USB Charging Explained
Originally designed as a data interface with only limited power delivery capabilities, Universal Serial Bus (USB) has become a primary method of powering and charging mobile devices. In this article, we cover the capabilities of the current crop of USB chargers.
There are four methods of distributing power between USB-enabled Hosts and Devices.
1 Default Power
The USB 2.0 specification allows Hosts to deliver 5V at 500 mA, for a total power output of 2.5 watts. USB 3.0 and 3.1 allow 5V at 900 mA (4.5W). Certified Hosts and Devices must limit their power delivery and consumption to these "default" power levels.
|Specifications||Max. Voltage||Max. Current||Max. Power|
|USB 3.0 / USB3.1||5V||900mA||4.5W|
|USB Battery Charging (BC) 1.2||5V||1.5A||7.5W|
|USB-C Current Mode (non-PD)||5V||3A||15W|
|USB-C / Power Delivery (PD)||20V||5A||100W|
2 USB Battery Charging Specification
The USB Battery Charging Specification allows devices to draw current in excess of the default power limits. The first version of the specification (BC 1.0) was released in 2007, followed by version 1.1 in 2009, and the current, BC 1.2, in 2010.
BC 1.2 introduced three types of downstream ports:
- Standard Downstream Port (SDP) - power is limited to the default power of the applicable USB specification (USB 2.0 or USB 3.x)
- Dedicated Charging Ports (DCP) - delivers power only (no data) up to 1.5A
- Charging Downstream Port (CDP) - capable of delivering both data and power up to 1.5A.
3 USB-C® Current Mode
USB-C ports are capable of delivering more power than the default power levels of USB 2.0 or USB 3.x. Downstream Facing Ports (DFPs) signal to Upstream Facing Ports (UFPs) their ability to deliver a higher current.
4 Power Delivery Protocol
USB Power Delivery (USB-PD) refers to the protocol that allows a "power provider," a cable and a "power consumer" to agree on the current and voltage levels. Because power can flow in either direction, the role of provider and consumer can change at any time.
This smart charging protocol enables devices to negotiate voltage, current and direction of power and data flow over the USB cable. Negotiations are governed by power rules and offer a range of voltage and current configurations. For example, a phone needing 18W might negotiate 9V and 3A from the power source.
USB-PD is used by Apple® iPhone®, iPad®, MacBook Pro®, Google Pixel™, and other smartphones and portable devices.
Do All USB-C Cables Support PD?
Yes. All USB-C to USB-C cables have the Configuration Channel (CC) required to support PD communication, but don't necessarily support the full range of voltage/current levels specified by USB-PD. The current on a passive USB-C cable is limited to 3A so the most it can support is 3A x 20V = 60W. The length of a passive cable is also limited to 4 meters (13 ft). Cables that support power delivery greater than 3A are electronically marked and rated up to 100W (5A x 20V).
Do all USB-C ports have the same functionality?
No. Even though all USB-C ports look the same, the features they support can vary widely. For example, a USB-C port on a wall charger will only charge devices. Ports on laptops may vary in the level of power they supply and the speed of data transmission. Some laptop USB-C ports support data only, power only or a combination of the two so check your machine's specifications before buying peripherals.
What is Fast Role Swap?
The USB Power Delivery 2.0 specification includes Fast Role Swap (FRS), a feature that ensures connected devices can continue to function when power is interrupted. As the name implies, Fast Role Swap allows a provider of power (a Source) to quickly and automatically become a consumer of power (a Sink). For example, let's say a hub, acting as a Source, is providing power to a laptop and an external hard disk drive (HDD) when it is unexpectedly disconnected from the AC outlet. The hub signals to the other devices that it needs to swap roles and become a Sink. The laptop, detecting the Fast Swap signal from the hub, switches its role from Sink to Source and begins supplying power from its battery to the hub and (indirectly) the HHD.
Other Fast Charging Standards
To make a device charge more quickly, most manufacturers will either increase the current (amps) or the voltage using one of the fast charging standards (e.g. USB-PD, Qualcomm Quick Charge™) or a proprietary fast charging protocol like Samsung® Adaptive Fast Charging or Motorola™ Rapid Charging.
Qualcomm Quick Charge™ (QC)
Phones and tablets that use the Qualcomm Snapdragon chipset can use the company's Quick Charge standard. The current version, Quick Charge 4.0, requires a USB-C cable and a device that uses the Snapdragon Series 8 chip. Older devices can use Quick Charge 2.0 and 3.0.
In many ways, the QC standard is similar to USB-PD, particularly in its use of power negotiation protocols and variable voltage selection. But while Quick Charge is limited to phones and tablets that use Qualcomm's System on a Chip (SoC), USB-PD is more of an industry standard adopted by a wide swath of the market, including laptops.
What's Next for USB Charging?
As more devices integrate USB Type-C® ports for data transfer, expect to see manufacturers to take advantage of its power and Alt Mode capabilities too. For example, Apple's MacBook Air® ships with a 30W USB-C Power Adapter. Google's Pixelbook has a USB-C port on either side so you can charge and connect to an external monitor at the same time.
Currently, USB is still a confusing maze of specifications, data transfer rates, wattages and video resolutions. But as USB-C, PD 3.0 and USB4 converge in a new crop of laptops and peripherals due early 2021, a single port world may be just around the corner.
If I have a GaN charger, can I leave my laptop AC adapter at home?
You absolutely can. If your laptop has a USB-C port, you can use your GaN wall charger to power and recharge the laptop and leave the bulky AC adapter at home. Small, lightweight GaN wall chargers can also provide power for your phone and tablet.