Choosing snubber capacitance

[Caraffa]

Hi, I'm building my first DRSSTC and I've encountered a problem with choosing snubber capacitor, mainly because the waveform looks a bit strange. I don't have much more than 1uF capacitors and any package will take a long time to arrive so I want to be sure before I order anything.

Back to the topic, I took some screenshots with different subbers. I think i should choose something bigger than 3uF but i'm not sure why square-wave has such sharp end. What do you guys think? 

[Caraffa]

I managed to solder another capacitor, so in total 4uF, but now it looks like too much? I'm only guessing. Maybe there is another problem that i didn't consider.

[davekni]

Back to the topic, I took some screenshots with different subbers. I think i should choose something bigger than 3uF but i'm not sure why square-wave has such sharp end. What do you guys think?

It would help to define what you are measuring.  Presumably a DRSSTC given the post category.  Given the waveform shape, perhaps you are scoping H-bridge output, perhaps differential between the two outputs, rather than Vge of an IGBT.
If this is differential across H-bridge output, are you using a differential probe, or a grounded probe and floating Vbus supply to H-bridge, or a floating scope chassis?  Scope captures of individual H-bridge outputs with probe ground to Vbus- are typically more useful.
If all my above guesses are correct, looks like you are getting resonance of snubber capacitance with lead (or bus bar) inductance to bulk caps.  A picture will help show what inductance might be there.  Pictures showing scope probe connections also help, as some artifacts can be due to inductive pickup in the loop between probe tip and probe ground lead.

[Mads Barnkob]

Did you try a few simple calculations to this problems? https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/snubber-capacitor/

[Caraffa]

It would help to define what you are measuring.

Sorry! I totally forgot. Yes you guessed everything correctly. It's differential between the two outputs of H-bridge.

If this is differential across H-bridge output, are you using a differential probe, or a grounded probe and floating Vbus supply to H-bridge, or a floating scope chassis?

It's grounded probe and floating Vbus.

A picture will help show what inductance might be there.

It's TO-3PN H-bridge so there is a little bit of cables that are maybe sub-optimal. I probably won't leave it as it is now, I wanted to build it mainly for testing and when I find more time I will design something better than a mdf board and a couple of screws. But it is unlikely to come soon.

Did you try a few simple calculations to this problems? https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/snubber-capacitor/

These calculations are mainly based on stray inductance, and I don't even know how to estimate it because I don't have much experience in this subject.

[Mads Barnkob]

Did you try a few simple calculations to this problems? https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/snubber-capacitor/

These calculations are mainly based on stray inductance, and I don't even know how to estimate it because I don't have much experience in this subject.

The estimates are in another article, there is a link between them at the start of the Snubber article

https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/busbar-and-primary-circuit/

[Caraffa]

The estimates are in another article, there is a link between them at the start of the Snubber article

Thanks, according to those formulas and my calculations 1,2-1,5uF should be enough. Do you think there will be much of a difference between 1,5uF and 1/2uF that i tried?

[Mads Barnkob]

The estimates are in another article, there is a link between them at the start of the Snubber article

Thanks, according to those formulas and my calculations 1,2-1,5uF should be enough. Do you think there will be much of a difference between 1,5uF and 1/2uF that i tried?

I also made a snubber capacitor calculator, you can play around with the numbers to get 1.5uF and 0.5uF, by adjusting the allowed transient voltage, that way you can judge if you find that transient level acceptable.

https://kaizerpowerelectronics.dk/calculators/snubber-capacitor-calculator/

[davekni]

Thanks, according to those formulas and my calculations 1,2-1,5uF should be enough. Do you think there will be much of a difference between 1,5uF and 1/2uF that i tried?

Given your earlier scope traces, I'd suggest using at least 2uF.  Standard snubber design guides such as Mads' well presented one are based on an assumption that snubber ring has decayed before the next H-bridge output transition.  That is typically the case in commercial uses of IGBTs, generally at much lower frequency than DRSSTC use.
When snubber ring has not decayed, resonance can make the ring amplitude build each cycle.  That is what shows up in your scope plots especially at 1uF.  There are two or three other threads on this forum where designs have had the bad luck of snubber resonant frequency hitting very close to 2x operating frequency.  That's the worst case, as Vbus ripple current is dominated by 2nd harmonic of operating frequency for full H-Bridges.

[Mads Barnkob]

Thanks, according to those formulas and my calculations 1,2-1,5uF should be enough. Do you think there will be much of a difference between 1,5uF and 1/2uF that i tried?

Given your earlier scope traces, I'd suggest using at least 2uF.  Standard snubber design guides such as Mads' well presented one are based on an assumption that snubber ring has decayed before the next H-bridge output transition.  That is typically the case in commercial uses of IGBTs, generally at much lower frequency than DRSSTC use.
When snubber ring has not decayed, resonance can make the ring amplitude build each cycle.  That is what shows up in your scope plots especially at 1uF.  There are two or three other threads on this forum where designs have had the bad luck of snubber resonant frequency hitting very close to 2x operating frequency.  That's the worst case, as Vbus ripple current is dominated by 2nd harmonic of operating frequency for full H-Bridges.

This sounds like something I just add to the guide

[davekni]

This sounds like something I just add to the guide

That would be a great addition to your well written tutorial.  I expect you'll write a clearer and more general explanation than what I ramble on about.

[Mads Barnkob]

This sounds like something I just add to the guide

That would be a great addition to your well written tutorial.  I expect you'll write a clearer and more general explanation than what I ramble on about.

I can see my wording is confusing, I meant to say NEED to add to the guide, not JUST add to the guide

Something like with SGTCs NSTs to take into account for LTR (larger than resonant, to avoid destroying transformer) in the calculation or calculator.