Interupting xenon flash current ?

On a sunny day (Wed, 05 Dec 2007 12:52:21 -0500) it happened Mike wrote in :

that in modern units IGBTs are

to flash a tube and measured

current peak to be about 1900A and am

about 6us and still at 300A after

reasonably priced/sized IGBT to

but the only IGBT I have is one I

My test setup isn't all that

about why it takes 20us from the

but that just seems like and

large, 80mm long and 8mm dia.

I always tought a SCR, not an IGBT, was fired parallel to the flash tube. And the flash tube is connected via a small inductor to the cap, the inductor limiting the current rise. But it has been at least 20 years ago I looked.... Anyways SCRs can have huge peak currents.

that in modern units IGBTs are

to flash a tube and measured

current peak to be about 1900A and am

about 6us and still at 300A after

reasonably priced/sized IGBT to

but the only IGBT I have is one I

My test setup isn't all that

about why it takes 20us from the

but that just seems like and

large, 80mm long and 8mm dia.

I did that in the '60's, but used an SCR, read my patent at...

...Jim Thompson

that in modern units IGBTs are

to flash a tube and measured

current peak to be about 1900A and am

about 6us and still at 300A after

reasonably priced/sized IGBT to

I believe there is a mistake with your measurement. In the common cameras, the flash current is at the order of 100A and the duration is ~tens of milliseconds. They handle it with IGBT of SOT-223 form factor.

However there are the special pulse tubes for use with lasers, etc. They have higher rate of the current increase and the shorter flash duration.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

that in modern units IGBTs are

to flash a tube and measured

current peak to be about 1900A and am

about 6us and still at 300A after

reasonably priced/sized IGBT to

but the only IGBT I have is one I

My test setup isn't all that

about why it takes 20us from the

but that just seems like and

large, 80mm long and 8mm dia.

The IGBT itself will limit the current. And waste some energy, I assume.

John

That was my frst reaction also since I expected to see something less than 400A, but I can't find any reason for such a major error. I have a 200mv 200A shunt in series with the tube and am simply grabbing the voltage waveform across the shunt with an HP 54502A storage scope. So with 1mv/A and reading 1.9v peak that's

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

Yep, but the reason for opening the circuit instead of shunting the tube is to leave the unused charge in the cap to allow much quicker recharge time. If there is no absolute truth then nothing can be known.

and I fear, fusing to a blob of silicon in the process. :)

Mike

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

This isn't a case of "current limit". It actually does indeed interrupt the flash tube current abruptly.

That's the only way that modern flash can work predictably, controllably and repeatably.

I looked into this (for a non-commercial application) some time back and couldn't find any off the shelf (one piece) semis that could do it. I couldn't find any specific data on the way it was done, so suspected they used two devices, an SCR as per normal, and a more traditional semi switch to turn it off after the predetermined delay.

voltage waveform across the shunt

amplitude and time.

two possible sources of error

or use a larger value shunt...

Mark

I bet it is either inductive or capacitive parasitic pickup or a common ground problem.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

voltage waveform across the shunt

amplitude and time.

Well, if that's what the 'scope shows, it's *got* to be true.

Best regards, Spehro Pefhany

A couple of people make ICs that do the whole function: flyback converter, trigger stuff, IGBT gate driver with timing.

Shunt igbt, wastes energy:

Series:

There are several others.

John

voltage waveform across the shunt

amplitude and time.

Current shunt inductance can be a serious error in this time domain. A current transformer would probably be more accurate, and avoid ground loops too.

John

Yes, but where were they when I needed them back then? :-)

Maybe they didn't exist?

The older controlled-flash units had two flashtubes, one to actually flash, and a second shunt tube, inside, that shorted out the cap when enough light had been generated.

John

Thanks, I had no idea anyone made a controller like that. In looking at the data sheet and their demo board info, I see they are dealing with an 80A pulse with only 1J and small tube. So maybe my measurment isn't all that far off.

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

They show a flash-tube current of 60A peak (with a short little extra bit to 80A) decaying with an approximately 70us time constant. They suggest a CY25BAH-8F IGBT, which is a 400V "150A" Renesas part in a tiny tssop-8 package.

I wonder how I might calibrate such a homebrew current transformer? Shouldn't I see some serious ringing in the current waveform if my shunt is that inductive?

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

This is a very low inductance shunt. It consists of 2 straps of resistance material about 1" wide and 5" long stacked on one above the other in brass blocks with Kelvin sense terminals. I just measured the inductance to be 0.2 to 0.4uH. It wants to drift around a bit. I have used this same shunt to measure a 3000A pulse in an impulse magnitizer that I built, but that was a much longer pulse. Somewhere around 8ms if I remember correctly. Yes the shunt is located between the capacitor neg terminal and the flash tube. The scope gnd, the charge power neg, capacitor neg, and shunt are all connected to the same node. I connected the scope probe tip to the that same node and the scope showed zero voltage.

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