Standardized Approach Feeds Wireless Power

At the turn of the 20th Century, Nikola Tesla contributed much in the fields of alternating current (AC) and electromagnetism. Tesla had a vision of transmitting electrical signals through the airas Marconi had demonstrated using Tesla's conceptsas well as wire-free electrical power. The transmitted wireless electricity could be used to power local streetlamps and households. It also would provide power to ships at sea and cities anywhere in the world.

Tesla was able to demonstrate his vision on a small scale at his Colorado laboratory. Due to withdrawal of support from his financial backers, however, he was never able to complete a grand-scale demonstration from his Long Island, NY laboratory. Today, various forms of Tesla's work with AC and wireless power have been implemented in applications ranging from induction motors, transformers, and power-plant generation to the wireless-charged electric toothbrush.

The ability to power an electronic device without the use of wires provides a convenient solution for portable device users (see sidebar, Energy Is Increasingly Drawn From Surroundings"). Among the common applications for wireless power include energizing mobile/smartphones, digital cameras, portable medical equipment, headsets, industrial equipment, and power tools. A practical solution to address efficiency concerns over large distances is to keep the wireless power transmission to a short, controlled, near-field environment that limits radiated emissions, such as with inductive power transfer.

Unfortunately, initial wireless power solutions have been implemented with applications that are unique to those solutionsand therefore not widely compatible to other wireless power systems.

To address this problem, the Wireless Power Consortium (WPC; www.wirelesspowerconsortium.com) was formed. It now consists of 91 companies that cut across many industries (OEMs, retailers, magnetics, silicon suppliers, etc.). It is open to all companies with the goal of creating an international standard similar to other industry standards, such as Universal Serial Bus (USB), Bluetooth, and Wi-Fi. The WPC standard enables interoperability between the device providing power (power transmitter, charging station) and the device accepting the power (power receiver, portable device). Devices meeting the WPC standard requirements use the "Qi" logo to denote compliance, thus assuring interoperability with other WPC-compliant devices.

Products that are Qi-compatible are a significant evolution from the old toothbrush application. That device was limited in terms of power transfer. The toothbrush-type device was placed on the charging stand for many hours with milliwatts of power transfer at low efficiency. It was then used for only a few minutes. For today's Qi-compliant system, the power-delivery capability can be 5 W at the point-of-load (POL) with the capability for much higher efficiencies. And higher wireless power standards are possible in the future. In fact, they are in WPC committee discussions today.

There are several key features to the WPC standard. One is the transmitter and receiver coil dual roles of power transfer. In addition, the closed-loop communications' control-signal protocol provides the following: compatible device recognition, efficient closed-loop operation of the inductive power transfer, efficiency optimization, and discontinuation of power for safety and end-of-charge. Other WPC standard benefits include recommended coupling requirements, configurations, and interoperability testing.

System Overview

A typical application for a WPC-compliant wireless-powered system is a mobile device with rechargeable battery. As shown in Figure 1, the mobile device contains Qi-compliant power-receiver electronics along with a wireless power-receiver coil. During power transfer, the receiver is placed on a charging pad or the wireless power transmitter. The charging pad detects the presence of the mobile device and establishes communications with the receiver. It then begins power transfer. That transfer ends when the charge is complete or power is no longer needed. Power transfer and control take place seamlessly without assistance from the user.

The transmitter charging pad contains the DC-to-AC converter, which drives a transmitter coil, along with communications circuits to receive commands from the mobile WPC-compliant receiver device. Power transfer between transmitter and receiver devices is through magnetic induction between the planar transmitter and receiver coils.

Wireless Power System

While other transmitter and receiver configurations are possible, this article addresses a WPC-compliant solution. Here, the transmitter solution uses a single A1-type coil with magnetic attractor driven by a half h-bridge circuit used in the bqTESLA bq51013 receiver and bq500210 transmitter IC solutions.

Power transmitter: Typically, the wireless power transmitter is the fixed or non-portable portion of the system operating from line power (Fig. 1). The key circuits and operations of the power transmitter are the receiver recognition, communications demodulation, coil power driver, intelligent power control, operation status reporting, and safety operations. For a WPC low-power-transmitter solution, a switching power supply drives the transmitter coil. It converts the DC input voltage (typically either +19 or +5 VDC) power to an AC voltage between 110 and 205 kHz. The coils of a WPC-compliant device operate as a resonant half-bridge with a 50% duty cycle. Two transmitter-coil drive circuits are recommended in the WPC specification: a half-bridge and a full-bridge. The transmitter adjusts the transferred power level by changing the operating frequency between 110 and 205 kHz with a lower frequency for more transferred power (Fig. 2).

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Power receiver: Usually, the power receiver is a portable device. The key circuits of the power receiver are the secondary coil, rectification, voltage conditioning, and communications circuits. The receiver's rectifier output voltage is monitored by the receiver. It generates signals to control the modulation circuit to pass coded information from the receiver to the transmitter. The coded information is organized into information packets, which have preamble, header, message, and checksum bytes. Per the WPC specification, information packets can be related to identification, configuration, control error, rectified power, charge status, and end-of-power transfer information. A low-dropout (LDO) regulator buffers the unregulated DC voltage from the rectification circuit into a regulated receiver output voltage. The coil voltage at the power receiver is full-wave rectified, with typical efficiency of 70% at 5 V and 500 mA.

Magnetics

WPC systems use inductive coupling between two planar coils to transfer power from the power transmitter to power receiver. Figure 3 shows an example of the receiver and transmitter coil where the coil is a fixed position A1 type with magnetic attractor. Coil assemblies for the transmitter and receiver are a combination of coil, shield, and magnet/attractor. Several magnetic suppliers provide WPC-compliant magnetic solutions for transmitters and custom solutions for receivers. Magnetic solutions can be purchased as complete assemblies or as individual components.

Owing to the WPC standard, the transmitter-side coil design allows consistent field strength to be applied to the receiver coil. It enables reliable operation over a number of interoperable devices. The receiver-coil assembly design is very flexible with a few restrictions, which permits the shape to be adapted easily to the mobile-device form factor (see table).

A WPC-compliant, A1-type transmitter uses a magnet in the center of the transmitter coil to assist with the alignment of the receiver coil over the transmitter coil for maximum power-transfer efficiency. The receiver coil is required to have a magnetic attractor to assist in this alignment.

The distance from the top of the transmitter coil and transmitter interface surface is 2.0 to 2.5 mm. Again, the receiver coil is more flexible. But distance between the receiver interface surface and receiver coil should not exceed 2.5 mm. Therefore, the maximum distance between the two coils is 5.0 mm (Fig. 4).

Communications System

The transmitter sends out an analog "ping" (query signal) approximately three times a second to determine if a receiver is present. When the power transmitter detects the presence of a device on the transmitter interface surface, it "wakes up" and begins interrogating the object placed on the interface area. If the receiver properly identifies itself as a WPC-compliant device, power transfer is initiated. When more or less power is required by the power receiver, the receiver sends communications packets to the power transmitter to request more or less power.

This communication continues throughout the power transfer until one of the following occurs: the receiver transmits an "end power" message or the transmitter does not detect any communications packets for more than 1.25 s. When no power is being transmitted, the power transmitter enters low-standby-power mode. No inductive field is emitted. Figure 5 shows a typical block diagram.

Communications from the receiver to the transmitter relies on backscatter modulation. The receiver modulates its load, which is reflected from the receiver coil to the transmitter coil. The voltage at the transmitter coil shows that the modulation and specialized circuits in the transmitter are then used to demodulate the data.

There are two options for modulating the receiver coil. One choice is resistive while the other is capacitive. Resistive loads, switched-in at the data rate, add a small load to the rectified voltage. This additional load is reflected through the receiver coil to the transmitter.

When capacitors are used for modulation, they are connected before the rectifier and deliver an AC load. An advantage of using capacitors is a reduction in power dissipation and improved efficiency. The capacitors, when switched at the data rate, shift the receiver's resonant frequency. The shift is reflected to the transmitter. With both techniques, modulation appears on rectified voltage and is present at the input to power conditioning.

The power receiver uses communications packets to control the transmitter power. Each packet contains a preamble, header, message, and checksum. Clock frequency for the communications data is 2 kHz 4%.

The communication sequence is divided into four parts: ping, identification, configuration, and power transfer. During the normal power-transfer phase, the receiver sends error packets every 250 ms. During large signal changes, the packets are sent every 32 ms or about 31 times a second. Power packets are sent every five seconds during normal operation, thereby telling the transmitter how much power it is receiving. To end the power transfer, an "end power" message is sent.

When a power receiver is placed on a power transmitter, the system steps through a predefined sequence:

1. Analog ping from power transmitter detects the presence of an object.
2. Digital ping (a longer version of the analog ping) gives the power receiver time to reply with a signal-strength packet. If the signal-strength packet is valid, the power transmitter continues to provide power to the coil and proceeds to the next phase.
3. Identification and configuration packets identify the power receiver and send configuration and setup information to the power transmitter.
4. In the power-transfer phase, the power receiver controls the power-transmitter operating point. It sends control error packets every 250 ms during normal operation and every 32 ms during large signal changes. The transmitter sends a power packet every five seconds during this mode.
5. To stop power transfer, the power receiver sends an end-power transfer packet, which commands the transmitter to enter its low-power state and remain in that state.

As Tesla's dream of wireless power continues to be embraced, the need for designing wireless power devices to an interoperable industry standard grows. The WPC standard fills that need.

Bill Johns
Applications Engineer
bqTESLA Wireless Power
Texas Instruments, Inc.

Michael Knight
Power Management Product Marketing Manager
TI's bqTESLA Products
Texas Instruments, Inc.
P.O. Box 660199
Dallas, TX 75266-0199
(972) 995-2011