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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Having slept on it, I think the simplest way to introduce dead time while turning both transistors 'off' would be to use the conventional astable circuit, but drive a pair of NPN's from the 'usual' outputsso that each 'power transistor' is switched off when the associated transistor in the astable circuit is 'on'.
However, in order for this to work, the Td(off) of the astable transistors would have to be longer than the Td(off) of the power NPN's.
Using a TL494 may be a better approach here.
BUT, the other thing that occurred to me, and I'm not sure about this, is that an inductor similar to the inductor used in the mazilli ZVS circuit might have a suitable current limiting effect, opposing the change in current when both transistors are on in my original circuit.
Would this be worth trying?
Would this work at 25kHz, with both transistors 'on' for around 250nS?
How would I calculate a suitable value inductor? (could I simply use one of the same spec. as used in the Mazilli circuit, or would I need a higher or lower value inductor?)
I hope this makes some sense. This approach should be more reliable than using a current limiting transistor and more efficient than using a current limiting resistor or light bulb.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
If I understand correctly you want to effectively add leakage inductance to your GDT primary. This is a bad idea for an ideal, square-wave driven transformer like a GDT. Look up any GDT design documentation to find out why.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Oh, for goodness' sake, enough theorising Build it, scope it and you'll understand better than 100 forum posts worth of explanation. This is the last post you're getting from me until you post some experimental results.
Having both transistors on and shorting the supply might be acceptable. This is after all how the old ferroresonant Royer worked. When the transformer core saturated, the induced EMF collapsed to zero, dragging the "on" transistor out of saturation by effectively shorting its collector to the supply. This caused the flip-flop to flip, beginning the next half cycle. The saturation removed the transistor's base drive too, but because of storage time, it still got a nasty forced turn-off. Old editions of the famous Pressman book had a whole chapter on selecting transistors with the fastest storage time, for efficient operation in these circuits.
The DC bus inductor greatly reduces the shoot-through current, at the cost of softening the edges of the square wave and slowing the rise time. In the limit, adding a tank capacitor too, it becomes a sine wave. Maybe not so good for gate drive.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve McConner wrote ...
Oh, for goodness' sake, enough theorising Build it, scope it and you'll understand better than 100 forum posts worth of explanation. This is the last post you're getting from me until you post some experimental results.
OK. My next post on this subject will be when I finish the heatsinks.
Steve McConner wrote ...
The DC bus inductor greatly reduces the shoot-through current, at the cost of softening the edges of the square wave and slowing the rise time. In the limit, adding a tank capacitor too, it becomes a sine wave. Maybe not so good for gate drive.
That's pretty much what I was thinking. Whether 'only' running this at 25kHz and 'clipping' the sine wave-ish output with zeners will work will be determined by experiment.
Lower frequency = lower switching losses.
EDIT: very useful and relevant thresd here, explains a lot about the Mazilli driver as well.:
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Should I worry about shoothrough?
Build it, scope it and you'll understand better than 100 forum posts worth of explanation. This is the last post you're getting from me until you post some experimental results.