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Ringing on GDT. Can it be stopped with RC snubber? (no choice but to dupilcate)

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teravolt

Mon Apr 24 2017, 02:22PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

one problem that I have had with dip drivers is that they can over heat driving transformers at that frequency so I converted to using TO 220-5 packages. a mosfet totem pole "half bridge" could be added to to the mosfet drivers

Gregary Boyles

Wed May 10 2017, 02:14PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

My GDT waveform looks more or less like this:
Gdtwav02

39 ohms is about the largest damping resistance before is start degrading the output from my top load.....according to a fluorescent light bulb test.

And increasing the resistance further does not seem to make any further difference to the ringing any way.

Are there any more tricks I can try to reduce the rining a bit more apart from increasing damping resistance?

teravolt

Wed May 10 2017, 09:44PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

have you tried another core and try using cat-5 wires

teravolt

Thu May 11 2017, 03:46AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

you could try this and hear are the mosfet for Q1,Q2 mosfet driver fets

and use cat-5 for T1 around a toroid. your toroid should have a frequency response 50k to 500kHz. I chose low ohm mosfet and a 1000uf cap to drive the transformer.

1494474410 195 FT179563 Tesla Interupter 2

Gregary Boyles

Thu May 11 2017, 04:20PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

Referring to this page: Typical gate drive waveforms

Sorry I didn't specifically state it but the ringing problem is on my GDT primary - is there any benefit to puting a low value resistor in series with the GDT primary cap?

I believe I have this problem with the GDT secondary:
Gdtwav07

But I am not sure because the droop looks worst on shorter oscilloscope time scales with wider square waves than longer time scales with narrower square wave.

Far too low primary inductance. Either too few turns or wrong core material used.

It would be nice to have some idea of what sort of primary inductance you need - at least ball park.
300uH?
800 uH?
3000 uH?
30,000uH?

I have used an N30 toroid which is one of the recommended ferrite materials.

I have used about ASA 22 magnet wire to create a trifiller winding and I have put enough turns on the torroid with this to get an inductance of about 32mH......to see how much improvement I get with the droop in the square wave.

teravolt

Thu May 11 2017, 08:02PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

a resistor in series with your gdt primary may save your 8 pin dip 37322 Ic but will your gats get enough initial current to turn them on properly. I prefer

and

because they have more balls. Higher current capability, higher rail voltage and a heat sync ready. what is available to you the circuit that you are using at the top is kinda antiquated

Gregary Boyles

Mon May 15 2017, 11:12AM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

This is the wave form I am getting now:

1494846468 9039 FT179604 Trace

This is with enough turns on my N30 toroid to give an inductance of 32,000uH.

According to http://www.richieburnett.co.uk/temp/gdt/gdt2.html I still don't have enough primary inductance for flat tops and bottoms for my square wave.

How much inductance do you need exactly??????

Is there any other causes for dropping square wave tops and bottoms that is not mentioned on this web page?

Gregary Boyles

Mon May 15 2017, 01:43PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

Well it has taken me some weeks of mucking around with my GDT but I just figured out why my square wave on the secondaries of my GDT are drooping some what and it has nothing to do with the GDT itself.

As it turns out, and as is not mentioned by the author of this page, this sort of wave form:

Gdtwav06

can also be caused by too little capacitance in the capacitor in series with the GDT primary.

After swapping my 100nF caps for 1uF caps this is now my secondary waveforms:
1494855489 9039 FT179604 Trace

And that is with only 1mH inductance in my GDT or 14 turns, as per this document: http://www.ferroxcube.com/FerroxcubeCorporateReception/datasheet/gate%20drive%20trafo.pdf
About as good as I am likely to get them probably.

teravolt

Mon May 15 2017, 07:20PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

congrats thanks for reporting it I guess there was a self resonating between your cap and the transformer inductance. what time scale are you using and how well does it work with your bridge.

Gregary Boyles

Tue May 16 2017, 02:27AM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

teravolt wrote ...

congrats thanks for reporting it I guess there was a self resonating between your cap and the transformer inductance. what time scale are you using and how well does it work with your bridge.

Time scale is on 1uS minus what ever the fine adjustment knob does.
I had it working with my half bridge powered with 12V from an ATX power supply - the secondary coil excites a fluorescent bulb nicely with the square wave at around 450kHz.

My secondary resistors are 100 ohm so now I will need to see how far I can back them off in value before the waveform degrades.

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