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Power Train

Understanding wire-less charging

May 11, 2012 | Markus Huschens, Murata Electronics Europe | 222902249
Understanding wire-less charging The increasing popularity of battery-powered consumer electronic devices such as portable media players, smartphones and tablets has led to a host of different chargers and a tangle of wires littering the home. The concept of wirelessly charging the devices, i.e. without any direct-wired connection, has been around for a while but is now rapidly gaining interest to make it more flexible and useable. But what are the different techniques available, and what are the design challenges an engineer needs to deal with?
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Being able to remove the need for charger cables and wirelessly charge your consumer device has many attractions. Perhaps we should be more specific and say that the goal here is to provide a way of charging an application’s battery by other innovative means other than by wires or connectors.

Already popular in a number of consumer devices such as an electric toothbrush, the approach has been dominated by an inductive method based on Maxwell’s law. The variation in a magnetic field from a coil induces a current in another coupled coil. While the inductive approach using magnetic fields is suitable for a number of small applications like the one above, the use of it in more modern consumer electronics such as tablets and smartphones creates several engineering design challenges. As the power to feed the battery increases, the related efficiency or the flexibility in positioning the coupling coil also arises. The main concern with an inductive approach is how to control EMI generated by the signal creating or “transmitting” the energy, using an inductive field, to the “receiving” device. The receiving device then converts magnetic energy into electric energy so that it can charge the battery. WiFi, Bluetooth, NFC, Cellular systems, and FM radio are just some of the many wireless voice and data connectivity methods that could suffer interference from such electro-magnetic fields.

Another concern of course is to keep the efficiency of the power transmission as high as possible, even under such challenging constraints of increased power levels and wider positioning tolerance. Over the past few years there have been many new ideas to implement an inductive charging technology, yet progress to avoid the impact of EMI has not been as forthcoming as hoped since immense efforts are necessary to achieve EMI compliance.

Recently this challenge has gained further momentum thanks mainly due to the efforts for the Wireless Power Consortium (WPC). The WPC is an initiative from the Consumer Electronics America (CEA) organisation in the US. Their remit has been to encourage further research and development into making Wireless Power marketable so that it is available to a larger consumer audience.

Another well-known constraint for the inductive approach is the need to precisely align the charger and charged device. This is best illustrated with the electric toothbrush example. The charger base has a small tower rising from its base on which you place the toothbrush to be charged. Using this approach the two coils mate perfectly ensuring the transfer of magnetic power. Any slight difference of alignment however results in completely losing the ability to transfer power. This will definitively not be so easy with other devices such as smartphones or tablets requiring slightly higher power levels. Finally there is the constraint of how to deal with the electrical heat loss. With higher wattage chargers, the level of heat loss increases. This is particularly an issue for highly temperature sensitive Li-Ion batteries and could introduce component stress in today’s highly compact form factor consumer electronics.

An alternative approach to magnetic-field wire-less charging, is by applying an analogy of Maxwell’s laws of electric field by using a capacitor configuration instead. This concept, adopted by Murata and now being widely introduced into new designs, uses a quasi-static electric field to transfer energy through a capacitor that is formed by electrodes belonging to physically separate devices. Bringing the devices closely together forms a capacitor array that can be used to transfer energy. Figure 1a shows the basis of this approach.

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