An Adaptive Power Harvester with Active Load Modulation for ... - MDPI

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An Adaptive Power Harvester with Active Load Modulation for Highly Efficient Short/Long Range RF WPT Applications Abdullah M. Almohaimeed 1,2, *, Rony E. Amaya 3 , Jose A. Lima 3 and Mustapha C. E. Yagoub 1 1 2 3

*

School of Electrical Engineering and Computer Science, Ottawa University, Ottawa, ON K1N 6N5, Canada; [email protected] Department of Electrical Engineering, Qassim University, Qassim 51431, Saudi Arabia Department of Electronic, Carleton University, Ottawa, ON K1S 5B6, Canada; [email protected] (R.E.A.); [email protected] (J.A.L.) Correspondence: [email protected]

Received: 31 May 2018; Accepted: 17 July 2018; Published: 23 July 2018

Abstract: After demonstrating, in previous works, the proof of concept of adaptive rectifiers with active load modulation to operate simultaneously for short/long range RF Wireless Power Transfer (WPT) while maintaining a high Power Conversion Efficiency (PCE), the authors introduced in this paper a power link budget of the proposed adaptive rectifier with a compromise between distance and efficiency. Then, to further exhibit its capabilities and enhance its performance, this paper first introduced a discussion about the parameters preventing the rectifier from operating over a wide range of input powers was performed. Furthermore, active load modulation was implemented and its co-simulation results presented. Finally, an adaptive rectifier was fabricated and its results successfully compared to measured data. It exhibits 40% of PCE over a wide dynamic input range of incident RF power levels from −6 to 25 dBm at the 900 MHz in the Industrial Scientific Medical band (ISM band), with a maximum PCE of 66% for an input power of 15 dBm. The proposed devices are therefore suitable for WPT applications to harvest energy from a controlled source. Keywords: rectifier; schottky diode; efficiency; wireless sensor network; WPT

1. Introduction Nowadays, Wireless Power Transfer (WPT) and energy scavenging have been attracting researchers and industry due to the ever-growing need for flexible, sustainable, and unfailing sources of energy. In addition, with recent advances in wireless sensor networks (WSN), e.g., the Internet of Things (IoT), such intelligent systems are further highlighting this necessity [1,2]. The aim of wireless power transfer is to allow devices to operate for an extended period of time without having to charge/replace their batteries, a critical issue because of the inherent costs and difficulties of reaching inaccessible areas. Furthermore, WPT is seen as an environmental solution and a cost-effective approach since it drastically reduces the need to recycle dead batteries [2]. WPT techniques can be classified into two main classes: (i) inductive and magnetic resonant coupling methods to transfer power over short distances (a few centimeters); and (ii) RF electromagnetic techniques utilized to transport the energy over long distances (a few meters) [3,4]. RF electromagnetic techniques are considered in this work due to their benefits in powering portable devices over longer distances. In order to estimate the power levels that can be transmitted, received, and harvested as well as the maximum distance that can be reached, a power link budget is essential. The common method to calculate the capability of power transmission is the power link. The propagation in the free space

Electronics 2018, 7, 125; doi:10.3390/electronics7070125

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Electronics 2018, 7, 125

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between aa transmitter transmitter and and aa receiver receiver can can be be expressed where the the between expressed via via the the well-known well-known Friis Friis equation equation where received power available to the rectifier can be estimated as in (1) received power available to the rectifier can be estimated as in (1) 2 λ𝜆 2 (1) 𝐺 ( ) Pr𝑃𝑟==Pt𝑃G𝑡 𝐺 G (1) t 𝑡 r𝑟 4𝜋𝑅 4πR where 𝜆 is the signal wavelength. (𝑃 , 𝐺 ) and (𝑃 , 𝐺 ) state, respectively, the transmitted and received where λ is the signal wavelength. (Pt𝑡, Gt𝑡) and (P𝑟r , G𝑟r ) state, respectively, the transmitted and received power and gain of antennas separated by a distance R. power and gain of antennas separated by a distance R. In the following calculations, we used the Federal Communication Commission (FCC) In the following calculations, we used the Federal Communication Commission (FCC) regulations regulations with a maximum transmitting power of 30 dBm and an Effective Isotropic Radiated with a maximum transmitting power of 30 dBm and an Effective Isotropic Radiated Power (EIRP) Power (EIRP) of 36 dBm at the UHF ISM band. Therefore, the applied transmitted power was set as of 36 dBm at the UHF ISM band. Therefore, the applied transmitted power was set as 23 dBm and 23 dBm and the gains of the transmitted and received antennas were set as 6 dBi and 2 dBi, the gains of the transmitted and received antennas were set as 6 dBi and 2 dBi, respectively. Table 1 respectively. Table 1 summarizes the received power link budget of the proposed adaptive rectifier summarizes the received power link budget of the proposed adaptive rectifier and trade-off between and trade-off between distance and efficiency. Note that the system using the concept of an adaptive distance and efficiency. Note that the system using the concept of an adaptive rectifier reaches an rectifier reaches an efficiency greater than 40% for distances up to 2 m and 49% of efficiency at 1 m. efficiency greater than 40% for distances up to 2 m and 49% of efficiency at 1 m. Table 1. RF system power link budget. Table 1. RF system power link budget.

Specifications

Value

Specifications Frequency operation

Value 915 MHz

Frequency operation 9152MHz Received antenna gain dBi Received antenna gain 2 dBi Transmitted antenna gain 6 dBi Transmitted antenna gain 6 dBi Transmitted power (dBm) +23 dBm Transmitted power (dBm) +23 dBm Distance (m) 1 2 3 Distance (m) 1 2 3 44 Received Power (dBm)(dBm) 0 −06.72 −6.72−10.3 12.7 Received Power −10.3 −−12.7 Efficiency (%) (%) 49 42 2828 Efficiency 49 42 33 33

55 6 6 −14.7 −16.3 −16.3 −14.7 25 25 2020

WPT through an electromagnetic technique has been applied in a diverse range of applications and fields, such as Radio Frequency Identification Identification (RFID) (RFID) [5], Wireless Sensor Networks Networks (WSN), (WSN), Wearable and Implantable Medical Devices (WIMD) [6], Structural Health Health Monitoring Monitoring (SHM) (SHM) [7], [7], and the Internet of Things (IoT) [8]. [8]. Such Such applications applications can can involve involve aa wide wide range of power levels depending on the targeted applications and and the distance from the controlled source: RFID RFID (μW (µW to mW), bio-implants bio-implants (mW (mW to to W), W), wirelessly wirelessly charging charging systems systems for for user user electronic electronic devices devices (a (a few few W). W). Consequently, theWPT WPT process is accomplished through a rectenna, an Consequently, the process is accomplished through a rectenna, which which mostly mostly containscontains an antenna, antenna, a rectifier, a matching network, and an output load as illustrated in Figure 1. Once the RF a rectifier, a matching network, and an output load as illustrated in Figure 1. Once the RF wave is wave is received by the receiving will be converted direct (DC) current (DC) to voltage either received by the receiving antenna,antenna, it will beitconverted to directto current voltage eitherto directly directlydevices power or devices or store theinenergy in batteries [3]. power store the energy batteries [3].

Figure 1. Diagram Power Transfer Transfer (WPT) (WPT) concept concept [4]. [4]. Reproduced Reproduced with with Figure 1. Diagram illustration illustration of of Wireless Wireless Power permission from [4], Copyright IEEE, 2016. permission from [4], Copyright IEEE, 2016.

Several approaches have been explored to improve the efficiency of WPT systems. The rectifier Several approaches have been explored to improve the efficiency of WPT systems. The rectifier is is one of the most widely used sub-circuits in a WPT system to optimize its performance for one of the most widely used sub-circuits in a WPT system to optimize its performance for maximum maximum Power Conversion Efficiency (PCE). Numerous rectifier configurations have been Power Conversion Efficiency (PCE). Numerous rectifier configurations have been suggested to improve suggested to improve system efficiency, including conventional configurations, such as series/shunt system efficiency, including conventional configurations, such as series/shunt diodes configurations, diodes configurations, voltage doublers, and bridge diode rectifiers [4]. voltage doublers, and bridge diode rectifiers [4]. However, all these topologies have noticeable limitations, such as the inability to attain a high efficiency over a wide range of input powers. This is mainly due to restrictions in diode properties,


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However, all these topologies have noticeable limitations, such as the inability to attain a high efficiency over a wide range of input powers. This is mainly due to restrictions in diode properties, including the incapability for a diode to exhibit a low threshold voltage and a high breakdown voltage simultaneously, resulting in a degradation in power efficiency in RF–DC conversion [4]. The effect of breakdown voltage, identified as early breakdown voltage [9] can be, in fact, one of the significant issues affecting the rectifier’s efficiency. Therefore, some approaches have been proposed to obtain a rectifier capable of handling a wide dynamic range of input power levels while exhibiting high Power Conversion Efficiency (PCE). In [1,2], the concept of a resistance compression network (RCN) has been introduced to improve the overall rectifier efficiency by attaining a small discrepancy of the input impedance while varying the output load and input power level, thus reducing the sensitivity of the input impedance rectifier versus the input power level and output load. However, the targeted input power range is still narrow. In [3,10], a rectifier with a tunable configuration has been proposed to work over a wide dynamic input power range but at the cost of a complex and bulky design where different rectifiers operating at different power level capabilities are combined in a single-pole four-throw RF switch structure. In [9,11], a topology utilizing a series diode associated with a pseudomorphic High-Electron-Mobility Transistor (pHEMT) transistor has been suggested to extend the breakdown voltage. However, such a configuration needs to be improved to cover wider input power levels. Also, maximum power point tracking (MPPT) approaches have been utilized to expand and enhance the overall conversion efficiency of the rectifier [12,13]. In [14], a nonlinear model has been proposed to rectify and recycle the harmonics with the aim of increasing the RF–DC power conversion efficiency. Moreover, an automatic load control has been proposed in [15] to improve the PCE by changing the load according to the input power variation. Furthermore, a rectifier array has been presented to extend the range of the rectification process as in [16]. Also, to extend the power range of the rectifier, an asymmetric power divider has been proposed in [17]. So, to efficiently address the issues of low efficiency and narrow input power dynamic range in conventional rectifiers, an adaptive/reconfigurable rectifier technique has been proposed to simultaneously exhibit a low threshold voltage and a high breakdown voltage [4,18]. Then, to extend its voltage breakdown, an active load modulation block [19] was added to the adaptive rectifier [20]. From that, after introducing the proof of concept of the adaptive rectifier with active load modulation in the above previous works [4,20], and to further demonstrate its capabilities and enhance its performance, this paper first introduces a power link budget of the proposed adaptive rectifier as well as an estimation of a compromise between distance and efficiency. Then, a discussion about the parameters preventing the rectifier to operate over a wide range of input powers is performed through the active device I-V characteristics and the circuit Power Conversion Efficiency. Furthermore, the active load modulation is implemented and its co-simulation results presented. Finally, an adaptive rectifier was fabricated and its results successfully compared to measured data. The adaptive rectifier concept was therefore used to extend the power input range of RF WPT systems operating at 915 MHz in the ISM band for Short/Long range WPT applications from controlled sources. This enhancement allows the RF WPT system through an electromagnetic technique to operate with true spatial freedom and be insensitive to range and location. 2. Rectifier Analysis While used as energy harvesters, the most important characteristics of power rectifiers can be summarized below.


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2.1. Efficiency The RF–DC power conversion efficiency (PCE) is an important figure of merit to characterize and estimate a rectifier’s performance (Figure 2). It is defined as the ratio between the output DC power ( PDC ) over the rectifier input RF power ( Pt ) [1] η (%) = Electronics 7, xoutput FOR PEER Vout 2018, is the DCREVIEW voltage

1 PDC (V )2 × 100 = Out × × 100. Pt RL Pt

across the load resistance R L .

(2) 4 of 13

Figure Illustrationofofthe theconcept concept efficiency. Here Pr the state the radio input frequency radio frequency Figure 2. Illustration of of efficiency. Here PRF P and Pr state input power RF and power andreflected input reflected respectively. and input power,power, respectively.

2.2. Output Load 2.2. rectifier is is an an alternating alternating current current(AC)/DC (AC)/DC converter circuit. The output voltage will drive drive The rectifier the load for specified applications, such as charging batteries or connecting it directly to electronic the load for specified applications, charging batteries connecting electronic devices. Consequently, Consequently,thethe output or resistance load resistance plays role a critical role inperformance. the overall devices. output loadload or load plays a critical in the overall performance. (2), the load 𝑅𝐿 , highly affectsPCE the overall PCE and output voltage. As seen in (2),As theseen loadinresistance, R Lresistance, , highly affects the overall and output voltage. Therefore, Therefore, an optimal value of to assure maximum power conversion an optimal value of R L should be 𝑅determined to determined assure maximum power conversion efficiency. 𝐿 should be efficiency. 2.3. Sensitivity 2.3. Sensitivity In WPT and harvesting energy systems, sensitivity is a key factor in designing rectifiers. It is utilized to define Minimum Desirable Signal (MDS) that canindeal with. rectifiers. It is In WPT andthe harvesting energy systems, sensitivity is a rectifier key factor designing utilized to define the Minimum Desirable Signal (MDS) that a rectifier can deal with. Sensitivity = PdBm = 10 log( PmW ) (3) (3) Sensitivity = 𝑃𝑑𝐵𝑚 = 10 𝑙𝑜𝑔(𝑃𝑚𝑊 ) With With the the aim aim of of capturing capturing low low input input power power levels, levels, the the rectifier rectifier components components should should show show aa low low threshold voltage to attain high sensitivity [9,18]. threshold voltage to attain high sensitivity [9,18].

3. Rectifier Efficiency Limitations 3. Rectifier Efficiency Limitations Designing a rectifier with high efficiency and that operates over extensive dynamic input power Designing a rectifier with high efficiency and that operates over extensive dynamic input power levels is essential for harvesting energy and wireless power transfer systems. In fact, the rectifier levels is essential for harvesting energy and wireless power transfer systems. In fact, the rectifier circuits have a diversity of factors affecting the rectifier efficiency performance while attaining a circuits have a diversity of factors affecting the rectifier efficiency performance while attaining a wide wide range of input power levels, such as active device parasitics, low breakdown voltage, high range of input power levels, such as active device parasitics, low breakdown voltage, high threshold threshold voltage, nonlinear effects, mismatched impedances, and sub-optimum output load [21]. As a voltage, nonlinear effects, mismatched impedances, and sub-optimum output load [21]. As a result, result, a system with optimal PCE would require a thorough set of trade-offs among these parameters. a system with optimal PCE would require a thorough set of trade-offs among these parameters. Therefore, to understand the rectifier operation and the efficiency limitations, these parameters and Therefore, to understand the rectifier operation and the efficiency limitations, these parameters and their impact on the circuit performance need to be investigated. In fact, the parameters effecting the their impact on the circuit performance need to be investigated. In fact, the parameters effecting the rectifier to be operated over a wide range of input powers are mainly the threshold voltage, Vth , and the rectifier to be operated over a wide range of input powers are mainly the threshold voltage, 𝑉𝑡ℎ , and voltage breakdown, Vbr [21,22]. the voltage breakdown, 𝑉𝑏𝑟 [21,22]. However, note that the threshold voltage, which can be defined as the voltage value from where However, note that the threshold voltage, which can be defined as the voltage value from where the diode can operate and detect the input signal, is controlled by the saturation current. Consequently, the diode can operate and detect the input signal, is controlled by the saturation current. Consequently, having a diode with a high saturation current is desirable in WPT and harvesting energy to attain and sense low signals. Therefore, the effect of the saturation current on the overall PCE is illustrated as in Figure 3, which shows that the loss at low input power is due to the lower saturation current and vice-versa.


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having a diode with a high saturation current is desirable in WPT and harvesting energy to attain and sense low signals. Therefore, the effect of the saturation current on the overall PCE is illustrated as in Figure 3, which shows that the loss at low input power is due to the lower saturation current Electronics 2018, 7, x FOR PEER REVIEW 5 of 13 and vice-versa. Electronics 2018, 7, x FOR PEER REVIEW 5 of 13

Figure 3. Saturation current effect on the overall Power Conversion Efficiency (PCE). Figure 3. Saturation current current effect Figure 3. Saturation effect on on the the overall overall Power Power Conversion Conversion Efficiency Efficiency (PCE). (PCE).

On the other hand, when the rectifier device is driven by a reverse bias, the electric field near the On the other hand, whenthe the rectifierdevice device driven a reverse bias, electric field when rectifier isis driven byby a reverse bias, thethe electric field nearnear the junction canother reachhand, its maximum allowable value. Once a breakdown occurs, the current will suddenly the junction can reach its maximum allowable value. Once a breakdown occurs, the current will junction can reach its maximum allowable value. Once a breakdown occurs, the current will suddenly increase while the DC output voltage remains almost constant. Therefore, voltage breakdown leads suddenly increase the DCvoltage output remains voltage remains constant. voltage breakdown increase while thewhile DC output almostaalmost constant. Therefore, voltage breakdown leads to significant efficiency degradation and attaining rectifier with aTherefore, high breakdown voltage is leads to significant efficiency degradation and attaining a rectifier with a high breakdown voltage is to significant efficiency degradation and attaining a rectifier with a high breakdown voltage required to achieve high efficiency [21,23]. As noticed in Figure 4, as the voltage breakdown increases, required to efficiency [21,23]. the voltage voltage breakdown increases, required to achieve achieve high efficiency [21,23]. noticed in Figure 4, asinput the the overall efficiency becomes broader andAs able to handle a wider power breakdown range, thus increases, achieving the overall efficiency efficiency becomes becomes broader broader and and able to handle a wider input power range, thus achieving higher efficiency. higher efficiency. efficiency.

Figure 4. Voltage breakdown effect on the overall PCE. Figure 4. Voltage breakdown effect on the overall PCE. Figure 4. Voltage breakdown effect on the overall PCE.

For this aim, a matching circuit should be taken into account to minimize mismatch loss between For this aim, matching should be taken into account to minimize mismatch loss between the antenna and aathe rectifiercircuit as highlighted in Figure 1. In WPT and harvesting energy circuits, For this aim, matching circuit should be taken into account to minimize mismatch loss between the antenna and the rectifier aswill highlighted in Figure 1.since In WPT and harvesting energy circuits, designing a matching network be more challenging the input impedance of the rectifier the antenna matching and the rectifier aswill highlighted in Figure 1. since In WPT and harvesting circuits, designing network be more challenging the input impedanceenergy of the with rectifier needs to beaa matched for a particular frequency operation since whilethe its input impedance varies the designing matching network will be more challenging input impedance of the rectifier needs to be matched for a particular frequency operation while its input impedance varies with the input power level [21]. needs to be matched for a particular frequency operation while its input impedance varies with the inputFurthermore, power level [21]. since the diode is a nonlinear component, generating harmonics at the output can inputFurthermore, power level [21]. since the efficiency. diode is a nonlinear component, generatingisharmonics at the output can reduce the overallsince rectifier In addition, the load resistance an important parameter to Furthermore, the diode is a nonlinear component, generating harmonics at the output can reduce the overall rectifier efficiency. In addition, the load resistance is an important parameter to consider as well; as the output load changes, the output voltage varies as well as the overall PCE. reduce the efficiency. In addition, the load resistance is an important to consider asoverall well; asrectifier the output output voltage varies as well as theparameter overall PCE. Accordingly, an optimal outputload load changes, should bethe applied [21]. Accordingly, an optimal output load should be applied [21]. 4. Design Methodology 4. Design Methodology In WPT, the usual technique to design a rectifier begins by selecting the appropriate active device, In WPT, the usual technique design a rectifierrectifier begins by selecting active typically a diode. Based on theirtobias techniques, diodes canthe be appropriate classified into twodevice, main typically a diode. Based on their bias techniques, rectifier diodes can be classified into two clusters, namely zero-biased diode configurations and non-zero-biased configurations [22]. main Non-


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consider as well; as the output load changes, the output voltage varies as well as the overall PCE. Accordingly, an optimal output load should be applied [21]. 4. Design Methodology In WPT, the usual technique to design a rectifier begins by selecting the appropriate active device, typically a diode. Based on their bias techniques, rectifier diodes can be classified into two main clusters, namely configurations and non-zero-biased configurations [22]. Non-zero-biased Electronicszero-biased 2018, 7, x FORdiode PEER REVIEW 6 of 13 configurations require an external source to operate, which is not appropriate in harvesting energy and wirelessenergy power and transfer applications. Among applications. the zero-bias Among diodes, P-N Schottky harvesting wireless power transfer the junction zero-biasand diodes, P-N barrier devices are the most widely used. junction and Schottky barrier devices are the most widely used. A of their their I-V I-Vcharacteristics characteristics(Figure (Figure5)5) shows that Schottky diode A comparison of shows that thethe Schottky diode can can turnturn ON ON at lower voltages (0.1 V–0.4 V) can thanthe can thejunction P-N junction (e.g., 0.7 V for silicon). In addition, at lower voltages (0.1 V–0.4 V) than P-N (e.g., 0.7 V for silicon). In addition, Schottky Schottky barrier have diodesfaster have faster switching capabilities a much highersaturation saturationcurrent current [22,23]. barrier diodes switching capabilities and and a much higher Therefore, the Schottky barrier diode is suitable for wireless power transfer and harvesting Therefore, the Schottky barrier diode is suitable for wireless power transfer and harvesting energy energy systems. systems.

Figure 5. 5. I-V I-V characteristics characteristics of of P-N P-N junction junction (pn) (pn) and and Schottky Schottky barrier barrierdiodes diodes(SB). (SB). Figure

There are several commercially available Schottky diodes that can be used for a rectification There are several commercially available Schottky diodes that can be used for a rectification process at the desired frequency operation (915 MHz), such as the HSMS-28xx family series and the process at the desired frequency operation (915 MHz), such as the HSMS-28xx family series and the SMS-76xx series [21,24], which can handle different input power levels. The HSMS2850 diode can SMS-76xx series [21,24], which can handle different input power levels. The HSMS2850 diode can work at low levels of input powers, i.e., less than 0 dBm (with a turn-on voltage of 150 mV and a work at low levels of input powers, i.e., less than 0 dBm (with a turn-on voltage of 150 mV and a breakdown voltage of 3.8 V), whereas the HSMS2860 diode is aimed at operating at higher levels of breakdown voltage of 3.8 V), whereas the HSMS2860 diode is aimed at operating at higher levels of input powers, i.e., 0 to 20 dBm (with a turn-on voltage of 350 mV and a breakdown voltage of 7 V) [3]. input powers, i.e., 0 to 20 dBm (with a turn-on voltage of 350 mV and a breakdown voltage of 7 V) [3]. When the diode device operates as a rectifier, the maximum output DC voltage is restricted by When the diode device operates as a rectifier, the maximum output DC voltage is restricted by the voltage breakdown, 𝑉𝑏𝑟 , and can be expressed as [21] the voltage breakdown, Vbr , and can be expressed as [21] 𝑉𝑏𝑟 (4) 𝑉𝑜,𝐷𝐶 = V . Vo, DC = 2br . (4) 2 the input AC signal amplitude is limited. Note that, due to output DC signal saturation, Therefore, thedue diode breakdown voltage determines theAC maximum peak voltage of Therefore, the input Note that, to output DC signal saturation, the input signal amplitude is limited. waveform. As stated voltage in [21], determines for a given load RL, the maximum output DCinput power is determined by the diode breakdown the maximum peak voltage of the waveform. As stated in [21], for a given load RL , the maximum output DC power 𝑉𝑏𝑟 2 is determined by (5) 𝑃𝑚𝑎𝑥,,𝐷𝐶 = . 4𝑅𝐿2 V Pmax, , DC = br . (5) 4R L 5. Adaptive Rectifier Design for Short/Long Range Since a single diode cannot cover a wide input power range because of voltage breakdown limitations in the semiconductor properties, diodes with a low threshold voltage and a high breakdown voltage can be combined to achieve a rectifier operating over a wide range of input powers. The proposed rectifier, shown in Figure 6, contains three shunt Schottky diodes (D1 to D3)


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5. Adaptive Rectifier Design for Short/Long Range Since a single diode cannot cover a wide input power range because of voltage breakdown limitations in the semiconductor properties, diodes with a low threshold voltage and a high breakdown voltage can be combined to achieve a rectifier operating over a wide range of input powers. The proposed rectifier, shown in Figure 6, contains three shunt Schottky diodes (D1 to D3 ) and a Field-Effect Transistor(FET) T1 (NE3210S1) [4], which aims at the rectifier working as a switch amongst low and high input powers. The operation procedure of the adaptive/reconfigurable rectifier can be explained as follows: once the input power level is low, the gate-source voltage across the transistor Vgs is close to zero, thus turning the transistor ON and allowing the current to pass through the channel, hence short-circuiting D2 and D3 . In this case, the current pathway passes through T1 and D1 . Once the input power rises, the transistor is turned OFF and D2 and D3 are working with D1 . Therefore, by carefully selecting the proper diodes to operate at low and high input power levels, a low Electronics 2018, 7, x FOR PEER REVIEW 7 of 13 threshold voltage and a high breakdown voltage can be achieved [4,10]. At low input power, D1. Once the input power rises, the transistor is turned OFF and D2 and D3 are working with D1. ( Therefore, by carefully selecting the proper diodes to operate at low and high input power levels, a T1 is ON, VD1 < Vth,D1 VGsvoltage > Vth,T , a high , the OFF ⇒ Vout = Vin (6) low threshold and breakdown voltage can be current achievedis[4,10]. D2 , D3 are shorted At low input power, 𝑇1 𝑖𝑠 𝑂𝑁, 𝑉𝐷1 < 𝑉𝑡ℎ,𝐷1 T is ON, V ,> 𝑉𝐺𝑠 > 𝑉𝑡ℎ,𝑇 , { theVcurrent th,D1 is OFF ⇒ 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 VGs > Vth,T𝐷 , 2 , 𝐷3 1are shortedD1 , the current is ON

(

(6)

D2 , D3 are OFF

𝑇 𝑖𝑠 𝑂𝑁, 𝑉 > 𝑉𝑡ℎ,𝐷1 , the current is ON 𝐷2 , 𝐷3 are OFF

and the output voltage𝑉can expressed 𝑉𝑡ℎ,𝑇 , { 1 as in (8)𝐷1 𝐺𝑠 >be

(7)

(7)

and the output voltage can be expressed (8)− (VD1 + VDS,T ). Vout as = in Vin

(8)

𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 − (𝑉𝐷1 + 𝑉𝐷𝑆,𝑇 ).

(8)

As the power increases, the voltage increases and T1 turns OFF As the power increases, the voltage increases and 𝑇1 turns OFF

Vout = Vin − (VD1 + VD2 + VD3 ). 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 − (𝑉𝐷1 + 𝑉𝐷2 + 𝑉𝐷3 ).

(9)

(9)

Theoutline outlineof of adaptive rectifier concept be illustrated the characteristics I-V diode The the the adaptive rectifier concept can becan illustrated through through the I-V diode characteristics and Power Conversion As seen in Figure 7a,b, stacking two or three diodes and Power Conversion Efficiency. AsEfficiency. seen in Figure 7a,b, stacking two or three diodes will increase will increase thevoltage; breakdown voltage; thevoltage threshold will as well. Therefore, the breakdown however, thehowever, threshold willvoltage increase asincrease well. Therefore, by applying by configuration applying the of configuration ofsystem the adaptive system Figure 6c, a and low high threshold and high the the adaptive in Figure 6c, a in low threshold breakdown can be breakdown can be accomplished, which leads the rectifier to achieve a high efficiency while operating accomplished, which leads the rectifier to achieve a high efficiency while operating over wide range of over wide range of input powers as can6d,e. be seen in Figure 6d,e. input powers as can be seen in Figure

1K

D3

NE321 0S01

HSMS2860

T1

D2

HSMS2850

D1

Figure 6. Adaptive rectifier configuration Reproduced with[4]. permission from [4], Copyright Figure 6. Adaptive[4]. rectifier configuration IEEE, 2016.

D3 D2

NE3210S01 T1

T1

D2

T1

HSMS2850


D1

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Figure 6. Adaptive rectifier configuration [4].

D3 D2

NE3210S01 T1

T1

D1

D1

Electronics 2018, 7, x FOR PEER REVIEW (a)2018, Stack Diodes Electronics 7, xtwo FOR PEER REVIEW

T1

D2

D1

(b) Stack three Diodes

(c) Adaptive System

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(d) (e) (d) (e) Figure 7. Adaptive rectifier technique compared to a conventional diode rectifier, (a) Stacking two Figure Figure 7. 7. Adaptive Adaptive rectifier rectifier technique technique compared compared to to aa conventional conventional diode diode rectifier, rectifier, (a) (a) Stacking Stacking two two diodes, (b) Stacking three diodes, (c) Adaptive system, (d) I-V curves, (e) Power Conversion Efficiency. diodes, diodes,(b) (b) Stacking Stackingthree threediodes, diodes,(c) (c)Adaptive Adaptivesystem, system,(d) (d)I-V I-Vcurves, curves,(e) (e) Power Power Conversion ConversionEfficiency. Efficiency.

6. Active Load Modulation for Simultaneous Short/Long Range WPT 6. 6. Active Active Load Load Modulation Modulation for for Simultaneous Simultaneous Short/Long Short/Long Range Range WPT WPT To further enhance the ability of adaptive/reconfigurable rectifiers to operate over a wide input To Tofurther furtherenhance enhancethe theability abilityof ofadaptive/reconfigurable adaptive/reconfigurable rectifiers to operate operate over over aa wide wide input input power range, an active load modulation has been applied to actively modulate and vary the output power range, an active load modulation has been applied to actively modulate and vary the output power range, an active load modulation has been applied to actively modulate and vary the output load as in Figure 8. Because the output DC power is mainly dependent on the voltage breakdown load load as as in in Figure Figure 8. 8. Because Because the the output output DC DC power power is is mainly mainly dependent dependent on on the the voltage voltage breakdown breakdown and the output load, the rectifier output load is indeed one of the crucial parameters that can affect and and the the output output load, load, the the rectifier rectifier output output load load is is indeed indeed one one of of the the crucial crucial parameters parameters that that can can affect affect the overall power efficiency [4,9]. Accordingly, the total PCE can be studied as a function of the load the overall power efficiency [4,9]. Accordingly, the total PCE can be studied as a function of the the power efficiency [4,9]. Accordingly, the total PCE can be studied as a function of the load impedance (Figure 9). Here, the concept of active load modulation can be explored using active load impedance (Figure 9). Here, concept active loadmodulation modulationcan canbe be explored explored using active impedance (Figure 9). Here, the the concept of of active load active components, such as transistors, that can adjust the circuit load to increase and enhance the circuitry components, components, such such as as transistors, transistors, that that can can adjust adjust the the circuit circuit load load to to increase increase and and enhance the circuitry circuitry efficiency of a predefined input power range. efficiency efficiency of ofaapredefined predefinedinput inputpower powerrange. range.

Active Load Active Load

D3 D3

HSMS2860 HSMS2860

D2 D2 HSMS2850 HSMS2850

R1 R1 NE3210S01 NE3210S01T1

T1

R3 R3

R2 R2

D1 D1

Figure 8. Illustration of the adaptive rectifier configuration with the active load technique. Figure 8. 8. Illustration Illustration of of the the adaptive adaptive rectifier rectifier configuration configuration with with the theactive activeload loadtechnique. technique. Figure

iency(%) cy(%)

80 80 70 70 60 60 50 50 40

300 ohm 300 ohm 700 ohm 700ohm ohm 1300 1300 ohm

HSMS2860 T1

D2

HSMS2850

R2

D1


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Figure 8. Illustration of the adaptive rectifier configuration with the active load technique. 80 300 ohm 700 ohm 1300 ohm

70

Efficiency(%)

60 50 40 30 20 10 0 -20

-10

0

10 Pin(dBm)

20

30

40

Figure 9. 9. Sweeping Sweeping the the adaptive adaptive rectifier rectifier loads loads versus versus input input power power level. level. Figure Electronics 2018, 7, x FOR PEER REVIEW

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The design keys of having active load modulation are (i) to achieve a high voltage at a low input The keys ofefficiency having active load are (i) voltage to achieve highinput voltage at a to low input power to design maximize the and (ii) to modulation reduce the output at aahigh power prevent power to maximize the efficiency and (ii) to reduce the output voltage at a high input power to the rectifier from reaching its breakdown voltage. Therefore, at low input powers, a large output load prevent the to rectifier from reaching its voltage breakdown voltage. Therefore, low input large is desirable produce a high output and thus to increase the at overall PCE. powers, Once thea input output load is desirable to produce a high output voltage and thus to increase the overall PCE. Once power increases, it is appropriate to minimize the output load to avoid the rectifier reaching its voltage the input power it is high appropriate to minimize the of output to avoid rectifier reaching breakdown, thusincreases, maintaining PCE over a wide range inputload power levelsthe [15]. its voltage breakdown, thus maintaining high PCE over a10a, wide range of input power (R levels [15]. The proposed active load block, illustrated in Figure contains three resistors 1 , R2 , and R3 ) The proposed active load block, illustrated in Figure 10a, contains three resistors (R 1 , R 2 and R3) and a switching transistor. The transistor is utilized to modify and adjust the load impedances ,amongst and a switching transistor. transistor utilized modify isand adjust the loadConsequently, impedances low and high input powers. The For low powerislevels, the to transistor in the OFF-state. amongst high input to powers. For low power levels, the10b, transistor is in the OFF-state. the outputlow loadand corresponding impedance, as depicted in Figure is Consequently, the output load corresponding to impedance, as depicted in Figure 10b, is Req OFF state = R1 + R2 . (10) 𝑅𝑒𝑞 𝑂𝐹𝐹 𝑠𝑡𝑎𝑡𝑒 = 𝑅1 + 𝑅2 . (10) power rises, rises, the the transistor transistor changes changes to to an an ON-state ON-state situation situation as as shown shownin inFigure Figure10c. 10c. As the input power Thus, the output load corresponding to impedance is then +R𝑅2)( )(𝑅3 + 𝑅𝑇 ) R 1+ ((𝑅 2 R3 + R T ). (11) 𝑂𝑁 𝑠𝑡𝑎𝑡𝑒== 1 R𝑅 (11) eq𝑒𝑞 ON state 𝑅 + 𝑅 +R 𝑅3 + +R 𝑅𝑇 . R11+ R22 + 3 T Note that at high driving voltages, the transistor equivalent resistance (RT) can be neglected Note that at high driving voltages, the transistor equivalent resistance (RT ) can be neglected compared to R3, leading to a simplified load equivalent impedance expression as in (12). compared to R3 , leading to a simplified load equivalent impedance expression as in (12). (𝑅1 𝑅3 ) + (𝑅2 𝑅3 ) 𝑅𝑒𝑞 𝑂𝑁 𝑠𝑡𝑎𝑡𝑒 = ( R R ) + ( R R ). (12) 3 𝑅2 + 𝑅 2 33 𝑅1 1 + Req ON state = . (12) R1 + R2 + R3

R1

R3

R1

R2

BJT

R2

(a) active load circuit

R3

(b) OFF-state

R1

R3 RT

R2

(c) ON-state

Figure 10. 10. Operation Operation simplification simplification of of the the active active load load equivalent equivalent circuit circuit configuration: configuration: (a) (a) active active load load Figure circuit, (b) (b) OFF-state, OFF-state, (c) circuit, (c) ON-state. ON-state.

Therefore, by applying active load modulation, a high and low load impedance can be achieved to produce a high output voltage at low power and prevent the rectifier from reaching its voltage breakdown at a high input power, thus maintaining high PCE over a wide range of input power levels.


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Therefore, by applying active load modulation, a high and low load impedance can be achieved to produce a high output voltage at low power and prevent the rectifier from reaching its voltage breakdown at a high input power, thus maintaining high PCE over a wide range of input power levels. 7. Experimental Validation of the Adaptive RF WPT Harvester Operating in the ISM band at 915 MHz, the designed adaptive rectifier (without the active load modulation technique) was optimized to operate for maximum power conversion efficiency over a wide range of RF input power levels. Simulated in Keysight’s Advanced Design System simulator [22], it uses a 31-mm-thick Rogers 5880 substrate and two transistor design kits (NE3210S1 and NE68133) from the Renesas Electronic Active Device Library. Also, the diode SPICE models (for HSMS2850 and HSMS2860) were extracted from Avago Technologies datasheets [20]. The S-parameters of the passive Electronics 2018, 7,(capacitors, x FOR PEER REVIEW of 13 components inductors, and resistors) were taken from the datasheets provided in10 [23]. Electronics 2018, 7, x FOR PEER REVIEW 10 of 13 The fabricated adaptive rectifier circuit (Figure 11) attained a co-simulated maximum power ofconversion PCE over a wide dynamic range of incident RFdepicted power levels from12. −6 to 25 dBm. Also, the coofdynamic 66% atinput 15 dBm input as Figure it exhibits of PCE over(PCE) a wide input range ofpower incident RF powerin levels from −6Furthermore, to 25 dBm. Also, the cosimulated results are in good agreement with the measured data. However, due to equipment 40% of PCEresults over a are wide input range incident RF power levels from −6totoequipment 25 dBm. and Also, simulated in dynamic good agreement withofthe measured data. However, due and test bench limitations, only measurements up to with 15 dBm were performed. The rectifier achieved 4V the co-simulated results are in good agreement the measured data. However, due to equipment test bench limitations, only measurements up to 15 dBm were performed. The rectifier achieved 4 V with an input power of 15 dBm. and limitations, withtest an bench input power of 15 only dBm.measurements up to 15 dBm were performed. The rectifier achieved 4 V with an input power of 15 dBm.

(a) (a)

(b) (b)

Figure 11. Adaptive rectifier circuit: (a) co-simulation layout view (without the active load Figure11. 11.Adaptive Adaptive rectifier circuit: (a) co-simulation layout view the (without the modulation active load Figure circuit: (a) co-simulation layout view (without active load modulation block); (b)rectifier the rectifier’s printed circuit board. modulation block); (b) the rectifier’s printed circuit board. block); (b) the rectifier’s printed circuit board.

4.5

70 70 60 60

Simulated R=1K ohms Simulated R=1K ohms measured R=1K ohms measured R=1K ohms

4.5 4 4 3.5 3.5

50 3

50

3 Vdc(V)

40 40

30 30

2.5 Vdc(V)

Efficiency (%) Efficiency (%)

DC voltage at 915 MHz DC voltage at 915 MHz DC Simulated DC Simulated DC measuerd DC measuerd

2.5

2 2

1.5 1.5

20 20

1 1

10

0.5

10 0 -300 -30

0.5

-20

-10 -20

-10

0 10 0 10 Pin(dBm) Pin(dBm)

(a) (a)

20

30 20

40 30

40

0 -25 0 -20 -15 -10 -5 0 -25 -20 -15 -10 -5 0 Pin(dBm) Pin(dBm)

5

10 5

15 10

15

(b) (b)

Figure 12. Simulated and measured parameters atat915 MHz for the designed adaptive rectifier with Figure Figure12. 12.Simulated Simulatedand andmeasured measuredparameters parameters at915 915MHz MHzfor forthe thedesigned designedadaptive adaptiverectifier rectifierwith with RR L = 1 kΩ: (a) Efficiency at 915 MHz; (b) output direct current (DC) voltage at 915 MHz. = 1 kΩ: (a) Efficiency at 915 MHz; (b) output direct current (DC) voltage at 915 MHz. L RL = 1 kΩ: (a) Efficiency at 915 MHz; (b) output direct current (DC) voltage at 915 MHz.

Figure 13 shows the adaptive rectifier with the active load modulation block as well as a Figure 13 shows the adaptive rectifier with the active load modulation block as well as a comparison of the expected PCE while using active load modulation versus fixed load values. As comparison of the expected PCE while using active load modulation versus fixed load values. As expected, by including the active load, the circuit presents a higher co-simulated efficiency, over 40%, expected, by including the active load, the circuit presents a higher co-simulated efficiency, over 40%, and within a wider range of RF input power levels (−7 dBm to 32 dBm) compared to fixed loads. The and within a wider range of RF input power levels (−7 dBm to 32 dBm) compared to fixed loads. The active load resistors were adjusted to attain maximum efficiency over the desired wide range of input active load resistors were adjusted to attain maximum efficiency over the desired wide range of input


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Figure 13 shows the adaptive rectifier with the active load modulation block as well as a comparison of the expected PCE while using active load modulation versus fixed load values. As expected, by including the active load, the circuit presents a higher co-simulated efficiency, over 40%, and within a wider range of RF input power levels (−7 dBm to 32 dBm) compared to fixed loads. The active load resistors were adjusted to attain maximum efficiency over the desired wide range of input power levels (R1 = 1 kΩ and R2 = R3 = 0.3 kΩ). Therefore, the load varies from 0.3 kΩ to 1.3 kΩ. A low pass filter, L, and C2 were added to the rectifier circuit. Since the real part is almost constant in the Smith chart, the series capacitor (C1 ) is enough to match the rectifier circuit to the antenna input (50 Ω). The adaptive active load modulation achieved a DC output voltage of around 3.25 V at Electronics 2018, 7, x FOR PEER REVIEW 11 the of 13 input power of 15 dBm. The reconfigurable rectifier consumes an area of 30 × 17 mm2 .

(a) 70 60

10

300 ohm 700 ohm 1100 ohm adaptive

10 Output DC (V)

Efficiency(%)

50 40 30

10

20

10

DC voltage at 915 MHz

2

1

0

-1

10 0 -20

-10

0

10 Pin(dBm

(b)

20

30

40

10

-2

-20

-10

0

10 Pin(dBm)

20

30

40

(c)

Figure13. 13.Adaptive Adaptiverectifier rectifiercircuit circuitwith withan anactive activeload loadconfiguration: configuration:(a) (a)Co-simulation Co-simulationlayout layoutview; view; Figure (b) Efficiency with active load and fixed passive load values; and (c) output DC voltage at 915 MHz. (b) Efficiency with active load and fixed passive load values; and (c) output DC voltage at 915 MHz.

To conclude, as reported in Table 2, the proposed rectifier performance was demonstrated To conclude, as reported in Table 2, the proposed rectifier performance was demonstrated through through a successful comparison with existing published designs. The proposed designs a successful comparison with existing published designs. The proposed designs demonstrated an demonstrated an extensive range of input power levels to address the issue of conventional rectifiers extensive range of input power levels to address the issue of conventional rectifiers by simultaneously by simultaneously utilizing the adaptive reconfigurable configuration along with the active load utilizing the adaptive reconfigurable configuration along with the active load modulation approach. modulation approach.


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Table 2. Rectifier Performance Comparison. Ref.

Technology

Freq. (GHz)

Maximum Efficiency

Pin (dBm) Range for PCE > 40%

[1]

Resistance compression networks

0.915/2.45

60%

−15 to 5

[2]

Proposed multi-stage voltage doubler

0.915

70%

−12 to 16

[9]

Extended power range

0.915/1.8

62%

−10 to 18

[15]

Automatic Load Control

2.45

70%

3 to 18

[16]

adaptive power distribution array

0.915

66%

−8 to −18

[25]

Schottky diode

2.4/5.8

65%

−6 to 13

[26]

Shunt diode

2.45

73%

−20 to 10

[27]

Full-wave Greinacher rectifier

2.45

75%

−16 to 7

Adaptive shunt diode

0.915

70%

−10 to 26

Adaptive shunt diode with active

0.915

66%

−6 to 32

This work

8. Conclusions In this work, an adaptive rectifier concept was implemented to address the early breakdown voltage issue of conventional rectifiers while exhibiting high efficiency over a wide range of RF input power levels. The proposed rectifier achieved an efficiency of over 40% of the RF input power level ranging from −6 to 25 dBm and accomplished 66% of maximum peak power efficiency at 15 dBm. To further improve the adaptive rectifier efficiency, an active load impedance block was added to expand the efficiency of the adaptive rectifier’s performance. The results from the active load modulation topology demonstrated a 40% RF-DC power conversion efficiency over a wider range of input power levels (−6 dBm to 32 dBm) while keeping the maximum peak power efficiency at 66% at 15 dBm. The rectifier circuit was designed to extend the power input range of RF WPT Harvesters operating at 915 MHz in the ISM band for Short/Long range WPT applications from controlled sources to operate with true spatial freedom and be insensitive to range and location. Author Contributions: Conceptualization, A.M.A.; Methodology, A.M.A., R.E.A., J.A.L., and M.C.E.Y.; Supervision, R.E.A. and M.C.E.Y.; Validation, A.M.A., R.E.A., J.A.L., and M.C.E.Y.; Writing (review & editing), A.M.A., R.E.A., and M.C.E.Y. Funding: This research received no external funding. Acknowledgments: The authors would like to express their thanks to the Saudi Cultural Bureau in Ottawa for their support. They would also like to thank Eqab Almajali and M. A. Le Hénaff, from the School of Electrical Engineering and Computer Science, University of Ottawa, for their help in PCB fabrication. Conflicts of Interest: The authors declare no conflict of interest.

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