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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, I had this idea of using an inductor in series with the bridge to decrease the maximum current during the first peak of the burst. Why would I do this? The peak topload voltage would be decreased, so you could theoretically get longer sparks from a smaller coil without arcing it over. But the lower peak voltage on the other hand could decrease the spark length again... See the schematic and before/after(red line) illustration of how could a burst look like (borrowed from MadsKaizer). What do you think?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I imagine that the interrupter MUST be disabled before the main bridge or there will be an enormous voltage spike across the main bridge. Just something to bear in mind when experimenting.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well, as I think about it, the gate drive of the IGBT bridge should be enabled synchronously with the auxiliary transistor, but the bridge should be left to oscillate for a while after the auxiliary transistor was switched off to discharge the bridge decoupling capacitor, so it starts from zero volts the next pulse. However there would be no voltage spikes, the lower diode in the schematic prevents this.
My current belief is, that you can avoid the primary current peak by tuning the primary fres not too much below secondary fres. That seemed to work with my coil, although it reduced arc length somewhat. The peak also depends a bit on the size of the breakout point. A larger one will cause earlier breakout, which pulls the secondary earlier into tune, reducing primary current.
You probably have experimented with tuning without being able to get rid of the peak. Any idea why it didn't work?
EDIT: Possibly the scheme of reducing primary current rampup speed might not work as expected. The current peak is partially caused by the primary being tuned low relative to the secondary. Initially this leads to low secondary voltage, delaying breakout until primary current is high enough so that breakout finally occurs. This then pulls the secondary into tune reducing primary current. So the current peak might not be that dependent on rampup speed.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
The concept you are describing is basically the same idea behind a QCW, in the original QCWs we used a buck that essentially acted like a current source (like an inductor). Controlling the rate of rise in the tank coupled with the slightly higher than normal tank Z made for a more controllable ring-up with lower top voltage and more efficient spark growth.
I also agree with Udo, how you tune also plays a part in this as well, tuning and coupling lower or high, can cause energy to "linger" in the primary before it fully transfers to the secondary side.
An inductor i could sorta see doing the same thing, but only for a very short time unless it was ungodly huge. The issue is you need more time to grow the same length spark with the lower peak voltages, that kind of duration would be hard to get out of an inductor.
I personally think the better solution is to use the bridge it's self to limit the ring-up rate of primary current. This can be done with some special modulation techniques, rather than a bunch of additional expensive and heavy hardware like inductors or bucks.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks for the replies guys. Eric - I always try to maintain ZCS if possible, the losses in the additional transistor would probably be much smaller compared to hard switching the bridge. But you are right, I can as well use the auxiliary transistor in a PWM mode and basically make the coil into a QCW.
As for the tuning - I'm not sure this is always possible, especially with low coupling. You can use a high-Z tank circuit, but again it stores more energy, which would othewise be stored in my inductor to do the same thing. Now it depends which of these approaches would be more practical. In the tank circuit you would need more expensive RF caps, but with my inductor you need an additional transistor.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Dr. Dark Current wrote ...
Thanks for the replies guys. Eric - I always try to maintain ZCS if possible, the losses in the additional transistor would probably be much smaller compared to hard switching the bridge. But you are right, I can as well use the auxiliary transistor in a PWM mode and basically make the coil into a QCW.
As for the tuning - I'm not sure this is always possible, especially with low coupling. You can use a high-Z tank circuit, but again it stores more energy, which would othewise be stored in my inductor to do the same thing. Now it depends which of these approaches would be more practical. In the tank circuit you would need more expensive RF caps, but with my inductor you need an additional transistor.
Soft-switching is possible with bridge power modulation. Typically ZVS is used (which is more efficient anyways). Other modes like pulse skip modulation can still run ZCS and modulate power by simply not adding energy every cycle, but rather on odd harmonics.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Eric, could you point me to info on
"Soft-switching is possible with bridge power modulation. Typically ZVS is used (which is more efficient anyways). Other modes like pulse skip modulation can still run ZCS and modulate power by simply not adding energy every cycle, but rather on odd harmonics"
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Sulaiman wrote ...
Eric, could you point me to info on
"Soft-switching is possible with bridge power modulation. Typically ZVS is used (which is more efficient anyways). Other modes like pulse skip modulation can still run ZCS and modulate power by simply not adding energy every cycle, but rather on odd harmonics"
Take a look on Google, there are a whole slew of papers on ZVS. Particularly when you search class DE. (think of class E applied to a half bridge or full bridge).
Pulse skipping I’m not sure I have any links on. It's a concept that just made sense to me and I tried it and it works reasonably well. The basic idea is that you drive the bridge normally on the first cycle and then you "skip" 2,4,6 or whatever amount of cycles you want and then drive again on the following cycle. ie you drive the bridge every 3,5,7, or 9 cycles depending on how much power you want to transfer.
During the times that you are skipping, you open up both high side OR both low side switches and allow the current to freewheel threw the bridge diodes and switches. This way it's not recycled back into the bus cap when you are not driving.
Pulse-skip modulation could be done with either ZVS or ZCS. While phase shift modulation can only be done with ZVS (if soft switching is desired)
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Using an inductor to limit the rate of current rise in a DRSSTC
