How Power Delivery Works on USB-C: A Comprehensive Guide

Preface

USB Power Delivery (USB PD) is a USB protocol that enables voltage and current regulation for USB devices, providing faster charging and maintaining signal integrity through a power negotiation algorithm based on the needs of each device. USB Power Delivery is designed to enable the maximum functionality of USB by providing more flexible power delivery with data by a single cable. The USB Power Delivery (PD) defines the mechanisms for pairs of directly Attached Ports to Negotiate voltage, current, and/or direction of power flow over the USB cable. It uses the USB Type-C® connector's CC wire as the communications channel. The PD mechanisms operate independently of and supersede other USB methods defined in [USB 2.0], [USB 3.2], [USBBC 1.2] and [USB Type-C 2.4].

USB Power Delivery also defines sideband mechanisms used for configuration management of USB Type-C devices and cables. Using Structured Vendor Defined Messages (Structured VDMs), PD facilitates the discovery of device and cable features and performance. Structured VDMs are also used to enter/exit some Active Modes, either USB-based (e.g., USB4® Mode) or USB Type-C Alternate Modes. Alternate Modes are associated with Standard or Vendor IDs (SVIDs) and can be standard either.

USB Power Delivery (USB PD) policies are structured into two levels:

• System Policy
  The System Policy is managed by the optional System Policy Manager (SPM), which oversees power distribution across multiple devices via USB communication using UCSI. It enables advanced power management but is not required for basic functionality, such as charging.
• Local Policy
  Local Policy is enforced on individual devices by the Device Policy Manager (DPM), which controls power ports and implements shared power algorithms.

The Policy Engine operates at the port level, handling power negotiation and executing Local Policies. Together, these components ensure efficient and scalable power delivery, whether the SPM is present.

The Protocol Layer forms the Messages used to communicate information between a Port Pair. It is responsible for forming Capabilities Messages, requests, and acknowledgments. Additionally, it forms Messages used to swap roles and maintain presence.

The Physical Layer (PHY Layer) defines the Signaling technology for USB Power Delivery such as defining the electrical requirements and parameters of the PHY Layer required for interoperability between PDUSB Devices. The USB PD PHY Layer consists of a pair of transmitters and receivers that communicate across a single signal wire (CC). All communication is half duplex. The PHY Layer practices Collision Avoidance to minimize communication errors on the channel

Power Delivery is used in various devices such as smartphones, hubs, laptops, mobile phones, chargers, etc.

Key Features of USB Power Delivery:

1. Increased Power Levels: USB PD can deliver up to 100 watts (20V and 5A) in Standard Power Range (SPR) or 240W (48V and 5A) in Extended Power Range (EPR).
2. Bidirectional Power: Power can flow in either direction. A device can act as a power source (like a charger) or a power sink (like a laptop).
3. Dynamic Power Negotiation: Devices can determine how much power is needed and even can adjust power to the needs of each device.

How USB PD Works:

1. Cable and Connector: USB Type-C connectors are used, which can handle higher power and have a reversible design.
2. Power Role Configuration: Whenever two USB PD devices connect, so according to the device’s situation, one of them will be as a Source for providing power and another one will be as a Sink.
3. Communication: The devices use the USB PD protocol to send messages over the CC lines (cc1, cc2) to introduce their specifications to each other. These messages include information about supported power levels, voltage, current, and whether they can be a power source or sink.
4. Negotiation Process:
   • Initial Connection: Upon connection, devices exchange basic capabilities.
   • Power Request: The sink device sends a request amount of power based on its charging needs. For example, a mobile phone requires 5 volts and 3 amps to charge, so it sends a message requesting that specific amount of power.
   • Response: The source responds with the power it can provide. The power supply device provides the required power based on the requested amount of power.
5. Power Delivery: Once the negotiation is complete, the source starts delivering power at the agreed voltage and current levels and initiate explicit a contract.
6. Monitor and Adjust: The devices can continue to communicate and adjust power delivery as needed during the charging process, ensuring efficiency and safety.

USB Power Delivery over USB Type-C significantly improves the capabilities of power transfer between devices, making it a standard for modern electronic devices.

Conclusion

USB Power Delivery (USB PD) over USB-C is a transformative technology that enhances power management and data transfer capabilities. By allowing devices to negotiate power requirements dynamically, USB PD ensures efficient, safe, and flexible charging and operation. With support for higher power levels, bidirectional power flow, and streamlined communication, USB PD has become a vital standard for modern electronic devices, promoting interoperability and simplifying connectivity with a single, versatile cable.