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No limits to DRSSTC

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Steve Conner

Mon Mar 20 2006, 05:00PM

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

EVR makes a good point about the logic. I used 74HC logic running off 5v for the OLTC2, and I found that the reverse recovery spikes from the diodes in my bricks would couple into the logic and drive it crazy. I had to seriously slug some things with capacitors to make it work.

I changed to 4000 series running off 15V for the DRSSTC. I still had some trouble: the 4046 PLL chip and current limit comparators would sometimes latch up in the event of a primary strike or flashover. Changing the clamp diodes to zener diodes, so that they clamped even before the signal reached the rails, fixed that and the controller seems pretty bombproof now. The downside is that flashovers are now much hotter because the controller keeps on driving: I burnt clean through the secondary wire with one

I do NOT like isolated heatsinks. The reason being that they could pick up enough voltage around a Tesla coil (either capacitively or by a streamer hit) to break down the IGBT's internal insulation. I have had strikes to the heatsink on my small DRSSTC before. (but then the heatsink is so oversized they could hardly miss it...)

I ground the heatsink to green wire ground, and use capacitors between the DC bus and green wire ground (these caps could be part of a commercial EMI filter) to make sure no high RF voltage appears there.

HV Enthusiast

Mon Mar 20 2006, 05:45PM

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

wrote ...

Best idea is to lock everything in a big metal box, as I did on my small DRSSTC (ATX SMPS box).

You need to be careful about enclosing everything in a metal box. You may also trap noise and have it resonate inside a metal box. Remember, the enclosure is a tuned cavity and if not done properly can be worse than unshielded.

EEYORE

Mon Mar 20 2006, 06:52PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Ill second that about shielded enclosures. For quite some time i was hard-headed about wanting one(mainly for aesthetic reasons) but got problem after problem. Seemed solving one only lead to another(kinda like C programming

)

Just leave the driver open and enclose the interruptor/make it remote via BNC or similar.
Matt

Marko

Mon Mar 20 2006, 08:23PM

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

Shielding with metal cage/box can do only good, faraday cage nullifies influence of huge electric field from the coil, outside and also blocks potentialy lethal arcs.
So I don't see why is it so bad.

We don't have any significant EM/EF emmision inside that could be ''trapped'' by the box (unless we make antennas on our bridge

EM field never seemed to cause too much trouble.?

Actually its best use is just to keep arcs from blowing anything sensitive.

Far bigger problem seems noise that couples into supply grid, disrupts IC's and it can be only fought by filters and decoupling caps.
I could get tiny burns from supply rails from SSTCs and I doubt logic ICs would like that.

cheers..

EEYORE

Mon Mar 20 2006, 08:35PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Firkragg wrote ...

Shielding with metal cage/box can do only good, faraday cage nullifies influence of huge electric field from the coil, outside and also blocks potentialy lethal arcs.
So I don't see why is it so bad.

We don't have any significant EM/EF emmision inside that could be ''trapped'' by the box (unless we make antennas on our bridge

I beleive the problem comes from currents being switch by the driver and bridge becoming trapped inside the box. Decoupling capacitors ought to solve alot of the trouble with interference from an unshielded driver, no?

HV Enthusiast

Mon Mar 20 2006, 08:44PM

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

wrote ...

Shielding with metal cage/box can do only good, faraday cage nullifies influence of huge electric field from the coil, outside and also blocks potentialy lethal arcs.
So I don't see why is it so bad.

This is why most engineers tend to have noise problems, because they don't understand the basic principles of EMI/EMC design. Shielding, if done without understanding what is going on, can have very bad effects on the system. Simply putting some electronics in a box doesn't solve anything. In fact, both the shield and/or box can act like an antenna radiating noise as well as receiving noise depending on the type of shield / box used. Also, internal noise sources can amplify due to the physical structure of the box creating new problems.

wrote ...

We don't have any significant EM/EF emmision inside that could be ''trapped'' by the box (unless we make antennas on our bridge EM field never seemed to cause too much trouble.?

There is quite significant EM/EF emission within a DRSSTC. The magnetic fields alone caused by the power traces is quite considerable, especially considering the way most amateurs route their full-bridges with large loop areas.

Dr. Drone

Mon Mar 20 2006, 09:23PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

Marko

Mon Mar 20 2006, 09:49PM

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

I guess im pwned then.
yeah and I said 'if we dont make antennas around our bridge', and surely interferences wouldnt stop but they must be diminutive compared to circuit open-to coil.

Exscept you stated how bad 'engineers' we all are you didnt explain the point, what makes good/bad cage? I want to know the most I can before I screw up my DRSSTC too...

shield and/or box can act like an antenna radiating noise

Something that well grounded cant act as antenna, box doesnt have some huge inductance and it behaves like faraday cage, so it shouldnt ''transmit'' anything.

If im running out of topic i would like to start a new one, this really interests me

HV Enthusiast

Mon Mar 20 2006, 11:10PM

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

For further information, please check out the materials by Henry Ott. He is one of the leading authorities of the subject and i have spent much time in training seminars, classes taught by him as well as private consultations.

Here are some good public files:
http://www.hottconsultants.com/papers.html

And his book is probably the best out there for EMI/EMC design. Its usually free when you attend his course, but here is the ordering information. Something all engineers should have . . .

http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471850683.html

Steve Conner

Mon Mar 20 2006, 11:48PM

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

The components used in DRSSTCs (4000 series logic, power MOSFETs and gate drivers) really don't generate enough energy at high enough frequencies to make an average sized shielding box resonate like an antenna. So I say that is a red herring. Even if it was a problem (and I admit it might be a problem in the other direction since sparks generate lots of HF) I think star grounding would go a long way to mitigate it: ie have a shield that encloses your whole circuit and only connect it to your circuit ground in one place.

On my DRSSTC driver I put all the connectors close to each other and screwed the shells of them to a copper plate that became one side of the box. This was also my star ground- pins inside the D connectors that were ground, were tied to the shells here, and the AC supply went in via a filtered IEC inlet screwed into a hole in the same copper plate.

I didn't allow any of the circuit ground wires to contact the shield anywhere else, so even if the shield was acting as an antenna, the energy wouldn't couple directly to the circuit (of course it could still couple capacitively or inductively.)

EMI still got in, but I traced it to differential mode getting in through the CT. Flashovers put lots of high frequency hash on the primary current and I guess this was hitting a resonance in my CT winding and creating high voltages on the CT output.

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