Author Topic: Primary-Strike Protection Grounding Scheme - Insanity?  (Read 546 times)

Offline nabzim

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Primary-Strike Protection Grounding Scheme - Insanity?
« on: February 18, 2019, 05:06:18 AM »
OK. So we are all familiar with the standard method of primary-strike protection, that is, connecting the RF-ground to the negative DC Bus via capacitive-coupling...
I have come to the realization, that this does NOT make sense.

If a streamer strikes your primary coil, the streamer-current must travel through one/both of your low-side IGBT's (full bridge), or through your low-side IGBT and/or Bus Capacitor (half bridge) to get from the Primary coil to the -Vbus rail.

Why is it not instead the standard to capacitively-couple the RF-ground directly to the Primary Coil? (a.k.a. the center-point, between high-side and low-side IGBT's)
This would keep the high-energy current from traveling through the semiconductor.

Are my assumptions here correct? If there is any logical reasoning behind the standard grounding-scheme (other than copying Steve Ward), then can someone please educate me? ???

Offline Hydron

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Re: Primary-Strike Protection Grounding Scheme - Insanity?
« Reply #1 on: February 18, 2019, 08:44:19 AM »
Coupling the non-MMC end of the primary would put a significant load on the bridge - take a look at the impedance of the typical coupling caps (e.g. 0.1uF) at the switching frequency. I suspect it would also cause EMI and safety issues, as any current through this capacitor would have to flow via the mains wiring to wherever the earth-neutral bond point is.

Given that the issue doesn't seem to be the strike current (on the order of a few amps only) flowing through the igbts (which have anti parallel diodes included to provide a path when off) but was more about voltage stress between the bridge and heatsink (ground) when hit, the standard method seems to do the trick.

Offline Teravolt

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Re: Primary-Strike Protection Grounding Scheme - Insanity?
« Reply #2 on: February 18, 2019, 09:25:24 PM »
adding beafy varistors from the primary to ground should increase robustness and a decent strike rail to ground is common practice hear

Offline nabzim

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Re: Primary-Strike Protection Grounding Scheme - Insanity?
« Reply #3 on: March 06, 2019, 08:33:48 AM »
Coupling the non-MMC end of the primary would put a significant load on the bridge - take a look at the impedance of the typical coupling caps (e.g. 0.1uF) at the switching frequency. I suspect it would also cause EMI and safety issues, as any current through this capacitor would have to flow via the mains wiring to wherever the earth-neutral bond point is.

Given that the issue doesn't seem to be the strike current (on the order of a few amps only) flowing through the igbts (which have anti parallel diodes included to provide a path when off) but was more about voltage stress between the bridge and heatsink (ground) when hit, the standard method seems to do the trick.
Thank you. I'm most likely going with the negative rail.

Offline nabzim

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Re: Primary-Strike Protection Grounding Scheme - Insanity?
« Reply #4 on: March 06, 2019, 08:47:41 AM »
adding beafy varistors from the primary to ground should increase robustness and a decent strike rail to ground is common practice hear

MOV's in addition to the capacitor? and in parallel, I'm assuming?

Grounded strike rail is a certainty! It sure comes in handy on my spark-gap coil: youtu.be/3d4ZDvA5Kcc   

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Re: Primary-Strike Protection Grounding Scheme - Insanity?
« Reply #4 on: March 06, 2019, 08:47:41 AM »

 


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