Wireless Charging

A 10 to 15 watt extension is in the works to enable rapid phone charging and charging of consumer tablets. In development is a medium-power specification that will deliver up to 120 watts for charging larger devices such as laptop computers and power tools.

For applications requiring greater separation between the charger and receiver, such as through a desk, there is magnetic resonance technology. Though magnetic induction and magnetic resonance are both based on similar principles involving coils, AC currents, and magnetic fields, a magnetic resonance implementation offers several advantages including, 1) a range of several inches or more, 2) charging is possible through an obstruction between the charger and receiver, such as a magazine, 3) multiple devices on a charging pad can charge at the same time, and 4) flexibility in orientation and positioning of the receiving devices on the pad (Qi can achieve expanded free positioning of the receiver using a three coil transmit array). The Alliance for Wireless Power, or A4WP, champions WiPower™, the first wireless charging standard that is based on magnetic resonance. WiPower was approved in January, 2013, and devices using this standard are projected to be available in late 2014. The WPC is also working on Qi-compliant magnetic resonance technology to enable longer range charging.

Wireless power technology is still evolving, and it remains to be seen which direction the technology will go, what influences proprietary charging approaches will exert, whether standards will merge, whether manufacturers will offer multi-mode solutions that support more than one standard, and ultimately which solutions will be embraced by consumers.

Silicon is naturally available in two categories: transmitter solutions, and receiver solutions.

Look for Qi-compliant wireless power transmitter ICs that maximize power transfer efficiency while offering features such as:

• Advanced foreign object detection (FOD) and parasitic metal object detection (PMOD) methods, with safety controls to protect the system from excessive power loss and heat in the event coins or keys, for example, interfere with charging
• Over-current, over-voltage, and over-temperature protection
• Operation from a USB port or 5-V adapter
• Intelligent power management for standby and power-down modes
• Status indicators (standby, power transfer both visual and audio, charge complete, fault)
• Compact, single-chip solutions

Available silicon for the receiver side includes:

• AC-DC rectification ICs, to convert the AC voltage generated by the transmitter to DC voltage to power the mobile system or charge the battery. These provide design flexibility, but must be paired with additional circuitry to complete the power receiver.
• Highly-integrated power receiver ICs that output a conditioned 5V DC. Functionality includes rectification, output voltage conditioning with an LDO, and digital control.
• Highly-integrated power receivers with built-in battery charger. These single-chip implementations enable the smallest total solution size.

Given that multiple wireless charging protocols exist today, it may be advantageous to consider multi-mode devices that support both Qi and other formats, such as proprietary protocols, for even broader application.

Give careful consideration to the matching capacitor, as it is critical to achieving the desired resonance frequency and proper system operation. The total resonant tank capacitance value is defined by the Qi specification and is a requirement, not a guideline. Choose capacitors of high quality dielectric and sufficient voltage rating in order to pass Qi-compliance certification.

The reason for the limited design freedom on the power transmitter side is to ensure interoperability of the charging pad with the largest number of mobile devices.

Due to greater variance in the design requirements for mobile devices (for example, a smart phone is very different from a wireless headset), the Qi standard provides minimal guidelines for the receiver parameters to enable greatest design flexibility. Designers can choose from catalog products that meet the design criteria, or create a custom receiver coil. Application notes such as Texas Instruments’ SLYT479 "Designing a Qi-compliant receiver coil for wireless power systems" help guide designers through the design process.

Close alignment of the transmitter and receiver coils is critical to achieving efficient power transfer. The closer, the better, however the Qi standard allows a maximum separation of 40 millimeters.

Adequate shielding at the bottom face of the transmitter coil and the top face of the secondary coil helps to direct the magnetic field to the coupled zone to maximize efficiency. Shielding also contains the fields to avoid interfering with the device or adjacent systems. It also limits the exposure of users to the magnetic field, although at Qi’s low operating frequencies (100 to 205 kHz range) the magnetic waves are not harmful to humans. The two basic methods for shielding at low frequencies are diversion of the magnetic flux with high-permeability materials such as ferrite or copper, and counteracting the magnetic flux by generating opposing flux according to Faraday’s law. A combination of materials and techniques can be used to achieve the desired shielding effectiveness. The Qi specification establishes the dimension, placement, and materials of the shielding on the power transmitter side. Shielding is recommended, but optional on the receiver side.

Cost and thickness are key drivers when selecting the appropriate electromagnetic shield.