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3v isolated outputs from lipo, buck/boost isolating converters, or a better way?

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Hi all, so annoyingly the post I had written up got nuked going through my history, I've been trying to find the chip that had looked good for the buck/boost side, but no joy so far.

Basically, I'm looking at a small project to rid myself of D cell batteries, and I want to replace two pairs of them with a lipo pack in a drop in case, so I don't have to modify the device in any way. Chances are it won't run as long but it will be massively more convenient.

So, I need two 3v outputs, fed from a lipo battery, that's fairly easy with a buck/boost converter chip, however, I need the two outputs to be isolated, as there's a tap off of the middle of the two sets of batteries, and connecting a common ground to the output of a converterwould be a dead short, no bueno.

So, I was looking for a magic chip that handles buck/boost and isolation duty to provide a 3v output from 2.7-4.2v in, but I haven't had any luck, so my next best guess is using a buck/boost converter to obtain the steady 3v (or possibly higher), and then run that through another chip which isolates the output.

I am hoping to avoid linear regulators to make this more efficient since I'll be going from the substantial rating of four d cells down to only 4000mah of lipo. I only need around 36ma on the output overall, so it's nothing particularly intensive.

It does seem that I may need to go for a 3.1 or 2.9v output since straight 3v isn't available for the things I'm currently looking at.

Unfortunately at the tail end of a long day so definitely struggling to find what I had before and probably missing something output. I had been hoping to figure this out solo but loosing my place didn't help. My 3d printing now layer shifting like crazy for no reason is really starting to piss me off too.

Aren't rechargeable D type cells available off-the-shelf?
If the tap current is small then you could split voltage in half/quarter with ICL7660.

For a quick and dirty Isolated supply the Mornsun range is worth a look. I use a few of the 0505 range which is 5V 1W rated but there is some at 3.3V 1-2W here to look at https://www.mornsun-power.com/html/products/38/sip-dip-regulated-output--0.75-1w.html

If you need a stable 3V then add a LDO Reg after them will cost you a little in waste but still an easy fix.

EDIT: With the power of Beer another quick and sort of dirty option is a pair of these and two 18650's. Providing you remove any leads from the charge side they will be isolated then hook up to charge. They run DC-DC for the 5V boost and an LDO Reg for the 3V output.

Or one of them and add in one of the Mornsun modules for your second and power it from the Holder.

David Hess:
As an alternative, many simple switching topologies can be configured with a second negative output using a dual inductor so +/-3 volts is possible.

A simple inverter could drive a pair of identical transformers to produce two floating unregulated 3 volt outputs, or a single transformer with a center tapped output to make unregulated +/- 3 volts.


--- Quote from: Alti on June 15, 2021, 08:10:31 am ---Aren't rechargeable D type cells available off-the-shelf?
If the tap current is small then you could split voltage in half/quarter with ICL7660.

--- End quote ---

I'm sure they are, but then I need to have a dedicated charger for them and I'll no doubt loose it at some point. I have a usb type c lipo charger pcb with low voltage protection, and I can easily add a barrel plug as well, so that makes it dead easy to charge without having to dig around for something I haven't used in months.

--- Quote from: beanflying on June 15, 2021, 09:36:08 am ---

--- End quote ---

18650's won't fit unfortunately, so I'm going to run four 1000mah pillows in parallel. I do already have a charging circuit solution but that one does look nice, cloning that circuit and adding the regulator in would probably be pretty nice. I suppose I could order a couple of those (so I had spare parts), then reverse engineer it to make a schematic and chop out/add in the bits I need, though honestly I'd be fine with paying more for new parts from digikey for the speed, aliexpress has been iffy for me lately with packages being returned to the sellers before they even leave china for some reason.

The batteries normally get discharged at slightly different rates, so this is why I'm running all four lipos in parallel, and wanting two isolated outputs by the way.


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