Research and design of underground miniaturized microwave energy transmission rectification circuit

    At present, more and more research on wireless energy transmission is being conducted at home and abroad. Wireless energy transmission will definitely become the direction of energy transmission in the future, especially in environments where wiring is difficult. There are many sensors underground, they are widely distributed, and the environment is complex and harsh, making it difficult to replenish sensor energy. Therefore, it is of great significance to study wireless charging of sensors. As a key technology for wireless charging, the design of the rectifier circuit directly affects the rectification efficiency of the antenna. This paper proposes a design scheme for the implementation of a microstrip rectifier circuit with an operating frequency of 5.8 GHz, and designs a rectifier circuit with small size, high rectification efficiency, and suitable for sensors. Combined with high-frequency simulation software HFSS and ADS, the implementation scheme of the rectifier antenna is studied, and the correctness of the design principles and steps is verified through experiments.
1 Principle and design of microwave rectifier circuit
    In order to meet the requirements of small-size and high-efficiency rectifier circuits for wireless sensors, this paper uses an adjustable output pass-through filter to achieve the matching effect, and there is no matching network between the input filter and the diode, thereby greatly reducing the size of the rectifier circuit and improving the rectification efficiency.
    The schematic diagram of the rectifier circuit is shown in Figure 1. The input filter uses a 5th-order Chebyshev low-pass filter to filter out harmonics, cut off harmonics, and prevent harmonics from flowing back to the antenna [1]. The rectifier circuit uses a Schottky diode (HSMS286), which has a low forward turn-on voltage and small junction capacitance at an operating frequency of 5.8 GHz.

1.1 Analysis of input and output filters
    The input filter allows the fundamental wave to pass through without loss, suppresses the higher harmonics generated by the diode, and prevents it from flowing back to the antenna and reducing the rectification efficiency. A step impedance low-pass filter is used. According to the transmission line theory, a short high-impedance transmission line can be equivalent to a series inductor, and a low-impedance transmission line can be equivalent to a parallel capacitor. The distributed parameter filter composed of alternating high and low impedance microstrip lines is a microstrip line step impedance filter [2]. The electrical length of the inductor is determined by the following formula.

    The output filter consists of a microstrip line with a length of L and a capacitor connected in parallel at both ends of the load. By adjusting the length of L, the input impedance of the diode end is made into a pure impedance, resonance occurs, and only direct current is allowed to pass, blocking the fundamental wave and higher harmonics [3]. Through simulation, it is found that when the length of the microstrip line is 5.3 mm, the pure impedance of the diode input end is 40. By adjusting the input and output impedances of the input filter, the impedance matching with the receiving antenna and the diode is achieved respectively, so as to achieve the purpose of reducing the size of the rectification circuit.
1.2 Analysis of rectifier diodes
　As the key technology of rectifier antennas, the design of rectifier circuits directly affects the efficiency of antennas, and rectifier diodes are the core components of rectifier circuits. When Schottky diodes are used as nonlinear resistors, their parasitic parameters will affect the performance of the circuit. When designing microwave circuits, the influence of these parasitic parameters should be fully considered [4].
    The three parameters of reverse breakdown voltage Vbr, zero bias junction capacitance Cj0 and series resistance Rs determine the rectification efficiency of the diode [5]. In order to improve the rectification efficiency of diodes, reference [6] studied the influence of Rs and Cj on the rectification efficiency. Since Rs and Cj play the role of voltage division and current shunting on the nonlinear junction resistance, when Rs decreases, the energy it consumes decreases and the rectification efficiency increases; when Cj decreases, the rectification efficiency increases because the input impedance of the Cj branch becomes larger and the reverse current is difficult to pass. At the same time, the time constant τ (τ=Rs×Cj0) determined by Rs and Cj0 decreases. According to references [5-8], through the analysis of the diode equivalent circuit, Figure 3 shows the curve of the diode rectification efficiency and input power change under different ? values ​​using theoretical formulas and Matlab simulation.

    Therefore, when selecting a rectifier diode, the effect of the diode parameters on the rectifier efficiency should be fully considered, and a diode with a relatively small value should be selected as much as possible.
    Avago's HSMS-286 family is designed and optimized for applications with an operating frequency of 915 MHz to 5.8 GHz, and can be perfectly applied to RF to DC conversion [9]. This paper selects the Avago HSMS-2860 Schottky barrier diode, Bv = 7.0 V, Cj0 = 0.18 pF, Rs = 5 Ω, and Vj = 0.7 V. The input pin of the diode is connected to the microstrip line input, and one of the other two is grounded through a via, and the other is connected to the microstrip line output.
    The performance of the rectifier diode is the key factor in determining the rectifier efficiency. The rectifier diode operates in a large signal state, so its nonlinear characteristics must be analyzed first, and the effects of parameters such as operating frequency, input power, and DC load on the rectifier efficiency must be studied [10].
    HSMS-2860 is usually used for high-power signals. It is necessary to use harmonic balance simulation in ADS software to study the nonlinear behavior of Schottky diodes to input power changes. According to the rectifier circuit design theory, the input impedance of the diode must be determined first before the subsequent design, especially the matching circuit design, can be carried out. The relationship between the diode input impedance and input power is simulated as shown in Figure 4. At low input power (less than -5 dBm), the input impedance remains basically unchanged when the input power changes. When the input power is large (greater than -5 dBm), the diode input impedance changes significantly with the input power. This is because at high power, the circular current generated by the load is comparable to the diode's own saturation current, which affects the diode's S parameters.
    The principle of measuring the diode rectification efficiency in the case of system impedance mismatch is to use the power that actually enters the diode as the denominator. Otherwise, the reflected power caused by the mismatch will greatly reduce the conversion efficiency of the diode, making the measurement result inaccurate. As shown in Figure 5.

    The diode rectifier circuit was tested. At the operating frequency of 5.75 GHz~5.85 GHz, the input power was 16 dBm (the maximum output power of the vector network), the load was 680 ?, the rectifier voltage reached 4.4 V, and the rectifier efficiency was 71.2%.
    The test efficiency is lower than the simulation efficiency. The main reasons are as follows: (1) The circuit welding is not accurate, the solder covers part of the signal line, which interferes with the signal and causes energy loss; (2) The SMA connector pin is a thick round shape, and the parasitic capacitance generated by welding has a greater impact on the rectifier efficiency; (3) The line width difference between the filter microstrip lines is large, causing signal reflection and increasing line loss; (4) The rough manual welding of the diode and the addition of the adapter cause partial energy loss.
    By improving the welding accuracy, changing the SMA connector with one pin and two grounds and covering the ground pin with copper paper to reduce interference, the most accurate measurement results can be achieved.
    This paper proposes a design method for a microstrip rectifier circuit suitable for charging underground wireless sensors. The circuit is small in size and has high rectification efficiency. By matching the diode and the output filter, the matching network is removed, the size is reduced, and the rectification efficiency is also improved. The simulation verification is carried out through ADS software. The reasons affecting the rectification efficiency are analyzed and improved. After testing, the rectification efficiency of the rectifier circuit is significantly improved, and it can be applied to the rectification circuit of wireless sensor energy transmission.
References
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