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Solar battery charger: buck VS boost

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Yes, DCM is pretty violent, but I can get away with it because my current is low.
At less than full output, even CCM will revert to DCM which creates some problems with the transition, and loop stability.
I decided to just stay with DCM all the way, as its going to operate DCM at very light loads anyway.

Here is shot of my inductors that have been hiding out of sight.
inductors.jpg

These are U cores with cross sectional area of 30mm x 30mm.
Air gap is 3mm and 18 turns on the main buck boost flyback winding to produce +235v, and an 18 turn secondary winding to produce -235v.
Switching frequency is 20 Khz and the two interleaved windings are five thou copper foil 35mm wide which are good for about 18A rms.
These enormous things run stone cold and saturate at about 55A.

They are larger than necessary, but I used what I already had rather than buying something.

The eight ugly black wires are from my four groups of solar panels. (east, west, north, and up)
 
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Looks impressive, indeed! The airgap is glued or something? (looks like there is some black resin in the picture).

Otherwise, aren't they buzzing/vibrating during operation? I've noticed that you didn't use any surrounding case for the ferrite core, to keep those pieces in place.

I really like your build (kept simple and robust) but I'm not sure if it's applicable to my situation.

Btw, I don't need output voltage control hence it doesn't need to run in DCM at low loads (as I only use it to charge the battery bank, not to feed any inverter). Am I wrong?
 

The airgap is two thickness’s of 1.5mm flat rubber sheet cut to size.
When clamped up, the rubber kind of "grips" to the ferrite.

I originally clamped up the core halves with some long 10mm steel bolts, but fringing around the air gap caused massive eddy current heating of the bolts. Hot smells and the bolts were too hot to touch. soldering iron type temperatures !!!

So I found some half inch fibreglass rod, drilled and tapped the ends, and used short steel bolts which seem o/k. Although I had some nylon bolts on standby, just in case.

One interesting thing I discovered from all of this, was to keep the airgap well away from the windings. That is easy with a long legged U core, but with gapped EE cores, the fringing around the airgap is right in the middle of both windings.

That produced some ringing, and horrible cross regulation between +ve and -ve dc outputs.
As I can only regulate the +ve dc output, my problem was that the inverter alternately loads the +ve side, and then the -ve side, as it is a half bridge topology inverter.

But keeping the airgap fringing far away from the interleaved windings, produces super clean waveforms, no ringing, and absolutely terrific cross regulation.

I can load the secondary (unregulated) side to 2Kw and there is only a volt or two difference between the regulated and unregulated dc outputs, probably about 1% cross regulation, which is amazingly good. But the fringing around the airgap is huge and needs some caution.

Another thing to think about is using a large low ESR capacitor direct across the solar panel input. The panel is a weak current source and cannot supply high current peaks. I used some (white) Evox Rifa low ESR 2,200uF electrolytics to supply my 40A peak current.

These are also hiding behind the inductors, and supporting the flat and very inefficient heat sinks and the PCBs on the capacitor terminals.

These electrolytics are a very important feature of any switching power supply, buck, boost, or whatever you decide to use when running off solar panels.

fullfrontal.jpg.
 
For that matter, the most "solar friendly" topology should be the boost type because the input current is continuous (so you need a small input capacitor bank, too).

Speaking of capacitors, I have used with best results (even for my 5 kW inverter) the polypropylene (high pulse) ones. I still have some 20uF capacitors available and they are rated at 30A/20kHz, if I remember corectly.

Once again, being a battery charger, I don't need that much regulation hence the whole circuit should me more tolerant to an imperfect design.
 

The whole topic is wide open to whatever is needed to meet specific requirements.
And requirements and circumstances can vary rather a lot. Every project is different.

A lot of thinking and planning is required....................
But its all very worthwile.

All I can do is perhaps give people some ideas to considder.
 
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