Bridge output

[davekni]

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

Positive spikes are less common.  Perhaps a scoping artifact (Vce switching coupling into scope probe/ground loop)?  If real, increasing gate series resistance should fix them.  When you get a chance, a scope capture and a picture of probe connection might be helpful.

[Felix M.]

Yeah, scope shots and pics of the GDT will come tomorrow.

[Felix M.]

Well, no scope shots today. After seeing that sine signals on ch1 are in fact shiftet by smth like 60° (they were not a few days before), I stopped doing test since the current also shifts slightly when I adjust the phase and it's pretty pointless doing it with the ref function. New equipment will arrive next week so I'll probably won't do much until then. I did however take some pictures of my setup.

GDT:


Wiring of the GDT between driver box and gates:


Feedback CTs:


Overview:


Overview of the setup with the MMC and the Siemens MKV capacitor in place:


If you see some flaws in the pics, just let me know. I also cut out some unnecessary cable length between driver and GDT to reduce innductance. The spikes I saw yesterday disappeared so that's good.

[davekni]

I did however take some pictures of my setup.

Those detailed images are great.  Leads to two suggestions:

Is your GDT wound from CAT5 cable or something similar containing 8 wires as 4 twisted pairs, with one wire of each pair used for GDT secondary and other wire of each pair for GDT primary?  Presuming so, it is best to keep each of the four primary winding leads as twisted pairs all the way back to driver, paralleling them at driver.  Makes a significant reduction in leakage inductance.  Primary lead inductance counts 4x because it has current of all four secondaries added together.  Here's my little tutorial on GDT winding.  It's for a half-bridge, but easy to extend to full H-bridge:
    https://highvoltageforum.net/index.php?topic=1854.msg13949#msg13949

Only other possible concern is routing of MMC output lead.  Looks like it passes across MMC.  Node between MMC and primary coil is much higher voltage than H-bridge output.  May be OK if wire insulation is good.

[Felix M.]

The wire is some kind of wire (I guess its cat 5) I found at my local junkyard. It has 6 internal wires with 4x2 going to the gates and emitters and 2x2 going to the driver allthough I changed it to 1x2 and just cut one of the 6 wires off so it won't do anything. All cables of the same colour are twisted together after leaving the wire mantle at the core.

Don't worry the cable does not acually reach over the MMC. I guess a normal car jumping cable like the one used in my build can handle up to 5kV because I had a similar one under these conditions in a larger pulse capacitor but normally the primary would be mounted on top of the wooden frame. There is a little feed-through hole in the wooden base plate of the primary where it's fed through. Here's a picture:


It'll look like this later with no coables or conductive materials near the aluminium rails of the MMC. Even the mouting screws are plastic to have no contacts you can accidentally touch.


(Those LEDs are somewhat shieldet in an aluminium profile. I'm curious to see how long they can stand those fields  )

[davekni]

The wire is some kind of wire (I guess its cat 5) I found at my local junkyard. It has 6 internal wires with 4x2 going to the gates and emitters and 2x2 going to the driver allthough I changed it to 1x2 and just cut one of the 6 wires off so it won't do anything. All cables of the same colour are twisted together after leaving the wire mantle at the core.

That isn't CAT5 cable.  Sounds like old wired telephone cable.  You will have significantly lower GDT leakage inductance if you rewind with CAT5 cable (or any such ethernet cable, which has 4 twisted pairs, 8 wires total) per tutorial.  Ethernet cable is common in scrap wire recycling around my area.  Not that expensive to purchase new if necessary.

(Those LEDs are somewhat shieldet in an aluminium profile. I'm curious to see how long they can stand those fields  )

That will depend on how LEDs are driven.  If in series strings, best to add an anti-parallel diode across each LED (or a capacitor across each LED to limit AC pick-up).  Reverse voltage is the most common cause of LED failure.

[Felix M.]

I‘ll try to get my hand on some cat5 cable, in fact I might even have some at home. What causes such a big difference between ordenary cables and cat5? How would you reccomend wiring it? Keep two in paralel or just one cable per gate/emmiter?

It‘ll be quite hard to put a reverse diode at each LED because it‘s just such an LED string with a silicone cover you can put in your rooms but I can add one at the input. 

[davekni]

What causes such a big difference between ordenary cables and cat5?

With 4 twisted pairs, each secondary winding is paired physically-close to one primary winding.  Makes less loop area for leakage inductance.

How would you reccomend wiring it? Keep two in paralel or just one cable per gate/emmiter?

If the tutorial isn't clear, please post a question on that thread.  Then answers will help anyone looking at the tutorial.

It‘ll be quite hard to put a reverse diode at each LED because it‘s just such an LED string with a silicone cover you can put in your rooms but I can add one at the input.

If this is a 12Vdc string, the one diode might be useful.  If for line voltage, diode would short half-cycles, so be problematic.

[Felix M.]

It's been a while. After having problems with the driver's PCB (the contacts for the inductor got bad after changing the inductor that often). I redid it and it's finally running again.

Meanwhile I remade my GDT:

I think it should work just fine. The phasing (high and low sides) are also how they should be.

The bridge also received some changes. The old design measured over half a meter in width. I was able to immensely reduce this size and thus the stray inductance

Old bridge (without the Siemens MKV film cap and the 5th and 6th snubber):


View on the bottom of the new busbars:


Overview of the whole finished bridge:


I'm going to do some test tomorrow after work with my new scope and differential probe to see which Slot7 inductor I need.
Please let me know your thoughts about the new bridge and GDT.

[flyingperson23]

For the driver board inductor pads - desoldering the inductor every time you want to change it is a huge pain. Assuming you're using a slot 7 inductor, you can use these: https://www.digikey.com/en/products/detail/mill-max-manufacturing-corp/0555-0-15-01-30-27-10-0/158204
They can be a little tricky to solder but once they're installed it's super easy to change inductors.
You don't even need to solder the case tabs; the 5 pins in those sockets are enough friction to keep it solidly in place.

[Felix M.]

I‘ll keep them in mind but hopefully this will be the last time soldering in a slot 7 inductor. I‘ll measure my offset between current and bridge out in order to calculate the right inductor directly.

[davekni]

Please let me know your thoughts about the new bridge and GDT.

Both look great!  Hope your testing goes well.

[Felix M.]

Good news and bad news. Good one first: The whole system is running again although there are some things to change like some wiring in the tank circuit as well as at the GDT.

(Channel 3 is just to give me a 0 reference line)
I captured this shot with the slot7 inductor shorted (no phase lead) in order to see how much delay I have:


After then putting in the right inductor I got it almost to zero. It's probably just a matter of fine adjustment:



I used the 7M3-154 with a range of 113-188uH.
(Turns out it's not necessary to buy extra pins when you have old IC sockets you can take appart. It's so much easier to change the inductors so thaks for the advice flyingperson23  )

Bad news is the bridge output signal. After a complete redesign it's still that wobbly. My guess is my 64V 5A lab bench power supply because the red current limit LED lights up at even 600mA which is much less than 5A. Maybe it can't supply the pulse current the coil needs or something like that. The wave also changes when the dummy load is changed so it would mayby just disapper at larger power and load with a propper mains supply but before the thing ever sees 3 phase power, I need to finish my softstarter.

[davekni]

After then putting in the right inductor I got it almost to zero.

This second scope capture shows ~0.7us long pause in H-bridge output transitions.  Looks like the common "triple transition".  Just to be certain, does this same pause show up on both H-bridge outputs individually?  I suggest measuring the two outputs individually (even with a differential probe) just to rule out the possibility that the pause is caused by one output switching before the other.  Presuming both outputs show this pause, its 0.7us length suggests longer dead time than necessary.  Reducing gate resistor value should shorten this pause (move second half of transition earlier).  With that change, phase lead is probably good at this UD2.7 inductor value.

Bad news is the bridge output signal. After a complete redesign it's still that wobbly.

Wobble is due to inductance of bus bar between bulk caps and IGBTs resonating with snubber caps.  It is better.  Notice that the wiggle is now 7th harmonic of operating frequency rather than previous 5th harmonic.  Bus bar inductance is lower so resonant frequency is higher and impedance lower.  This amount of 7th harmonic ringing is very unlikely to cause problems.

My guess is my 64V 5A lab bench power supply because the red current limit LED lights up at even 600mA which is much less than 5A. Maybe it can't supply the pulse current the coil needs or something like that.

Pulse load is likely causing current limit LED to light up.  Supply is hitting current limit during pulse load as it should.  That is not a cause for any concern and not related to 7th harmonic ringing.

The wave also changes when the dummy load is changed

Load makes slight changes in frequency as well as damping.  Probably moving 7th harmonic closer or farther from snubber cap resonant frequency.  Interesting detail to explore if you have the interest, but very unlikely to be of any real concern for coil operation.

[Felix M.]

It's right, the PSU has nothing to do with the wobbly signal. If I charge the bus and do one puls without the psu, it stays wobbly. For the test today I grounded the negative bus rail, attached the probe ground to it and measured each output of the two bricks. This is how it looks:



I have no experience with dead times etc. so you tell me if this looks good or bad. All gates have identical gate resistors and diodes. The wires between them and the GDT are twisted and have the same length.

In addition I captured the GE signal of one IGBT:



There's a hint of the positive spike I saw a few weeks back but it doesn't exceed the zener diode's 33V, it stays at 24V. The rise time was about 730-740ns. I could try changing the gate resistors to make it rise faster.

[Felix M.]

I scoped almost everything possible on both bricks. Negative busbar was grounded, all scope grounds were attached to it. For the one gate with it's emmiter being the output terminal of the IGBT, I used my differential probe.

CH1=Vge1, CH2=Vge2 (Measured on IGBT Brick 1):


CH1=Vge1, CH2=Vge2, CH3=primary current at 51R burden resistor (Measured on IGBT Brick 1):


CH1=Vge1, CH2=Vge2, CH3=primary current at 51R burden resistor, CH4=Brick output with ground ref. (Measured on IGBT Brick 1):


CH2=Vge, CH4=Brick output with ground ref. (Measured on IGBT Brick 1):



CH1=Vge1, CH2=Vge2 (Measured on IGBT Brick 2):


CH1=Vge1, CH2=Vge2, CH3=primary current at 51R burden resistor (Measured on IGBT Brick 2):


CH1=Vge1, CH2=Vge2, CH3=primary current at 51R burden resistor, CH4=Brick output with ground ref. (Measured on IGBT Brick 2):


CH1=Vge, CH4=Brick output with ground ref. (Measured on IGBT Brick 2):


A couple of things I noticed: Why does Brick 2's output look so much smoother and less wobbly than the one from Brick 1? Are the bricks output signals supposed to lagg behind that much (t on+t off)? Are current and Vge supposed to be out of phase like that? Could the problem just be one malfunktioning brick (likely Brick 1)?

[davekni]

A couple of things I noticed: Why does Brick 2's output look so much smoother and less wobbly than the one from Brick 1? Are the bricks output signals supposed to lagg behind that much (t on+t off)? Are current and Vge supposed to be out of phase like that? Could the problem just be one malfunktioning brick (likely Brick 1)?

Yes, I'd guess that brick 1 has a problem.  It is much slower than brick 2.  Brick 1 may have more problems too, but for sure it is much slower than brick 2.
Brick 1 problems might also be causing shoot-through current when it switches, which may be exciting that 7th harmonic ring more than normal operation would.

[Felix M.]

I'll change the brick tommorow and see how it performs. Can you tell if the bricks are legit by this pic?

[davekni]

Can you tell if the bricks are legit by this pic?

I can't tell, but don't see anything that looks suspicious.

Presuming the replacement brick is appropriately fast like brick 2, you may have too much phase lead.  Previous adjustments were trying to compensate for an overly-slow brick.

[Felix M.]

Good thing I have a lot of inductors on hand and can swap them easily