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I have looked at various USB power delivery (PD) adapters that supply a certain number of watts and are powered by 230V mains voltage. Always, they seem to have either a USB-C port or a full USB-C cable permanently connected to the adapter. I have never seen a USB power delivery adapter with USB-A port, so that one could use it with USB-A to USB-C cable.

Does a USB-A connector even have the pins required for negotiating power delivery (PD)? I know it can supply at least 5V voltage, but I'm not interested in that; I'm interested in power delivery standard with much higher voltages.

Wikipedia says a PD aware cable needs to be used. Can a USB-A to USB-C cable even be PD aware? If it can, is it possible in practice to find such PD aware cables in the marketplace?

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If one were to look long and hard enough for the specifications on USB Power Delivery version 1.0 then you will see a means by which a compliant source and sink could negotiate up to 20 volts at 5 amps on a USB-A connector. There were no extra pins for this negotiation, the communications was by BFSK modulated signals sent on the Vbus and Vgnd lines.

Use of USB-PD on USB-A was short lived, and I am not aware of seeing any devices that operated in this manner. One issue I heard was that the communication was not well isolated and so one device could have this power negotiation induced onto an adjacent port and that device would see it's voltage change even though it made no such request. This could damage hardware. Another problem was that the cables would have to be certified for this higher voltages and current, meaning the cables would have to be built for this as well have a means for the USB controller to know that the cables met the spec.

USB-PD 1.0 was not required for USB-A but it was required for USB-C, and that likely affected adoption. USB-C had a different means to handle PD negotiation, the Configuration Channel, which used a different protocol called BMC. USB-C could optionally support BFSK on Vbus for communication with USB-A ports which meant that to assure compatibility the cables would have to translate the PD negotiation, or do no PD negotiation at all and supply only the default 5 volts.

At the same time USB-PD 1.0 was introduced so where a couple competing systems for negotiating power over USB-A. USB-PD was a bit late to the game and so QuickCharge and others took over that space. I believe that they simply negotiated the power over the USB D+/D- lines. This violated the USB spec, or perhaps not, and because it was in a grey area the USB specification enforcers could not, or would not, do much about it.

Not many people cared much of USB-PD until version 2.0 and it was required for USB-C. USB-PD v1.0 was technically supported but not much people even cared. PD v1.0 was limited to 2 amps in certain configurations and so that's why you will find older 10 watt USB-A power supplies, which used USB-BC as an alternate means to negotiate power. Anyone wanting more power seems to have moved on to USB-PD v2.0 or some third party protocol like QC or VOOC. When USB-PD v3.0 came along all features from USB-PD v1.0 were gone and third parties started to incorporate at least some support of USB-PD into their specs.

The means by which these protocols negotiated the power transfer was by communicating over the D+/D- data lines, rather than the USB-PD way, use of the data lines for power negotiation meant the cables were not capable of transferring data. Some used the D+/D- lines to carry power as well.

The latest USB-C spec, USB-PD spec, and perhaps other USB specs, makes use of a third party power transfer system a violation of the spec. That's not going to make it all stop because there's plenty of old gear out there, and not all manufacturers care much about complying with the USB spec. Those that do care about compatibility will support USB-PD fully as well as portions of the third party spec as a selling point and/or means for a smooth transition from old to new.

The direct answer is that the pins are there but USB-PD no longer uses them. Some USB-A to USB-C cables have circuitry in them to convert third party power from USB-A to USB-PD for providing power to USB-C devices. Personally I'd stay away from them but for people with Android devices this may be something of a necessary evil.

Explaining this is in a way a history lesson of USB-PD. I know I didn't get every detail right, but the highlights are there. If dates on when these things happened concern you then maybe someone can come along to fill that in. As I see it the wired charging boils down to the old Apple and USB-BC on "dumb" USB-A chargers, some "smart" USB-A chargers use some mix of USB-PD and their own protocol, and USB-C chargers are going to be USB-BC. A new development is USB-PD PPS, or Programmable Power Supply, which means they can vary the output voltage by small increments. With PPS there's plenty of room to support third party charging which may or may not technically violate the USB spec.

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From a physical connector rating standpoint, it is possible to use a USB-A port for delivering more than the standard 5V - the quick charger for my phone (HTC brand) does just that. It can provide 5V @ 3A, 9V @1 .7A or 12V @ 1.25A when attached to the phone using a USB-A to USB-C cable.

However, that particular charger is not using USB power-delivery to do so. Instead it uses Qualcomm® Quick Charge 3 (Note 1), which is a proprietary technology that is not officially part of the USB specification. It is not the same as the USB power delivery specification.

There was a power delivery extension to USB 2.0, however that was essentially just allowing USB 2.0 ports to provide a higher current (>2A), but still not exceeding 5V. That was possible based on using resistor values on the data lines to select power delivery current levels.

As far as I'm aware using USB-A connectors to deliver (or select) higher voltages is not possible under the official USB standard, power delivery or otherwise. To do so requires some other proprietary technology that deviates from the standard.


Note 1: Other such proprietary standards exist.

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  • \$\begingroup\$ I have seen power supplies with USB-A ports that appear to use USB-PD to negotiate power to USB-C devices which indicates to me that the USB-A to USB-C cable has some electronics in it to perform some kind of translation. This means USB-PD works but only if used with a non-standard cable, the cable that came with the charger or a cable that is from the same manufacturer. The USB-C end of the cable has USB-PD but the USB-A end does not. \$\endgroup\$
    – MacGuffin
    Commented Oct 6, 2022 at 22:52

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