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"MMI 1" - New DRSSTC

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tesla500

Sat Feb 07 2009, 09:35AM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Hey guys,

I recently got a lot of work done on my first DRSSTC. I went for a medium size coil, about as big as is practical to run indoors.

Specs:
Primary: 6T 3/8" copper tube, 1/4" spacing, 15" diameter vertical helix. Tapped at around 5T
Primary cap: 0.52uF 4.8kV made of 1.2kV 0.68uF GE film caps
Secondary: 10"x32" 26AWG on sonotube
Toroid: 4x14" really banged up toroid from an old coil, for testing. Probably 6x16 or so for the final version.
Frequency: 64kHz with current toroid
Control: My own PCB layout, based heavily on Steve Ward's controller with beefed up FET drive and a metal housing fiber receiver.
Inverter: Full bridge, currently using 3x HGTG20N60A4D IGBTs on each leg, room for 5.

I wasn't keen on using expensive or hard to find IGBTs, so I went for an "MMI" approach, with several cheap TO-247 IGBTs in parallel in a full bridge. The bus cap is also composed of many small caps in parallel, 30x 150uF 400V high ripple electrolytics, giving a very low inductance, in about 1/4 the volume of a single large cap of similar ratings.

Schematics of the control and inverter boards are attached below the pictures. I can post Altium source files or gerbers if you'd like, and I have a few of the PCBs available if anyone is interested.

Currently I'm running it off a rectified 120V variac, but eventually I'll modify the boost PFC stage from a scrap battery charger from work to supply 385VDC. Max sparks achieved so far are about 3' long, running at 150VDC and around 500Apk for 200uS burst length, 120Hz pulse rate. I'm starting to have problems of arcs breaking out of the strike rail and hitting the secondary when going longer than about 3', perhaps the wood primary mounts weren't such a good idea, or the strike rail needs to be lowered.

David

]control.pdf[/file]
]power.pdf[/file]

101111

Sat Feb 07 2009, 04:59PM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

Oh my!.... :O

This looks fantastic, keep on the good work.

Tom540

Sat Feb 07 2009, 05:19PM

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

Hey that looks awesome! I like that bridge layout. I love those IGBT's too. I'm suprised about your wood primary supports. Ive tried it a few time and both times I had the wood break between turns. I wasn't using plywood though. I might have to give that another try. I would say lower or widen the strike rail. im not sure what wood supports would have to do with those strikes you're getting.

MRacerxdl

Sat Feb 07 2009, 05:32PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Wow, that is a very impressive setup! Very good work!

tesla500

Sat Feb 07 2009, 07:04PM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Thanks guys!

I'll try lowering the strike rail and see how that works. I was pretty careful with the wood supports, I baked them at 125C for several hours, then dipped them in varnish to keep them dry. I was almost going to use plain wood instead of plywood, probably a good thing I went with plywood instead, if plain wood breaks easily.

David

...

Sat Feb 07 2009, 07:06PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Wow that really is amazign looking!

Good job with getting all of your bridges working together happily, I have seen a few failed projects trying to make that work!

Sounds like a geat coil

101111

Sat Feb 07 2009, 07:27PM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

There is one thing I think you can improve: The Alu bars going over the IGBTs to keep them tight to the heat sink should have one screw between each IGBT for best thermal contact, and you should also use thermal grease.

tesla500

Sat Feb 07 2009, 08:49PM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Yep, moving the strike rail down fixed it, at least for operation at 200VDC.

The Al bars are solid 1/2" square, have silicone model airplane fuel line under them to provide even pressure, and there's a phase change thermal material between the IGBTs and the heatsink. It's a 2mil Kapton sheet with aluminum oxide powder mixed with wax coated on each side. When the devices heat up and the wax melts, it becomes just like thermal grease.

I'm currently removing the PFC stage from the scrap battery charger, then I can test at higher bus voltage.

David

GeordieBoy

Sun Feb 08 2009, 12:13AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Neat layout and neat PCBs too. 3ft sparks sounds good for only 150VDC bus voltage.

You shouldn't have any problems with the active PFC pre-converter provided that you keep it's own DC bus capacitor present, and then connect it's output to your inverter boards. You can put a ferrite common-mode choke over this cable carrying power from the PFC to the inverter. This choke will prevent hash from the DRSSTC inverter from getting back into the PFC controller.

The only problem you might experience is one of high inrush current due to all of those paralleled electrolytics on your DRSSTC inverter boards!!! This inrush might blow the delicate fast-diode in your PFC pre-converter if it's from a dinky power supply and doesn't incorporate soft-start resistors.

-Richie,

tesla500

Sun Feb 08 2009, 07:59AM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Good progress on the PFC, it still works after cutting the charger in half. I replaced the stock silicon PFC diode with a 10A 600V silicon carbide schottky for operation at higher power. This PFC should be good to about 12A mains draw from 120V to 240V now.

I was hoping to get away with a smaller cap on the PFC output, using the inverter bus caps to do most of the filtering, but it looks like that won't work, the small electrolytic I tried got warm quickly. Shouldn't have cut the heatsinks so short, I'll need to cram the original caps back into the case somehow.

Inrush shouldn't be a problem. This charger has a precharge resistor with a relay that shorts it out about 1 second after plug in. A bigger resistor certainly will be needed, there's 10x as much bus capacitance now! The original was 100ohm 1W, I've got a 16ohm 30W wirewound that should do nicely. I'll also add a diode from input bridge positive to bus positive, so if the PFC shuts down for some reason the SiC diode won't have to take the large peak currents.

David

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