It's been my experience that the best way to generate a well defined current pulse is to use a current steering scheme, i.e. long tail pair. Since you do laser circuits, do you have a better idea?
With lasers it's usually done this way: A current source dumps a constant current into the laser diode. Then an RF transistor switches across the laser diode, robbing part or all of that current, depending on what you have to build.
-- Regards, Joerg http://www.analogconsultants.com/
I can see that scheme working well too. The idea behind both schemes is to keep the current source running and gate the current as it is needed. Trying to create a fixed current or regulate one on the fly requires feedback, and thus has all the associated problems such as settling time, overshoot, etc.
However, I don't think such a circuit is what the original poster wanted.
Hi,
yes, my pulses are of the order 0.3-1ms so a feedback loop (including a boost converter in current regulation) actually has time to regulate with minor overshoot. It is easy to design a "conservative" circuit with a separate op-amp + BJT and sense resistor for each LED string, providing good regulation, I've done this and measured and it works well (without the super-capacitor-noise the 80V boost generated :). But it takes up too much PCB space really, perhaps the best solution is to simply try to optimize the size of the component selections instead of the circuit topology.. choosing the smallest possible power ratings etc. Also I can mitigate it by boosting 12->24V for example and have half the number of strings, so I have plenty of suggestions and ideas to go forward with now thanks!
Nevertheless it is interesting to read about the laser and other short- pulse scenarios! If the pulses had been in the microsecond range, something like that would have been necessary definitely..
Best regards, Bjorn
Why can't you just charge the inductor, then dump. That is, make it a one-shot. You have at least 100ms to charge the inductor. The current pulse would not be flat of course. It would fall off the cliff from a peak. You would probably want to diode clamp the string to make sure you don't reverse bias the LEDs.
You mean like a buck or boost converter without the output capacitor, with the load directly on the inductor output and another FET for pulse-shaping in series with the LED string ? And then skip output current regulation, but instead make sure to switch when the first phase of current through the inductor reaches the desired peak ?
So when the inductor current reaches say 0.6 A, you turn off the switch FET and turn on the output FET for the desired time, then the inductor will "discharge" a shape with peak at 0.6 and below - the shape is depending on the inductance of L.
Would this scheme work equally well with a buck or boost topology at the switch-side ? The important thing here seems to be the measurement of the current through the inductor to cap the output current.
How would the slow charging affect the power-efficiency btw (actually my pulses are more like 15 ms apart than 100 ms)? Time for some LTspice simulation maybe..
/Bjorn
15ms apart is not a camera flash. The camera lag is closer to 100ms. It would really help if you wrote a specification.
Use the boost configuration. I don't see the need for two fets. Just the standard one FET and a fast recovery diode.
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