Understanding USB Power Standards
USB Power Overview
Upstream USB connectors supply power at a nominal 5 V DC via the V_BUS pin to downstream USB devices. Over time, USB power standards have evolved significantly, enabling everything from low-power peripherals to high-power devices such as laptops and monitors to be powered through a single USB cable.
Low-Power and High-Power Devices
Before the introduction of USB Power Delivery (USB PD), USB devices relied on predefined power limits based on the USB specification. Traditional USB power is provided at 5 V, with power capacity varying across USB versions and connector types.
Unit Load Concept
USB power distribution follows the concept of a unit load, where devices draw power in discrete steps. A device starts as a low-power device and can negotiate for higher power if the host supports it.
- USB 2.0: A unit load is defined as 100 mA (500 mW), with hosts providing up to 2.5 W.
- USB 3.x: A unit load increases to 150 mA (750 mW), with hosts providing up to 4.5 W.
- USB-C (pre-PD): Allows a unit load of 250 mA (1250 mW) and can support up to 15 W.
Devices that draw no more than one unit load are categorised as low-power devices, while those drawing more are considered high-power devices. Examples include keyboards (low-power) and external hard drives (high-power).
USB Power Standards
The following table summarises power specifications across different USB versions:
| Specification | Max Current | Voltage | Max Power |
|---|---|---|---|
| Low-power device | 100 mA | 5 V | 0.50 W |
| Low-power SuperSpeed / USB 3.x device | 150 mA | 5 V | 0.75 W |
| High-power device | 500 mA | 5 V | 2.5 W |
| High-power SuperSpeed / USB 3.x single-lane device | 900 mA | 5 V | 4.5 W |
| High-power SuperSpeed / USB 3.x dual-lane device | 1.5 A | 5 V | 7.5 W |
| Battery Charging (BC 1.2) | 1.5ā2.4 A | 5 V | 7.5ā12 W |
| USB4 | 1.5 A | 5 V | 7.5 W |
| Type-C current (1.5 A) | 1.5 A | 5 V | 7.5 W |
| Type-C current (3 A) | 3 A | 5 V | 15 W |
| Power Delivery SPR | 5 A | Up to 20 V | 100 W |
| Power Delivery EPR | 5 A | Up to 48 V | 240 W |
Note: Higher power outputs (above 60 W) require electronically marked cables rated for 5 A. Extended Power Range (EPR) cables are mandatory for voltages exceeding 20 V.
Battery Charging (BC) Mode
In Battery Charging (BC) mode, USB ports can deliver higher currents specifically for charging devices. Devices recognise a Dedicated Charging Port (DCP) through a resistance not exceeding 200 Ī© across the D+ and Dā terminals. This enables efficient charging without requiring active data communication.
BC 1.2 and Higher Current Support
The BC 1.2 specification allows devices to draw higher currents for faster charging. While the standard specifies up to 1.5 A, many devices and power adapters support currents of up to 2.4 A at 5 V, providing up to 12 W of power. This increased current capability is particularly common in smartphone and tablet chargers.
USB Power Delivery (USB PD)
USB Power Delivery (USB PD) revolutionised power delivery by allowing devices to negotiate voltage and current dynamically. With USB PD:
- Devices can request up to 240 W (USB PD 3.1 EPR).
- Power flows bidirectionally, allowing devices to send or receive power.
- Voltage can be adjusted dynamically (5V, 9V, 15V, 20V, and up to 48V).
Proprietary USB Power Systems
In addition to standard USB power specifications, proprietary systems such as PoweredUSB exist. Developed in the 1990s, PoweredUSB is used primarily in point-of-sale (POS) systems, such as cash registers, to supply higher currents and voltages beyond standard USB limits.