I have read a bit already about USB C specification. One thing is not clear for me though. Please take a look at this picture:

enter image description here

A device with profile from 1 to 5 can provide a higher voltage on the Vbus. From profile 4 to 5 it is specified that up to 20 V. The question is:

Can the Vbus voltage vary from 12 to 20 V freely or is it always 12V or 20V for profiles 4 and 5? I am not certain about this especially because even the AC adapters for laptops provide 19.5V, meaning that it would be hard (from the power electronics point of view) for a laptop to provide 20 V. Note: take under consideration only USB C to USB C cable connection, not USB A to USB C, as this one clearly can provide only 5V.

When designing a device that would like to be charged from USB C, should it have the ability to be charged from 5V and 12 - 20 V? I would appreciate all help.

  • \$\begingroup\$ It should be mentioned in the fine manual, there is no standard requirement saying "if it is a laptop it must provide X". my laptop only offers profile 1 \$\endgroup\$ – PlasmaHH Aug 30 '18 at 7:41
  • \$\begingroup\$ Sorry I forgot to mention that I meant "USB C <-> USB C" connection. In that case I could imagine that there is a device that offers only profile 1 but thats unlikelly. \$\endgroup\$ – Łukasz Przeniosło Aug 30 '18 at 7:44
  • \$\begingroup\$ I think it is more unlikely to see many devices that offer anything but profile 1 in the near future. As to your actual question, if your device can be charged from 5V why would you ask for more? That would only increase the complexity of your circuit and won't increase available charging current until you get to profile 3 and up \$\endgroup\$ – Maple Aug 30 '18 at 10:35
  • \$\begingroup\$ This is a complicated non standard device in fact. But about the question- I would only really like to understand the actual theory behind how the profiles work. \$\endgroup\$ – Łukasz Przeniosło Aug 30 '18 at 11:19

I would only really like to understand the actual theory behind how the profiles work.

The actual "theory" is changing every year or so. So it is impossible to understand.

The fixed power profiles are from PD Rev. 2.0, which is superseded by PD 3.0 specifications. In PD 3.0 there is an option to have variable voltage profiles.

The limitation of "19.5V" in laptops is not something of major concern, since laptops are not meant to be a major source (provider) of power. The higher power profiles are envisioned as a feature for main universal AC-DC PSU that can feed a chain of devices without a need for additional AC-DC adapters. If a particular laptop can't handle 20V input, than it won't be offering this profile downwards, that's it, although step-up DC-DC converters are not something unheard of, if somebody wants to make a really crazy laptop.

The full PD specifications can be found as a part of total USB 3.x zip file from USB.ORG site.

  • \$\begingroup\$ Of course but I never assumed anyone would use a step up converter only for that purpose. But consider this situation: why wouldnt you be able to power a.3,5 inch hdd from a laptops usb c port? You can provide 12V easilly and the sata adapter can buck it down to 5V as well. You cant really say laptop is not a major power provider here? \$\endgroup\$ – Łukasz Przeniosło Sep 12 '18 at 4:12
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    \$\begingroup\$ @Bremen, the 3.5" HDDs are not really a mainstream for portable storage these days, 4 and 6 TB in 2.5" formfactor have replaced them, with 5 V DC power, and running well from standard USB 3.0 900 mA port. Next are SSDs. If you mean something like NAS, then you better power them from own solid power, and from a good UPS, not from laptop. Note that I specifically mentioned never-ending change in PD specifications, and the adoption rate of PD relative to alternatives as QC is miserable as of near end of 2018. \$\endgroup\$ – Ale..chenski Sep 12 '18 at 4:27
  • \$\begingroup\$ This then yields a different question: can a laptops usb c connector even provide higher voltage than 5V? \$\endgroup\$ – Łukasz Przeniosło Sep 12 '18 at 4:34
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    \$\begingroup\$ @Bremen, yes, in theory, if it is designed so. But in practice it will be highly unlikely, because any higher power will eat into laptop's battery life, which is one of the major selling points, and OEMs won't like it if somebody find that it goes down. It can be flexible though and depend if the AC-DC adapter is used or not, like supplying power to extra monitors, but again it will be major customer issue if his/her extra monitor would stop working, there are a lot of "use cases" that are not very well thought by PD founding fathers. \$\endgroup\$ – Ale..chenski Sep 12 '18 at 4:44
  • \$\begingroup\$ I gues the best way to find out is to grab some usb c sink device eval board and test. Thanks. \$\endgroup\$ – Łukasz Przeniosło Sep 12 '18 at 4:46

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