Wenlong Qi

Integration of an Active Filter and a Single-Phase AC/DC Converter With Reduced Capacitance Requirement and Component Count

Existing methods of incorporating an active filter into an AC/DC converter for eliminating electrolytic capacitors usually require extra power switches. This inevitably leads to an increased system cost and degraded energy efficiency. In this paper, a concept of active-filter integration for single-phase AC/DC converters is reported. The resultant converters can provide simultaneous functions of power factor correction, DC voltage regulation, and active power decoupling for mitigating the low-frequency DC voltage ripple, without an electrolytic capacitor and extra power switch. To complement the operation, two closed-loop voltage-ripple-based reference generation methods are developed for controlling the energy storage components to achieve active power decoupling. Both simulation and experiment have confirmed the eligibility of the proposed concept and control methods in a 210-W rectification system comprising an H-bridge converter with a half-bridge active filter. Interestingly, the end converters (Type I and Type II) can be readily available using a conventional H-bridge converter with minor hardware modification. A stable DC output with merely 1.1% ripple is realized with two 50-μF film capacitors. For the same ripple performance, a 900-μF capacitor is required in conventional converters without an active filter. Moreover, it is found out that the active-filter integration concept might even improve the efficiency performance of the end converters as compared with the original AC/DC converter without integration.

A Single-Stage Two-Switch PFC Rectifier With Wide Output Voltage Range and Automatic AC Ripple Power Decoupling

Conventional single-phase power-factor-correction (PFC) rectifiers with active power decoupling capability typically require more than three active switches in their circuits. By exploring the concept of power-buffer cell, a new single-stage PFC rectifier with two active switches, one inductor and one small power-buffering capacitor is reported in this paper. The proposed converter can achieve high-power factor, wide output voltage range, and power decoupling function without using electrolytic capacitor. Additionally, an automatic power decoupling control scheme that is simple and easy to implement is proposed in this paper. The operating principle, control method, and design considerations of the proposed rectifier are also provided. A 100-W prototype with ac input voltage of 110 Vrms and a regulated dc output voltage ranging from 30 to 100 V has been successfully designed and practically tested. The experimental results show that with only a 15 μF power-buffering film capacitor, the proposed converter can achieve an input power factor of over 0.98, peak efficiency of 93.9%, and output voltage ripple of less than 3%, at 100-W output power.

Enhanced Automatic-Power-Decoupling Control Method for Single-Phase AC-to-DC Converters

Existing control schemes for single-phase ac-to-dc converters with active power-decoupling function typically involve a dedicated power-decoupling controller. Due to the highly coupled and nonlinear nature of the single-phase system, the design of the power-decoupling controller (typically based on the small-signal linear control techniques) is cumbersome, and the control structure is complicated. Additionally, with the existing power-decoupling control, it is hard to achieve satisfied dynamic responses and robust circuit operation. Following a recently proposed automatic-power-decoupling control scheme, this paper proposes a nonlinear control method that can achieve enhanced large-signal dynamic responses with strong disturbance rejection capability without the need for a dedicated power-decoupling controller. The proposed controller has a simple structure, of which the design is straightforward. The control method can be easily extended to other single-phase ac-to-dc systems with active power-decoupling function. Simulation and experimental results validate the feasibility of the proposed control method on a two-switch buck–boost PFC rectifier prototype.

Bi-directional active-filter-integrated AC/DC converter without electrolytic capacitor and extra power switches

In this paper, a cost-effective single-phase bidirectional active-filter-integrated AC/DC converter is proposed. It provides simultaneous functions of power factor correction (PFC), DC voltage regulation and active power decoupling for mitigating the low-frequency DC voltage ripple, without an electrolytic capacitor and extra power switches. Such a converter is readily available from a conventional H-bridge converter with minor hardware modification. Detailed principles of operation, controller design of the converter are provided in the paper. The feasibility of the converter is then confirmed experimentally through a 210 W rectification system. A stable DC output with merely 1.9% ripple is realized with two 50 μF film capacitors. In contrast, for the same ripple performance, a 514 μF capacitor is required in conventional converters without an active filter. Additionally, the uncompromised or even improved efficiency performance is another unique feature of the proposed converter.

A General Approach to Programmable and Reconfigurable Emulation of Power Impedances

Starting with a brief review on the existing methods of impedance emulation, this paper addresses a general and systematic approach to programmable and reconfigurable emulation of power impedances. The proposed approach not only enables the impedance value to be programed, but also allows the characteristics (i.e., type) of the impedance to be reconfigured instantly during the operation. Based on the proposed control method, emulation of at least six types of emulated power impedances (EPI) can be easily attained. In particular, it is theoretically and practically demonstrated that the impedance characteristic can be emulated through a combination of different functions. The systematic derivation of these functions is explained. New techniques that compensate the circuit power losses are introduced. This general approach has been practically verified in several EPI. Both steady-state and dynamic performance of these EPI confirm the programmability and reconfigurability. It is envisaged that the proposed method can be applied to a range of functions, such as power filtering, energy storage, and even power conversion based on direct impedance control.

Parabolic-Modulated Sliding-Mode Voltage Control of a Buck Converter

Conventional hysteresis-modulation-based sliding-mode (SM) controller exhibits a varying switching frequency. Existing SM controllers for achieving constant-switching-frequency operation often have complicated structures and/or are less straightforward to design to meet different steady state, transient, and loading requirements. In this paper, a new direct SM voltage controller based on parabolic modulation (PM) is proposed and applied to a buck converter. The objective is to achieve constant-switching-frequency operation of a switching converter with simple control structure and good operating performance. Both theoretical study and experimental work on a buck converter show that the proposed PM-based SM control can achieve 1) constant switching frequency in the continuous-conduction mode (CCM) and quasi-constant switching frequency in the discontinuous-conduction mode (DCM) of operation, 2) inherent input-disturbances rejection and superior reference tracking performance, and 3) seamless mode transition between CCM and DCM.

A constant-frequency parabolic-modulation-based sliding mode controller for buck converters

In this paper, a direct sliding mode (SM) voltage controller based on parabolic modulation is proposed for a buck converter operating in the continuous conduction mode of operation. The objective is to achieve SM control of a switched-mode power converter with a relatively constant switching frequency through a simple control structure. This is achieved by directly modulating the constructed state trajectory through a pair of parabolic carriers. Both the steady-state and the transient performance of the buck converter with the proposed controller are tested. It is shown that the maximum switching frequency deviation throughout the testing condition is merely 3.8%. In addition, the proposed controller achieves good load regulation and dynamic performance.

A two-switch buck-boost PFC rectifier with automatic AC power decoupling capability

In this paper, a single-stage power-factor-correction (PFC) rectifier with active power decoupling function is proposed. The proposed rectifier has a low component count as compared to existing solutions. Only two active switches, one inductor and one small power-buffering capacitor are needed. High power factor, wide output voltage range and active power decoupling can be simultaneously obtained. In addition, the rectifier has an inherent automatic power decoupling capability, and no dedicated active power decoupling control is required. Therefore, the control of the rectifier is simple and easy to implement. A 100 W prototype of the proposed rectifier with 110 Vrms/50 Hz input and a regulated DC output voltage ranging from 30 V to 100 V has been constructed and tested. The results show that with only a 15 μF power-buffering capacitor, a power factor of over 0.98, peak efficiency of 93.9% and output voltage ripple of less than 3% has been achieved.