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Im baack...SSTC bridge question

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LAN_403

Killa-X

Sat Jan 15 2011, 06:24PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Well..Today i thought id put my DRSSTC dedicated secondary on my SSTC. It did kick out slightly longer sparks. Once i got to about 60VAC, i had a mosfet fail, as well as another trace blow. Under a 8mm diameter washer, turned all PCB....Evaporated the copper trace. Nothing was hot, it just randomly decided to explode.

So. This time, I come back with questions and details.

First, I lost two fets, and as always, the positive contact is what exploded on the bridge.

Second. In the SSTC fullbridge diagram found on steves site, there is a 10uF capacitor that goes directly from the positive input voltage, to the negative. How vital is this capacitor? Because i just noticed i never put that capacitor back on.

Third. Again, his circuit calls for a 1.62 uF capacitor going from the bridge to the secondary, how vital is that? I have 0.68uF, figured it wouldn't make a difference.

The fets die cold, and short across pin 2 and 3. My only thoughts is I'm having voltage spike issues. I have NO Diodes (MUR1560) across my mosfets. Just the ones built into them.

Also, I run my coil in Burst mode...On a SSTC. Where every 1 second or 0.5 seconds, it kicks a spark out at a selected duty and frequency. Just a quick blip. But is it true this can cause MASSIVE voltage spikes when you get into the higher voltage areas (60VAC)? Which could be causing my issues with constantly blowing bridges every time i go to higher stages? Long ago i was able to spark for like 30 mins, but when I went to 100VAC, Bang. This time, it didnt last long at all, on my DRSSTC dedicated secondary.

If you have any tips, suggestions, or ideas, Let me know. I'm considering rebuilding the bridge seeing how messed up it is now after all these traces exploding.

I noticed in CW mode, it shows a near-sine looking 350khz waveform when i scope the primary, but its very messy. Not sure if it matters, but the CT for feedback, just goes from 0V on the driver, to a capacitor, to the 74HC14N. No other stuff.

Gregory

Sat Jan 15 2011, 09:43PM

Registered Member #2922 Joined: Sun Jun 13 2010, 12:08AM
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Posts: 226

1 - I think that the less robust contact will blow before ( it can be aleatory)

2 - The 10u is snubber capacitor, that reduce the "voltage spikes" in the hard switching of the bridge, with not this you can blow the bridge more easy.

3 - 1,62u capacitor from bridge to secondary??? oO are you saying about the 1,32u capacitor in series with the primary? it is for bypass, only AC will pass in the circuit. You need to know that with low capacitance you can tune the primary, making your SSTC a DRSSTC.

4 - The sine wave in primary is normal, isn't a feedback problem, but here I always put a resistor between the 0,1uf capacitor and the 74HC14, like 1k or 470R..

Killa-X

Sat Jan 15 2011, 10:42PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

I went off this for my fullbridge design. But i have a 0.68uF capacitor there instead.

So..that may be one reason i keep killing fets at the high powers during burst mode. I dont have any capacitors across the + and -..that 10uF capacitor, I have NOTHING at all. So I will fix this.

If it wasnt for my traces exploding, i would have mosfets exploding violently. So in a way its nicer, but it does bad damages that cant be repaired besides soldering wires on it.

I did speak to people and found the possible main issue here. My wire from the Driver to the GDT, is 6 inches. Not even twisted. I was told that 6 inches is EXTREAMLY horrible. It should be like an inch at most. So. I plan to redo my boards. So i can have things very close.

If you guys have other suggestions, let me know. I will be building the boards when i get parts next week, and will let you know how it turns out.

Also, I put like 5 ohm resistors between the GDT and gate. I think i recalled 2.1ohms actually had the best square form.

Gregory

Sat Jan 15 2011, 11:19PM

Registered Member #2922 Joined: Sun Jun 13 2010, 12:08AM
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Posts: 226

with .68uf your primary can be tuned man..! and the high current peaks kill the mosfet

dmg

Sat Jan 15 2011, 11:42PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

Killa-X wrote ...

Id also put a resistor before that cap, and a pair clamping diodes right before the gate's input
(with a 5V source, relative to to the line, and another diode from ground)

also, your MUR1560 diodes... Get some
you need freewheeling diodes in there.

While its not critical to the bridge's function, you should really use that snubber.

If you can, Id also use some TVS diodes for each fet, as there are still fast transients that the snubber could miss.

Arcstarter

Sun Jan 16 2011, 12:23AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Gregory wrote ...

with .68uf your primary can be tuned man..! and the high current peaks kill the mosfet

Well, a sstc is usually tuned with many primary turns, so the resonance would be much lower freq than the running freq.

That 10uf cap is very vital, there are plenty of inductances in your circuit, and any of them could ring and make bad voltage spikes. You could use something lower, like 1uf even, but it should be good quality/low ESR.

Killa-X

Sun Jan 16 2011, 01:26AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Well for now I have a 3uF snubber installed. So that will help, I can hear the arc sounds a lot cleaner now that I fixed it.

I still need to fix the length of the feed wire from the driver to the GDT though...

GBD, I can try a resistor in series. I might have a set of TVS diodes actually. I was going to make a SSTC long ago, and the schematic called for a TVS. I still have these, so might as well use them! The 1.5KE220

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