Actually I am stuck in the bring-up stage... It is pulsed current source using buck topology.
The load is a spark gap (modeled by ~40V MOV). Pulse duration
100-300us, current ~20A with ~10A ripple (exact values - TBD). Input voltage source is ~50uF capacitor charged to ... whatever it needs to be charged. The feedback signal is taken from two current sense resistors (one - in series with input energy storing capacitor and another - with diode)... The problem I am facing is unstable operation and there is no easy way to check feedback loop.
Any ideas/suggestions? Thanks Michael snipped-for-privacy@rcn.com, remove 999
Actually I am stuck in the bring-up stage... It is pulsed current source using buck topology.
The load is a spark gap (modeled by ~40V MOV). Pulse duration
100-300us, current ~20A with ~10A ripple (exact values - TBD). Input voltage source is ~50uF capacitor charged to ... whatever it needs to be charged. The feedback signal is taken from two current sense resistors (one - in series with input energy storing capacitor and another - with diode)... The problem I am facing is unstable operation and there is no easy way to check feedback loop.
Any ideas/suggestions? Thanks Michael snipped-for-privacy@rcn.com, remove 999
I get a glimmer of what you're trying to say, but there are some contradictions, too.
First: A 40V spark gap? I'd accuse you of missing a few zeros but you say you need to buck the voltage down to make it work. I assume this is a very _small_ spark gap? Can you tell us how it's built to operate at just 40V?
So whatever it is, you want to give it constant current for 300us, around 20A, and you want to accomplish this with a 50uF capacitor in parallel with the spark gap?
You are currently monitoring the current going into the capacitor, and also (presumably) the peak current in the gap -- yes?
If my assumptions so far are correct its no wonder that you're seeing unstable operation. Spark gaps, at least in a gaseous environment, have a negative resistance characteristic once they start. Driving one from a capacitor would result in a current that's controlled more by the ESR of the cap than anything else.
None the less, could you describe what is unstable, and in what way? Could you explain why you can't check your feedback loop, and for that matter expand on what the feedback is and what it's controlling?
I would be tempted to recommend that you use a different regulator topology with an inductive output. Particularly since you can stand a
10A ripple you should be able to use a topology that puts an inductor in series with the spark gap and controlls the average current to the inductor. This should tend to null out the effects of the negative resistance of the spark gap to good effect.
You may be able to achieve this simply by tossing your cap and driving the gap straight from the inductor.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
I don't think that a 40V metal-oxide varistor is much of a model for a spark-gap.
For a start there is a minimum in the Paschen curve of break-down voltage versus gap length at around a few hundred volts, whcih depends on the nature of the gas and the pressure.
Once your single electron has strayed into the spark-gap and initiated the avalanche of ionisation that constitutes your spark, the discharge is sustained by positive ion bombardment of the negative electrode, for a while at least. With your 20A current, this would be followed by a rapid (sub-microsecond) transition to the arc mode, where the discharge is sustained by field emission from the spikes on the almost molten surface of the negative electrode. Sustaining a glow discharge needs something of the order of 100V, while an arc can get by with a lot less.
If you do want a constant current discharge you'd be better off with energy stored in an inductor rather than a capacitor, as Tim Westcott says. Because of the large voltage excursions as your discharge goes from spark to glow to arc, you are going to have to worry about the interwinding capacitance of your inductor which is going to be self-resonant with an appreciable Q (if you don't do anything about it).
The whole circuit will tend to ring at this resonant frequency as soon as the gap has broken down.
Getting from there to a constant current could take some ingenuity.
In fact, in the very early days of radio there were no vacuum tubes and the high-tech transmitters used the negative resistance of the arc to create a sustained oscillation. AFAIK this was _not_ just a spark-gap transmitter exciting a resonant filter; it was a true oscillation that depended on the arc as an active element.
Hmm. Gotta research that. Sometime when its not past bed time...
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Arcs do tend to have a slightly negative incremental resistance - and that could have been enough to offset the resistive losses in the coil. There might have been some mildly odd effects as the arc current went through zero, but your conducting channel is running at about 6000K and won't cool off much while the current is changing polarity.
I will answer the questions I was asked. The spark gap is ~0.75-1mm, filled with Argon. I was told (by our chemical analyst, actually she referred me to an article) then when spark is "on" (arc is present) the voltage across it is ~48V, somewhat dependent on the current. The current polarity must not be changed - no ringing allowed. I cannot measure the spark characteristics as ~10kV spike is applied to the gap to initiate the plazma (...this looks like really iteresting ee challenge though).
I used 2.5Ohm resistor as the load until ordered TVS arrived yesterday. In general, the system works (up to 250us wide output pulse), it is not very stable (not shippable :o) )
I modeled this thing (in Excel): switching frequency, and energy storing capacitor voltage drop vs. time for different input/plasma voltages, inductance, capacitance, min/max current values. I will be more than happy to email the file if you want to play with it. (mkogan1999 at rcn.com, remove 999) :o)
Until now we used ~100uF capacitor charged to 90-250V duscharged directly into the gap through ~0.1-0.3 Ohm (resistor and whatever ESR the capacitor has). The problem: peak current exceeding 200A . People use inductors to spread the pulse, but these things are BIG (~100+uH for tens of Amps), we cannot use it. I cannot say how soon the capacitor and HV diode (to isolate the trigger pulse) will fail as 200A+ current is way outside any spec.
I was told that the amount of charge per pulse is what needs to be delivered into spark gap, so controlling current by placing reasonably sized inductor (~12uH@20A) in series with the gap seems to be the right way to do it. I think I found the suspect: one of (two) comparators in the feedback circuit seems to be ringing (2mV built-in hysteresis does not seem to be sufficient). This will be easy to do (cut some traces and glue couple 0805 resistors) Thanks again Michael Kogan
If precision rate of charge delivery and no ringing is required then you need a current source. The 20A by 250us is 5,000 uC of charge dump and allowing for 10V overhead potential means that Vinit-5,000u/C=60V so that for 100u capacitor this would make Vinit=110V or so, which is not so bad. An arrangement like this should be only slightly more than a Radio Shack project- nothing extraordinary about it: View in a fixed-width font such as Courier.
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