Author Topic: My First DRSSTC  (Read 2383 times)

Offline sjsimmo

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Re: My First DRSSTC
« Reply #20 on: December 18, 2018, 02:08:57 AM »
Thanks for the guidance :)

I just finished doing the calculations, and got the following values for my coil at 350A peak, 100uS on-time, 1000BPS:
- Fmax1=1290kHz
- Fmax2=984kHz
These values seem about 5 times bigger than what I was expecting. Am I doing something wrong, or are these IGBTs invincible?


Thanks again,
-sjsimmo
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Offline Mads Barnkob

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Re: My First DRSSTC
« Reply #21 on: December 18, 2018, 09:47:51 AM »
1000 BPS is really extreme to calculate with, if you want long sparks I think its better to use longer on-time to transfer more power and maybe maximum 500 BPS, a 1 kHz tone is not that pleasant to listen to anyway :)

I looked at this datasheet: https://www.mouser.com/pdfdocs/FGY75N60SMD1.pdf

Did you extrapolate "Figure 15. Switching Loss vs. Collector Current" to get the real mJ losses for the higher current? Or where did you get those low values? Just by eye it looks more like it should be 10-30 mJ (before derating)
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Offline sjsimmo

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Re: My First DRSSTC
« Reply #22 on: December 18, 2018, 10:45:01 AM »
I'm confused now, that is the datasheet I was using...


The calculated current at switch on is 11.6A, and switch off current is 22.7A. These values are both below the minimum current shown on the graph of figure 15. extending the graph gives a switching loss of about 0.25mJ and 0.4mJ at 80 degrees celsius respectively, or am I reading the graph wrong?  :-\


These values can then be multiplied by 0.2, giving the final values that I wrote down.


The reason for using 1000BPS for the calculations, is because I want to midi modulate it eventually, and I figure that 1kHz should be a sensible limmit... (or is this actually too high?) ??? . As for on-time, I might make that a bit longer, at 200A anything above 80uS has little effect on spark length, but I suppose at 350A this time will grow (although by how much, I'm not sure, since I can almost double bus voltage still.)
« Last Edit: December 18, 2018, 10:56:49 AM by sjsimmo »
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Offline Mads Barnkob

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Re: My First DRSSTC
« Reply #23 on: December 18, 2018, 12:50:45 PM »
I'm confused now, that is the datasheet I was using...


The calculated current at switch on is 11.6A, and switch off current is 22.7A. These values are both below the minimum current shown on the graph of figure 15. extending the graph gives a switching loss of about 0.25mJ and 0.4mJ at 80 degrees celsius respectively, or am I reading the graph wrong?  :-\


These values can then be multiplied by 0.2, giving the final values that I wrote down.


The reason for using 1000BPS for the calculations, is because I want to midi modulate it eventually, and I figure that 1kHz should be a sensible limmit... (or is this actually too high?) ??? . As for on-time, I might make that a bit longer, at 200A anything above 80uS has little effect on spark length, but I suppose at 350A this time will grow (although by how much, I'm not sure, since I can almost double bus voltage still.)

You are right, I jumped the first step there.

I will try your numbers in a excel spreadsheet I got on my PC at home, I will take a look at it tonight.

Edit: I used a bit more conservative extrapolations on the graphs, noticeably 6V CEsat and Ton 30mJ, Toff 20mJ, (derated by 0,05% on-time and 0,2 for pulses) and I am getting 144 kHz

« Last Edit: December 18, 2018, 08:56:11 PM by Mads Barnkob »
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Offline sjsimmo

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Re: My First DRSSTC
« Reply #24 on: December 19, 2018, 12:19:32 AM »
Thanks for doing the calculation :) 

It's interesting to see how much effect changing those values had on the maximum frequency, especially since they're all from reading the same graphs. I guess it's a pretty big guessing game once we go outside the values shown on the data sheets...

So should I proceed to turn up the current until I either get 350A, or an explosion? (going by our two wildly different results...)
Out of curiosity, how much can an IGBT overcurrent failure affect other components? Is it likely for example, to kill the UCCs?

I'm also a little concerned about the fact that my gate waveform still has about 500 nS rise/fall times. Whilst this is a lot better than before, I'm not sure what effects it will have on reliability at higher peak currents. Should I be concerned about this?

EDIT: I've attached a scope shot of my gate waveform (1uS/div)
« Last Edit: December 19, 2018, 01:10:11 AM by sjsimmo »
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Offline Fumeaux

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Re: My First DRSSTC
« Reply #25 on: December 22, 2018, 10:25:25 PM »
EDIT: I've attached a scope shot of my gate waveform (1uS/div)

You might already know about this page: http://www.richieburnett.co.uk/temp/gdt/gdt2.html
If not, it looks like your waveform is overdamped so i guess you should reduce the inline resistance.

The rest of the topic goes over my head so that's that.

Offline sjsimmo

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Re: My First DRSSTC
« Reply #26 on: December 24, 2018, 01:15:00 AM »

Yeah, I've seen that page. It's a really good resource  :)


I've previously tried reducing the gate resistance from 6.8R to 3.3R with no effect (it's still using the 3.3R). This resistance change was before I doubled up the UCCs which had dramatic improvements, resulting in the waveform in my previous post. Previously it was more of a sawtooth wave. I'd think that needing less than 3.3R would be very unusual, considering I'm using a TO-247 IGBT full-bridge.
One thought that occured to me is that maybe I need to run 3 UCCs in parallel, but this also seems a bit ridiculous, given the IGBTs that I'm using.

On a side note, the biggest spark so far is now 68cm to a grounded object with 300A peak current, 240VAC input and ~50uS on-time. (increasing on-time causes the OCD to fire constantly, may need tuning?)
« Last Edit: December 24, 2018, 01:35:14 AM by sjsimmo »
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Offline sjsimmo

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Re: My First DRSSTC
« Reply #27 on: January 19, 2019, 08:08:58 AM »
So I had my first IGBT failure recently  :(

One of my IGBTs has always noticeably run warmer than the others (for unknown reasons), and I suspect this to be the cause of the failure. The coil had not been run in over a week, and so had been packed up since the last full power run. I ramped the coil up to 240VAC input. Everything seemed normal for about 5 seconds. However, before i could turn the interrupter from 10uS on-time up to 50uS on-time, one half of the bridge failed short circuit (Gate, Collector and Emitter now read ~0 ohms on both IGBTs), blowing up the 3A slow-blow fuse in my variac. Neither of the IGBTs have any sign of physical damage on the package. Is it normal for IGBTs to fail without any visible damage, and when not operating at full-power?

Thanks again for your help, everyone :)
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Offline Hydron

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Re: My First DRSSTC
« Reply #28 on: January 19, 2019, 11:07:22 AM »
The 75N60s were fine hard switching 225A (their rating), I suspect you could get away with a bit more than this with resonant switching but haven't tested them at higher currents than this. They won't be as resilient when grossly overloaded as a brick, but if you pop a set then they are also cheap to replace!

Edit: I wrote this reply based on the first page, didn't see page 2. Comments still stand but it sounds like you've already done some testing to failure :P
« Last Edit: January 19, 2019, 11:09:05 AM by Hydron »

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Re: My First DRSSTC
« Reply #28 on: January 19, 2019, 11:07:22 AM »

 


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