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Feathers

Sun Apr 07 2013, 07:50PM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

Thanks!

I understand why the bus connections have to be as close as possible (preferably laminated), and I have plenty of 1/8" copper bar ready to make a low-inductance setup. I also have some big 2uf snubbers that will sit on top of the positive and negative rails on the IGBTs.

I understand that the RIFA capacitors are sealed particularly well, have a long life, and extremely low ESR.
I would mount one of these on the bridge, and then feed the coil ~400vdc from a doubler on the controll end.

OR

I would series up the electrolytics I have right on top of the bridge, and use that as my voltage doubler. It would be slightly larger and higher inductance (I would need 3 extra inches of copper to series up the caps), as well as using normal sprague electrolytics (no fancy low-ESR super-sealed, etc.).

My question is, what would a more experienced coiler suggest I do? Putting the doubler right on top of the bridge would make the design more compact, but may not be as efficient as a single RIFA cap.

I don't think I have enough of a "feel" for these components to make an informed decision.

Any input welcome!

Finished a gate drive transformer:

Primary supports laser cut:

Physics Junkie

Sun Apr 07 2013, 08:41PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

The secondary looks great! One thing you must do though is shorten up the GDT secondaries. No more than 2" if you can manage.

Regarding the DC bus caps, the single rifa cap would be a better choice -BUT- dont use a voltage doubler with the sprague caps in that case, it's just not necessary and very bulky. Find a 120/240 step up transformer. The step up transformer would obviously be powered by the variac, and the rifa cap on the secondary side of the transformer. Doing it this way is less inductance on the bridge using the one rifa cap, less bulky, etc..

-OR-

Leave the rifa cap out of it, and use only the voltage doubler configuration with sprague caps in series as close to the bridge as possible.

Feathers

Sun Apr 07 2013, 10:54PM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

Thanks.

I'm just leaving them long untill I get everything fitted up.

I think eric goodchild used a step-up transformer untill he got his solid state SCR variac/boost converter up and running. Worked well for him!

I'll go with the RIFA on the bridge, the setup would have a larger capacitance, smaller footprint, and lower inductance, than the doubler, and will look better.

I appreciate the input!

Steve Conner

Mon Apr 08 2013, 10:40AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

HighVoltageFeathers wrote ...

I know we run with a safety margin, and that AC rating is substantially less than dc ratings, but why do I see so many tank caps with >8kvDC ratings, on coils with primary voltages under 500 volts? I realize that peak current handling capacity is essential, but have been confounded by the voltages.

Resonant rise. The voltage across the tank capacitor is theoretically almost unlimited if things go wrong. I've always recommended that you design for a loaded Q of 10, which means that the voltage across the tank cap could be 10 times the bridge's output voltage. In practice it's probably somewhat less.

The voltage across a capacitor is a function of the current passing through it and the frequency. So the overcurrent trip on your driver, if it's set properly, also protects the tank capacitor from overvoltage.

Feathers

Mon Apr 08 2013, 04:27PM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

I see. I'm going to use the CD 942 .15uf 2000v caps.

Feathers

Sun Apr 21 2013, 11:26PM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

So I've made some significant progress.
The base is finished, CNC routed from 3/4" MDF, and the primary coil is sitting happily in it's supports.
I've also got a beautiful 18"X4.5" seamless aluminum toroid from EasternVoltageResearch.

Untitled by HighVoltageFeathers, on Flickr

I'm no electrical engineer here, so most of what I'm going to do will be based on a general systematic understanding of the coils operation, and by studying schematics.

Given that, I've run into another question, in studying others' build photos and coils:

I'm planning on (a p6ke30ca) 25.6V TVS diode from gate to emitter (bridging the two pins), as I've seen done, to protect from spikes on the emitter capacitively coupled to the gate. I'm using Steve Ward's UD2.1 with a 1:1 GDT, so I think I'm okay here.

The part I'm really wondering about is the diode/resistor parallel pair I always see on the gate pin of the IGBT.

Apparently its common to add something like a 1n5819 Schottky diode for polarity protection (?) paralleled with a low-value resistor (I think Paul Slawinski used an 11 Ohm resistor on his CM300dy-12h's) from the GDT to the gate pin.

What is their purpose? I can only guess fast polarity protection/gate charge regulation or something along those lines.

I know things get complicated, looking at G-E capacitance, and all the other discrete relationships that arise from the electrical architecture of an IGBT.

Does anybody have any suggestions as to the values and components I should be using? Right now, I would just go with the components I've listed above.

HV Enthusiast

Mon Apr 22 2013, 12:56AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Beautiful coil!

Very nice toroid indeed!!!!! I accidentally dropped one on the floor today that i was shipping overseas to a customer and put a big dent in it. DOH! Now i'm the happy owner of one dented toroid!

One quick comment - i can't see your entire strike rail, so i'm not sure if there is a break in it behind the secondary, but you want to make sure the strike rail isn't an enclosed loop otherwise it will act as a shorted turn. But you could be okay - i just can't see behind the secondary.

But beautiful!!!!

Feathers

Mon Apr 22 2013, 01:15AM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

Ouch! I'm sorry to hear that. These things are works of art.

It is closed. I suppose I didn't think about that. I'll have to get some brass knobs to terminate them.

What can you recommend regarding gate protection? I'm using a 25V TVS diode from gate to emitter, but I always see a Schottky diode in parallel with a low-value resistor on the gate terminal. Just wondering how to pick/size these components (especially the resistor) for a CM300dy-12h.

Physics Junkie

Mon Apr 22 2013, 01:25AM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

The gate resistor is best determined through trial and error. start with 5.1 ohm 2W carbon film. You want to find a resistance that best reduces the voltage spikes on your gate waveform while still allowing the fastest rise/fall times possible with those cm300's. stick with 1N5819 in parallel. I assume you have an oscilloscope or access to one?

Ben Solon

Mon Apr 22 2013, 02:40AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

HighVoltageFeathers wrote ...

What is their purpose? I can only guess fast polarity protection/gate charge regulation or something along those lines.

I know things get complicated, looking at G-E capacitance, and all the other discrete relationships that arise from the electrical architecture of an IGBT.

Does anybody have any suggestions as to the values and components I should be using? Right now, I would just go with the components I've listed above.

gate resistors are used to dampen the oscillations on the gate of the igbt due to parasitic LC elements. they can also help relieve stress on gate drivers if the impedance to the gate is low enough to cover-current them.

parasitic ringing during deadtime:

(the half sine wave that occurs when crossing 0) It can be a lot worse

The schottkey diode enable the gate to discharge a lot faster than it can charge, therefore minimizing shoot-through in the bridge. The bridge will have cross conduction if the gates ever see voltage above the on threshold at the same time, so if you shut one device off extremely fast, then begin charging the other [relativity] slowly at the same time, they won't turn on at the same time. This method is used where a large gate capacitance is present, ie. brick igbts.

Physics Junkie wrote ...

The gate resistor is best determined through trial and error. start with 5.1 ohm 2W carbon film. You want to find a resistance that best reduces the voltage spikes on your gate waveform while still allowing the fastest rise/fall times possible with those cm300's. stick with 1N5819 in parallel. I assume you have an oscilloscope or access to one?

Sort of. In this application, they're for dead time control, but if the resistor stood alone it would be a damping resistor. That's not to say that when current flows through the resistor it doesn't behave that way. But whats the point of damping the turn on, but not the off?

If you need the dead time, then use this topo, and probe the gates to be sure that they act as they should. If not, then just tune the damping resistance for minimal overshoot and rise/fall time. Richie's page may help with the damping part.

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