© 2016 Integrated Device Technology, Inc
Theory of Operation
General System Architecture
A wireless power transfer system has two sub-systems: the wireless power transmitter (Tx) and the wireless power receiver (Rx). The
transmitter makes power available through a full bridge/half bridge driven LC resonant tank. It transmits power through the generation of an
AC magnetic field. Once the receiver coil is placed near the magnetic field, the field will induce an AC current through the receiving coil
where it is converted into a DC current.
High Level Control Scheme
Wireless power systems adopt a set of pre-defined in-band communication commands as the close loop control strategy. The amount of
power transferred is controlled by the receiver. The receiver sends out Control Error Packets (CEP) to the transmitter to increase power,
decrease power, or maintain the power level. The transmitter responds by adjusting the switching frequency and/or duty ratio. The receiver
requests more power by sending out a CEP, which includes a positive numerical value. The communication is digital. The communication 1’s
and 0’s ride on top of the power link that exists between the two coils.
Wireless Power Communication
When the transmitter is not transferring power to the receiver, it is in the low power Standby Mode. While in this mode, in order to detect a
receiver, the transmitter sends out periodic analog and digital pings.
Analog pings are very short AC detection pulses. These short pulses do not transmit enough energy to wake up the receiver, only to detect its
presence. Digital pings, on the other hand, do transmit enough power to enable the receiver to wake up and begin communication. The
transmitter uses digital pings to listen for a response from a receiver. After the transmitter detects a receiver, it may extend the digital ping.
This causes the system to proceed to the Identification and Configuration phase.
Once the receiver is detected and powered up, it will send out communications packets to handshake with the transmitter. The first
communication packet the receiver sends out is the Signal Strength packet, followed by Identification packets and Configuration packets.
Once the handshake process is done, the receiver will send out periodic Control Error packets and Received Power packets to adjust the
power.
If the receiver needs to stop the power transfer, it will send out an End of Power Transfer (EPT) communication packet. The transmitter stops
transmitting power immediately, and starts pre-defined routines according to the information decoded from the EPT packet.
System Fault Protection
The wireless power transfer system implements system level protection. These include over voltage, under voltage, over current, and over
temperature protection. On the transmitter side, whenever a fault condition is detected, it shuts down the whole system immediately and
protects itself. If the receiver detects a fault condition, it will send the End of Power Transfer packets to shut down the system. The transmitter
will continue to transmit power from the time of the receiver fault detection to the reception of the End of Power Transfer packet.
Over voltage protection: If the transmitter VIN is greater than 5.5 V, and the system is not in the Power Transfer mode, then the transmitter will
shut down until the VIN is in the range of 4.5 V to 5.5 V. If the system is already in the Power Transfer mode, then the transmitter takes no
action.
Under voltage protection: If the transmitter VIN is less than 4.5 V, the transmitter will shut down for five minutes, or until the VIN is cycled. off/on.
Over current protection: The transmitter uses a 20 mΩ sense resistor (RSENSE, R6) to monitor the current. If the transmitter detects a current
greater than the programmed current limit, it will shut down for five minutes, or until VIN is cycled off/on.
Over temperature protection: If the TS pin (pin 18) voltage falls below 600 mV (typical) then the transmitter will shut down. It will restart once
the TS voltage rises above 800 mV (200 mV hysteresis).