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GDT and UCC driver Problems

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Mike

Sat Mar 17 2007, 01:19AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

So I am trying to make my Full Bridge work, since I have already got half bridges working fine. Basically Im going to stack ucc's and use a GDT with 1 primary and 4 secondaries. For testing I dont have stacked UCC's.
Anyways I have been having an odd problem, I had to disconnect all the secondaries and withonly one connected to one of the fets, I get this signal:

Now if I unsolder one side of the secondary from the fet I get this signal:

Also I have checked the conductance of the fet and circuit and they all seem correct.
With a 100nF capacitor on one of the secondaries I get a simmilar waveform to the

And finally another odd problem is that the UCC37322 blows up if I leave the circuit on for 15 seconds. It is the ucc with the .1uF ceramic DC blocking cap on the output of it. I have the UCC's being fed by a tl494 squarewave at 50khz.

Anyone have the slightest idea as to what is wrong here?

Thanks,
Mike

Bennem

Sat Mar 17 2007, 07:08AM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

Have you got decoupling caps of 0.1uf and at least
10uf tantalums on each UCC's voltage supply?
I also put an extra 100uf on each of my UCC's.
It also needs some anti-latch up diodes
on the Ucc's output.

Mike

Sat Mar 17 2007, 07:14AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

i guess i could try doing the 10uF tants and .1uF decoupling

But I know i dont need the diodes on the output because I can see the waveform fine if I put the scope on the pins 6,7 of each UCC even with the GDT connected, so they arent latching up. I get a nice squarewave from that. Its just right after the .1uF dc blocking cap it gets ugly
So I dunno if its the GDT or what.

Mike

cjk2

Sat Mar 17 2007, 07:19AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

Check that the GDT is suitable for the frequency it is being used at. You may need more or fewer turns for proper performance. I also strongly suggest the use of caps on the UCC chips.

...

Sat Mar 17 2007, 07:53AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

yes, caps on the ucc's is definitely a good idea. As are latchup protection diodes. You see, when teh ucc has no load (right now you aren't driving jack with the gdt) the waveforms will look fine, but when you go hook that up into a nice fat fet then everything will go to hell.

As to your problem, if you run the ucc's push-pull (or whatever it is called when you have the gdt connected across 2 ucc's that are 180degrees out of phase) you can eliminate that cap (at least for testing at 50% duty cycle). From there you can see if it is just a poorly suited cap or if your gdt is b0rked.

Also, looking at your scope shots, I think that part of your problem is in measurement error. That 2nd waveform (with the 'filled in' waves) looks like you are lacking in the gnd department and the circuit is floating at 60hz, or some incredibly high harmonic of something. In general, to get decent feedback you need to provide a lot more info on the setup, like where the scope probes are for the pics you posted. A pic of the gdt would be nice as well...

Just taking a rough guess, I would say do the following:
1. power down circuit
1. disconnect all secondaries from the GDT
2. remove dc block cap between ucc/gdt
3. pull out ucc's
3. power up circuit, use scope with channel 1 on the 1st ucc input, channel 2 inverted on the 2nd ucc input, then move the 2 (hopefully square) waves to be on top of each other. Then adjust the duty cycle to make then exactly the same. power it down.
4. put a 1nf loading cap on one output of the gdt, and put a scope across the cap.
5. populate ucc's, power back up
6. check for proper wave across the test cap. If everything looks like hell and the ucc's explode add more turns to the GDT.

Marko

Sat Mar 17 2007, 02:02PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

And finally another odd problem is that the UCC37322 blows up if I leave the circuit on for 15 seconds. It is the ucc with the .1uF ceramic DC blocking cap on the output of it. I have the UCC's being fed by a tl494 squarewave at 50khz.

It's generally not so easy to blow up an UCC. It looks like you either have a bad short somewhere or you used improper core material like one of those yellow-white toroids.

What number of turns you used for all windings?

Mike

Sat Mar 17 2007, 03:54PM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

about 5 turns -6 turns of insulated wire twined together wrapped around a ferite core.

Tom540

Sun Mar 18 2007, 12:11AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

Sounds like that DC blocking cap is a little small. Maybe you don't have enough current to push that gate. 50Khz seems a little low, try speeding it up a tad. The waveforms get kinda crummy at lower speeds through a GDT possibly even on the primary side.

Mike

Sun Mar 18 2007, 05:54AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Yeh I have tried most of those things, but it really just ends up having a shitty wave form on the primary and secondary , even with nothing connected to the secondary
So basically I have removed the GDT, and the UCC's both run fine with good outputs after the DC blocking cap and such, So I am convinced it is the GDT, it was a rather crappily made one anyways. I shall try a new one soon.

Thanks for the help

Marko

Sun Mar 18 2007, 10:20AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Mike wrote ...

about 5 turns -6 turns of insulated wire twined together wrapped around a ferite core.

I'm not sure if I understood you right, is this for two or one UCC? With two, I'm pretty much sure you are going into saturation at 50kHz with only 6 turns (on any typical core cross-section). Use higher frequency!

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