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yet another DRSSTC - reality check

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Steve Ward

Mon May 29 2006, 10:26PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Guys, if you havent tried already, reverse your feedback phasing from the CT. My coils have absolutely no problems with the primary feedback, they never lose feedback, and never have difficulty starting up.

If you are using an antenna, you can reverse the feedback phasing by swapping the connections to your primary coil. This only works if your feedback is based off of the seconary of the TC! If you are using primary feedback, you must either 1) invert the gate drive signal (you can swap your gate drivers around), or 2) swap the output leads from the feedback CT, or 3) pass the conductor through the CT in the opposite direction. Either of these 3 will change the phasing by 180 degrees. One phasing will not work, the other will.

If this doesnt resolve your problems, let me know.

Wolfram

Mon May 29 2006, 10:31PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Are the IGBTs IRG4BC20UDs?

Dr. Shark

Tue May 30 2006, 07:46AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Yes Anders, indeed they are. And just like you probably did, I got them by the 100s of ebay.de, for just a few cents a piece, so I don't mind blowing them like popcorn.

I think my feedback problem is somewhat more fundamental, maybe a bug in the circuit. I'll worry about that before I follow Firkraggs advice and go for (hopefully) bigger sparks. But really, I think I wont get that much more than the current 4" from the TO220s

Marko

Tue May 30 2006, 06:54PM

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

Joe, what voltage were you using for that streamer from the pic?

Im fighting for mere few centimeters at 30V...

Your antenna definitely should work, try replacing 4049 if it is blown.
Problem is definitely somewhere in the circuit...

HV Enthusiast

Tue May 30 2006, 07:43PM

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

Uh . . .

Fighting for a few centimeters arc with a bridge voltage of 30V should be expected and is quite normal.

On my DRSSTC II system, I need to have about 70V on the bridge for 1 inch output.

Sounds to me, if you are getting only a few cm at 30V bridge voltage, your coil is working perfectly fine.

vasil

Tue May 30 2006, 09:03PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

4 kHz PRF...wow. Thats explain the waste of IGBTs. And what ON time?

teravolt

Wed May 31 2006, 03:29AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

is your circuit zero crossing at the right time

Dr. Shark

Thu Jun 01 2006, 11:29AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

@Vasil, the IGBTs stopped blowing up after I put Zeners across the Gate.
@Teravolt, certainly not, since it is running open-loop.

Still no progress, and I am running out of ideas... While the coil runs perfectly (giving about 1cm streamers at 50V and up to 10cm at 300V), I just cant get the PLL to lock. I have replaced the chip, traced all the connections... and the funny thing is that by turning the phase pot, I can slighly change the frequency, so the VCOin at pin 9 is working, only SIGin on pin 14 is completely ignoring what I feed into it. Which is a nice sqare-wave, as checked with a scope. I am using Steves dirty circuit

just instead of the capacitor on the very left I have the antenna. The problem is that I haven't got the slightest clue as to how this circuit works, what locking range the PLL should have, and if the values of the resistors and capacitors (other than the timing capacitor) have to be adjusted for my operating frequency of 350kHz, which is probably twice of what Steve was running at.
I am grateful for any ideas!

Steve Conner

Thu Jun 01 2006, 11:47AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you're using the circuit with an interrupter and small pulse widths (as in a DRSSTC) the PLL doesn't really work so well. You'll never see any behaviour that looks like "snapping into" lock with a sudden increase in output, no matter how hard you tweak it.

The problem is that the PLL always trying to achieve lock, even when the interrupter signal is off and the coil isn't doing anything, hence the phase detector has no input. During these "off" periods, it tends to wander away, and the signal it gets during the "on" periods doesn't last long enough to help it much. So on the whole, it really runs more like a VCO.

Also, due to phase shifts between primary and secondary, when you're using antenna or secondary base current feedback, you won't necessarily get zero-current switching even if it does lock.

These are some of the reasons why I made the Mk2 driver. It has the PLL linked to the interrupter so that it only self-tunes during the burst. Also, the phase shift issue is the reason why I switched to primary current feedback.

Dr. Shark

Fri Jun 02 2006, 04:37PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Whoa, thats harsh! Maybe the project titel "Dirty..." should have put me of, but in my infinite confidence for the brand that is "made in Scotland", I did not for one second doubt that the design was good.
Well, at least this explains why I dont seem to get lock, and I suppose once I change back to primary feedback, it will run well.

So just to make sure, I tweak the frequency for maximum output and then use the phase pot to fine-tune the frequency, or do I need to really adjust the phase?

If I use primary feedback, I will get ZCS no matter how the phase is adjusted, or even if the frequency is off?

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