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Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
On a day where I could have gotten the motor for "Thumper" put all together, Daniel called and said he had made one of my boards, only changed it to put out +-15V, so that it could drive an intermediate H-bridge, which in turn drives his power bridge made up with Semikron SKM 400GB124D trough another big gate transformer. So lots of delays, just perfect for the Predikter, which is well described down under projects, in the "Thumper" thread.
We had little time to set things up really organized, so this is the ghetto setup that we ran things with:
closer,
The ringup produced, and showing tuning to be mediocre:
But did we make sparks? With 320V into the bridge (no voltage doubler either) we made 5 footers, the ones hitting the ladder in the video below. We had only a 3A variac so only pulsed mode. Btw. It is a really nice feature to have the precision rectifier in the current overload circuit. We set the desired voltage on the reference pin of the comparator, and the over current protection cuts in at the exact corresponding current.
I think this proves the basic soundness of the predikter driver.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Hi Finn,
Looks good from the pictures. How much propagation delay do you think the predicter is compensating for in the whole feedback loop? In other words, how much "timing advance" is the predicting part of the circuit contributing?
It is always good news if the IGBT voltage waveforms look clean when running under power. The absense of overshoot and ringing must mean that the switching transitions are occuring somewhere near the current zero-crossings. Clean voltage waveforms also indicate that there is no significant shoot-through.
It will be interesting to see how it performs in continuous runs, where the spark channels are given more time to grow and wander around!
BTW. What is the DC bus supply voltage to your "intermediate H-bridge" ? The idea here is to use a higher voltage than the IGBT gates require, and use a "step-down" GDT to boost the drive current at the IGBT gates. This technique is highly effective for driving multiple MOS devices in parallel where the total input capacitance is large. It also minimises the detrimental effect of any remaining leakage inductance when a GDT. This is because the leakage inductance from the driver side (primary) is reduced by the square of the turns-ratio when it actually appears at the IGBTs on the secondary windings!
Nice work, and will be interested to see how it develops...
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
GeordieBoy wrote ...
Hi Finn,
Looks good from the pictures. How much propagation delay do you think the predicter is compensating for in the whole feedback loop? In other words, how much "timing advance" is the predicting part of the circuit contributing?
We did not measure it, but since it used all of the available capacity there was, I think about 2µS!.
GeordieBoy wrote ...
It is always good news if the IGBT voltage waveforms look clean when running under power. The absense of overshoot and ringing must mean that the switching transitions are occuring somewhere near the current zero-crossings. Clean voltage waveforms also indicate that there is no significant shoot-through.
This we did measure, and even at low voltage, where there usually is a lot of spiking on the voltage waveforms, there was nil. -That is: The first transition spikes a little bit, but the others are clean.
GeordieBoy wrote ...
It will be interesting to see how it performs in continuous runs, where the spark channels are given more time to grow and wander around!
Having to leave the machine without running it flat out was very hard, but time did not allow more experiments. I'm sure Daniel will do this tomorrow.
GeordieBoy wrote ...
BTW. What is the DC bus supply voltage to your "intermediate H-bridge" ? The idea here is to use a higher voltage than the IGBT gates require, and use a "step-down" GDT to boost the drive current at the IGBT gates. This technique is highly effective for driving multiple MOS devices in parallel where the total input capacitance is large. It also minimises the detrimental effect of any remaining leakage inductance when a GDT. This is because the leakage inductance from the driver side (primary) is reduced by the square of the turns-ratio when it actually appears at the IGBTs on the secondary windings!
This intermediate driver does not take advantage of any of these techniques. it is just a booster at 30 or so voltage.
GeordieBoy wrote ...
Nice work, and will be interested to see how it develops...
Registered Member #289
Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154
Congratulations on testing Finn! Your project is showing very promising results; I am going to have to reconsider using this approach in future systems. Can't wait to see a full power video!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Looks great Finn! =) I'd also love to see footage of a continuous run at lower break rate and higher bang energy. Get those peak junction temperatures to the redline
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
All,
Thanks for your kind words. I was away on family get together, but Daniel recorded this video. 380V or so in, 1000A max primary current, 80BPS and a bit more when it hits the lighting armature, for a full 6 feet discharge.
Everything is running cool, so looking forward to winding current transformers with 0.002V/A as the ones we are using now are hitting the input voltage limit very soon. And voltage doubler, of course.
Hope this is something near what you were looking for, although no junctions got near the red line.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Robert2 wrote ...
You can give pcb or page with this drsstc ?
Robert,
Files are down in projects, but please understand that this design primarily addresses problems encountered with large coils, which has long delays, due to powerfull, but slow switching devices. Be also aware that this project has not yet been described to the point where it is ascessible to the first time builder.
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Hammertone "Predikter", the DRSSTC driver for the rest of the century.
