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Half bridge sstc revisit

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teslacoolguy

Tue Aug 12 2008, 08:10PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Thanks for the tips. I have some new fets on order so until they come i am at a standstill. For the grounding i have mine grounded to a copper water pipe that is about 5' away from the coil. I would guess that the copper piping would have a similar effect to shaun's workbench ground. I cant ground mine to the workbench because it is wood. Also Shaun mentioned something about the heatsink. I am actually using proper heatsink insulator material but when the fet blew it also kinda burned that up.

Shaun

Tue Aug 12 2008, 08:27PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I've never actually measured continuity between any part of an FET and the heatsink, even when I was using thermal grease and the FETs were on the same heatsink. Then again my meter only reads using about 9V, and mains voltage could fare differently.

I don't know if any of the failures I had were because of conduction between the two, but it didn't seem like a good idea to keep both FETs on the same aluminum slab separated only by a few mm of thermal grease.

You appear to be using thermal pads which are probably a much better alternative to thermal grease. I wouldn't expect any problems in that area, provided you replace them if they get burned.

Brett Miller

Fri Nov 14 2008, 12:57AM

Registered Member #593 Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50

Shaun,

Your experience in tweaking your half-bridge SSTC (extremely) closely mirrors my own. In fact my longest sparks to date were around 14 inches. You (and teslacoolguy) should be able to read my specs on page 8 of the post own coil section. They are typical specs for a Ward design of this type, but I certainly learned a lot and had fun building this about a year and a half ago or so...well, maybe it was two years, since after that I built the Terry Fritz SISG TC and was side tracked from "conventional" SSTC work for a while. Anyway, since then I've been out of circulation in the coiling community for a while. Perhaps my corroboration of Shawn's advice will help underscore some of these truths.

I think I might convert mine to an H-bridge as well, something I always wanted to do, or maybe even start from scratch, so I can make a compact and presentable system for once. Shaun, I bet those 28 or 30 inch sparks are really loud. My halfbridge system with it's sparks half that length is really deafening at the higher PRF settings.

Let's see, other issues: I haven't blown a single one of my IRFP260N MOSFETs, even though I logged countless hours fooling around with that thing, including running CW and two different interrupter designs. I think one reason is that I used a monster heat sink, which was isolated from the FETs by thermal pads. I can feel the FETs after a lengthy run and they are room temperature, which the exception of CW operation, after which they are a bit warm. Of course, I only run CW for short bursts seldom exceeding 10 seconds or so.

Ok, well, if I think of anything to post regarding this design (and if this thread resurrects) I will. I've been really out of coiling for a little over a year, but everything is coming back rapidly and I am quickly finding that old SSTC project ideas are once again teasing my emotions...and new ones are emerging from the creative nebula.

Shaun, I'd like to see any documentation you have on your H-bridge (if you did anything non-standard)...do you have a website?

-Brett

Nicko

Fri Nov 14 2008, 09:46AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

This was our first SSTC (first coil actually) and worked flawlessly (respect to SW). My earth was to mains ground as per Steve's specs - worked well - see getting an SW SSTC to work. Many thanks to Karim & others at the UK Cambridge 2008 Teslathon - they helped us tune the coil to about 10" sparks, with some improvements still to go. Runs at about 450kHz (too high). There are some photos of our setup in the link above.

2.5" x 14" secondary, 4" diameter, 7 turns of 12AWG primary. 6" topload - see pre-tuning photo. Get the earth wrong, as I did at the start by having a coiled extension lead powering it, and this is what you get. Here's a photo of the coil setup - by moving the primary about 4" up the secondary, reducing to 7 turns and leaving about 0.25" between each primary winding, we got a far better result - moved it up until we got flashover, then very rapidly moved it back down a bit. Coil has 4 coats of polyurethane to help it a bit...

I did build a decent control box (schematic) to try to limit problems with poor earth - everything to a common point with 12AWG tri-rated cable...

Now producing some nice PCBs for it with changeable bridges, so we can easily experiment with full vs. half bridge, different GDT drive capacitors, half/full wave rectification & variable/no smoothing on the HV DC etc.

Cheers!

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