Wireless Power Consortium standards and TI compatible solutions

　　introduction

　　Wireless charging technology shows great potential in the consumer market. Charging electronic devices without the use of wires not only provides a convenient solution for portable device users, but also allows designers to find more innovative problem-solving methods. Many battery-powered portable devices can benefit from this technology, ranging from mobile phones to electric vehicles .

　　The inductive coupling method enables efficient and universal wireless charging. To facilitate use and benefit both designers and consumers, the Wireless Power Consortium (WPC) has developed a standard. It creates interoperability between power supply devices (power transmitters, charging stations) and power consumption devices (power receivers, portable devices). WPC was founded in 2008 and is composed of companies from various industries in Asia, Europe and the United States, including electronic equipment manufacturers and original equipment manufacturers (OEMs). The WPC standard defines the type of inductive coupling (coil structure) and the communication protocol used by low-power wireless devices . Any device operating under this standard can be paired with any other WPC-compliant device. An important benefit of this method is that it uses these coils to achieve communication between the power transmitter and the power receiver. See Figure 1 for a typical application diagram.

　　Wireless charging WPC standard

　　Under the WPC standard, "low power" for wireless transmission means power consumption of only 0 to 5W. Systems that meet this standard range use inductive coupling between two planar coils to transfer power from the power transmitter to the power receiver. The distance between the two coils is generally 5mm. The output voltage regulation is handled by a global digital control loop, at which time the power receiver communicates with the power transmitter and requires more or less power. The communication is a one-way communication from the power receiver to the power transmitter through backscatter modulation. In backscatter modulation, the power receiver coil is loaded, which changes the current consumption of the power transmitter. We monitor these current changes and demodulate them into the information needed for the two devices to work together.

　　The WPC standard defines three main aspects of the system – the power transmitter that provides the power, the power receiver that consumes the power, and the communication protocol between the two devices. Below, we will look at each of these aspects in detail.

　　Power Transmitter

　　The direction of power transmission is always from the power transmitter to the power receiver. The key circuits of the power transmitter are the primary coil for transmitting power to the power receiver, the control unit to drive the primary coil, and the communication circuit to demodulate the voltage or current of the primary coil. The flexibility of the power transmitter design is limited to provide consistent power and voltage levels to the power receiver.

　　The power receiver presents itself as a compatible device to the power transmitter and also provides configuration information. Once the transmitter starts power transfer, the power receiver sends some error packets to the power transmitter to request more or less power. Once a "terminate power" message is received, or if no packets are received for more than 1.25 seconds, the power transmitter stops providing power. When there is no power transfer, the power transmitter enters a low-power standby mode.

　　The WPC specification allows for both fixed and mobile configurations. A single fixed coil (called Type A1) is a solution supported by TI.

　　The power transmitter (which is typically a flat surface on which the user places the power receiver) is connected to a power source . The coil of a WPC-compliant device acts as a 50% duty-cycle resonant half-bridge with a 19-VDC (±1 V) input. If the power receiver requires more or less power , the coil frequency changes but remains between 110 and 205 kHz, depending on the power demand.

　　Power Receiver

　　The power receiver is typically a portable device. The key circuits of the power receiver are the secondary coil for receiving power from the power transmitter, the rectification circuit for converting AC to DC, the power conditioning circuit for converting the unregulated DC to regulated DC, and the communication circuit for modulating the signal to the secondary coil. The power receiver is responsible for all communications of its identity and power requirements, as the power transmitter is just a "listener".

　　Although we constrained the design of the power transmitter to make it conform to the WPC standard, we have more freedom when designing the power receiver. We can adjust the coil size of the power receiver to meet the volume requirements of the device. With a typical efficiency of 70% for a 5-V, ​​500-mA output, we full-wave rectify the coil voltage of the power receiver. Since the communication between the two devices is unidirectional, WPC chose the power receiver as the "talker". Inductive power transfer works by coupling the magnetic field from the primary to the secondary coil. Uncoupled magnetic lines of force rotate around the primary coil, and there are no losses as long as the magnetic lines of force do not couple parasitic loads (for example, eddy current losses in metal, etc.).

　　Communication Protocol

　　The communication protocol includes analog and digital pinging; identification and configuration, and power transfer. The typical startup sequence that occurs when the Power Receiver is placed on top of the Power Transmitter is as follows:

　　1. A simulated ping from the power transmitter detects the presence of an object.

　　2. The digital ping from the power transmitter is a lengthened version of the analog ping and gives the power receiver time to reply with a signal strength packet. If the signal strength packet is valid, the power transmitter keeps the coil powered and proceeds to the next step.

　　3. During the identification and configuration phase, the power receiver sends some data packets to identify itself and provide configuration and setting information to the power transmitter.

　　4. During the power transmission phase, the power receiver sends control error packets to the power transmitter to increase or decrease power. During normal operation, these packets are sent every approximately 250ms, and every 32ms during large signal changes. In addition, during normal operation, the power transmitter sends a power packet every 5 seconds.

　　5. To terminate power transfer, the Power Receiver sends a "Terminate Charging" message or does not communicate for 1.25 seconds. Either of these events causes the Power Transmitter to enter a low power state.

　　TI's WPC-compliant solutions

　　TI is one of the founding members of WPC and has played an active role in developing robust wireless charging specifications. TI uses three newly developed ICs to provide reliable solutions for both power receivers and power transmitters. The power receiver uses the MSP430bq1010 and bq25046 devices. The power transmitter is based on the bq500110, which supports the A1 type (single coil) structure. Both the receiver and transmitter ICs are interoperable with other WPC-compliant solutions.

　　The MSP430bq1010 in the power receiver handles all logic functions and communications. On-board analog-to-digital converters monitor the voltage levels entering the bq25046 and the current levels flowing out of the bq25046. The bq25046 provides load current information to the MSP430bq1010, which then uses this information to control the operating point of the power transmitter. The bq25046 has a low-current 3.3-V low-dropout regulator (LDO) that powers the MSP430bq1010 and logic circuits, while a larger 5.0-V LDO can provide up to 1A of current to the main output.

　　The power transmitter solution is implemented using the bq500110. This device demodulates and decodes the serial data from the power receiver. The control circuitry first confirms that the power receiver is actually a WPC compliant device and then configures the power transmitter accordingly.

　　TI's BQTESLA100LP EVM kit combines separate transmitter and receiver designs into one kit including the mechanical package. The kit can be used for both IC evaluation and as a design example. The WPC has confirmed that these power transmitter and receiver solutions meet the 1.0 specification. No software is required to operate the EVM, which requires only a 19-V input. The EVM kit outputs 5V at up to 1A. The transmitter EVM includes multiple LED options for visual indication of the power transmitter status. In addition, two buzzer options provide an audible indication of the start of power transfer.

　　in conclusion

　　The WPC standards are a set of guidelines that give manufacturers confidence that their components will work in conjunction with the various other WPC-certified components designed for inductive power transfer, allowing for the development of a wide range of solutions.

　　By Bill Johns,