Interupting xenon flash current ?

found

John, Thanks for the CT info. I tried the CT that I found and it seems to work ok. At 1:200 I used a 2ohm 1/4w metal film resistor (all I have) and it shows a

Mike When truth is absent politics will fill the gap.

found

I think your CT is probably under responding to the actual current; your shunt measurement was more believable IMO.

I tried a ferrite core CT wound approximately to John Larkin's suggestion, and it says my 600 volt 70 uF flash with 4.5 uH 0.1 ohm ballast delivers 1800 amps peak to the tube. The current amplitude and shape are very close to what the theoretical discharge of the RLC circuit should be. Trace posted on ABSE.

(My lamp is 12 mm OD, 10 mm ID, 12 mm between the tips of about 8 mm dia tungsten tipped copper electrodes, and uses a 20 kV trigger.)

Glen

I finally attempted this to see how fast I could get with parts on hand. With two parallel 1 uF 1000 VDC oil/paper capacitors mounted on the flash head with short leads I got a 250 amp peak pulse approx 7 us duration, starting about 3 us after the trigger. At half amplitude it is 3.5 us wide. I expect that lower inductance capacitors would not help with the slow 3 us turn on time but might shorten the pulse width a bit.

Incidentally 10 us is the exposure time claimed for Edgerton's famous football photo (the first widely published high speed strobe photo):

Judging from the availability of a flash with a 250 ns pulse, it must quit pretty fast, but I did not find the lifetimes of the excited states of xenon in a quick search, only a bunch of papers of questionable use for sale.

Right, although all good studio strobe lighting equipment allows for adjustable light output, preset by the photographer rather than depending on a photosensor, but using similar methods to turn the flash off before the capacitor is fully discharged.

In my case this is just a pleasant diversion from real work, and as such the requirements are rather arbitrary.

There are several options for speeding things up. I must reject the options of using a tube with lower pressure, a different gas or different geometry due to parts on hand limitations, leaving only the most entertaining option: use of a simmer current to keep the tube conducting while "off" (actually only almost off). Then either a switched DC supply or a pulse transformer should be able to get current through the tube very quickly, and the trigger can be dispensed with entirely if the simmer current source is of high enough voltage, like perhaps a HeNe laser power supply connected through a ballast resistor, fitting the parts on hand requirement. Alas the fast 1000 volt 2000 amp mosfets are not found in my junk box, so I will have to try it driven by a pulse transformer.

The simmer current technique is used in the huge arc tubes which switch the main beam amplifier mirrors at the LLNL National Ignition Facility, rather quickly IIRC though I can't find the specs right now.

But by the time I get around to trying it this thread will be long forgotten. Time to get back to real work again for a while :-).

Glen

I'll be watching for further developmens.

I have changed to pretty small tube I scrounged from an old Polaroid camera because the large tube was just too slow turning on. Even after removing all current measurment components, shortening the leads, and increasing the charge voltage the light pulse, as measured with a fairly slow photo transistor, took a little over 10us to peak. The small tube using 15uf charged to 575V measures 4.5us to peak light output and the total light pulse length is 60us. I'll have more accurate measurments when the PIN photo diodes I ordered arrive. In the mean time I think I'll see if I can blow up an IGBT by trying to open the dischagre path with it. The one I have is rated at 300A for 1ms and I'm hoping it can withstand the current since it only has to conduct for 10us worse case. The current shouldn't be nearly as high as yours with the smaller caps and higher impedance tube. I'll lower the charge voltage to the minimum for the tube to flash and increase it if the IGBT survives.

Mike

Television is a medium because it is seldom well done.

(responding to someone else who wrote this)

I have a Photogenic Powerlight 1500SL, and it appears to me to work differently.

The main energy storage capacitor is charged to a variable voltage. It appears to me that this big capacitor is in parallel (except for see below) with a smaller capacitor that is charged to the full 500 volts or so.

A diode separates the big capacitor from being truly in parellel with the small capacitor (or this is the way things here appear to me to be). The small capacitor is in parallel with the flashtube. The small capacitor gets the arc in the flashtube going once the trigger pulse (kilovolts applied to the trigger lead) ionizes the flashtube. Once the small capacitor's voltage drops below that of the large capacitor, the large capacitor starts conducting through that diode. Of course, this has to be a hefty diode!

This scheme is used because, with a lower flash intensity setting, the large capacitor is only charged to a voltage that can sustain the arc once it gets good and rolling, but not enough voltage to get enough current through the thin streamers initiated by the trigger pulse to get them to "blow up" into an arc.

It is easier to terminate the charging of a big capacitor at some selectable voltage than to interrupt the current being discharged from such a capacitor through a big flashtube.

One side effect of this flash output adjusting scheme: The color temperature of the flash varies slightly with flash output. At minimum, my unit is closer to 5,000 Kelvin, and at maximum it is closer to 6,000 Kelvin.

One other thing: If you turn down the flash setting and use the flash before the big capacitor bleeds down to the new corresponding voltage setting, the next flash will be brighter than you have the unit set for.

- Don Klipstein ( snipped-for-privacy@misty.com)

I'm not familiar with that brand but it is obviously similar to many others. In the units I'm familiar with this is generally termed the firing assist voltage and works as you describe. The firing assist capacitor is typically 100nF, c.f. the main storage capacitors being 1000's of uF, whatever is needed to store the required energy. The trigger circuit is also usually powered from this supply.

That's correct.

I did once have to deal with a 400J head that had automatic exposure control by terminating the discharge. There was an SCR in the main current path that was triggered on at the same time as the tube. There was a second SCR + 5.5uF capacitor series combination in parallel with the main SCR. To terminate the flas, the second SCR was triggered, the current into the 5.5uF cap allowed the main SCR to cease conducting. That unit doesn't seem to be in their catalogue anymore.

There are some units that claim constant colour temp & the ability to vary discharge duration. eg Grafit A2 at

I don't have any experience with them.

The solution to this problem is to either automatically initiate a trigger so the unit then charges up again to the new reduced setting (sometimes called flashmatic), or to automatically discharge the storage caps via a transistor & power resistor. This latter is slower but seems to be the prefered method nowadays.

-- Regards Malcolm Remove sharp objects to get a valid e-mail address

OK, I'll post any results I get.

because

measurment

pulse,

peak. The

the total

photo diodes

trying to

I'm hoping

higher

flash and

For short accurately timed flashes it looks like pseudosimmer could be the best mode of operation for small tubes. If you haven't already, get dts_flashcatalog.pdf from

OK, I stand corrected on that. How about "at least one major brand of studio flash lighting equipment uses interruption of lamp current to control output" instead.

I recall perhaps 3 decades ago reading advertisements for studio flash equipment that bragged about no color change due to no voltage adjustment, with duration limiting instead. But I had completely forgotten about the voltage adjustment method until you reminded me.

Glen

Thanks, I had missed that document.

Btw, My igbt has survived a few flashes with a 550V charge, but with the gate constantly turned on, I.E. tied to +12 thru a low value resistor. Now if it'll survive the turn off. The pseudo simmer is interesting, but it may be a problem for some high speed photos since it requires at least 100ms advance notice of when the event will occur.

Oh yeah, my igbt is only rated for 150A peak. I've been looking at too many data sheets I guess.

Mike If there is no absolute truth then nothing can be known.

Hi, I've been working on a high energy flash system for some time now (about 4000 Joules) and just come across this forum. Like some others here I've been having difficulty with interrupting the current to terminate the discharge current. I had originally intended to run the tubes at 1400 volts but I have had to scale that down to 700 volts. I am currently using 24 IGBTs in parallel each rated at 60 Amps 900 Volts (will increase to 50 eventually to handle the full energy). The flash tube draws some 900 amps once the curent stabilises after the initial surge.

Unfortunately the IGBTS are not surviving (I have blown 3 already) and I have had to limit the voltage further to 500 volts, reducing the steady current to about 600 amps. I'm still trying to find out why I'm getting the failures at 700 volts. The current sharing scheme is admittedly quite primitive (or perhaps I should say non existent) but it does not seem to be the problem and neither does the initial 5000 amp current surge. What does seem to be the problem is some very ugly oscillation when the IGBT is turned off which probably means I need to work more on the gate drive circuitry.

I've been reading further back in this forum and noted some discussion on the initial surge of current when a flashtube is first triggered. I have also found that phenomenon and with a very low resistance path between the capacitor bank and the flash tube I was getting an estimated 30 KA surge at 1400 volts, the duration of the surge being about 30 to 40 microseconds. I had initially suspected an inductance problem, as was also suggested here, but to eliminate that possibility I tried using different shunts, ranging from a flat piece of lead to a short length of 10mm2 copper cable and got similar results in each case.

Hi, I've been working on a high energy flash system for some time now (about 4000 Joules) and just come across this forum. Like some others here I've been having difficulty with interrupting the current to terminate the discharge current. I had originally intended to run the tubes at 1400 volts but I have had to scale that down to 700 volts. I am currently using 24 IGBTs in parallel each rated at 60 Amps 900 Volts (will increase to 50 eventually to handle the full energy). The flash tube draws some 900 amps once the curent stabilises after the initial surge. Unfortunately the IGBTS are not surviving (I have blown 3 already) and I have had to limit the voltage further to 500 volts, reducing the steady current to about 600 amps. I'm still trying to find out why I'm getting the failures at 700 volts. The current sharing scheme is admittedly quite primitive (or perhaps I should say non existent) but it does not seem to be the problem and neither does the initial 5000 amp current surge. What does seem to be the problem is some very ugly oscillation when the IGBT is turned off which probably means I need to work more on the gate drive circuitry.

I've been reading further back in this forum and noted some discussion on the initial surge of current when a flashtube is first triggered. I have also found that phenomenon and with a very low resistance path between the capacitor bank and the flash tube I was getting an estimated 30 KA surge at 1400 volts, the duration of the surge being about 30 to 40 microseconds. I had initially suspected an inductance problem, as was also suggested here, but to eliminate that possibility I tried using different shunts, ranging from a flat piece of lead to a short length of 10mm2 copper cable and got similar results in each case.

(PS: sorry if this post appears in duplicate. I had to post it several times for it to appear - I'll check later and delete the surplus ones if they actually appear)

Have you tried snubbers and softening the turn-off to reduce di/dt?

Best regards, Spehro Pefhany

Have tried but I probably do need to put some more effort into that. All I tried was a 75nF capacitor in series with a 1 ohm resistor and it is connected across the emitter-collector of the IGBT group. It was there already though when I burnt out the IGBTs. It had made a considerable improvement but not nearly enough it seems. Without it I had a huge over/undershoot at cutoff - so much that it went offscale on my scope (and even crashed the laptop sometimes ! ) even though I was using a 100:1 probe.

I probably need to do some more research into designing a better snubber as the one I made was based more on what I had in the junk box than any calculation. With the snubber I now have only some 30% voltage overhsoot but the cutoff is plagued with ringing - some 50 to

My worry with lower resistance/higher capacitance in the snubber is that I will cause problems on turn on when the IGBT will have to discharge the snubber capacitor through just one ohm - not very kind to the IGBT given the stresses it already faces with the flash tube. I have been thinking about some snubbers incorporating diodes (for different turn on/off behaviour) but it looks like the cost/size of the diodes may be a problem.

What I was thinking could be the problem is that rate of change of voltage rise on the collector causing feedback into the gate. If that is the case the snubber will certainly help, however I have been trying to approach the problem more from the gate side than the Collector side.

By applying capacitance across the gate and emitter and playing around with different gate resistors I managed to tame the ringing somewhat such that I now have less than 20 ringing cyles with an amplitude of about 200 V. However I am just trying random things at this point but I cannot honestly say that I know what I'm doing.

I have an excel file with the trace of one of the latest tests and I would post it here but I can't quite figure out how to do that.

About 40 years ago, SCRs, ignitrons (and some transistors) had I*I*T ratings that were necessary to use for designs in impulse circuits. Demand that data from the vendors. Also try an SCR in place of a FET, but watch tat eye-squared-tee rating!

ting!- Hide quoted text -

I*I*T seems not to be a problem. I have succesfully fire the tube many times at 700 volts through the IGBTs, keeping thjem turned on long enough for the capacitor to drain almost fully and it seems the IGBTS can take that in their stride. The problems happen when I try to turn off the IGBTs at an early stage while there is still a high voltage and current. In fact all three failures have occured during test firings with a duration between 50uS and 200uS. After such a discharge the capacitor voltage would not have reduced appreciably. Whereas with the longer duration pulses (which seem not to cause problems) the voltage would have gone down to some 400 volts or lower.

I have actually even tried a stupid experiment using just one single IGBT to handle the full current without switching it off and surprisingly it survived. The transistor is rated at 60A continuous or

That makes me think that it is what goes on during the actual switching off that is killing the transistors, perhaps the VxI during parts of the ringing when both the voltage across the transistor and the current through it are high. ANother possibility I am thinking about is overvoltage on the gate due to the high dv/dt at the collector pulling up the gate through the gate-collector capacitance.

I had originally planned to use thyristors but it tuirned out that they would be even more difficult to get to work properly. The two major problems where how to switch off the thyristor and how to limit the di/dt enough to avoid destruction of the thyristor on turn on. The switching off problem is the most difficult though as one needs to shunt current away from it (by another equally rated thyristor) and then ensure that it actually stays off when it experiences the fast voltage rise after cutoff. There used to exist so called 'gate turnoff thyristors' which were a bit easier to use and were used in many consumer camera flashes but it seems they no longer exist and in any case seem toi have never been made with sufficiently high current and voltage ratings for what I need.

Next thing I am thinking of trying is to get a 'softer' turnoff as Spehro suggested, but the downside of that is potentially destructive V*I*T due to the period of partial conduction if I overdo it. I'll also need to learn a lot more about how to design a better snubber.

A current snubber or other RCD clamping configurations allow the capacitor to absorb current during the voltage rise-time only.

If you know the amount of energy that you wish to deliver prior to switching, this could be adjusted by separating the ignition source from a ( lower voltage ) programmed energy source, allowing the tube to self-extinguish on the discharging sources. It would also allow use of SCRs.

You can't count on parallel IGBTs sharing without external assistance. Perhaps you might consider a cascode HV bipolr/ LV mosfet.

I'd also invest in a current transformer to assist in monitoring.

RL

Isn't the point of SCRs to shunt the flash tube? Obviously, through some sort of energy damping medium, like an inductor between the cap and tube. You only have to deal with the flyback energy later.

(Hmm, if you just let it swing, the inductor will run the capacitor's voltage negative by however much, no complicated snubber or switching or damping, the SCR only conducts peak current. That might be useful. Flip polarity with a beefy relay and recharge the difference?)

Tim

-- Deep Fryer: A very philosophical monk. Website @

Hide quoted text -

I had originally planned to use thyristors but it tuirned out that they would be even more difficult to get to work properly. The two major problems where how to switch off the thyristor and how to limit the di/dt enough to avoid destruction of the thyristor on turn on. The switching off problem is the most difficult though as one needs to shunt current away from it (by another equally rated thyristor) and then ensure that it actually stays off when it experiences the fast voltage rise after cutoff. There used to exist so called 'gate turnoff thyristors' which were a bit easier to use and were used in many consumer camera flashes but it seems they no longer exist and in any case seem toi have never been made with sufficiently high current and voltage ratings for what I need.

Actually, you can. IR wrote an AN somewhere that describes the process. At low current, it's true, but presumably, at these current levels, the paralleled devices are each rated for enough that there is no problem. At higher current levels, the resistive component of the IGBTs comes into play, and current shares to an increasing degree. What you get is a roughly constant difference in current between transistors, and as long as this difference is less than the rated current of each device, you're fine.

Tim

-- Deep Fryer: A very philosophical monk. Website @

The RCD snubber seems like a good solution but the problem is the cost of the diodes needed. I have three high current diodes on the way in the post and I may be able to spare one of them for that. However even with that I am having some difficulty getting the values right so that the snubber can be used multiple times, for instance to have three 100uS pulses spaced at 100uS. The snubber capacitor finds itself still charged on the second current disconnection and will not have much effect. However that is probably something that can be solved with the right values.

The capacitor self discharging is not an option because thje pulse widths and spacing need to settable to arbitrary values controlled by a pic from some 50 uS all the way up to 5mS, so it is not possible to tailor component values to fit the timings required. Also, I need the pulses to have a very sharp cutoff, something that cannot be achieved if one depends on the capacitor voltage decaying to below the holding voltage to cut off.

As for current sharing I am currently not doing anything except paralleling the emitters and collectors of all 24 transistors on two thick copper busbars. The gates are separated into four groups of six to reduce to some extent the interaction between the transistors on turn off. In future I intend to have a 0.25 ohm resistor on the collector of each transistor to help distribute the current. It will cause some 15 volts drop at maximum load, but that is a small price to pay if it does its job. The plans are also for each gate to have its own optically isolated driver to completely eliminate any interaction between the gates of the various transistors.