DRSSTC design procedure - draft

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Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> 

Hi all

After much messing around with simulations and theory, I think I've finally
come up with a usable design procedure for DRSSTCs. It has been tested by
simulation but not by building a real coil. So I'd be grateful if any other
DRSSTC builders could give me their opinion. If you compare your measured
primary current, spark length, etc. with the predictions that my method
makes, we can see if it's accurate.

This method is based on the transient behaviour of the coil and uses nothing
more complex than conservation of energy. The simulated coils that I
designed with it seemed to transit quite happily into a steady state after
breakout, with the primary current stabilising around the design value. They
seemed quite insensitive to streamer load impedance, although very low
impedances caused dangerously high primary current.

This article doesn't cover driver circuits and power supplies as there is a
lot of information on them already.

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1: Decide what length of spark you want to produce.

2: Use Freau's equation (spark length=1.7*sqrt(power)) to calculate the
power you will need to do this. Then divide it by two because DRSSTCs are
twice as efficient :)

3: From the power, and knowing the breakrate which is usually 100 or 120,
calculate the bang energy.

4: Now the fun starts. In a DRSSTC the primary and secondary resonators ring
up together. When breakout occurs they both empty into the streamer load
together. The split of energy between the primary and secondary is
approximately (1-k):k. So use the tightest coupling possible, we will assume
that you managed to get k=0.33. Knowing this we can size our primary and
secondary for energy storage.

5: We now know enough to design the resonator. At breakout time, 0.33 of the
total energy is in the resonator. Use this to calculate the topload voltage,
and choose the toroid so that its breakout voltage is about the same as the
voltage you worked out. You may have to iterate here, as changing the toroid
changes the capacitance. Also check that the voltage doesn't add up to more
than about 1.1kV per mm of resonator (to avoid hassles with flashover)

6: Now it's time to do the primary. 0.66 of the energy is in the primary, so
use E=0.5*L*I^2 (where I is the maximum current you dare to put through your
IGBTs) to give the required primary inductance. Now choose the primary
capacitance to resonate this at the frequency of the secondary, and add 10%
more capacitance for luck. Finally, choose the voltage rating of the
capacitor by using E=0.5*C*V^2 to find V.

7: Light blue touchpaper and retire.


Steve C.