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New DRSSTC, secondary to primary arcing is OK!

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Steve Ward

Tue Aug 19 2008, 05:48PM

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

This is all really weird. My coils would survive strikes to the primary all the time but what was getting destroyed was the MMC but it wasnt right away. Then I added the TVS primary protection to save my MMC. I'm now wondering if the primary strikes are effecting your MMC.

Tom, were you exceeding the VAC rating of the caps? Then they will FAIL NO MATTER WHAT. Ive performed life-time testing on MMC caps, if you exceed the VAC rating, you will hear them "snap" internally. When running at 2kVpk on a 2kvDC cap, id give the caps about 50-100 hours of run time if you are lucky. I call bullshit on the primary strikes causing the MMC failure directly because depending on the phase of the streamer current, it would only add or subtract like a couple of peak amps. Compared with a few hundred peak amps, thats nothing. Cant break the rules of kirchoffs current laws...

Now, what can boost primary current is a short heavy secondary arc, so if your current limiter wasnt set to act immediately, then the extra primary current/voltage of having lots of short heavy arcs coulda helped blow up the caps extra fast.

As far as ive seen, mostly everyone abuses the voltage rating of their MMC, and they are definitely running them where they are likely "failing" every few minutes to even 10's of times per second. You may not notice it because you cant hear the capacitors snapping internally over the sound of the sparks. After awhile, some caps tend to dive-bomb and then it becomes noticeable when they catch fire or something :P. It can take a long time for this to happen, many 10's of hours if you arent thrashing your caps too badly.

Also on a side note I build up some half bridge coils and they would die every time when hit with primary strikes. Took out both igbt's. Never happened on a my fullbridge. I even had a strike directly to full bridge PCB and still nothing. Don't think I'll be using half bridges anymore.

This seems unscientific. There must be something about how you had the other side of the primary terminated that made it different. Maybe its the direct coupling to the AC line neutral. Did you have RF ground and electronics ground linked together like ive done? And how about a decoupling cap from the bridge rails to ground? If you were missing that cap, i could see big problems.

Tom540

Tue Aug 19 2008, 08:41PM

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

Steve Ward wrote ...

Now, what can boost primary current is a short heavy secondary arc, so if your current limiter wasnt set to act immediately, then the extra primary current/voltage of having lots of short heavy arcs coulda helped blow up the caps extra fast.

I wasn't even using a current limiter because I'm using secondary feedback. Current draw is super low. I'm using some wimpy 20 watt heat sinks and they stay cold so I figure why bother with current limiting. The IGBT's arent even running at half their rated pulse current.

Steve Ward wrote ...

As far as ive seen, mostly everyone abuses the voltage rating of their MMC, and they are definitely running them where they are likely "failing" every few minutes to even 10's of times per second. You may not notice it because you cant hear the capacitors snapping internally over the sound of the sparks. After awhile, some caps tend to dive-bomb and then it becomes noticeable when they catch fire or something :P. It can take a long time for this to happen, many 10's of hours if you arent thrashing your caps too badly.

Well for the MMC failures I had zero problems with them before and realized they were failing from strikes because when i got a primary strike the caps would flash internally. I'm pretty sure I wasn't exceeding their AC rating because I had my TVS string set just above that rating. Ive since added more caps for a higher voltage as well. Since all this I haven't had any failures. Adding tvs's is kind of a lame solution though. If I used a breakout point I never had failures and primary strikes were rare. Before all this they would almost always fail.

Steve Ward wrote ...

This seems unscientific. There must be something about how you had the other side of the primary terminated that made it different. Maybe its the direct coupling to the AC line neutral. Did you have RF ground and electronics ground linked together like ive done? And how about a decoupling cap from the bridge rails to ground? If you were missing that cap, i could see big problems.

Hmm so you got your Earth ground tied to logic ground? Maybe I'll try this but I might just convert back to full bridge.
My methods are always unscientific

. Mostly trial and error. You're probably right about the coupling to AC neutral this is what I was thinking. I did not have any RF grounding but as far as the caps from DC bus. I never had any problems not using it in the past. I am having other problems with this bridge as well. Too much heating so I much just dump the design and stick with stuff I know works since I am now unemployed and need to prioritize my time.

Edit:
I tried out the things you did to my Half bridges and they still failed but not from primary strikes. I have no idea what from but I would just like to add one thing about this here because I've been thinking about it today. I am thinking that all you have done to modify your circuit is not the reason it was surviving primary arcs. The reason I say this is because my original DRSSTC full bridge board has always survived primary strikes and it has none of the additions or features you have added. The half bridges I was referring to earlier in this post were the ones not surviving but they had other problems so I'll ignore them for now. I don't doubt that what you have done is an improvement but as far as being the sole reason it survives I am not convinced. As I stated above The TVS's were only added to protect the MMC. The IGBT's have always survived. Some of the things you added ive seen on power supplies at my last job so I know they are not in vein, with that said, I do not have a clue in hell why they survive just yet. I just know that the extra features you added are not required for primary strike protection.

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