Capacitors are widely used in dc links of power electronic converters to balance power, suppress voltage ripple, and store short-term energy. Condition monitoring (CM) of dc-link capacitors has great significance in enhancing the reliability of power converter systems. Over the past few years, many efforts have been made to realize CM of dc-link capacitors. This article gives an overview and a comprehensive comparative evaluation of them with emphasis on the application objectives, implementation methods, and monitoring accuracy when being used. First, the design procedure for the CM of capacitors is introduced. Second, the main capacitor parameters estimation principles are summarized. According to these principles, various possible CM methods are derived in a step-by-step manner. On this basis, a comprehensive review and comparison of CM schemes for different types of dc-link applications are provided. Finally, application recommendations and future research trends are presented.

/pdf/an-overview-of-condition-monitoring-techniques-for-332haqdmou.pdf

An Overview of Condition Monitoring Techniques for Capacitors in DC-Link Applications

A single-inductor dual-output (SIDO) buck–boost power factor correction (PFC) converter operating in critical conduction mode is proposed in this paper. By multiplexing a single inductor, each output of the SIDO buck–boost converter can be regulated independently. Compared with a conventional two-stage multiple-output converter, the SIDO buck–boost PFC converter benefits from significant overall cost saving, small size, and light weight. Moreover, the efficiency of the SIDO buck–boost PFC converter can be improved due to single-stage power conversion. The control strategy and characteristics of the proposed converter are analyzed. The efficiency, power factor, total harmonic distortion, and output accuracy are verified using the experimental results.

Single-Inductor Dual-Output Buck–Boost Power Factor Correction Converter

Passive or active current balancing circuits are usually used to mitigate current imbalance in driving multiple light-emitting-diode (LED) strings. Passive current balancing schemes adopting capacitors with high reliability, small size and low cost are very popular in many applications. However, the high reactive power of the capacitive balancing scheme with variable frequency control will bring high power stress on the VA rating of the main switches which drive this passive current balancing circuit and decrease the overall efficiency. Fixed frequency control does not permit zero-voltage switching (ZVS) under load variations. Hence, this paper proposes a current-source-output LED driver based on LCLC resonant circuit to provide a constant output current regardless of variations in LED parameters. In the LCLC circuit, the number of additional capacitors is scalable with the number of LED strings for current balancing. Moreover, the input impedance of the improved LCLC circuit is designed to be resistive at the operating frequency to minimize reactive power. The conventional duty cycle control can easily incorporate ZVS. The analysis, implementation and verification are detailed in this paper.

An Improved LCLC Current-Source-Output Multistring LED Driver With Capacitive Current Balancing

A 1 MHz half-bridge resonant dc/dc converter based on GaN FETs and planar magnetics is proposed in this paper, which improves the system efficiency and power density. The resonant network can achieve satisfactory soft-switching characteristics based on a small impedance angle, which greatly reduces the losses of switches and diodes. The losses characteristics during the turn-on and turn-off transitions are analyzed in detail. The calculation results show that the GaN FETs with low output capacitance and on resistance can achieve fast switching speed and low losses in high-frequency conditions. To reduce the profile and increase the power density of the system, planar magnetics are used in this paper. The response surface methodology (RSM) and modular layer model (MLM) are adopted to help design the planar inductor and transformer, respectively. Both of the methods offer clearer and more effective ways to design the planar magnetics. A 25-W prototype is built to verify the feasibility of the proposed high-frequency converter.

A 1 MHz Half-Bridge Resonant DC/DC Converter Based on GaN FETs and Planar Magnetics

There is an emerging application, which uses a mixture of batteries within an energy storage system. These hybrid battery solutions may contain different battery types. A dc-side cascaded boost converters along with a module-based distributed power sharing strategy has been proposed to cope with variations in battery parameters such as, state-of-charge (SOC) and/or capacity. This power sharing strategy distributes the total power among the different battery modules according to these battery parameters. Each module controller consists of an outer voltage-loop with an inner current-loop where the desired control reference for each control-loop needs to be dynamically varied according to battery parameters to undertake this sharing. As a result, the designed control bandwidth (BW) or stability margin of each module control-loop may vary in a wide range, which can cause a stability problem within the cascaded converter. This paper reports such a unique issue and thoroughly investigates the stability of the modular converter under the distributed sharing scheme. The paper shows that a cascaded PI control-loop approach cannot guarantee the system stability throughout the operating conditions. A detailed analysis of the stability issue and the limitations of the conventional approach are highlighted. Finally in-depth experimental results are presented to prove the stability issue using a modular hybrid battery energy storage system prototype under various operating conditions.

/pdf/control-of-cascaded-dc-dc-converter-based-hybrid-battery-125l0f318q.pdf

Control of Cascaded DC–DC Converter-Based Hybrid Battery Energy Storage Systems—Part I: Stability Issue

Summary

The system of magnetically coupled resonant wireless power transfer (MCR-WPT) for a rectifier bridge light-emitting diode (LED) load is modeled, and its transmission characteristics for different LED arrays are analyzed and validated in this paper. On the basis of the derived equivalent steady-state circuit model consisting of the high-frequency alternating current source, the WPT networks and the rectifier bridge LED load, transmission efficiency and transmission distance related to the LED connection forms are obtained in theory. In our concerned scheme, the total power of LED arrays is chosen as its rated one so as to guarantee a rated current of each LED. More accurate system parameters are measuring, and some theoretical results for two feasible connection forms of LED arrays are validated in experiment. The theoretical and experimental results at the two working points are consistent approximately, which validates the feasibility of the derived equivalent circuit model of the concerned MCR-WPT system. Copyright © 2014 John Wiley & Sons, Ltd.

Modeling and analysis of magnetically coupled resonant wireless power transfer system with rectifier bridge LED load

For the average current mode (ACM)-controlled boost power factor correction (PFC) converter, there is line frequency instability under light load and small output capacitor conditions. The line frequency instability is characterized as self-oscillation in the form of period-doubling, which drastically increases the total harmonic distortion (THD) and additional losses. Regarding this line frequency instability, this paper proposes a stability control scheme incorporating an outer-voltage proportional controller and an additional third current-harmonic feedforward compensation (TCFC). Specifically, the outer-voltage proportional controller replaces the traditional low-pass filter (LPF) to mitigate the line frequency instability, and the TCFC aims to ensure an acceptable THD. Moreover, the dynamic response of the proposed current-feedforward-based stability control scheme outperforms the existing voltage-feedback-based schemes. Furthermore, the control parameters are determined by the harmonic averaged model, and the feasibility of the proposed scheme is verified by experiments based on IC UC3854BN. The experimental results demonstrate that the THD with the line frequency instability is 56.60%, while it is improved to 15.58% by using the proposed scheme.

Mitigating Line Frequency Instability of Boost PFC Converter Under Proportional Outer-Voltage Loop With Additional Third Current-Harmonic Feedforward Compensation

Circuit modeling and efficiency analysis for wireless power transfer system with shielding

In this paper, a two-period frame transient switching control scheme for buck converters using coupled-inductor auxiliary circuit to improve the load dynamic response is proposed. The proposed auxiliary circuit only works during a transient event, while in steady state only the main converter side of the coupled inductor is used with a constant-frequency capacitor-current hysteresis control (CF-CCHC) scheme. The transient event consists of two periods, in the first of which, the coupled inductor is inversely connected in parallel using a logic switching control scheme, so as to reduce the equivalent inductance sharply and achieve a small voltage-deviation transient regulation. In the second transient period, the auxiliary-circuit side inductor works in a freewheeling state and the converter behaves as in an approximate steady state using the CCHC scheme. With three switching actions of the auxiliary circuit, the proposed scheme is implemented using a digital processor and an analog control circuit. With the application of the proposed control scheme to a 24–12 V synchronous buck converter, its feasibility is validated in simulation and experiment. Experimental results demonstrate that the settling time of the proposed scheme, with its coupled coefficient of 0.57, has improved by more than 83% compared to the time-optimal control scheme, and the voltage deviation has improved by more than 77% as well.

Two-Period Frame Transient Switching Control for Buck Converter Using Coupled-Inductor Auxiliary Circuit

For the case with electrolytic capacitors as the output capacitor, the traditional capacitor current hysteresis (CCH) controlled buck converter is always in over-damping state due to the ESR, and has a poor dynamic performance. Moreover, there is a problem of variable switching frequency operation in the CCH controlled buck converter. To solve these problems, an improved constant-frequency CCH (CF-CCH) control scheme using the reconstructed ideal-capacitor voltage is proposed in this paper. The ideal-capacitor voltage is reconstructed through a low-pass filter and an integrator, and introduced into the control loop to replace the actual output voltage. In the proposed CF-CCH control scheme, the damping ratio is designed as the optimum value 0.707, which is in under-damping state. Furthermore, the switching frequency is kept constant by disturbing the hysteresis bands with a pair of periodic ramp signals. In addition, the circuit design of the proposed CF-CCH control scheme is presented. Finally, the validity and superiority of the proposed CF-CCH control scheme are verified by simulation and experiments.

Weiguo Lu

Papers

Modeling and analysis of magnetically coupled resonant wireless power transfer system with rectifier bridge LED load

Mitigating Line Frequency Instability of Boost PFC Converter Under Proportional Outer-Voltage Loop With Additional Third Current-Harmonic Feedforward Compensation

Circuit modeling and efficiency analysis for wireless power transfer system with shielding

Two-Period Frame Transient Switching Control for Buck Converter Using Coupled-Inductor Auxiliary Circuit

Constant-Frequency Capacitor Current Hysteresis Control of Buck Converter Using Reconstructed Ideal-Capacitor Voltage