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Third SSTC - Mosfet Drivers Exploding

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SuperZanti

Sun Aug 05 2012, 02:58AM

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

My first tesla coil was simply a spark gap tesla coil, and that was real easy to get working,
My second was an audio modulated tesla coil based off steve's sstc 5.

Both of those I was able to get working, but I designed a coil based off of steve's and it doesn't seem to be working, the only thing I changed from my last design, is I made it an H bridge. I've completely re-made it three times now, and something goes wrong every time. Based off of how hot my variac gets, I thought it was a short, but I double checked all my connections and I still can't seem to find what's wrong.

I was wondering if anyone could look over my schematic and confirm that its correct:

Also, I just wanted to say thank you to everyone on these forums. I've learned so much from just lurking here for a long time. I really appreciate how everyone records their steps and how 4HV is constantly growing as a community with their knowledge base.

Dr. Dark Current

Sun Aug 05 2012, 09:18AM

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

Hi,
the biggest problem I see is that the lower diode on the input of your first inverter gate is connected wrong. You don't need these diodes anyways, as the inverter has built in diodes for up to 20 mA of current.
Then,
-tantalum capacitors are useless for power applications, use ceramic or non inductive film on the supply rails of the gate driver chips
-the cap in series with the GDT primary is .47uf or 100pf? If the latter, it will not work.

SuperZanti

Sun Aug 05 2012, 12:39PM

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

Thank you so much for the fast reply. Most of those just looked like errors on my schematic. except I have been using tantalum capacitors. I just changed that. Also it was 0.47uF not 100pF.
Again thank you so much for your help. Other than that, does it look like it will work?

Dr. Dark Current

Sun Aug 05 2012, 12:57PM

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

The 10k pot on the audio input is connected wrong, I haven't examined the rest of the TL494 circuit, but other than that it looks OK

PS. I'm not sure how you think the audio modulation will work.

SuperZanti

Sun Aug 05 2012, 05:07PM

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

That chip should pulse width modulate the audio right?

Dr. Dark Current

Sun Aug 05 2012, 07:03PM

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

Yes, but at the interrupter frequency? This way you will get just some "interference modulation"

Forty

Sun Aug 05 2012, 07:05PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

your tl494 section looks identical to plasmana's single fet plasma speaker (including the 10k pot.) so with that output signal eventually feeding the gates i imagine the audio modulation should work the same as it does in plasmana's speaker.

HV Enthusiast

Sun Aug 05 2012, 07:50PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

I don't know the specs of the MOSFETs you are driving, but since this an audio modulated coil you are likely going to be driving the bridge at CW (100% duty). Which means you're asking a lot of those UCC3732x gate drivers to drive (4) MOSFETs especially if you're switching at high frequency. They will get smoking hot!!!

You're better off in my opinion to divide each half of the bridge with its own set of UCC drivers and separate gate transformer.

With DRSSTCs, its not as bad because you are driving at very low duty cycles (usually 5-10% max), but with a CW coil you're driving at 100% duty and its a lot of power to do that. (again, more so if its high frequency)

You'll also want to think about heatsinking of the gate drivers - however it does depend on the actual load the MOSFETs are presenting as well as the switching frequency and voltage you are switching at.

SuperZanti

Sun Aug 05 2012, 11:09PM

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

Once again, thank you all for your replies (so fast too). It means a lot that you take time out of your schedule to help some random person on the internet xD

Anyway, as far as the tl494 chip, I'm nearly 100% positive that that's not my problem. I also have tested the audio modulation on an o-scope and it seems to be doing what I want it to. I should also mention that the audio that I'm feeding the chip is filtered. I filter out all frequencies under 200Hz and all frequencies over 2kHz. That's just not included in my schematic because that part of it is also part of another project.

My biggest problem, is just that I've built this schematic 3 times now, and the first time it worked for a little bit until it shorted out. I bought all new parts, and tried it again, and then it just didn't work. I debugged for the longest time, and I couldn't figure it out, so I decided to just try again (it was worth my time/money) and I bought all new parts and built the circuit 1 more time.
I just feel like, even with the problems stated, that when I turn it on, I should see something. But I don't get anything, no sparks, or noise.

The mosfets are 500V 20A (IRFP460), and I am only running them at a max 120V, originally I think this H bridge is designed to run off of 365V. I may be wrong, but I don't think they should over heat, and I have some huge heat sinks on them too (I actually used a processor heat sink for each mosfet). As far as the mosfet drivers go, this might be the problem, but they don't get hot at all.

Right now I think I'll triple check my wiring to make sure I've hooked it up correctly. I fear that this might be the only problem and I havn't been careful enought.

Once again, thank you all.

SuperZanti

Fri Aug 10 2012, 07:33AM

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

Update: I built this schematic:

I'm 100% positive that I built it correctly. (correctly meaning all the wires go exactly where there supposed to, as shown in the schematic, and none of them go where they're not supposed to). Now, I fired up the coil... and nothing happened. Or, so I thought, I decided to touch the coil with a ground wire and I was getting sparks that were less than a millimeter in length.

Now, My only thought is that maybe i'm not providing enough power? Most of these go over 300 volts. and mine is about 165.
Hopefully somebody has an idea as to why this isn't working....

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