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