Three wireless charging reference designs should suit most requirements

4 min read

While consumers in general are in favour of portable electronic devices, they are less happy with having to recharge them by plugging them into a mains adaptor. For many years, this has been inescapable; you either plugged in or your device failed through lack of power. But recently, the old concept of wireless power has been updated and applied to charging mobile phones, tablets and similar equipment.

Wireless power transmission was originally demonstrated by Nikola Tesla in the last few years of the 19th Century. Tesla wasn't without ambition: one of his goals was intercontinental wireless transmission of industrial levels of power. For various reasons, his ideas failed to connect and the concept lapsed until revived in recent times in such small scale applications as charging electric toothbrushes. Now, wireless power is back on the technological agenda. Two industry consortiums have been formed – the Alliance for Wireless Power, bringing together Samsung, Qualcomm and five others, and the Wireless Power Consortium, whose 109 members includes Freescale. And Freescale has recently unveiled three reference designs for wireless charging devices. According to Geoff Lees, general manager of Freescale's industrial and multimarket mcu business: "Wireless charging frequently tops the lists of 'hot' features for future smart phones. Our three new reference designs offer wireless charging solutions for a range of battery powered devices. Once wireless charging is broadly deployed in the public infrastructure, charging could become an afterthought for users." The attraction of wireless charging is underlined by study recently released by IMS Research, which suggests that more than 100million devices with wireless power will be shipped in 2015, with the market worth $5billion by 2016. "Though wireless power is still in its infancy," said Jason de Preaux, research manager at IMS Research, "there is massive potential for use in consumer applications. The availability of specialised components for wireless power is a critical step in the evolution of this market by making the technology easier to implement while also driving down cost." The three reference designs introduced by Freescale address smartphones, tablets and industrial products, with power capacities of 5W, 30W and 120W respectively. In fact, the industrial reference design is a 4 x 30W layout, capable of charging four devices simultaneously. Yves Legrand is marketing manager, consumer and industrial markets, with Freescale. "The 5W design is following the standards layed out by the Wireless Power Consortium," he said. "This is the area which should prove the volume driver for wireless charging." He added the other two designs were similar in terms of implementation. "The 30W charger for tablets is a one coil, one receiver design, while the mcu in the 120W design can control four channels at once; there's more than enough computing power available." While the Wireless Power Consortium has already published a standard for wirelessly charging smartphones under the name of Qi, there is, as yet, no standard for higher power systems. In this respect, Freescale is working ahead of the market, although it is a member of the Consortium's steering group. "We expect the Consortium to publish a medium power standard early in 2013," Legrand commented. "The draft has already reached version 0.5 and we are working towards version 0.9 in Q4 of 2012. After that, the specification will be published and available online. The fact that Freescale has released three reference designs demonstrates the potential for wireless charging and the fact that different product types will need different charging arrangements. "In the tablet design," Legrand noted, "the transmitter and receiver pads are the same size. This ensures that when the tablet is placed on the transmitter pad, it is aligned correctly and the two coils fit together." But the smartphone design offers more flexibility in terms of where the device sits on the charging mat. "The 5W design uses an array of seven coils to provide for more X-Y freedom of placement," Legrand explained. One centre coil is surrounded by another six; each overlapping its neighbours. "One, two or three coils can be activated simultaneously to create a magnetic field that is best centred with the receiver coil, wherever it might be," Legrand added. However, Freescale is not working alone; it has partnered with wireless power specialist Fulton Innovation to build on the latter's eCoupled technology. "Fulton is pleased to work with Freescale as it creates products to enable wireless power devices that support the Qi standard," said Bret Lewis, director of Fulton Innovation. "We'll continue to expand the capabilities of eCoupled technology and work with Freescale to support the changing needs of the consumer electronics market." While the two higher power designs are supplied encapsulated in black plastic, the 5W smartphone design comes in a clear plastic housing, with an obvious nod towards the consumer. How complete are the designs? "We will be providing schematics and a bill of materials for each design," Legrand confirmed, "and we also plan to deliver software, although this is not in place at the moment." He said this should be available within the next three months or so. "When the software is complete, users will be able to download it from the Freescale site or access it via Code Warrior." Legrand says the reference designs can take customers as close to a final product as they wish. "From the starting point of the reference design, it will need little engineering effort to bring a product to market," Legrand explained. But he added that the reference designs are open. "We will not be delivering parts in which the firmware is embedded," he asserted. "Customers can add whatever they want to their design, compile it and download it to the host processor. We believe we should offer as much flexibility as possible in terms of design implementation." One reason for this is to allow wireless charging to be applied in different markets. "We've been talking with customers who want to do in car charging consoles. They want to offer additional features, such as a Bluetooth connection or a link to a roof antenna," said Legrand. "We see demand for extra functionality on top of wireless charging and customers will be able to do this." All three designs are constructed around Freescale's 56800E based digital signal controller family. The single chip parts include a small, efficient dsp core, microcontroller functionality and a flexible set of peripherals. "The part is good at running closed loop algorithms to control power transmission and to control the analogue loop digitally," Legrand explained. Receivers feature the 8bit MC9S08P. In the industrial design, which is aimed at power tools and handheld radios, the receiver manages and converts the incoming power and then transfers the power to the battery by implementing a charging algorithm. Each transmitter channel adjusts its energy transfer independently by responding to commands from the receiver embedded in the battery pack. Through software control, power transfer can be adjusted dynamically. Benefits include the elimination of exposed electrical contacts and the ability to charge various types of batteries on a common platform. According to Freescale, customers can be confident that products based on the reference designs will be compatible with other devices and infrastructure based on the Qi standard. All reference designs should be available during the next three months, Legrand concluded.