Wireless power transfer systems having concentric coils
A conventional wireless power transfer (WPT) system based on resonant inductive coupling typically operates at a peak power efficiency at the expense of a energy transfer rate. Impedance matching circuits can increase the energy transfer rate, but tend to increase the complexity, form factor, and weight of the WPT system. To overcome these limitations, a WPT system is described herein that includes a resonant circuit with integrated impedance matching. The resonant circuit includes a first coil, a first capacitor in series with the first coil, a second coil in series with the first coil and the first capacitor, and a second capacitor in parallel with the first coil and the first capacitor. The inductor coils and capacitances are tailored to increase the voltage gain and, thus, the energy transfer rate. The inductor coils also transmit or receive power, thus increasing the energy transfer rate and the power efficiency.
1. A wireless power receiver, comprising:
a resonant circuit to receive power from a wireless power transmitter via wireless
magnetic resonance charging at a voltage gain of about 1 and an efficiency of at least 95% at a resonant frequency between about 80 kHz and about 90 kHz, the resonant circuit comprising:
a first coil;
a second coil coupled in series and concentric with the first coil and stacked on the first coil, the first coil and the second coil receiving the power from a third coil and a fourth coil in the wireless power transmitter at the resonant frequency during operation of the wireless power receiver;
a first capacitor coupled in series with the first coil; and
a second capacitor coupled in parallel with the first coil and the first capacitor,
wherein:
the first coil is magnetically coupled to the second coil;
the first coil and the second coil are operable to, when physically aligned with the wireless power transmitter, be magnetically coupled to the third coil and the fourth coil; and the first capacitor and the second capacitor do not act as an open circuit during operation of the wireless power receiver.
2. The wireless power receiver of claim 1 , wherein:
the first coil has an inductance of about 0.1 μH to about 100 μH,
the first capacitor has a capacitance of about 0.01 μf to about 100 μf,
the second coil has an inductance of about 0.1 μH to about 100 μH, and
the second capacitor has a capacitance of about 0.01 μf to about 100 μf.
3. The wireless power receiver of claim 1 , wherein:
the first coil has an inductance of about 1μμH to about 20 μH,
the first capacitor has a capacitance of about 0.05 μf to about 2 μF,
the second coil has an inductance of about 1 μH to about 20 pH, and
the second capacitor has a capacitance of about 0.05 μf to about 2 μF.
4. The wireless power receiver of claim 1 , wherein the first coil and the second coil are tuned such that electric currents flowing through the first coil and the second coil have a phase difference of less than 20 degrees.
5. A method of transferring power wirelessly to a wireless power receiver comprising:
a resonant circuit to receive power from a wireless power transmitter via wireless magnetic resonance charging, the resonant circuit comprising:
a first coil;
a second coil in series and concentric with the first coil and stacked on the first coil, the first coil and the second coil being physically aligned with the wireless power transmitter and magnetically coupled to a third coil and a fourth coil of the wireless power transmitter;
a first capacitor coupled in series with the first coil; and
a second capacitor coupled in parallel with the first coil and the first capacitor,
wherein:
the first coil is magnetically coupled to the second coil; and
the first capacitor and the second capacitor do not act as an open circuit durinq operation of the wireless power receiver,
the method comprising:
receiving the power, from the third coil and the fourth coil, with the first coil and the second coil at a resonant frequency between about 80 kHz and about 90 kHz, a voltage gain of about 1.0, and an efficiency of at least 95%.
6. The method of claim 5 , further comprising:
positioning the first coil within about 50 mm of the third coil.
7. The method of claim 5 , further comprising:
running a first current through the third coil and the fourth coil, the first current inducing a second current running through the first coil and the second coil.
8. The method of claim 7 , wherein a phase difference between the first current at the third coil and the first current at the fourth coil is less than about 20 degrees.
9. The method of claim 7 , wherein a phase difference between the first current at the third coil and the first current at the fourth coil is less than about 10 degrees.
10. A wireless power receiver, comprising:
a first coil having a first inductance;
a first capacitor in series with the first coil and having a first capacitance, the first coil and the first capacitor connected together at a first node and forming a first branch of a parallel circuit;
a second capacitor having a second capacitance and forming a second branch of the parallel circuit that is in parallel with the first branch; and
a second coil, in series with the parallel circuit and concentric with the first coil, having a second inductance,
wherein:
the product of the second capacitance and the second conductance does not equal the product of the first capacitance and a difference of the first inductance and the second inductance; and
the first coil and the second coil are tuned such that electric currents induced in the first coil and the second coil by the third coil and the fourth coil in the wireless power transmitter have a phase difference of less than 20 degrees.
11. The wireless power receiver of claim 10 , wherein the resonant frequency of the wireless power receiver is about 87 kHz.
12. The wireless power receiver of claim 10 , wherein the wireless power receiver has a voltage gain of about 1 over a band from about 80 kHz to about 90 kHz.
13. The wireless power receiver of claim 10 , wherein the wireless power receiver has a power transfer efficiency of at least 95% over a band from about 80 kHz to about 90 kHz.
14. The wireless power receiver of claim 10 , wherein the first coil is co-planar with the second coil.
15. The wireless power receiver of claim 10 , wherein the first coil is stacked on the second coil.
16. The wireless power receiver of claim 10 , wherein the first coil is a first flat spiral coil and the second coil is a second flat spiral coil.
17. The wireless power receiver of claim 10 , wherein the first coil and the second coil have outer diameters equal to or less than 220 mm.
18. The wireless power receiver of claim 10 , wherein the first coil is connected to a second node and the second coil is connected to the second node.
19. The wireless power receiver of claim 10 , wherein the first capacitor is connected to a second node the second coil is connected to the second node.
20. The wireless power receiver of claim 10 , wherein:
the first coil has an inductance of about 0.1 μH to about 100 μH,
the first capacitor has a capacitance of about 0.01 μf to about 100 μf,
the second coil has an inductance of about 0.1 μH to about 100 μH, and
the second capacitor has a capacitance of about 0.01 μf to about 100 μf.
21. The wireless power receiver of claim 10 , wherein:
the first coil has an inductance of about 1μμH to about 20 μH,
the first capacitor has a capacitance of about 0.05 μf to about 2 μF,
the second coil has an inductance of about 1 μH to about 20 pH, and
the second capacitor has a capacitance of about 0.05 μf to about 2 μF.
22. The wireless power receiver of claim 10 , wherein:
the first coil is magnetically coupled to the second coil; and
the first coil and the second coil is operable to, when physically aligned with the wireless power transmitter, be magnetically coupled to the third coil and the fourth coil.
23. The wireless power receiver of claim 10 , wherein the first coil and the second coil are configured to provide impedance matching to increase a voltage gain at an operating frequency of the wireless power receiver.