I need to design a constant current buck switcher. I need to power a Perkin Elmer krypton long arc lamp, a QCW12. At the middle of the lamp's V-I curve, near where I want to run, the lamp voltage is155V DC @ 20A. I want to run at 80-100 Khz off rectified 220 line. I've serviced these systems since 1988, so I have a idea what needs to be done to support the lamp, but I never ripped out a customer's inductor and measured it.
For the curious who will ask, why do you want this?, the lamp drives a Nd:YAG laser that is frequency doubled to 3.5 watts of green light. Since the next question will be, jeeze, thats a tought design, why don't you just buy one? There is only one remaining maker of these supplies in the US and my lamp is too small for their unit, which is designed for a 40 amp 160V lamp. Everything else is solid state, pumped by laser diodes, these days.
These lamps start degrading from minute one, so the simple answer of using a series resistor is foolhardy, if you dip into the negative slope portion of the lamp I/V curve, the system oscillates and the lamp explodes, taking the laser rod with it. I've tried it, I didn't blow the rod or lamp, but the output power is all over the place.
As I look at various on line tutorials, I keep seeing a constant for ripple factor for picking the inductor. I've looked at three different tutorials and have seen three different numbers. The one that makes the most sense so far is .4 x the DC design current, or 8 Amps.
Anybody have a known proven rule for finding a value for this constant?
Thank you for your time,