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calculating bang energy in drsstc tank circuit

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bob golding

Tue Mar 18 2008, 12:11AM

Registered Member #1084 Joined: Mon Oct 29 2007, 08:58PM
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Posts: 28

from reading steve wards drsstc pages concerning filter cap sizing he says go for 50 times the bang energy as a rough guide. how is this calculated? i presume the standard john freau model doesnt work for a drsstc. i am trying to design my coil around the hard to get bits, which seem to be the filter caps. i have located some 5800uF 400volt ones which look about the right sort of size so i am trying to work out how big a bridge and secondary i can get away with using 4 of these. initially i am limiting myself to 3 kw input power. not sure how much energy might be flowing back from the primary into the caps so not going to go over 300 volts to start with. sorry if this has been answered a 100 times before. if it has i cant find it,and i have been looking:-))

regards
bob

HV Enthusiast

Tue Mar 18 2008, 01:54AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

The bang energy is essential the amount of energy transferred into the primary from the MMC during each pulse. Basically you would just calculate the discharge from the MMC (over the pulsewidth you'd expect) given the primary tank impedance.

You just want to size your DC bus capacitors so they can supply enough energize and not droop excessively once the system reaches steady state over many pulses.

Kolas

Tue Mar 18 2008, 03:40AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

energy in joules... should be easy to find on google

J = (V^2*C)/2

where C is capacitance in farrads...

this kinda has me confused though. do DRSSTC's still have coupling less then .5?

if so, then the secondary will never see even 50% of the mmc's energy. 50% being the theoretical max any system can put into a resonant rize system

Steve Conner

Tue Mar 18 2008, 11:50AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think as long as it's at least 10 times the bang energy, you're OK. Any more than this just makes the shrapnel fly further when it goes wrong.

The John Freau equation does indeed work, though it's mostly useful in a backward sense. You decide what spark length you want first, and then calculate what bang energy you'll need using Freau's equation, then design everything else around that.

What I do is to design the inverter with enough peak power to deliver the desired bang energy in 150-300us or less, and then choose the tank capacitor value so that the primary will have a loaded Q of 10 at that power level and some frequency, which I guess because I don't know yet.

I then design the secondary with a size and shape similar to a classic coil of the same bang energy (though stubbier) and a characteristic impedance (Z0) of around 20 to 50k, which gives it a loaded Q around 10 too. Richie and I have tried Z-noughts between 20 and 100k with our various coils and they all seemed to work well, so it's probably not that critical.

Finally I choose the primary inductance so it resonates at the same frequency as the secondary I just designed, with room for adjustment, which I do by trial and error.

PS: It's impossible for more energy to flow back into the DC bus capacitors than was drawn out of them in the first place. (Where would it come from?!)

Steve Ward

Sat Mar 22 2008, 07:57AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

PS: It's impossible for more energy to flow back into the DC bus capacitors than was drawn out of them in the first place. (Where would it come from?!)

How about the twin DRSSTC blasting out kWs of RF only 20 feet away? You can see the bus voltage rising, its kind of scary, even if the power transfer is just a few percent.

On a related note to this topic, i quite often simulate my DRSSTC designs in spice and simply integrate the "power" from the main filter cap to determine the bang energy. You can set the initial condition on your bus cap to be whatever voltage you want, and simply dont connect it to a power supply so that you can measure the difference in voltage after one burst of RF and work out the energy. Or you can use the math functions in spice to integrate the power in the primary or tank cap over a burst. This is how i got an estimate of bang energy vs number of operating cycles. It seemed to correlate fairly well with my wall power measurements at the time.

Perhaps my statement of 50X the bang energy is a bit extreme, though i find its often necessary to have monster capacitors simply to handle the RMS current. But that also comes down to how long you intend to run the coil for, you can push the bus caps harder for a short time because they wont get hot fast enough to degrade them. In any case, my monster sized lytics that i use only get slightly warm after an hour of run time, and that heating might mostly be due to eddy currents from the primary coil :P.

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