Bench Talk

The smartest move made by the authors of the Universal Serial Bus (USB) specification was to ensure the technology could handle power as well as data. Such forward thinking has allowed USB to evolve from a data interface that could deliver limited power to becoming a primary power source capable of deliver up to 100W, which can handle data if the occasion arises.

Without its power delivery capabilities, USB might have been consigned to history by fierce competition from high-bandwidth wireless data transfer technologies like Wi-Fi. (To be fair, the latest version of USB, 3.1, supports up to 10Gbps, compared to Wi-Fi IEEE 802.11n’s 450 Mbps; nonetheless, it remains a rare individual who plugs a USB cable into their smartphone or tablet for anything other than a battery boost.)

Back in 1994, the designers of USB were aiming to make it easier to connect devices to a PC by replacing the multitude of ports that existed at the time. In a flash of inspiration, designers that peripheral devices could eschew batteries if the new connectors and cables carried power as well as data. The computer mice of the time didn’t exactly gobble watts, so the original USB specification limited the maximum current to a modest 100mA at 5V for a power delivery of 500mW.

Apple’s iMac, launched in 1998, was the first desktop computer to incorporate USB ports and helped establish the standard. Apple’s decision prompted an expansion of PC peripherals to scanners, tape drives, microphones, and speakers that needed more power than USB 1.0 could supply. USB 2.0 satisfied that demand by pushing up the maximum current to 500mA, increasing power to 2.5W.

By 2005 mobile phone makers had latched on to USB’s convenience for battery charging, and the idea picked up greater momentum with the European Union’s (EU) drive to introduce a common external power supply (charger) standard that relied heavily on USB electromechanical components. The bad news was that although the feature phones and early smartphones of the time were designed with relatively modest batteries (of around 900mAh capacity), USB 2.0’s maximum power output extended charging the time for a full charge to nearly two hours.

Worse yet, USB hadn’t been specifically designed to cope with battery charging; for example, there was no way of charging the battery of a switched-off peripheral device. And even if a device was powered up, the USB port entered a suspend mode if no data passed for a finite time, reducing the maximum current to a pathetic 2.5mA.

The charging time problem was made worse as fully-fledged smartphones made their market debut. Apple’s original iPhone, for example, housed a 1400mAh Li-ion battery that extended charging time even further. A solution came in the form of an engineering change to USB 2.0, introduced in 2010, dubbed USB Battery Charging (BC) 1.0. USB BC recognized that battery charging was an important application for USB and overcame the charging problems inherent with earlier version of the technology. Later USB BC 1.2 pushed up the maximum current to 5A (increasing power to 25W), allowing a smartphone battery to be charged in well under an hour. (Rather ironically, when charging using maximum current, the USB link couldn’t be used to carry data.)

The USB Implementers Forum (the group responsible for the USB standard) really got into its power stride with another engineering change: USB Power Delivery (PD). USB PD was compatible with the main standard’s USB 2.0 and 3.0 electromechanical components, could coexist with USB BC, and offered 20V and 5A (100W) through the cable—sufficient to charge even the largest portable computer battery without undue delay.

USB PD coincided with the Li-ion batteries used in smartphones, tablets, and portable computers becoming much hardier. While fully-charging a Li-ion battery still took a while because of the fickle nature of the cell’s chemistry, it was now robust enough to accept high current for a good part of the charging cycle. With “quick charge” technology and USB PD, a typical smartphone battery could be charged from 10 to 70 percent in as little as 15 minutes—perfect for a quick boost when time was short. 

Enhancements in USB’s power delivery capabilities also came at a time when the technology’s USB-C electromechanical form factor was rapidly gaining in popularity. Introduced in late 2014, USB-C met the demand for a compact, reversible plug connector suitable for future applications. The 24-pin connector supported USB PD, as well as a USB 2.0 data bus, and USB SuperSpeed.

It’s important to note that USB PD and USB-C are independent; in other words, a device supporting USB-C doesn’t necessarily benefit from the higher power transfer offered by USB PD unless the equipment designer has built it in. However, many manufacturers are taking advantage of the benefits a USB-C port combined with those that USB PD can bring.   

For example—and completing a nice symmetry considering it was the first company to embrace USB for its computers—Apple has equipped its MacBook with just a single USB-C port for wired connectivity and charging, safe in the knowledge that the power provided by USB PD can charge the computer’s large battery in short order. Incidentally, The MacBook’s USB-C cable can also handle up to 5Gbps thanks to the SuperSpeed technology, quite handy if for some reason the portable computer’s 1Gbps Wi-Fi (IEEE 802.11ac) wireless link proves a little tardy.