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Semikron DRSSTC Twin Coils!

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loneoceans

Fri Apr 17 2015, 01:35AM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

Hi everyone,

I recently got some white Semikron IGBTs which seem to be more common in Europe than in the US, (vs the black Powerex / Mitsubishi IGBTs). So I thought it would be a good exercise to build a medium sized coil from these IGBTs, and I thought I'll start a thread to document the design and construction progress. These IGBTs are the ultra-fast variety (125 series 400A) which seem to be designed for high frequency resonant inverter operation. I have only used CMx00 IGBTs, so I'll find out how well these stack up against the competition, seeing that there aren't too many Semikron IGBT coils around the forum. Steve Conner's Odin coil comes to mind.

1429234687 4098 FT0 Semikronbridgerender1z

The first thing I did was to work around my constraints, and started designing the bridge in CAD as a very rough layout. I had a bunch of nice Itelcond capacitors on hand so these will form the main bus capacitors, which will be mounted on a laminated bus directly to the IGBTs for the lowest possible bus inductance. The frame will be made of basic 80-20 aluminium framing.

In deciding the operating frequency of the coil, I wanted to balance the frequency with being low enough for the IGBTs to handle, but not too low for it to become annoying to wind (too thin wire). I decided on a 8" secondary to reduce the number of turns, and the length to be about 30" - though the size was basically the largest pipe I could fit on a lathe I happen to have access to. Adding a suitably sized toroid about the same size of the coil, this gave me an operating frequency for about 60 - 65kHz with AWG 26 wire. I haven't decided how I would get the toroid done, but this could be a spun toroid, or a fancy spiral toroid inspired by zrg's large coil.

For the tank capacitors, I decided to use large snubber capacitors - again parts I had on hand, which will give me a good 375nF capacitor bank at lots of volts and lots of Irms capability. From this, I did a quick mockup of how the coil will look like and it should stand about 5 feet tall.

Since the coil looks about right, it's time for construction! Stay tuned as the project progresses.

Mads Barnkob

Fri Apr 17 2015, 07:57AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Looks good and you seem to have all the basic numbers in the proper ratios. A medium coil like this should pose no particular problem.

A few things you should consider

- The aluminium frame might be too close to the primary and could present a closed 1 turn loop that would eat power.

- Watch the voltage rating of the power resistors on the heat sink. If you are burning off two too different potentials, you might risk flash over to the heat sink .

loneoceans

Fri Apr 17 2015, 04:52PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

Mads Barnkob wrote ...

Looks good and you seem to have all the basic numbers in the proper ratios. A medium coil like this should pose no particular problem.

A few things you should consider

- The aluminium frame might be too close to the primary and could present a closed 1 turn loop that would eat power.

- Watch the voltage rating of the power resistors on the heat sink. If you are burning off two too different potentials, you might risk flash over to the heat sink .

Thanks these were indeed things I were considering during the design. I decided on 80/20 because they're easy to put together and the workshop I have access to has a lot of leftover small parts. Because of the same issue you talked about, you can see that I raised the primary up on stands away from that top square 'ring'. I believe it will eat power, but hopefully not too much. Depending on how this works, I'll also try to see if a gap is required or not.

For the resistor question, I was also concerned about this and checked the datasheet to make sure. It seems like the rated operating voltage is (P*R)^0.5, which puts me just around 1.2kV for the bleeder resistor. The 25W packages are rated for 1kVrms and the 50W packages 2kVrms, so I think they should hold up.

Right now I've got most of the parts of the bridge so I'll begin constructing that soon!

Question for people who have used Semikron bricks before - what sort of current do you push these guys to? Mine are 400A IGBTs so I think 800A would be good for robustness, and 1.2kA should still be ok, despite the Icrm being only 600A :)

Hydron

Fri Apr 17 2015, 07:45PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

I've pushed the 200A 125s to at least 650A at ~80kHz without issue. Nice and quick, much less ringing than with CM300s. 400A at 300kHz (!) worked too for a short run :).

Saturation voltage is pretty high, otherwise they seem great. I ended up putting CM300s into my coil instead, but that was mainly because they were more proven performers for ~1kA peak current.

Edit: actually it was 750A, as in this post:

Seems they've been pushed much harder too, that thread is probably quite useful for you.

Hazmatt_(The Underdog)

Sat Apr 18 2015, 01:46AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Hey I got a question here, When you guys build these things on a metal base, which acts like a shorted secondary turn, how do you deal with the current flowing through the metal?

At high power does the base even get warm? and how much actual power loss is there would you estimate?

loneoceans

Sat Apr 18 2015, 04:13AM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

Hydron wrote ...

I've pushed the 200A 125s to at least 650A at ~80kHz without issue. Nice and quick, much less ringing than with CM300s. 400A at 300kHz (!) worked too for a short run :).

Saturation voltage is pretty high, otherwise they seem great. I ended up putting CM300s into my coil instead, but that was mainly because they were more proven performers for ~1kA peak current.

Edit: actually it was 750A, as in this post:

Seems they've been pushed much harder too, that thread is probably quite useful for you.

Hydron thanks that was very useful! It's good to hear that the 200A 125s have had good performance from some coils! so I think I can safely push these IGBTs to 1.2kA I think! That's correct as well, it looks like conductive losses will be higher due to the higher saturation voltage, but they do seem to have less switching loss than the CM300s. It also seems like Steve Conner's Odin coil didn't have good luck with the T4 IGBTs, but his new bridge of 400A 123s look to be doing well making 9 feet of spark at around 1+kA, as with Eric reporting 1.2kA with 200A 125s. So with this coil, hopefully I'll be able to add another data point :) for these IGBTs.

Hazmatt_(The Underdog) wrote ...

Hey I got a question here, When you guys build these things on a metal base, which acts like a shorted secondary turn, how do you deal with the current flowing through the metal?

At high power does the base even get warm? and how much actual power loss is there would you estimate?

What do you mean metal base? I found that any metal plate near or middle of the primary coil gets blisteringly hot in a short amount of time (tens of seconds), since my first DRSSTC had a metal plate at the bottom of the secondary acting as the ground connector, and I quickly found out how hot it got! However, it didn't seem to cause much % power loss since when I removed it I didn't see any noticeable change in performance.

loneoceans

Wed Apr 22 2015, 08:37PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

More progress has been made. Here are continued updates.

17048996988 8f3066ecb7

I finalized the bridge design and it uses an integrated rectifier, laminated bus and voltage doubler. This allows me to put all connections on the bus plates and it turned out better than I thought it would in the computer! So it's time to build it.

16615362684 2c11d4be4d

I drilled and tapped the mounting holes on a large heatsink. This went easily enough since I had everything in CAD and I could just print out a to-scale drawing and punch out the holes :). The mounting brackets and bleeder resistors were also mounted, along with the two Semikron bricks.

16615362694 844c146cb9

The laminated bus was cut out from aluminium and fiberglass sheeting and glued together. The result is a little bit rough on the edges since it's hand made, but I think it turned out pretty good.

17050040618 C736b18388

In goes the bus caps. It's nice how everything finally came together exactly as it looked like in CAD.

16617597783 D393b138b0

The MMC is pretty large, but at 150Arms it should be more than enough for the coil. I made some nice acrylic supports for it.

17051626119 6b1925f9e3

Overall I think I'm quite happy with how this came out.

Onwards to continue on the rest of the coil!

Gabriel35

Thu Apr 23 2015, 03:42AM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

State of the art work! Pretty clean and good looking, Congrats!

hen918

Thu Apr 23 2015, 04:57PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Gabriel35 wrote ...

State of the art work! Pretty clean and good looking, Congrats!

+1!
I wish I had the attention span to get my stuff to look like that!

loneoceans

Fri Apr 24 2015, 05:32PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

Thanks for the comments everyone! Would also appreciate any suggestions or ideas as I go along.

Continuing on the project, the bridge and MMCs were getting hard to move around since they're quite heavy, so I got started working on the enclosure. I got some aluminium framing measured and sawed up, and started putting them together.

17049470477 E1b4cceeda

Framing is relatively easy to put together, if not somewhat expensive. However this got together quite fast with a cordless screwdriver.

16634422284 A0fe7c0986

I also bought some nice caster wheels for the coil since I expect this to be quite heavy when it's done.

17049470507 E6bd74cca7

Finally I cut up a piece of polycarbonate which will act as the main base for the coil. The nice thing about using this framing is that I plan to simply cut out side-panels which will slot directly into the grooves of the frame. I tried to make the sides to be the golden ratio; no idea if this makes it look nicer or not

Next up will be to make the primary coil platform and supports, and to wind the primary coil. The secondary coil needs to be done early as well since it'll take some time for the varnish to dry once it is wound.

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