USB Charging


Almost all portable or low power electronic devices are either charged or powered by USB ports. This trends has allowed power capabilities of USB to evolve quickly as higher wattage and fast charging are needed.


In USB power delivery system, there are two power ports: source and sink.

In USB type C port configuration, power source is associated with upstream facing port UFP and power sink associated with down-streaming facing port (DFP).

Dual role port (DRP) in USB type C port that supports either UFP or DFP functionalities interchangeably.

  • Note: DRP is powerful if implemented because it allows a USB device to act as either a power source or sink. For example, a smart phone acts as a power sink during charging and acts as a power source when it is used source power to drive USB headphone.

Type of USB Charging

Passive Charger

A USB power sink device detects and identify the correct power source type by monitoring positive (D+) and negative (D-) terminals of USB data line. It is mainly used with USB Type A interconnect.

    • Note: USB Type A D+/D- are configured on each USB power source with a different of resistor network to advertise charger capability

USB Power Specification:

  • USB1.0 5V/100mA

  • USB2.0 5V//500mA

  • USB3.0 5V/900mA

  • BC 1.2 (an extension of USB3.0) 5V/1.5A

USB Power Source Types

Dedicated charging port (DCP)

Follows BC 1.2 spec with maximum 1.5A output current. An example of DCP charger is USB Wall charger.

    • Note: this type of port does not enumerate with a device that connects to it, and it's job is to act as "dedicated" power supply.

    • SDP and CDP is capable USB enumeration for any devices connected to it, the reason is that SDP and CDP are host/hub ports found on PC/laptop or USB hub.

Standard charging port (SDP)

Follows USB2.0 spec with maximum 500mA current output is typically found in computer USB host ports/ USB hub.

Charging downstream port (CDP)

Extension with USB2.0/3.0 current limit allowing up 1.5A output current associated with USB host port found on computer or USB Hub.

Active Charger

A USB power sink device uses digital communication to do power negotiation and establish power delivery with a power source device.

In USB Type C interconnect, channel configuration (CC) pin are used for power communication.

It follows USB Charger Power Delivery (PD) 3.0 Standard

  • Note: USB charger power delivery standard should coexists with BC 1.2 spec and is solely designed to work with USB Type C devices; however this does not mean all type c device supports USB PD charging protocol.

USB PD 2.0/3.0 Power Specification main features:

  • Up to max 100W 20V/5A

  • supports 5V/3A, 9V/3A, 12V/3A, 15V/3A, and 20V/3A.

  • support programmable power supply (PPS) allowing more efficient charging hence reducing charger heat dissipation.

Example Implementation

Phone Type C Charger 18W

    • voltage current configurations: 5V/3A, 9V/3A.

Laptop Type C Charger 60W

    • voltage current configurations: 5V/3A, 9V/3A, 12V/3A, 15V/3A, 20V/3A

Summary and Conclusion:

  • Passive USB charger uses resistor network to advertise power capabilities

  • Active USB charger uses digital communication interface to advertise power capabilities

  • DCP/SDP/CDP are three common types USB power source type.

USB charging is prevalent and widely adapted as the to go charging port. As a system design engineer, understanding USB charger type and select the right capabilities is essential to design portable and low power devices.

Further Reading

"USB Charging Introduction"

"What is fast charging?"

"USB Charger (USB Power Delivery)"