# Mosfet keeps getting stuck on



## CPLTECH (Sep 14, 2007)

You are to be applauded for your desire, but it becomes an issue of time [lots of it], money for blown parts [lots of it], test equipment [more than a DMM], and the list goes on. Some in the EV world have persevered and conquered the multitude of problems that arise due to the voltage we run at, due to the various motors involved, and the circuitry required to make it all go without it all going up in smoke a mile or two from home [literally]. I see that you are new to the forum and suggest you continue to stick around. Hang out at the “Controller” section, and note the trials & hardships Tesseract and software partner Qer have overcome to put a reliable product on the market. On the other end of the spectrum, there is the LogiSystems controller that many are disappointed with. And some others along the way. Learn to become an expert of the “Search” function.

There are a lot of good available products out there that give good performance and reliability. Go with them and enjoy your EV ride… now.


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## Russco (Dec 23, 2008)

Your 6 volt gate drive is way too low and operating the FET in an analog mode, so boom. 

Purchase a ready made controller. You'll be years ahead. 

Russ Kaufmann

RUSSCO Engineeering

http://russcoev.com


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## Snakub (Sep 8, 2008)

Is there any particular reason why you are switching your mosfet on and off manually? I would suggest you get some kind of PWM circuit even if it is a 555.


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## az_tom (Oct 23, 2009)

Snakub said:


> Is there any particular reason why you are switching your mosfet on and off manually? I would suggest you get some kind of PWM circuit even if it is a 555.


 
I went to the manual switch circuit just to test things out. I had a PIC generating a PWM waveform tied to the gate of the MOSFET. I did this just to see what would happen w/o using a mosfet driver. This circuit actually worked for some small DC motors just fine. I then tried this same circuit on my 28 Amp ebike motor and it worked at low amps, but once it started to pull some good amps, it fried the PIC and/or MOSFET. This is what I was trying to figure out, what was causing the PIC/mosfet to blow. I then removed the PIC from the equation by manually switching the MOSFET and I suffered similar problems. Up to this point it has been fun and I have learnt quite a bit. I have looked at the open source controller design and it is way more than I wish to do and understand. I was just curious if there was something simple that was blowing the mosfet. Other than my obvious lack of understanding. LOL! And yes, it has become obvious that this type of project is a major undertaking.


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## pyrogaz (Jan 3, 2010)

Like Russco says you need to raise the gate voltage, this will lower the mosfet's resistance making it less prone to failure. Also, have you got a heatsink on the Mosfet? It may be rated at 50A but depending on spec it may not even manage 10A without a heatsink.

It looks like you have no capacitors in the circuit? I've built several 24v 60A (150A peak) controllers for small electric endurance racers and use two 2200uF 100v capacitors between the +ve and -ve power rails, without these you get voltage spikes that will quickly destroy the mosfet.

Connecting the batteries to a common ground is correct, in the absence of a Fet driver pulling the gate to ground is sensible to prevent it floating.

When you used a PIC it will onlysource 20mA which is nowhere near enough to switch a Mosfet on quickly enough if using PWM, something like a TC4427 Mosfet driver is fairly cheap and works well at even 30Khz. Without it the Mosfet never switches on properly (exacerbated by the low gate drive voltage) and the PWM waveform will be more like a sawtooth.


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## az_tom (Oct 23, 2009)

pyrogaz said:


> Like Russco says you need to raise the gate voltage, this will lower the mosfet's resistance making it less prone to failure. Also, have you got a heatsink on the Mosfet? It may be rated at 50A but depending on spec it may not even manage 10A without a heatsink.
> 
> It looks like you have no capacitors in the circuit? I've built several 24v 60A (150A peak) controllers for small electric endurance racers and use two 2200uF 100v capacitors between the +ve and -ve power rails, without these you get voltage spikes that will quickly destroy the mosfet.
> 
> ...


 
Thanks for all of your input!


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## ngrimm (Oct 19, 2007)

I second the mosfet driver suggestion. I am using a Pic 4431 and three 4422 mosfet drivers for a three phase bldc motor project. Haven't fried any mosfets yet


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## jehan12413 (Feb 4, 2010)

Do you know how much gate current is flowing when you turn it on? I noticed you have no resistor between the battery and gate, I would try one between 75 and 120 ohms and see how it works. Also what is the peak current to the motor when you turn it on? You could be well over 100 amps.


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