Understanding wire-less charging
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.
Figure 1a: Principle of a wireless power transmission transmitter-receiver pair. Click image to enlarge.
By using two sets of electrodes, or plates, power is transferred by electrostatic induction. Effectively for each, the charger or “transmitter”, and the portable device or “receiver”, are used to create a longitudinal quasi-electrostatic coupling among properly dimensioned metal surfaces forming capacitors. One electrode, the driven or active one, is smaller than the other and has a larger voltage applied to it, while the passive longer one is subjected to a low voltage. Normally of course, the amounts of energy involved in capacitive transfer are minute, as is the area of the electrodes. So, in order to accommodate the power levels required to charge consumer devices (for example from 5 to 25 W) the size of the electrodes and the voltage level at coupling needs to be larger depending on the exact configuration.
Figure 2a shows an example block diagram of a charge transfer charger approach using a receiver and transmitter module that has recently been developed by Murata. The module approach aids the quick development of a wire-free charging capability allowing the engineer to concentrate on the coupling zone electrode design. The amount of energy you can transfer by electrostatic means is directly proportional to the frequency used. So, driving the electrode pairs with a higher frequency provides the design options to handle higher power. However, there are regulatory limits for the frequency and electric field strengths to consider. You have, in effect, a configuration that can be a very effective antenna structure, so EMI aspects will naturally limit the design flexibility. A proper design is needed to retain the emission between the couplings and minimise any radiation to the outside. This is where some work is needed to understand and determine the correct electrode dimensions, their design, operating voltage, power level, optimum operating frequency and the overall geometric constrains. Typically, the ideal frequency range is between 200 kHz to 1 MHz and the voltage levels are in the range of 800 V to 1.52 kV at the active coupling area.
Figure 2b shows the voltage step-up and step-down as part of the transmit-to-receive capacitive coupling for a 10 Watt charger meeting EMI compliance requirements. By using modules, the Murata concept is to allow the set maker to use it as a black-box approach for easy integration for the transmitter and receiver. The transmitter design covers the link to the power source, the control of the wireless power transfer and the active coupling electrodes in any form factor according to the positioning flexibility targets. On the receiver side the battery interface determines the design to receive the power appropriately through the down-conversion module from the active coupling electrodes area. As there is a wide variety of batteries used in portable devices, a standardisation of the battery interface would represent a big step forward towards a very easy design, also to think about even more challenging concepts like faster charging speed. Already, thanks largely due to the continued pressure from the European Commission, the micro-USB 5V charging interface is becoming a standard for all mobile phones in Europe.
One of the key advantages of using the quasi-electrostatic transfer over the inductive method is that the placement of the device to be charged on the charging base (or mat) is much less critical. The high efficiency of energy transfer, typically about 80% for any design, even cabled chargers is maintained relatively flat as the receiver is moved away from the transmitting source which results in a very high positioning tolerance possibility by design in x-y (surface), whereas z (height) remains the most challenging design parameter.
Also, using a flat square or rectangular table-top mat, or a near vertical docking stand allows placement of the charging device in any orientation rather than a precise engagement. In addition, since the main active receiving electrode can be constructed of a simple thin copper film, in the order of several micrometres thick embedded in plastic cover materials, it is much easier to incorporate it into the housing of the consumer device than a power inductor.
As mentioned early, heat transfer close to batteries can be a serious consideration for the inductive approach. However, since the electric field as an energy carrier in the capacitive coupling configuration does not show any large current flow. The absence of such direct current flow results in removing the heat problem from the coupling area: All ohmic losses are concentrated in the modules, the driver electronics, but not in the coupling area. So the set manufacturer gets much more design flexibility for the integration of tiny modules into the set while keeping design freedom for the coupling design, power level and positioning tolerance to reach.
Considering all those challenges, the capacitive coupling wire-less power transfer offers possibilities to achieve even higher power transfer, greater positioning flexibility, the ability to meet EMC compliance, and finally, to provide greater design flexibility to manufacturers. Together these factors present a strong case to encourage manufacturers to integrate the capability of charging portable devices without any cable connection.
The author, Markus Huschens is New Business Research Manager at Murata Electronics Europe – www.murata.eu.