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Troubleshooting my first SSTC...

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Peyn

Thu Feb 03 2011, 12:08AM

Registered Member #3566 Joined: Mon Jan 03 2011, 10:46PM
Location: California
Posts: 5

So I'm building my first SSTC, and I'm following Steve Ward's miniSSTC design, like a lot of new coilers do. Now, I've got my circuit set up (minus the power section) so that I can test it, but it doesn't seem to be behaving. I'm using a 24VAC transformer for the logic power section instead of a 20VAC transformer, but I don't think that should cause any issues. Today I connected a function generator to the antenna input to emulate some oscillations, and I connected an oscilloscope to the output of the schmitt trigger and to the +5V voltage regulator. When I did, however, the signal gets "messy". Take a look at my pictures for a better idea; the first image is when I don't have the oscilloscope hooked up (The yellow channel is the +5V power supply, which appears to be operating just fine, and the blue channel is nothing) and the second image is when I have the o'scope monitoring the output of the schmitt trigger (Again, the yellow channel is the +5V power supply and the blue channel is the output of the schmitt trigger). It seems like when I connect probe the +5V supply has major spikes every time the schmitt trigger switches. Everything here SHOULD have a shared ground. Anyone have any ideas?

5558d5

UPDATE: Okay, so I went back and tested the circuit with my oscilloscope again. This time, I removed the sn74hc14n, and only had the 555 timer ic in. I also left the gate drivers out of their sockets as well. I still used a function generator for the input on the antenna, and everything seemed to be working great.

Then, I added the sn74hc14n back in, and checked the signal from the 555 as well as the logic power supplies. All seemed well until I checked the output of the sn74hc14n, which was outputting a signal similar to the one shown above. I'm still not sure what this means, but I'm suspecting that it is a measurement issue.

UPDATE AGAIN!: Alright, I realize now that it WAS a measurement issue...I shouldn't have grounded the function generator that was acting as the input for the antenna. When I don't ground it, the output of the sn74hc14n is perfectly clean. Unfortunately, I've come across another bug:

When I try to measure the output of one of the UCC3732x ICs, the signal I get is somehow messed up. This is what the output of the gate driver is like when the ENA signal is high, and with a 90kHz antenna signal:

122ne9v

Now I'm really not sure what is wrong...I've had problems with bad bypassing before, and I remember the symptoms were similar to this. But this time, I have more bypassing caps on the gate drivers than are shown in Steve Ward's schematic, so I'm not sure if that is it. If anyone has any clues, help would be greatly appreciated.

UPDATE 3: Okay, now I'm making more progress. It seems that when I measure the output of the gate drivers with no reference to ground, the signal is much cleaner although not perfect by any stretch of the imagination. I'm going to work on having better grounding, and I'll post updates as they happen. Thanks in advance.

Peyn

Tue Feb 08 2011, 06:22PM

Registered Member #3566 Joined: Mon Jan 03 2011, 10:46PM
Location: California
Posts: 5

Okay, so here's what is going on: I've redone a little bit of my circuit in the hopes that better grounding and more direct traces will give me better results. Pics of the top and underside of it is shown below (note that I haven't added any of the power section/MOSFETS/GDT yet).

23lfqmg

I've got some long wires to my interrupter pots, I know, but they are going to be mounted.

Now, back to my issue: when I input a 100kHz wave into the feedback antenna and measure the output of the UCC3732x with my oscilloscope, it looks like this when my interrupter signal is high:

2hcpafd

Here's a closer look:

6eq1y0

When I crank up the amplitude of the 100kHz feedback signal, some of that overshooting seems to go away, but not enough of it. I'm not sure where to go from here; I've put several decades of decoupling caps at both ends of the +15v trace, so I'm not even sure how I could have better decoupling. Anyone have any idea what my issue is?

ScotchTapeLord

Tue Feb 08 2011, 06:54PM

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

What are your UCCs loaded down with? I do not see a gate drive transformer.

Peyn

Tue Feb 08 2011, 08:21PM

Registered Member #3566 Joined: Mon Jan 03 2011, 10:46PM
Location: California
Posts: 5

Uhh, nothing at the moment. I figured I'd test the outputs before loading them.

ScotchTapeLord

Tue Feb 08 2011, 11:55PM

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

You're lucky that they are still in one piece! UCC37322/1's are notorious for strange behavior unloaded. Many, including myself, have reported instances where, unloaded, they enter a mode of operation akin to that of a firework.

I suggest putting your GDT and a DC-block/AC coupling capacitor onto your UCC bridge's output, and loading the transformer's secondaries with capacitors that match the values of your switches' total gate charge.

Peyn

Wed Feb 09 2011, 12:31AM

Registered Member #3566 Joined: Mon Jan 03 2011, 10:46PM
Location: California
Posts: 5

Thank you! I've been stumped with this for so long, I can't thank you enough. I'll post progress later on.

ScotchTapeLord

Wed Feb 09 2011, 01:43PM

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

Thank me after you get sparks! There may still be problems to come... but hopefully not!

If you don't have GDTs handy yet you could just put a few nF capacitor on the ucc output to make sure everything's working right.

Peyn

Sat Feb 12 2011, 06:47AM

Registered Member #3566 Joined: Mon Jan 03 2011, 10:46PM
Location: California
Posts: 5

Well, it's been a few days since I posted last, but today I'm here to report that I got sparks! Granted they were only ~6 inches long at best, and my secondary actually caught fire (just at the tip, nothing particularly bad) but they were awesome sparks nonetheless!

I plan on getting a nice spun topload for it, winding a less leaky GDT and polishing off the bits that need polishing. Thanks again for the help!

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