BUILD BETTER DEVICES: USB POWER DELIVERY EXPLAINED

How USB-PD (USB Power Delivery) Works 

USB-PD operates through a sophisticated communication system between devices and chargers. We can break it down into 4 phases.

1. Device Detection Phase

• When a device is connected to a USB-PD charger, the system first detects the connection at the hardware level.
• No power is delivered during this initial detection.

2. Communication Phase

• The device and charger establish a dedicated communication channel.
• This channel is separate from the power delivery lines.
• The device sends its identity and capabilities to the charger.
• The charger responds with its available power options.

3. Negotiation Phase

• The device requests its preferred power configuration.
• The charger evaluates the request against its capabilities.
• Both devices agree on the optimal power level.
• If no agreement is reached, they default to a safe power level (5V, 500mA).

4. Power Delivery Phase

• Once negotiation is complete, power delivery begins.
• The charger supplies power at the negotiated voltage and current.
• The device monitors and controls the power reception.
• Power delivery continues until either device requests a change or the connection is broken.

Understanding the CC Line

This negotiation happens on the CC line (Configuration Channel), which also serves as a continuous power management signal that indicates the current level after initial negotiation. It uses specific voltage levels to indicate power delivery status:

• Different voltage levels on the CC line represent different power states
• The line remains active during power delivery
• Changes in CC line voltage can trigger immediate power adjustments

Role Negotiation

In USB Power Delivery (USB-PD), role negotiation allows devices to dynamically change their power roles (source/sink) and data roles (downstream/upstream). This is useful for dual-role power (DRP) devices like smartphones, laptops, and docking stations that can act as either a power provider (source) or a power consumer (sink).

Types of Roles in USB-PD

Power Roles

• Source → Provides power.
• Sink → Consumes power.
• Dual-Role Power (DRP) → Can switch between Source and Sink.

Data Roles

• Downstream Facing Port (DFP) → Acts as the host (provides data to a peripheral).
• Upstream Facing Port (UFP) → Acts as the peripheral (receives data from a host).
• Dual-Role Data (DRD) → Can switch between DFP and UFP.

Although source and sink refer to the power roles, and DFP and UFP refer to the data roles, you see source → DFP and sink → UFP used interchangeably in documentation. However, these roles are actually independent.

Role Negotiation Process in USB-PD

Initial Role Determination

1. When two USB-C devices connect, the CC (Configuration Channel) lines determine the initial power and data roles:

• The device with the pull-up resistor (Rp) on CC becomes the Source (provides power).
• The device with the pull-down resistor (Rd) on CC becomes the Sink (consumes power).
• The device that detects an Rd on its CC line acts as DFP (host), while the other becomes UFP (device).

2. If both devices are Dual-Role Power (DRP), a randomized process decides the initial roles.

Power Role Swap (PRS)

• A device can request to switch roles using the Power Role Swap (PRS) command.
• The current Source sends a Power Role Swap Request message.
• The current Sink responds with Accept or Reject:
  • If Accepted, the devices swap roles.
  • If Rejected, the original roles remain.

Example:

• A phone (Sink) charging from a laptop (Source) can request to become the Source and supply power back to the laptop.

Data Role Swap (DRS)

• The data role can be swapped using the Data Role Swap (DRS) command.
• The current DFP sends a Data Role Swap Request.
• The UFP either Accepts or Rejects the request.
• If accepted, the devices switch roles:
  • The DFP becomes UFP (host → device).
  • The UFP becomes DFP (device → host).

Example:

• A laptop (DFP) connected to a dock (UFP) may want to switch roles so the dock becomes the DFP and controls peripherals.

VCONN Swap

• If an Electronically Marked Cable Assembly (EMCA) is present, one device must provide VCONN power to the cable’s circuitry.
• A device can send a VCONN Swap request to change which device provides VCONN.

USB Cables

• USB-C unmarked cables: These do not support USB-PD and typically only populate one of the CC lines (which are not technically “CC” lines in USB-C 2.0). This allows the host to detect the orientation of the cable (for D+/D-).
• Electronically Marked (E-Marker) Cables: These cables contain an E-Marker chip that communicates the cable’s capabilities, such as power delivery and data transfer rates.
  • Power Transmission: E-Marker cables can support up to 100W of power transmission, ensuring safe and efficient charging for high-power devices.
  • Identification: E-Marker cables are essential for devices that require higher power levels, such as laptops and high-performance peripherals.

You can only have 1 e-marked cable per connection, and cable extenders are not supported for two main reasons.

First, USB-C and USB-PD Rely on fixed cable length and integrity

• In USB-C, the CC lines are critical for detecting cable orientation, power negotiation, and communication between the Source and Sink.
• USB-PD expects a direct connection between the devices (or an e-marked cable), and adding an extender disrupts the expected topology.

Second, extenders Break USB-C Connection Logic

Problem 1: CC Line Disruption

• A standard passive USB-C cable only passes one CC line.
• If you add an extension cable, the devices may not detect orientation properly.
• If both CC lines are passed, it could confuse role detection and disrupt negotiation.

Problem 2: E-Marker Detection Fails

• E-marked cables have an internal chip that is queried during power negotiation.
• If an extension cable is added, the Source may fail to detect the e-marker, leading to power limits (e.g., 60W max instead of 100W).

Problem 3: Increased Resistance and Voltage Drop

• USB-PD supports up to 5A at 20V (100W), which requires carefully controlled resistance in the cable.
• An extender increases resistance, causing voltage drops and potential overheating (and therefore fire hazard).
• This is why 5A cables require an e-marker to ensure proper power transmission—and extenders aren’t accounted for in this system.

This can be a challenge for testing or test systems where you need intermediate connections to simply be passed through. However, some test cables are available for compliance testing. ($200 for a single 1-foot cable!) In addition, there are bulkhead connectors with flying leads which pass through all the signals.

Conclusion: USB Power Delivery Explained

USB Power Delivery (USB-PD) has significantly improved the way modern devices charge and communicate power needs—making fast, flexible, and intelligent charging possible over a single USB-C connection. Whether you’re powering a smartphone, laptop, or docking station, USB-PD dynamically negotiates the optimal voltage and current to deliver safe and efficient energy transfer. The nuances of cable integrity, role negotiation, and the communication protocol can be a challenge during development, but this complexity is what allows us to power such a wide variety of devices. Using USB-PD allows us to use the same cables for many different devices, reducing electronic waste and saving money for consumers.