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Comment by jjeaff | Hacker News Reader

I have recently found that converting higher DC voltage (48v or higher) down to 5v is rather difficult. Or at least there are not many isolated options on the market. I have a diy home battery system that runs on 48v and it has been very difficult to get anything to work and not fail within a week or two. I started with various sizes of buck converters, but it seems that they are too noisy and get blown out by voltage spikes or dips when the inverter kicks in or cuts off. I finally found some isolated converters that convert dc-ac-dc through induction. But the largest I could find can do 5v at 1amp. So I had to get two of them just to handle a raspberry pi zero and an esp8266.

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As you get close to 5% duty cycle you start having tough problems keeping the control loop stable. You might have better luck stepping down 48V to 12V and 12V to 5V cascaded. Less efficient, but cheaper than blowing up your supplies regularly. And now you get 12V for other stuff.

> I started with various sizes of buck converters, but it seems that they are too noisy and get blown out by voltage spikes or dips when the inverter kicks in or cuts off.

Dips shouldn't kill it (unless it's inherently unstable) but spikes definitely can.

Spikes can be addressed with appropriate input protection. A big, low-value resistor with a TVS behind it should take care of spikes.

But this shouldn't be a difficult problem to solve. Converting 48V to ~5V is extremely common in PoE equipment. There are numerous off the shelf designs designed for it. It's likely you were looking in the wrong place (e.g. searching for "buck" instead of "flyback").

I’ve been running into this problem too, my solution has been to use parts and reference designs originally created for PoE applications. Inductor selection is also more finicky. When assembling a buck circuit to take 12v down to 5v, I can be pretty cavalier about the inductor, so long as it supports my max current x1.5 (my minimum derating value), but once you get to covert 48v down to 12, 5, or 3.3, ripple becomes a huge concern, but also the inductor behaves like a gigantic flywheel, plowing current down regardless of upstream switches and in general doing its own thing. I tend to over-protect my circuits with camping diodes and zeners to dump excess voltage to ground - something you don’t usually do when you’re controlling the voltage source, but I lack the knowledge on how to make a minimum design possible here.

Pro tip:

Most Wall AC Adapters will operate at 48V. This has been used by the ebike community for a while to power lights and accessories. I use an 120VAC molex hdd ac adapter to get 12V and 5VDC out of 48V batteries.

You can put 48vdc into a 120vac adapter?

Did you consider adding a voltage-protection circuit, e.g. based on zener diode?

If not for being mechanical, I could imagine using a 48V motor and a 5V generator; both can be 95%-98% efficient.

On at least one of my cousin's farm fields, there's a single phase induction motor connected to a 3-phase alternator out on the power line pole across the dirt road from the field, to provide 3-phase power where there's only single-phase service. It's not crazy.

(I think perhaps his irrigation pivot needs 3-phase power. I'm pretty sure all of his well pumps are run by de-tuned automotive engines converted to run on natural gas.)

At some point it makes sense to not use a simple buck converter, but e.g. a flyback converter with a small transformer. Then you can step-down much higher voltages, of course it will also increase your complexity.

Hi-Link on Aliexpress has decent modules, also isolated ones, that don’t cost too much and push more than a few milliamps.

I'd honestly just get 48V to 5V non-isolated then 5V to 5V isolated if it osn't anything high current

Interesting. Although it's the non-isolated converters that keep getting blown out from spikes. Luckily, they seem to die without blowing up my MCUs.

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