A power supply for a Raspberry Pi 3

[Ian.M]

If you are starting from an 18-20V laptop PSU, then you need a regulator suitable for float charging between it and the battery.  Many of the EBAY CC-CV buck converters with two presets (voltage and current) can do that.  Simply set to the correct float voltage for the battery technology you are using (typically 13.6-13.8V for 12V SLA) before connecting the battery, and set the current limit to C/5 where C is the battery capacity, or 50% of the nominal buck converter rating.   You can use the same type of module to power the Pi - set that one to 3A, and at least you have some protection if something shorts out.   The only remaining thing you need is a low battery warning circuit and a way for the PI to shut off the power so it doesn't kill the battery by deep discharging it if mains power is off for an extended period.   

There are commercially available Pi UPS 'hats' -  Its worth studying them carefully before you start hacking something together as by the time you've mounted and wired two buck converters, modded one of them to access the shutdown pin, and built a latching comparator board for the low battery warning and lockout, you'll probably have spent as much as the commercial 'hat'.

[linux-works]

I thought the battery output was regulated (to charge the battery); it was the pass-thru port that is not regulated (which is why I have that buck converter to step down whatever it sees down to 5v).

I could not imagine anyone designing a battery charger and not having a limit on its output voltage!

are you sure about this, on this circuit?

[timb]

That's 18W at 13.5V. Once you step it down to 5V, it should be at least 3A. (18W @ 5V = 3.6A, so assuming a worst case 80% buck converter efficiency you're left with 2.88A. ~90% would be more likely, so you'd be okay.

Good luck getting 90% efficiency out of LM2576. Even the datasheet states typical 77% efficiency at VIN = 12V, ILOAD = 3A. 90% is in the realm of ICs with synchronous rectifier.

Yeah, totally true for the LM2576. The board I was using didn't have that controller though, some other chip that I measured at about 85%.

I've been working on the design of a 4-switch synchronous buck-boost converter using an LT part, which does get above 90% efficiency, so I must have had that on the brain when writing the above post.

[gore]

A small update. So, I got one of those drok UPS power supplies ~ 50W version. Unfortunately the PSU was in a pretty bad shape on arrival. As soon as I took it out of the box something was rattling inside. A piece of aluminum to hold down a power diode for heat dissipation was loose. The perfect scenario for a short. Not a good start. Well, I took care of that and turned it on. A somewhat loud and periodic tick was audible as I hooked up my 12V 7aH battery. The output voltage was alright though.

Wasn't difficult to pin point the source of the noise. Sucker's coming from the flyback transformer! The core of the flyback was... loose, to put it mildly. At that point I thought about returning it, but I really dislike messing around with shipments with a relatively low cost. What am I going to do? I still need a PSU/UPS for the Pi!

Opened the damn thing up and disassembled it. What a mess! First thing came to my eye - a couple of schottky barrier rectifiers weren't properly attached to the heat sink. I also found a 220ohm resistor on the output soldered in instead of a capacitor. The silkscreen clearly shows an output cap, as it should, right? I checked some of their gut photographs of the product on amazon. There's a cap in there, yup.

The poor resistor was smoking hot. I knew I smelled something... Replaced it with an output cap. Actually I replaced just about anything that looked suspicious. What else? Some of the solder joints looked pretty bad. After all the 'mods' and cleaning up the ticking was gone. Now it seems to work just fine.

As for the further design. Well, I'm still going with the LM2576's. Got all the parts on my bench to finish off the DC-DC converter. I know they are inefficient in the world of SMPS's (~75%), but in this case I do have the power to spare. Wouldn't do it a second time though. What else? Apart from a crowbar for over voltage protection - that's about it. Everything seems to be working just fine. The next step is a low voltage disconnect to keep the battery safe. Until that arrives, I have to finish the board. It's a split board with two adjustable outputs. I need the second source for other piece of hardware with a different voltage output.

P.S the two TO-220's in the picture aren't properly secured to the heat sink yet.

[timb]

Huh. My DROK UPS didn't have any rattling or loose parts. Which seller did you buy from? If it was Fulfilled by Amazon I would have requested a refund. It's super easy, you don't even have to dick around with shipping labels and the like; you simply print the RMA sheet (or write the number down on a slip of paper), stick it in the box with the product, seal the box up and stick it in your front porch. UPS will arrive the next day, collect the box and apply the label for you. (I write "Broken Item - Amazon Return for UPS Pickup" on the box so there's no confusion.)

[gore]

Actually mine's not a "Drok". It's a "Deok", if it makes any difference.  That's what they sell on UK amazon. Anyway, yeah. I thought about doing it, but... curiosity happened. Well, that plus I wasn't really up to waiting for another week or two. I marked some of the things to watch out for, for anybody who's willing to buy one. Perhaps I had a faulty unit and there's nothing to worry about. Transformer noise was my biggest concern and now it's fixed.

EDIT: About the resistor. That's probably a load resistor to keep it stable. Some builds of this power supply have the resistor, while others don't. Weird. I hadn't planned on turning the PSU on without a load anyway.

[gore]

The step-down portion is complete (13.5V to 5V @ 3A). Now I need another design for stepping-up (13.5V to 19V @ 1.6A). Any ideas what I could use to achieve that? I have to fit that on the left side of the board.

[gore]

Hello. It's been a while. I've put together a case from organic glass to house all the goods. The first wave of boards have arrived from production. A combined step up and down DC-DC converter for the RP, some add-ons and a router too. It has adjustable voltage outputs on both channels; adjustable over voltage protection and a temperature based fan control. The efficiency isn't top notch (70 – 80%), but good enough for its purpose. The PSU has a few spare watts to waste away.

The low voltage disconnect boards are on the way. It’s a key piece to keeping the battery safe. Well, so that’s that. There’s still work to be done on placement, enclosure routing and mounting.