Offering more power and greater flexibility than conventional USB charging, the USB Power Delivery specification (USB PD) is moving into the mainstream. In this article, we explain some basics of USB PD and show how it enhances the convenience of USB charging.
USB Is Replacing Earlier Modes of Charging
Everywhere we look today, we see USB being used to charge smartphones, digital still cameras, and similar devices. In the previous era, until about ten years ago, keeping one’s devices charged was a tricky and cumbersome endeavor. Most devices came with their own dedicated AC adapters. These bulky “power bricks” could be very different from one another, utilizing different connectors and wiring, and requiring different power levels. It was difficult, if not impossible, to use a single adapter for multiple devices. Users were often searching for misplaced adapters or trying to select the correct one from a large disordered assortment. To prepare for a business trip, you needed to arrange and pack multiple, often heavy adapters—one for each portable device you carried.
Things changed rapidly with the appearance of USB interface. This new serial interface was also capable of delivering power, hence it quickly became the power-supply solution for a wide range of digital devices. Smaller devices, particularly smartphones and small digital still cameras, were now charged through USB adapters. This is because a single adapter could be used with many different devices - it is no longer necessary to carefully store and select among different adapters.
USB PD Raises Power Supply Capacity to 100W
Under USB standards, the USB interface implements both data communication and power supply, which is the main differential point from the earlier standard serial and parallel interfaces. USB 1.1 mandated power delivery of 2.5W (5V, 500mA) and USB 3.0 brought this up to 4.5W (5V, 900mA).
The USB Battery Charging Specification (USB BC), released in August 2007, increased the power further up to 7.5W (5V, 1.5A)—accelerating the marketplace transition from device-specific charging to USB. USB BC 1.2, released in December 2010, established a more robust set of rules that increased general confidence in the use of USB charging. On the other hand, the power capability was only suitable for small devices such as smartphones and digital cameras, but remain insufficient to provide the power (typically 20W or more) required by typical notebook computers and monitors.
USB Power Delivery 1.0 (USB PD 1.0), released in July 2012, corrects this shortcoming by allowing power delivery up to 100W over a single USB cable. Consider, for example, that a 15-inch notebook sinks about 60W, while an A3-grade multifunction printer needs about 30W. Under the new specification, both can now be powered up through a single USB hub based on the condition that both support USB PD. Notice also that the new standard no longer limits voltage to 5V (Fig. 1).
Prior to this new standard, it would not have been possible to power up the notebook through a hub configured as shown below. However, USB PD supports role swapping (explained below), allowing each device to switch between “host” and “device” roles, and between “power provider” and “power consumer” roles. Hence, the notebook PC in the illustration can now receive power from the hub while also serving as the USB host. As USB PD becomes the mainstream, users will no longer need to utilize multiple AC adapters and wires to supply power to a host computer and each of its peripherals. Instead, simple USB cabling will be sufficient to handle both power supply and communication, eliminating tangled wires and multiple AC adapters.
USB Type-C™ Cable Enables Maximum Functionality
USB PD 1.0 (2012) included specifications for compliant connectors and cables. While conventional USB connector shapes were maintained, subtle differences were introduced in the internal wiring. PD-compliant cables remained compatible with common USB ports. However, standard USB cables, if connected to power-capable ports, could still only carry power up to the USB BC limit (while supporting standard data communication at the same time).
The following figure shows the familiar, standard USB connectors. Type A connectors are used on hosts, while Type B are used on printers and other peripherals. Micro-B connectors are used on smartphones, tables, and other small devices.
Note that the connectors in Figure 2 are non-symmetrical; plugs can only fit if inserted right-side up. In addition, the host side will only fit into a host and the peripheral side to a peripheral. The newer connector called USB Type-C™ (Fig. 3), in contrast, is symmetrical (either-side up), and can connect to both hosts and non-hosts. These USB Type-C connectors have more pins than conventional connectors and supports more protocols. A single USB Type-C cable, connecting a computer to a monitor, can carry a video signal while delivering the required power at the same time.
USB PD 1.0, released in 2012, focused on power supply. PD 2.0 was released on 2014, the same year that saw the introduction of USB Type-C connector. PD 2.0 stipulated the use of USB Type-C connectors, and introduced related power rules from USB PD 2.0 rev1.2. Specifications were further revised with the release of PD 3.0 in 2015—which in particular added safety features in particular, such as those related to device authentication.
The power rules introduced in USB PD 2.0 stipulate multiple normative voltages and current, to promote smoother power delivery and consumption among devices. The rules also established five supportable power-supply levels—15W, 27W, 45W, 60W, and 100W—with normative voltage and current determined by the power supply to be supported. For example, a provider capable of 15W must support 5V, 3A; while a provider of 45W must support all the following: 5V 3A; 9V 3A; and 15V 3A. Please note that the main concern of USB PD is power specifications, and can coexist with any relevant USB data communication standard (such as USB 3.1).
Capabilities Unleased by USB Type-C
USB PD 3.0 enables sophisticated use of Type-C power capabilities and multiprotocol functionality. One of the more noteworthy features is role swapping. For example, consider the case where a notebook computer is charging an external battery via USB. If you continue using the computer, the internal battery is likely to eventually run down. Prior to Type C, the only way to feed power back from the external battery would be to remove, reverse, and reconnect the cable; and this would only work if the internal battery can connect through both a Type-A port (to supply power) and a Type-B port (to receive power). However, notebook computers supporting this capability are not readily available in the market.
Role swapping, as supported by Type-C, eliminates this problem. If power conditions are satisfied, the computer and external battery can rapidly swap roles, with the computer changing from provider to consumer, and the battery changing the other way. There is no need to reverse the cable and no need for multiple ports.
Note that role swapping supports not only for charging, but also for data communication. In addition, data communication can continue without interruption even as the power role swap proceeds. In other words, data can continue to flow in a single direction even as the power direction changes.
Built-In Safety Management
The USB PD standard, used together with Type-C cable, greatly reduces the need for AC adapters and allows for much simpler wiring—removing considerable clutter and providing a cleaner digital device environment. However, Type-C cable could supply up to 100W, hence steps must be taken to prevent overheating and similar problems. For this reason, USB PD 3.0 includes an authentication capability that allows interconnected devices, as well as the cables, to identify themselves. If the USB cable in use has not passed USB-IF certification tests, for example, system policy will not allow power flowing through that cable to exceed 2.5W (5V, 0.5A).
USB PD 3.0 can therefore be seen as an important USB extension that contributes greatly to convenience while also ensuring safety. Going forward, the new standard has the potential to address power-supply needs of other types of devices as well, such as bench-top tools and kitchen cooking appliances. The elimination of dedicated AC adapters in favor of easily purchased general-purpose adapters can be a considerable benefit across a wide variety of devices.
USB PD certainly offers potential convenience, but are there any issues or complications that we still need to be concerned about? In our next session, we will look at some of these.
- USB Power Delivery (1) Enhanced Convenience in USB Charging
- USB Power Delivery (2) The Technology - Convenience and Safety
- USB Power Delivery (3) USB PD Safety Implementation USB Type-C™ Authentication (C-AUTH) of Legitimate Devices
- USB Power Delivery (4) Renesas Solutions for USB PD
- USB Power Delivery (5) Faster Development with Renesas Solutions