Interupting xenon flash current ?

Actually, you may want to consider commutating the snubbers them selves. Say 10 us after turn off.

You actually want a few (probably no more than 4) milliohms between the emitter terminals and the bus bar.

Wild think. The voltages and currents make FET totem pole stacks difficult. IGBTs have speed and balance related issues. SCRs at that power level cannot be reasonably commutated fast enough. Have you considered radar modulator tubes? Or even more common high power UHF transmitting tubes?

Not doing anything yet for current sharing, just connecting all 24 in parallel. The gates are in four groups of 6 so at least I don't have all 24 driven in parallel off one driver (to reduce the interaction). Reason I am not using current sharing yet is that I am running them at significanlty lower current than the current I am designing for, so it is a low priority thing at this time so to speak. The plans are to have a 0.25 ohm resistance on the collector side (not the emitter), that is between each collector and the load busbar. That will be meant to compensate somewhat for the differneces in VI characteristics of the individual units.

An important differnece between what I am doing and more common applications using paralleled IGBTs is that one of the main difficulties is absent in this case - that of thermal equalisation. Whereas in most applications (motor controls, inverters and such) the IGBTS would be dissipating a large amount of power and heating up considerably, in my case the average power dissipation is very low and the IGBTs' case temperature will not increase noticeably, so I don't really need to worry about differences in the temperature coefficients between the devices. (They will in practice always be at 'room temperature').

I had originally though of putting it on the emitter side, the idea being that increased collector current will decrease the gate-emitter voltage reducing drive and thus current. However from some (admittedly superficial) research I've done I find that the emitter-busbar resistance (and especially inductance) should be kept as low as possible. I haven't fully understood why but I was sort of taking their word for it.

(Eventually each individual IGBT will have its own opto couple gate driver)

Regarding 'totem poles' - I'm not very familair with the term but I thought it related to series connections for higher voltage handling rather than paralleling. My original target was for a 1400V system which would have two groups of IGBTs in series but eventually I dropped that and settled for a less ambitious 700 volts target.

As far as using tubes of any type I haven't really considered that for various reasons, but probably most importantly because I don;t have any in my junk box :)

I was however considering the use of a spark gap to handle the turn on and main duration of the pules and then using the IGBTs to quench the spark gap and then turn off cleanly. That was meant to reduce the I*I*T load on the IGBTS while keeping the fats and clean turnoff that they should be able to give me. However I have more or less come to the conclusion that the IGBTs are having no difficulty with I*I*T and that the big problem is what is going on during the turn off so I will probably concentrate on getting that out of the way before tackling the other issues.

Regarding 'commutating the snubbers', I'm not sure what you mean. Would it involve something like switching in a resistance at the appropriate time to discharge the capacitor in preparation for the next turn-off ?

My ancient patent, 3496411, works the other way around.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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Actually shortly after the turn off was the idea, but that is the line of thought.

I agree that saturated switching can give results +/-20%, provided VCEsat is matched, per the old harris and fairchild app notes., however linear operation or operation during switching (particularly at turn-off) require better control of emitter current than is provided by the resistive DS path of the insulated gate input mos structure or beta of the output power structure.

The IR app note (990?)seems to concentrate on thermal limits produced by two devices exhibiting a 40% current imbalance under saturated conditions.

The OP doesn't appear to be dealing with a long-term thermal issue, but an energy issue experienced under dynamic conditions - notably turn-off, where the highest gain and most charge retentive device suffer the bulk of turn-off losses and possibly severe dv/dt.

RL

As you pointed out, my major difficulty is actually what goes on during the turn off. I have succesfully fired the tube many times without problem, provided that I ensure the turnoff happens only after the capacitor voltage has decayed substantially.

I would like to thank everyone for the good suggestions. I'm still trying to figure out what course of action to take, but probably it will be:

1. Insert a resistance of about 0.25 ohm between each collector and the collector commoning busbar.
2. Feed each gate independently with an opto coupled gate driver
3. Add an RCD snubber across the emitter-busbar and the collector busbar

I am hoping that the collector resistance should help significantly even in dynamic curent balancing. If any of the transistors were to be significantly slower than the others at turning off I am hoping that the resistor would take the brunt of the V*I*T as it will generally have a higher voltage drop across it than the IGBT. The loss of energy in the resistor is not really a concern in this application unlike it would be in motor drives, inverters and such.

With the independent feeds I am hoping to eliminate completely the interaction between the IGBTs, which I think could be part of the problem. Each driver will take the gate to about 20 volts positive and

5 volts negative for turn off. I'm still not sure what value resistor to put in series with the gate. From what I understand, too small a resistor could lead to oscillation, while too high a resistor will allow the turnoff DV/DT to turn on the gate. I've been looking for information on how to calculate gate resistance but I haven;t quite figured it out yet.

I will also be applying an RCD snubber as suggested by one of the posters. It would replace the current RC snubber. The diode will be placed such that on turnoff the diode will conduct and most of the tube current would go through the diode and charge the capacitor - with no series resistor (the tube itself is already limiting the current). The capacitor will then discharge through the IGBTs and a series resistor at turn-on through a limiting resistor placed across the diode. I considered having a separate IGBT or something to discharge the capacitor independently but I could not find a suitable way of doing it.

All this will take me some time to do, even because I do not have all the components yet, so I may be trying out some other things in the meantime.

Thanks to all for the help and good advice.

If device to device dynamic timing is considered a potential problem, you should probably use one opt-ocoupler and n x buffers.

RL

The opto coupler I was planning to use is the HCPL3120 - it is an opto coupler and gate driver combined onto one integrated circuit. It is meant to have a very predictable and constant propagation delay, varying only by a few nS between different units. I haven't verified this yet though as I have not yet received the components.

What I like about using the optocouplers is that it eliminates problems of ground bounce due to the small but finite inductance (and to a lesser extent) resistance in the emitter busbar. The output of the drivers will be referenced directly to the emitters thus eliminating the problem on the driver-gate side. Without the opto coupler however that problem is merely be transferred to the input side of the buffers. I was hoping that the opto isolator would eliminate this problem by making the drive signal completely isolated from the gate drive circuit.

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Do you know what the failure mode is of your IGBT ?

I'm finding out the major failure mechanism in this application is indeed during turn-off and the problem invariable is in the IGBT driver. You need a push pull driver with separate resistor for on and off function. If the resistor for the off function is too small, you risk blowing up the gate during the transistion. If it's too large, your turn-off will be slow and you'll run into potential thermal issues. As for the "on" resistor, shorter is better but one has to stay within the max gate current limit. I'm finding out that the resistor in the "off" leg is about 3 x larger than the "on" function.

On another note, as I read this early on in this thread ...

As for the delay in reaching the max light output of a xenon flash lamp, this is called the "ionization" delay. It is indeed in the 10us neighborhood for most photo flash lamps, can be shortened to maybe 3us if very low ESR caps are used and the ESL fo the circuit is low.

It is extremely difficult to make 1us flash pulses with xenon flash lamps, short of economical not feasible. If one is happy with very low output, then some of the analytical xenon flash lamps could be tried.

This is exactly the reason why spark gaps are used for extreme high speed photography.

YMMV,

BUT...one could try firing a spark gap and using the UV to illuminate the flashtube...

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What I have managed to find out until now is:

- The failure occurs during the turnoff

- The faulty device ends up being a dead short between all three terminals (only one device fails each time).

- The device is not noticeably warmer immediately after the failure.

- There seems to be no appreciable V*I*T heating during the turn-on and steady on state

- The turn on is very fast and very clean. The collector voltage goes down from 700 volts to practically 0 (given the resolution limitation of the scope at that scale) in less than a microsecond. This applies both to the first pulse (which has a gradual current rise) as well as the second pulse, where the tube is already hot and the current rises to its full level in about 2 microseconds.

- The gate drive voltage appears to be sufficient as even during the intitial pulse of several KA the collector voltage never rises significantly

- The turnoff is very 'dirty', with many high amplitude oscillations with a period of about 50 to 100nS.

I had captured one of the failures on the PC scope. Unfortunately I could not save it though because the EMP or something caused the PC to freeze so I could just look at the trace on the frozen display (with hinsight I realised I could have taken a photo of the screen!). What I saw was the same voltage rise initially as in other turn offs, but the voltage never went all the way up, instead hanging around at about half the capacitor voltage for a few microseconds and then went down again as if the transistors had again turned on. It stayed this way until the cap was fully discharged.

The gate drive is admittedly somewhat primitive and I am probably asking for trouble and need something better, probably on the lines of what you suggest. What I have some difficulty with that is regarding how to determine the minimum value of the turn off resistor. The problem is that in the datasheet I don't find any specification for what is the maximum allowable gate current. Is that a value that can be determined form the other specifications ?

Alternatively, do you think it would be OK if I were to take the maximum rating of the gate driver IC as the gate current limit I should aim for ? The drivers I intended to use have a pulse rating of about 2 amps, both on charge and on discharge. Can I just divide the gate drive voltage by that current to obtain the best value for the resistor?

"Turnoff very dirty" suggests (small) inductance and capacitance resonating fast enough to oscillate a number of cycles before the FET internal charge dissipates. But the fact turnon is so clean suggests different active circuitry during those two different times. So the fixed passive (wiring, resistors, etc) and semi-passive (maybe the capacitor in nonlinear mode) parts can be neglected as culprits. What is left? The FET which has some definitely non-linear gate capacitance and charge VS voltage characteristics - which must be different due to the internal stored charge (as a function of time). You could try a scaled-down system using only one FET and try an assymetrical gate drive: turnon as you have it now, and vary the turnoff voltage and current. Be prepared to use cheap FETs as you may need to do a lot of replacement. I would start with both ends, using a scope to monitor that turnoff characteristic. One end is a minimum turnoff: zero volts and hi Z (500 ohms max, maybe 100 ohms would be more preacitcal). The other end is blast the damn thing: maximum negative gate voltage at maximum gate current .or. maximum gate power (whichever comes first). The high turnoff drive should give better results, but most likely not be enough - at least this experiment will let you know where you stand. So, assuming that to be the case... Rewind the card reader, unpunch the mag tape and press restart on the Hoover Dam. Basic dumb circuit: Capacitor gets charged somehow from power supply; + is high end, - is ground; flash tube ties to + side of capacitor and top of turn-on FET. So add a second FET from capacitor to ground .or. from capacitor to first FET. Turn off first FET in "optimal" manner while turning on second FET. Nasty.

I tried the first suggestion you made, that is using just one single IGBT with a proportionally smaller load and various different gate resistances from 1 ohm to 10k. As a load I used 15 ohm 5 watt resistor instead of the tube since I do not have a tube that would draw a low enough curent for one IGBT to handle. Surprisingly I was getting a reasonably clean turnoff lasting jsut under a micro second for gate resistors up to about 1k ohm, only the turn on and turn off time grew longer with increasing resistance. I only use 0/12 volts drive in all cases. Over 1 K things started to get a bit ugly, but very different from what i see with the multiple IGBTs and flash tube. I blew up three load resistors (quite spectacularly :) , aparently because the IGBT went into latch up and the resistor could not really take the strain for more than a 100 uS or so. However that only happened with unrealistically high gate resistance.

I had a bigger surprise when trying something else - I went back to the multiple IGBT setup and replaced the flash tube with a 0.6 ohm load resistance (which is more or less what the tube shows when lit) made up of many 1/4 watt 1 ohm resistors in a series parallel configuration. Oddly enough the turnoff was reasonably clean, more similar to what I had with the single IGBT experiment than what I normally get with the tube, while handling the same current (about 1000 A). There were still oscillations, but only abou 20V p-p and the turnoff time was also shorter - less than 2uS compared to the 6uS with the tube. There was also very little overshoot.

This has got me very puzzled and I am now starting to wonder whether it is the tube itself that is doing something funny. One of the odd things (which I'm not sure I mentioned previously) is that apart from the high frequency ringing the turn off waveform also shows a rather clean overshoot (of the collector voltage) exceeding the capacitor voltage by some 25%. The duration of the overshoot is between 1 and 2 microseconds - much longer than the period of the oscillations during the turnoff and I never quite figured out what is causing it. I don't think it can be inductance as that would show up as a slow current rise during turn on and I don't think the IGBT's themselves have any way of creating a voltage higher than that of the supply. Now that with the tube replaced by a resistive load I do not have this overshoot the only explanation I can think of is that the tube is doing something strange, however to get such an overshoot it would have to momentarily generate a 300 or so volt potential opposite to what was applied to it.

The overshoot seems to contain quite a lot of energy as it manages to charge up the 0.66uF capacitor in the RCD snubber to the peak voltage, that is about 0.1 joules. It may not appear much but is certainly much more than I would think could be stored in stray inductances which I would have thought were the only potential source for the flyback energy.

One thing I would love to do would be to get a trace of voltage vs current through the tube, as that would tell me whether all the funny behaviour is originating there, however my measurement capabilities are too limited to achieve that.

I have not tried the 'hoover dam' :) experiment yet, partly because I made this new 'discovery' (the different behaviour between flash tube and resistor), but also because I will need to prepare a suitable high-side driver for the second IGBT.

You have done fabousouly! The evidence points directly to the flash tube. It is known that a gas arc has negative resistance and so that characteristic explains (almost) all of what you have seen. What is puzzling is that turn-on spike; maybe the arc is transitioning thru a negative resistance region. It should be fairly simple to see both the tube voltage and current; make a curect transformer using a toroid (fairly small ferrite - no larger than 1/4 inch diameter) and run the tube wire thru the center; say 100 turns on the toroid with 1-10 ohm load to scope. Tube voltage (if floating like i mentioned) would have to be done differentially: 2 probes A-B either via crude inverting of B or differential plugin like the old W or 7A13 or equivalent. Naturally, this helps you to better understand the problem source but does not help solve the turnoff problem. Bypassing the tube with a second FET to discharge the capacitor cannot solve the problem source; that shunt FET will also get sass fromthe tube. May i suggest a nasty solution? Use an artifical transmission line! "Perfect" for a fixed ON time, and modifiable for a "programmable" lengths of time.

I haven't read this entire thread, just bits occasionally, but if you consider the physics involved in the tube current, there could be quite a few electrons in flight when you turn off. Those electrons are still going to hit the end of the tube and impart their charge... except for some that have just started the journey and get turned back by the -ve voltage transient.

Someone whose physics is more recent than mine could calculate the size of the charge imparted based on the known current and voltage prior to turn-off.

Clifford Heath.

I'm no plasma physicist, but it seems to be the charges will neutralize pretty readily. There may be some seperation, due to ion drift towards the negative. Seems to me it would look capacitive, if anything.

Seems more likely to me that the tube's inductance would do it. I mean, come on, a kiloamp? How big is your current path? It only takes about, oh, 0.2uH to store 0.1J at that current -- about a 6" loop? How does the tube's current path compare with the resistor loaded experiement?

Tim

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

Hi, thanks for all the feedback.

This thing is getting curioser and curioser. :)

There does seem to be some interesting things going on in the tube. I never really gave much thought to how complex the process might be. Thinking of the current in terms of electrons flying one way and possibly xenon ions flying the other way with all the associated momentum, KE and stuff seems to have some potential for odd things happening. I mean like Clifford said somehow they have to stop at some time after the voltage is removed. What the implications of that are way beyond my knowledge of plasma physics, which is more or less on par with what my cat knows. Is there enough momentum in there to have any significant effect on the current voltage characteristics and is it even relevant - I really don't know, maybe it is far too small to be significant or maybe not. Also, in some other tests I had done some time ago I was looking at how the light output decays at turnoff and found that the light output only starts falling a few microseconds after the turnoff and takes quite a while (several uS) to fall appreciably, so I supose there is still quite a lot of chaos going on inside the tube even after the supply voltage is removed. It seems very plausible to me that all those hot ions and electrons would not just let the tube terminals float peacfully to zero volts. However that supposition could be simply based on my lack of knowledge of how such stuff works rather than anything else.

As Tim said there is also the possibility it could be plain and simple inductance at play though I do have my doubts since my load resistor was placed right instead of the tube leaving all the wiring just as it was. That leaves only the inductance of the tube itself being different between the two cases. Also, I have made a simple calculation on what an inductance of 0.1uH would need if made by winding a toroid. It resulting (if I calculated right that is) that a typical way of achieving that would be a toroid with 5 turns on a

100mm (4 inch) diameter air core. That is about 60 cm (2 feet) worth of wire, a lot more than I have between the various parts of the high energy circuit. I have also tried as much as possible to have all cabling in such a way that they are always in antiparallel, so for instance the pair of wires going to the tube are kept parallel to eachother all the way to the tube and so on, hoping that that will cancel out some of the effect of the inductance.

What I shall be trying to do to find out whether or not it is a matter of tube inductance is to construct a 0.7 ohm resistor which will have a shape very similar to the tube. This I will make out of a good number of 1 ohm 1/4 watt resistors trying to mimic as much as possible the shape of the tube. The tube is a U-shape with each leg about 140mm long and a tube diameter of some 10mm. I will stick this weird right instead of the tube and see what happens. Hopefully the resistor will last long enough to allow me some meaningful measurements.

I'll also be making that current transformer to get some hopefully more accurate current measurements. The differential voltage probe however is something that I haven't yet figured out. I checked on eBay and the cheapest ones cost a fortune so I'll have to bodge something together myself.

Not electrons, ions. Time of flight is not relevant here.