Fast charging is a convenient way to keep your phone topped up throughout the day, but it comes in a variety of flavours. Qualcomm has its Quick Charge technology in a number of handsets. OPPO and OnePlus offer varieties of VOOC. MediaTek has Pump Express. The list goes on. But there’s also USB Power Delivery, designed to be a more universal charging standard that allows a range of devices, not just smartphones, to charge quickly over a USB connection.

Google’s new Pixel 2 and Pixel 2 XL are the latest handsets to sport the USB Power Delivery specification. The phone will work with 27 watt charging accessories, but it’s capped at just 18 watts. To help make heads and tails of this increasingly common standard, here’s the important facts you’ll want to know.

USB Power Delivery and Type-C

Before looking at the Power Delivery specification, there’s a little confusion to be cleared up with the latest USB Type-C standard. Even though new phones sporting Power Delivery have a Type-C port, the two aren’t the same when it comes to charging power.

Starting at the bottom, all USB 2.0 devices provide a minimum of 500 mA at 5 volts. This increases to 900 mA with a USB 3.1 port. USB Type-C ports can be configured in fast charging modes at either 1.5 or 3.0 A for more power when connected to other Type-C devices or chargers. USB Power Delivery is a separate specification that can work across USB 2.0 and 3.0 ports and cables, but you’ll still need a USB Type-C port too due to the communication pins used by the standard.

It is up to device manufacturers to support USB Power Delivery in addition to the mandatory charging modes of the basic connection type. That means that both the device and charger have to be compatible with the specification to maximize charging speeds, just like with Quick Charge, Pump Express, and other standards.

A closer look at the specification

One of the primary motivations behind the development of USB Power Delivery was to produce a single charging standard that could be used across all USB devices. The goal is to reduce e-waste in the future by removing the need for chargers with various ratings for different products. The standard is now on its 3.0 revision, but is backwards compatible with 2.0 products.

As well as supplying up to 100W, USB PD is designed to reduce future e-waste by offering an interoperable charging standard for all USB devices.

Regardless of the version number, the specification caters to much higher power ratings than typical smartphone-based charging designs, as it’s intended to power hard drives, printers, and displays, as well as larger portable electronics like laptops. The specification is rated up to 100 watts, but most smartphone products will use the 10 to 18 watt modes. Making use of the highest power modes also requires specially rated USB cables, as standard cables are only rated for 7.5W at most.

For comparison, Qualcomm’s Quick Charge supports up to 36W, VOOC is 20W, and most others are either 15 or 18W.

In addition to faster charging, this USB standard enables both host and peripheral devices to provide power. You could use your phone to power a hard drive over the same port used for charging, for example. To figure out how much power to transfer to any device, the source and host need to be able to communicate their requirements. This is done by sending small packets of data between the host and source across the USB communication (CC) lines, although this is complicated somewhat by the potential for dual roles for charging and data sources.

In this case, a Dual-Role-Data (DRD) port can act as a Downstream Facing Port (DFP) to advertize its supported power levels to the power supply. This starts at the maximum support powered, but will scale down to other modes if the power supply can’t handle the requirement. If the device then switches to become a power supply for a peripheral, the port switches to an Upstream Facing Port (UFP) to listen for the charging modes before sending the correct amount of power.

Handshaking for power

USB Power Delivery 2.0 introduced some changes to the way that power ratings between devices are handled, making the standard more flexible than before. The old set of Power Profiles were removed in PD 2.0 and replaced by Power Rules that allow for a wider range of current negotiations. The USB 3.1 specification adopted Power Profiles instead, but is only rated up to 15W rather than 100W.

PD 3.0 made some tweaks to enhance power delivery, but the Power Rules are the same as PD 2.0 products. Rules are split into four target power categories; 7.5W, >15W, >27W, and >45W; each of which offers a range of voltage and current configurations. Sources supplying more than 15 watts offer voltages of 5 and 9 volts, those supplying more than 27 watts offer 5, 9 and 15 volts, and those supplying more than 45 watts offer 5, 9, 15 and 20 volts. The maximum 100W power supply is achieved with 20V and up to 5A, although all of the other modes cap out at 3A, depending on the required power.

Digikey

For example, a smartphone requesting 10W of power would negotiate a 5V, 2A configuration with the source. A laptop needing 30W of power may request 15V and 2A from its charger. In the case of the Google Pixel 2, the smartphone can still communicate with 27W capable chargers. However the phone, acting as the sink, will negotiate a power arrangement with the source that supplies the 18W of power that the phone can handle.

If two devices fail to communicate a suitable Power Rule, USB Power Delivery will default to the next power option supported by the relevant USB protocol to prevent damage. See the table above for how precedence is given to the various possible USB charging standards.

With more devices switching over to USB charging, Power Delivery adoption is growing.

Closing thoughts

USB Power Delivery is a useful specification, it enables a wide range of devices to share chargers and even power each other seamlessly. That’s a welcome change in a world where each new smartphone brings with it a new charger. However consumers won’t feel these benefits until they own several USB PD capable devices.

In the here and now, USB Power Delivery is a bonus luxury rather than a major shift in charging capabilities. Smartphones aren’t going to use the very high power charging modes, and so far it’s not used to charge phones any faster than other standards. It also adds additional complexities to the USB circuitry and development costs, yet USB Type-C’s 15 watt capabilities alone are enough to charge up a typical smartphone battery considerably in just 30 minutes.

Smartphones don’t need to deploy Power Delivery mode to charge any quicker, but support for universal USB fast charging makes this a nice future-proofed option to have.

Robert Triggs

Lead Technical Writer covering SoCs, displays, cameras, and everything in between. In his spare moments you’ll find him building audio gadgets.