Pdsink: USB Power Delivery Sink library for embedded devices

I looked at adding USB-PD as a replacement for a 12V barrel-plug power supply in a recent project.

One big issue that came up (and killed the idea) is that if you are not battery powered, then putting a USB-C power input on your device that will only work if you can negotiate 12V+ with adequate current will just cause confusion. In my case, I don't think I could even boot to an error message on 5V.

Phones and the like don't have this issue, since they are still usable (charging slowly) on 5V, but can make use of higher voltages and currents to charge faster.

So I guess my question for the implementer is how booting & negotiating on 5V and then accepting higher voltage is likely to work in practice.

In those cases you have to use a micro or purpose built controller chip to negotiate a higher power input while running off the 5V (bootstrap with dead battery mode). There are chips out for it including power regulation built in.

Add a small "charger error" LED which will be lit by your PD uC to indicate negotiation error or a charger unsuitable to your device due to voltage or current issues.

You could run on 5V with a boost voltage converter to 12V. For extra credit, you could run the USB-PD off 5V, negotiate 12V and only then switch it to the load.

If you absolutely need it, use a separate uC / “trigger” chip for PD negotiation.

I think the GP's point is that this requires a 12V-capable USB power supply.

I have converted pretty much everything I have to USB-C, from toothbrushes to old laptops, and am very happy with the results. My solution is to only own high-quality power supplies with good support for PD. Having done this, the question "Why isn't this thing charging?" doesn't really arise.

Here’s a tangent discussion from a while ago that I enjoyed, on bootstrapping PoE (Power over Ethernet) from UEFI: https://news.ycombinator.com/item?id=44111609

Looking back, funnily the top comment drew a parallel to negotiating USB-PD in u-boot, aka the bootloader. I suppose this wouldn’t have worked for your case though, since your device couldn’t boot at all on 5V.

The common device that this doesn’t work well for is the Raspberry Pi 5. For full power mode it needs an unusual 5V/5A power supply, and that is quite unusual.

If I need 12V/1A, then that suggests I need 5V/2.4A even with 100% efficiency. Without negotiating anything, a device shouldn't draw more than 5V/0.5A.

That's not to say that a boost converter doesn't have value, but it still leaves a gap where there could be confusion.

The confusion or complexity even multiplies if the device has additional USB-C for data transfer. In that case, you either have to mark one port as being the "power in" port, or you have to support power in and data out on all the ports, which gets complicated and expensive.

It would be a great move by the USB IF to think through this sort of thing more carefully. Right now the USB-c connector is so overloaded in terms of power, display modes, thunderbolt, speeds, etc. that it's very hard to predict whether two USB-c devices will connect and at what power or speed and with what capabilities. For power, it would be helpful to require supplies to have a standardized status LED so that e.g. green means that the supply is providing the highest power allowed by the device (not the supply), yellow means there's been a compromise, and red suggests an error condition.

Specifically it needs a supply that offers 5V/5A as a basic profile outside of PPS (programmable power supply), because the Pi doesn't support PPS negotiations. That is what's so rare, much more than the actual ability to do 5V/5A.

Well the question is how many watts you need to display an error message. You made it sound like voltage was the main issue.

And yeah you're supposed to negotiate before pulling 2.4 amps at 5v but that's not usually a big deal in practice. Especially when you're actually supposed to stick to 100mA at first, but who does that.

A diagnostic LED sounds nice but given how most cables don't even have a speed printed on them good luck at something more invasive.

I will say that thunderbolt support isn't often an issue beyond the basic speed rating, and should be even less of one since USB4. And that power ratings are pretty simple, 60W or moreW. I really don't think the overloading of many different types of feature is a big deal, I think the single feature of unknown speed is the big issue-causer.

You can also use various USB pd "decoy modules"

If you need 12V/1A, starting up and showing an error message at 12V/0.2A sounds quite feasible. Of course it depends on what's using up all your power. But at least microprocessors can usually be started at lower power levels (lower frequency) with a switch to high frequency once you've confirmed you have the power available. Display backlight can be dim until you have the power, and peripherals can be powered through a transistor so you can start delivering power after initial system checks.

But it's a bit more involved than just replacing a barrel jack with a USB-C port, and would require some design considerations early on