Bridge output

[davekni]

I'm not too sure how valid the next shot is, because channel 1 on my scope malfunctions and the sine wave is slightly behind to where it should be.

What leads you to believe channel 1 malfunctions?  How do you know that the malfunction is smaller than measured phase lead error?
If channel 1 can't be trusted, then use it as trigger only.  One side of UD2.7 output (one GDT primary wire) is a good trigger source.  Presuming coil behavior is repeatable from one burst to the next, measure other signals sequentially using channel 2:  One H-bridge output, other H-bridge output, current, low-side Vge, GDT input (one side).  If your scope can save and display reference signals, save each channel 2 measurement and display together.
How are you measuring current?  Do you have a separate CT?  What turns ratio?  What core?  What burden resistor?

Inductance can be calculated once phase is more accurately measured.

[Felix M.]

The signal on Ch1 is off by mby 1-5°. The current is measured with an external N87 CT with something around 10 windings and 1,5 ohms. It however doesn't matter if the current is probed with an external CT or across the 51ohm resistor on the feedback.

[davekni]

The signal on Ch1 is off by mby 1-5°.

Even 1 degree is ~35ns.  Are you using a 10x probe?  If so, have you checked compensation (square wave ~1kHz output from scope)?  Could be probe compensation issue (on one probe or the other).  Either that or scope's probe delay settings are way off, if your scope has that ability.

The current is measured with an external N87 CT with something around 10 windings and 1,5 ohms.

10:1 CT into 1.5 ohms will become a problem at higher currents.  Also, if 1,5 ohm resistor is wire-wound, resistor inductance may be causing phase error in current measurement.

It however doesn't matter if the current is probed with an external CT or across the 51ohm resistor on the feedback.

This works well as long as scope is directly across 51 ohm resistor, not across resistor + inductor (for phase lead).  I'd suggest staying with this method, with probe directly across resistor, and with probe ground lead running adjacent probe body/tip to avoid inductive loops that can pick up stray magnetic fields.  Some people wrap ground wire around probe to keep it close.  Or use tape or twist ties or ... to keep ground lead as close as possible to probe body.

[Felix M.]

The one scope probe on x10 mode and the channel 1 got fried because they saw too much voltage (there's some more information in the ud2.7c "sawtooth" output thread). Sine signals are fine to measure and the probe can only be used in x1 mode. (The scope reads just a few milivolts on a 7Vpp signal from my function generator).

I scoped the 51ohm resistor directly with the probe ground at the resistor's ground and the head on the other side. No matter what you try with shielding or reducing probe inductance, the current is still way off. I'll probably buy a new scope soon but until then I'm stuck with the broken channel.

I don't think that the measurement is uncorrect although the channel malfunctions under some conditions because I do have zcs with the Vge signals. For some reason Vce is lagging behind.

[davekni]

The one scope probe on x10 mode and the channel 1 got fried because they saw too much voltage (there's some more information in the ud2.7c "sawtooth" output thread.

OK, now I understand.

I probed the 51ohm resistor directly with the probe ground at the resistor's ground and the head on the other side. No matter what you try with shielding or reducing probe inductance, the current is still way off. I'll probably buy a new scope soon but until then I'm stuck with the broken channel.

Until you get a new scope, use channel 1 for trigger only.  Just because sine waves look like sine waves doesn't mean any useful data can be deciphered.  Use channel 2 to sequentially measure signals including current.

[Felix M.]

Is there any way to do phase lead adjustments with just one channel? (If there would be, I‘m not aware of it)

And what about the slot 7 inductor. I have to order a new one and it‘s better to order the correct one right away.

[flyingperson23]

I (kinda) did phase lead with effectively one channel 2 ways:
1 - look at the bridge output and adjust until theres minimal voltage spikes
2 - look at the primary current under high bus voltage/current, and there are noticeable pulses when the bridge switches. That may be an artifact of my bad probing and/or circuit design, but still it kinda worked

Likely neither of these got it perfect but I've been running the coil hard and it hasnt blown up yet, so better than nothing

[davekni]

Is there any way to do phase lead adjustments with just one channel? (If there would be, I‘m not aware of it)

I've been tried to describe this in two posts above, but apparently not successfully.  Easier in your case because you can use channel 1 for trigger only.  (Or, if your scope has an external trigger input, use that.)  Trigger on a signal with fast edges (not current).  UD2.7 output (one GDT primary lead) is best.  Does not matter that it looks like a trapezoid.  Just trigger on the first edge.  Then measure other signals one at a time using channel 2 (your good channel).

Flyingperson23's ideas will work for finer adjustment once you have a good inductor ordered.  I'd recommend (1) for fine adjustment as preferred to just comparing current and voltage phase.  Initial test as above will get close enough to order an appropriate inductor.

[Felix M.]

I‘m sorry davekni, I just didn‘t get it what you ment with the trigger and how it should help me adjusting the phase. Doing it until voltage spikes at the bridge are minimal would be an option but in comparison to my setup without any snubbers, there are not that many spikes left. Tuning the phase was reducing the Vmax by something around 2V at best. And I can‘t do much in terms of tuning it with the inductor. I‘d be glad to get some more information on what you mean davekni.

[davekni]

Does your scope have the ability to save and display reference waveforms?  Here's a youtube video showing how to do that on a Rigol DS1054z scope:
   

/>Useful part starts at about 3 minutes into video.
Will be a bit harder to measure if your scope doesn't have such ability.  I'll describe one or the other way in more explicit detail depending on your answer.

[Felix M.]

Fortunately my scope has this function. Here are some shots of the inverter's output and the primary current:



Doesn't look that different to me than the shots I did with the broken channel measuring the current but there's probably some difference.

And like before, the gate signal and current appear to match each other quite closely:


Is there any formula or table for the right Slot 7 inductor for specific phase shifts?

[flyingperson23]

https://kaizerpowerelectronics.dk/calculators/ud2-x-phase-lead-calculator/

[Felix M.]

The current is off by almost half of one half cycle. I'm not sure if this wasn't a problem with the CTs all the time.

[Felix M.]

Fortunately my scope has this function. Here are some shots of the inverter's output and the primary current

Yeah, I made the mistake to have my old broken inductor in the system. After re doing the test with the inductor shortet, the phaseshift is at around 1us which makes a lot more sence. I ordered a bunch of new ones but 60uH should be the value I approximately need.
It's still strange to me that it shifts the phase towards hardswitching when there's more inductance added.

[davekni]

I ordered a bunch of new ones but 60uH should be the value I approximately need.

Yes, that value sounds good.  Looks like about 1us of delay, so phase lead needs to shift about -1us (1us earlier).

It's still strange to me that it shifts the phase towards hardswitching when there's more inductance added.

UD2.7 inductance (inductor in series with 51 ohm feedback CT burden resistor) moves H-bridge switching earlier.  I haven't seen any evidence in your posted scope images of different behavior.

[Felix M.]

I did two shots, one with the inductor (at max 42uH) and one with the inductor shorted.

With the inductor:


Without the inductor:


I probably missread something in the shots but without any inductor, it almost couldn't switch off more current. I'd need something around 200uH for it so shift enough. Is this really an issure with the inductor or rather something else? I got a good hint to check my resistors and if they are wire wound but they are all metal film. I ordered such a large inductor with up to 200uH but it seems odd to me that the current and gate signal match but current and inverter is so much off.

[davekni]

I did two shots, one with the inductor (at max 42uH) and one with the inductor shorted.
With the inductor:

This capture looks fine.  However, looks like ~1us of phase lag even with 42uH.  If this is really with 42uH, then you will likely need over 100uH to get proper phase lead.  100-150uH may work fine.  Other option is to try reducing delay through H-Bridge (reducing GDT leakage inductance and perhaps reducing gate series resistor value).

Phase lag can be seen here in H-bridge output voltage.  Voltage increases slightly as current reverses.  IGBT voltage is changing from Vce forward drop to diode forward drop (opposite polarity).  I've circled these steps of the final cycle in your capture.  Easier to see at ground w/o Vbus ringing.
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Without the inductor:

Second capture is clearly not correct.  That is a risk of sequentially probing.  Either trigger didn't catch same edge or coil behavior changed between the two captures.  If you are using channel 1 for trigger, may be useful to display that channel in spite of distortion.  Would make it clear if trigger is same for the two captures being compared.
Another way to check:  Notice on the first image that current starts at roughly the same time as voltage.  If you panned to the left on the second image, presumably current and voltage would start at different times due to trigger change.  If they do start at the same time, then coil operation must have changed for some unknown reason.
Notice that steps on voltage waveform look roughly the same as on upper image.  Thus actual phase didn't change very much between the two.

[Felix M.]

I didn't have time to do mutch today. The inductors arrived and the big one with the 200uH range shifts the bridge output too mutch. Tomorrow I'll try a smaller one and also rework the GDT connections. Is it important that the cables to the pair of the high gates and the pair of the low gates are the same lenght to have the same inductance or delay? While doing test with the big inductor, I noticed some spikes in the gate signals that reached up to over 40V every time it turned on and off. That's probably a bad thing and I'd like to avoid them.

[davekni]

Is it important that the cables to the pair of the high gates and the pair of the low gates are the same lenght to have the same inductance or delay?

Yes, it is good to have matched GDT lead lengths.  Otherwise either rising or falling edge of H-bridge output will have more phase lag.  Correcting for that (adding phase lead) will cause other edge to have slightly too much phase lead.  (Too much is better than too little phase lead.)

While doing test with the big inductor, I noticed some spikes in the gate signals that reached up to over 40V every time it turned on and off.

I'm presuming Vge spikes are negative.  Negative spikes are common since diode is bypassing gate series resistor.  40V is not likely damaging gates.  However, I'd lower the value of your 33V zener diodes and perhaps change to TVS diodes (zener diodes intended for high peak current).  Other option to reducing negative spikes is adding some series resistance to diode across gate resistor.

BTW, spec for SKM200GB218D shows 3.8ohm internal gate resistance.  Combined with ~40nF of Cge for negative voltages, short Vge spikes external to brick will be filtered to lower voltage internally, so less likely to cause problems.

[Felix M.]

They are positive spikes. I do also have some negative spikes but they are not that big.