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Flyback driver destroying MOSFETs

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LAN_403

Dr. Dark Current

Fri Jan 30 2009, 10:55PM

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

If he really used the diode across the primary, the current would rise to insane levels and blow the transistor.

Antonio wrote ...

The lowering of the operating frequency really increases the current in the transistor, as mentioned. But it's perfectly safe to operate at audio frequencies, if the primary winding has sufficient inductance and the power supply voltage is not too high. Using the normal primary winding of the flyback, and a power supply of a few tens of Volts, it's really necessary to operate at a few kHz to have significant output power.

Operating at a lower frequency limits the maximum achievable power throughput. I found the optimum for the flyback converter driving TV transformers to be at around 20kHz.

ScotchTapeLord wrote ...

The post I was referred to says decoupling should be .1-1uF? Is there such a thing as too much decoupling? I have the decoupling capacitor before a 12V regulator, is that right or should it be after it?

It should be both before and after it, and it should be a film or ceramic cap, not electrolytic. There is no upper capacitance limit.

GeordieBoy

Fri Jan 30 2009, 10:59PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Connecting a diode across the flyback transformer's primary also acts to make the primary current free-wheel when the switch opens, instead of decaying. This will likely cause the primary current to integrate up to a very high value unless peak current-mode control was employed.

As Antonio said, a flyback converter cannot function correctly when the reflected voltage across the primary is clamped with a single diode! An RCD snubber is the preferred method to control the switch voltage.

-Richie,

ScotchTapeLord

Fri Jan 30 2009, 10:59PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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The reason I was lowering the frequency was because the spark gets very small at peak audible frequencies... I know I can increase voltage but I want it to be portable. I guess I'll have to settle for small arcs until I can afford another 20x AA NiMH pack

EDIT: ... Just read up on IGBT pins. So... the emitter goes to ground...? ^^ right...

Dr. Dark Current

Sat Jan 31 2009, 08:19AM

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

ScotchTapeLord wrote ...

The reason I was lowering the frequency was because the spark gets very small at peak audible frequencies... I know I can increase voltage but I want it to be portable. I guess I'll have to settle for small arcs until I can afford another 20x AA NiMH pack

EDIT: ... Just read up on IGBT pins. So... the emitter goes to ground...? ^^ right...

If you want higher voltages at higher frequencies, increase the air gap in the core and reduce the number of primary turns.

Yes the IGBT connects the same as bippolar transistor with the difference that it's voltage controlled.

ScotchTapeLord

Sat Jan 31 2009, 03:55PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Um... It is running at 13kHz but the spark is very small. The writing on the tv I got it from says 36KV which is far more than I am getting. I am not really sure enough of my abilities to take it apart though. I was getting huge sparks when I tapped on a potentiometer I was using for frequency... Before destroying the timer... So I know this can do better.

Dr. Dark Current

Sat Jan 31 2009, 04:11PM

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

ScotchTapeLord wrote ...

Um... It is running at 13kHz but the spark is very small. The writing on the tv I got it from says 36KV which is far more than I am getting. I am not really sure enough of my abilities to take it apart though. I was getting huge sparks when I tapped on a potentiometer I was using for frequency... Before destroying the timer... So I know this can do better.

If taking the core apart is not an option, you can always decrease the number of primary turns.

P.S. Are you sure you have the primary winding connected with the right polarity? Try reversing it and see what happens.

ScotchTapeLord

Sat Jan 31 2009, 04:47PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

I have switched the leads going to the primary and the other configuration yields a much weaker output. So introducing the plastic covering to my dremel is better than lowering the frequency? Can I wind my own primary on the other, exposed side of the core and just not used the built in primary?

EDIT: I have found a post by Dr KV including the advantages of winding your own primary. That is what I shall do.

Later that day (edit): I wound my own primary with 22 awg magnet wire, because it was all I had that would fit into the gap. It was suitable, as I found no heating. I measured the inductance to be .05 mH, which gave me about 4 ohms of reactance according to my calculations. I hooked it up and heard the usual frequency, but also some crackling which I deemed to be static charge. The sparks were great, not thick but very long. I played with it for maybe a minute and then it ceased to function as anything but a funny-smell producer. I opened up the foil pouch I had my circuitry in, which I noted to be quite hot, and found that the IGBT had melted through the plastic bag I had between the circuit and foil. I believe I see the value of higher frequencies now. They will put the voltage across the primary more than the transistor, correct? But I don't really want to halve my spark length...

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