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Interupting xenon flash current ?

R

Robert Baer

Jan 1, 1970
0
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.
I do not think that inductance is the issue / problem source.
Without very special precautions (gas pressure, current range, aging,
addition of radioactivity, and i do not know what else), gas in a tube
will have regions of negative resistance; in the right conditions, one
can see a "crawling worm" of plasma in a gas tube.
If you want to get "precise" data, then use a current-limited supply
set to (say) 50V (no less than 32V) and slowly adjust that limit from
(say) 1mA ("engineering zero") to the max of the supply, while
monitoring the tube voltage and current with a scope.
Use a trigger transformer to start the ionization.
 
J

JosephKK

Jan 1, 1970
0
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.

Sorry Robert, this is not any ordinary flash. OP is talking
kiloamperes and kilovolts.
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.

Robert, OP is trying to deal with megajoule pulse at microsecond and
shorter times. The instrumentation must necessarily be suitable for
those conditions.
 
J

JosephKK

Jan 1, 1970
0
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.

This makes me think that a different approach for commutation may be
more appropriate. Perhaps something more similar to how SCRs are
commutated. But at this scale normal approaches must be modified.
 
Just a quick note, the IGBTs are surviving with no problem turning off
up to about half a megawatt ( 550 V @ 900 A). There is of course the
very dirty turnoff but the IGBTs seem to handle it quite well at this
voltage. It is only when I go above that voltage/current that the
IGBTs start to die.

I'll probably be making the test resistor and doing some tests
tomorrow or Friday. I'll let you all know of the results.

(PS: My original target was actually much higher - 1400V @ 3000A. If I
eventually manage to get it working at 700V I'll probably move my
target back up to that again )
 
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?- Hide quoted text -

- Show quoted text -

Appears to me that you are frying the IGBT on turn-off. The situation
you described with turn-on where you crashed your PC could be a
fluke. I build quite a few flash lamp power supplies and the only
times I fried IGBT's are turn-on with too high of a gate resistor and
as such not fully turning the IGBT on with a thermal failure as a
result. The other failur is to have a gate resistor that is too low
and have the IGBT anode current try to exit through the gate.
 
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.

What is the total gate capacitance from your parallelled IGBT's ? Is
your driver capable of supplying the peak curent without any hick-ups ?
 
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.

What you are describing is "after glow" in the flash lamp. Keep in
mind that we are talking thermionic emission here. That cathode is
hot from the pulse and will keep sending electrons out making ions
along the way. This process is a multiple micro second process and
function of lamp, drive current etc.

What flash tube are you using again ?
 
   Not electrons, ions.
   Time of flight is not relevant here.- Hide quoted text -

- Show quoted text -

But time the cathode is hot is relevant as it will keep sending out
electrons, making ions in the process. That's where the afterglow
photons come from.
 
Just a quick note, the IGBTs are surviving with no problem turning off
up to about half a megawatt ( 550 V @ 900 A). There is of course the
very dirty turnoff but the IGBTs seem to handle it quite well at this
voltage. It is only when I go above that voltage/current that the
IGBTs start to die.

I'll probably be making the test resistor and doing some tests
tomorrow or Friday. I'll let you all know of the results.

(PS: My original target was actually much higher - 1400V @ 3000A. If I
eventually manage to get it working at 700V I'll probably move my
target back up to that again )

Are you trying to get micro second flash pulses at these operating
conditions ? I'm not checking this list often, it is OK to use my
emal address.
 
Just a quick note, the IGBTs are surviving with no problem turning off
up to about half a megawatt ( 550 V @ 900 A). There is of course the
very dirty turnoff but the IGBTs seem to handle it quite well at this
voltage. It is only when I go above that voltage/current that the
IGBTs start to die.

I'll probably be making the test resistor and doing some tests
tomorrow or Friday. I'll let you all know of the results.

(PS: My original target was actually much higher - 1400V @ 3000A. If I
eventually manage to get it working at 700V I'll probably move my
target back up to that again )

The shortest pulse I was ever able to drive a xenon flash lamp with is
2us. (laser pumping) At these pulse times your flashlamp doesn't even
come fully on. No use of an IGBT was made to obtain these times.
 
Correction to above post:

"I added a 33nF capacitor across the Gate-
Emitter of each group of six capacitors. "

should of course read:

"I added a 33nF capacitor across the Gate-
Emitter of each group of six IGBTs"


Someone also what flashtube I am using and I forgot to reply to that.

It is an IFK-2000, the 2000 representing its rating of 2000 Joules.
Meant to work up to 1000 V but is OK with 1400 at lower energies.
Makes a nice 'bang' discharging at that voltage. I thought the thing
exploded the first time i tried it 8)

IFK-2000
Picture of IFK-2000[/
url]


If/when I sort out the problem I'll actually be running two of them
simultaneously.
 
R

Robert Baer

Jan 1, 1970
0
JosephKK said:
Sorry Robert, this is not any ordinary flash. OP is talking
kiloamperes and kilovolts.




Robert, OP is trying to deal with megajoule pulse at microsecond and
shorter times. The instrumentation must necessarily be suitable for
those conditions.
Power level and timing do not "prevent" the use of an artificial
transmission line; just the part specs and wiring layout.
 
R

Robert Baer

Jan 1, 1970
0
But time the cathode is hot is relevant as it will keep sending out
electrons, making ions in the process. That's where the afterglow
photons come from.
Yes; "time of flight" is in nanoseconds or less - ion relaxation /
buildup is in hundreds of microseconds or so as a guess based on how
long it takes a tube plasma too die (seconds).
 
   Yes; "time of flight" is in nanoseconds or less - ion relaxation /
buildup is in hundreds of microseconds or so as a guess based on how
long it takes a tube plasma too die (seconds).- Hide quoted text -

- Show quoted text -

Some possibly dumb questions about what goes on in the tube:

If I calculated right (which I probably didn't) an electron would take
a minimum of about 50nS to travel from the cathode to the anode if it
didn't hit anything along the way. (assuming a 300mm path and 700
volts) The Ions I think would take much longer, being thousands of
times more massive. Is that anywhere near being correct, and would the
ion motion play a significant part in what appears on the terminals of
the tube?

Would electrons and ions actually travel all the way or would most
collide and recombine long before reaching the other end of the
tube ?

If I understand correctly, most of the emitted light is due to the
temperature of the gas rather than electron-ion recombination. If that
is the case quite a significant time is needed for the gas to cool
down, which I think accounts for the (relatively) slow decay of the
light output. Would the presence of the hot gas in itself cause
voltage/current through the electrodes? COuld that be causing the
overshoot (assuming it is not just inductance)?
 
T

Tim Williams

Jan 1, 1970
0
If I calculated right (which I probably didn't) an electron would take
a minimum of about 50nS to travel from the cathode to the anode if it
didn't hit anything along the way. (assuming a 300mm path and 700
volts)

That sounds about right. I've made calculations of a couple ns for vacuum
tubes (about 1/8" travel, near zero initial velocity, about 300eV final
velocity).

Since the electrons are surrounded by gas, they take a whole lot longer to
get there, ricocheting off atoms until they gain enough energy (heck, only
a few eV for xenon) to ionize them, at which point the collisions go from
elastic to inelastic and energy is transferred. Electrons spall off and a
plasma discharge is born.

Of course, the electrons have to come from somewhere. There are a few just
sitting around, but despite the unstable nature of a discharge, there
aren't enough. A high voltage pulse is usually added to a flash tube to
pull electrons from nearby surfaces with field emission. Once the
electrodes are hot, thermionic emission also serves to keep things
conducting.
The Ions I think would take much longer, being thousands of
times more massive. Is that anywhere near being correct, and would the
ion motion play a significant part in what appears on the terminals of
the tube?

Indeed, you can imagine the ions and atoms being in about the same place,
bumping around inside the tube, while the electrons dance around them
about, oh, 2 x 10^5 times faster. (That's slow, but it also doesn't take
much velocity to have a good kinetic energy on an atom that size.)

If you assume the ions and electrons fully migrated to opposite ends of the
tube, making assumptions about the number in the tube (pressure, internal
volume, etc.), ionization level, etc., you could guess what the literal
capacitance would be (since we're talking seperation of charge here). Such
a capacitor would discharge quickly!
Would electrons and ions ... collide and recombine long before reaching
the other end of the tube ?

Yup. Don't forget that the electric field isn't constant- each time an
electron has enough energy to ionize a neutral atom, it soon will. In low
pressure, low current situations, you can actually see this as glowing
bands above the cathode: electrons go so far, gaining kinetic energy in the
field, then they smash it away, then go on again and so forth... Each
ionized region has a nearly equal voltage across it, while the un-ionized
[not union-ized, heh] regions have somewhat more field in return.
If I understand correctly, most of the emitted light is due to the
temperature of the gas rather than electron-ion recombination. If that
is the case quite a significant time is needed for the gas to cool
down, which I think accounts for the (relatively) slow decay of the
light output.

That should be correct. If you have a spectrometer on hand, you could see
if it looks black body (at that temperature, probably centered around
blue?) or has any spectral lines characteristic of xenon (or anything else
for that matter!).
Would the presence of the hot gas in itself cause
voltage/current through the electrodes? COuld that be causing the
overshoot (assuming it is not just inductance)?

I don't think so. It's quite conductive before it cools down. It takes a
lot of energy to seperate charges, and that energy is soon released if it
can be!

Tim
 
I suggest a much smaller fan out on the driver chips, say 3 to 1. It
might help.


I do have plans for reducing the fanout - actually having either 1
driver chip for each IGBT or possibly one driver chip for every two
IGBTs. However I have reduced the priority of doing that move as it
is starting to appear that the problem I currently has nothing to do
with gate drive.

My priority is now how to get a clean turnoff. Originally I was
blaming poor design of the gate drive, but I have seen it work fine
with a resistive load rather than the flashtube so I will now be
looking at ways to 'tame' the flashtube.

I have now added a 'freewheeling' diode between the colectors and the
positive supply (of the capacitor). This has greatly reduced the
positive overshoot to about only 80 volts above the capacitor voltage.
I quite naively hoped this would solve the problem so I tried firing
the tube at 700 volts. Worked a few times, but only just the few.
After about the tenth discharge one of the IGBTs blew - full short
circuit between all three terminals.

It was suggetsed that it could be something as simple as the
inductance of the tube so I tried to make an experimnent to see
whether that was the case. I built a resistor out of 200 (!) small
resistors, giving it a shape as similar as possible to that of the
xenon tube . The idea was that the inductance would be very similar
and the resistance would be equal to that of the tube at the time of
turnoff. Unfortunately the resistor was no match for the current I
tried to put through it and just went up in a puff of smoke and a loud
bang. Turns out I was a bit over optimistic assuming that keeping the
pulse short enough would allow the resistors to survive - and I also
made a mistake setting the pulse length to 200uS instead of 20uS as I
had meant to.

I've now decided to take an opposite (and counter intuitive)
approach. I will be instead adding inductance in series with the
tube. One thing I think this might do is provide a high frequency
blocking between the tube and the IGBTs. ALthough this will not
necessarily solve the problem I am hoping it will allow me to figure
out better where the oscillations are originating. I will be able to
check the voltage on each side of the inductor. I should see much
larger oscillations on one side than the other. If it is on the IGBT
side then I will know there must be some very significant collector to
gate feedback. If on the other side it would confirm the suspicion
that the tube is causing the oscillation - due to its negative
resistance or some other weird plasma physics thing.
 
R

Robert Baer

Jan 1, 1970
0
I do have plans for reducing the fanout - actually having either 1
driver chip for each IGBT or possibly one driver chip for every two
IGBTs. However I have reduced the priority of doing that move as it
is starting to appear that the problem I currently has nothing to do
with gate drive.

My priority is now how to get a clean turnoff. Originally I was
blaming poor design of the gate drive, but I have seen it work fine
with a resistive load rather than the flashtube so I will now be
looking at ways to 'tame' the flashtube.

I have now added a 'freewheeling' diode between the colectors and the
positive supply (of the capacitor). This has greatly reduced the
positive overshoot to about only 80 volts above the capacitor voltage.
I quite naively hoped this would solve the problem so I tried firing
the tube at 700 volts. Worked a few times, but only just the few.
After about the tenth discharge one of the IGBTs blew - full short
circuit between all three terminals.

It was suggetsed that it could be something as simple as the
inductance of the tube so I tried to make an experimnent to see
whether that was the case. I built a resistor out of 200 (!) small
resistors, giving it a shape as similar as possible to that of the
xenon tube . The idea was that the inductance would be very similar
and the resistance would be equal to that of the tube at the time of
turnoff. Unfortunately the resistor was no match for the current I
tried to put through it and just went up in a puff of smoke and a loud
bang. Turns out I was a bit over optimistic assuming that keeping the
pulse short enough would allow the resistors to survive - and I also
made a mistake setting the pulse length to 200uS instead of 20uS as I
had meant to.

I've now decided to take an opposite (and counter intuitive)
approach. I will be instead adding inductance in series with the
tube. One thing I think this might do is provide a high frequency
blocking between the tube and the IGBTs. ALthough this will not
necessarily solve the problem I am hoping it will allow me to figure
out better where the oscillations are originating. I will be able to
check the voltage on each side of the inductor. I should see much
larger oscillations on one side than the other. If it is on the IGBT
side then I will know there must be some very significant collector to
gate feedback. If on the other side it would confirm the suspicion
that the tube is causing the oscillation - due to its negative
resistance or some other weird plasma physics thing.
As i mentioned, the flashtube plasma is the source of oscillation
(and cannot be tamed).
I also suggested the use of an artificial transmission line for
determining the on time; that could be programmable in steps as needed.
 
J

JosephKK

Jan 1, 1970
0
I do have plans for reducing the fanout - actually having either 1
driver chip for each IGBT or possibly one driver chip for every two
IGBTs. However I have reduced the priority of doing that move as it
is starting to appear that the problem I currently has nothing to do
with gate drive.

My priority is now how to get a clean turnoff. Originally I was
blaming poor design of the gate drive, but I have seen it work fine
with a resistive load rather than the flashtube so I will now be
looking at ways to 'tame' the flashtube.

I have now added a 'freewheeling' diode between the colectors and the
positive supply (of the capacitor). This has greatly reduced the
positive overshoot to about only 80 volts above the capacitor voltage.
I quite naively hoped this would solve the problem so I tried firing
the tube at 700 volts. Worked a few times, but only just the few.
After about the tenth discharge one of the IGBTs blew - full short
circuit between all three terminals.

It was suggetsed that it could be something as simple as the
inductance of the tube so I tried to make an experimnent to see
whether that was the case. I built a resistor out of 200 (!) small
resistors, giving it a shape as similar as possible to that of the
xenon tube . The idea was that the inductance would be very similar
and the resistance would be equal to that of the tube at the time of
turnoff. Unfortunately the resistor was no match for the current I
tried to put through it and just went up in a puff of smoke and a loud
bang. Turns out I was a bit over optimistic assuming that keeping the
pulse short enough would allow the resistors to survive - and I also
made a mistake setting the pulse length to 200uS instead of 20uS as I
had meant to.

I've now decided to take an opposite (and counter intuitive)
approach. I will be instead adding inductance in series with the
tube. One thing I think this might do is provide a high frequency
blocking between the tube and the IGBTs. ALthough this will not
necessarily solve the problem I am hoping it will allow me to figure
out better where the oscillations are originating. I will be able to
check the voltage on each side of the inductor. I should see much
larger oscillations on one side than the other. If it is on the IGBT
side then I will know there must be some very significant collector to
gate feedback. If on the other side it would confirm the suspicion
that the tube is causing the oscillation - due to its negative
resistance or some other weird plasma physics thing.

I will make you a deal. I think that the problem is some weird plasma
physics, thus a resistor pack does *_NOT_* constitute an equivalent
load. Accordingly i will offer you a bet, cut the driver to IGBT
ratio to no more than 1 driver to 3 IGBT and see what happens. If it
does not help i will apologize profusely, if it helps you apologize
profusely.
 
Robert,
Thanks, I have not ignored your suggestion. I have been trying to
figure out what the artificial transmission line would involve but I
haven't got very far.

With what little I know about it I have two concerns:
1. Something tells me that it will involve some rather largish
inductors and capacitors. I am already 'over budget' in terms of
space requirements so that can be a problem
2. I need the timing to be arbitrarily determined by software on the
PIC, one of the objectives being also a returned light exposure
control, as used in ordinary automatic flashguns.

Would it be possible to guide me with a few words in the right
direction as to what it would involve and whether these two concerns
are valid?


Joseph,
as I said, I do intend to increased the driver to IGBT ratio but that
is going to take me quite a while. Presently I have the IGBTs mounted
on copper bars and the drive consists of components soldered to copper
strips and bars directly on the IGBTs. To have the multiple drivers
this system is no longer practicable so I'll have to design and order
a PCB.
I was going to postpone this but maybe I'll put some more energy back
into it. Put it this way, I cannot lose either way. Either I'll be
rewarded by a solution to my problem or a profuse apology :) A
winner any way. :)
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
well it seems to me that power dissipation of the device is the criteria not the current.. once it get fired then the decay is the same as a rc time constant.
 
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