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Accelerating development of USB-C power systems

May 10, 2018

rushil-May 10, 2018

The introduction by the USB Implementers’ Forum (UBS-IF) of the most recent version 3.0 of the USB Power Delivery (USB PD) standard promises to spark a new wave of product development. We can expect to see a whole new range of power adapters, power banks, and chargers for consumer devices such as laptop computers, tablets and mobile phones.

USB PD 3.0 over a USB Type-C connector dramatically increases the power rating of the USB interface from a 7.5W maximum to 100W, via a maximum 20V/5A supply. Fast battery charging and the supply of power-hungry systems such as all-in-one PCs over USB Type-C have been made possible by the introduction of USB PD 3.0.

The ratification of the new USB PD 3.0 standard followed the introduction of the USB Type-C interface standard, which provided a new connector/receptacle design. The USB-C plug is reversible, making it more convenient for users. It is also smaller than the familiar USB Type-A connector and capable of carrying HDMI and DisplayPort traffic as well as USB traffic (see Figure 1).

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Fig. 1: The standard USB-C interface can replace multiple other interfaces in consumer devices. (Source: Cypress Semiconductor)

The USB-C connector is also compatible with USB PD 3.0. Now power adapter manufacturers have the potential to produce a power unit or charger that supplies as much as 100W over a small, reversible connector that has a standard form factor and protocol. This means that, in time, it will become familiar to consumers and well understood by them, and should be interoperable with any USB-C device.

It seems like a winning proposition, and indeed analysts suggest that the market for the key controller component in USB PD devices with a USB-C interface is set to grow at a compound annual growth rate of 89% between 2016 and 2021.

Producing a power device that realizes all the benefits outlined above, however, is more challenging than system designers might assume, especially in a compact unit and at a competitive bill-of-materials cost.

First, it might be expected that the existence of the USB-C and USB PD standards would guarantee interoperability between different manufacturers’ products, helping to maintain consumer faith in the USB interface as a channel for charging and powering devices of many kinds. In fact, the USB standard itself and standards which co-exist alongside UBS, such as the Qualcomm Quick Charge technology for fast charging of mobile devices, are subject to regular revisions and updates to take account of emerging user requirements and new technical capabilities. The latest revision of the Quick Charge technology is version 4.0. That means there have already been three major revisions since v1.0 was released – and v4.0 is not likely to be the last.

At the same time, the USB PD 3.0 standard is itself more than a power specification that sets limits for the input voltage and current to a power-consuming device. More of the standard is devoted to the communications protocols by which connected devices establish their identity, the functions which they are capable of performing, and the role they are going to play as either power provider or power consumer in any given session. The standard also includes provisions for programmable power supplies that can produce a variable output power for use with multiple end devices.  

In summary, then, the engineer responsible for designing a USB-C power adaptor or charger with USB PD 3.0 capability will often be required to:

  • achieve conformance to the USB PD 3.0 specifications

  • provide scope in the design for regular updates to take account of changes in specifications

  • implement additional features, including a programmable power supply and support for Quick Charge technology.

Drawbacks of discrete implementations

A USB-C PD 3.0 system that can meet the requirements above has certain functional requirements, some of which will be implemented in hardware and some in software.

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