# With a SMPS charging a capacitor bank. What is the power rating of the SMPS?

I'm working on a project to build a 24VDC to 250VDC switch mode power supply for the purpose of charging a capacitor bank. The capacitor bank will be rated at 250V and 180mF. So, the prime power source is a 24V battery and we need to bump up the voltage to 250V to charge the capacitor bank. The capacitor bank will initially have 0V. I'm trying to narrow down which SMPS topology I should use since they have different applications in terms of power output.

The capacitor bank needs to be fully charged in 1 minute. I calculated the resistance value (67 ohms) for the RC time constant. This would mean that the current draw from the capacitor bank would be 3.73A at time = 0. So, the instantaneous power at that time would be 250V * 3.73A = 933W. Is this the value I should use to decide which SMPS topology to use?

But as the capacitor bank is charging, the current draw from it decreases exponentially which would change the power output of the power supply. So in this case, would the average power output be the number I should use?

Thanks in advance for any help!

I'm not sure where you got the idea that the power output determines the topology. Power output can affect the topology, but unless you're working on something itty bitty, or absolutely huge (compared to my hand, to put it into engineering terms) the step-up or step-down ratio, the voltages involved, and whether you need isolation or not has greater impact.

You probably want a flyback*, and you probably don't want to think in terms of power output per se. You want to think in terms of the allowable current on your flyback transformer, because charging a capacitor from zero to some way-higher-than-source voltage could otherwise put some wildly varying currents on your inductor and switches.

You do not say what capacitance you are trying to charge to 240V in one minute, but from your quoted power levels I assume you can't use an off-the-shelf supply. Were I in your shoes I'd start with some white papers on photoflash units and extrapolate upward in power from there.

* but at a 10:1 ratio you could maybe get by with a buck-boost.

• From the reading that I've done, I've come across statements like, "For medium-power applications (100 to 400 W) the half-bridge topology becomes the pre-dominant choice." That's the reason I am trying to figure out the "power application" of charging a capacitor bank. Also, the capacitance of the capacitor bank is 180mF. Can you please elaborate on what you meant by "but unless you're working on something itty bitty, or absolutely huge"? Are you talking about the size of the SMPS itself? Btw, thanks for your answer! Oct 18, 2019 at 1:49
• You will get a lot more traction with your question if you edit it to state that you want to charge a 180mF capacitor bank to 240VDC. Say what voltage you want to charge it from, while you're at it. Oct 18, 2019 at 14:38
• If you find a "buck-boost converter" chip on the net, of the sort that's been coming out in the last few years, that's a half bridge topology. And some of them are designed to deliver less than 10W (possibly less than 1 -- I haven't been paying close attention to that market lately). SMPS technology is changing fast these days; you can't go by rules of thumb (except that it'll either be way easier or way harder than you think, or, strangely, both). Oct 18, 2019 at 14:41
• FWIW, the 'general opinion' seems to be that the simplicity of flyback designs ceases to be a compelling factor to adopt them as they pass ~100W and approach 200W. Oct 13, 2020 at 4:23

It might work with massive 100A inductors to not saturate with a CV boost converter.

So you want a CC boost linear voltage ramp, rather than exponential CV boost converter. Think like its rapid a 1kW CC CV battery charger for the performance then cut pack with compromises.

• Then split the current into 3 phases.

## This seems to do the topology job.

But you ought to buy it if this is your 1st rodeo.

• Yes, this is my first rodeo and I wish my group could buy the power supply but we need to build one ourselves. Oct 18, 2019 at 1:54
• It will cost you a lot more. I guess your time is free and so are the board spins. If I were you I'd get the price from DK.. I have given you all the clues to find it. your group is naive. But if you were to do it brute force from Vac, it could be done cheaply. with heater resistors diodes and cutoff regulator. Oct 18, 2019 at 2:22