Wireless charging eliminates the cable typically required to charge mobile phones, cordless appliances and so on. With a wireless charger, the battery inside any battery-powered appliance can be charged by simply placing the appliance close to a wireless power transmitter or a designated charging station. As a result, the appliance casing can be made completely sealed, even waterproof. Besides the inherent convenience it offers, wireless charging can also greatly enhance reliability, since the charging plug on the side of an appliance can suffer mechanical damage easily, or simply by someone inadvertently plugging in the wrong adapter. The underlying principle behind wireless charging is the well-known Faraday’s law of induced voltage, commonly used in motors and transformers.
Applications of Wireless Battery Charging
- Smart Phones, Portable Media Players, Digital Cameras, Tablets and Wearables: Consumers are asking for easy-to-use solutions, increased freedom of positioning, and shorter charging times. These applications typically require 2 W to 15 W of power. Multi-standard interoperability is preferred. Wireless charging can coexist with NFC (Near Field Communication) and Bluetooth, allowing for very creative solutions. For example, paired phones can charge each other up when placed back-to-back, after they negotiate the appropriate host and client.
- Accessories: Headsets, wireless speakers, mice, keyboards and many other applications can benefit from wireless power transmission. Plugging charging cables into the tiny connectors of ever-shrinking devices is an impediment to robust design. For example, Bluetooth headsets need to be sweat-proof to survive in a gym environment. Only wireless charging can enable that possibility.
- Public Access Charging Terminal: Deployment of charging pads (transmitters) in the public domain requires systems to be safe and secure. But smart charging systems can go well beyond stand-alone charging solutions. They can enable quick network-connectivity and create billable charging stations if desired. Many coffee shops, airport kiosks and hotels support these scenarios. Furniture manufacturers also design-in discreet wireless power transmitters into their end and side tables.
- Computer Systems: Laptops, notebooks, ultra books and tablet PCs are all candidates for wireless charging as either hosts or clients. The possibilities are endless.
- In-Cabin Automotive Applications: A wireless charger is ideal for charging mobile phones and key fobs by placing them either on the dash or the center console of the car, without inconvenient wires going to the cigarette lighter socket. Moreover, since Bluetooth and Wi-fi require authentication to connect phones to car electronics, combining NFC with wireless charging can enable the user to not only charge the phone, but to automatically connect it to the car’s Wi-fi and Bluetooth networks without going through any specific setup process.
- Electric Vehicles: Smart charging stations for EVs (electric vehicles) are also coming up, but require much higher powers. Standards are under development.
- Miscellaneous: Wireless chargers are finding its way into anything with a battery inside it. This includes game and TV remotes, cordless power tools, cordless vacuum cleaners, soap dispensers, hearing aids and even cardiac pacemakers. Wireless chargers are also capable of charging super capacitors (super caps), or any device that is traditionally powered by a low-voltage power cable.
Wireless Charging Standards for Compliant Wireless Power Transmission
There are three major competing wireless charging standards that have emerged in the last few years, including Qi, PMA, and AirfuelTM, as explained further below. All three are essentially based on Faraday’s law of induced voltage, and utilize inductive coils for wireless power transmission, but are defined to function at different frequencies with different control schemes. As such, each wireless power standard offers unique benefits in technology, with different levels of industry support and market share.
In traditional Chinese culture, Qi (pronounced “chee”) is frequently translated as “natural energy”, “life force” or “energy flow”. It is also the name of the industry standard created by the Wireless Power Consortium (WPC). Qi currently supports wireless power transfer of up to 5 W over distances up to 5 mm, but is being quickly extended to deliver up to 15 W, and thereafter to 120 W over much larger distances.
The main purpose behind creating any industry standard is interoperability. For example, any receiver with the Qi logo can be placed on any transmitter pad that displays a Qi logo. Perhaps even on a pad based on a different standard, provided the wireless receiver chip supports multi-standard interoperability. Soon there will be no need to carry proprietary chargers on long journeys anymore.
Whereas the Qi standard works over the approximate frequency range of 100-200 kHz, the PMA (Power Matters Alliance) standard delivers up to 5 W over almost twice that frequency. Both the PMA and Qi standards are actually quite similar, being based on “magnetic induction (“MI”) principles. They do use rather different methods of communication between the wireless power receiver and transmitter.
Recently PMA reached an agreement with A4WP to create a merged standard (now the Airfuel Alliance). This is based on a slightly different principle called “MR”, which stands for magnetic resonance. Early versions of the standard allowed power delivery of 3.5 W and 6.5 W, but recently this has been increased to 50 W. Though MR is also based on the underlying law of induction, it consists of much more loosely coupled, yet more tightly tuned receiver and transmitter coils, with a very high Q (quality factor), to enable resonant transfer at about 7 MHz. As such, Airfuel offers more spatial flexibility with respect to physical placement of transmitter to receiver.
Major Components of Wireless Battery Charging System
- The wireless charging transmitter is powered by an input DC rail of 5 V to 19 V, typically derived from a USB port or an AC/DC power adapter.
- A switched transistor bridge using two or four FETs drives a coil and series capacitor. A resonant frequency is set internally, by means of the series capacitor.
- The transmitter has a coil to transfer power by electromagnetic induction. Some transmitters support multi-coil arrays, driven by separate bridges which are automatically selected to deliver the highest coupled power into the wireless power receiver.
- The induced power is coupled to the wireless power receiver, which has a similar coil to collect the incoming power.
- The receiver rectifies the power by means of diode rectifiers, usually made of FETs for improving the efficiency. It also filters the power using ceramic output capacitors, and then applies it to the battery that needs to be charged, either through a linear stage or a switching regulator.
- The battery inside the portable device receives the power and charges up. The receiver can command the transmitter to adjust the charging current or voltage, and also to stop transmitting power completely when end of charge is indicated.
Primary Design Considerations
Wireless electricity is certainly a complex area, which is what Renesas excels in. When integrating a wireless charging system into a device, one must first decide which wireless power standard is most appropriate for the application. In some cases, Renesas offers dual-mode solutions to maximize interoperability and convenience.
Coil selection is defined by the standards. All major magnetic vendors provide the same standard coils (as defined). An engineer then typically picks coils based on the application, depending on input DC voltage and output requirements. However, the appropriate coil geometry and coil type is usually the exact one used in the evaluation kit of the particular receiver or transmitter IC solution.
Typically, only a few millimeters of space is required inside the receiver to accommodate the coil and associated electronics. Some shielding may be necessary to prevent noise and EMI pickup occurring inside the device. Fuel gauging is usually not integrated in wireless chargers, so this feature may need to be supported separately
Another consideration during integration is that power cannot be transferred across a metal enclosure, since metal effectively shields the receiver from the transmitter. Therefore the systems designer needs to have a relatively flat plastic interface available on the receiver casing, for the wireless charging coils to face each other. Furthermore, the plastic wall cannot be more than a couple millimeters thick, as that can affect the transfer of power too.
Lastly, some engineers realize the need to accurately detect a foreign metallic object if present in the power transfer path to avoid an overheating condition. To address this need, all of Renesas’s solutions feature robust foreign object detection and control circuitry making the solutions compatible with all major safety regulations.
Renesas is an Industry-leader in Wireless Charger Solutions
Renesas has taken a leadership position in wireless charging through its work with the three key standards groups-the WPC, the PMA, and the Alliance for Wireless Power. These relationships enable the company to work closely with other leading innovators to develop solutions addressing the challenges of wireless power delivery.
As a result, Renesas offers a range of WPC, PMA and WPC / PMA (dual-mode) compliant wireless power receiver ICs. The company’s dual-mode receivers deliver 5 W at 5 V, with either a step-down DC-DC switching regulator or a tracking LDO (low-drop regulator).
Renesas also offers several WPC-compliant transmitters, with a variety of input requirements ranging from 19 V to 12 V, or operating off a 5 V adapter or 2 A USB ports. All wireless charging products are supported by powerful software tools and design guides to aid in the design-in process.