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Comment by hazkoulia | Hacker News Reader

I'm looking forward to USB-C PD small format factor PC's. A decent amount of room in the PC cases is taken up by the power supply. And if USB-C could somehow provide a range of voltages to the motherboard, SFFPC's could be downsized even more

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I have a couple like that in my mini rack. The problem is the power supply: these take 65W, so I figured I could get a quality 140W charger and power them both, but it turns out that things like "unplugging something else" would kill their power. I had to spend a lot more on a StarTech charger to get the performance I needed

Yeah, it's due to power renegotiation.

It wants to supply the maximum power possible as you don't want a device which can charge faster stuck at a lower rate, so it wants to renegotiate with the sink, and the lazy way to do that is a full reset.

Which startech product did you end up getting? One of the hubs or chargers? I have a similar issue with various USB-C rack devices

Jeff Geerling has done a review and follow-up on one: https://youtu.be/twoAW0eLiXY

I've considered just getting a bunch of 65W USB-C buck converters and DIY one.

This charger: https://www.startech.com/en-us/computer-parts/424dna-usb-c-c...

A lot of SFF PCs already come with a power brick. Just with a 12V barrel jack instead of a USB-C port. Compared to that design you really wouldn't safe much space. Though I admit that USB-C would be convenient. Maybe with a tiny battery

The issue is that a USB charger is not a USB power supply. A charger does its best, but makes no guarantee of constant power delivery or duty cycle. The power supply absolutely must provide its rated output at 100% duty cycle.

There's also the fact that devices would still need an internal PSU to convert the 28v USB-C to the multiple voltages that all the parts inside need. It would be smaller without the AC to DC conversion though.

This is already the case today. Those mini pcs often take 19V, so it can use cheap and abundant laptop PSUs, which use that voltage for battery chemistry reasons.

Literally nothing inside it uses that voltage, so it'll just get downconverted to the single-digit voltages the chips actually need.

> And if USB-C could somehow provide a range of voltages to the motherboard, SFFPC's could be downsized even more

You reeeeeeally don't want to do that. Cable inductance is a big deal, among other issues. You want the main DC-DC regulators on the board, usually right at the load, for the main loads. Most of the PSU bulk is for dealing with mains itself: handling 50/60Hz or mains isolation is just physically large. Getting in secondary 20V DC (or so) from a single connector and then regulating it down on board is pretty much the ideal solution.

(I can't even begin to comprehend the horrors of a USB-PD negotiation involving multiple voltages. It's already the worst standard I've ever had to deal with.* Don't make it worse!)

(* Not hyperbole, it is truly, truly awful. At least things like 60601 are bad because, you know, they're covering lots of stuff like lifesaving medical devices. USB-PD is... holy hell, it is just bad.)

I quite like PD 2.0 when ignoring the legacy USB-A stuff. It's semi-elegant. 3.0 and further made it an abomination though.

> I can't even begin to comprehend the horrors of a USB-PD negotiation involving multiple voltages. It's already the worst standard I've ever had to deal with.

Yeah but usually you don't have to deal with that yourself but have some sort of chipset that handles all the USB-C PD and, optionally, even data-lane muxing stuff.

Existing solution @ https://github.com/vk2diy/hackbook-m4-mini

There are some SFF PCs that can take USB-C power.

Lenovo have some,but sometimes require adapter cards. And a few of the Chinese N150 units will take PD power

It's great for hot swapping and more portable than a laptop.

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