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Messages - ritaismyconscience

Pages: [1] 2 3 ... 5
1
I'm already tripping the 500A limit at around half voltage. I'll try reducing the tank capacitor to 50nF.

2
Just out of curiosity, is there a reason why 100nF works better than 200nF?

3
Looks like you're right.

Tested at half voltage, already working really well. Will have to go somewhere bigger to test.

Edit: This one's probably better. Moved the breakout point up a bit so it doesn't get cut off by the door.


Thanks for your help davekni!

4
The critical frequency is the lowest frequency where the on time is still less than the maximum set in one of the variables. Basically you figure out from duty cycle and frequency how long the on time is, it's 10^6us/freq*duty. Then you solve for max on time and you get max = 10^6us/freq*duty --> freq = 10^6us/max*duty.

Changing the MCU frequency from 1MHz to 8MHz will mess with the delay (since it'll take 8x less time or 8x more time)

The bottom code basically turns on the interrupter for a set period of time, then turns it off for a set period of time. It does this repeatedly.

Haven't looked at the other code enough to figure out why you divide by 5 or add 4 but in practice you're never going to use an ontime anywhere close to 6000us.

5
Can  you tell me the max current through the IGBTs when there's an arc? I'm planning on building this using H30R1103 IGBTs and I want it to be reliable.

6
Got it to work at full voltage without blowing up. Set overcurrent to 500A.



I'm only getting 1 foot arcs, which means that my coil is probably a bit out of tune.

7
Looks like I found the answer

https://www.microsemi.com/document-portal/doc_download/14696-igbt-tutorial

Pages 2-3 say that high collector current could cause the IGBT to latch on.

I'm wondering if I should go back to using 100nf for primary capacitor since the primary current rises really fast

8
So I did a bit more testing. This time, I shorted the OCP input which should disable it.

Here's a plot of bus voltage vs peak current:


Something interesting happens above 100V, the driver shuts off the output at 900A even though the OCP was shorted. I think it might be acting as an antenna or something.



Test at higher voltage. Notice the shorter pulse width. Both are 500A/div. Anyways, I'm pretty sure my TO-247 IGBTs are not designed to withstand 900A.

I guess if you look at the datasheet I posted on reply 39, you can see that the FB and OCP pin both go to OCP_n, which probably is a pin on the FPGA. I think this is how the driver figures out there's too much current. Interestingly, the red OCP led did not light up. Also, the datasheet says the max FB current is 900mA, which seems like too good of a coincidence.

9
The datasheet says 10 ohms and the resistor on the board says 100 on it, so I'm pretty sure that's not the problem.

I'll check with a multimeter to make sure.

Edit: Checked, measured 10 ohms. I'm using 3 cores for my feedback and OCP transformer. It looks like this:

Could the feedback be stealing current from the OCP?

10
I'm still trying to figure out why the overcurrent isn't working as intended.



Here's a scope shot with the output of the transformer connected to the OCP and another CT I made. I expected the first one to have a higher voltage because it should be equal to the CT voltage plus the 1V diode drop. That doesn't seem to be true, though. The flat sine wave is OCP signal, 1 div is 0.5V

Edit: Improved phase lead:






11
I fiddled around with the phase lead a bit:

I got it so that it switches before the peak.


Looks like there's still quite a bit of ringing, but I'm testing this at low voltage.


Here's a scope shot of the bridge at 5us/div instead of 2us.

12
Some pictures:


^GDT waveform vs voltage at capacitor


^Same thing, but at 10us/div


Left side gate drive and right side gate drive


Bridge output (looks like I need to increase phase lead)


Bridge output vs current measured with 1:1024 CT with 10 ohm resistor. 1 div is 100A. I don't know why, but looks like the OCP is trying to keep the current at 100A, not 25A. I tried it with OCP set to 50A, and the CT said the driver was trying to keep the current at 200A. If this is true, looks like I blew the IGBTs with overcurrent. That's probably also why 100nF worked but 200nF didn't.

13
So I tested the overcurrent by setting it to around 25A. It looks like there isn't a way to turn off pulse skip with this driver:



You can see that the current starts ramping down, then the driver turns on again until it trips the overcurrent again.

I'm running everything at about 2 BPS right now ( I manually press the single pulse button), so I got a really high value for the frequency. I used 200us pulse width when the IGBTs blew up (at around 90% of max voltage)

14
I'm using 2 paralleled TO247s, so the current should be 250 through each.

Also the driver I'm using seems to use a 9V supply for the op-amp, so 5V should work. See reply #39 for info about driver.

Think I told you guys I was using the 75T60 IGBT on one of the previous pages...

15
I turned off pulse skip mode. I'm now using the normal mode, with pulse width around 100-200us. I'm using the single pulse mode too, mainly to avoid more frying.

Edit: blew the transistors again, this time more violently.





Seems like the overcurrent might not be working. I'll scope that next.

Edit 2:
So it looks like the exact 5 IGBTs survive each time. It's always the 4 IGBTs on the left and the one on the top right. All the other 3 fail. I think this might be because I'm using 550V TVS, and maybe the IGBTs on the left got "lucky" and got the ones that triggered on lower voltage. I should probably switch to 440V. However, nothing bad seems to have happened yesterday, and I tested it at higher voltage. Maybe the extra current contributes to higher voltage spikes?

16
I tried doubling the capacitance and halving the primary inductance. It still works, I ended up with a resonant frequency of 151kHz. It's a bit too bright outside to see the sparks, so I'll test some more tonight.

17
I successfully got my coil to work at full voltage without blowing up:


I checked and the overcurrent LED never lit up. I'll have to measure the pulse width, but it's set to 75% of the max pulse width the controller lets me set.

Edit: tried it at the highest pulse width, still no overcurrent. I think my transistors blew up because of overcurrent. Could also be faulty OCP?

Actually, I think I need a higher capacitance. Right now I'm using 100nF, I should probably use 200nF or something.

150nF would get me 4.5 turns and reduce the surge impedance to 7 ohms (from 10)

200nF would get me 3.75 turns, surge impedance = 5 ohms

18
Electronic Circuits / Re: TVS diode selection for 400v transistor
« on: March 28, 2020, 01:02:41 AM »
You want higher voltage across primary for obvious reasons but the voltage at peak current is still smaller than 400. The 250V one definitely will work, but you could probably go up to 300 to 350V because the current probably won't be close to 1A.

19
General Chat / Re: g10 disk
« on: March 27, 2020, 11:24:19 PM »
I came up with this idea because I couldn't get a disk too.



Haven't tried it yet, so I don't know if it works or not

20
The left side of the half bridge didn't suffer any damage, but 3/4 IGBTs on the right side blew. I replaced all 4 just to be safe. None of the other pars seem to have failed, when I replaced the IGBTs, everything worked again up to 75% input voltage.

I lowered the strike rail, so I haven't gotten any strikes yet. I shorted the secondary and ran it at 50% input voltage, and I used your method to look at the voltage at the capacitor. It looked like a sine wave but it was considerably more fuzzy.

Also, during a ground strike, wouldn't the inductance of the primary drop, making the resonant frequency higher? Could that be a problem (like could it cause the IGBTs to switch at the wrong times?) I think the amount of phase lead produced by my circuit varies with frequency, and my phase lead circuit seems to work better at lower frequencies.

The IGBTs I'm currently using also don't have antiparallel diodes, so I'm currently using 4 fast diodes in place of them. Could this be a problem?

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