ZHCSEA8A October 2015 – October 2015
PRODUCTION DATA.
The principle of wireless power transfer is simply an open-cored transformer consisting of transmitter and receiver coils. The transmitter coil and electronics are typically built into a charger pad and the receiver coil and electronics are typically built into a portable device, such as a smart phone. When the receiver coil is positioned on the transmitter coil, magnetic coupling occurs when the transmitter coil is driven. The flux is coupled into the secondary coil, which induces a voltage and current flows. The secondary voltage is rectified, and power can be transferred effectively to a load, wirelessly. Power transfer can be managed through various closed loop control schemes.
After power is applied and the transmitter device comes out of reset, it will automatically begin the process of detecting and powering a receiver. The bq500511 sends a ping to detect the presence of a receiver on the pad. After a receiver is detected, the bq500511 attempts to establish communication and begin power transfer. The bq500511 is designed to operate with the bq50002 Wireless Power Transmitter Analog Front End to control a full-bridge power stage to drive the primary coil. Through a simple interface the bq500511 instructs the bq50002 how much to increase or decrease power. The full bridge power stage allows for higher power delivery for a given supply voltage.
The embedded demodulator outputs at least one valid bit stream under all modulation conditions that can be used by the accompanied bq500511 controller in order to comply with WPC specification.
More specifically, the WPC worst case condition is defined as the transmitter operating at the minimum modulation level, and the receiver coil is measured by certain x, y and z axis distance to the center of the transmitter coil.
Analog demodulation channel function diagram:
During a transition the primary cell current and primary cell voltages are undefined. See Figure 3.
bq50002 operates in two modes:
Direct PWM Control Mode and Self Switching. The mode of operation is set by the state of PWM_CTRL input when EN=HIGH. This is intended for control by bq500511.
bq50002 passes external PWM inputs to drive gate drivers when the following conditions hold:
bq50002 follows the commands from the controller to adjust the internal oscillator frequency up or down to adjust output power levels when the following conditions apply:
bq50002 follows the commands from the controller to adjust the internal oscillator duty cycle up or down to adjust power output levels when the following conditions hold:
To support foreign object detection (FOD), bq50002 senses the average input current to the device. The integrated current sense amplifier has voltage gain of 50.
bq50002 has an integrated low-dropout (LDO) voltage regulator which supplies power to the companion bq500511 controller. The BP3 pin supplies a regulated 3-V voltage supply and should have a 2.2-µF capacitor tied to GND.
Power transfer efficiency and robustness depends on coil coupling. Coupling depends on the distance between coils, alignment, coil dimensions, coil materials, number of turns, magnetic shielding, impedance matching, frequency. Most importantly, the receiver and transmitter coils must be aligned for best coupling and efficient power transfer. The smaller the space between the coils is, the better the coupling. 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 fields to avoid coupling to other adjacent system components.
Regulation can be achieved by controlling any one of the coil coupling parameters. However, for WPC compatibility, the transmitter-side coils and capacitance are specified and the resonant frequency point is fixed. In the bq500511/bq50002 system power transfer is regulated by changing the operating frequency between 110 kHz to 205 kHz. The higher the frequency, the further from resonance and the lower the power. Duty cycle remains constant at 50% throughout the power band and is reduced only once 205 kHz is reached.
Dynamic Power Limiting™ (DPL) allows operation from a 5-V supply with limited current capability (such as a USB port). When the input voltage is observed drooping, the output power is dynamically limited to reduce the load and provides margin relative to the supply’s capability.
Anytime the DPL control loop is regulating the operating point of the transmitter, the LED will indicate that DPL is active. The LED color and flashing pattern are determined by the LED Table. If the receiver sends a Control Error Packet (CEP) with a negative value, (for example, to reduce power to the load), the bq500511 in DPL mode will return to normal operation and respond to this CEP via the standard WPC control loop behavior.
Communication within the WPC v1.2 specification is from the receiver to the transmitter. For example, in order to regulate the output of the transmitter, the receiver sends messages requesting the transmitter to increase or decrease power. The receiver communicates by modulating the rectifier voltage and using amplitude modulation (AM) sends packets of information to the transmitter. A packet is comprised of a preamble, a header, the actual message, and a checksum, as defined by the WPC standard.
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. In the bq500511/bq50002 system, the bq50002 performs the demodulation function and passes a digitized version of the message to the bq500511 where the message is decoded and processed. For example in response to a Control Error Packet, the bq500511 calculates the required change in output power and in turn controls the bq50002 through the CLK_OUT, UP_DOWN, and MODE pins to adjust the operating point and thus its output power.
The modulation impedance network on the receiver can either be resistive or capacitive. Figure 4 shows the resistive modulation approach, where a resistor is periodically added to the load, resulting in an amplitude change in the transmitter voltage. Figure 5 shows the corresponding capacitive modulation approach.