Building a bigger DRSSTC

[davekni]

Looks like 3 diodes should be OK for peak current.  Whether they are OK for RMS current depends on burst length and repeat rate and how well energy is coupled to the secondary and dissipated in arcs.  At high burst repetition rate and poor energy coupling, the diodes could overheat, especially with just sheet-metal heat sinking.  Just check the sink temperature after some short runs to make sure it stays reasonable.  (I have a somewhat similar issue - IGBTs with internal diodes where the diode thermal resistance is about 3 times higher than the IGBT's.)

200A is probably OK as long as you have a bit of phase lead and gate drive above 15V (say 18V+).  That also presumes reasonable current sharing.

Be careful handling your H-Bridge as it is.  Reaching to pick it up and accidentally touching a gate lead before the bulk supply could easily fry an IGBT with ESD.

[Mads Barnkob]

Wow... that is a one of kind board, never seen such a large hack'n'slash circuit board! Kudos for going all the way with the tools and materials at hand

Did you do continuity test or high voltage test to check for large enough respect distance, seems there is some copper pieces still along the insulation zone in the middle?

Did you test the board for each IGBT soldered on? Could maybe have used terminals to make exchange of IGBTs easier, I foresee a few blown ones

[Max]

Short story: I'd not feel safe with that little insulation on your bridge. Get at least 6mm creepage distance. And remember, it does not matter if you have 20mm almost everywhere, electricity will find that one place where a tiny amount of copper was not scratched away and blow your IGBTs through the room.

Long story: I had to learn the above the hard way. Twice.
I've build two DRSSTC bridges so far. The first one was a 5 layer stack of 2mm Aluminium plates and 2mm polycarbonate plates as insulator and "filler". The stack was hold together with nylon screws which at the same time fixed the 10 bus capacitors and the 4 IGBT bricks to the bus bars. Performance was great, insulation not so much. Since the layers were only 2mm thick and since I didn't take any other precautions the creepage distance was 2mm, too. This was barely sufficient for 230VAC (325Vbus), but it blew up at 400V (566Vbus) - violently.




So I redesigned the whole thing, this time with more creepage distance. The inner ground layer had bigger holes to get even more creepage to the screw holes. This time I replaced the 2mm PC by silicone. The idea being that with its flexibility it would fill tiny gabs between the layers. All aluminium pieces had 3mm radiuses and all edges were sanded. No sharp points this time. During assembly I cleaned all parts with ethanol and made sure there was not even the tiniest amound of (conductive) dust left over. This, together with the increased 7mm creepage distance "inside" the layers should prevent any problems. Well, I had to reduce it to 4mm on the bottom surface because there was no space for the insulation layer. And guess what: it worked great. Until it blew up  . Bottom layer, right where I had to reduce the creepage distance.

Assembly (the easies part actually):

And finally, the most interesting part, when it blew up, and what damage that made:

After disassembling and reassembling everything I managed to add a small bar between those two rails. This seems to work so far, but all in all this bridge design is not optimal. I have new ideas which should be almost as good concerning inductance, but wayy easier to isolate properly.


Kind regards,
Max

[davekni]

I'd agree that it would be wise to clean up spots with tiny gaps between copper, such as this circled spot from your image and any similar short gaps:


Also make sure there are no slivers of copper (or other metal) around.  I construct many projects with dremel-tool cuts in copper-clad FR4.  Sometimes the cuts leave tiny whiskers of copper that need to be cleaned up.  Beyond that, at 325V, you are probably fine.

Max,
Are you certain there wasn't some conductive contamination initiating the arc?  There are 1200V IGBTs available in TO220 packages with only 1.3mm between leads (and at least one 1600V part).

Line-voltage spacing requirements are high because line-surge capability needs to be ~3kV, and because failure is a potential safety issue.  I think 600V is unlikely to jump a clean 2mm insulating gap.

BTW, here's an image of my latest copper-clad with cuts board, after adding labels:

This isn't a high-voltage board, just an example of technique.  Cuts are with a small rotory-tool cutoff wheel, with edges cleaned up using a knife and light sanding to finish.

[Max]

(@mods, @ritaismyconscience, sorry for the OT...)
The argument with the smaller TO packages is good... Didn't think about that. Honestly I can't guarantee you that there was really no contamination. As I said, I wiped everything with ethanol as good as I could.
The best theory was that a transienst initiated the arc over, and the bus capacitors then dumped all their energy in it. The first version had 2*440V = 880V TVS diodes for each IGBT, the newer has four 0.68uF snubber capacitors. After blowing up, too, I soldered 880V TVS diodes to the terminals of each snubber capacitor (only place where they fitted). That setup ran for about 10-20 minutes without problems so far. Won't have the opportunity to test it again til the end of the lockdown.


Kind regards,
Max

[ritaismyconscience]

It looks like I kinda messed up with the insulation, and there's a 1mm gap between the top and the bottom of the PCB.


Would coating the gap with something work?

[davekni]

At 325V I expect 1mm will be fine.  You could scrape the board edges with the edge of a knife to make sure there's no bits of solder or other conductive debris shortening the 1mm distance.

The bidirectional TVS diodes on the gates is great for making the board survive handling and associated ESD events.

[ritaismyconscience]

Would coating it in nail polish help? I read somewhere that coating the PCB can increase breakdown voltage.

Also I'm going to use a doubler to power it.



I'm using 8x 2700uF 200V

[davekni]

Nail polish is a good idea.  Make sure the board edge is clean so it adheres well.  Coating doesn't help if not bonded to the bare board between copper.  I'd use clear polish, although colors are probably fine, presuming none of the color pigments are conductive.

What is your line voltage?  I'd thought you were in a 230V part of the world.  Peak doubled would be 650V, too high for your IGBTs.  If 120V line, the peak doubled is 340V, so should be good.

[ritaismyconscience]

Line voltage is 110V (my previous DRSSTC had a doubler too)

[Max]

It is already hard to repair this in case of a crash, and I think nail polish won’t improve that. It‘d be right where you‘ll most likely have to solder: around the diodes which go through the board, and the board edge between the IGBT legs.

I‘d rather try to omit voltage doubling and stay at ‚easier‘ voltage levels. It should be possible to get similar results by reducing your tank impedance.


Kind regards,
Max

[ritaismyconscience]

I made sure there was >2mm insulation around the diodes, so the weak point is probably the edge of the board.

[ritaismyconscience]

Here's my plan: I basically cut out 7 wires of the same length to connect to all of the emitters, and I added 7 1n5819 diodes and 7 10 ohm resistors to each gate. I'm going to connect one output of my GDT to these wires.

Will this work? I'm worried about the emitters being at different voltage levels because of the high current.

Edit: Can I use a quarter watt 10 ohm resistor?

[davekni]

Yes, voltage drop variations are a concern, but probably not unreasonable for your build.  Your good layout will minimize inductive drops.  Resistive drops are likely within reason too.  Driving the gates with +-20V allows for +-5V variation while maintaining at least 15Vge.

If you want to be more sure of Vge, use multiple GDT windings, with each winding driving a smaller set of IGBTs.  I'd posted somewhere back in this thread about that option.  Even 28 winding pairs, one per IGBT, with all primaries paralleled isn't unreasonable.  (The other half of each twisted pair winding are all paralleled for the GDT primary.)

Ideal way to find gate resistor power is with simulation.  For a crude estimate, calculate the energy stored in the gate capacitance, then presume that energy is dissipated in the resistor every cycle.  (Worst-case, it could be 4 times as high, since the gate is swinging twice the voltage, from -20V to +20V.)

On an unrelated note, attaching TO247 packages to heat sinks with their "mounting" hole is not always effective for heat sinking.  The attachment force isn't where the die is located.  Most commercial designs use some form of clamp or spring clip applying force over the die, roughly 1/2 way from the leads to hole.  Depending on how compliant and thermally conductive the pads are and how consistent screw tightening torque is, this may or may not be a big issue.  All it takes is one hot (poorly heat sinked) part to fry shorted, and the failure will cascade.  (I think the mounting holes are there for historical reasons.  Designs from 50 years ago used the holes, until engineers figured out that thermal performance was unreliable that way.)

[ritaismyconscience]

I searched it up and I found this:
http://www.aosmd.com/res/application_notes/package/AN101_TO220_Guidelines.pdf

On page 4, there is a graph showing thermal resistance vs torque on screw, and it seems like 0.5Nm is sufficient. Also, overtightening the screw seems to make the thermal resistance worse for some reason.

[Mads Barnkob]

I searched it up and I found this:
http://www.aosmd.com/res/application_notes/package/AN101_TO220_Guidelines.pdf

On page 4, there is a graph showing thermal resistance vs torque on screw, and it seems like 0.5Nm is sufficient. Also, overtightening the screw seems to make the thermal resistance worse for some reason.

When tightened too hard, the part will start bending from the through-hole and bend upwards around it in a circle, so you start to loose contact again.

Please do remember to have cameras filming when you power up that bridge, its spectacular either way if it runs long sparks or huge explosions Multiply cameras from different angles would be great!

[Max]

I'd try to use the exact same gate resistors for all IGBTs. While different types might have sufficiently similar resistance, this might not be the case for their parasitic inductance. Especially the bottom one looks like it could be a wire wound resistor (= high inductance). I suggest to twist the gate and emitter wires for each IGBT to reduce stray inductance - and probably even more important here: to get similar stray inductance for all the gates.

Finally, I'd quadruple check every single signal before feeding any amount of energy in this thing. It looks like it would be an incredible amount of work to repair this thing.


Kind regards,
Max