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Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
To help clear up the discussion about Steve Ward vs EVR methods, this is what is on their respective web pages:
From (EVR site): "One (1) of the 0.1uF, 2kV capacitors is used as a primary strike transient suppressor and protects the half-bridge power electronics from secondary voltage strikes which may hit the primary coil. This capacitor is tied directly between the output of the half-bridge circuit and chassis ground via the heatsink."
Their diagram:
From (Steve's site): "Notice that if the coil arcs to the primary system, either the IGBTs or diodes (which are always there) will let the current go safely back into the DC bus caps, where it can couple through C3 (a 100nF PP film cap) to the ground connection of the secondary coil. If the capacitor was not there, then it would likely blast through to the heatsink and destroy the IGBTs insulation. Or, it will find some other way. The idea here is simply to provide an excellent path for the streamer current to take, simple. "
Steve's diagram:
They look different to me, though maybe EVR suggests the same method as Steve elsewhere. It looks like mbd found the same info on EVR's site as above, and had the same thoughts as I did about the capacitor loading the bridge.
Registered Member #43278
Joined: Sat Feb 22 2014, 09:18AM
Location: Sydney
Posts: 9
Hydron's reply identifies exactly the issue I was trying to raise.
Normally, I would ponder a response like:
To clarify, I mean it's exactly the same...
for quite a while before I posted a follow-up, but in this case I think that I will really struggle to work out the reason behind Eric's statement unless someone offers me a clue ... (My apologies if the reason is obvious to more enlightened beings.)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It is not "exactly the same"- in the EVR method the high frequency output of the bridge is injected onto the heatsink through the first capacitor, where it then gets shorted to ground, for extra stress, carnage and malfunction.
The methods are only equivalent in so far as the DC bus bypassing should look like a short circuit at high frequencies, so it shouldn't matter whether you bypass one pole of the DC bus to the heatsink, or both.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I don't get this as well. A relatively large cap from bridge output to ground would just cause a horrible RF interference as well as extra stress on the transistors, which could even be fatal. And now realize that these coils are being sold even for professional purposes, for no little money.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Good information everyone. I agree with your assessments of this bypassing scheme and that it definately warrants additional attention and re-examination.
Thanks for bringing it out and discussing it. This is what makes 4HV so great!
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