High-efficiency quad-band RF energy harvesting system with improved cross-coupled differential-drive rectifier

1 Introduction

Exploration of renewable energy sources has long been a focal point of research endeavors. Ambient RF energy sources, notably, present a promising avenue for harvesting and supplying power to low-power wireless devices, encompassing applications such as implantable devices [1], wireless sensor networks [2], RFID tags [3], wireless battery chargers [4], underwater wireless sensors [5], and IoT applications [6].

In the literature, various methods have been proposed to design single-band RF energy harvester (RFEH) systems with low power dissipation and high power conversion efficiency (PCE), thereby maximizing the collection of ambient RF energy [7], [8], [9]. However, the limitations inherent in single-band systems often impede their ability to meet the power requirements of consumers [1], [2], [3], [4], [5], [6].

In contrast, multi-band RFEH systems offer enhanced energy harvesting capabilities and improved conversion efficiency compared to their single-band counterparts [10]. Moreover, multi-band systems exhibit greater resilience to signal variations across different frequency bands, ensuring sustained power scavenging [11]. Consequently, the design and implementation of multi-band RFEH systems have been the subject of numerous studies, employing various topologies [12], [13], [14], [15], [16], [17].

The generic structure of a single-band RFEH system is shown in Figure 1. The system is typically comprises an RF generator or RF power source, a receiving antenna, an impedance matching network (IMN), a rectifier/voltage multiplier, power management, and a load. IMN plays a critical role in maximizing power transfer from the antenna to the rectifier/voltage multiplier, ensuring optimal energy harvesting efficiency. The rectifier is the core of the system, which converts the received RF power to DC power. Power management is responsible for optimal use of DC power by the load, which can be a sensor, battery, RFID, etc.

Figure 1:

Block diagram of a single band RFEH system.

Conventional single-band rectifiers are predominantly designed using diodes [18], [19], [20]. The diode-based designs suffer from the high dropout voltage problem. To overcome this issue, diodes are replaced with diode-connected MOSFETs [21]. However, the poor sensitivity of diode-connected MOSFETs to dropout voltages at low input power remains a concern in RFEH systems [22].

This paper aims to design a high-PCE RFEH system optimized for low input power and increased power availability. To this end, our focus lies on a circuit configuration based on a cross-coupled differential-drive (CCDD) rectifier to provide an RFEH system with four RF bands. The primary potential sources of scattered RF energy in the surrounding environment are radio frequency band signals (FM), digital television (DTV), mobile phones (GSM), and Wi-Fi [23]. The proposed quad-band system is capable of harvesting energy across the FM Radio-100, DTV-600, GSM-1800, and WiFi-2400 frequency bands. The harvested energy output is demonstrated to be adequate for powering low-power devices, even in scenarios involving signal absence or ultra-low power signals in specific frequency bands.

The rationale behind selecting FM Radio-100, DTV-600, GSM-1800, and WiFi-2400 as the frequency bands for our RF energy harvesting system is based on several factors, including the availability of RF energy, the prevalence of these frequencies, and the feasibility of harvesting energy from these bands.

FM radio signals are ubiquitous in both urban and rural areas, with consistent power levels due to the widespread use of FM broadcasting. Additionally, the lower frequency range of FM radio waves provides better penetration and propagation characteristics, allowing for more efficient energy capture even indoors.

DTV-600 remains a significant part of the digital television broadcasting spectrum in many regions, ensuring a stable and continuous source of RF energy. User density, making it a rich source for energy harvesting. Moreover, the GSM-1800 band is widely used for mobile communications, especially in urban areas with high user density, making it a rich source for energy harvesting.

In addition, the 2.4 GHz WiFi band is extensively used for wireless internet and other communications, creating a dense and widespread presence of RF signals in residential, commercial, and public environments, providing a robust and accessible source of RF energy for harvesting.

The selection of these specific frequency bands ensures that the harvesting system can operate effectively in various environments to provide a stable and continuous energy supply.

2 System overview and design

The primary performance metric for evaluating an RFEH system is PCE, quantified as the ratio of DC output power (P _out) to input RF power (P _in). Mathematically,

where P _diss is the power dissipation of rectifier parts and P _rvs is the power loss due to reverse leakage current.

The basic structure of a single-band CCDD rectifier is a bridge rectifier [24]. The bridge rectifier is widely used in RFEH systems due to its high efficiency and improved sensitivity. Figure 2 shows the bridge CCDD rectifier, which is employed in this research.

Figure 2:

CCDD basic structure and its forward and reverse current.

The output voltage of the CCDD circuit is obtained from

where V _onNMOS and V _onPMOS are drop voltages of NMOS and PMOS transistors, respectively, and V _RF+ is the peak voltage of the input RF signal.

According to Figure 2, the operation of the CCDD circuit is as follows: In the positive input half cycle, transistor MP1 is turned on and the forward current passes through it. In the negative half cycle, it is expected that the current from the output energy storage element does not return to the circuit. However, after passing the input voltage to the negative cycle, MP1 stays on until the gate voltage reaches the threshold voltage; as a result, the current returns to the input through MP1. This reverse current I _REV leads to a power loss in the system [13], [14], [15], [16]. As a result, the CCDD circuit’s operation is susceptible to reverse current flow in the negative input half-cycle, leading to energy losses and reduced PCE.

To mitigate this issue, one way is to reduce the leakage current by lowering the threshold voltage and forcing transistors to turn off in the opposite cycle. In [25] and [26], two modifications have been proposed in the CCDD rectifier: Incorporating self-biased diode and resistor is able to prevent I _REV and increase PCE. These modifications are integrated into our proposed CCDD rectifier design, improving system efficiency and sensitivity, particularly at low power levels, as shown in Figure 3.

Figure 3:

The modified CCDD circuit [25], [26].

We explain the reason why the self-biased diode and resistor improve PCE for MP1, while the proof can be extended for other transistors in a straightforward manner.

Initially, in the positive half cycle, MP1 and MN2 are on. This occurs because the gate voltage of MN2 is positive, while the gate voltage of MP1 is negative. Consequently, during the positive cycle, the source voltage of MP1 is higher than its gate voltage. When the difference between these voltages exceeds the threshold voltage (V _th), MP1 turns on. Similarly, MN2 turns on when its source voltage becomes lower than its gate voltage, and the difference between these voltages exceeds V _th. They remain on until the gate voltage of MP1 becomes lower than V _th.

In the absence of the self-biased diode, when the output voltage, which is the voltage held by capacitor C _L, exceeds the input voltage, the current returns to the input through MP1 and MN2, resulting in an energy loss in the circuit. Due to the fact that the diode conducts and suddenly raises the gate voltage, MP1 turns off and the leakage current effect is eliminated. As a result, power loss decreases and PCE increases. Besides that, the self-biased resistor has the same usefulness in such a way that at lower voltage differences which the diode cannot conduct, the resistor prevents MP1 to turn on in the negative half cycle. As a result, in addition to increasing PCE, sensitivity becomes high at the low power mode.

3 Quad-band RFEH structure design

The proposed quad-band RFEH system is shown in Figure 4. The system features four distinct paths, each equipped with an antenna to receive RF signals from FM-100, TV-600, GSM-1800, and WiFi-2400 frequency bands.

Figure 4:

Block diagram of proposed quad-band RF energy harvester.

The harvested signal of each input channel passes through a tuned IMN to transfer maximum power between the antenna and the CCDD rectifier. Each channel’s CCDD rectifier circuit converts received RF voltage to DC voltage, subsequently stored in a capacitor as a DC pass filter. Finally, the output signals from each channel are combined in series after rectification.

The proposed structure offers several advantages: It produces a higher output voltage amplitude and power compared to single-band systems. Besides, the output voltage is continuing as in cases one band has ultra-low power or no power, other bands compensate this shortcoming, resulting in enhanced output voltage continuity and stability across heterogeneous input scenarios.

The power transferred to the load can be calculated using

where P _DC.out is the DC output power of multi-band system, η _i is PCE of ith channel, f is frequency and P _RF.f1 to P _RF.f4 are the input power of the 1st to 4th channel, respectively.

A schematic diagram of the proposed quad-band RFEH circuit is shown in Figure 5. We can observe how to connect and combine the output of each single-band circuit. This approach allows the designer to easily feed the consumer.

Figure 5:

The schematic diagram of the proposed quad-band RF energy harvester.

In order to efficiently transfer the total received power from the antenna to the rectifier, a proper selection of IMN and its components parameters is required. To this end, the Π network is selected due to its extra degree of freedom to control the value of quality factor.

The elements of IMN must be fully adjusted by the circuit designer in the best mode of the relevant band for maximizing power transfer. If there is no match, a power transmission loss occurs between antenna and rectifier. In our design, a fixed IMN is implemented on-chip using a CAD-oriented design procedure to maximize the overall PCE [27].

The impedance matching networks (IMNs) are individually designed for each frequency band to align the antenna’s impedance, R _s, with the input impedance of the energy harvesting circuit, R _in, which is the CCDD rectifier, thereby maximizing power transfer. Figure 6 illustrates a Π-type impedance matching network utilized in this study.

Figure 6:

The π-type impedance matching network.

A notable advantage of a Π-type network is its capability to match non-linear loads to a transmission line with a complex impedance. This configuration offers superior impedance matching and enhanced power transfer efficiency compared to other matching network types.

Additionally, multi-band impedance matching networks can be employed. For instance, the single-line series multi-stub tuning configuration [28] has been considered, as it often requires fewer elements to implement these single-channel IMNs. However, implementing single-band IMNs in parallel paths generally achieves higher efficiencies.

The design of a multiband IMN can be achieved using a multiplexer [29], which functions similarly to individual IMNs for each frequency band. The advantage of using a multiplexer is that it allows the system to have a single input for the antenna. However, designing a multiplexer is more complex compared to individual IMNs. Due to the separate degrees of freedom in each band, higher efficiency is often achievable with individual IMNs.

In [30], a seven-band rectifier utilizes three optimized single Schottky diodes in parallel, paired with a broadband omnidirectional monopole antenna for precise impedance matching. A microstrip line adjusts the input impedance to enhance RF energy absorption, improving RF-DC conversion efficiency. A Π-type matching network ensures the rectifiers match the input impedance, integrating a dual/triband DC-pass filter. Radial stubs replace open microstrip lines to broaden the resonance frequency bandwidth, yet RF-DC efficiency ranged only from 28 % to 45 % across 1.67 GHz–5.92 GHz at −10 dB m input due to poor matching and Schottky diode nonlinearity.

We simulate the structure shown in Figure 6 to find the best values of elements covering capacitors and inductors. The process is performed such that the elements are obtained to optimize the output of the matching circuit while the values of S ₁₁ for each band are held below −10 dB m. The values of the IMN elements are included in Table 1.

As shown in Figure 5, reverse diodes have been added in parallel with the output capacitors in each stage. Our evaluations show that without the reverse diode, at times the power of inputs are not the same, an imbalance and instability are observed in the output waveform. By using these diodes, this issue can be solved. The reason is because if a signal is absent in one of the inputs channels, no voltage exists at the output capacitor of that channel; hence, the capacitor starts charging through the output of other channels and DC current is blocked or cannot pass evenly. However, with a diode, when the output current of one channel is higher than that of another channel, the diode conducts and the current continues in a clockwise direction without stopping and distorting the output load. These traits are shown in the simulation results.

4 Simulation results and discussion

The proposed multi-band rectifier is designed in 130 nm standard CMOS technology and simulated in ADS. The S ₁₁ curves and corresponding Smith charts of each bands are plotted in Figure 7. As can be seen, the values of S ₁₁ in the desired frequency interval for each channel is less than −10 dB and the Smith chart curves are located near the center, indicating a relatively good match in the corresponding frequency range.

Figure 7:

The S ₁₁ curves and corresponding Smith charts for (a) first channel, (b) second channel, (c) third channel, and (d) forth channel.

The PCE curve of the proposed quad-band system in terms of variation of the input power is shown in Figure 8 for different load resistances from 1 kΩ to 10 kΩ. The PCE is calculated using

where, P _in is the total power received by input channels and V _out is the output voltage. In this simulation, we consider equal input powers for each band. As can be seen in Figure 8, for the input power range of – 5 dB m to +13 dB m and output load of 5 kΩ, the efficiency achieves above 80 %, which is suitable for a power supply system, e.g., a wireless sensor. The maximum PCE in this simulation equals to 85.87 % at the input power of 10 dB m with a load of 5 kΩ.

Figure 8:

PCE to P _in at different load resistances.

Figure 9 depicts the output voltage variation with respect to the input power. As before, we consider equal input powers for each frequency band. As can be seen, the output voltage increases with increasing input power. The output voltage for the input power of −10 dB m is higher than 1 V, which is usable for the today’s CMOS technology devices. It is noteworthy that at conventional input power levels, the reverse voltages appearing on the active elements in our design are less than their breakdown voltages.

Figure 9:

DC output voltage to input power.

One of the advantages of the proposed system, as mentioned before, is stability of the output voltage under different inputs conditions as well as heterogeneous inputs. To verify, we put different RF inputs for each band such that we short-circuit two input channels and apply the other two, the same inputs with a power of −10 dB m. Then, we check the output in two modes, i.e., with and without the reverse diode.

The simulation results show that in the absence of a reverse diode with two output heads, it leads to instability at the DC output voltage, as shown in Figure 10(a). To solve this issue, a reverse diode is used at the output. The result can be seen in Figure 10(b), which stabilizes the output voltage.

Figure 10:

Time variations of V _out (a) without output reverse diode, (b) with output reverse diode.

As shown in Figure 10, V _out2 and V _out4 exhibit negative values. This occurs because we did not apply input power to the stages related to V _out2 and V _out4, while we applied −10 dB m input power to the stages corresponding to V _out1 and V _out3. Considering the current path to the load, the reverse diodes connected to V _out1 and V _out3 conduct and generate rectified DC output, resulting in the voltage polarity direction as shown in Figure 11. Therefore, in Figure 11, the stages connected to V _out1 and V _out3 are represented as DC voltage sources. Since these outputs are in series, their voltages combine to yield a DC output given by V _out = V _out1 − V _out2 + V _out3 − V _out4 in this scenario.

Figure 11:

Equivalent circuit where two inputs are inactive (no power), while the remaining two inputs are active.

The layout of the proposed quad-band rectifier is depicted in Figure 12, which is designed using ADS. The dimensions of the layout measure 1.61 mm × 1.94 mm.

Figure 12:

The layout of the proposed quad-band rectifier.

The performance of the proposed quad-band rectifier is compared with those of various related multi-band designs, as shown in Table 2. It can be seen that most results from previous studies have lower PCE at the input power of −5 dB m, while the maximum PCE is at the input power of 10 dB m. Our proposed design clearly yields higher RF to DC power conversion efficiency in comparison with those of multi-band counterparts.

5 Conclusions

In this paper, a highly efficient quad-band RFEH system has been presented for ambient RF energy harvesting applications. The quad-band approach utilizes the highly congested frequency bands of FM-100, TV-600, GSM-1800 and WiFi-2400. This structure addresses the problem of ultra-low power level or lack of signal in one or more frequency bands, which is the main problem of single-band RFEH systems, ensuring sustained power output continuity and stability for consumers. The maximum power conversion efficiency of approximately 86 % has been achieved for a load of 5 kΩ with a sufficient power stability. The system hence demonstrates significant power conversion efficiency, making it suitable for a wide array of practical applications. Future work will focus on enhancing system dimensions through the integration of single wideband antenna.