Author Topic: IGBT problems  (Read 532 times)

Offline skishlish

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IGBT problems
« on: April 20, 2022, 12:06:36 AM »
Greetings,

This is my first post on this forum and I send greetings to everyone from Croatia.

I am working on my first SSTC system which is mostly based on schematic from a similar system I found on the internet, which I adopted for the components available to me at this time.

It is based on a half-bridge of two IGBTs. The ones I used are FGH40N60-SMD, and the have a built in antiparallel diode.

The first time I turned it on, both IGBTs blew instantly (yet with no sound or smell) and my 16A household circuit breaker dropped.
It seems that it was a full short circuit.

Next I tried running it (with replaced IGBTs) through some resistors (amounting to about 800 ohms) trying to figure out what's going on. Nothing blew up this time but it seems
that there was again a short circuit through both IGBTs.

Next I tried using a 36V DC source without the protective resistors. It seems that the current through the IGBTs dances around between 0 and 3.5 A in low intervals (something like 2 Hz frequncy...) on my ammeter, but that is probably
not an accurate representation of what's really going on (I assume it is some sort of "rounding error" from a high frequency signal). When I tune the circuit on the variable resistor connected to the gate driving chip, the intervals shorten and the current
jumps faster (maybe 4 Hz?) I have no way of knowing if I'm getting a high frequency output or not as I don't have an oscilloscope.

The other thing is that the primary output has no voltage difference regardless of what I'm doing (using 220V AC with resistors or 36V DC source).

I'd be happy to provide any useful additional information if someone has any idea what could be going on.

Any input or suggestion for further experimentation would be appreciated

thanks

Edgar

 

Offline davekni

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Re: IGBT problems
« Reply #1 on: April 20, 2022, 06:02:49 AM »
Quote
I'd be happy to provide any useful additional information if someone has any idea what could be going on.
Yes, will require much more information for us to be of assistance.  Working without a scope is difficult, but there are examples of success without access to a scope.

Information that would help:
1)  Is the schematic you posted after your modifications?  Or as you obtained from the internet?  If your modifications are not shown, please list them, or edit the schematic to show them.
2)  Pictures of your coil and circuitry.
3)  Details of test equipment you have available.  You mentioned ammeter.  Is that a DMM?  Analog meter?  If DMM, can it measure frequency?  Any sort of signal generator?

That circuit is self-oscillating with no feedback to force oscillation frequency to match SSTC secondary resonant frequency.  That makes success unlikely.  Will need some modification to add feedback from secondary coil, either by antenna or monitoring current in bottom lead (in the connection to ground).

If you haven't used JavaTC before, try entering your coil parameters into that tool.  It will estimate (fairly accurately) secondary frequency and coupling factor between primary and secondary.  Frequency will be necessary to adjust oscillation frequency close, so that feedback can take over and hit the exact correct frequency.

Good luck!
« Last Edit: April 20, 2022, 06:08:48 AM by davekni »
David Knierim

Offline skishlish

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Re: IGBT problems
« Reply #2 on: April 20, 2022, 05:46:33 PM »
Hi! Thanks for your reply!

OK, here we go with the additional info.

1) The schematic is my own version, but the only main difference between the working example is that I used slightly lower power IGBTs (40A continuous rating in my case vs. 60A continuous) and I had to change
    some capacitor and resistor values (up to max 10% value), and in other cases I combined multiple resistors or capacitors to get the ballpark values. I'm uploading a new corrected schematic because I forgot to
    add one resistor in the first version.

2) I'm uploading the photo of my circuit. It's a mess right now because I removed the heat sinks and the IGBTs and suspended in air now (for easier access when testing). I also didn't remove an old weaker diode, but I marked that with an X
    in the photo. The new diode is connected to the automatic circuit breaker on top of the image. That goes though a chain of protective resistors which I can clamp at different points to regulate the current, but that is not the part of the actual
    circuit. The large white capacitors on the right are for the voltage dividing, and the second layer on top of that with the resistors and 4 smaller red capacitors, that's the snubber network for the IGBTs. It's difficult to understand from the photo
   what's going on there, but I have checked many times that the circuit follows the schematic. Here is also a photo of my coil with the topload. Mind you that I didn't actually run it there beside the wall. I just took a photo of it on that spot.

3) Yes, it's a DMM, but it can't measure frequency. At this point I'm not that concerned about tuning the circuit because I'm getting zero voltage at the connectors for the primary, and there still seems to be a short circuit between the IGBTs. That's that part
    I'm first trying to solve. Whether I get actual sparks later is a different problem altogether.

As far as it being a self-oscillating circuit: In theory the chip can be tuned to get up to 1 MHz, by using different resistor and capacitor values on two of its pins. I have tried using JavaTC and It says that my resonant frequency of the secondary is about 440 kHz with the topload. Right now the chip is oscillating way off, but that doesn't solve the mystery of the short circuit.

 I'd like to get any sort of HF output on the coil connectors first, before I proceed with getting the frequency right.

Gate voltages V-GE) seem to show a reasonable value (8V or so) which would seem the chip is working fine, but I can't see the signal, nor can I know if the chip is switching the IGBTs interchangeably or for some reason both are turned on at the same time
which would explain the short circuit.


Offline Mads Barnkob

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Re: IGBT problems
« Reply #3 on: April 20, 2022, 08:34:45 PM »
Hi Edgar and welcome to HVF :)

Do you have a neon/fluorescent bulb or tube? You can use that to detect HF oscillations in the secondary coil. The high frequency electromagnetic field will excite the gas in the bulb/tube and make it light up.

As Dave mentions, oscilloscopes are needed to troubleshoot on circuits like this, everything else it just guesswork. Would it be possible for you to find some old used 20 MHz 2 channel oscilloscope near you?

I looked for a Croatian sell-used-stuff-website and found this https://www.njuskalo.hr/?ctl=search_ads&keywords=osciloskop but they do seem a bit expensive, In Denmark I would expect to buy a old 20 MHz 2-ch for about 300-500 kn, depending on condition.
https://kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics
https://www.youtube.com/KaizerPowerElectronicsDk60/join - Please consider supporting the forum, websites and youtube channel!

Offline skishlish

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Re: IGBT problems
« Reply #4 on: April 21, 2022, 03:58:00 AM »
Hi Mads. I do have a fluorescent bulb, but I'm fairly certain such testing is futile because there is zero signal on the output. I don't even have the primary connected any more. I'm doing my testing right now unloaded, in order to figure out why my IGBTs are shorting.

I had one suspicion which I eliminated just now. I figured maybe the IGBTs are too slow for the operating frequency of the chip due to sluggish raise and fall times. But it actually turns out that combining the raise and fall times of the gate driver and the transistors I have hundreds of ns of spare dead-time at 450 kHz...and right now I'm running it at about 150 kHz, so dead time issues cannot be the problem. In fact my IGBTs seem to be quite fast compared to the ones used in the original design. And even if there was some overlap between the pulses on the 2 IGBTs they would short for a short while (in hundreds of ns I guess), which would even out to a small current and would cause overheating eventually and not instant failure.

I guess you are right about using an oscilloscope. I have no choice but to bring the circuit to someone who has one to probe the gate outputs of the chip for starters. And you are right, people hold high prices for decent oscilloscopes over here. Eventually I have to get one, but it will have to wait for now.


Offline skishlish

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Re: IGBT problems
« Reply #5 on: April 21, 2022, 05:36:57 AM »
I seem to have found the main problem. You wouldn't believe what it was. The chip was constantly outputting DC 15V on both gate driving pins for some mysterious reason, which is its full operating voltage. I removed it from the socket to make some resistance measurements only to find that some of the pins were bent and actually ended up shorting with neighboring pins, this gave full operating voltage across the entire chip on all pins and gave a constant "open" signal to the IGBTs.
When I fixed this problem I actually got a nice 150V output (at the points where the primary is connected). Though I don't know the shape of that signal. At least I got SOMETHING out of the circuit for starters.

The bad news is that one of the wires which was unscrewed from the time I was making current measurements touched one of the gate wires blowing one of the IGBTs. But that is a problem as silly as the first one, so I guess tomorrow I have to replace the transistor and continue where I left off.

Either way finding the main source of the problem is a huge relief.

The rest of the journey will have to do with tuning the circuit to get sparks.
« Last Edit: April 21, 2022, 05:38:41 AM by skishlish »

Offline davekni

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Re: IGBT problems
« Reply #6 on: April 21, 2022, 06:42:04 AM »
Quote
I seem to have found the main problem.
Great that you found that problem.

Quote
I'm uploading the photo of my circuit. It's a mess right now because I removed the heat sinks and the IGBTs and suspended in air now (for easier access when testing).
Even with the heat sinks replaced, I suspect you will have more fried parts in the future due to wire lengths and resulting parasitic inductance, especially as you get up to full voltage.  Depending on your tolerance for fried parts, you certainly can continue experimenting with this layout.  (Coil looks great.  It is driver and half-bridge wiring that's concerning.)

For extreme of low-parasitic-inductance half-bridge construction, here's my example:
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795
This example shows GDT (gate drive transformer) topology, but could be adapted to IR21531 use.  You don't necessarily need to go to this extreme.  However, the general idea of minimizing parasitic inductance is important.  Most problematic is IGBT emitter inductance that is common between the high-current C-E path and gate voltage (G-E).  That is why GDT wires connect close to IGBT body, avoiding much common emitter lead length.

Parasitic inductance is problematic in the IR21531 circuit as well.  At least construct it over a "ground" plane.  For example, use a piece of copper foil (foil tape on a piece of wood or ...) covered with insulating tape (mylar or kapton or ...).  Make a hole through the insulation for any "ground" connections such as IR21531 COM lead.  Solder those leads to the foil.  Remaining wires and parts can be over the insulating layer, but close to it to reduce inductance (reduce distance between wires and "ground" plane, thus restricting magnetic field path).

Have fun experimenting!  I tend to be a perfectionist, so see what you can get away with in reality.
David Knierim

High Voltage Forum

Re: IGBT problems
« Reply #6 on: April 21, 2022, 06:42:04 AM »

 


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