The beginning.

A project log for Avalanche

A high-voltage power supply and high-bandwidth pulser

alexwhittemorealexwhittemore 08/02/2014 at 02:580 Comments

Obviously this project is a bit more underway than "just starting," but for the sake of starting a proper project log, I'll update with where it's at now.

The pulser bit is done. I mean, it's like 4 components, it's trivial. Although really, it's not done. It needs a PCB, proper RF connector, and probably some kind of source impedance. Realistically that's actually a number of details to work out.

Even so, the much-harder part is the power supply, and that's where I really plan to put the value of this project. At present, the supply/pulser system is MOSTLY self-contained, meaning it only needs two power supplies. A 5V supply runs a 555 to generate a mostly-50% duty cycle mostly-square wave, and that is then used to drive a SN754410NE H bridge. This is where the second supply comes in - the H bridge is switched by the 5V from the 555, but it's PUSHING a much higher second supply of anywhere from 0 to 35-odd volts. In practice, I'm currently finding best functionality around 12 or so volts, but you'll see in the next part why it even matters. And actually, I lied, the H bridge isn't driven directly by the 555 but rather by a pair of signals coming from an inverter sitting in the middle - one of the signals is inverted from the other, basically driving two outputs of the quad half-H bridge in a push-pull configuration.

So this push-pull output of the H bridge, equal to the second supply voltage (so peak to peak, double that) is then fed into a Cockroft-Walton multiplier of (as it happens to be now) 7 stages. The exact output of this multiplier, set by the second supply voltage (since it's a multiple of that), is used to drive the pulser. By adjusting it up, one can tune the frequency that the pulser recovers and thus re-pulses, though the exact timing is random. In general, if the output is just past the avalanche breakdown threshold of the device used (here, a 3904), you'll get one, infrequent pulse. By increasing it, the output capacitor charges more quickly and thus the device avalanches more frequently, though again, the timing is quite non-deterministic. In the final design, I very much intend to maintain adjustability of the power supply so one can fine-tune the output pulse frequency, since I'd like to do some experiments with that as a generator of random timing.

Thing is, this power supply topology in general isn't particularly great. I don't really like using the multiplier, if only because it's terribly low-current. I've also had some issues with poor isolation breaking the H bridge device - for example, if you try to spark the multiplier output to its ground reference, you can do that, but the H bridge tends not to survive. I suspect it's probably because of some kind of high voltage ground bounce. I'd like the supply to be more resilient to this kind of abuse, so I'm working on a different topology. That'll be the next update, I think.