Impedance tuning device and method and rectifier circuit, for a wireless power transfer apparatus

Technical field

The present invention relates to impedance tuning, more particularly to a impedance tuning device and method and rectifier circuit for a wireless power transfer apparatus.

Background

Wireless Power Transfer (WPT) has emerged as one of the most promising solutions to achieve self-sustainable electronic device operation for the Internet of Things (loTs) applications. With the increasing number and ubiquity of connected wireless sensor networks (WSNs), together with enhanced capacity for data management, intelligent monitoring can be achieved in a wide variety of applications, such as environment, weather forecasting, biomedical treatment, and security. Devices (e.g., sensor devices and routing devices) in such WSNs typically have batteries as their primary power sources. With the ubiquity of such devices, battery replacement and maintenance can become difficult and costly particularly where such sensors are employed in remote locations or permanently embedded in rigid structures. WPT technologies are suitable for extending and prolonging battery life of such devices, reducing the need for battery replacement and maintenance. WPT systems operate by wirelessly transferring energy in the form of electromagnetic (EM) waves for receipt and use by such devices. Due to the penetrative properties of EM waves, WPT systems are well suited for powering those sensors that are otherwise hard to reach and that need to be powered up and interrogated periodically.

Figure 1 shows an example conventional sensor system 100. It includes an antenna 110 to receive an RF power signal (marked by“PRF”), a rectifier 120 to convert the received RF power signal into a first direct current (DC) signal (marked by “PDCI”), a power management (PM) module 130 with a boost converter (not shown) to store the charges of the first DC signal in an energy storage element (e.g., a rechargeable Li-ion battery or supercapacitor, not shown), and a sensor node 140 powered by a second DC signal (marked by “PDC2”) drawn from the energy storage element for sensor data communication. In this system 100, the RF-to-DC conversion efficiency is an important factor that determines the amount of energy available for the sensor node 140, the charging duration of the energy storage element of the PM module 130, and the performance of the sensor node 140. Conventional rectifier designs cannot achieve high conversion efficiencies over a wide range of input powers. In addition, the dynamic nature of the PM module 130 and the sensor node 140 causes the loading conditions of the rectifier 120 to vary, which may deteriorate the overall performance.

US Patent Publication Nos.: 2018/0062419A1 , 2015/0372541 A1 , and

US2018/0062418A1 provide different impedance tuning solutions. However, they suffer from poor performance.

It is desirable to provide an impedance tuning device and method and a rectifier circuit, which address at least one of the drawbacks of the prior art and/or to provide the public with a useful choice.

Summary

According to one aspect, there is provided an impedance tuning device for a wireless power transfer apparatus, comprising: a rectifier circuit arranged to be electrically coupled to a load; and a reflection power detector configured to detect magnitude of a reflection power of a load reflected signal from the load; the rectifier circuit’s input impedance configured to be tuned based on the detected magnitude of the reflection power.

The described embodiment is particularly advantageous. By tuning the input impedance of the rectifier circuit based on magnitude of the detected reflection power, the rectification performance of the rectifier circuit can be tuned according to needs, including varying input power and load conditions, for example. The rectifier circuit may be implemented as part of a wireless powering solution to improve wireless power transfer efficiency of a sensor node in various loT applications. This is particularly desirable for applications where the sensor node is located in a remote location or embedded in a rigid structure where battery replacement and maintenance may be challenging, and such a wireless powering solution may address such problems.

The impedance tuning device may further comprise a controller configured to determine a biasing voltage from the detected magnitude of the reflection power; and further configured to tune the input impedance based on the determined biasing voltage. Preferably, the rectifier circuit may include a first rectifier configured to receive an alternating current (AC) signal and further configured to convert the AC signal into a first direct current (DC) signal corresponding to the load reflected signal.

The first rectifier may include a first varactor arranged to tune the first rectifier’s input impedance in response to a first rectifier biasing signal relating to the detected reflection power. This arrangement of the first varactor is particularly advantageous. In response to the first rectifier biasing signal, the first varactor tunes the input impedance of the first rectifier based on the detected reflection power.

It is preferred that the first rectifier may include first and second impedance modes, and the first DC signal may be arranged to be converted in one of the first and second impedance modes. The first rectifier may also include additional impedance modes different from the first and second impedance modes, achieving a wider and/or finer range of adjustment of the input impedance and hence the first DC signal. In such a configuration, the first rectifier can be tuned to operate with an input impedance that results in a lower reflection power, achieving an improved RF- DC efficiency. The controller may be configured to generate the first rectifier biasing signal based on a determination of the one of the first and second impedance modes. The rectifier circuit may further comprise a second rectifier arranged to convert the AC signal into a second direct DC signal corresponding to the load reflected signal; the impedance tuning device further comprises a switch configured to route the AC signal to one of the first and second rectifiers. This is advantageous in that one of the first and second rectifiers may result in a higher RF-DC conversion efficiency and may be selected accordingly to perform the conversion. The RF switch efficiently routes or directs the AC signal to one of the first and second rectifiers.

Preferably, the second rectifier may include a second varactor arranged to tune the second rectifier’s input impedance in response to a second rectifier biasing signal relating to the detected reflection power. This arrangement of the second varactor is particularly advantageous. In response to the second rectifier biasing signal, the second varactor tunes the input impedance of the second rectifier based on the detected reflection power.

The second rectifier may include third and fourth impedance modes, and the second DC signal may be converted in one of the third and fourth impedance modes. The second rectifier may also include additional impedance modes different from the third and fourth impedance modes, achieving a wider and/or finer range of adjustment of the input impedance and hence the second DC signal.

Preferably, the controller may be configured to generate the second rectifier biasing signal based on a determination of the one of the third and fourth impedance modes. It is preferable that the impedance tuning device may further comprise a supply power detector configured to detect a supply power of the AC signal, wherein the switch may be configured to route the AC signal to one of the first and second rectifiers based on the detected supply power. This is particularly useful in that the first and second rectifiers may be optimised or tuned for respective, different ranges of input power. By detecting the supply power, a more suitable one of the first and rectifiers can be selected to perform the RF-DC conversion to achieve in an improved conversion efficiency. According to a second aspect, there is provided an impedance tuning method for a wireless power transfer apparatus, comprising detecting magnitude of a reflection power of a load reflected signal from a load, the load being coupled to a rectifier circuit; and tuning an input impedance of the rectifier circuit based on the detected magnitude of the reflection power.

In a third aspect, there is provided a rectifier circuit for a wireless power transfer apparatus, the rectifier circuit comprising a rectifier having a varactor configured to tune the rectifier’s input impedance in response to a rectifier biasing signal. In a fourth aspect, there is provided a wireless power transfer apparatus comprising the impedance turning device, a wireless receiver arranged to receive a wireless RF signal for the rectifier circuit, and the load powered by the rectifier circuit’s output. It is envisaged that features relating to one aspect may be applicable to the other aspects.

Brief Description of the drawings

Example embodiments will now be described hereinafter with reference to the accompanying drawings, wherein like parts are denoted by like reference numerals. Among the drawings:

Figure 1 shows a conventional wireless power transfer system; Figure 2 shows an impedance tuning device of one embodiment of the present disclosure;

Figure 3A shows an RF switch in association with a source of an AC signal and rectifiers of the system of Figure 2;

Figure 3B shows a schematic circuit diagram of a rectifier representative of either one of the rectifiers of the system of Figure 2;

Figure 3C shows a schematic circuit diagram of a supply power detector (or a reflection power detector) used in the system of Figure 2;

Figure 3D shows a line graph of the input power to output voltage of the supply power detector (or the reflection power detector) of Figure 3C.

Figure 4 shows Smith charts of impedance measurements using the system of Figure 2 with different load impedances and input powers;

Figure 5 shows line charts of rectifier reflection coefficients versus input powers for different load impedances for each of the rectifiers of the system of Figure 2;

Figure 6 shows a flowchart of a method performed by a microcontroller (MCU) of the system of Figure 2;

Figure 7 shows line charts of RF-DC efficiencies versus input powers obtained using the rectifiers according to the method of Figure 6; and

Figure 8 is a block diagram of a variation of Figure 2, which illustrates the impedance tuning device including one rectifier.

Detailed Description

Figure 2 shows a schematic diagram of an impedance tuning system (or impedance tuning device) 200 according to one example embodiment of the present disclosure. The system or device 200 includes a directional coupler 210, a supply power detector 220, a reflection power detector 230, a radio frequency (RF) switch 240, a signal rectification part (or rectifier circuit) 250, and a microcontroller (MCU) 260.

The directional coupler 210 is electrically coupled (indicated as solid“Power Flow” lines in Figure 2) to the rectifier circuit 250 for sampling input and reflected signals from the rectifier circuit 250. The input and reflected signals are then fed to the supply power detector 220 and the reflection power detector 230 which are electrically connected to the directional coupler 210 to output corresponding DC voltages.

The MCU 260 is communicatively coupled (indicated as dotted“Signal Flow” lines in Figure 2) to the supply power detector 220, the reflection power detector 230, the RF switch 240, and the rectifier circuit 250. The corresponding output DC voltages from the supply power detector 220 and the reflection power detector 230 are measured by the MCU 260, and are representative of the incident and reflected signal power levels. Based on the output DC voltages, the MCU 260 selectively powers a rectifier 251 , 252 from the rectifier circuit 250 via the RF switch 240, and tunes the input impedance of the selected rectifier 251 , 252 to match with the characteristic impedance of the system. This minimizes the reflected signal from the rectifier circuit 250, and thus system performance including RF-DC conversion efficiency is optimized over a wide range of input power, as well as loading conditions.

The system 200 is employed in association with a sensor node (also referred to as “load”) and an antenna (also referred to as “source”, not shown). The antenna is arranged to receive a source supply signal (such as a radio frequency AC signal). Other types of controller may be used in place of the MCU 260. The foregoing description of Figure 2 provides an overview of the system 200. Each component of the system 200 is further described in the following paragraphs with reference (and where applicable, cross-reference) to Figures 2, and 3A to 3D. With reference to Figure 2, the directional coupler 210 has a coupler input port 211 , a coupler output port 212, a coupled port 213 and an isolated port 214. The directional coupler 210 receives the source supply signal from the antenna via the coupler input port 211. The directional coupler 210 is configured to sample the source supply signal for detection by the supply power detector 220. Specifically, the directional coupler 210 is configured to split the received source supply signal into an intermediate supply signal and a sample supply signal of a predetermined ratio (e.g., 1 :1 or 9:1 ) for output via the coupler output port 212 and the coupled port 213, respectively. The intermediate supply signal and the sample supply signal differ from the source supply signal only in magnitude in this embodiment. That is, the intermediate supply signal and the sample supply signal are respective AC signals that differ from the source supply signal only in magnitude. The coupler output port 212 is connected electrically to the RF switch 240. The coupled port 213 is connected electrically to the supply power detector 220. In such a configuration, the directional coupler 210 is arranged to sample the source supply signal corresponding to the intermediate supply signal to provide the sample supply signal for detection by the supply power detector 220.

The supply power detector 220 has a supply detector input port 221 connected (or coupled) electrically to the coupled port 213, and a supply detector output port 222 connected electrically to the MCU 260. The supply power detector 220 receives the sample supply signal from the directional coupler 210 via the supply detector input port 221 , and detects a power (referred to as“supply power”) of the received sample supply signal to generate a supply power detection signal. The detected supply power is representative of a power (referred to as “RF power”) of the source supply signal. The supply power detector 220 outputs the supply power detection signal to the MCU 260 via the supply detector output port 222. In an alternative embodiment without the directional coupler 210, the supply power detector 220 may be otherwise arranged to detect the source supply signal, which may serve as the intermediate supply signal.

The RF switch 240 is a MOSFET-based single-pole-double-throw (SPDT) switch with a switch input port 241 connected electrically to the coupler output port 212, first and second switch output ports 242, 243 connected electrically to the signal rectification part 250, and first and second switch control input ports 244, 245. The RF switch 240 is operable in a first switch state and a second switch state. In the first switch state (as depicted in Figure 2), the RF switch 240 routes the intermediate supply signal from the switch input port 241 to the first switch output port 242. In the second switch state, the RF switch 240 routes the intermediate supply signal from the switch input port 241 to the second switch output port 243. The RF switch 240 receives switch control signals via the switch control input ports 244, 245, respectively, and operates in one of the first and second switch states according to the received switch control signals. The RF switch 240 is shown in Figure 3A to be associated with the source (marked by“S”) of the source supply signal. In this embodiment, the system 200 adopts an active design where the switch control signals are generated and provided by the MCU 260 to the RF switch 240, which is described in detail below. With a high isolation, a low insertion loss, and a low power consumption, the RF switch 240 can efficiently toggle between the first and second switch states to route the intermediate supply signal to one of first and second rectifiers 251 , 252 for signal rectification, respectively, according to the switch control signals. The signal rectification part 250 is configured to receive and rectify the intermediate supply signal in order to generate a load supply signal for provision to the load. The signal rectification part 250 includes first and second rectifiers 251 , 252 in this embodiment. The first rectifier 251 has a first rectifier input port 251 a connected electrically to the first switch output port 242, and a first rectifier output port 251 b connected electrically to the MCU 260. When the RF switch 240 is in the first switch state (as depicted in Figure 2), the first rectifier 251 receives the intermediate supply signal from the directional coupler 210 via the RF switch 240. The first rectifier 251 has first and second impedance modes, and is responsive to the intermediate supply signal to convert the intermediate supply signal into a first direct current (DC) signal in one of the first and second impedance modes. That is, the first rectifier 251 performs the conversion when it receives the intermediate supply signal. In other words, the first rectifier 251 converts the AC signal into the first DC signal in one of the first and second impedance modes in response to the AC signal. The first rectifier 251 outputs the first DC signal via the first rectifier output port 251 b to the MCU 260. In this embodiment, the first rectifier 251 includes a first varactor (not shown in Figure 2) responsive to a first rectifier biasing signal to cause the first rectifier 251 to operate in one of the first impedance mode and the second impedance mode. The first rectifier 251 further has a first rectifier control input port 251 c for receiving the first rectifier biasing signal. In this embodiment, the first and second impedance modes are unbiased and biased impedance modes, respectively.

The second rectifier 252 has a second rectifier input port 252a connected electrically to the second switch output port 243, and a second rectifier output port 252b connected electrically to the MCU 260. When the RF switch 240 is in the second switch state, the second rectifier 252 receives the intermediate supply signal from the directional coupler 210 via the RF switch 240. The second rectifier 252 has third and fourth impedance modes, and is responsive to the intermediate supply signal to convert the intermediate supply signal into a second DC signal in one of the third and fourth impedance modes. That is, the second rectifier 252 performs the conversion when it receives the intermediate supply signal. In other words, the second rectifier 252 converts the AC signal into the second DC signal in one of the third and fourth impedance modes in response to the AC signal. The second rectifier 252 outputs the second DC signal via the second rectifier output port 252b to the MCU 260. In this embodiment, the second rectifier 252 includes a second varactor (not shown) responsive to a second rectifier biasing signal to cause the second rectifier 252 to operate in one of the third impedance mode and the fourth impedance mode. The second rectifier 252 further has a second rectifier control input port 252c for receiving the second rectifier biasing signal. In this embodiment, the third and fourth impedance modes are unbiased and biased impedance modes, respectively. The MCU 260 in this example embodiment has a controller input port 261 , first and second rectifier control output ports 262, 263, first and second switch control output ports 264, 265, a supply detection port 266, and a reflection detection port 267. In this embodiment, the ports 262-265 are respective general purpose input output (GPIO) ports.

The controller input port 261 is connected electrically to the first rectifier output port 251 b and the second rectifier output port 252b, and receives one of the first DC signal and the second DC signal at any given time, depending on the state of the RF switch 240. That is to say, the MCU 260 receives the first DC signal when the RF switch 240 operates in the first switch state to direct the intermediate supply signal to the first rectifier 251 ; and the MCU 260 receives the second DC signal when the RF switch 240 operates in the second switch state to direct the intermediate supply signal to the second rectifier 252. In this embodiment, whichever one of the first DC signal and the second DC signal received by the MCU 260 serves as a load supply signal supplied to the load. In other embodiments, the DC signal may be subjected to additional signal processing prior to serving as the load supply signal.

The first rectifier control output port 262 is connected electrically to the first rectifier control input port 251c. The second rectifier control output port 263 is connected electrically to the second rectifier control input port 252c. Generation and provision of the first and second rectifier biasing signals by the MCU 260 to the first and second rectifiers 251 , 252, respectively, are described in detail below in relation to Figure 6.

The switch control output ports 264, 265 are connected electrically to the switch control input ports 244, 245, respectively. In this embodiment, the MCU 260 is configured to determine said one of the first and second rectifiers 251 , 252 based on the detected supply power of the sample supply signal. Determination of said one of the first and second rectifiers 251 , 252 and provision of the generated switch control signals to the RF switch 240 are described in detail below in relation to Figure 6.

The supply detection port 266 of the MCU 260 is connected electrically to the supply detector output port 222 of the supply power detector 220, allowing the MCU 260 to receive the supply power detection signal from the supply power detector 220.

The load receives the load supply signal through the controller input port 261 of the MCU 260. Impedance mismatch causes a portion of the load supply signal to be reflected by the load back to the antenna through the system 200. This reflected portion is referred to below as “load reflected signal”. In this embodiment, the load reflected signal corresponds to the first and second DC signals. Specifically, the load reflected signal corresponds to the first DC signal when the first rectifier 251 converts the intermediate supply signal into the first DC signal, and corresponds to the second DC signal when the second rectifier 252 converts the intermediate supply signal into the second DC signal. The load reflected signal traverses in reverse from the MCU 260 through the signal rectification part 250 via the RF switch 240 and through the directional coupler 210 back to the antenna.

The directional coupler 210 is further configured to sample the load reflected signal for detection by the reflection power detector 230. More particularly, the directional coupler 210 splits the load reflected signal received via the coupler output port 212 into a source return signal and a sample reflection signal for output via the coupler input port 211 and the isolated port 214, respectively. The source return signal and the sample reflection signal differ from the load reflected signal only in magnitude in this embodiment. The source return signal is received by the antenna. The isolated port 214 is connected electrically to the reflection power detector 230. In such a configuration, the directional coupler 210 samples the load reflected signal for detection by the reflection power detector 230. The reflection power detector 230 has a reflection detector input port 231 connected electrically to the isolated port 214, and a reflection detector output port 232 connected electrically to the reflection detection port 267 of the MCU 260. The reflection power detector 230 receives the sample reflection signal from the directional coupler 210 via the reflection detector input port 231 , and detects a power (referred to as “reflection power” and more specifically, magnitude of the reflection power) of the sample reflection signal. In this embodiment, the reflection power detector 230 detects magnitude of the reflection power of the load reflected signal from the load. The reflection power detector 230 outputs a reflection power detection signal associated with the detected reflection power via the reflection detector output port 232. The MCU 260 is arranged to receive the reflection power detection signal from the reflection power detector 230 via the reflection detection port 267 connected electrically to the reflection detector output port 232. In an alternative embodiment without the directional coupler 210, the reflection power detector 230 may be otherwise arranged to detect the load reflected signal.

In this embodiment, the supply power detector 220 and the reflection power detector 230 are implemented using identical circuits. The supply power detection signal generated by the supply power detector 220 is in the form of a direct current proportional in magnitude (e.g., voltage or current) to the sample supply signal. The reflection power detection signal generated by the reflection power detector 230 is also in the form of a direct current proportional in magnitude (e.g., voltage or current) to the sample reflection signal

The MCU 260 includes two analog-to-digital converters (ADCs, not shown) associated with the supply detection port 266 and the reflection detection port 267 for digitising the received supply power detection signal and the received reflection power detection signal, respectively. In such a manner, RF measurement equipment, which is typically costly and complex to implement, may be omitted. The first and second rectifiers 251 , 252 are similar in component arrangement and different in component parameters. Figure 3B shows a schematic representation of a rectifier 310 representative of component arrangement of each of the rectifiers 251 , 252, in association with an AC supply signal source 320 and a load 330 according to another embodiment. It can be seen that the rectifier 310 includes a varactor 313 (varactor diode, marked by“D _v ”) with a cathode terminal. The cathode terminal receives a rectifier biasing signal with a bias voltage (marked by“Vbias”) via an inductor (marked by“L2”). The rectifier 310 is operable in an unbiased impedance mode (corresponding to the first and third impedance modes) and a biased impedance mode (corresponding to the second and fourth impedance modes) based on the bias voltage to convert an AC signal into a DC signal. In the configuration shown in Figure 3B, the varactor 313 is responsive to the rectifier biasing signal to adjust an impedance of the rectifier 310. For example, if the bias voltage falls in one range, the varactor 313 causes the rectifier 310 to operate in the unbiased impedance mode to perform the conversion. If the bias voltage falls in another range, the varactor 313 causes the rectifier 310 to operate in the biased impedance mode to perform the conversion.

In this example, the varactor 313 is shown to be connected in series between two LC networks 311 , 312. To bias the varactor 313 with the rectifier biasing signal, the varactor 313 is arranged between the LC networks 311 , 312 in order to be isolated from any DC signal incident from the AC supply signal source 320. The rectifier 310 is further shown to include a delay line 314. One end of the inductor of the bias voltage is connected between the varactor 313 and the delay line 314. In this embodiment, the bias voltage for the unbiased impedance mode is 0V and that for the biased impedance mode is 3V. The bias voltages may be different in other embodiments. In addition, in other embodiments, the rectifier 210 may have any number of impedance modes corresponding to respective bias voltages. With reference to Figure 3B, the first rectifier 251 of this embodiment includes a first varactor responsive to the first rectifier biasing signal to cause the first rectifier 251 to perform the conversion in one of the unbiased impedance mode (first impedance mode) and the biased impedance mode (second impedance mode). In such a configuration, the first varactor tunes the input impedance of the first rectifier 251 in response to the first rectifier biasing signal relating to the detected reflection power. Similarly, the second rectifier of this embodiment includes a second varactor responsive to the second rectifier biasing signal to cause the second rectifier 252 to perform the conversion in one of the unbiased impedance mode (third impedance mode) and the biased impedance mode (fourth impedance mode). In such a configuration, the second varactor tunes the input impedance of the second rectifier 252 in response to the second rectifier biasing signal relating to the detected reflection power. A schematic circuit diagram 350 of the supply power detector 220 (or the reflection power detector 230) used in the system 200 is shown in Figure 3C. The supply power detector 220 receives the input signal, Pin from the directional coupler 210 and outputs a corresponding DC voltage, Vdetect for the input signal, Pin. Figure 3D shows a line graph 360 of the input power to output voltage of the supply power detector 220 (or the reflection power detector 230) with the following circuit component values (C3 = 0.7pF; C4 = 100pF; l_3 = 27nFI; RL = 10MW; D2 = FISMS 2850) and operating frequency of 869 MFIz.

Figure 4 shows first to third Smith charts 410-430 obtained for the rectifier 310 with load impedances of 2 kQ, 5 kQ and 12 kQ, respectively. For each load impedance, measurements are made for the unbiased impedance mode and the biased impedance mode with RF powers of 0 dBm, -10 dBm and -20 dBm. Each of the Smith charts 410-430 shows a dotted line 411 , 421 , 431 , a dashed line 412, 422, 432, and a solid line 413, 423, 433. Each of these lines mark triangular, square and round symbols corresponding to the input powers of -20 dBm, -10 dBm and 0 dBm, respectively. For each chart 410-430, a first arrow 414, 424, 434 (marked by“delay”) is shown to extend from the dotted line 411 , 421 , 431 towards the dashed line 412, 422, 432; and a second arrow 415, 425, 435 (marked by“3V” and“0V”) is shown to extend from the dashed line 412,

422, 432 towards the solid line 413, 423, 433. The first arrow 414, 424, 434 marks a transitional effect of the delay line 314 on the impedance of the rectifier 310. The second arrow 415, 425, 435 marks a transitional effect of the varactor

313 on the impedance of the rectifier 310 when the varactor 313 receives the bias voltage of 3V, causing the rectifier 310 to operate in the biased impedance mode. In this embodiment, the rectifier 310 is optimised for a load impedance of 2 kQ. With reference to the first Smith chart 410 (2 kQ), it can be seen that the dashed line 412 is proximate to the centre point of the chart 410, indicating a low level of reflection of the input powers by the load 330 at 2 kQ when the rectifier 310 operates in the unbiased impedance mode. When the rectifier 310 transitions to the biased impedance mode, an increased level of reflection is observed, indicated by a larger distance from the centre point to the solid line 413. Thus, for a load of 2 kQ, the rectifier 310 should operate in the unbiased impedance mode. With reference to each of the second and third Smith charts 420, 430 (5 kQ and 12 kQ, respectively), it can be seen that the dashed line 422, 432 is distal from the centre point of the chart 420, 430, indicating a high level of reflection of the input powers by the load 330 at 5 kQ and at 12 kQ. When the rectifier 310 transitions to the biased impedance mode, a decreased level of reflection is observed, indicated by a smaller distance from the centre point to the solid line

423, 433. Thus, for loads of 5 kQ and 12 kQ, the rectifier 310 should operate in the biased impedance mode

It can therefore be appreciated that the varactor 313 can be selectively biased based on the load impedance and the RF power to reduce impedance (e.g., input impedance) mismatch and to improve performance. In other words, the varactor 313 is employed to adjust or tune the impedance of the rectifier 310 to better match, for example, a characteristic system impedance (typically at 50 W).

In this embodiment, the first and second rectifiers 251 , 252 are designed, by way of component parameter adjustment, to correspond to high and low power ranges, respectively. Each of the first and second rectifiers 251 , 252 has the unbiased and biased impedance modes. Depending on the supply power detection signal, one of the first and second rectifiers 251 , 252 is selected by the MCU 260 by way of the switch control signals for rectifying the intermediate supply signal into the respective one of the first and second DC signals for provision to the load. The varactor of the first rectifier 251 is a high-power diode (e.g., model number HSMS-2860 by Broadcom Ltd.) and that of the second rectifier 252 is a low-power diode (e.g., model number HSMS-2850 by Broadcom Ltd.) Determination of the selected one of the first and second rectifiers 251 , 252 is described in further detail below in relation to Figure 6.

Figure 5 shows first to sixth line charts 510-560 of rectifier reflection coefficient performance (Sn) measured using a conventional rectifier, the first rectifier 251 and the second rectifier 252, each in association with a range of RF powers and a range of load impedances. The first to third line charts 510-530 correspond to the second rectifier 252 with the low-power diode, and the fourth to sixth line charts 540-560 correspond to the first rectifier 251 with the high-power diode. The first to third line charts 510-530 correspond to input RF powers of -20 dBm, -10 dBm and 0 dBm, respectively. The fourth to sixth line charts 540-560 correspond to input RF powers of -20 dBm, -10 dBm and 0 dBm, respectively. Each line chart 510-560 shows a dash-dotted line 511 -561 , a dashed line 512- 562 and a solid line 513-563 of Sn performance for load impedances ranging from 0 kQ to 20 kQ. Each dash-dotted line 511 -531 represents measurement results of a conventional low-power rectifier without a varactor. Each dash- dotted line 541-561 represents measurement results of a conventional high- power rectifier without a varactor. Each dashed line 512-532 represents measurement results of the second rectifier 252 in the unbiased impedance mode. Each dashed line 542-562 represents measurement results of the first rectifier 251 in the unbiased impedance mode. Each solid line 513-533 represents measurement results of the second rectifier 252 in the biased impedance mode. Each solid line 543-563 represents measurement results of the first rectifier 251 in the biased impedance mode.

It can be understood that, for each of the rectifiers 251 , 252, a significant improvement in Sn performance can be achieved over a wide range of RF powers and a wide range of load impedances. In general, with the exception of the fourth line chart 540, the rectifiers 251 , 252 achieve a better performance in a lower range of load impedances when operating in the unbiased impedance mode, and a better performance in a higher range of load impedances when operating in the biased impedance mode. For the fourth line chart 540, the first rectifier 251 performs better across the entire range of load impedances when operating in the biased impedance mode.

Figure 6 shows a method 600 of impedance tuning according to one example embodiment of the present disclosure. The method 600 is performed by the MCU 260.

In step 610, the MCU 260 registers a value (referred to as“supply power value”) of the digitised supply power detection signal received by the MCU 260, and proceeds to step 620. In step 620, the MCU 260 determines whether the registered power supply value is greater than a predetermined threshold value (e.g., 0 dBm), proceeds to step 630 if affirmative, and proceeds to step 640 if otherwise.

In step 630, the MCU 260 selects the first rectifier 251 for handling the intermediate supply signal. Specifically, the MCU 260 causes the RF switch 240 to operate in the first switch state by way of the switch control signals, and proceeds to step 650. In the first switch state, the RF switch 240 routes or directs the intermediate supply signal to the first rectifier 251.

In step 640, the MCU 260 selects the second rectifier 252 for handling the intermediate supply signal. Specifically, the MCU 260 causes the RF switch 240 to operate in the second switch state by way of the switch control signals, and proceeds to step 650. In the second switch state, the RF switch 240 routes or directs the intermediate supply signal to the second rectifier 252. In step 650, for the selected rectifier 251 , 252, the MCU 260 causes the selected rectifier 251 , 252 to operate in the unbiased impedance mode by providing the corresponding rectifier biasing signal with the bias voltage of 0V, registers a first value (referred to as“first power reflection value”) of the digitised reflection power detection signal corresponding to the unbiased impedance mode of the selected rectifier 251 , 252, and proceeds to step 660. That is, the first rectifier biasing signal provided by the MCU 260 relates to the detected reflection power.

In step 660, for the selected rectifier 251 , 252, the MCU 260 causes the selected rectifier 251 , 252 to operate in the biased impedance mode by providing the corresponding rectifier biasing signal with the bias voltage of 3V, registers a second value (referred to as“second power reflection value”) of the digitised reflection power detection signal corresponding to the biased impedance mode of the selected rectifier 251 , 252, and proceeds to step 670. That is, the second rectifier biasing signal provided by the MCU 260 relates to the detected reflection power.

In step 670, the MCU 260 determines whether the first power reflection value is greater than the second power reflection value, proceeds to step 680 if affirmative, and proceeds to step 690 if otherwise. In step 680, the MCU 260 causes the selected rectifier 251 , 252 to operate in the biased impedance mode in the manner described above in relation to step 660. In step 690, the MCU 260 causes the selected rectifier 251 , 252 to operate in the unbiased impedance mode in the manner described above in relation to step 650.

The order of performing steps 650 and 660 may be reversed. That is, the second power reflection value may be registered prior to registering the first power reflection value.

Through performing steps 610 to 640, the MCU 260 determines and selects a suitable one of the first and second rectifiers 251 , 252 for performing the conversion of the intermediate supply signal into the corresponding DC signal. Through performing steps 650 to 690, the MCU 260 determines one of the unbiased and biased impedance modes for the selected rectifier 251 , 252 based on the registered values of the digitised reflection power detection signal corresponding to operations of the selected rectifier 251 , 252 in the unbiased and biased impedance modes, respectively. That is, the MCU 260 determines, for the selected rectifier 251 , 252, said one of the unbiased and biased impedance modes based on reflection powers detected by the reflection power detector 230 and corresponding to operations of the selected rectifier 251 , 252 in the unbiased and biased impedance modes, respectively. In such a manner, the MCU 260 can be considered to determine the biasing voltage for the selected rectifier 251 , 252 from the detected magnitude of the reflection power and, to tune the input impedance based on the determined bias voltage.

In this embodiment, an adjustment or tuning of the input impedance of the selected rectifier 251 , 252 occurs in the event of a transition of the selected rectifier 251 , 252 from one of the unbiased and biased impedance modes to the other of the unbiased and biased impedance modes. The method 600 ends upon completion of step 680 or step 690 in this embodiment. In other embodiments, the method 600 may return, upon completion of step 680 or step 690, to step 610, which is particularly useful if the source supply signal or the load condition may vary. The method 600 may return, upon completion of step 680 or step 690, to step 650, which is particularly useful if the load reflected signal may vary.

By performing the method 600, the MCU 260 can achieve an optimal configuration for the impedance tuning system 200. The MCU 260 not only selects a suitable one of the rectifiers 251 , 252 but also selects a suitable one of the unbiased impedance mode and biased impedance mode for the selected rectifier 251 , 252, which advantageously reduces the rectifier reflection coefficients |Sn| for various input and load conditions. As a result, a reconfigurable impedance tuning device 200 may be achieved since the input impedance may be tuned and configured based on magnitude of the detected reflected load power. The rectification part 250 (or rectifier circuit) thus has an input impedance configured to be tuned based on the detected magnitude of the reflection power.

Figure 7 shows line charts 710, 720, 730 of measured rectifier conversion efficiency (or RF-DC efficiency) for respective load impedances of 2 kQ, 5 kQ and 12 kQ and for a range of input powers from -15 dBm to 10 dBm. Each chart 710, 720, 730 shows a first line 711 , 721 , 731 of connected square symbols, a second line 712,722, 732 of connected triangular symbols, and a third line 731 , 732, 733 of connected round symbols. The first line 711 , 721 , 731 represents measurement results obtained using only the first rectifier 251 with the higher- power diode (FISMS-2860). The second line 712, 722, 732 represents measurement results obtained using only the second rectifier 252 with the low- power diode (FISMS-2850). The third line 731 , 732, 733 represents measurement results obtained using the first and second rectifiers 251 , 251 under control of the MCU 260 performing the method 600. From each of the charts 710, 720, 730, it can be appreciated that the MCU 260 selects, through the switch control signals, the first rectifier 251 for a high range of input powers (e.g., greater than 0 dBm) and the second rectifier 252 for a low range of input powers (e.g., smaller than or equal to 0 dBm). The unselected rectifier 251 , 252 can be considered to be disconnected due to the high isolation of the RF switch 240. Further, for the selected rectifier 251 , 252, the MCU 260 further selects, through the corresponding rectifier biasing signal, a suitable one of the unbiased impedance mode and the biased impedance mode to improve the rectifier conversion efficiencies.

In Figure 7, it can be the seen that an efficiency gain of up to 13% can be achieved by virtue of the varactor of the selected rectifier 251 , 252. This allows the system 200 to achieve peak efficiencies of 69% (at 4 dBm), 58% (at 3 dBm) and 38% (at 1 dBm) for load impedances of 2 kQ, 5 kQ, and 12 kQ, respectively. For the load impedance of 2 kQ, the improvement in efficiency is limited because the rectifiers 251 , 252 are already optimized for 2 kQ. That is, the varactor 313 cannot provide a significant improvement in terms of return loss. Under such circumstances, the MCU 260 through performing the method 600 will arrive at step 690 where the selected rectifier 251 , 252 operates in the unbiased impedance mode.

The source supply signal corresponding to the intermediate supply signal may vary in use. When the detected power of the source supply signal (or the intermediate supply signal) increases to exceed the threshold value (e.g., 0 dBm), the second rectifier 252 enters a breakdown region. As this occurs, the MCU 260 selects the first rectifier 251 to perform power rectification. The MCU 260 further selects a more suitable one of the first and second impedance modes for the first rectifier 251 to further improve the efficiency. Conversely, when the detected power of the source supply signal decreases to fall below the threshold value, the first rectifier 251 is not operating optimally at the low-power region, with the low-power region being the low range of input powers of smaller than or equal to 0 dBm. As this occurs, the MCU 260 selects the second rectifier 252 to perform power rectification. The MCU 260 further selects a more suitable one of the third and fourth impedance modes for the second rectifier 252 to further improve the efficiency. Therefore, the impedance tuning system 200 can achieve efficient power rectification for both low-power and high-power ranges, even where the conditions of impedance mismatch vary (e.g., varying source and load conditions).

With the improved RF-DC rectification efficiency, the impedance tuning system 200 can advantageously operate with a lower duty cycle. That is, to supply a given amount of energy in DC for a given operation of the load, the impedance tuning system 200 can operate with a duty cycle lower than that required by a conventional impedance tuning system. This reduces energy consumption.

Figure 8 illustrates an impedance tuning system 200’ with a single rectifier. In particular, the system 200’ includes a directional coupler 210’, a supply power detector 220’, a reflection power detector 230’, a rectifier 250’ (forming a rectifier circuit), and a microcontroller (MCU) 260’. Similar to the system 200 of Figure 2, the system 200’ of Figure 8 is employed in association with a sensor node (also referred to as“load”) and an antenna (also referred to as“source”, not shown). The antenna is arranged to receive a source supply signal (such as a radio frequency AC signal).

The directional coupler 210’ has a coupler input port 21 T, a coupler output port 212’, a coupled port 213’ and an isolated port 214’. The ports 21 T, 213’ and 214’ of the directional coupler 210’ are similar in configuration to the ports 211 , 213 and 214 of the direction coupler 210, respectively. The coupler output port 212’, in contrast with the coupler output port 212, is connected directly and electrically to the rectifier 250. Operation of the directional coupler 210’ is similar to that of the directional coupler 210, splitting a source supply signal received via the coupler input port 21 T into an intermediate supply signal and a sample supply signal for output via the coupler output port 212’ and the coupled port 213’, respectively.

Configurations and connections of the supply power detector 220’ and the reflection power detector 230’ with respect to the directional coupler 210’ and the MCU 260’ are identical to those described in relation to the embodiment of Figure 2, and are thus not described again for the sake of brevity.

The rectifier 250’ has a rectifier input port 250a’ connected electrically to the coupler output port 212’ for receiving the intermediate supply signal from the directional coupler 210’, and a rectifier output port 250b’ connected electrically to the MCU 260’. Similar to the first and second rectifiers 251 , 252 of Figure 2, the rectifier 250’ of Figure 8 has two impedance modes, and is responsive to the intermediate supply signal to convert the intermediate supply signal into a direct current (DC) signal for output via the rectifier output port 250b’ in one of the impedance modes. The rectifier 250’ outputs the DC signal via the rectifier output port 250b’ to the MCU 260. In this embodiment, the rectifier 250’ includes a varactor (not shown) responsive to a rectifier biasing signal to cause the rectifier 250’ to operate in one of the two impedance modes. That is, the varactor tunes the input impedance of the rectifier 250 in response to the rectifier biasing signal. The rectifier 250’ further has a rectifier control input port 250c’ for receiving the rectifier biasing signal. In this embodiment, the two impedance modes of the rectifier 250’ are unbiased and biased impedance modes, respectively.

The MCU 260’ in this example embodiment has a controller input port 26T, a rectifier control output port 262’, a supply detection port 266’, and a reflection detection port 267’. The controller input port 26T is connected electrically to the rectifier output port 250b’ to receive the DC signal, which serves as a load supply signal supplied to the load. The rectifier control output port 262’ is connected electrically to the rectifier control input port 250c’. Generation and provision of the rectifier biasing signal by the MCU 260’ to the rectifier 250’ are similar to those of the first and second rectifiers 251 , 252 of Figure 2, and are not described again for the sake of brevity.

The supply detection port 266’ and the reflection detection port 267’ are connected electrically to the supply power detector 220’ and the reflection power detector 230’, respectively, in the same manner as the embodiment of Figure 2.

The load receives the load supply signal through the controller input port 26T of the MCU 260’. Impedance mismatch between the source and the rectifier 250’ (typical characteristic system impedance of 50 W) causes a portion of the load supply signal to be reflected by the load back to the antenna through the system 200. This load reflected signal corresponds to the DC signal provided by the rectifier 250’, and traverses in reverse from the MCU 260’ through the rectifier 250’ and via the directional coupler 210’ back to the antenna. Sampling of the load reflected signal by the directional coupler 210’ for detection by the reflection power detector 230’ in this embodiment is identical to that in the embodiment of Figure 2, and is not described again for the sake of brevity. The MCU 260’ is configured to perform steps 650 to 690, with the rectifier 250’ serving as the selected rectifier. Since there is only one rectifier in the embodiment of Figure 8, steps 610 to 640 may be omitted. That is, the supply power detector 220’ may be omitted in the embodiment of Figure 8 without adversely affecting operation of the system 200’.

Some other alternative arrangements are described below. In an alternative embodiment, the second rectifier 252 may have a single impedance mode and converts the intermediate supply signal into the second DC signal in response to the intermediate supply signal in the single impedance mode.

In one arrangement, the first rectifier 251 may not have the varactor and may have any other components for tuning the impedance of the first rectifier 251 based on the detected reflection power. In one alternative embodiment with only one rectifier (e.g., the first rectifier 251 ), the RF switch 240 may be omitted.

In alternative embodiments, the rectifier may have two or more biased impedance modes of different non-zero biasing voltages.

A switch of another type may be used in place of the RF switch 240.

It should be noted that the term“connect” and its derivatives may mean“couple” and its corresponding derivatives.

As used herein, the term“bias voltage” and“biasing voltage” have the same meaning.