How USB-C Chargers Decide How Much Power to Send
USB Power Delivery lets a charger and a device agree on the power they need. See how the negotiation works, what the 100 W and 240 W limits mean, and why the cable matters.
In this article
One USB-C charger can power a phone, a tablet and a laptop, and it does not blast the same amount of electricity into each of them. The devices and the charger talk to each other first and agree on how much power to send. The system that makes this possible is called USB Power Delivery, or USB PD. This article explains how the conversation works, what the power limits are, and why the cable you use matters.
What USB Power Delivery does
The USB Implementers Forum, the organisation that looks after the USB standards, says USB Power Delivery provides more flexible power along with data over a single cable. The direction of the power is no longer fixed, so either the host or a peripheral can supply it. Devices can take only the power they need, and they can ask for more when an application requires it.
A menu and an order
The negotiation is simpler than it sounds. The charger offers a menu, and the device places an order. A guide from the electronics company Acroname describes the sequence.
- The charger lists what it can do. First the source checks the capabilities of the USB-C cable. It then sends a message listing its power options. Each option is called a power data object.
- The device chooses. If the device supports USB PD, it replies with a request data object. That request names one of the offered options and gives extra detail about the voltage or current it wants.
- The charger says yes or no. The source answers with an accept or a reject message.
- The power changes. If the request is accepted, the source changes its voltage and current, then sends a power supply ready message. That message tells the device it can draw the power it requested.
Because the device can only choose from the menu the charger offers, a small charger with a short menu cannot supply what a power-hungry laptop asks for. Acroname also notes that the traditional 5 volt output of a USB port counts as one of the fixed options, so the plain 5 volt supply that older devices expect is part of the menu.
How much power can it carry?
The USB Implementers Forum describes the limits introduced in revision 3.1 of the USB PD specification.
- Up to 100 watts was the limit before this revision, using 20 volts and cables rated for 5 amps. (20 volts times 5 amps is 100 watts.)
- Up to 240 watts is supported by revision 3.1 over a full-featured USB Type-C cable and connector.
- New fixed voltages of 28, 36 and 48 volts allow up to 140, 180 and 240 watts respectively.
- An adjustable voltage mode lets a device request intermediate voltages between 15 volts and the charger's maximum fixed voltage.
A phone needs far less than any of these figures. The point of the negotiation is that a device asks only for what it needs.
Why the cable matters
The cable is part of the system. The USB Implementers Forum says that 240 watt operation requires a full-featured USB Type-C cable and connector, and that the USB Type-C specification, release 2.1, defines the requirements for such cables. Acroname's description of the sequence also starts with the source checking the capabilities of the cable. A cable that cannot carry the current for a high-power option limits what the charger and device can agree on.
This is worth remembering when something charges more slowly than expected. The charger, the device and the cable all have to support the power level. The weakest of the three sets the limit.
What certification means
According to the USB Implementers Forum, products must pass compliance testing, and Power Delivery products must be listed on its Integrators List. Companies that want to use the certified USB logos need a current trademark licence agreement with the organisation. When a charger or cable claims USB PD support, the Integrators List is the place where the claim can be checked.
In short
USB-C charging works because the charger and the device agree on terms before any significant power flows. The charger offers options, the device chooses one, the charger confirms, and then the voltage and current change. The result is a single charger that can serve many different devices, as long as the charger, the device and the cable all support the power level you are after.
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