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4hv.org :: Forums :: Tesla Coils
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Opinions on DRSSTC power board design

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tesla500
Tue Oct 21 2008, 07:02AM Print
tesla500 Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106
Hi All,

I just completed a schematic and PCB layout for a DRSSTC power board, and I'd like your opinion on it. I'm building my first DRSSTC and I'm trying to come up with the most bulletproof power board possible.

I've tried to go with common, cheap parts as much as possible, and used SMT parts to save space and keep inductance down. I plan to have the board made with 4oz copper, as opposed to the normal 1oz thickness.

This full bridge uses 5 TO-247 IGBTs in parallel as each switch element, to allow the use of cheap IGBTs, and give a high peak current rating. I'm thinking of using HGTG20N60A4D IGBTs which are rated 600V 280A maximum pulse, for a bridge rating of 1400A max without exceeding datasheet spec. These cost about $4.73 each. The board has room for terminal strips to allow easy IGBT replacement.

I've opted to go for many small parallel bus capacitors for low internal resistance and good heat dissipation (large surface area to volume ratio) The main bus cap will be 30 of either 150uF 400V or 100uF 450V KXG series from United Chemi-con.

There are some MCX RF connector jacks installed to provide easy and accurate measurements of the low side gate drives and the outputs, using MCX-BNC and BNC-scope probe adapters.

The schematic and board layouts are in PDF format below.

Schematic
PCB top layer
PCB bottom layer


The unknowns I can think of are:

-Is the inductance of the HF current loop from the bus caps, through the IGBTs, and back to the bus caps low enough?

-If the above inductance is low enough, are TVSs necessary?

-Will the electrolytics have low enough ESL to allow operation without other capacitors? I've made provisions for 20 x 500V 0.22uF SMT ceramic caps Those caps are expensive and I'd like to avoid using them if possible.

-Will GDTs be suitable? Each 5 IGBT bank has a Qg of about 1uC. I'm thinking one GDT on each half bridge.

-Will shoot though be a problem with the individual gate resistors on each IGBT? Those are there to prevent oscillations that can occur when directly paralleling gates.

Any opinions/critique/advice is much appreciated!

David
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Mates
Tue Oct 21 2008, 10:09AM
Mates Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
I think that paralleling 5 IGBTs is too big piece of cake... I made a lot of experiments of paralleling the IGBTs in a single switch design and the power peak you can switch is not following the linear additive function. I personally think that three transitors is the rational maximum unless you have very precisly matched components (including resitors). Otherwise you can use the GDT for all three (or five?) transitors, the limiting factor is rather in the gate drive. There are few schematics for really powerful gate drivers around this board...
Good luck with your project - It looks very attractive wink


BTW: Always use the TVS! - they are cheap and can save a lot...
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tesla500
Wed Oct 22 2008, 06:55AM
tesla500 Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106
Hi Mates,

I was thinking that the ZCS of a primary current feedback DRSSTC would help with any imbalances during switching by allowing the switching to occur at relatively low current, hopefully completely within the rating of 1 IGBT. Was the application you were referring to with parallel IGBTs ZCS or hard switched?


David
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Mates
Wed Oct 22 2008, 10:39AM
Mates Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
tesla500 wrote ...

Hi Mates,

I was thinking that the ZCS of a primary current feedback DRSSTC would help with any imbalances during switching by allowing the switching to occur at relatively low current, hopefully completely within the rating of 1 IGBT. Was the application you were referring to with parallel IGBTs ZCS or hard switched?


David
Once you are able to keep the system all the time close to ZCS than you can parallel IGBT's without limits... But driving 5 gates in parallel will be challenge - I would go for min 30A, 20V per five transitors, each gate its own resitor and zeners. You should also find some rather big toroids for your GDT's. Otherwise, your scheme and PCBs proposals look great. I'm thinking of using it for my own future projects - good job.
Good luck...
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Dylan
Thu Oct 23 2008, 10:58AM
Dylan Registered Member #1024 Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
nice layout
If I was to change anything I would make the gate tracks thicker and try to extend the emitter tracks underneath them, but really it looks good.
I have a few tubes of those 20n60's so I may well put one of your boards together!
I would probably add some big poly caps instead of the expensive ceramic smt's.
And I would probably drive the gates with one or two big gdt's wound with shielded audio cable, and driven with a descreet driver for good power.
Anyway looks very good,let us know how it goes!
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