Wireless power transfer based on transmitter coil voltage sensing
In a wireless power transfer system for supplying power to a wireless device, e.g., charging a battery, wireless power transmitter coil voltage is used in place of coil current for communication and power control reference. The transmitter coil voltage waveforms provide phase information, with reference to the pulse width modulation (PWM) waveforms, that can be used to demodulate digital packet communication from a wireless power receiver used to provide a voltage to a device. The DC voltage amplitude of the wireless power transmitter coil is used for controlling the power injected to the wireless power receiver coil, while the phase of the transmitter coil voltage PWM signal is used for demodulation of signal packets from the wireless power receiver to control the transmitter coil voltage and thereby wireless power transfer to the power receiver.
1. A system for wireless power transfer based on transmitter coil voltage sensing, said system comprising:
a wireless power transmitter comprising
a DC-to-AC power inverter, wherein an input of the power inverter is adapted for coupling to a DC power source,
a transmitter tuned circuit comprising a series connected capacitor and a transmitter power coil, wherein the series connected capacitor and transmitter power coil are coupled to an output of the DC-to-AC power inverter,
a circuit for sampling voltage on the transmitter power coil, whereby amplitude and phase of the voltage are determined; and
a wireless power receiver comprising
an AC-to-DC power converter,
a receiver tuned circuit comprising a receiver power coil and at least one capacitor, wherein the receiver tuned circuit is coupled to an AC input of the AC-to-DC power converter,
a voltage control circuit coupled to a DC output of the AC-to-DC power converter and providing a DC voltage to a load, and
a data communications circuit for modulating voltage control data onto the receiver tuned circuit;
wherein the receiver power coil is inductively coupled to the transmitter power coil, receives power therefrom and transmits modulated voltage control data thereto for controlling the voltage amplitude on the transmitter power coil.
2. The system for wireless power transfer according to claim 1 , wherein the data communications circuit modulates the voltage control data onto the receiver tuned circuit by changing current drawn by the load.
3. The system for wireless power transfer according to claim 1 , wherein the data communications circuit modulates the voltage control data onto the receiver tuned circuit by changing the receiver tuned circuit resonant frequency.
4. The system for wireless power transfer according to claim 1 , wherein the voltage control data is demodulated by comparing the phase of the voltage on the transmitter power coil with a pulse width modulation (PWM) signal from the output of the DC-to-AC power inverter.
5. The system for wireless power transfer according to claim 1 , wherein the wireless power transmitter further comprises:
a positive voltage detector coupled at a node between the series connected capacitor and transmitter power coil and providing a positive voltage therefrom,
an analog-to-digital converter (ADC) having an analog input coupled to an output of the positive voltage detector, and
a microcontroller having a digital input coupled to the ADC and an output coupled to and controlling the DC-to-AC power inverter, wherein the microcontroller monitors the positive voltage from the positive voltage detector and controls the AC voltage from the DC-to-AC power inverter, wherein the positive voltage comprises a positive voltage waveform from the DC-to-AC power inverter and the modulated voltage control data from the wireless power receiver.
6. The system for wireless power transfer according to claim 1 , wherein the wireless power receiver further comprises:
a circuit for providing an error value from a comparison between the DC output voltage from the AC-to-DC power converter and a reference voltage;
an N-bit digitizer for digitizing the error value to a digital error value; and
a serializer for converting the digital error value into a serial data word and coupling the serial data word to the data communications circuit for transmission as the modulated voltage control data to the wireless power transmitter.
7. The system for wireless power transfer according to claim 6 , wherein the demodulation of the voltage control data comprises:
a de-serializer for converting the serial data work back to the digital error value;
a digital divider for dividing the digital error value (e) by 128;
an adder for adding one (1) to e/128;
a first sampler for sampling the voltage on the transmitter power coil at a delay time instance i to produce a voltage reference, Vref;
a multiplier for multiplying Vref with 1+e/128 to produce a voltage reference error value, Vreferr;
a second sampler for sampling the voltage on the transmitter power coil at a delay time instance j to produce a coil voltage, Vcoil;
a subtractor for subtracting Vcoil from Vreferr to produce an error voltage, Verr; and
a proportional-integral-derivative (PID) compensator having an input coupled to the subtractor and receiving the error voltage, Verr.
8. The system for wireless power transfer according to claim 7 , further comprising a gain block to scale an output of the PID compensator.
9. The system for wireless power transfer according to claim 7 , wherein the first sampler takes at least one sample at delay time instance i, wherein i is a positive integer value.
10. The system for wireless power transfer according to claim 9 , where i is from 1 to 20 for every voltage control data received.
11. The system for wireless power transfer according to claim 7 , wherein the second sampler takes at least one sample at delay time instance j, wherein j is a positive integer value.
12. The system for wireless power transfer according to claim 1 , where j is from 1 to 20 for every voltage control data received.
13. The system for wireless power transfer according to claim 9 , wherein the voltage reference, Vref, is an average of i samples per voltage control data received.
14. The system for wireless power transfer according to claim 11 , wherein the coil voltage, Vcoil, is an average of j samples per voltage control data received.
15. The system for wireless power transfer according to claim 6 , wherein N is equal to eight (8).
16. The system for wireless power transfer according to claim 1 , wherein the DC-to-AC power inverter is a metal oxide semiconductor field effect transistor (MOSFET) H-bridge inverter.
17. A method for wireless power transfer based on transmitter coil voltage sensing, said method comprising the steps of:
generating an alternating current (AC) signal from a DC power source and coupling the AC signal to a transmitter tuned circuit comprising a series connected capacitor and a transmitter power coil;
inductively coupling the AC signal from the transmitter tuned circuit to a receiver tuned circuit comprising a receiver power coil and at least one capacitor;
generating a DC voltage with an AC-to-DC power converter coupled to the receiver tuned circuit, wherein the receiver tuned circuit is coupled to an AC input of the AC-to-DC power converter;
supplying DC voltage from the AC-to-DC power converter to a load;
modulating voltage control data onto the receiver tuned circuit whereby a voltage on the transmitter power coil changes according to the voltage control data modulation;
sampling the voltage on the transmitter power coil, whereby amplitude and phase of the voltage are determined;
demodulating the voltage control data from the phase of the sampled voltage on the transmitter power coil; and
controlling the amplitude of the voltage on the transmitter power coil from the demodulated voltage control data.
18. The method for wireless power transfer according to claim 17 , wherein the step of modulating voltage control data onto the receiver tuned circuit comprises the step of changing current drawn by the load with the voltage control data.
19. The method for wireless power transfer according to claim 17 , wherein the step of modulating voltage control data onto the receiver tuned circuit comprises the step of changing the receiver tuned circuit resonant frequency with the voltage control data.
20. The method for wireless power transfer according to claim 17 , wherein the modulating voltage control data comprises serial digital data.
21. The method for wireless power transfer according to claim 17 , wherein the step of demodulating the voltage control data from the phase of the sampled voltage comprises the step of comparing the sampled voltage to a pulse width modulation (PWM) signal of the generated AC signal.