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Third SSTC - Mosfet Drivers Exploding

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Sigurthr

Fri Aug 10 2012, 12:44PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Okay, now test your SSTC's power side of things before changing anything. Take the line/trace to Pin 3 of the UCC chips and pull it up to the low voltage DC rail with a high value (10k) resistor. This will give a continuous ON signal and let you see if your coil is functioning, and what output power you should be seeing. It is possible your audio input isn't strong enough, or there is an impedance mismatch, or even a problem in the audio modulation section. Checking the coil in CW mode without modulation lets you at least rule out the power side of things.

SuperZanti

Fri Aug 10 2012, 02:09PM

Registered Member #6006 Joined: Sat Aug 04 2012, 08:50AM
Location:
Posts: 22

Unfortunately, I already tried that. I got the same results as I described above. Thanks though.

Sigurthr

Fri Aug 10 2012, 03:02PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

You're inputting rectified 120V mains (about 170V dc) to the fullbridge right? Something is wrong if you're only getting 1kV out. On my smallest tabletop SSTC based on Steve's designs I got 22.5mm output from only 50W input (25V 2A).

1) How much current is the TC drawing when run CW?
2) Have you tried using a breakout point?
3) Check placement of the antenna and verify the clamping diodes have not shorted. (if the feedback circuit cannot receive a strong enough signal to maintain resonance you'd essentially be untuned (and my UCC chips like to run at 8MHz when no feedback is applied!) and get only very poor output.

We need more details about the construction of the tesla resonator to diagnose your coil further.

Some possible causes:
Coupling; Solid state coils require very tight coupling, if you are using the same setup one uses for a SGTC (flat primary below the secondary with large spacing) it will likely not work very well at all.

Primary Inductance; The impedance of the primary circuit is the main limiter of the current draw for SSTCs, so if you're using very thin wire, or poor layout deisgn with high stray inductance you could be severely hurting your coil's performance. Far too many turns will also greatly reduce current draw and reduce output. Remember magnetic coil power is measured in amp turns, just like with a resistor if you increase the turns (impedance/resistance) it requires more voltage to push the same current through. Though, I cannot imagine you have something crazy like 60 turns on your primary. You'd need something like that on a small-ish coilform in order to limit current so severely as to only get 1mm output. I get 1mm output at like 7watts input (14Vrms @ 0.5A).

Secondary Grounding: a true RF ground is essential to the proper performance of TCs. Some builds/locations can get away with poor RF HF grounds and still get decent performance, but it is always a performance boost to get a proper low-impedance-at-RF-frequencies ground going. If your secondary's base connection is poor or if your ground is high impedance your output will be greatly reduced.

SuperZanti

Fri Aug 10 2012, 03:56PM

Registered Member #6006 Joined: Sat Aug 04 2012, 08:50AM
Location:
Posts: 22

I haven't measured how much current my system is drawing, I always use a breakout point, and I've played around with the antenna placement quite a bit. However your idea of the TC not resonating properly sounds really good to me and now I think that that might be the problem. I just don't know how to fix it...

Anyway, here is the picture of my setup

This is my circuit up to the two drivers

Here is the H bridge

And you can see the antenna in this shot. Its the white wire

Thanks again.

SuperZanti

Mon Aug 13 2012, 01:51AM

Registered Member #6006 Joined: Sat Aug 04 2012, 08:50AM
Location:
Posts: 22

Quick update:
It looks like I blew 1 or both of my gate drivers while just trying to tune the antenna.

So I'm going to be ordering parts again soon, but I'd like to know what all of you have to say before I order, just in case I need to order something different.

brandon3055

Mon Aug 13 2012, 03:04AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

The wires on your bridge are waaay to thin and you should also ad some snubber caps but I don't think thats why its not working that should just make the fets expload

also I haven't done a lot of work with antenna feedback but based on some other coils I've seen your antanna looks to short also the primary wire looks a bit thin

and do you have a dc blocking cap there sonwear?

SuperZanti

Mon Aug 13 2012, 03:44AM

Registered Member #6006 Joined: Sat Aug 04 2012, 08:50AM
Location:
Posts: 22

I'm really sorry, but I'm quite new with tesla coil design (my first 2 i simply copied other's schematics). I'm aware that my bridge wires are too thin, and I'm working on replacing those soon (I just took those ones out of an old computer power supply).

Could you define 'snubber caps' for me, I'm not even sure what those are (I'll see what research I can do on the forum to find out).

Based on the schematic I roughly based mine off of (Kaizer sstc II) the antenna is supposed to be 6-8 inches, and mine is about 8 inches.

Where would I need the DC blocking cap? I don't think I've seen many schematics with one...

Thank you!

brandon3055

Mon Aug 13 2012, 05:28AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

I'm also fairly new to Sstc's the snubber cap or caps are smaller caps around 10uf (im not shore of the exact type or value) that are attached to the dc rails of the bridge as close to the fets as possible and the dc blocking cap goes in seiries with the primary and its purpose is to protect the bridge if the output goes dc

SuperZanti

Mon Aug 13 2012, 05:38AM

Registered Member #6006 Joined: Sat Aug 04 2012, 08:50AM
Location:
Posts: 22

Hmm, I've never seen that before, but, just like you said, I don't think that's my issue... I'm really stumped at the moment.

I still can't figure out what's wrong. I rebuilt everything and as soon as I turned out on, the mosfets blew up almost instantly. There has to be something wrong with the h bridge, because I used almost the exact same driver circuit for the half bridge that I built. Here aresome pictures:

SuperZanti

Thu Aug 23 2012, 07:41AM

Registered Member #6006 Joined: Sat Aug 04 2012, 08:50AM
Location:
Posts: 22

Do these two things work together? I think they should, but my system just doesn't seem to be working. These are the closest schematics I could find to what I'm doing. Obviously the driver is modified to support 4 mosfets.

Driver:

Bridge:

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