Author Topic: Bridge output  (Read 4003 times)

Online flyingperson23

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Bridge output
« on: May 29, 2023, 08:46:02 PM »
I'm trying to tune zvs on my tesla coil (bsm150 full bridge, 300A ocd, ud2.9), and the way I read to do this was to minimize ringing on the bridge output. Scoping the output got me the attached image, which seems off to me. Shouldn't it more or less resemble a square wave? The signals to the gates were all normal. Any ideas?

It looks like maybe the transistors are turning on too slowly? Which doesn't really make sense because the design is based on franzoli electronics' alphaDRs, and I used the same igbts and 5.1ohm gate resistors he did.
« Last Edit: May 29, 2023, 10:41:06 PM by Mads Barnkob »

Offline Mads Barnkob

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Re: Bridge output
« Reply #1 on: May 29, 2023, 10:44:47 PM »
Are you testing at a low DC bus voltage? The IGBT output capacitance is constant, which makes it a major contributor to switching spikes at low voltages. Get up around 100VDC and the spikes should percentage wise to inverter output, be much smaller.

Also, please use edit when adding new information, instead of double posting.
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Offline davekni

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Re: Bridge output
« Reply #2 on: May 30, 2023, 03:58:51 AM »
Quote
Shouldn't it more or less resemble a square wave?
Parasitic inductance of interconnect (wiring and/or ECB and/or bus bars) is causing the non-square wave shape.  Connecting the scope probe and ground clip as close as possible to low-side IGBT collector and emitter will help.  Also helps to keep scope probe ground lead as short as possible and tight against probe body to avoid inductive loops that couple to H-bridge current.  There are tutorials somewhere here about scoping techniques.
If problems persist, images of H-bridge layout and probe placement will help.  And some scope images zoomed in to one or two cycles for better viewing of details.
David Knierim

Offline Manz

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Re: Bridge output
« Reply #3 on: May 30, 2023, 10:19:14 PM »
It would be useful to see primary current and inverter output at the same time to ceck if the ZCS is correct, my coil's inverter output looks almost the same when the phaselead is way off.
Also as Mads mentioned the bus voltage could be a problem if too low

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Re: Bridge output
« Reply #4 on: May 30, 2023, 11:44:22 PM »
Image 1 shows the gate of one of the upper transistors in yellow and the bridge in blue. Image 2 is c-e of the same transistor, but all the transistors look similar. Image 3 shows current in yellow and bridge output in blue. I have 7m3-223 and 233 inductors and sweeping through both their ranges does nothing noticeable except reduce the spikes seen in image 4, which is what it looks like with no phase lead. Image 5 is a close up of one of the transitions and definitely shows something, but I'm not sure what. All pictures were taken with a bus voltage >100V, and the problem seems to get worse with higher voltage.

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Re: Bridge output
« Reply #5 on: May 31, 2023, 01:27:17 AM »
Image 3 makes me think there is some kind of phasing problem
The inverter is switching at peak current, it's probably 90° out of phase. What are you using for your CT? A wrong core material and permeability might be the cause of that phaseshift


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Re: Bridge output
« Reply #6 on: May 31, 2023, 04:31:36 AM »
I used EVR's CT kit with their ferrites.

Offline davekni

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Re: Bridge output
« Reply #7 on: June 01, 2023, 04:52:33 AM »
Quote
Image 1 shows the gate of one of the upper transistors in yellow and the bridge in blue.
Do you have a differential scope probe to measure upper IGBT Vge?

Quote
Image 3 shows current in yellow and bridge output in blue.
How is current measured?  Do you have a 3rd CT (or 3rd CT second-stage)?  What burden resistors are on each CT?  Is UD2.9 feedback resistance 51 ohms as default?  What resistance is on OCD feedback after diode bridge in UD2.9?  What resistance is on current measurement CT?  What are the CT ratios?  Are the EVR parts just cores or pre-wound CTs?

Quote
The inverter is switching at peak current, it's probably 90° out of phase.
Yes, something is quite wrong.  More details are needed to determine if this is a measurement issue or performance issue.  I'm guessing measurement.  Besides above questions, images of probing will help, including where scope probe ground leads are connected and how differential probe wires are routed.
David Knierim

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Re: Bridge output
« Reply #8 on: June 11, 2023, 04:16:10 AM »
I don't have a differential probe, I just leave my scope ungrounded and only measure 1 non-floating thing at a time. The current is measured with a third 1:100 CT with .25ohm burden. The feedback resistance is 51ohm. The OCD burden is 5.1 ohms, and seems to work fine. The main CTs are cascaded 1:1024, and I'm fairly certain that's not the issue as I tried the inverter with a known working CT from a different coil and it gave the same output. The EVR parts were cores with wire to wind them, but not prewound. It's a little hard to see but I have the scope ground connected to one side of the bridge output and probe connected to the other, and the second probe is connected straight to the output of the CT.

Offline davekni

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Re: Bridge output
« Reply #9 on: June 13, 2023, 04:52:49 AM »
Quote
It's a little hard to see but I have the scope ground connected to one side of the bridge output and probe connected to the other, and the second probe is connected straight to the output of the CT.
Certainly accurate to say "hard to see".  Is the upper probe measuring bridge output?  If so, is the probe ground connection around the probe ring normally used for ground lead, or at the probe tip?

Either way, large loops between scope probe tip and ground on lower probe adjacent high current (primary current) wiring (also with large loop area) is likely making measurements way off.

Best to have primary coil wires paired as close together as voltage allows.  Twisted together is great if primary lead wire insulation is thick enough to handle primary voltage.

Scope measurements are best made from opposite side of H-bridge, or at least kept away from primary current wiring (away from MMC and leads to primary coil) and away from primary coil itself.

For measuring H-bridge output, best to connect scope probe ground lead to Vbus- (low side of H-bridge power) and measure one side of H-bridge output.  Avoids high-frequency signal on scope case, which couples to other measurements.  Make a second measurement of other output if you suspect a problem with bridge causing outputs to not be complimentary.

Measuring CT voltage across 0.25 ohm resistor would be best with coax all the way from 0.25 ohm resistor to scope input.  That relatively-low-voltage signal will be sensitive to any scope probe tip-to-ground loops.
David Knierim

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Re: Bridge output
« Reply #10 on: June 29, 2023, 12:07:22 AM »
I got better results switching measurement positions to be closer. That was borrowing a primary from a different coil, which was far away. Now with the actual primary right on top of it, no measurements positions seem normal, even directly on the IGBT outputs. The image attached shows the output of one transistor in blue and a floating probe next to the primary in yellow, so I'm going to assume it's just the high current throwing everything off.

Offline davekni

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Re: Bridge output
« Reply #11 on: June 29, 2023, 03:58:03 AM »
Quote
a floating probe next to the primary in yellow
Around high voltage, floating probes will always pick up significant signal.

Reasonable scope measurements are possible.  Requires minimizing wire loop areas for both high current signals (entire H-bridge output through MMC and primary coil) and for measurement connections (scope probe and scope ground connection.  If primary coil is close to h-bridge, may also require a metal shield plate (or foil) between primary and h-bridge.  Needs to be a minimum of primary-coil-radius below primary coil to minimize blocking primary field necessary for TC operation.
David Knierim

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Re: Bridge output
« Reply #12 on: June 29, 2023, 04:12:11 AM »
I unfortunately wasn't the brightest designing this. The primary is right above the bridge, so I'll try making extensions and moving it away. I routed the hv cables to try to minimize loop area and there's not much from the mmc to the primary but there is a good bit between the inverter and mmc pointing a field right at all possible measurement points. I'll see if I can move the gdt and reroute stuff but idk if it'll be possible. Assuming I got phase lead and primary tuned with minimal interference, would there be any long term problems running the coil as it is with the interference?

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Re: Bridge output
« Reply #13 on: June 29, 2023, 08:39:09 PM »
Quote
Assuming I got phase lead and primary tuned with minimal interference, would there be any long term problems running the coil as it is with the interference?
If your driver circuitry is on an ECB with good ground planes, probably no issue.  If driver is hand-wired breadboard, may cause issues.  Shielding around driver would help if there are issues.
David Knierim

Offline Felix M.

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Re: Bridge output
« Reply #14 on: July 29, 2023, 07:18:39 PM »
Hi all,
I have some similar problems with my bridge. I'm using a SKM200GB218D fullbridge in combination with a 324nF MMC. The operating frequency is at 80kHz. The driver is an UD 2.7C THT. I have gate resistors with 5.1Ω, a diode for faster discharge and some 33V zener diodes for protection.

After doing some static load test with 64V on the bus, I got this for a bridge output:

(Notice how the maximum voltage reaches double the Vbus)

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. It still confirms that the coil is pretty much hardswitching:

Tuning the phase lead with the inductor doesn't work at all


I think gate signal's rise and fall times are also not as fast as they are supposed to be:

It's a 15:15 GDT on a N30 ferrite core. (Here is a link: https://www.mouser.de/ProductDetail/871-B64290A0040X830 )

Here is a shot of the UD 2.7C output with the GDT attached:



Back to the brigde output: Is there any explanation for such a funky looking signal? Any idea on how to fix it?

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Re: Bridge output
« Reply #15 on: July 29, 2023, 09:33:09 PM »
Quote
After doing some static load test with 64V on the bus, I got this for a bridge output:
Is this measurement using a differential scope probe?  Or with either the scope or 64V power supply isolated from line ground?  Even with scope or supply floating, connecting scope probe ground clip to an active signal (bridge output) can cause problems.  Scope or supply internal Y caps pass high frequency current to line.  Unless you have a differential probe, I'd recommend connecting scope ground to Vbus- and measuring one output of the bridge at a time.

If not a scoping artifact, I'd guess the ~400kHz ringing is due to wiring inductance between H-bridge and bulk capacitors resonating with film cap(s) close to IGBTs.  Pictures of your setup are helpful, including showing scope probe connections.

The end of this scope capture also looks odd, as if Vbus were ramping towards 0.  Possible GDT output phasing (lead reversal) issue?

Quote
I think gate signal's rise and fall times are also not as fast as they are supposed to be:
Quote
It's a 15:15 GDT on a N30 ferrite core.
Probably fine for an 80kHz coil.  Pictures of GDT and wiring would help here.  Reducing GDT leakage inductance (including lead wire inductance) would improve Vge fall time.  Rise time is intentionally a bit longer due to 5.1 ohm gate resistors.  Added delay avoids IGBT cross-conduction (provides dead-time).
« Last Edit: July 29, 2023, 09:35:37 PM by davekni »
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Offline Felix M.

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Re: Bridge output
« Reply #16 on: July 29, 2023, 10:49:46 PM »
Thanks Dave

I do have an isolated power supply and was just scoping the output with ground attached to one and the probe head to the other terminal at the bridge. There are also some WIMA FKP sunbber caps on the bridge. I'll do some further test tomorrow and post them along with some pictures of my setup.

Offline Felix M.

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Re: Bridge output
« Reply #17 on: July 30, 2023, 03:15:18 PM »
First of all, here is a picture of my setup:

The inverter bridge is quite huge which is likely the reason for such a wobbly looking signal because of the large stray inductance. A whole redesign of the busbars would be quite a project but if it's the issue, I guess there is no real way around it.

Doing the output scope shots with the ground to the negative rail gives the same signal.


The shots from yesterday were done with 4x 0.47uF of snubber capacitance (2 caps on each IGBT).

I've put the Siemens MKV cap along with 2 more snubbers back in place today and did the shots with them in the setup (not sure if it changed anything)

Offline davekni

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Re: Bridge output
« Reply #18 on: July 30, 2023, 07:30:57 PM »
Quote
Doing the output scope shots with the ground to the negative rail gives the same signal.
Looks better to me.  Does the other H-bridge output look similar?   Presuming so, I think you'll be fine with this build.  Output swing is not dropping to half near the end as with previous capture.
What is the H-bridge output connected to for this test?  Is it MMC and primary only?  Or is secondary in place with top load?  Or is there a metal pot or other dummy load on primary instead of secondary?  I don't see the typical higher-frequency ring at the end of the normal frequency ring-down square wave (caused by IGBT output capacitance and primary inductance).  Perhaps induction load is dampening that ring.

Quote
The shots from yesterday were done with 4x 0.47uF of snubber capacitance (2 caps on each IGBT).

I've put the Siemens MKV cap along with 2 more snubbers back in place today and did the shots with them in the setup (not sure if it changed anything)
That explains some of the improvement.  With 4x 0.47uF it appears that bus/snubber resonance was at ~400kHz, fifth harmonic of TC frequency.  With more capacitance, bus/snubber frequency is no longer hitting a harmonic of primary frequency.
David Knierim

Offline Felix M.

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Re: Bridge output
« Reply #19 on: July 30, 2023, 08:45:50 PM »
Looks better to me.  Does the other H-bridge output look similar?
The output of the second brick looks almost the same.

The test were performed with the tank circuit at the output of the inverter. The static load was mainly a large isolation transformer along with a mot.


There is some ring-down accuring, I just missed it on the scope shot. I'm really not too sure if more snubber capacitance fixed the wobbly signal, since the scope can now only capture the positive half of the squarewave and you can't see as mutch as in the shot from yesterday. It would be great however  :D

The next problem is the phaselead:


A friend mentioned that the CTs could cause the phaseshift. However if I lay the sine wave from the current over the gate signal of the IGBTs, it's almost perfectly in the zero crossing. Unfortunately the ferrite peace inside my slot 7 inductor broke which makes tuning impossible right now. Tuning the phase with the inductor before when it was still functional didn't really work. I was never able to get it to the zero point. Bypassing it would also not make a huge difference. If you have any suggestion on the right slot 7 inductor, please let me know.

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Re: Bridge output
« Reply #19 on: July 30, 2023, 08:45:50 PM »

 


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davekni
September 19, 2023, 06:29:00 AM
post Re: Drsstc feedback startup
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 19, 2023, 06:15:40 AM
post Re: My QCW DRSSTC, small questions.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 19, 2023, 05:56:13 AM
post Re: Measuring electric voltage in a Tesla coil
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
September 19, 2023, 05:05:57 AM
post Re: Migatronic FocusStick 161E Stick Welder Teardown
[Electronic Circuits]
orac
September 19, 2023, 04:52:30 AM
post Re: Part identification for a Esab Es300i renegade Machine
[Beginners]
orac
September 19, 2023, 04:48:02 AM
post Re: Enamel on secondary breaking down?
[Dual Resonant Solid State Tesla coils (DRSSTC)]
orac
September 19, 2023, 04:29:14 AM
post Measuring electric voltage in a Tesla coil
[Dual Resonant Solid State Tesla coils (DRSSTC)]
omrid13
September 19, 2023, 01:14:00 AM
post Re: First sstc is not working
[Solid State Tesla Coils (SSTC)]
NyaaX_X
September 19, 2023, 12:59:52 AM
post Re: Drsstc feedback startup
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
September 19, 2023, 12:45:37 AM
post Re: Interrupter pulling ct signal
[Solid State Tesla Coils (SSTC)]
ZakW
September 18, 2023, 11:16:41 PM
post Re: Interrupter pulling ct signal
[Solid State Tesla Coils (SSTC)]
Egg
September 18, 2023, 09:55:08 PM
post Re: My QCW DRSSTC, small questions.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Lucasww
September 18, 2023, 08:15:37 PM
post Re: First sstc is not working
[Solid State Tesla Coils (SSTC)]
Recep talip
September 18, 2023, 07:47:21 PM
post Re: Drsstc feedback startup
[Dual Resonant Solid State Tesla coils (DRSSTC)]
RoamingD
September 18, 2023, 07:46:46 PM
post Re: My QCW DRSSTC, small questions.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
All Science
September 18, 2023, 07:39:16 PM
post Re: Profdc9 Ud2.7C jumper question
[Dual Resonant Solid State Tesla coils (DRSSTC)]
dexter
September 18, 2023, 06:37:35 PM
post Re: Drsstc feedback startup
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
September 18, 2023, 06:20:30 PM
post Re: Drsstc feedback startup
[Dual Resonant Solid State Tesla coils (DRSSTC)]
RoamingD
September 18, 2023, 05:01:22 PM
post Re: Drsstc feedback startup
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
September 18, 2023, 04:29:15 PM
post Drsstc feedback startup
[Dual Resonant Solid State Tesla coils (DRSSTC)]
RoamingD
September 18, 2023, 02:59:10 PM
post Re: My QCW DRSSTC, small questions.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Lucasww
September 18, 2023, 10:07:22 AM
post Re: First sstc is not working
[Solid State Tesla Coils (SSTC)]
davekni
September 18, 2023, 06:07:25 AM
post Re: Anyone around with good coding skills (for the Flipper Zero)
[Dual Resonant Solid State Tesla coils (DRSSTC)]
ako
September 17, 2023, 09:46:42 PM
post Anyone around with good coding skills (for the Flipper Zero)
[Dual Resonant Solid State Tesla coils (DRSSTC)]
ako
September 17, 2023, 09:42:52 PM
post Re: First sstc is not working
[Solid State Tesla Coils (SSTC)]
NyaaX_X
September 17, 2023, 06:05:59 PM
post Re: What Cable Thickness for Capacitor Discharges?
[Capacitor Banks]
MRMILSTAR
September 17, 2023, 04:48:28 PM
post Re: First sstc is not working
[Solid State Tesla Coils (SSTC)]
Recep talip
September 17, 2023, 11:24:49 AM
post Re: What Cable Thickness for Capacitor Discharges?
[Capacitor Banks]
klugesmith
September 17, 2023, 07:00:02 AM
post Re: First sstc is not working
[Solid State Tesla Coils (SSTC)]
davekni
September 17, 2023, 05:29:59 AM
post Re: Odd MOSFET Driver Behavior
[Solid State Tesla Coils (SSTC)]
davekni
September 17, 2023, 05:19:47 AM
post Re: My QCW DRSSTC, small questions.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 17, 2023, 05:13:58 AM
post Re: First sstc is not working
[Solid State Tesla Coils (SSTC)]
Recep talip
September 16, 2023, 09:28:50 PM
post Re: Odd MOSFET Driver Behavior
[Solid State Tesla Coils (SSTC)]
KrisPringle
September 16, 2023, 04:33:30 PM

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