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Mosfet failure mechanism

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ragnar

Thu Apr 19 2007, 05:03AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Transient voltage and current spikes on the drain due to your 100nF( !! ) capacitor across drain and source.

Dr. Dark Current

Thu Apr 19 2007, 02:30PM

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

New fet- guess what- popped again. Again all pins shorted together. This time it died in a few seconds. I have absolutely no idea what could be causing that, in my previous drivers I also used the irfp250 also with 100n cap across d-s, used to much greater powers and with greater supply voltage than now, and it never popped just got hot. The only thing I changed is the drive ic- before I used tl494 and now sg3525, the only real difference is that the latter has a push-pull output stage. I think I will scope the gate waveform, as I have no idea what else could be causing the deaths.

edit> gate waveform looks perfect. I'm out of ideas

ragnar

Thu Apr 19 2007, 11:53PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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That 100nF cap across drain/source sounds far too big for any purpose.

Dr. Dark Current

Fri Apr 20 2007, 01:10PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
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BlackPlasma wrote ...

That 100nF cap across drain/source sounds far too big for any purpose.

I tried out many caps and I get the longest arcs with least mosfet heating (and with least input power) with the 100n cap.
btw I tried it with irf740 which is 400v, and so far it hasn't popped, but gets noticably warm. What I don't get is why the irfp250's were popping, even with the 100 cap it should avalanche and heat first before the death no?

ShawnLG

Fri Apr 20 2007, 02:09PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
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Why do you guys use a capacitor across the source and drain?

Ken M.

Fri Apr 20 2007, 10:18PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
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I'd say its because current would go through the coil (producing a flux feild) and charge the cap, once the cap is charged current stops flowing. When the fet is turned on the current flows through the coil as well as discharges the cap. At least thats my guess on the system.

Marko

Fri Apr 20 2007, 10:41PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

The snubber capacitor reduces mosfet's turn-off switching loss by slowing down dv/dt across mosfet, wich can then turn off while switching much smaller current.

This also means that maximum amplitude of voltage spike that will be seen across the primary is reduced.

Jmartis; if you are just using the capacitor directly across the DS, it won't do much alone.
The capacitor discharges it's energy back into mosfet each time it turns on again, so you'l just convert some possible avalanche loss into resistive loss while having reduced output and efficiency.

With proper DCR snubber where the cap is blocked by diode and discharged with paralell resistor you could effectively move that heat away from the mosfet, but that's all you can do.

Zener clamp would do pretty much the same.

Generally, with stepup flyback converters and fast mosfets I don't prefer to use any snubber at all. I just use a device of adequate voltage rating and this seems to give best efficiency regardless of higher on resistance.

Dr. Dark Current

Sat Apr 21 2007, 07:01AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
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Firkragg, I don't know any theory behind this, but why do you think I use the capacitor? It's because it increases output voltage and decreases fet's heating greatly. I have no idea why is that (my explantation is that the cap resonates with the primary), as well I have no idea why the fets were dying. That's the reason why I will probably use different topology in my driver. (my original goal was an adjustable driver with high voltage output from the flyback at low current, but with this ic I will hopefully be able to achieve that too with an offline halfbridge driver using low duty cycles

)

Sulaiman

Sat Apr 21 2007, 07:29AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Every time the switching transistor turns ON
it short-circuits the 100nF capacitor that you apparently have from Drain to Source.
If it's a good quality capacitor then the peak current will exceed the current rating for the IRFP250,
so step one is ; put a 10 Ohm resistor in series with the 100 nF capacitor.

If you look at the datasheet for IRFP250N you will see that the maximum repetitive avalance energy is 21 mJ
so you MUST ensure (0.5 x Lpri x Ipri x Ipri) < 21 mJ

Personally I try to never avalanche my switching transistors so some kind of circuit is required to ensure this.

A diode-capacitor-resistor system works fairly well, but I prefer a zener (or TVS) voltage clamp.

With a simple circuit of the kind that I use ... (555 + MOSFET)
you should realise that the circuit is CONSTANT POWER, REGARDLESS OF THE LOAD.

So if the power isn't going into a load (open circuit secondary for example), where does it go?
Without some kind of load it goes to the switching transistor.

For hobby use I use several 1.5KE TVS epoxied to a heatsink to absorb any 'spare' power.
In your case use 1.5KE150CP or similar across the PRIMARY. (not Drain-Source)

Dr. Dark Current

Sat Apr 21 2007, 07:44AM

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

I'll definately try the series resistor, thanks Sulaiman!

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