform with an oscilloscope if you can. You might be greatly surprised at how slow it is and how much time is spent in the linear region.
It can supply about 40mA/IO pin. Hmmm, i guess that does seem a bit low. should i try driving the FET with a little 2n2222?
 

ronv

Joined Nov 12, 2008
3,770
It can supply about 40mA/IO pin. Hmmm, i guess that does seem a bit low. should i try driving the FET with a little 2n2222?
I think people are confused. You stated the temperature at 140F - not C, but F.
The FET is only dissipating about .3 watts not 48 watts - that's the coil.
You can calculate the switching time by dividing Nc by drive current. So with a Nc of 7 from the data sheet the switching time should be around 175 nanoseconds. Not a problem considering it only switches at a 1 ms rate.
If it bothers you put a little heat sink on it.
 

Thread Starter

Kuhny1

Joined Jul 2, 2015
20
I think people are confused. You stated the temperature at 140F - not C, but F.
The FET is only dissipating about .3 watts not 48 watts - that's the coil.
You can calculate the switching time by dividing Nc by drive current. So with a Nc of 7 from the data sheet the switching time should be around 175 nanoseconds. Not a problem considering it only switches at a 1 ms rate.
If it bothers you put a little heat sink on it.

Yeah, I probably should of used C. I've been using F, so ill just stick with it for the rest of the thread I have kept the FET running for a while (about 50 seconds to a minute) and the temp climbed to about 173F and was still climbing. Not only that, but the whole processor browns out and resets if I connect the Source of the FET directly to ground. I have to have the Source go through my meter and then to ground (with the meter on current reading of course), then it runs until it I disconnect it so it doesn't do damage to itself or the board.
 

Thread Starter

Kuhny1

Joined Jul 2, 2015
20
More details about this coil, please. What inductance?

I cant say the exact but its roughly 60 turns on very thin magnet wire on a iron core.

I would say its probably around 0.0183 H. I made the electromagnet so i can't really tell because its on a oddly shaped core.

Im going to go to bed now, I'll check back when I wake up :)

Thanks for every ones help!
 

OBW0549

Joined Mar 2, 2015
3,566
...and then there was a nice puff of smoke from the coil...
Are you still using that coil? Because that puff of smoke was probably insulation burning-- which may mean that the coil is now partially or completely shorted, making its resistance a LOT lower than 4 ohms. And why did the coil smoke at all? Didn't you design it for 12V/3A continuous (i.e., wire size)?

Im stumped as it stands right now lol.
So am I. That FET should run barely warm to the touch. I'm ruling out switching losses caused by inadequate FET drive from the microcontroller; I was playing around with a similar circuit a few months ago to test a PID algorithm on an Arduino PWMing an IRLZ34 MOSFET connected to a heating element, and the Arduino had no trouble switching the MOSFET on and off at the PWM frequency.

I will put all 4 FETs in parallel if I have to.:cool:
You shouldn't have to; one FET should handle this job easily.

Do you have access to an oscilloscope? If you do, check the waveforms in the circuit; that will most likely tell you what's wrong.

I'm still thinking wiring error (i.e., the circuit you actually have is not what you think you have), or possibly shorted coil.
 
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Papabravo

Joined Feb 24, 2006
18,128
To me the strongest evidence is the construction of the circuit BEFORE the production of the schematic. The chances of a wiring error are substantial.
 

Thread Starter

Kuhny1

Joined Jul 2, 2015
20
Are you still using that coil? Because that puff of smoke was probably insulation burning-- which may mean that the coil is now partially or completely shorted, making its resistance a LOT lower than 4 ohms. And why did the coil smoke at all? Didn't you design it for 12V/3A continuous (i.e., wire size)?




So am I. That FET should run barely warm to the touch. I'm ruling out switching losses caused by inadequate FET drive from the microcontroller; I was playing around with a similar circuit a few months ago to test a PID algorithm on an Arduino PWMing an IRLZ34 MOSFET connected to a heating element, and the Arduino had no trouble switching the MOSFET on and off at the PWM frequency.


You shouldn't have to; one FET should handle this job easily.

Do you have access to an oscilloscope? If you do, check the waveforms in the circuit; that will most likely tell you what's wrong.

I'm still thinking wiring error (i.e., the circuit you actually have is not what you think you have), or possibly shorted coil.
I wound a new coil after that happened (yes, it was the insulation burning off XD), the reason it happened was because of a wiring problem.

And I dont think that it can handle 3A at 12v but its not running continuous. Whats weird is that, when i measured the current going through the FET it only averaged about 500mA. Unfortunately i dont have access to a scope, but I'm saving up for one now. And being 17, working at minimum wage will make it take a while to get it.

I'll go rewire everything and see if that helps.

Well... As I was tearing out the wiring, I saw that I had the positive going through FET instead of ground! Would that make heat because of the internal diode? If thats the case then I'Il Invert my code, and make the ground go through the FET and then that should fix it right?

So it was actually wired like this...
 

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OBW0549

Joined Mar 2, 2015
3,566
So it's in the ballpark.
Max on resistance of the FET with logic level drive = 16 milliohms
4.3 amps squared X.016 ~ .3 watts.
62C / watt junction to case ~ 18C or 64 F rise plus 76F for the room ~140F.
60C is okay, but uncomfortable. You can add a little heat sink if you like, but it will run a long time at 60C.
I think the diode is okay.
18°C is 32°F, not 64°F; that, plus the room temperature, gives 108°F-- barely warm to the touch. (And that 62°C/Watt thermal resistance is the junction-to-ambient, not the junction-to-case, which is only 2.3°C/Watt.)
 

Thread Starter

Kuhny1

Joined Jul 2, 2015
20
18°C is 32°F, not 64°F; that, plus the room temperature, gives 108°F-- barely warm to the touch. (And that 62°C/Watt thermal resistance is the junction-to-ambient, not the junction-to-case, which is only 2.3°C/Watt.)

So apparently when I fried the first coil, that was the FET working correctly and wired correctly, but it was on too long (on the programming side) and it fried the coil and the FET just shrugged it off. The coil draws about 5 amps. The wire simply cant handle that. And what do you know, the FET stays cool; I cant even pick it up on the thermal camera.

Edit: Once I did the above, I noticed that I had the one end of the coil into the source of the mosfet, making the diode inside keep the coil on 100% of the time, frying the coil. So now i put the coil on the drain and the FET runs cool and the coil doesnt fry but will if i keep it on to long. So now im going to make a stronger coil :)

Thanks for all your help everyone!

Problem solved :D
 
Last edited:

Papabravo

Joined Feb 24, 2006
18,128
18°C is 32°F, not 64°F; that, plus the room temperature, gives 108°F-- barely warm to the touch. (And that 62°C/Watt thermal resistance is the junction-to-ambient, not the junction-to-case, which is only 2.3°C/Watt.)
Excuse me, but 0 °C is 32 °F and 18 °C is 64.4 °F
 

OBW0549

Joined Mar 2, 2015
3,566
Excuse me, but 0 °C is 32 °F and 18 °C is 64.4 °F
You're excused.

We're not talking here about the alignment of the Celsius and Fahrenheit temperature scales; if we were, you'd certainly be right. But we're not; we're talking about temperature rise above ambient, in which each increase of 1 degree Celsius corresponds to an increase of 1.8 degrees Fahrenheit.

My 108°F result is correct (assuming, as ronv did, an ambient temperature of 76°F).
 

SLK001

Joined Nov 29, 2011
1,548
Edit: Once I did the above, I noticed that I had the one end of the coil into the source of the mosfet, making the diode inside keep the coil on 100% of the time, frying the coil. So now i put the coil on the drain and the FET runs cool and the coil doesnt fry but will if i keep it on to long. So now im going to make a stronger coil :)

Thanks for all your help everyone!

Problem solved :D
If you use an N-Channel MOSFET, it must be the element closest to ground (or LOW SIDE control). If you want a HIGH SIDE control, you need to switch to a P-Channel device.
 

ronv

Joined Nov 12, 2008
3,770
18°C is 32°F, not 64°F; that, plus the room temperature, gives 108°F-- barely warm to the touch. (And that 62°C/Watt thermal resistance is the junction-to-ambient, not the junction-to-case, which is only 2.3°C/Watt.)
Hmm, your right.:confused: Glad I didn't do the conversion all these years.:D
What does junction to case have to do with it his readings?
 

OBW0549

Joined Mar 2, 2015
3,566
Hmm, your right.:confused: Glad I didn't do the conversion all these years.:D
What does junction to case have to do with it his readings?
Not much, as the junction-to-case thermal resistance would, in this case, mean barely a 1 °C temperature difference between the two.

I only made that comment as a clarification, because in post #17 you had identified the 62 °C/Watt spec as the junction-to-case thermal resistance, whereas it is actually the junction-to-ambient thermal resistance. IOW you cited the right number, but called it the wrong thing. Nothing more.
 

ronv

Joined Nov 12, 2008
3,770
Not much, as the junction-to-case thermal resistance would, in this case, mean barely a 1 °C temperature difference between the two.

I only made that comment as a clarification, because in post #17 you had identified the 62 °C/Watt spec as the junction-to-case thermal resistance, whereas it is actually the junction-to-ambient thermal resistance. IOW you cited the right number, but called it the wrong thing. Nothing more.
Wow, I used up a weeks worth of mistakes on one thread that was a wild goose chase.:(
 
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