System for ambient energy harvesting
A method and apparatus is disclosed herein for harvesting ambient energy. In one embodiment, an energy harvester comprises: a first RF rectifier to output a first voltage determined by rectified RF energy in response to received RF energy; a first energy reservoir coupled to the first RF rectifier to store energy at the first voltage; a DC/DC converter coupled to the first energy reservoir to convert the first voltage to a second voltage; a second reservoir coupled to the DC/DC converter to store energy at the second voltage, the second voltage being greater than the first voltage; and a third reservoir coupled to the second reservoir to receive energy transferred from the second reservoir periodically.
1. An energy harvester to convert incident radio frequency (RF) energy associated with an RF signal to direct current energy, the energy harvester comprising:
a first RF rectifier to output a first voltage determined by rectified RF energy in response to received RF energy;
a first energy reservoir coupled to the first RF rectifier to store energy at the first voltage;
a DC/DC converter coupled to the first energy reservoir to convert the first voltage to a second voltage;
a second reservoir coupled to the DC/DC converter to store energy at the second voltage, the second voltage being greater than the first voltage; and
a third reservoir coupled to the second reservoir to receive energy transferred from the second reservoir periodically and coupled to a load, wherein the third reservoir comprises a capacitor having a capacitance value chosen to control an amount of usable energy stored in the third reservoir as a function of an RF transmission duration, the impedance of the load, and a threshold voltage, the usable energy being a portion of the stored energy above the threshold voltage.
2. The energy harvester defined in claim 1 wherein the third reservoir is coupled to the second reservoir via a first switch, the first switch being closed periodically to transfer energy from the second reservoir to the third reservoir.
3. The energy harvester defined in claim 2 wherein the first switch is controlled at least in part by an envelope of the received RF energy signal.
4. The energy harvester defined in claim 1 wherein the first voltage is insufficient to operate the load directly, and the third voltage is sufficient to operate the load.
5. The energy harvester defined in claim 1 wherein the DC/DC converter comprises a self-excited oscillator.
6. The energy harvester defined in claim 5 wherein frequency of the self-excited oscillator is controlled at least in part by an envelope of the received RF energy signal.
7. The energy harvester defined in claim 1 wherein the DC/DC converter comprises a boost converter.
8. The energy harvester defined in claim 1 wherein the first reservoir comprises a capacitor with a capacitance determined at least in part by an envelope of the received RF energy signal.
9. The energy harvester defined in claim 1 wherein the DC/DC converter is a switch mode converter with switching intervals controlled at least in part by an the envelope of the received RF energy signal.
10. The energy harvester defined in claim 1 wherein the RF rectifier comprises one or more diodes selected from a group consisting of: a Schottky diode, a P-N junction diode, a diode-connected field effect transistor, and a tunnel diode.
11. The energy harvester defined in claim 1 wherein the third reservoir has an energy storage capacity that is greater than an energy storage capacity of the second reservoir.
12. The energy harvester defined in claim 1 wherein the second reservoir comprises an inductor, wherein energy stored in the inductor is approximately equal to energy stored in the first reservoir.
13. The energy harvester defined in claim 1 wherein the capacitance value is chosen to maximize the amount of usable energy stored in the third reservoir as a function of the RF transmission duration, the impedance of the load, and the threshold voltage.
14. An energy harvester to convert incident radio frequency (RF) energy to direct current energy, the energy harvester comprising:
a first RF rectifier to output a first voltage determined by rectified RF energy in response to received RF energy;
a first energy reservoir coupled to the first RF rectifier to store energy at the first voltage;
a DC/DC converter coupled to the first energy reservoir to convert the first voltage to a second voltage;
a second reservoir coupled to the DC/DC converter to store energy at the second voltage, the second voltage being greater than the first voltage; and
a third reservoir coupled to the second reservoir to receive energy transferred from the second reservoir periodically, wherein the first reservoir comprises a first capacitor and the second reservoir comprises a second capacitor, wherein capacitance of the second capacitor is approximately equal to a product of the first capacitor capacitance times a square of a ratio of an expected value of the second voltage divided by an expected value of the first voltage.
15. A device comprising:
an antenna;
a load; and
an energy harvester coupled to the antenna and the load, wherein the energy harvester comprises
a first RF rectifier to output a first voltage determined by rectified RF energy in response to received RF energy;
a first energy reservoir coupled to the first RF rectifier to store energy at the first voltage;
a DC/DC converter coupled to the first energy reservoir to convert the first voltage to a second voltage;
a second reservoir coupled to the DC/DC converter to store energy at the second voltage, the second voltage being greater than the first voltage; and
a third reservoir coupled to the second reservoir to receive energy transferred from the second reservoir periodically, wherein the third reservoir comprises a capacitor having a capacitance value chosen to control an amount of usable energy stored in the third reservoir as a function of an RF transmission duration, the impedance of the load, and a threshold voltage, the usable energy being a portion of the stored energy above the threshold voltage.
16. The device defined in claim 15 wherein the third reservoir is coupled to the second reservoir via a switch, the switch being closed periodically to transfer energy from the second reservoir to the third reservoir.
17. The device defined in claim 15 wherein the third reservoir is coupled to the load, and wherein the first voltage is insufficient to operate the load directly, and the third voltage is sufficient to operate the load.
18. The device defined in claim 15 wherein the DC/DC converter comprises a boost converter.
19. The energy harvester defined in claim 15 wherein the capacitance value is chosen to maximize the amount of usable energy stored in the third reservoir as a function of the RF transmission duration, the impedance of the load, and the threshold voltage.