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Increasing the avalanche capabilities of a MOSFET

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Rey

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It's what it says on the tin. Does anybody have any ideas how to do this? Is it possible to use two MOSFETs to double the avalanche rating?
 
In series it's possible but two Mosfets in series driving the gates would be problematic.
 
Snubber cap between drain and source. Use a small value resistor if the current is too high for the cap. This will reduce the voltage ringing when the MOSFET turns on and off.
If you're having a di/dt issue, you might need a very low value inductor inline with the drain or source to slow it down.

What are you driving with it, and what kind of avalanche issues are you having?
 
It's being driven by a 555 timer, and the item being driven is a flyback transformer. What should the capacitance value be? And will it slow down my switching times? It runs at about 11 kHz.

Let's just say that the problem I'm having is..'Explosive'...
 
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Simple guideline is to make it twice the output capacitance of the switch (Coss on the datasheet) and at least twice the voltage your driving.

The flyback transformers leakage inductance makes it worse too, but I'm not sure how much bigger to make the cap to make up for it. If you know what the leakage inductance is, I think you can factor that in, but not sure where the formulas are for that.

It won't change the switching speed, but the disadvantage is you have to dissipate the amount of power in the cap every cycle, so your efficiency goes down as the cap size goes up.

Did you hand wind the transformer, or is it commercial?
 
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It's a TV flyback, just the primary is hand wound. I'm pretty sure that the capacitance for an irfp460 is 3000 pf, although Im not able to look up the datasheet right now. Also, I have no clue what the exact avalanche voltage is, just that a 45 volt input with 8 turns on the primary caused explosive ignition of the MOSFET and surrounding breadboard (Ruined!). The MOSFET was an irfp460 with a Vds of 500v. Perhaps that can give you a general idea of the avalanche voltage.

Saying as how it blew up that MOSFET, I'm a bit reluctant to just go hooking my fluke up to this thing. Any other options?
 
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It has an output capacitance of 870pF, so try something close to 1740pF. Although since you're driving a flyback, make it the next larger common value.

I'm not sure this is a voltage avalanche condition though. If you're only driving 45V, even a flyback shouldn't hit over 500V in ringing. I would still put the snubber caps on though.

8 turns of what gauge wire? What's the DC resistance of the primary? Maybe it's a di/dt issue?

I'm assuming they are heatsinked as well?

How are you're gates setup? Could there be transients being induced into them from the transformer being to close?
 
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The wire is 18 guage, and the MOSFET is protected by a CPU heatsink with a fan. The DC resistance is about 2 ohms I think, I'll take a look tomorrow. What exactly is di/dt? The flyback is sitting in a plastic container of mineral oil about a foot from the breadboard, so I doubt that any voltage is being induced.
 
di/dt == delta I(current) over delta Time. How fast current is changing. If it changes too fast, it can turn on the parasitic BJT and cause a self-latching effect. This is the other condition that causes avalanche failure.

45V at 2ohms would be 22.5A. Although the inductance of the coil would reduce the di/dt some, I don't think it's slowing it down enough. The mosfets are only rated for 20A@25C. As they heat up, they will be able to handle less and less current. At 100C it's only 13A. I would guess that you need to handle more current.

The only other thing I can think of is they aren't turning on completely (full 10V at the gate-source) and they are frying just due to heat dissipation.
 
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22amps ???
A TV flyback (there are many different types) probably only has 0.5 amps on the primary. There is a large tuning capacitor across the transistor to make the pulse half sign wave. Normally the pulse is 700 to 1000 volts to get 25,000 volts. They are designed to run at 15750hz with a certain pulse width. (frequency depends on where you are) In a TV there is a large current in the horizontal yoke but not much in the flyback. Flyback transformers were never designed to be used with out a tuning capacitor.
 
As it turns out, my guess of 2 ohms is a bit off. The real resistance is 0.2 ohms. Also, even if it was 2 ohms, 22 amps just isn't right. My current flow meter shows that only 4 amps is flowing, even at higher voltages. If 22 amps was actually being drawn, my variac would've been smoldering. Even stranger, I was able to push an NTE2377 MOSFET to 50 volts before it overheated (In a NON-explosive fashion). The NTE2377 has a Id of 8 amps, and a Vds of 900 volts, so for me the logical conclusion is that my problem lies within the voltage, not the current draw. Can you tell me more about this parasitic BJT?

Also, ronsimpson, would a tuning capacitor be a blue, ceramic looking cap? Or a tall, yellow, slighly transperent one?
 
Do you have a protection diode on the flyback primary? None of this is making sense yet.

Also, what duty cycle is your PWM at during this time?

A schematic would make this much easier. :)
 
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The first schematic on this page is what my design is based off of. The schematic has a few errors, but it should give you the general idea. The duty cycle is about 65 percent, for the MOSFET.

_-= Uzzors2k =-_ Project Site

No, there is no diode on the primary.
 
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