Wireless charging technology shows great potential in the consumer market. Charging Electronic devices without wires not only provides a convenient solution for portable device users, but also enables designers to find more innovative solutions to their problems. Many battery-operated portable devices can benefit from this technology, from cell phones to electric vehicles.
The inductive coupling method enables efficient and versatile wireless charging. The Wireless Power Consortium (WPC) has developed a standard for ease of use and benefits for both designers and consumers. Interoperability is created between powered devices (power transmitters, charging stations) and powered devices (power receivers, portable devices). Founded in 2008, WPC is comprised 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 that works under this standard can be paired with any other WPC-compatible device. An important benefit of this approach is that it utilizes these coils to enable communication between the power transmitter and power receiver. See Figure 1 for a typical application diagram.
Wireless charging WPC standard
Under the WPC standard, the “low power consumption” of wireless transmission means that the power consumption is only 0-5W. Systems that meet this standard range use inductive coupling between two planar coils to transfer power from a power transmitter to a power receiver. The distance between the two coils is generally 5mm. Output voltage regulation is handled by a global digital control loop, where the power receiver communicates with the power transmitter and requires more or less power consumption. 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, changing the current draw 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 uses the power, and the communication protocol between the two devices. Below, we will describe these three aspects in detail.
The direction of power transfer 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 for driving the primary coil, and the communication circuit for demodulating the voltage or current of the primary coil. We have limited the flexibility of the power transmitter design 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, requesting more or less power. The power transmitter stops supplying power upon receipt of a “terminate power” message, or if no data packets are received for more than 1.25 seconds. When no power is being delivered, the power transmitter enters a low-power standby mode.
The WPC specification allows the use of fixed and mobile configurations. A single fixed coil (called Type A1) is a TI supported solution.
The power transmitter (which is usually a flat user on which the power receiver is placed) is connected to the power source. The WPC compliant device coil 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 205kHz, depending on the power requirements.
The power receiver is usually a portable device. The key circuits of the power receiver are the secondary coil for receiving power from the power transmitter, the rectifier circuit for converting AC to DC, the power conditioning circuit for converting unregulated DC to regulated DC, and A communication circuit for modulating a signal to the secondary coil. The Power Receiver is responsible for all communications of its identity and power requirements, since the Power Transmitter is just a “listener”.
Although the design of the power transmitter has been restricted to conform to the WPC standard, there is more freedom in 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% on 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 chooses the power receiver as the “speaker”. Inductive power transfer works by coupling a magnetic field to the secondary coil. The uncoupled magnetic lines of force rotate around the primary coil, and as long as the magnetic lines of force do not couple parasitic loads, there will be no losses (eg: eddy current losses in metals, etc.).
letter of agreement
Communication protocols include analog and digital pinging; identification and configuration; and power transmission. 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 a power transmitter detects the presence of an object.
2. The digital ping from the power transmitter is an extended version of the analog ping and gives the power receiver time to reply with a signal strength packet. If the information strength packet is valid, the power transmitter keeps the coil energized and proceeds to the next step.
3. During the identification and configuration phase, the power receiver will send some data packets to identify it and provide configuration and setup information to the power transmitter.
4. In the power transfer phase, the power receiver sends a control error packet to the power transmitter to increase or decrease the power. During normal operation, these packets are sent every 250ms or so, and every 32ms during large signal changes. Also, during normal operation, the power transmitter will send power packets every 5 seconds.
5. To terminate the power transfer, the power receiver will send a “terminate charging” message, or no communication for 1.25 seconds. Either event will put the power transmitter into a low power state.
TI’s WPC Compliant Solutions
TI is a founding member of the WPC and has played an active role in developing a robust wireless charging specification. TI uses three newly developed ICs to provide reliable solutions for both power receivers and power transmitters. The power receiver uses MSP430bq1010 and bq25046 devices. The power transmitter is based on the bq500110, which supports the A1 type (single coil) configuration. Both receiver and transmitter ICs are compatible with other WPC-compatible solutions.
The MSP430bq1010 in the power receiver handles all logic functions and communications. An on-board analog-to-digital converter monitors the voltage level into the bq25046 and the current level 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 is capable of delivering up to 1A to the main output.
The power transmitter solution is implemented with the bq500110. This device demodulates and decodes serial data from a power receiver. The control circuit 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 a single kit that includes a mechanical package. This kit can be used both for IC evaluation and as a design example. WPC has confirmed that these power transmitter and receiver solutions are compliant with the Version 1.0 specification. No software is required to operate the EVM, it only requires a 19-V input. The output of the EVM kit is 5V at currents up to 1A. The transmitter EVM includes several LED options for visual indication of power transmitter status. Additionally, two buzzer options provide an audible prompt for the start of power transfer.
The WPC standard is a set of guidelines that convince manufacturers that their components can work in harmony with other various WPC-certified components designed for inductive power transfer, leading to the development of numerous solutions.
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