Battery Charger SMPS, How'd they do it?

Ti or Onsemi has good app notes on SEPIC converters. I think the coupling Cap ESR will limit your effiency.

Cheers

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Why particularly SEPIC? There are lots of app notes around on buck/boost switchers -just make sure you understand the technologies involved before spending money on, or marketing , anything.

Why particularly SEPIC? There are lots of app notes around on buck/boost switchers -just make sure you understand the technologies involved before spending money on, or marketing , anything.

I'm just wanting to be able to take 12V from a lead acid battery and charge from 0V (shorted cell) to ~30V-50V. I don't care if it's SEPIC or buck/boost. Most applications I've seen are for variable input and fixed output. I need ~11-16V in and 0- ~30V out.

Thanks!

RogerN

Since this is a battery charger, every thing but the current limiting can be slowish. This could make doing the whole PWM process within the micro practical.

250W is at the high end but within the reach of the SEPIC topology. The same can be said for the transformer coupled flyback. You have said that you want to make it open source so I assume that this means that you would rather not use any custom parts. For this reason I suggest that the boost-buck be your choice.

If you look at the buck regulator, you will notice that all the switching happens on the input end of the inductor. If you look at a booster, all the switching happens on the output side.

You can do both types of switching with the same inductor. When you are bucking, the boost circuits inductor is always off. When you are boosting, the buck transistor is always on. This means that the micro only needs to PWM one device at a time.

The current limit system needs to be able to turn the buck transistor off quickly. It may be practical to allow the micro to be in this path. An interrupt can be generated by the over current circuit and the micro could get the transistor off before any damage is done.

The inductor you are going to use will have to be somewhat huge. Getting inductors this large off the shelf is not very easy. You can use a string of lower value inductors in series.

So, instead of a shelf system, think in terms of pallets, forklifts, and the like...

Seriously, most large inductors are carefully designed and custom-made for an application, not stocked at distributors. In onesies, cannibalizing a PC power supply is the easiest way to find a suitable bag of components for a gizmo like this. I've designed and wound my own inductors, but the safety-critical nature of the high voltage/high current parts means extra pitfalls. Just remove the filter capacitors from a PC power supply and replace its voltage sense feedback with something battery-charge-relevant, it'll deliver all the current your batteries could want.

And when you make a mistake, the builtin fuse will indicate and no house-burn-down event is expected.