For years, the electrical relationship across a USB connection was relatively easy to understand. A host normally supplied power and the peripheral consumed it. The amount of power available was limited, and the roles of the devices were closely connected to what each end of the cable was expected to do.
USB Type-C and USB Power Delivery made that relationship much more flexible. Two compatible devices could communicate before settling on how power should move between them. The connection could determine which side would provide power, which side would receive it, and how much electrical power the receiving device could request.
That meant a USB connection was becoming more than a powered data port. It was becoming a negotiated power system.
The Connector No Longer Told the Entire Story
The reversible USB-C connector made it possible to plug the cable in without choosing an orientation, but the connector itself did not automatically mean that every USB-C device supported every available power feature.
USB Type-C established the physical connection and basic methods for identifying the relationship between attached devices. USB Power Delivery added a communication system capable of negotiating more advanced power behavior.
Five Volts Was No Longer the Only Possibility
Traditional USB power was closely associated with a 5-volt supply. That worked well for small peripherals, but it placed obvious limits on how much useful power could be delivered without requiring very large currents.
USB Power Delivery allowed compatible equipment to establish power contracts using voltage and current conditions beyond ordinary USB power. Higher voltage made it practical to transfer substantially more power through the connection.
Compatible USB Power Delivery equipment could operate at power levels reaching 100 watts, using up to 20 volts and 5 amperes when the source, receiving device, and cable supported the required conditions.
This expanded the kinds of equipment that could realistically receive power through USB. The same connector family used by small accessories could also participate in powering much more demanding devices.
The Devices Had to Talk Before Using More Power
A receiving device could not simply connect to a USB-C port and assume that the highest voltage or current was available. The source first had to advertise what it was capable of supplying.
The sink could then request an appropriate operating condition from those available capabilities. Only after the negotiation succeeded would the connection move into the agreed power state.
The Type-C configuration channel helps the attached devices recognize the connection and establish their initial relationship.
A Power Delivery source can tell the receiving device which electrical operating conditions it is prepared to provide.
The receiving device chooses a suitable option from the capabilities offered by the source.
Once the request is accepted and the transition is completed, the devices operate according to the negotiated conditions.
The Cable Became Part of the Decision
Negotiating a high-power connection involved more than the charger and the device being charged. The cable between them also had electrical limits.
A standard USB Type-C cable could carry the current levels required for many applications, but operation above 3 amperes required a cable capable of supporting the higher current. Electronically marked cables could provide information about their capabilities so the system did not have to treat every cable as electrically identical.
Two cables could use USB-C connectors while supporting different electrical capabilities. A high-power contract therefore depended on compatible devices and a cable suitable for the required current.
This was an important change from thinking of a cable as a passive piece of wire whose only meaningful characteristics were its plugs and length.
Power Could Move in Either Direction
USB Power Delivery also weakened the old assumption that one particular kind of device must always supply the power.
A product designed for dual power roles could be capable of acting as either a source or a sink. Which role it used depended on the devices connected and the capabilities implemented by each side.
A portable computer, for example, might receive power when attached to a compatible power adapter. In another situation, a USB port on that same computer might supply power to an attached peripheral.
The device providing data-host functions did not always have to be understood simply as the permanent source of electrical power. USB Type-C and Power Delivery allowed the connection to manage these relationships more independently.
A Power Role Could Even Be Exchanged
Once two Power Delivery devices were communicating, the original source and sink relationship did not necessarily have to remain unchanged for the entire connection.
USB Power Delivery defined mechanisms for changing power roles when the attached equipment supported that behavior. A device that had been receiving power could potentially become the provider, while the previous provider became the receiver.
This capability was particularly useful as computers, docks, displays, batteries, and mobile devices began sharing the same connector type. Their electrical relationships could vary according to the situation rather than being permanently dictated by connector shape.
Data Direction and Power Direction Were Different Questions
The increased flexibility also made USB terminology more important. Supplying power was not exactly the same responsibility as controlling the data connection.
USB Type-C could distinguish between data roles and power roles. This allowed the electrical direction of power to be considered separately from which device was operating in a particular USB data role.
The same physical connection could involve USB data, negotiated power, cable identification, and optional alternate functions. The Type-C configuration channel helped coordinate capabilities that previously would have been associated with more rigid connector arrangements.
The Small Configuration Pins Had a Large Responsibility
The Type-C connector included Configuration Channel connections known as CC pins. Their role went far beyond detecting which way the reversible plug had been inserted.
The configuration channel helped establish attachment and orientation information, identify basic Type-C current capability, and provide the communication path used for USB Power Delivery signaling.
This meant that power negotiation did not require the normal USB data lines to perform the conversation. Power Delivery communication could take place through the configuration channel while the connection managed the electrical state of VBUS.
The Charger Was Becoming a Negotiating Partner
A conventional power adapter can be understood largely by its fixed electrical output. USB Power Delivery introduced a different model. A compatible source could describe what it was able to provide, and the receiving equipment could participate in deciding which available operating condition was appropriate.
The cable was no longer merely carrying power from one fixed end to another. The devices could agree on how that power should be delivered.
That negotiation helped USB expand from powering small peripherals toward supplying equipment with dramatically different electrical requirements. Phones, accessories, portable computers, displays, docks, and other devices could begin sharing a common connection while still asking for different amounts of power.
The most important change was therefore not simply that USB could provide more watts. Power itself had become part of the protocol.
