Drsstc feedback startup

[RoamingD]

Hello, it may sound like a stupid question, but do I need to have my secondary coil in place for my ud2.7c to be able to pick up the feedback signal from the primary coil?
Currently my driver works fine only when it is fed a 120khz square wave, and the feedback transformer when loaded with a 51ohm burden resistor appears on the oscilloscope

[flyingperson23]

No, the UD operates on primary current feedback; all it needs is for current to be flowing through the primary. If you operate without a secondary, the UD will oscillate normally as current will still flow through the primary.

[RoamingD]

Some quick scope shots, this is the halfbridge output working when the driver is fed a signal
I fed it 170 vac on the bus at some point and nothing blew up so the halfbridge is switching right

This is the feedback transformer loaded on a 51ohm burden resistor and it looks good 

but the really strange output is this "starting pulse" that I measured on the gdt when it has no feedback connected, to me it looks inverted, how is this possible?

[flyingperson23]

Those waveforms look normal enough to me. What do you mean by 'inverted'?

[RoamingD]

I mean the output is almost high and has just a short off time, kinda like a high duty cycle, and on the internet I saw quite the opposite, a short positive pulse, not a short gnd pulse like mine, I really don't know, but the circuit isn't able to start it's oscillation from the current transformer

[flyingperson23]

Did you try changing the phasing on the driver? If your feedback phase is wrong it won't oscillate under its own feedback.

[davekni]

Some quick scope shots, this is the halfbridge output working when the driver is fed a signal
I fed it 170 vac on the bus at some point and nothing blew up so the halfbridge is switching right

Do you have a bleed resistor pulling half-bridge output low?  Something is causing output to end up at 0V after enable pulse ends.
Looks like Vbus is 60V for this scope capture.  Is that accurate?  Is Vbus 60V for the current transformer output capture too?

but the really strange output is this "starting pulse" that I measured on the gdt when it has no feedback connected, to me it looks inverted, how is this possible?

Where on GDT are you probing?  Is this low-side Vge (or Vgs if using FETs)?  If so, it looks appropriate, other than possible CT inversion issue as flyingperson23 mentioned.  What does half-bridge output look like?  What does voltage across UD2.7 51-ohm resistor look like?
If this is low-side Vge, then high side Vge should be a matching positive pulse.  That should make a low-to-high transition on half-bridge output.  That output transition should cause current that should trigger another transition.

[RoamingD]

Do you have a bleed resistor pulling half-bridge output low?  Something is causing output to end up at 0V after enable pulse ends.
Looks like Vbus is 60V for this scope capture.  Is that accurate?  Is Vbus 60V for the current transformer output capture too?

This horrible thing I drew is the schematic of the bridge, I dont use any bleed resistor except on the bus capacitor

and this is how I connected the probe

This is how I connected and measured the feedback transformer with a 120khz signal from a 555 timer fed in the feedback connector, the variac was set to 50v ac

With no feedback transformer/signal generator, this is the startup pulse that I can measure on the gates of the igbt, is this normal?


And I dont think the gdt is the problem because with external feedback, the circuit didnt blow at 170vac input and also no real heating, at some point I was even tripping the ocd
I really dont know what to do, but for a last resort can I run the coil based on the signal generator and ditch primary feedback, keeping in mind the ocd does still work?

[flyingperson23]

Typically the non-igbt end of a half bridge tank circuit is connected to a capacitive voltage divider, not Vbus-

[RoamingD]

I asked previously about this in this thread https://highvoltageforum.net/index.php?topic=2348.0

[Mads Barnkob]

I replied to your thread "Re: Inverter question" and I think your problem here is primary coil saturation, from leaving out the DC blocking capacitors. Often they are mistaken for only being used for voltage doubling in 120VAC countries, but that is just a double use of them.

[RoamingD]

Oh, I see, but I have one more question, when adding the DC blocking caps does their capacity add up to the MMC?

[Mads Barnkob]

Oh, I see, but I have one more question, when adding the DC blocking caps does their capacity add up to the MMC?

Not if you calculate it correctly as I linked to in the other thread.

[RoamingD]

Not if you calculate it correctly as I linked to in the other thread.

I am very sorry if my physics or mathematics skills are lacking ) the resonant freq is 122khz with a 3.808 uH primary coil inductance
I have run the calculations and a 1uF capacitor will have 1.305 Ohms of reactance but the frequency comes at around 0.08164 kHz, will this work?
Also, another beginner question, what should the voltage rating of those caps be? Can I get away with 630v?

[davekni]

I dont use any bleed resistor except on the bus capacitor

Then I'm puzzled as to why half-bridge output returns quickly to 0V (to Vbus-) shortly after a burst.  (This is from first scope capture of reply #2 above.)  Only explanations I'm thinking of at the moment are either there is some glitch in low-side IGBT Vge or the low-side IGBT is partially damaged (leakage current).

With no feedback transformer/signal generator, this is the startup pulse that I can measure on the gates of the igbt, is this normal?

I'll guess the probe is really 10x, so 20V/div.  Even then it does not look normal.   Initial -24V pulse looks somewhat reasonable, but too short.  Subsequent ring appears clipped, more square than sine wave, and too high frequency for your 122kHz coil.  Are you sure GDT is on ferrite core and not powdered iron?  It may work barely well enough for operation at 122kHz, but not for the initial start transition.

Typically the non-igbt end of a half bridge tank circuit is connected to a capacitive voltage divider, not Vbus-

Agree.  But I don't think this is a significant cause of issues showing up here.

[RoamingD]

Sadly I am sure the gdt is proper ferrite, I have used this type of core in my 200khz sstc and it was fine, here is a link of it
https://www.tme.eu/ro/details/b64290l0618x830/miezuri-de-ferita/epcos/
Also, I think the output returning to 0 was some kind of weird glitch with that cheap oscilloscope, my trusty analog scope doesn't show it falling, but staying in the middle

[davekni]

Sadly I am sure the gdt is proper ferrite, I have used this type of core in my 200khz sstc and it was fine, here is a link of it

Material is fine.  How many turns are on your GDT?  Core is rather small for 120kHz.  I'm guessing core is saturating after 5us.  That should be just enough for 120kHz (4us half-cycle).  Not much margin, but probably not the main issue.

Also, I think the output returning to 0 was some kind of weird glitch with that cheap oscilloscope, my trusty analog scope doesn't show it falling, but staying in the middle

Puzzling, but I'll ignore that one capture showing quick return to Vbus-.  You still might need bleed resistors to keep half-bridge output at half way point.  Without scope load, if upper IGBT happens to have slightly higher leakage current, output may drift high.  Initial start pulse tries to pull it high.  If output is already high, no initial change, so no feedback.

I still suggest scoping UD2.7 51ohm CT burden resistor during that startup pulse, then half-bridge output.

[RoamingD]

Hello, I made a huge discovery, the whole thing works fine when I touch the 2nd or 3rd pin of the AD8561ANZ  ultra fast comparator that I used instead of the lt1001 with the tip of a oscilloscope probe
Now I am confused as to why does this solve the problem and what should I change on the board to make it work by itself
With red is where I probed and it stared working (the ground lead was not connected to anything)

[davekni]

Hello, I made a huge discovery, the whole thing works fine when I touch the 2nd or 3rd pin of the AD8561ANZ  ultra fast comparator that I used instead of the lt1001 with the tip of a oscilloscope probe
Now I am confused as to why does this solve the problem and what should I change on the board to make it work by itself
With red is where I probed and it stared working (the ground lead was not connected to anything)

Scope probe and lead cable are acting as an antenna.  You are picking up enough either random noise or more likely voltage from secondary.  That pickup makes extra driver output transitions to start oscillation.

Three options come to mind for fixing this without a scope probe:

1) Add an antenna.  NOT recommended.  Hard to control what phase lead will be or how much signal may be picked up and over-drive comparitor input.

2) Fix the issue(s) preventing normal startup.  I've already twice suggested next scope measurements to take, so won't repeat here.

3) Modify your driver to be self-oscillating.  I described this for UD2.7 in third paragraph of this post:
    https://highvoltageforum.net/index.php?topic=1373.msg10197#msg10197