Energy Harvesting System

Abstract

In the energy harvesting system 400, a power extraction circuit 404 extracts power from an environmental power source that is prone to providing fluctuating power levels. A power storage device 406 stores the power extracted from the environmental power source by the power extraction circuit 404. A sensing circuit 407 provides an indication 409 of the level of power the power extraction circuit 404 can extract from the environmental power source. A controlled switch circuit 408 electrically disconnects the power extraction circuit 404 from the power storage device 406 when the indication 409 indicates that the level of power the power extraction circuit 404 can extract from the environmental power source is insufficient to allow the power extraction circuit 404 to charge the power storage device 406. This prevents leakage of harvested power.

Description

[Technical Field]

[0001] One aspect of the present invention relates to an energy harvesting system that can extract power from an environmental power source and store the power. This energy harvesting system can be used, for example, in devices that form part of the so-called Internet of Things infrastructure through wireless power transmission. This can enable battery-less operation of such devices. Further aspects of the present invention relate to a wireless system including a wireless power receiving device that includes the energy harvesting system, and a method of energy harvesting. [Background technology]

[0002] Ambient energy is present virtually everywhere, in the form of, for example, electromagnetic waves, light, heat, wind, vibration, and other mechanical energy. These ambient energies provide the ambient power sources from which energy harvesting systems extract power. Such energy harvesting systems typically include a transducer that converts one or more of the aforementioned types of ambient energy into electrical energy. The harvested electrical energy can then be stored and used to power one or more electrical circuits. Typically, the transducer forms part of a power extraction circuit that conditions the power signal provided by the transducer to obtain a power signal suitable for storage and power delivery.

[0003] U.S. Patent No. 9,197,143 describes an apparatus including a plurality of energy harvesting circuits, an energy storage device, a first diode, and a second diode. The plurality of energy harvesting circuits includes a first energy harvesting circuit and a second energy harvesting circuit. The first energy harvesting circuit includes a first transducer, and the second energy harvesting circuit includes a second transducer. The first transducer has a structure for converting a different type of energy into electricity than the second transducer. The first diode is electrically connected between the first energy harvesting circuit and the energy storage device, and the second diode is electrically connected between the second energy harvesting circuit and the energy storage device. The first diode is oriented to prevent energy from the second energy harvesting circuit from being dissipated in the first energy harvesting circuit, and the second diode is oriented to prevent energy from the first energy harvesting circuit from being dissipated in the second energy harvesting circuit. Summary of the Invention

[0004] There is a need for improved energy harvesting systems that allow for relatively high efficiency in power harvesting, particularly when extracting power from ambient sources that tend to provide fluctuating power levels.

[0005] The present invention takes into consideration the following: An environmental power source may provide a varying power level. For example, if the environmental power source is in the form of electromagnetic waves, the electromagnetic waves may vary in intensity. There may be time intervals (bands) when the electromagnetic waves are relatively strong and time intervals (bands) when the electromagnetic waves are relatively weak. As another example, if the environmental power source is mechanical in nature, e.g., vibration, the mechanical force may vary in intensity. There may be time intervals (bands) when the vibrations are relatively strong and time intervals (bands) when the vibrations are relatively weak.

[0006] In the aforementioned conditions, the power extraction circuit of the energy harvesting system may therefore extract relatively high levels of power from the ambient power source during some time intervals and relatively low levels during other time intervals. Consequently, the power storage device of the energy harvesting system may receive varying levels of power from the power extraction circuit. As a result, the power extraction circuit may provide an insufficient level of power to charge the power storage device. In such conditions, stored power may leak from the storage device through the power extraction circuit. That is, rather than power flowing from the power extraction circuit to the power storage device, power may flow from the power storage device to the power extraction circuit, resulting in a leakage that constitutes a loss of stored power.

[0007] According to one aspect of the invention as set forth in claim 1, there is provided an energy harvesting system comprising: a power extraction circuit adapted to extract power from an environmental power source prone to providing fluctuating power levels; a power storage device adapted to store power extracted from the ambient power source by the power extraction circuit; The power extraction circuit includes: - sensing circuitry adapted to provide an indication of the level of power that the power extraction circuitry is able to extract from the ambient power source; and a controlled switch circuit adapted to electrically disconnect the power extraction circuit from the power storage device when the indication indicates that, for the level of power that the power extraction circuit can extract from the ambient power source, the power extraction circuit is insufficient to charge the power storage device. This means that, when the indication indicates that, for the level of power that the power extraction circuit can extract from the ambient power source, the power extraction circuit is sufficient to charge the power storage device, the controlled switch circuit can electrically couple the power extraction circuit to the power storage device.

[0008] Thus, in the energy harvesting system according to the present invention, when the level of power extracted from the environmental power source is too low to charge the power storage device, the power extraction circuit is electrically disconnected from the power storage device. Electrically disconnecting the power extraction circuit from the power storage device electrically isolates them from each other, thereby preventing stored power from leaking from the power storage device through the power extraction circuit (which may otherwise occur, as described herein). Preventing leakage and loss of harvested energy can increase the efficiency of energy harvesting.

[0009] According to a further aspect of the invention as set forth in claim 14, there is provided a radio system comprising: a power emitting device adapted to wirelessly emit a power signal having a high peak-to-average power ratio; a wireless power receiving device including an energy harvesting system as described herein, wherein the power extraction circuit includes an antenna adapted to extract power from ambient radio frequency power.

[0010] For purposes of illustration, some embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which additional features will be set forth, some of which will be recited in the dependent claims and whose advantages will become apparent. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a block diagram of a wireless system including a wireless power receiving device. [Figure 2] FIG. 2 is a block diagram of a basic implementation of an energy harvesting system in a wireless power receiving device. [Figure 3] FIG. 3 is a waveform diagram of a high frequency power signal that may be radiated in a wireless system. [Figure 4] FIG. 4 is a block diagram of an improved embodiment of an energy harvesting system. [Figure 5] FIG. 5 is a combined block and circuit diagram of an improved embodiment of an energy harvesting system. [Figure 6A] FIG. 6A is a circuit diagram of a comparator circuit in an improved embodiment of the energy harvesting system. [Figure 6B] FIG. 6B is a time diagram of the various signals in the comparator circuit. [Figure 7] FIG. 7 is a graph plotting power harvesting efficiency as a function of incident RF power for various power harvesting systems. [Figure 8] FIG. 8 is a graph plotting the output voltage of the power extraction circuit as a function of incident RF power for various power harvesting systems. [Figure 9] FIG. 9 is a block diagram of an expanded and improved embodiment of the energy harvesting system. DETAILED DESCRIPTION OF THE INVENTION

[0012] FIG. 1 illustrates a schematic diagram of a wireless system 100. FIG. 1 provides a block diagram of the wireless system 100. The wireless system 100 comprises a power emitting device 101 and a wireless power receiving device 102. The wireless system 100 may, for example, constitute the infrastructure of a so-called Internet of Things through wireless power transmission. Thus, the wireless system 100 may include one or more additional wireless power receiving devices that are not depicted in FIG. 1 for the sake of simplicity and convenience. For illustrative purposes, the wireless power receiving device 102 shown in FIG. 1 constitutes a smart low-power sensor, although such a device may have different functionality in alternative embodiments.

[0013] More specifically, the smart low power sensor 102 includes an energy harvesting system 103 and a sensing system 104. The sensing system 104 may include a sensor device 105, a sensor interface 106, a processing and control circuit 107, and a transmission circuit 108. The transmission circuit 108 may be, for example, of the Bluetooth® low energy type.

[0014] The wireless system 100 can basically operate as follows: A power emitting device 101 radiates a high frequency power signal 109 toward a smart low power sensor 102. To that end, the power emitting device 101 can include an antenna array and beamforming circuitry. The high frequency power signal 109 can be in an unlicensed frequency band for industrial, scientific, and medical purposes, such as the 2.45 GHz band. In addition to power, the high frequency power signal 109 can also carry information to the smart low power sensor 102.

[0015] The radio frequency power signal 109 is an environmental power source for the smart low power sensor 102, and the ambient radio frequency power is available to the smart low power sensor 102. The energy harvesting system 103 within the smart low power sensor 102 extracts power from the ambient radio frequency power and stores the extracted power. The energy harvesting system 103 can generate a power supply voltage based on the extracted and stored power. The sensing system 104 can receive and operate entirely based on this power supply voltage. In this manner, the smart low power sensor 102 can perform battery-less operation thanks to the ambient radio frequency power and the energy harvesting system 103.

[0016] FIG. 2 schematically illustrates a basic implementation 200 of the energy harvesting system 103 in the smart low-power sensor 102. FIG. 2 provides a block diagram of this basic implementation 200, which will be referred to hereinafter for convenience as the basic energy harvesting system 200. The basic energy harvesting system 200 includes an antenna 201, an impedance matching circuit 202, a rectifier circuit 203, and a power management circuit 204 including a power storage device 205. The antenna 201, the impedance matching circuit 202, and the rectifier circuit 203 collectively form a power extraction circuit 206. The power management circuit 204 may be, for example, an integrated power management circuit model number AEM30940 proposed by e-peas semiconductors.

[0017] The basic energy harvesting system 200 basically operates as follows: The antenna 201 absorbs a portion of the ambient radio frequency power. The impedance matching circuit 202 aims to absorb as much of the ambient radio frequency power as possible. The impedance matching circuit 202 also aims to transfer as much of the absorbed ambient radio frequency power as possible to the rectifier circuit 203. The rectifier circuit 203 receives the reduced portion of the ambient radio frequency power and converts it to DC power. The power management circuit 204 receives and stores this DC power. The power management circuit 204 further generates the supply voltages described herein based on the received and stored DC power.

[0018] The basic energy harvesting system 200 has relatively low power harvesting efficiency when the ambient RF power is relatively low. It has been observed that the power harvesting efficiency can drop significantly when the ambient RF power is below -10 dBm, where dBm is a unit of level used to indicate a power ratio expressed in decibels (dB) relative to 1 milliwatt (mW). This power harvesting efficiency problem is primarily due to the following reasons:

[0019] The smaller the ambient RF power, the smaller the input RF signal voltage in rectifier circuit 203, resulting in a larger equivalent input resistance of rectifier circuit 203. For example, it has been observed that when the ambient RF power decreases from +5 dBm to -20 dBm, the equivalent input resistance increases exponentially from 0.1 kΩ to 1 MΩ.

[0020] The higher the equivalent input resistance of the rectifier circuit 203, the larger the impedance difference that the impedance matching circuit 202 must match. If the equivalent input resistance of the rectifier circuit 203 is relatively high, the impedance matching circuit 202 may not provide sufficient matching, may induce relatively large losses, or both. As a result, only a relatively small portion of the ambient RF power reaches the rectifier circuit 203, resulting in a relatively small amount of extracted DC power. This problem with impedance matching is exacerbated at relatively high frequencies, such as 2.45 GHz, due to parasitic impedances that may be present in, for example, printed circuit boards and device packages.

[0021] One constraint to consider in relation to the issue of power harvesting efficiency is regulations regarding maximum radiated power. These regulations are commonly referred to as EIRP regulations, where EIRP stands for equivalent or effective isotropically radiated power. The power emitting device 101 in the wireless system 100 shown in FIG. 1 must comply with the applicable EIRP regulations, which means that the high-frequency power signal 109 must not exceed the maximum radiated power defined by these regulations. Therefore, when the power emitting device 101 emits its maximum radiated power, there is a maximum distance from this power emitting device at which the smart low-power sensor 102 can still operate. The higher the power harvesting efficiency of the energy harvesting system 103, the greater the maximum distance.

[0022] A possible solution to the problem of power harvesting efficiency given the constraints imposed by EIRP regulations is to provide the RF power signal 109 with a high peak-to-average power ratio. That is, the RF power signal 109 can be given a non-constant envelope waveform consisting of relatively short, high-power bursts interleaved with relatively long periods of relatively low radiated power. Such a waveform can be synthesized by combining multiple tones, such as 64 BPSK-modulated tones, where BPSK stands for binary phase shift keying. More specifically, the waveform may be generated from an inverse 64-point fast Fourier transform (IFFT) of a Walsh-Hadamard code of size 64.

[0023] FIG. 3 is a schematic diagram illustrating an example of a radio frequency power signal 109 that may be radiated by the power radiating device 101 in the wireless system 100 illustrated in FIG. 1 . FIG. 3 is a waveform diagram of the radio frequency power signal 109. The radio frequency power signal 109 includes relatively short power bursts 301 that may occur periodically, for example, every 21 μs. That is, the relatively short power bursts 301 may have a period 302 of 21 μs. The power bursts 301 may have a duration 303 of 0.65 μs. The time interval between two consecutive power bursts 301 with relatively low radiated power may have a duration 304 of 20.35 μs. The radio frequency power signal 109 may have a peak-to-average power ratio 305 of 18.24 dB.

[0024] The energy harvesting system 103 may have a higher power harvesting efficiency when the high frequency power signal 109 has a relatively high peak-to-average power ratio 305, as illustrated in FIG. 3, compared to an embodiment in which the high frequency power signal 109 has a constant envelope 306, which implies a peak-to-average power ratio of 0 dB. That is, the equivalent input resistance of the rectifier circuit 203 is relatively low during the power bursts 301 of the high frequency power signal 109 illustrated in FIG. 3. This allows the impedance matching circuit 202 to provide a satisfactory impedance match, or reduce its power loss, or both, as described herein, contributing to the power harvesting efficiency.

[0025] However, as shown in FIG. 3 , when the high frequency power signal 109 has a relatively high peak-to-average power ratio 305, a phenomenon detrimental to the power harvesting efficiency of the basic energy harvesting system 200 shown in FIG. 2 may occur. During a time interval 304 in which the radiated power is relatively low, the input high frequency signal voltage at the rectifier circuit 203 may be relatively small, while a relatively large DC voltage may exist at the output node of the rectifier circuit 203. In this situation, a leakage current may occur from the output node of the rectifier circuit 203 to the signal ground. This leakage current is drawn from the DC power extracted from the power burst 301. The leakage current may be relatively large, particularly when the high frequency power signal 109 has a relatively high peak-to-average power ratio 305. Therefore, although a relatively high peak-to-average power ratio 305 may contribute to power harvesting efficiency, it may also induce current leakage problems that are detrimental to power harvesting efficiency.

[0026] A solution to the current leakage problem may include inserting a diode between the rectifier circuit 203 and the power storage device 205 to prevent leakage current from flowing from the power storage device 205 to signal ground. However, such a diode has a relatively large forward on-resistance that causes power loss, which is also detrimental to power harvesting efficiency, although to a lesser extent than the leakage current. Power loss can be mitigated by reducing the on-resistance, but this practically means increasing the diode size or selecting a specific discrete diode, which is detrimental to cost and compactness. Increasing the diode size to reduce the forward on-resistance induces a relatively large reverse leakage current, which is also detrimental to power harvesting efficiency.

[0027] FIG. 4 schematically illustrates an improved embodiment 400 of the energy harvesting system 103. FIG. 4 provides a block diagram of the improved embodiment 400 of the energy harvesting system 103, hereinafter referred to as the improved energy harvesting system 400 for convenience. The improved energy harvesting system 400 comprises various elements present in the basic energy harvesting system 200 shown in FIG. 2 , namely, an antenna 401, an impedance matching circuit 402, and a rectifier circuit 403, which collectively constitute a power extraction circuit 404. The improved energy harvesting system also comprises a power management circuit 405 including a power storage device 406. The power management circuit 405 may be based, for example, on the integrated energy management circuit model number AEM30940 proposed by e-peas semiconductors. The power storage device 406 may be comprised, for example, of a capacitor.

[0028] The improved energy harvesting system 400 further includes a sensing circuit 407 and a controlled switch circuit 408. In this embodiment, the sensing circuit 407 is coupled to receive an input radio frequency signal voltage present at the rectifier circuit 403. The controlled switch circuit 408 can be coupled between an output node of the rectifier circuit 403 and a power storage device 406 included in the power management circuit 405.

[0029] The improved energy harvesting system 400 basically operates as follows: The sensing circuit 407 provides an indication 409 of the level of power that the power extraction circuit 404 can currently extract from the ambient power source. The controlled switch circuit 408 electrically disconnects the power extraction circuit 404 from the power storage device 406 when the indication 409 indicates that the level of power that the power extraction circuit 404 can extract from the ambient power source is insufficient to allow the power extraction circuit 404 to charge the power storage device 406. Referring to FIG. 3 , this latter condition occurs during the time intervals 304 between two consecutive power bursts 301, during which the radiated radio frequency power is relatively low. Because the power storage device 406 is disconnected from the rectifier circuit 403 during these time intervals, the aforementioned leakage current is significantly reduced.

[0030] Additionally, the controlled switch circuit 408 may have an on-resistance lower than the typical on-resistance of a diode, which is another solution for preventing leakage current as described herein. Because the on-resistance of the controlled switch circuit 408 can be relatively low, there is less forward power loss during closed switch mode when the indication 409 indicates that the power extraction circuit 404 is sufficient to charge the power storage device 406 for the level of power that the power extraction circuit 404 can extract from the ambient power source. Additionally, the controlled switch circuit 408 may have an off-resistance higher than the typical off-resistance of a diode. Because the off-resistance of the controlled switch circuit 408 can be relatively high, there is less reverse leakage loss during open switch mode when the indication 409 indicates that the power extraction circuit 404 is insufficient to charge the power storage device 406 for the level of power that the power extraction circuit 404 can extract from the ambient power source. These allow the improved energy harvesting system 400 to have a relatively high power harvesting efficiency compared to the basic energy harvesting system 200, and even compared to the latter system with the addition of anti-leakage current diodes.

[0031] Figure 5 illustrates a schematic diagram of one embodiment of the improved energy harvesting system 400 presented herein with reference to Figure 4. Figure 5 provides a combined block and circuit diagram of this embodiment, which for convenience is also referred to as the improved energy harvesting system 400. The improved energy harvesting system 400 can be designed for ambient high frequency power at 2.45 GHz, for example, generated by the radiated high frequency power signal 109 as shown in Figure 3, which is generated in the power emitting device 101 shown in Figure 1.

[0032] In this embodiment, the antenna 401 includes a so-called balun circuit 501 for converting a single-ended antenna signal into a differential antenna signal. However, in another embodiment, the antenna 401 may be a differential antenna without a balun circuit. As shown in FIG. 5, the impedance matching circuit 402 is composed of an input capacitor 502, an output capacitor 503, and a pair of inductors 504 and 505 coupled between the capacitors. The input capacitor 502 may have a capacitance of, for example, 2 pF (picofarads). The output capacitor 503 may have a capacitance of, for example, 0.45 pF. The parasitic capacitance of the package and bonding wires may form the output capacitor 503. The pair of inductors 504 and 505 may have a total inductance of 7 nH, with each inductor having an inductance of 3.5 nH (nanohenries).

[0033] In this embodiment, the rectifier circuit 403 is a so-called cross-coupling rectifier circuit 403 and has dynamic threshold voltage compensation capability. Therefore, the rectifier circuit 403 in this embodiment will be referred to as the cross-coupling rectifier circuit 403 hereinafter. The cross-coupling rectifier circuit 403 has a relatively small equivalent output resistance, potentially resulting in a relatively large leakage current, as described herein. However, the leakage current problem is prevented in the improved energy harvesting system 400 by a smart gating solution provided by the sensing circuit 407 and the controlled switch circuit 408, as described herein.

[0034] The cross-coupling rectifier circuit 403 is composed of two NMOS transistors 506, 507 and two PMOS transistors 508, 509 coupled to operate as diodes. The NMOS transistors 506, 507 may have, for example, a gate width of 24 μm and a gate length of 60 nm. The PMOS transistors 508, 509 may have, for example, a gate width of 48 μm and a gate length of 60 nm. The cross-coupling rectifier circuit 403 further includes a ripple filtering capacitor 510 at the output node.

[0035] In this embodiment, the sensing circuit 407 is Miniaturized Prepaid card circuit 407. Therefore, the sensing circuit 407 in this embodiment is hereinafter referred to as a rectifier circuit. Miniaturized Prepaid card circuit This will be called 407. Miniaturized Prepaid card circuit The smaller 407 is, the less power this circuit 407 will consume, which is generally preferable. Downsizing A factor may be desirable. Maximum achievable Downsizing The factor may be determined by the manufacturing process characteristics that set the minimum transistor width.

[0036] Rectifier circuit Miniaturized Prepaid card circuit In this manner, 407 is also configured to couple two NMOS transistors 511 and 512 and two PMOS transistors 513 and 514 to operate as a diode. Miniaturized Prepaid card circuit The two NMOS transistors 511 and 512 and the two PMOS transistors 513 and 514 are significantly smaller than the two NMOS transistors 506 and 507 and the two PMOS transistors 508 and 509 of the cross-coupling rectifier circuit 403 . Miniaturized Prepaid card circuit The NMOS transistors 511 and 512 may have, for example, a gate width of 0.4 μm and a gate length of 60 nm. This corresponds to a size ratio of approximately 1:60 relative to the NMOS transistors 506 and 507 of the cross-coupling rectifier circuit 403. The PMOS transistors 513 and 514 may have, for example, a gate width of 0.8 μm and a gate length of 60 nm. This also corresponds to a size ratio of approximately 1:60 relative to the PMOS transistors 508 and 509 of the cross-coupling rectifier circuit 403. Thus, in this example, a gate width of approximately 60 nm is used. Downsizing The coefficient is applied.

[0037] Rectifier circuit Miniaturized Prepaid card circuit 407 also has a ripple filtering capacitor 515 at the output node, which is the same as the ripple filtering capacitor 510 in the cross-coupling rectifier circuit 403. Miniaturized Prepaid card circuit The same scaling factor can be applied to this reduced ripple filtering capacitor 515. Thus, the reduced ripple filtering capacitor 515 can have a capacitance that is approximately 60 times smaller than the capacitance of the ripple filtering capacitor 510 at the output node of the cross-coupling rectifier circuit 403.

[0038] In this embodiment, the controlled switch circuit 408 is composed of a comparator circuit 516, a buffer 517, and a PMOS transistor 518 that operates as a switch between the cross-coupling rectifier circuit 403 and the power storage device 406 in the power management circuit 405. In this embodiment, the power storage device 406 is in the form of a capacitor. Hereinafter, the PMOS transistor 518 operating as a switch will be referred to as the PMOS switch 518 for convenience. The size of the PMOS switch 518 can be twice that of the PMOS transistors 508 and 509 in the cross-coupling rectifier circuit 403. This contributes to the PMOS switch 518 having a relatively low forward on-resistance, and therefore contributes to power harvesting efficiency. The PMOS switch 518 also provides an advantage when the improved energy harvesting system 400 must start up. That is, at start-up, the PMOS switch is closed by default because its gate voltage is low, e.g., 0 V.

[0039] In this embodiment, the power management circuit 405 further includes a maximum power point tracking circuit 523 and a DC / DC converter circuit 524, where DC stands for direct current. The maximum power point tracking circuit 523 adjusts the voltage of the power storage device 406 to improve power extraction efficiency. The DC / DC converter circuit generates a stable DC power supply voltage 525 for the sensing system 104 shown in FIG. 1 regardless of voltage fluctuations of the power storage device 406. The maximum power point tracking circuit 523 and the DC / DC converter circuit 524 may be based on, for example, an integrated power management circuit model number AEM30940 proposed by e-peas semiconductors.

[0040] The improved energy harvesting system 400 illustrated in FIG. 5 basically operates as follows: Miniaturized Prepaid card circuit 407 provides an output voltage 519 that represents the output voltage 520 of the cross-coupled rectifier circuit 403 when the PMOS switch 518 is in an open state. Miniaturized Prepaid card circuitThe output voltage 519 of 407 represents the output voltage 520 when no load is coupled to the output node of the cross-coupling rectifier circuit 403. Miniaturized Prepaid card circuit The output voltage of 407 is referred to as sensed voltage 519. In this embodiment, sensed voltage 519 may therefore constitute the indication 409 referred to herein with respect to the improved energy harvesting system 400 illustrated in FIG.

[0041] If the sensed voltage 519 is higher than the voltage present in the power storage device 406 of the power management circuit 405, the power extraction circuit 404 can charge the power storage device 406. This is typically the case during the power burst 301 in the high frequency power signal 109 illustrated in Figure 3. In that case, the PMOS switch 518 can be set to a closed state, providing a low resistance path between the cross-coupled rectifier circuit 403 and the power management circuit 405.

[0042] Conversely, if the sensed voltage 519 is lower than the voltage present at the power storage device 406, the power extraction circuit 404 may not be able to charge the power storage device 406. This is typically the case during the time interval 304 between power bursts 301 in the high frequency power signal 109 illustrated in FIG. 3. In that case, the current leakage problem described herein may occur, and a relatively large leakage current may flow from the power storage device 406 to signal ground, constituting a loss of harvested energy. In that case, the PMOS switch 518 may be set to an open state to prevent or at least mitigate this loss.

[0043] Comparator circuit 516 compares sense voltage 519 to the voltage present at power storage device 406, applied to non-inverting input 521. If sense voltage 519 is higher, comparator circuit 516 causes buffer 517 to apply a relatively low voltage, close to signal ground, to the gate of PMOS switch 518, which sets the PMOS in a closed state and allows power extraction circuit 404 to charge power storage device 406. Conversely, if sense voltage 519 is lower than the voltage present at power storage device 406, comparator circuit 516 causes buffer 517 to apply a relatively high voltage, close to supply voltage level 522, to the gate of PMOS switch 518, which sets the PMOS in an open state, thereby combating current leakage issues. Power supply voltage 522 for comparator circuit 516 may be the voltage of power storage device 406 or the output voltage 525 generated by power management circuit 405.

[0044] In practice, the comparator circuit 516 exhibits rising and falling edge delays and voltage signal propagation delays corresponding to the delays for switching the PMOS switch 518 from a closed state to an open state and vice versa. These delays can affect power harvesting efficiency. During the delay for switching the PMOS switch 518 from a closed state to an open state, leakage current can occur. During the delay for switching the PMOS switch 518 from an open state to a closed state, power that could be harvested is lost.

[0045] 6A is a schematic diagram of an embodiment of the comparator circuit 516 in the improved energy harvesting system 400 illustrated in FIG. 5. FIG. 6A provides a circuit diagram of this embodiment, which for convenience will be referred to hereinafter as the comparator circuit 516. The comparator circuit 516 is comprised of a differential transistor pair 601 providing an inverting input and a non-inverting input. The differential transistor pair 601 may be comprised of relatively narrow, low threshold voltage MOS transistors having a length of, for example, 0.2 μm, so that the aforementioned delay for switching the PMOS switch 518 is relatively small.

[0046] The delay can generally be reduced by increasing the bias current of the differential transistor pair. However, this comes at the expense of more power being consumed by the comparator circuit 516, which is detrimental to power harvesting efficiency. Therefore, a compromise must be made regarding the bias current. A satisfactory compromise can be achieved with a bias current of tens of nanoamperes (nA), e.g., 30 nA. The bias current 602 can be generated from a beta multiplier reference circuit or a bandgap reference circuit. The bias current 602 is mirrored to different circuit branches by a current mirror circuit 614.

[0047] More specifically, comparator circuit 516 comprises switch 603, switch 604, transistors 605 and 606 forming a differential transistor pair 601, transistors 607 and 608 coupled to a current mirror formed by transistors 609 and 610, circuit line 611, and a delay circuit 612 followed by a buffer providing a voltage signal 613. Switch 603 may be in the form of, for example, a PMOS transistor. Switch 604 may be in the form of, for example, an NMOS transistor. Voltage signal 613 may be applied to the gate of PMOS transistor 603 and the gate of NMOS transistor 604. Delay circuit 612 may be comprised of, for example, a resistor and a capacitor.

[0048] 6B is a schematic representation of the signals in comparator circuit 516, generally depicted as a time diagram. The time diagram has a horizontal axis representing time T and a vertical axis representing signal level L. The time diagram is divided into sections, each with a horizontal axis representing a respective signal.

[0049] Comparator circuit 516 shown in FIG. 6A essentially operates as follows, thereby referencing the signals shown in FIG. 6B. When voltage signal 613 is high during time interval 304 between power bursts 301 in radio frequency power signal 109 shown in FIG. 3, switch 604 shorts transistor 608 to transistor 610, and switch 603 disconnects transistor 607 from transistor 609. As a result, comparator circuit 516 has a threshold voltage 618 that is lower than voltage signal 615 at the gate of transistor 606, which may be the voltage of power storage device 406. Voltage signal 619 on circuit line 611 of comparator circuit 516, which is the input voltage of the buffer, goes from high to low when voltage signal 616 at the gate of transistor 605, which is voltage 519 shown in FIG. 5, rises above threshold voltage 618. After a time delay 621 introduced by the limited transient response of the comparator 516 and the buffer, the PMOS switch 518 closes when its gate voltage signal 620 goes from high to low.

[0050] After a time delay introduced by delay circuit 612, voltage signal 613 transitions from high to low. Delay circuit 612 ensures stability in the transition of the comparator's threshold voltage. When voltage signal 613 transitions low, switch 604 disconnects transistor 608 from transistor 610, and switch 603 shorts transistor 607 to transistor 609. The comparator has a threshold voltage 617 higher than voltage signal 615 at the gate of transistor 606, which may be the voltage of power storage device 406. Voltage signal 619 on circuit line 611 of this comparator circuit 516 transitions from low to high when voltage signal 616 at the gate of transistor 605, voltage 519 shown in FIG. 5, falls above threshold voltage 617. After a time delay 622 introduced by the limited transient response of comparator 516 and the buffer, gate voltage signal 620 transitions from low to high, opening PMOS switch 518.

[0051] Thus, comparator circuit 516 exhibits a delay hysteresis that can match the duration that voltage signal 620 is low with the duration that voltage 616 is greater than or equal to voltage 615. In this manner, comparator circuit 516 can compensate for, or at least mitigate, the delays described herein, thereby achieving proper timing in controlling the PMOS switches to coincide with power bursts 301 in high frequency power signal 109 illustrated in FIG.

[0052] FIG. 7 illustrates the relationship between power harvesting efficiency and incident RF power for different power harvesting systems. FIG. 7 provides a graph 700 with a horizontal axis representing average incident RF power expressed in dBm. The vertical axis represents power harvesting efficiency expressed as a percentage (%) of the average incident RF power that can be effectively harvested. Graph 700 includes three curves 701-703.

[0053] A first curve 701 with circular dots illustrates the relationship between power harvesting efficiency and average incident RF power for the improved energy harvesting system 400 shown in Figure 5 when the system receives a non-constant envelope waveform such as that shown in Figure 3, with a peak-to-average power ratio 305 of 18.24 dB. A second curve 702 with triangular dots illustrates the relationship between power harvesting efficiency and average incident RF power for the basic energy harvesting system 200 shown in Figure 2 when the system receives the same non-constant envelope waveform. A third curve 703 with square dots illustrates the relationship between power harvesting efficiency and average incident RF power for the basic energy harvesting system 200 shown in Figure 2 when the system receives a constant envelope waveform, and therefore has a peak-to-average power ratio of 0 dB.

[0054] 7 illustrates that the improved energy harvesting system 400 illustrated in FIG. 5 exhibits relatively high power harvesting efficiency when the average incident RF power is relatively low, below −10 dBm. That is, curve 701, which represents the power harvesting efficiency of the improved energy harvesting system 400, is located well above curve 702, which represents the power harvesting efficiency of the basic energy harvesting system 200 receiving the same non-constant envelope waveform with the same peak-to-average power ratio. This is primarily due to the smart gating solution of the improved energy harvesting system 400 provided by the sensing circuit 407 and the controlled switch circuit 408, as described herein. This solution combats the leakage current problem in the basic energy harvesting system 200, which is detrimental to power harvesting efficiency.

[0055] 7 further shows that when the average incident RF power is relatively low, less than −10 dBm, power harvesting efficiency can be improved by using a non-constant envelope waveform with a relatively high peak-to-average power ratio instead of a constant envelope waveform. That is, curves 701 and 702, which show the power harvesting efficiency when using a non-constant envelope waveform with a relatively high peak-to-average power ratio, are both located above curve 703, which shows the power harvesting efficiency when using a constant envelope waveform. As explained above, this is mainly due to the impedance matching and power loss issues that arise when the incident RF power is relatively low and the rectifier input impedance is relatively high.

[0056] FIG. 8 illustrates the relationship between the output voltage of a power extraction circuit and incident radio frequency power for different power harvesting systems. FIG. 8 provides a graph 800 with a horizontal axis representing the average incident radio frequency power in dBm. The vertical axis represents the output voltage (V) from the power extraction circuit that the power management circuit can accept as an input voltage. The output voltage is measured when the power extraction circuit is an open circuit with no load. Graph 800 includes three curves 801-803.

[0057] A first curve 801 with circular dots illustrates the relationship between the power extraction circuit output voltage and the average incident RF power for the improved energy harvesting system 400 shown in Figure 5 when the system receives a non-constant envelope waveform such as that shown in Figure 3 having a peak-to-average power ratio 305 of 18.24 dB. A second curve 802 with triangular dots illustrates the relationship between the power extraction circuit output voltage and the average incident RF power for the basic energy harvesting system 200 shown in Figure 2 when the system receives the same non-constant envelope waveform. A third curve 803 with square dots illustrates the relationship between the power extraction circuit output voltage and the average incident RF power for the basic energy harvesting system 200 shown in Figure 2 when the system receives a constant envelope waveform and therefore has a peak-to-average power ratio of 0 dB.

[0058] 8 illustrates that the output voltage of the power extraction circuit of the improved energy harvesting system 400 illustrated in FIG. 5 is relatively high when the average incident RF power is relatively low, below −10 dBm. That is, curve 901, which represents the output voltage of the power extraction circuit of the improved energy harvesting system 400, is located well above curve 902, which represents the output voltage of the power extraction circuit of the basic energy harvesting system 200, which receives the same non-constant envelope waveform with the same peak-to-average power ratio. This is primarily due to the smart gating solution of the improved energy harvesting system 400, provided by the sensing circuit 407 and the controlled switch circuit 408, as described herein. This solution prevents leakage current problems in the basic energy harvesting system 200, which are detrimental to the output voltage of the power extraction circuit.

[0059] 8 further shows that when the average incident RF power is relatively low, less than −10 dBm, using a non-constant envelope waveform with a relatively high peak-to-average power ratio instead of a constant envelope waveform results in a relatively high output voltage of the power extraction circuit. That is, curves 801 and 802, which represent the output voltage of the power extraction circuit when a non-constant envelope waveform with a relatively high peak-to-average power ratio is used, are both located above curve 803, which represents the output voltage of the power extraction circuit when a constant envelope waveform is used. As explained above, this is mainly due to the problem of impedance matching and its power loss, which occurs when the incident RF power is relatively low and the rectifier input impedance is relatively high.

[0060] The higher the output voltage of the power extraction circuit, the easier it is to activate the power management circuit and the higher the efficiency of the power management circuit, especially the power conversion efficiency. Thus, Figure 8 also demonstrates the advantages of the improved energy harvesting system 400 shown in Figure 5 over other energy harvesting techniques discussed above.

[0061] FIG. 9 schematically illustrates an enhanced and improved implementation 800 of the energy harvesting system 103. FIG. 9 provides a block diagram of this enhanced and improved embodiment 900, hereinafter referred to as the enhanced and improved energy harvesting system 900 for convenience. The enhanced and improved energy harvesting system 900 is comprised of multiple power extraction circuits 901-903 coupled to a power storage device 904. For simplicity and convenience, FIG. 9 illustrates three power extraction circuits 901, 902, and 903. The three power extraction circuits 901, 902, and 903 may have similar topologies but different parameters to extract power differently from the environmental power source. For example, the three power extraction circuits 901, 902, and 903 may be optimized for different frequencies that the environmental power source may have, or for different power levels that the environmental power source may provide, or for various combinations thereof. The three power extraction circuits 901, 902, 903 may share a common transducer 905, such as an antenna, that picks up ambient power and converts it into an electrical input signal to a rectifier.

[0062] The first power extraction circuit 901 is coupled to the power storage device 904 via a first controlled switch circuit 906. A first sensing circuit 907 provides an indication of the level of power the first power extraction circuit 901 can extract from the ambient power source. Similarly, the second power extraction circuit 902 is coupled to the power storage device 904 via a second controlled switch circuit 908. A second sensing circuit 909 provides an indication of the level of power the second power extraction circuit 902 can extract from the ambient power source. The third power extraction circuit 903 is coupled to the power storage device 904 via a third controlled switch circuit 910. A third sensing circuit 911 provides an indication of the level of power the third power extraction circuit 903 can extract from the ambient power source.

[0063] The sensing circuitry and controlled switch circuitry described above may operate in a manner similar to that described herein above with respect to the sensing circuitry and controlled switch circuitry in the improved energy harvesting system 400 illustrated in Figure 4. The enhanced improved energy harvesting system 900 thus has smart and adaptive operation that allows the system to efficiently harvest energy under a variety of conditions.

[0064] (Note) The embodiments described herein with reference to the drawings are offered by way of example. The invention can be embodied in many different ways. To illustrate this, some alternatives are simply presented.

[0065] The present invention can be applied to a wide variety of products or methods related to power harvesting. In the embodiments described herein, the environmental power source is a radiated high-frequency signal. In other embodiments, the environmental power source may be in the form of, for example, light, heat, wind, vibration, and other types of mechanical energy. Regardless of the form of the environmental power source, the present invention prevents leakage, and therefore loss of harvested energy, in situations where the level of power that can be extracted from the environmental power source fluctuates. The fluctuations in power level may be relatively rapid, e.g., having a frequency greater than 1 kHz, or may be more gradual. For example, assume that the environmental power source is in the form of mechanical vibrations of varying intensity. In such an alternative embodiment, the energy harvesting system according to the present invention may include a transducer for converting the mechanical vibrations into an electrical alternating current signal. Additionally, many details of the implementation and operation of the embodiments described herein may be substituted for such alternative embodiments.

[0066] There are many different ways to implement a power extraction circuit in an energy harvesting system in accordance with the present invention. For example, referring to the embodiment shown in FIG. 4 , the impedance matching circuit 402 may be tunable or another impedance matching network structure, whereby the power extraction circuit may include tuning control circuitry for tuning the impedance matching circuit 402 in order to maximize power harvesting efficiency. As another example, the power extraction circuit may include multiple antennas, or multiple impedance matching circuits, or both, whereby the power extraction circuit may include selection control circuitry for selecting an antenna, an impedance matching circuit, or both in order to maximize power harvesting efficiency.

[0067] There are many different ways to implement a controlled switch circuit in an energy harvesting system according to the present invention. In the embodiment shown in Figure 5, the voltage present at the power storage device 406 is used as the comparator threshold voltage. Other embodiments may use a different threshold voltage that reflects the boundary between when the power extraction circuit is likely to charge or discharge the power storage device. As another example, the controlled switch circuit may be configured as a comparator with hysteresis, or a general continuous-time static comparator, or an auto-zero comparator.

[0068] Generally, there are many different ways to implement the present invention, and various implementations may have various topologies. In any given topology, a single entity may perform multiple functions, or multiple entities may jointly perform a single function. In this regard, the drawings are highly schematic. For example, while FIG. 1 illustrates a wireless power receiving device 102 with two antennas, the wireless power receiving device 102 may also consist of a single antenna with a radio frequency switch for switching between receiving and transmitting via the single antenna.

[0069] The remarks made herein demonstrate that the embodiments described with reference to the drawings illustrate the invention, rather than limit it. The invention can be embodied in numerous alternative ways within the scope of the appended claims. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. Any reference signs in the claims should not be construed as limiting the claims. The verb "comprise" in the claims does not exclude the presence of other elements or steps than those stated in the claims. The same applies to similar verbs such as "include" and "contain." In a product claim, the recitation of an element in the singular does not exclude the possibility that the product comprises a plurality of such elements. Similarly, the recitation of a step in the singular in a method claim does not exclude the possibility that the method comprises a plurality of such steps. The mere fact that each dependent claim recites each additional feature does not exclude combinations of additional features other than those reflected in the claims.

Claims

1. A power extraction circuit (404) including a rectifier circuit (403) adapted to convert AC power to DC power, the power extraction circuit (404) adapted to extract power from an environmental power source (109) prone to providing fluctuating power levels; a power storage device (406) adapted to store the DC power; a sensing circuit (407) coupled to receive a signal present at an input of the rectifier circuit and adapted to provide an indication (409) of the level of power that the power extraction circuit is currently able to extract from the ambient power source; and a controlled switch circuit (408) adapted to electrically disconnect the power extraction circuit from the power storage device when the indication indicates that the level of power currently available for the power extraction circuit to extract from the environmental power source is insufficient to charge the power storage device.

2. The energy harvesting system of claim 1 , wherein the rectifier circuit (403) is a cross-coupled rectifier circuit.

3. The energy harvesting system of claim 1 or 2, wherein the sensing circuit (407) includes a miniaturized replica circuit of the rectifier circuit.

4. 4. The energy harvesting system of claim 3, wherein the power extraction circuit (404) includes a capacitor (510) at an output node of the rectifier circuit, and the sensing circuit (407) includes a miniaturized replica circuit (515) of the capacitor at an output node of the miniaturized replica circuit of the rectifier circuit.

5. 5. The energy harvesting system of claim 4, wherein the same miniaturization factor is applied to the miniaturized replica circuit of the rectifier circuit and the miniaturized replica circuit of the capacitor in the sensing circuit (407) for each of the rectifier circuit and the capacitor in the power extraction circuit (404).

6. An energy harvesting system as described in any one of claims 3 to 5, wherein the rectifier circuit (403) includes semiconductor elements (506-509) that function as rectifier elements, and the miniaturized replica circuit of the rectifier circuit includes miniaturized replica circuits (511-514) of the semiconductor elements that function as rectifier elements.

7. 7. The energy harvesting system of claim 3, wherein the controlled switch circuit (408) is adapted to electrically disconnect the power storage device (406) from the power extraction circuit (403) when the voltage (519) at the output node of the miniaturized replica circuit (407) of the rectifier circuit falls below a threshold level.

8. The energy harvesting system of claim 7 , wherein the threshold level corresponds to a voltage of the power storage device (406).

9. 9. The energy harvesting system of claim 7 or 8, wherein the controlled switch circuit (408) includes a comparator with delayed hysteresis adapted to compare the voltage (519) at the output node of the miniaturized replica circuit (407) of the rectifier circuit with the threshold level.

10. 10. The energy harvesting system of claim 1, further comprising a power management circuit (405) adapted to generate a power supply voltage from the DC power stored in the power storage device (406).

11. 11. An energy harvesting system as claimed in any one of claims 1 to 10, comprising a plurality of power extraction circuits (901-903) of which the power extraction circuit is a part, the plurality of power extraction circuits having similar topologies but different parameters for extracting power differently from the environmental power source.

12. The energy harvesting system of any one of claims 1 to 11, wherein the power extraction circuit (404) includes an antenna (401) adapted to extract power from ambient radio frequency power.

13. 13. The energy harvesting system of claim 12, wherein the power extraction circuit (404) includes an impedance matching network (402) coupled between the antenna (401) and the rectifier circuit (403).

14. a power emitting device (101) adapted to wirelessly radiate a power signal (109) having a high peak-to-average power ratio (305); A wireless system (100) comprising: a wireless power receiving device (102) comprising the energy harvesting system (400) according to any one of claims 1 to 13.

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