Open circuit-based Rx power limiter for wireless power transfer

1. A circuit in a wireless power transfer system that receives a wirelessly-generated input voltage at an input and produces a constant DC voltage at an output to drive a load, the circuit comprising:

an electronic switch connected in series between the input and the output;

a comparator, configured to receive a representation of the wirelessly generated input voltage and a reference, and to provide an output to control conduction of the electronic switch, wherein the conduction of the electronic switch is configured to provide the constant DC voltage; and

a voltage selector circuit configured to receive two input voltages and to provide one of the two input voltages as a supply voltage of the comparator.

2. The circuit of claim 1 wherein the two input voltages are the voltages on each side of the electronic switch.

3. The circuit of claim 1 , wherein the voltage selector circuit is a maximum voltage selector circuit; and

wherein the output of the maximum voltage selector circuit connects to one of the power inputs of the comparator.

4. The circuit of claim 3 wherein the electronic switch is a PMOS electronic switch.

5. The circuit of claim 1 , wherein the voltage selector circuit is a minimum voltage selector circuit; and

wherein the electronic switch is an NMOS electronic switch.

6. The circuit of claim 1 wherein the input voltage is an AC voltage and the circuit further comprises a rectifier that rectifies the input voltage.

7. The circuit of claim 6 further comprising a filter capacitance coupled across the load to filter and to a first node of the electronic switch, the filter capacitor configured to filter the rectified AC input voltage.

8. The circuit of claim 7 further comprising a second capacitance coupled to a second node of the electronic switch, the second capacitance configured to filter the rectified AC input voltage.

9. The circuit of claim 1 further comprising a rectifier in series with the electronic switch.

10. A wireless power transfer system for wirelessly transmitting and receiving power comprising:

a transmitting coil that wirelessly transmits the power;

a receiving coil magnetically but wirelessly coupled to the transmitting coil that wirelessly receives the power and generates an AC input voltage;

a rectifier that rectifies the AC input voltage;

a capacitance that filters tile rectified AC input voltage;

an electronic switch connected in series between tile rectified AC input voltage and an output;

a load connected to the output;

a comparator configured to open and close the electronic switch so as to cause the output to be at a constant DC voltage, notwithstanding variations in the load; and

a voltage selector circuit configured to receive two input voltages and to provide one of the two input voltages as a supply voltage of the comparator.

11. The wireless power transfer system of claim 10 wherein the voltage selector circuit comprises:

a maximum voltage selector circuit that receives two input voltages and outputs the higher of the two input voltages; or

a minimum voltage selector circuit that receives two input voltages and outputs the lower of the two input voltages.

12. The wireless power transfer system of claim 11 wherein the two input voltages are the voltages on each side of the electronic switch.

13. The wireless power transfer system of claim 11 wherein the output of the maximum voltage selector circuit or the minimum voltage selector circuit powers the comparator.

14. The wireless power transfer system of claim 11 wherein the system includes the maximum voltage selector circuit and the electronic switch is a PMOS electronic switch.

15. The wireless power transfer system of claim 11 wherein the circuit includes the minimum voltage selector circuit and the electronic switch is an NMOS electronic switch.

16. The wireless power transfer system of claim 10 wherein the load includes a battery charger connected to a battery.

17. The wireless power transfer system of claim 10 wherein the capacitance includes a first capacitance coupled to a first node of the electronic switch and a second capacitance coupled to a second node of the electronic switch.

18. A circuit for a wireless power transfer system that receives a wirelessly-generated input voltage at an input and produces a constant DC voltage at an output to drive a load, the circuit comprising:

switch means connected in series between the input and the output for controllably opening and closing a connection between the input and the output;

controller means for causing the switch means to open and close so as to cause the output to be at the constant DC voltage; and

means for receiving two input voltages and for providing one of the two input voltages as a supply voltage of the controller means.

19. The circuit of claim 18 , wherein the means for receiving two input voltages includes:

a first diode configured to receive a first input voltage of the two input voltages; and

a second diode configured to receive a second input voltage of the two input voltages.

20. The circuit of claim 18 , wherein the means for receiving two input voltages includes one of a maximum voltage selector circuit or a minimum voltage selector circuit.

21. A wireless power transfer system for wirelessly transmitting and receiving power comprising:

transmitter means for wirelessly transmitting the power;

receiver means magnetically but wirelessly coupled to the transmitting means for wirelessly, receiving the power and for generating an AC input voltage;

rectifier means for rectifying the AC input voltage;

capacitance means for filtering the rectified AC input voltage;

switch means connected in series between the AC input voltage and an output for controllably opening and closing a connection between the AC input voltage and the output;

load means for providing a load connected to the output;

controller means for opening and closing the switch means so as to cause the output to be at a constant DC voltage, notwithstanding variations in the load; and

a voltage selector means to provide one of two input voltages as a supply voltage of the controller means, the two input voltages derived from a voltage across the switch means.

22. The wireless power transfer system of claim 21 , wherein the voltage selector means includes:

a first diode configured to receive a first input voltage of the two input voltages; and

a second diode configured to receive a second input voltage of the two input voltages.

23. The wireless power transfer system of claim 22 , wherein the switch means includes a PMOS transistor; and

whereon the voltage selector means includes a maximum voltage selector circuit.

24. The wireless power transfer system of claim 22 , wherein the switch means includes a NMOS transistor; and

wherein the voltage selector means includes a minimum voltage selector circuit.