# MOSFET keep burning



## rwaudio (May 22, 2008)

nirbaer said:


> Hi,
> I built a small controller for 24VDC/350W motor with PWM input for gokart, used 2x IRF3205 mosfets which are rated 110A (link)
> They have good contact to the heat sink and they are keep burning as I test the controller (not always). I am driving them with only few amps.
> 
> Which mosfet would better fit my needs?


Assuming the design is sound it's possible the mosfets are fake, ebay isn't the most reliable place for IC's etc. 

By "good contact to the heat sink" do you mean they are touching? Or are clamped/screwed/bolted along with heatsink compound???


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## nirbaer (May 25, 2010)

There is a screwed metal holding them tight and I also used thermal grease.

I was not aware of fake mosfets, can you recommend a good source to buy them that ships international?


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## jackbauer (Jan 12, 2008)

How are you driving the fets?


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## nirbaer (May 25, 2010)

I am using 4093 schmitt trigger as a pwm generator and driving both mosfets with 100 ohm serial resistor (R1 and R2 in the attached schematics are actually this resistor).


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## jackbauer (Jan 12, 2008)

As they say on Mythbusters "Well there's your problem!" 

You are not driving the fets hard enough. They are spending time in the active region , acting like resistors and blowing up. You would need a mosfet driver ic between the pwm output and the fet gates.


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## nirbaer (May 25, 2010)

That's the answer I hoped for.
I will have to do some reading, anyhow from a quick search here I found the IR2110 is seldom used, will it fit?


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## globivogel (May 5, 2010)

There are hundreds of gate drivers; I think, the Ir2110 will fit. 
It,s a low and high side driver. 
If you only need the low side driver, you can leave the high side part unconnected; you optionally can connect the pin Vs with COM.
Place the driver close to the FET's, there should be short connections.

When you replace the two upper diodes with FET's in a later time, you could make regenerative braking.
But for the first step, I would use the diodes. (BTW the diodes should be fast types)

For braking, you short the motor with the relais.
If the motor runs fast, and you short it, there can be a high current,
which isn't good for the motor.


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## nirbaer (May 25, 2010)

Thanks! this is a very helpful info.

The brake relay is not done yet but I plan to use power 10 ohm resistor instead of a short for this reason.


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