Mark Patrick, Mouser Electronics
In our increasingly ‘wireless’ world we can connect many of our devices wirelessly by various available protocols. The one area where wires are still needed is to charge these devices, but things are starting to change. Wires are inconvenient, they can fray or break – and get left at home while travelling. In today’s ultra-small wearable devices there may not be enough space to place a charging connector and, even if there is, the connector is a point where moisture can get in and damage internal circuitry.
With implantable medical devices, such as defibrillators, replacing a battery would normally require some form of surgery, with the associated costs, inconvenience and potential risk to the patient.
As many of today’s electronic devices operate on very low levels of power, it is becoming easier to replace a physical cable with a non-contact approach such as wireless power transfer (WPT). There are several approaches to this, but the basic principle is that the power supply (power adapter) transmits the power via an antenna to a receiving antenna in the device. In order to deliver the maximum power efficiently, the system should operate in resonance – however, this can be affected by factors such as spacing of the antennae and environmental conditions. As a result, in many cases, power transfer may be less than optimal.
The principles of WPT
The principle that drives WPT is commonly used and present in all transformers. When current passes through a conductor, a magnetic field is generated and conversely, when a conductor is placed in an alternating magnetic field, a current is generated.
By creating two coils and attaching one to an alternating power source, placing the second in reasonably close proximity to the first will cause a current to flow in the second that can be used to charge a device.
As WPT becomes more popular, semiconductor manufacturers are offering devices that will ease the task of designers. ROHM’s BD57015GWL and BD57020MWV are integrated devices for wireless charging comprising a matched transmitter and receiver pair for powering the coils.
The distances between the coils can be increased if resonant frequencies are used, as these give highly efficient transmission. In general, WPT does not use magnetic materials such as ferrite as part of the coils, as this can make them bulky – which is not desirable in small, modern electronic devices. A good-sized air coil can make effective transmission when operating in resonant mode, and can generate a magnetic field up to four times the width of the coil itself.
WPT protocols: Qi standard
For a transmitter and receiver to be able to work together they must be operating in the same way with regard to frequencies and other parameters. Device interoperability is critical for WPT to reach its full potential, and so a number of large manufacturers got together and formed the Wireless Power Consortium (WPC). The WPC developed a popular standard known as ‘Qi’ (pronounced ‘chee’). This operates at frequencies in the range 100kHz to 200kHz, with definitions in the standard for the base station that provides the power, and the receiver which is located in the device to be charged.
In the standard itself are comprehensive details for the coils and their physical and electrical properties. The standard also addresses the requirements for materials that can be used in the case of the transmitter and receiver. By following the standard closely, WPT manufacturers can ensure their devices will interoperate with any other devices that are compliant with the standard.
The Qi standard allows for the transmitter to contain one or more transmitting coils, while there is always just a single coil placed in the receiving device. When placed closely together the coils effectively form a transformer with an air gap that is capable of transferring power. Once resonance is achieved, this arrangement can reach performance that is similar to that achieved with a galvanic conductor.
While many semiconductor manufacturers now produce chipsets for WPT, they all perform the basic functions required by the Qi standard. Many of the chipsets available are certified by the WPC to be ‘Qi-compliant’, thereby giving designers confidence that their design will interoperate with similar compliant products from other manufacturers.
One such Qi-compliant solution is STMicroelectronics’ STWLC WPT receiver series. These devices support direct charging and are able to charge almost any battery type, including popular chemistries such as Li-ion or Li-polymer. With miniature wearable devices being one of the main applications, the STWLC04 is specifically designed with this application in mind, allowing up to 1W to be transferred – easily sufficient for these highly efficient devices. The STWLC devices include onboard digital control as well as an I2C bus for communication with the main microcontroller (MCU).
The magnetic fields can cause eddy currents in any metallic object placed close to the transmitter, causing heat to be generated – sometimes to a dangerous level. Sophisticated devices such as the STWLC incorporate a function for foreign object detection (FOD) that allows the transmitter to reduce or shut off power if foreign objects are detected, thereby improving safety. Even in these low-power systems, efficiency is an important consideration, and these controllers keep power usage to a low level until the transmitter and receiver are in close proximity.
The associated STWBC-MC 15W wireless transmitter for battery charging applications is capable of powering and controlling several charging coils in parallel. The modern device meets the requirements of the Qi MP-A15 extended power profile (EPP) and is compliant with the latest iteration of the Qi specification (Qi 1.2.4).
The NXQ1TXH5 from semiconductor manufacturer NXP is able to intelligently detect other Qi-compliant devices through an incorporated analogue ping function. The device is a fully integrated Qi-compliant (A5, A11, A12 and A16 versions) transmitter that offers direct coil driving via a full-bridge power stage running at 5V.
As many consumers have multiple devices that need to be charged at the same time, a technology to address this is very valuable, and this is exactly what the LinkCharge from Semtech does. Its TS51223 is a low-cost space-saving solution for applications such as low-power wearable devices. It is capable of charging multiple devices at the same time, while having sufficient electrical performance to give the devices spatial freedom.
Design support tools
Evaluation boards and reference designs are an invaluable source of support for designers, especially those new to WPC and the associated technology. Texas Instruments is well known for the wide range of boards it offers, and its TIDA-00881 enables designers to add Qi-compliant charging to these boards.
Based on a Qi-compatible 19mm loop, the TIDA-00881 allows a 3.6V battery to be charged by simply placing the coil on any compatible charger. The Launchpad gets its 3.3V from the battery and also supplies the 5V necessary for additional modules. The option of using a coin-cell LIR2032 to provide currents up to 50mA is also included in the TI package.
IDT is another manufacturer that supports the prototyping phase with hardware platforms. Its WP3W-RK supports wireless charging applications up to 3.0W, achieving efficiencies up to 80%. The comprehensive WP3W-RK reference kit includes the wireless transmitter (P9235A-R-EVK) and corresponding receiver (P9027LP-R-EVK).
Summary
Wireless charging is not only highly convenient, but is becoming necessary as devices such as wearables and hearables become too small to contain charging ports. It also goes a long way towards facilitating the waterproofing that is a popular feature on expensive devices such as smartphones.
Longer term, WPC is expected to gain significant traction as standards converge and infrastructure grows in public spaces such as airports, hotels and other facilities, as well as homes and offices. While the technology is currently focused on low-power applications, as it develops it is anticipated that higher-powered devices will also benefit from ‘going wireless’.
