Airsoft MOSFET Burning out

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ian field

Joined Oct 27, 2012
6,536
I build little MOSFET systems for Airsoft guns to save the trigger from arching and decrease overall resistance. I having the issue of the FETs getting really hot as the replica is fired, until it finally locks closed and burns itself out.

http://unconventional-airsoft.com/wp-content/uploads/2009/08/Simple-MOSFET-Setup-2009-06-22.jpg


I have been building these little trigger chips for some time now, but one particular gun is giving me some trouble. I have tried five different versions of the circuit, making slight variations to the resistors and solder arrangements.
The gun houses an m120 spring on a standard gear ratio with a stock motor, which means that there is a smallish ferrous motor pulling an 8 inch 80lb spring. The system draws about 35A sustained with an initial spike somewhere around 350A, and the battery can safely put out 65A sustained.

I've been dealing with this problem for far too long and am coming to this community in an effort to avoid spending $40 on a commercial Airsoft trigger controller. Please help me figure out why my FETs are burning out.

Super simple system. I'm not very familiar with schematics, so some of this may be off, but I am following the basic idea of the video linked above. The Diode is 18V, placed to absorb voltage spikes.

View attachment 111202
I didn't bother doing the equations - but your resistor pair could be cutting it fine on VGSthr.

Likely causes of getting too hot include; too much current generally, not enough gate voltage to get RDSon down or in high speed switching - prolonged transitions between on and off.

If the back emf is excessive - failing short happens without it getting hot.

If the only issue is contact arcing, you can use the MOSFET in grounded gate configuration. The gate is directly to Vcc and the source is grounded through the switch, clearly the switch has to carry the full current - but as long as the MOSFET is rated to handle any back emf; it stops at the MOSFET.

I used this for a transistor assisted ign contactor unit on a small motorcycle - there was no spare current for "wetting" the ponts.
 

walkura

Joined Jan 11, 2007
3
With currents like this it is important to open your mosfet as fast as possible, and keep it cooled.
I know from experience that this mosfet can handle this current given that your cooling is good enough,
the gate drive is in order and the frequency not to high (not to much transitions)
But there is as always a but, the current specified in the datasheet is the current the mosfet inside can switch.
Meaning, without wires and kept at 25 degrees,
in real life really high currents the leads will generate more heat then the mosfet and heat it.
(Look in the IRF white papers about switching high currents)
Use a gate driver like the Microchip TC442* and like said before, put a clamping diode over the motor, devide the current over 2 mosfets.
(even them keep them cool)
How long is this surge current, how much milliseconds are we talking about?
 

DickCappels

Joined Aug 21, 2008
10,169
The time has come to put an end to this discussion. There is no reason why those interested cannot continue in private, but after some discussion, it is clear that this tread is in violation of the user agreement. To wit:

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