Is xenon flash tube being multiple-triggered and if so, why?

A contract Engineering company have designed us some 15W xenon flash tube lamps.

I have noticed that there are several 250V capacitors providing the 210uF of capacitance used to flow current through the xenon tube when its struck….however, one 22uF , 250V capacitor is connected to the others via a diode , such that it cannot actually discharge via the xenon tube…..thus when the xenon tube strikes, this particular 22uF capacitor stays charged up.
….this particular 22uF capacitor is used to provide the voltage to the primary of the trigger transformer.

But why, I ask, have they bothered to do this?, …because our 5W xenon flash lamp does not bother to have a separate capcitor like this for provision of tube trigger transformer primary voltage.
…..so I believe that they must have been having trouble triggering the tube, and so are triggering it multiple times, and due to the multiple triggering, they need this extra capacitor to provide adequate trigger voltage to the primary of the trigger transformer as the tube flows current and discharges the 250V capacitors.

Anyway, I am asking myself why on earth they would need to trigger the tube multiple times?
…….well, i believe that the answer lies in the stray inductance in the connections linking up to the xenon tube……….that is, when the xenon tube suddenly starts flowing current, this suddenly rising current flows in the stray inductance of the wires, and temporarily (for just 1.5us) reduces the tube voltage to near 0V.
……on falling to 0V, the tube goes out, and then needs retriggering to make it strike properly.

The following schematic and waveform indicate the problem………in the waveform you can see how the tube voltage suddenly falls to near zero as the tube starts conducting…..


Schematic (xenon tube represented with current source):
https://i45.tinypic.com/ops8kg.jpg


Waveform showing tube voltage.
https://i48.tinypic.com/2vsnf4h.jpg

We are triggering the tube with Capacitive external triggering as on page 4 of the following…..



Anyway, do you believe my diagnosis is correct…..ie that it’s the stray inductance that’s making them need to multiple trigger.?

Also, do you know a better solution to this problem than multiple triggering?

That all sounds a bit far-fetched. It would certainly be more useful to look at the circuit and the waveforms of the actual lamp, rather than looking at a simulation of an imaginary problem.

Why guess whether it's being triggered multiple times when you can just examine it and find out?

i can't examine it and find out because the tube has broken........the contractors can't get us another prototype for at least 3 weeks, and maybe longer.

It would certainly be more useful to look at the circuit.

Click to expand...

Exactly. Unfortunately you didn't show the circuit you are talking about.

Instead of multiple triggering, applying a higher anode voltage during triggering, e.g. by dymically increasing the voltage as shown in the Excelitas datasheet, can be reasonable.

grizedale said:

the tube has broken

Click to expand...

How does this prevent you from looking at the circuit to see how it works?

because without having the tube there to take down the cap voltage, i cant see why they needed this extra holdup cap...i cant see why they just didnt use the same circuit as our lower wattage lamps

All we are asking for is a clear schematic of the circuit you are talking about...

i cant give exact circuit as id be sacked.........

but the trigger circuitry is like in the following thread..............

https://www.edaboard.com/threads/260467/

and the topology is simply a buckboost charger circuit to charge up the tube capacitor.

..from that , you can build up an exact picture of it.

the trigger pulse os initiated from a ground refernced microcontroller, whereas the tube is referenced to vin.