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Mark

Wed Jun 25 2008, 09:01AM

Registered Member #558 Joined: Thu Mar 01 2007, 10:28AM
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Posts: 9

The resistor I'm using is 47ohm 3Watts. So how delicate the FET is for overvoltage? I'm powering the drive part..TL494 and componets around it with a benchsupply. It has analog reading hence not very accurete, so the voltage can be easily set accidently to 13 or even 14volts...but that can't kill the FET, can it?
For driving FEts and Coil I use another bench supply with higher amp ratings. Connecting both to same ground of course...like in schematic.

From datasheet of IRF540

Absolute maximum ratings
VGS Gate- source Voltage Â± 20 V

So the overvoltage can't be the reason why already three FETs have been destoyed

[Moderator edit: fixed double-post]

Dr. Dark Current

Wed Jun 25 2008, 09:53AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Mark wrote ...

The resistor I'm using is 47ohm 3Watts. So how delicate the FET is for overvoltage? I'm powering the drive part..TL494 and componets around it with a benchsupply. It has analog reading hence not very accurete, so the voltage can be easily set accidently to 13 or even 14volts...but that can't kill the FET, can it?
For driving FEts and Coil I use another bench supply with higher amp ratings. Connecting both to same ground of course...like in schematic.

your FET dies with just gate driver powered or with the bridge powered too? If with just gate drive powered, you probably had a damaged FET in the first place.

If with the bridge powered too, does it die with or without load on the output?

Mark

Wed Jun 25 2008, 02:49PM

Registered Member #558 Joined: Thu Mar 01 2007, 10:28AM
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Initially I started with load connected and both powers on. After few seconds the 47ohm resitor started to smoke. I mesured the FET - it was shorted. I replaced it with the a new one...I tested it with ohmmeter and drived it with 12V to gate. Multimeter showed D-S 0.1Ohm...so it was definetely working. But same thig happened again. I don'd understand?? I have checked the circuit dozens of times. And the design is good as well I suppose.
What is the purpose for diode and 47ohm meter -to bias the second FET?

Mark

Fri Jun 27 2008, 03:39AM

Registered Member #558 Joined: Thu Mar 01 2007, 10:28AM
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Crap, I found the problem. The heatsink insulation of FETs had failed!! Maybe the nut was too tight or so.
But now it works - with at least two inch spark.

Antonio

Fri Jun 27 2008, 12:00PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

A problem with this circuit are that the upper mosfet is still conducting when the lower mosfet turns on, drawing a large current spike from the bridge capacitors and the power supply.
There is also no way of controlling the output power. All or nothing.
What is the initial spark length between smooth surfaces (balls)?

Dr. Dark Current

Fri Jun 27 2008, 12:04PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Antonio wrote ...

A problem with this circuit are that the upper mosfet is still conducting when the lower mosfet turns on, drawing a large current spike from the bridge capacitors and the power supply.

As soon as the lower FET starts turning on, it turns the upper one off. When the lower fet is fully on, the upper one is fully off. The cross conduction is very little.

Antonio wrote ...

There is also no way of controlling the output power. All or nothing.

There is, vary the duty cycle.

Mark

Sat Jun 28 2008, 12:32PM

Registered Member #558 Joined: Thu Mar 01 2007, 10:28AM
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Posts: 9

So there is a short moment where both FETs are on? That might be the reason why FETs are blowing constantly after running the spark (around 2inch) for more that 5 sec. Im driving FETs with 12-13V so D-S resistance should be very low and FETs shouldn't even warm up. I have limited power to 4amp with 32V. So even short circuit shouldn't break FETs (IRF540)

Dr. Dark Current

Sat Jun 28 2008, 12:57PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Mark wrote ...

So there is a short moment where both FETs are on? That might be the reason why FETs are blowing constantly after running the spark (around 2inch) for more that 5 sec. Im driving FETs with 12-13V so D-S resistance should be very low and FETs shouldn't even warm up. I have limited power to 4amp with 32V. So even short circuit shouldn't break FETs (IRF540)

1) there is, but it is very little (they are not fully on at that moment) and it will NOT cause any problems.
2) The reactive current in primary circuit can be well many times your supply current. I would advise use higher supply voltage (safe up to about 70% of your FET voltage rating, in your case 70V) and increase primary turns accordingly.

Mark

Sun Jun 29 2008, 11:50AM

Registered Member #558 Joined: Thu Mar 01 2007, 10:28AM
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Posts: 9

Thanks. Much better now. I changed powersource to 75Volts and primry turns from 5 to 14. Current is less than 1 amp.

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