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Ultra Short, Ultra Fast LED Flash

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Marco:
With a switched current source with ballast you retain the problem of the amount of circulating current.

If you go the low voltage COB way, you could easily be talking about 1000A of current. That current isn't started a ms or so before the trigger either. Much of the time in these photography setups the events are kicked off by slow humans, so count on that current to be circulating for minutes rather than milliseconds. Even if you went for the very complex solution of circulating the current through a regenerative power supply so you don't have to burn the full Vs*I, keeping a 1000A running through some fast MOSFETs is going to burn way too much power to run this on a couple of AA batteries.

It's not worth it, he's not looking for single digit ns rise/fall times or 1% precision regulation of the current. Just dump some voltage on and take it off again with a MOSFET, good enough.

David Hess:

--- Quote from: Marco on March 11, 2019, 04:02:59 pm ---With a switched current source with ballast you retain the problem of the amount of circulating current.
--- End quote ---

Sorry, in this case I meant driving a current into a cascode transistor (emitter/source switching) which then handles the voltage compliance.  An actual switched current source could be much faster but would also dissipate the peak power continuously.

The advantage is that the switching transistor can be much smaller for lower capacitance and higher speed and the fixed voltage across the resistor yields a more accurate and regulated output current.

Marco:
I guess a little overvoltage with active current regulation makes sense given the likely high parasitic inductance of a COB (anyone measured it?). I don't really see the need for a cascode though, a simple source resistor with a BJT to pull down the gate should be enough to get decent  regulation. Or just a source resistor period for that matter.

Zero999:
If a triangle wave will do, an inductor can be used to limit the current. I did a quick simulation in LTSpice and a 50nH series inductor with a freewheeling diode could theoretically limit the current to 90A into the Luminous devices PT-121-B, which is similar to one of the LEDs I've tested. 50nH is small and theoretically could be an air core.

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