How USB-C Power Delivery works: the short conversation before your phone charges

Where the conversation happens

A USB-C cable carries more than power. Alongside the power line, known as VBUS, it has configuration channel wires, known as CC. Texas Instruments describes the CC lines as the path for PD data packets: this communication lets the source advertise what power levels it can support, and the sink then requests a supported level.

Source and sink are the standard's words for the side giving power and the side taking it. They are usually the charger and the phone, although USB-IF notes that with PD the direction of power is no longer fixed.

The negotiation, step by step

Microchip's introduction to the standard sets out the sequence, written for version 2.0 of the specification. Each message below is also acknowledged by the other side.

1. The charger lists its optionsIt sends a Source_Capabilities message: a menu of power options, starting with the 5 V default.
2. The device choosesA device that speaks PD replies with a Request message naming the option it prefers.
3. The charger agreesIt checks that the request is valid and sends an Accept message.
4. The charger switchesIt moves its supply to the agreed voltage and current limit, then sends PS_RDY, short for power supply ready. The device then starts drawing power under the agreement.

The cable can be part of this too. A PD charger can ask the cable to identify itself before the negotiation starts, and if the cable does not answer, the charger has to assume it can carry only 3 A. The guide to USB-C cables covers that step.

The power levels

PD now covers two ranges. USB-IF says the original standard was limited to 100 W at 20 V, and that revision 3.1 raised the ceiling to 240 W with new fixed voltages of 28 V, 36 V and 48 V.

PPS and AVS: finer control

Fixed voltages are not the only option. Programmable Power Supply, or PPS, lets a device ask for a voltage in steps of 20 mV, which Texas Instruments describes as useful for fast charging that needs a fine-tuned level. Adjustable Voltage Supply, or AVS, works in 100 mV steps. Texas Instruments says the two have different uses.

This matters when you match a charger to a phone. Google recommends a compatible USB-C PD or PPS charger for Pixel phones. Apple says that to reach its fastest times on iPhone 18 Pro models and iPhone Duo, a comparable third-party adapter has to be 60W-capable, use AVS and support USB-PD 3.2.

Why a phone can charge below the number on the box

• The device asks for lessThe device picks from the menu, so a charger's top figure only applies to a device that requests it.
• The cable limits itWithout a cable rated above 3 A, a charger will not offer more than 3 A.
• The phone slows itselfApple says its phones fast charge to 80% and then slow down, and that software may limit charging above 80% when recommended battery temperatures are exceeded. The fast-charging guide has more on this.