Study of a Matching Circuit Effect on a Microwave Rectifier

Yuwei Zhou, Bruno Froppier, Tchanguiz Razban IREENA, University of Nantes, Nantes, France

yuwei.zhou@etu.univ-nantes.fr Abstract—An effective method is proposed to design a matching circuit for a microwave rectifier in order to manage the frequency bandwidth and the power bandwidth. Although the input impedance is affected by the nonlinear behavior of diodes with the variation of input power levels, the analysis on a variety of matching circuit configuration brings more evidence to improve the performance of rectifiers. This method has been applied in a low cost, zero-bias Schottky diode rectifier having RF-DC conversion efficiency of 55 %. The nonlinear simulation of rectifier circuit and an analytic description of multiple dimensional parameters are the key to study the relation between the nonlinear performance of a rectifier and its matching circuit.

Keywords-Schottky diode; rectifier; matching circuit

I. INTERODUCTION People have searched for some ways to obtain the energy

from nature resources, as shown in Fig. 1, such as thermal effect, mechanical vibration, photovoltaic cells, and electromagnetic energy receivers. These methods have provided efficient and practical solutions to consumer, industrial, and military needs [1]. Taking into account the impact of external environments and the constraint on natural conditions, the reception of microwave energy has been introduced by the usefulness of wireless devices as well as the potential range of applications. The search for new energy harvesting device is driven by sensor networks and green communications. The autonomous sensors exchange their data cooperatively through the network to a main location thanks to the technology of energy self-sufficiency at the terminals. The energy harvesting is needed to increase the life time of sensors and to minimize the energy impact of communication.

Figure 1. Schematic of the power harvesting system

In recent years, the current trend in wireless communication system has been to develop low cost, light weight rectennas (rectifier antenna) which are capable of capturing the microwave energy from surrounding environments and converting the power into DC energy with a high performance over a large spectrum of frequencies.

This technological direction has focused much effort on the design of microstrip rectennas. We take this developing technology as the starting point. A typical response of a rectifier, using this technology, is shown in Fig. 2. This figure shows the link between the reflection coefficient and the conversion efficiency against the variation of input power levels.

(a) Pin=10 dBm (b) Pin=0 dBm

Figure 2. S11 parameter and efficiency for given input power designs

For the high power level (Pin=10 dBm), the maximum efficiency point is inside the well matched power span (S11<-10 dB), as shown by the shaded area in Fig. 2(a). The matching circuit improves RF-DC conversion efficiency. To the contrary, for the low power level (Pin=0 dBm), the maximum efficiency point is beyond the well matched power span, as shown by the shaded area in Fig. 2(b). In this case, the contribution of matching circuit to the conversion efficiency is negligible. This first study shows that the rectenna can operate under both high and low power levels. The simulations show that high power levels can be defined to be the region of Pin>5 dBm and low power levels (Pin≤5 dBm). Approaches to design a matching circuit are different for each situation.

In some high power applications, the wireless energy transmission is directly done from a base station to a custom designed antenna optimized specifically for a narrow portion of ISM (Industrial-Scientific-Medical) frequency due to the free bandwidth principle and the low cost technology [2]. By this way, wireless devices can operate with the rechargeable battery technology for longer durations far away from centralized power sources. On the other hand, in low power applications, wireless devices are allowed to operate with a well designed rectenna which absorbs the electromagnetic energy from low ambient radiation sources at multiband frequencies [3]. Unfortunately, the existing Schottky diode packages are not recommended for these low power

Mechanical Vibration

Photovoltaic Cells

Electromagnetic Energy Receiver

Thermal Effect

Other Method

Power Receiving System

Power Management

applications because of the low conversion efficiency. At this point, we choose to study the high power level and take into account the effect of matching circuit.

In this paper, we present a zero-bias Schottky diode rectifier for the high power applications. The rectifying circuit is constructed from a single Schottky diode, an output resistor, and a capacitor parallel with the load, as presented in Fig. 3. The behavior of rectifiers is determined by the nonlinear process of Schottky diode, by the barrier losses, and by the series resistance losses [4]. In order to improve the conversion efficiency, the matching technique is studied by way of conjugate matching between the input impedance of rectifying circuits and the antenna impedance. The matching circuit design is complicated due to the input impedance variation versus the input power level.

Figure 3. Microwave rectenna configuration

II. RECTIFIER CIRCUIT The software ADS (Advanced Design System from

Agilent) is used for the nonlinear simulation of rectifiers [5]. Rectification circuits were designed, built, and tested in order to convert RF input signals into a DC signal via Schottky diodes. At the same time, these circuits were optimized to have the highest efficiency possible to increase overall system performance.

Studies on various rectifier configurations show that the conversion efficiency can not be considered without matching. In fact, the input power from antennas may not transfer entirely to rectifiers and consequently, the mismatching maybe lead to a low efficiency. As presented in Fig. 4, Pin is the output power from the sum of power sources, Pt is the power effectively inside the diode, Pr is the reflected power at the diode input, PDC is the output DC power. Therefore, we proposed a model by taking into account mismatching and estimated the efficiency regardless of mismatching losses.

Figure 4. Structure of the rectifier

The efficiency of the overall system, named by global efficiency, is defined by

The efficiency of the rectifying circuit, named by effective efficiency, is defined by [6]

A. Choice of Diodes The first step in making a diode rectifier is to select the

diode type. We used the packaged diode model in the Agilent ADS libraries and tested four kinds of diodes, such as HSMS-2810, HSMS-2820, HSMS-2850, and HSMS-2860, as presented in TableⅠ. Each diode has a specific set of characteristics, which can be compared easily by consulting the SPICE parameters given on each data sheet [7]. The diode HSMS-2820, encapsulated in a SOT23 package, is optimized for zero-bias detector diodes in RF applications. In addition, the parasitic series resistance of this product is low to 6 Ω involving the high efficiency of rectifying operation. And the breakdown voltage is high to 15 V involving the output power.

B. Choice of the Load The next step is to choose the value of load resistor in

order to optimize the performance of this rectifier [8]. The scattering parameter simulation expresses the reflection coefficient in the frequency domain and the harmonic balance simulation makes it easy to observe the nonlinear behavior of this rectifier at different input power levels, as shown in Fig. 5.

Figure 5. Input impedance versus input power

The choice of output load was done to get the best value of effective efficiency ηeff. With the simulation parameters, we made the load resistor vary and found out the highest efficiency located at the load between 500 Ω and 1000 Ω, as shown in Fig. 6. When the capacitor is greater than 30 pF, the capacitor increment doesn’t influence the rectifying effect any more. We chose a capacitor which was large enough to export continuous DC electricity.

Figure 6. Rectifying efficiency versus transmitting power

With the aim of obtaining a good efficiency, other types of diodes and lumped elements were simulated in the same way, as presented in TableⅠ. We chose the rectifying circuit consisting of a single Schottky diode HSMS-2820, a load resistor 820 Ω, and a parallel capacitor 82 pF. The maximum effective efficiency reaches 70 % when the input power is 22 dBm although the conversion efficiency is low to 23 % without matching. Therefore, the matching circuit has great significance for the global efficiency.

Pin

Pt

PDC

Pr

Microstrip Antenna

Matching Circuit

Rectifying Circuit

in

DC

PP

=η (1)

t

DCeff P

P=η (2)

TABLE I. TYPICAL PARAMETERS FOR FOUR COMMON DIDOES

Diode Type HSMS-2810

HSMS-2820

HSMS-2850

HSMS-2860

RS (Ω) 10 6 25 5 BV (V) 25 15 3.8 7

CJ0 (pF) 1.1 0.7 0.18 0.18 IS (A) 4.8E-9 2.2E-8 3E-6 5E-8

Input Power Level

Low flicker noise >-20dBm <-20dBm >-20dBm

Frequency Band RF RF <1.5GHz HF Max Efficiency

(%) 54.3 70.3 71.3 76.1

Load Resistance (Ω) 600 800 2000 5000

Parallel Capacitance(pF) 100 100 100 100

C. Measurement By using the Hybrid optimization in ADS, the double

stubs matching circuit was designed corresponding to the input power 10 dBm. DC output voltage increases until getting the maximum value about 11 V, as shown in Fig. 7, because of the breakdown voltage when the reverse bias voltage is beyond the peak inverse voltage. The efficiency of the entire rectifier reaches 55 % compared to 65 % theoretically, as presented in Fig. 8. The frequency response, measured by a vector network analyzer, has a bandwidth 80 MHz with the power span 14 dBm in a good matching level. The first study on a RF-DC rectifier shows that the matching circuit is critical to a good performance.

Figure 7. DC output voltage versus input power

Figure 8. Efficiency versus input power

The rectifier was modified in order to integrate with a microstrip patch antenna and to easily extract the value of DC voltage. A preliminary testing on the rectifier was done by measuring DC output voltage against the input power. Then, the rectenna experiment was carried out in an anechoic chamber to measure DC output voltage with a transmitter-receiver configuration [9]. A horn antenna, having a gain 20 dB, was used to transmit the power from the generator. The location of the rectenna was kept away from the horn in the far field distance.

Figure 9. Rectenna experiment set-up

The rectenna can receive the power from -11 dBm to 13 dBm and obtain the maximum conversion efficiency close to 20 %. The rectenna curves fit the measurement of the rectifier, as shown in Fig. 10. However, the over all efficiency of the rectenna is low to 0.1 % due to the attenuation in the cables and also in the space. The long term goal is obviously to increase this global efficiency by controlling each step of the design.

Figure 10. Measurement of a rectifier and a rectenna

III. ANALYSIS OF MATCHING CIRCUIT The matching circuit not only improves the transfer

power from an antenna to a diode, but also prevents harmonic signals generated by the rectification process from reaching the antenna [10]. In the former work, the diode is matched in a narrow bandwidth for a specific RF power. A comparison among the different configurations of the matching circuit shows the link between its physical limitations and available power levels.

A. Single Stub Matching Circuit We present an approach of “multiple parameter sweeps”

in ADS simulation. This method is based on scattering parameter simulation at the operating frequencies. The sweeps of several parameters are combined into a hierarchical sweep plan. Due to the nonlinear performance of diodes, the input impedance changes with the variation of input power levels. The harmonic balance simulation shows the nonlinear voltages and currents, in the frequency domain, which include one fundamental frequency and the high order of harmonics. The input impedance is calculated by the ratio between input voltage and input current at the fundamental frequency. We sampled the input impedance corresponding to different input power levels and used the index of real part and imaginary part of the input impedance to label each situation of input power.

For the single stub matching circuit, its structure is relative to several elements, including length and width of a series microstrip line, and length and width of a stub. We constructed a variety of sweep controllers to sweep the parameters of interest. The scattering matrices are stored in the dataset in the form of multiple coordinates. The relation between the physical dimension and the performance of

Voltmeterd>2D²/λ

HornAmplifier

Generator

matching circuit is obtained in the frequency domain, as shown in Fig. 11, and also in the power span, as shown in Fig. 12. The dimension of microstrip lines controls the frequency response, the power response, and the matching level. The electromagnetic energy flux depends on the microstrip dimension and the operating frequency. The frequency response is not sensitive to the width variation of microstrip lines. But it is sensitive to the length variation and this sensitivity increases for a wider microstrip line. The length of a series microstrip line determines the available range of input power. And the length of a single stub affects the matching level. Besides, Fig. 13 shows the matching level for the optimized design both in the frequency domain and at specified power levels. These figures show that the matching circuit can control both the power bandwidth and the frequency bandwidth.

Figure 11. Frequency response of single stub matching for Pin=10 dBm

Figure 12. Power response of single stub matching for Freq=2.45 GHz

Figure 13. S11 parameter variation of single stub matching

B. Tapered Line Matching Circuit By way of “multiple parameter sweeps”, the length of a

tapered line, the input power, and the operating frequencies are optimized. The tapered line, taking the exponential tapered line as an example in Fig. 14, is a good way to get a broad frequency bandwidth. The higher power level is easily matched by a longer tapered line and in this case, the power span is larger, as shown in Fig. 15. Meanwhile, Fig. 16 shows the matching level for the optimized design either in the frequency domain or at specified power levels. The tapered line matching circuit, capable of a good efficiency with a wide frequency bandwidth and a large span of input power, is suitable for the higher level of power supplies.

Figure 14. Frequency response of tapered line matching for Pin=10dBm

Figure 15. Power response of tapered line matching for Freq=2.45GHz

Figure 16. S11 parameter variation of tapered line matching

IV. CONCLUSION We proposed an effective method to design a matching

circuit in order to control the frequency and power bandwidth. The input impedance matching depends on both the nonlinear behavior of diodes against the input power and the operational frequency affecting the electromagnetic field distribution on transmission lines. This method has applied in a low cost, zero-bias Schottky diode rectifier having a measured RF-DC conversion efficiency of 55 %. The analysis on a variety of matching circuit configuration brings more evidence to improve the overall performance of rectifiers. This study is the beginning of a global optimization of microwave energy harvesting designs.

ACKNOWLEDGMENT The author gratefully acknowledges the assistance of

many colleagues at IREENA for the fabrication and measurement.

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