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Half Bridge DC Blocking Caps - Does Size Matter?

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Sigurthr

Mon Apr 08 2013, 05:05AM

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

I have been working on a new mini SSTC and I've run in to a problem. I've tested the driver, secondary, primary, and GDT with other SSTCs and all check out fine. When I power up the coil, it oscillates well, the half bridge runs cool, and all waveforms look good. However, this bridge is only drawing 500mA at 120Vac input! I've pushed 1.5kW through this primary and secondary before, so I know that isn't it. This half-bridge is my least inductive yet and is certainly overdesigned with current capacity in mind. I'm only getting about 5kV output from the breakout point on the toroid at full power. There is no streamer unless I pull an arc from it. I first thought it was not oscillating until I powered up my scope and grabbed a neon tube. The RF E field is just fine, considering it is only about 50VA!

The only thing I've done differently than my other half bridges is I used different DC blocking caps (capacitive divider that connects one side of the primary to the + and - sides of the DC bus). I was using 0.68uF 600V CDE foil caps but this time I decided to spend the money for large electrolytics and picked up some nice 2200uF 400V caps to use in their place. Capacitive reactance is inversely proportional to capacitance, so these caps should have a lower impedance than the 0.68uF ones. I have no idea why the bridge would be so severely limited in power. Oh, and I did test these caps, they work fine. I discharged it using a pair of aligator jumpers and scared the crap out of myself in the process. 10Mohms really is too high for a bleeder resistor, I had thought I waited long enough for the voltage to drop to a non-scary level, but man that was a loud bang!

It will be a multi-hour effort to take apart and then cut apart my bus bars and desolder these massive caps (couldn't get screw terminals, only crappy snap-in tabs, so I soldered the bus bars straight to them!), but I do have two spare CDE 0.68uF caps to swap them with. Before I invest the time to do it, does it make sence that the caps are the issue? I'd hate to tear it all apart and find out it wasn't the caps!

Andy

Mon Apr 08 2013, 05:32AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

I'd stay with 0.68uF caps, if the frequency is right. I think haveing do high a Farad would flatting out the sinewave .

0.002mF*60 = 0.12(500hertz)

Steve Conner

Mon Apr 08 2013, 08:52AM

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

wrote ...
I think haveing do high a Farad would flatting out the sinewave .

I think this sentence makes no sense whatsoever. :/ Large electrolytics should work fine, except bear in mind that your bridge will now be fed with smooth DC instead of 120Hz pulsating DC. So the spark appearance will be different and the power factor on the mains will be much worse.

The only way I can think of that the capacitors might reduce performance, is if your 0.68uF caps were originally resonating with the primary, making it run like a DRSSTC. But even 0.68uF is somewhat too big for resonance, considering typical SSTC primary inductances and operating frequencies.

I think it's more likely you made some other unrelated mistake while wiring it up. Maybe your driver picked up some unwanted phase shift somehow. Try swapping the primary terminals over.

Ash Small

Mon Apr 08 2013, 08:54AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm just guessing here, but could it be something to do with resonant frequency?

220uF is a lot more than 0.68uF, so maybe the resonant frequency of primary and capacitors is too low?

(I'm on painkillers at the moment, so I'm not sure how much sense this makes)

EDIT: Looks like Steve beat me to it regarding the point about resonance.

Sigurthr

Tue Apr 09 2013, 03:49AM

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

I've spent far too many hours today trying to debug this setup. In the end I took this new coil's driver and tested it on my big SSTC; worked fine. I then took the new coil's bridge/GDT and hooked it up to my big SSTC's driver and resonator; it works beautifully! By process of elimination there must be an issue with the new resonator.

Since this new bridge runs better than my old one on my big SSTC it is now going to live with the big SSTC.

I now have a spare auto-tuning SSTC driver w/ fiber optic interrupter and my tried and true old halfbridge needing a new resonator. Thank you all for saving me from tearing apart a perfectly good bridge!

zzz_julian_zzz

Wed Apr 10 2013, 02:10AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Like what Dr.Conner's said, My bet too is on the phasing, Primary coil Phasing, GDT phasing, and/or CT Feedback Phasing..

Sigurthr

Wed Apr 10 2013, 02:56AM

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

I tried reversing the phasing of the primary, gdt primary, and UCC chips individually and in all combinations (still not using CT feedback due to no experience in choosing a proper CT for TC applications - they're all rated under 200kHz!), still no change. To top it all off when I put the new bridge on the big SSTC resonator it worked no matter the primary phasing.

The important part is that now I know I was right in thinking the half bridge caps can be polar and bigger is better.

I'll play around with the small resonator more when I get some time and see exactly what is up with it. I'm starting to wonder if some of the primary turns shorted or were arcing to the internal secondary ground lug or what have you. I kept hearing "ticking" from the secondary while it was operating.

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