Wireless Power Transfer relies on magnetic induction between a planar receiver and transmitter coils. When the receiver coil is positioned over the transmitter coil, magnetic coupling occurs when driving the transmitter coil. The resultant flux is coupled into the secondary coil, which induces a voltage and current flows. The secondary voltage is rectified, and power is wirelessly transferred to a load. Two new Texas Instruments ICs manage this transfer: one transmits and the other receives the transferred power, as shown in Fig. 1. The power transfer receiver IC is a bq51050B secondary-side direct li-ion battery charge-controller (Fig. 2).
This 20V receiver IC provides:
•Efficient ac/dc power conversion
•A digital controller that complies with the WPC 1.1 Standard
•The necessary control algorithms needed for li-ion and li-pol battery charging
The bq51050B’s self-contained charger eliminates the need for the separate battery charger circuit used in older generation systems. This inductor-free, single-stage design delivers high efficiency and saves board space compared with implementations requiring the separate charger IC.
Fig. 1. Wireless power transfer using receiver and transmitter circuits.
Fig. 2. The bq5105x secondary-side 20V receiver is a digital controller that provides ac/dc power conversion, WPC 1.1 communication protocol and an integral li-ion battery charger.
The bq51050B integrates a low-impedance synchronous rectifier, low-dropout regulator (LDO), digital control, li-ion charger controller, and accurate voltage and current loops. The entire power stage (rectifier and LDO) utilize low-resistive NMOS FET’s (100-mΩ typical RDS(ON)) ensuring high efficiency and low power dissipation. Its features include:
•Wireless power receiver, rectifier and battery charger in one small package
•4.2V or 4.35V battery output voltage options
•Support for up to 1.5A charging current
•93% peak ac-dc charging efficiency
•20V maximum input voltage tolerance, with input overvoltage protection clamp
•Thermal shutdown and overcurrent protection
•Temperature monitoring and fault detection
•Power stage output that tracks rectifier and battery voltage to ensure maximum efficiency across the full charge cycle
•Either small WCSP or QFN packages
The bq500410A wireless power transmitter features:
•Expanded “free-positioning” using a three-coil, A6, transmit array
•Intelligent control of wireless power transfer
•Wireless Power Consortium (WPC) compliance
•Digital demodulation the reduces components
•A signal output that indicates the start of power transfer, which can activate a ceramic buzzer
•An End-of-Power Transfer signal that causes an LED indicator to illuminate
•LED indication of charging state and fault status
•Overcurrent monitoring threshold that can halt power transfer for one minute.
•Power-On Reset (POR) that monitors the supply voltage and sets the device startup sequence.
•A 48-pin, 7 mm x 7 mm QFN package
•Operating temperature range of –40°C to 110°C
Power transfer depends on coil coupling that depends on:
•Distance between coils
•Number of turns
•Frequency Duty cycle
Receiver and transmitter coils must be aligned for best coupling and efficient power transfer. The closer the space between the two coils, the better the coupling. However, to account for housing and interface surfaces the practical distance is set to be less than 5 mm, as defined within the WPC Standard (see sidebar, “Wireless Power Consortium 1.1 Standard”). Shielding is added as a backing to both the transmitter and receiver coils to direct the magnetic field to the coupled zone. Magnetic fields outside the coupled zone do not transfer power. Thus, shielding also serves to contain the wireless fields and avoid coupling to other adjacent system components.
You can control regulation by varying any one of the coil coupling parameters. However, for WPC compatibility, the transmitter-side coils and capacitance are specified and its resonant frequency point is fixed. Power transfer is regulated by changing the frequency along the resonance curve from 112 kHz to 205 kHz, (the higher the frequency, the lower the power). Duty cycle remains constant at 50% throughout the power band and reduces only when it reaches 205 kHz.
The bq500410A uses the A6 coil arrangement to achieve greater than 70-percent efficiency. The WPC Standard establishes coil and matching capacitor specification for the A6 transmitter. Although the bq500410A is intended to drive an A6 three-coil array, it can also be used to drive a single coil. For single coil operation the two outer coils and associated electronics are simply omitted. Fig. 3 shows the A6 three-coil configuration that allows 70 x 20 mm charge surface area as well as the A1 single-coil 18 mm x 18mm “bull’s-eye” charge space. The 70 mm by 20 mm charge area is 400-percent larger than 18-mm by 18-mm area now being used. Use of the A6 coil configuration provides a “free-positioning” digital wireless power controller.
Fig. 3. The bq500410A can employ either of two types of coil configurations. On the left is the A1 single coil and on the right is the A6 three-coil configuration used for free-positioning.
The performance of an A6 transmitter can vary based on the design of the A6 coil set. For best performance with small receiver coils under heavy loading, it is best to design the coil set so that the distance between the centers of the outer coils is on the low end of the specified tolerance (49.2 mm).
The WPC standard describes the dimensions, materials of the coils and information regarding the tuning of the coils to resonance. The value of the inductance and its associated resonant capacitor are critical for proper operation and system efficiency. The resonant tank circuit requires a total capacitance value of 68 nF plus a 5.6 nF center coil, which is the WPC system compatibility requirement. Capacitors chosen must be rated for at least 100 V and must be of a high quality C0G dielectric (sometimes also called NP0). They typically have a 5% tolerance, which is adequate. The designer can combine capacitors to achieve the desired capacitance value. Various combinations can work, depending on availability.
The bq500410A drives three independent half-bridges. Each half-bridge drives one coil of the A6 coil set. A TPS28225 is the recommended driver IC that features high-side drive capability and enables use of N-channel MOSFETs throughout. You can derive the gate-drive supply from a primitive, active voltage divider. The MOSFET gate driver does not require a highly regulated power supply.
POWER TRANSFER EFFICIENCY
The bq500410A supports both Parasitic Metal Detection (PMOD) and Foreign Object Detection (FOD) by continuously monitoring the efficiency of the established power transfer. PMOD and FOD protect against power lost due to metal objects in the wireless power transfer path. The bq500410A compares input power, known losses, and the amount of power reported by the receiver IC. This yields an estimate of unaccounted power presumed lost due to misplaced metal objects. Exceeding this unexpected loss generates a fault and halts power transfer. PMOD and FOD share the same threshold-setting resistor for which the recommended starting point is 400 mW.
The FOD algorithm uses information from an in-system characterized and WPC1.1 certified receiver and it is more accurate the previous PMOD obtained from WPC1.0. WPC1.0 specification required merely the rectified power packet, whereas the WPC1.1 Standard additionally uses the received power packet that more accurately tracks power used by the receiver.
WPC 1.1 is intended for 12 V systems, but the bq500410A requires 3.3 VDC input supply. Therefore, use a buck regulator or a linear regulator to step down from the 12V system input. Either supply voltage choice is WPC-compatible. You can use a low-cost buck regulator, such as the TPS54231, to step the 12 V down to 3.3 V.
Communication within the WPC is from the receiver to the transmitter, where the receiver tells the transmitter to send power and how much. For regulation, the receiver must communicate with the transmitter whether to increase or decrease frequency. The receiver monitors the rectifier output and using Amplitude Modulation (AM) to send packets of information to the transmitter. A packet consists of a preamble, header, actual message and a checksum, as defined by the WPC standard.
The bq500410A starts power transfer by pinging the surrounding environment looking for WPC compliant devices waiting to be powered. If it finds a compliant device it safely engages it, reads the packet feedback from the powered device, and manages the power transfer.
The receiver sends a packet by modulating an impedance network. This AM signal reflects back as a change in the voltage amplitude on the transmitter coil. The signal is demodulated and decoded by the transmitter-side electronics and it adjusts the frequency of its coil-drive output to close the regulation loop. The bq500410A features internal digital demodulation circuitry.
The bq500410A pings the surroundings in 400-ms intervals by sequentially firing the three coils in the A6 array. A COMM feedback signal is multiplexed through analog switches and synchronized to the coil being driven. To select the best coil match, the bq500410A looks for the strongest COMM signal. The coil is engaged and driven, only one coil is driven at a time. The driven coil is tolerant of slight misalignment of the receiver while power is being transferred. Actually displacing the receiver to an adjacent coil while charging is allowable, the sequential ping sequence and detection to determine the best matching coil to drive continues to repeat.
You can use an MSP430G2101 as a low-power supervisor to provide accurate ping timing, retain charge state, operating mode, fault condition and all relevant operation states. Using the suggested circuitry, you can expect a standby power reduction from 300 mW to less than 90 mW, making it possible to achieve an Energy Star rating.
WIRELESS POWER CONSORTIUM 1.1 STANDARD (JULY 2012)
The Wireless Power Consortium (WPC) is an international group of companies from diverse industries. They developed the WPC Standard to facilitate cross compatibility of compliant transmitters and receivers. The Standard defines the physical parameters and the communication protocol used in wireless power transfer.
Power transfer involves two device types. Those that provide the wireless power are designated as Base Stations, and those that consume wireless power are called Mobile Devices. Power transfer always occurs from a Base Station to a Mobile Device. The Base Station contains a power transmitter with a primary coil. The Mobile Device contains a power receiver with a secondary coil. The primary and secondary coils form two halves of a coreless resonant transformer. Shielding at the bottom face of the primary coil and the top face of the secondary coil, as well as the close spacing of the two coils, ensures that power transfer occurs with an acceptable efficiency.
Typically, a Base Station has a flat surface—referred to as the interface surface — on top of which a user can place one or more Mobile Devices. This ensures that the vertical spacing between primary and secondary coils is sufficiently small.
“Free-positioning” using the A6 coil arrangement does not require active participation in alignment of the primary and secondary coils. One implementation of free-positioning uses an array of primary coils to generate a magnetic field at the location of the secondary coil. Another implementation of free-positioning uses mechanical means to move a single Primary Coil underneath the Secondary Coil.
Main features in wireless power transfer are:
•Contactless power transfer from a Base Station to a Mobile Device, based on near field magnetic induction between coils.
•Transfer of about 5 W, using an appropriate secondary coil (having a typical outer dimension of around 40 mm).
•Operation at frequencies in the 100…205 kHz range.
•Free-positioning enables arbitrary placement of the Mobile Device on the surface of a Base Station.
•A simple communications protocol enables the Mobile Device to take full control of the power transfer.
•Very low stand-by power (implementation dependent).