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I've got a laptop which charges through USB Power Delivery. The charger supports the following profiles:

  • 5V x 3A
  • 9V x 3A
  • 12V x 3A
  • 15V x 3A
  • 20V x 3.25A

I know USB PD devices and chargers enumerate and negotiate the profile they will use together. I suspect my laptop to use the last, 65W profile, although I cannot guarantee it.

My question is: what happens when there's no exact match between the profile(s) the laptop expects and the ones the charger proposes? I am asking because I need to buy a second charger, but most of the ones I find only provide 60W (20V x 3A, not 3.25A). What would be the result of the PD negotiation if I plugged one of those chargers to my laptop? If it would work (which I suspect), why? Do the charger and device negotiate only based on voltage? If not, is there any limit of amp difference for a profile to be accepted?

In conclusion, would it work, and why? :-)

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    \$\begingroup\$ iirc, the negotiation is over voltage, the amps depend on source capability. I would expect the OEM charger to provide a little more than the laptop actually needs. Since such input basically demands an SMPS in the laptop, you can assume one, and with that, the watts are most critical; the battery can be charged "less hard" if too much draw drops the voltage. In short, i think you're over-thinking it, usbpd is supposed to be fool proof. The replacement might not be as fast as OEM, but it should still work. \$\endgroup\$
    – dandavis
    Commented May 5, 2018 at 13:27

2 Answers 2

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The entire intent of Power Delivery specifications is to "provide simple and consistent user experience". The main idea of PD is to provide interoperability and compatibility of all PD-enabled devices, regardless of their manufacturer. The PD devices are supposed to be "un-bundled" from power adapters, this is the whole idea.

Section 10 of PD specifications provides the rules for sources and sinks. The main rule is the advertising of source power capability - PDP, or PD Power, Power Delivery Power. The rest is revolving around, all contracts are based on PDP. So if your sink capability is 65W, but the new source capability is 60 W only, the sink should adapt that if it can. Are there limits? Surely there are, but not to the degree that will affect "user experience". In theory the laptop will likely just charge itself 10% longer, that should be it, since Li-Po batteries can be charged at any current below their maximum.

In short, it should work by definition.

However, the entire specification is the hell of a mess, electrically and logically, it requires a communication stack nearly as complex as USB itself. Some simple examples of PD contract negotiations contain up to 80 steps to conclude. The specification itself evolves to new substantial revision every two years. I somehow doubt that the environment of such complexity can have uniform interpretations between different product development teams (list of contributing engineers count more than 280 persons!!!), and implementations might have "glitches".

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Short answer, the Power Deliver Power Rules state that if a pair of devices cannot agree on a voltage and current, then it should drop down to the next compatible profile. Whether this means same voltage lower current or next voltage down (15v 3a) or simply the usb default of 5V, depends on how well both the supply and the laptop were designed to spec.

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