Re: Interupting xenon flash current ?

On Feb 29, 6:57 am, Lister6...@xxxxxxxxx wrote:
On Feb 28, 7:15 pm, rblon...@xxxxxxx wrote:





On Feb 24, 1:20 am, Lister6...@xxxxxxxxx wrote:

On Feb 23, 10:30 pm, legg <l...@xxxxxxxxxxxxxxx> wrote:

On Wed, 20 Feb 2008 07:50:38 -0600, "Tim Williams"

<tmoran...@xxxxxxxxx> wrote:
"legg" <l...@xxxxxxxxxxxxxxx> wrote in message
news:mpaor3dhjgjtelmp437mvnjtrl1g1jrbt3@xxxxxxxxxx
You can't count on parallel IGBTs sharing without external assistance.

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

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 emittercurrentthan 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%currentimbalance 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 thecurrentRC snubber. The diode will be
placed such that on turnoff the diode will conduct and most of the
tubecurrentwould 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.- Hide quoted text -

- Show quoted text -

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 gatecurrentlimit.  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 axenonflash
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 withxenonflash
lamps, short of economical not feasible.  If one is happy with very
low output, then some of the analyticalxenonflash lamps could be
tried.

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

YMMV,- Hide quoted text -

- Show quoted text -

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 data*** 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.
.

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