IEEE Access

Bài báo này trình bày một kỹ thuật giảm thiểu tổn thất chuyển mạch độc lập hiệu quả cho bộ biến đổi nguồn điện 3 pha 2 mức. Một thuật toán điều chế độ rộng xung dựa trên sóng điều chế đã được sửa đổi, mang tên điều chế độ rộng xung không liên tục theo từng pha, được sử dụng để tạo ra các khoảng thời gian không chuyển mạch khác nhau cho pha có trạng thái lão hóa tồi tệ nhất bằng cách tiêm một điện áp không chuỗi thích hợp vào các điện áp tham chiếu. Các khoảng không chuyển mạch dẫn đến việc giữ một pha nhất định không chuyển mạch nhằm giảm tần suất chuyển mạch và tổn thất tương ứng. Việc xác định cẩn thận các khoảng kẹp thay đổi nhằm đạt được sự giảm bớt mong muốn về tần suất chuyển mạch theo từng pha của bộ biến đổi nguồn điện 3 pha 2 mức trong khi vẫn đảm bảo rằng hiệu suất đầu ra không bị suy giảm nhiều. Sự giảm tần suất chuyển mạch và tổn thất chuyển mạch tương ứng có tác động tích cực đến áp lực nhiệt của các thiết bị bán dẫn công suất, dẫn đến việc cải thiện độ tin cậy của bộ biến đổi nguồn điện. Để đánh giá hiệu suất của phương pháp đề xuất, phương pháp điều chế véc tơ không gian dựa trên sóng điều chế thông thường, điều chế độ rộng xung không liên tục 3 pha tổng quát, và điều chế độ rộng xung không liên tục sử dụng điện áp bù hybrid đã được triển khai cùng với phương pháp đề xuất. Độ chính xác và hiệu quả của phương pháp đề xuất đã được xác nhận bằng cách sử dụng cả dữ liệu mô phỏng và dữ liệu thực nghiệm.

The Cramér-Rao Bound (CRB) is a powerful tool to assess the performance limits of a parameter estimation problem for a given statistical model. In particular, the Gaussian CRB (i.e., the CRB obtained assuming the data are Gaussian) corresponds to the worst case; giving the largest CRB among a large class of data distributions. This makes it very useful in practice since optimizing under the Gaussian data assumption can be interpreted as a min-max optimization (i.e., minimizing the largest CRB). The Gaussian CRB is also the corresponding bound of Second-Order Statistics (SOS)-based estimation methods, which are frequently used in practice. Despite its practicality, computing this bound might be cumbersome in some cases, particularly in the case where the input is assumed deterministic and has a large number of samples. In this paper, we address this computational issue by proposing a fast computation for the deterministic Gaussian CRB of Single-Input Multiple Output (SIMO) blind system identification. More precisely, we exploit circulant matrix properties to reduce the cost from cubic to quadratic with respect to the sample size. Moreover, we derive a closed-form formula for the asymptotic (large sample size) Gaussian CRB and show how it can be computed using the residue theorem.

Nowadays, active rectifiers are increasingly used, particularly in grid-connected applications. The high efficiency and reliability of active rectifiers become crucial for increasing the system performance. Due to the lack of model predictive control methods considering the different aging levels among phase legs of rectifiers, this paper thus introduces a direct power control-based model predictive control for independent loss reduction in each phase of the active rectifier. Different from conventional techniques, the target of proposed model predictive direct power control scheme is lowering switching loss in the leg having highest aging level of active rectifier, resulting in increasing its corresponding lifetime and lifetime of entire rectifier. As opposed to the model predictive control method based on incorporating additional terms regarding switching loss in the cost function, a switching state preselection strategy is proposed in this study to determine clamping region and corresponding preselected switching state for the leg having highest aging level of active rectifier. The use of the proposed method lowers thermal stress in the leg having highest aging level and increases lifetime of active rectifier. Simulation and experimental results for the proposed technique are presented and compared with conventional model predictive direct power control method to validate the accuracy and feasibility of the proposed method.

This paper presents a dual-sense circularly polarized (CP) antenna with compact size, high isolation, and wideband characteristics. The antenna consists of a microstrip patch, an array of parasitic metal plates positioned around the patch, and a 90° branch line coupler as feeding structure. Unlike the conventional design method, we demonstrate that the wide isolation bandwidth can be achieved with an imperfect coupler and imperfect matching at the radiating element. The final design with overall dimensions of 0.59λc× 0.59λc × 0.06λc (λc is the free-space wavelength at center operating frequency) was fabricated and tested. Measurements on the fabricated prototype show that the antenna has a bandwidth of 16.5% (5.0-5.9 GHz) with port isolation of better than 15 dB. In addition, a peak broadside gain of about 6.6 dBic is attained. In comparison with other related works in literature, this antenna has advantages of wide operating bandwidth with high isolation while possessing a compact footprint.

A wideband small-footprint circularly polarized Fabry-Perot resonator antenna is presented in this paper. The design employs a partial reflecting surface (PRS) consisting of thin homogeneous dielectric slabs separated by a small air gap. The PRS is analyzed using transmission line theory, which gives more insightful into the theoretical concept and provides a simple tool for design and optimization. An antenna prototype with overall size of 1.2λfmin × 1.2λfmin × 0.6λfmin has been fabricated and experimentally validated (λfmin being the free-space wavelength at the minimum operating frequency). The measured impedance matching and axial ratio bandwidths are approximately 55.7% and 47.7%, respectively. In addition, the antenna achieved 3-dB gain bandwidth of 50.9% with a peak gain of 15 dBi. Compared to previous designs targeting similar applications, the antenna proposed here possesses a simple geometry with light weight, ease of optimization and fabrication, as well as enhanced axial ratio bandwidth and 3-dB gain bandwidth.

This paper introduces a compact antenna with bandwidth reconfigurable and flexible characteristics for cognitive radio applications. The antenna consists of a monopole as a primary radiator and is excited by a coplanar waveguide to have a single-layer design. To adjust the operating bandwidth (BW) by fixing the lower frequency and varying the upper one, an additional stub is connected to the feeding line. This stub causes a disturbance in the current distribution, which significantly degrades the matching performance. By controlling its length, the rejected band can be changed and thus, the upper frequency is varied. To validate the proposed approach, two stubs are used, and they are connected to the feeding line through two PIN diodes. By switching the ON/OFF state of the diodes, three different states with different operating BW are attained. The fractional BWs for these states are 18.9, 33.2, and 64.3% respectively. Besides, it is also worth noting that the antenna achieves good performance when bent over a curved surface.

A method to significantly improve the isolation of multiple-input-multiple-output (MIMO) antenna is presented in this paper. The proposed design consists of multiple patches arranged in the H-plane configuration. Satisfactory isolation is obtained by using microstrip lines that are positioned along the radiating edges of the patches. The function of the decoupled lines is to excite an orthogonal polarization mode on the non-excited patch. A two-element array is fabricated to demonstrate the effectiveness of the proposed decoupling structure. The experimental results illustrate that the isolation of the 2-element MIMO antenna can be significantly improved by about 26 dB with an extremely small element spacing of $0.017 \lambda _{0}$ . Besides, the antenna exhibits good diversity performances. The decoupling method is also applicable to a multiple-element MIMO antenna. Compared to other related works, the proposed design has the smallest element spacing while achieving high isolation. The antenna can be used for 5.2 WLAN applications.

This paper shows a method to reduce the mutual coupling between extremely closely spaced circularly polarized (CP) multiple-input-multiple-output (MIMO) antenna. The strong coupling between dual-sense CP MIMO elements is thoroughly investigated and a method to effectively suppress the mutual coupling by controlling the phase of the coupled signal is proposed. High isolation can be achieved by using a grounded stub, which is positioned along the edges of radiating elements. An antenna prototype is fabricated and measured to verify the feasibility of the utilized decoupling scheme. The measured results show an operating bandwidth of 2.1% (5.21-5.32 GHz) with isolation of better than 20 dB. The poor isolation of the coupled MIMO is significantly enhanced to 38 dB with the use of the proposed decoupling network. A 33 dB improvement can be attained with an extremely small edge-to-edge distance of $0.017\lambda _{c}$ and center-to-center spacing of $0.23\lambda _{c}$ at the center frequency. Compared to other related works, the proposed antenna features the smallest element spacing while achieving high isolation enhancement.

This paper presents a simple design of high gain Fabry-Perot (FP) antenna with polarization diversity. The proposed antenna is composed of a reconfigurable microstrip patch, a partially reflective surface, and a planar metallic reflector. By electrically controlling the ON/OFF states of two p-i-n diodes, the polarization of the antenna can be switched among dual-sense circular polarization and single linear polarization. Notably, the biasing wires, which are generally used in designing reconfigurable antennas, are not necessary in the proposed design. Thus, the undesirable effects of the biasing wires on the antenna's operating characteristics can be eliminated. An antenna prototype, whose overall dimensions are 2.0λ×2.0λ×0.6λ at the center operating frequency, is fabricated and measured to validate the design concept. The measured overlapped bandwidth for -10 dB |S11| and 3-dB AR bandwidths of all polarization states is 4% (7.3-7.6 GHz). The desired radiation patterns are also observed with a peak gain of 15.1 dBi.

This paper proposes a predictive control method with offset voltage injection for achieving a neutral point (NP) voltage balance of three-phase three-level neutral point clamped (NPC) inverter, without employing a weighting factor. In order to ensure the proper and reliable operation of the NPC inverter, the NP voltage balance should be regulated in addition to the sinusoidal output current. The conventional predictive control methods for NPC has suffered from tedious weighting factor selection. Besides, when the converter's parameters value and control condition changes, it is cumbersome to empirically redesign the weighting factor. Therefore, the proposed predictive control method without the weighting factor can successfully maintain the balance of NP voltage by utilizing an offset voltage, which is determined according to the difference between the upper and lower capacitor voltages. As a result, the proposed algorithm using the offset voltage injection can control the output currents and maintain the balance of NP voltage. Simulation and experiments are presented to prove the validity of the NP voltage balancing of the proposed control method.