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Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
So, i got finally my UD2 build kind of working but im still experiencing some unstability and i would greatly appreciate some expert guidance with this process.
First thing is the feedback circuit, as default, there is a LR burden network made from 51ohm resistor and adjustable inductor. Since im using relatively large transistors i chose my phase lead inductor to be the good old SLOT TEN-5-09 which gives me the range of 16 to 42µH
According to LTSpice simulation (45kHz) at 16µH, voltage across the resistor is leading the voltage over whole network about 315ns and at 42µH it is 810ns which i think is good range for compensating for CM600 bridge and logic delays.
But the issue is that this adjustment seems to be really fiddly and driver behaviour more or less unstable every time i try running it with phase lead enabled. It works beautifully with just resistive burden (minus the obvious ringing from missed zero crossings..). With phase lead enabled i seem to be getting yet again really glitched burst starts and as Mikeselectristuff would say "general not-working-ness"
Second question is about OCD burden resistance and acceptable input voltages for OCD comparator. I do understand this is a 5V common negative refenced logic so its good idea to have the trip level set at something like 1.5V...3.5V. Does this sound right? Right now I have 1:1089 CT loaded with 1 ohm and i want the OCD to trip at 2500A so i set the reference voltage to 2.3V.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Couple of thoughts about the inductor:
- Is it saturating with your high primary current? (at 1089:1 it could get >2A). This might be a question for those with larger coils, I've only gone up to ~700A in mine, and coilcraft don't specify saturation current.
- Are the right pins on the inductor being used? I had a few in my junk box which looked similar to a SLOT-10 but didn't match the UD board's pinout.
As for the OCD trip level, that sounds right to me, though you could check the LM311 datasheet to make sure (look for it's common mode input range at 5V single supply).
Bear in mind that if your tank rings up very fast then there could be another larger pulse after the trip is hit, so set the limit conservatively. (UD2.x design only disables the IGBT drive after a full cycle completes, with a rising edge from the TL3116 to the 74HC74, so you can get another half cycle after tripping the OCD)
I ended up using a scope to adjust my OCD, if you use the testpoint then it would be easy to run the tank with no secondary to check it with a scope.
Edit: You may be able to test with an air-cored inductor to eliminate saturation concerns (I think Steve or someone might have used one for his first phase-lead circuit), but you'd have to be careful to orientate it so that primary currents don't couple with it (make sure it's off-axis and at a 90-degree angle to the primary).
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
wrote ...
Couple of thoughts about the inductor:
- Is it saturating with your high primary current? (at 1089:1 it could get >2A). This might be a question for those with larger coils, I've only gone up to ~700A in mine, and coilcraft don't specify saturation current.
- Are the right pins on the inductor being used? I had a few in my junk box which looked similar to a SLOT-10 but didn't match the UD board's pinout.
Comparator input waveform looks beautiful and i cant see anything that would point towards inductor saturating. And im sure Im using slot-10 because i got mine from coilcraft :)
wrote ...
As for the OCD trip level, that sounds right to me, though you could check the LM311 datasheet to make sure (look for it's common mode input range at 5V single supply).
Bear in mind that if your tank rings up very fast then there could be another larger pulse after the trip is hit, so set the limit conservatively. (UD2.x design only disables the IGBT drive after a full cycle completes, with a rising edge from the TL3116 to the 74HC74, so you can get another half cycle after tripping the OCD)
I ended up using a scope to adjust my OCD, if you use the testpoint then it would be easy to run the tank with no secondary to check it with a scope.
Im not too concerned about couple extra pulses here and there, i have tested bridge up to pretty brutal currents and nothing broke so it wont matter if the burst ends at 2500 or 3000A but i will check that with current transformer soon.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Yeah those ideas were long shots. Even if it were saturating it should still work, and scoping the comparator eliminates that. If anything else springs to mind I'll post, but from the other thread it seems you've covered most possibilities.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
OK We got more results.
I took a small crappy video that shows what im seeing right now. It starts with phase lead enabled, running at about 100Vbus and ends up to about 400Vbus. Lower trace is feedback comparator output measured with low inductance probe right at the comparator. Upper trace is CM600 bridge output voltage 200V/Div
Second run is phase lead inductor disabled, with just resistive burden. Without phase lead i get tons of switching noise everywhere which is expected, but also I get nice clean glitch free run form 20Vbus to 700Vbus and up to 3kA. Phase lead enabled I seem to be getting nothing but different kind of glitches and that good old machine gunning i was talking about in the other UD2.x thread. Tested inductances between 20 and 100µH and none of those worked like the resistor alone.
Why? Are any of you guys seeing something like this? Or is this normal? Its difficult to tell because i have no reference systems
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
Kizmo wrote ...
OK We got more results...
This problem went away when i added even heavier bleeder resistors over the bridge output (4x 10k in parallel). 2x 10k in parallel worked great without phase lead. Wonder why.
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Setting up UD2 based driver, is there a method in this madness?
