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Dead time circuits always slowed everything down so we avoided them like the plague. But now with Finn's Predikter, dead time is far far more friendly!! Shoot through is very bad and I give it 1mS in a far different application
We really need to take a whole new look at gate drive timing now!
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
I was thinking about your circuit again and the dead time has the potential to be non uniform. Theoretically if the charging resistor was very small and the discharge resistor very large, you could end up with almost a 50% duty cycle out of the non inverting driver, and much less out of the inverting driver. Of course you can adjust this so the duty cycles are uniform with pots. If I were using the circuit I would use a comparator with the threshold set at 1/2 VCC, and a single pot and capacitor. That way the duty cycle of each driver is always equal. My 2 cents.
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Z28Fistergod wrote ...
Theoretically if the charging resistor was very small and the discharge resistor very large, you could end up with almost a 50% duty cycle out of the non inverting driver, and much less out of the inverting driver. Of course you can adjust this so the duty cycles are uniform with pots.
Any circuit has the potential to be broken by bad component choice - If you look at the two resistor values I use, you will see that they are asymmetric - the reason for this is explained above - the values chosen result in a perfectly symmetrical output.
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
Sure most circuits won't work right without the correct component values. The difference is that you have two variables to produce one adjustment. It could be a pain to watch the scope and try to detect a 1% difference in the duty cycle between the two drivers.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Nicko -
Interesting approach to the dead-time issue ........ have you fired up this driver & bridge combination yet, powering either a dummy load or a real TC?
Not sure I understand your last post ...... is there more information available somewhere (complete driver & bridge schematics, driver board photos, scope shots showing delay timing, etc)?
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Herr Zapp wrote ...
Interesting approach to the dead-time issue ........ have you fired up this driver & bridge combination yet, powering either a dummy load or a real TC?
Not sure I understand your last post ...... is there more information available somewhere (complete driver & bridge schematics, driver board photos, scope shots showing delay timing, etc)?
I've tested the bridge on a dummy load with a frigged set of drivers, but the real controller board is still being made - its all very small SMD - the total size of the controller is 53mm x 68mm approx - double sided with a few components on the bottom. I don't know why, but I'm drawn to "small & neat" not "big & scary"...
I'll happily put up the schematics when the board is built, probably in two weeks time as I'm buried at work and the board house takes 10 days to do fine-pitched boards. There will undoubtedly be errors which I'd like to fix. I have no time yet to update my web site with this but will do as time permits. I'm driven mainly by the autumn UK Cambridge Teslathon which last year had quite a dramatic conclusion
I have all the simulation stuff in LTspice which looks good - If you want I'll post that - of course, practical realisation is another world, so "first light" may make the whole thing catch fire....
I updated the original post in this thread with a photo of one of the bridge options - the FET FB - not rocket science - I also changed the name of the thread - I've added the schematic to the top post.
Should I start a new project thread or just keep updating this one? Should I update "in-line" or the first post?
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Herr Zapp wrote ...
... scope shots showing delay timing, etc...?
Here's the LTspice simulation output showing the current through the GDT primary:
With the component values selected in the schematic clip in the top post, you get an approx 80nS shoulder on each crossing. I will post proper scope shots as soon as I get them...
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
New boards finally arrived today. Hopefully start assembly this weekend (under the microscope...). On a course all week, so I'm going to have to wait... It's small, very small...
uDR/SSTC Controller board This is a daughter board for the full bridge board below. When roatated by 90 degrees, it sits across the bridge board about 3cm up.
uDR/SSTC Full bridge board (IGBT or FET) This can be seen assembled in the first post of this thread.
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uDR/SSTC design - updated with full bridge testing and current monitoring..
