Supply for pulsed load

"Anno Siegel" wrote in message news:d8v0gh$3gl$ snipped-for-privacy@mamenchi.zrz.TU-Berlin.DE...

It would have to operate at 16 V according to your description.

One stage is certainly doable, depending on your actual regulation and ripple requirements. Does your volume justify a custom design of the initial converter?

A supply designed to deliver 4 A pulses at 5 W average power would be considerably smaller than a 50 W supply. Think of a supply with semiconductors good for the higher current but with smaller heatsinks than would be required to deliver that current continuously. I would be tempted to exploit the fact that input resevoir capacitors in a switching supply can be depleted quite a bit before regulation suffers. At the timescale you mentioned, getting a switching regulator to convert (for example) an input resevoir ranging from 300 V down to 150 V during the pulse into 12 V with 10+ KHz load response would be doable. By keeping the efficiency up, supply dissipation could be kept to less than 1 W, which should get bulk and cost down.

I've seen supplies custom designed for such requirements. They are in equipment that is costly to service, and have not been a problem, so I suppose they are tried and proven.

I wonder if you could use a smaller capacitor at a much higher voltage, and use a switch-mode regulator to bring the voltage down. Maybe something like a pure-hysteresis converter. Then you would need an inductor and a filter cap, too, obviously.

I think you need to deliver 12V * 4A * 0.03 seconds = 1.44 Joules in your worst case pulse. (Double-check my notoriously poor arithmetic and even my basic equation.)

So, if you wanted to go with a Voltage of, say, 36 Volts, you could drop all the way to, say, 15 Volts, and you wanted that delta V to correspond to a delta E of 1.44 Joules, you would need a capacitor of what size?

dE = E2-E1 = (C/2) * (V2*V2-V1*V1)

So:

C = 2 * (E2-E1)/(V2*V2-V1*V1)

C = 2 * 1.44 / (36*36 - 15*15)

C=~ 2700 uF.

This seems potentially workable. I don't know if it would save you money or not. And you still have to go through the exercise of trying out different inductor and output filter capacitor values (analytically, I mean) to see what works, and what frequencies and duty cycles you will get for the converter.

The advantage of a pure hysteresis converter is that you don't have any problems with stability or minimum currents or duty cycles. It can go all the way down to zero duty cycle, and in this case it probably will in between pulses. But any step-down converter that can handle the range of input Voltages and output currents should do.

Just my first thought on the subject.

--Mac

Suppose a supply has to deliver a considerable load, but only for short pulses with a low overall duty cycle. One could use a standard supply for the full load and be done with, or try to take advantage of the low duty cycle.

To be concrete, I'm looking at a supply for around 12 V (voltage isn't fixed yet) whose load comes in pulses of up to 4 A with 10 - 30 ms duration and 10% duty cycle, tops. Regulation can be lax and ripple isn't critical, if it matters.

My immediate idea is to use a first-stage supply with some extra voltage, say 16 V, that feeds a capacitor large enough not to lose more than 4 V per pulse. The power of the first stage (with 10% duty cycle) is (ideally)

That would need a capacitor C with 4A*20ms/C = 4V, or C = 20,000 uF, if my arithmetic is right. That's not out of the world, but it won't come for nothing at 12 V (does it have to see 12 V?). The idea is to save bulk and money, not to spend more. The whole design looks rather clumsy. Could the functionality be realized in one stage? Are there tried-and- proven solutions?

Anno

Suppose a supply has to deliver a considerable load, but only for short pulses with a low overall duty cycle. One could use a standard supply for the full load and be done with, or try to take advantage of the low duty cycle.

To be concrete, I'm looking at a supply for around 12 V (voltage isn't fixed yet) whose load comes in pulses of up to 4 A with 10 - 30 ms duration and 10% duty cycle, tops. Regulation can be lax and ripple isn't critical, if it matters.

My immediate idea is to use a first-stage supply with some extra voltage, say 16 V, that feeds a capacitor large enough not to lose more than 4 V per pulse. The power of the first stage, at 10% duty cycle, is (ideally)

That would need a capacitor C = 4A*20ms/4V = 20,000 uF, if my arithmetic is right. That's not out of the world, but it won't come for nothing at

Anno

Power line.

Thanks. Your keywords about what to dimension accoding to what were what I needed to solidify the vague idea of "make it for 4.8W *and* for 4A, it won't overheat".

No problem at all (I think). It was the amateurish idea I had to add something to exploit the duty cycle. The right solution is to leave some things off, i.e. to exploit it, period. Thanks again.

Anno

Larry Brasfield wrote in sci.electronics.design:

Right. I corrected that in a superseding posting, but Usenet (well, my server) seems to spin faster these days. I used to be able to safely catch a posting before it left my server. No more, it seems. Sorry for the confusion, if any.

Both are relaxed. The pulses drive solenoids that make a ferromagnetic peg jump against a metallophone plate to make it sound. Ripple and poor regulation won't be a big deal.

Oh dear. I don't see a huge market for MIDI metallophones :) No, if at all, this is going to be a one-off thing, amateur level. However, that may also justify custom design.

That seems to be essentially the solution Tim Wescott suggested in the other branch: Get the time constants right and dimension for full current but reduced power. Very well.

I should have been clearer about my purpose from the start. No need for industrial strength, it just has to work. If it can be simple, I want it simple. I think I over-estimated the power problem. I'm glad to know it won't be very hard to solve.

Thanks, Anno