Power over USB

The Universal Serial Bus (USB) is designed to provide power as well as data communications. It can support 127 devices and has developed over the years from 1.5 Mb/s to 480 Mb/s. Currently (2014) a normal USB 2.0 has a low current limit of 100 mA and a high current limit of 500 mA at 5 V although a Thunderbolt connector is able to deliver 2 A at 5 V. Several years ago USB 3.0 was introduced which increased the theoretical bit rate by 10x’s to 5 Gb/s and raised the low current limit to 150 mA and the high current limit to 900 mA. More recently the USB 3.0 Promoter Group has announced a new specification which will allow USB 3.0 compatible sockets and cables to deliver up to 100 W by changing the voltage from 5 V to 20 V and increasing the current to 5 A.

USB also has limitations on the maximum cable length and in most cases this is restricted to 3m. With special cables this can be increased to over 6 m and with a USB bridge this can be extended to over 30 m. With such a large increase in power a single USB 3 SuperSpeed hub could be charging a laptop, powering an external hard drive as well as a second monitor. In future, TV manufacturers could include high-powered USB hubs to power nearby AV and communication equipment thereby reducing the need for multiple wall outlets.

Both USB and PoE (power over ethernet) bring power and communication functions together by using the data exchange capabilities to negotiate power need with the device in a number of incremental stages. This level of power control allows a common supply to deliver a range of different power profiles best suited to whichever device is connected to that particular port. For example, in the future a USB 3 might provide 60 W (20 V at 3 A) for a laptop or 5 to 10 W (5 V at 1 or 2 A) for an average smart phone.

The benefit of both of these systems is that they are well developed solutions that are supported by appropriate standards, research and testing. Components are readily available and system design expertise is relatively common.

In terms of network distribution efficiency both PoE and USB are still subject to conversion and distribution losses as any other network. Whilst a modern PoE injector may have a very efficient power supply, power is still converted from 230 V AC to approximately 50 V DC, distributed to the point-of-load and then converted again to what’s required by the end device. In the case of PoE the voltage is higher than USB potentially reducing its distribution losses although conductor sizes are relatively small.

In the domestic environment USB ports are much more common and could in the future be used not just for connecting between devices, for example a laptop and a peripheral, but also as an additional low voltage power outlet complimenting the existing 230 V wall outlet. This may reduce the need for individual power adapters and bulky 230 V plugs and sockets although where this is the case, USB sockets are still likely to be associated with a central device such as a TV or computer.

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This article was created by --BRE. It was taken from The future of electricity in domestic buildings, a review, by Andrew Williams, published in November 2014.

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