Woojin Choi

Performance Improvement of LCL -Filter-Based Grid-Connected Inverters Using PQR Power Transformation

The demand for three-phase pulsewidth modulation (PWM) inverters in applications such as power control or grid connecting has been on the increase in recent years. Such inverters are connected to the grid via an L filter or an LCL filter to reduce the harmonics caused by the switching. An LCL filter can reduce the harmonics induced by low switching frequency and generates a satisfactory level of grid-side current using a relatively low inductance, as compared to an L filter. The additional poles introduced by the LC part induces resonance in the system, leading to stability problems; this paper presents a compensation method using power theory to improve these issues, so that the performance of the designed LCL filter system can be improved. The effectiveness of the proposed algorithm is verified by simulations and experiments.

Soft-Switched Interleaved Boost Converters for High Step-Up and High-Power Applications

This paper proposes a generalized scheme of new soft-switching interleaved boost converters that is suitable for high step-up and high-power applications. The proposed converter is configured with proper numbers of series and parallel connected basic cells in order to fulfill the required output voltage and power levels, respectively. This leads to flexibility in device selection resulting in high component availability and easy thermal distribution. Design examples of determining the optimum circuit configuration for given output voltage and power level are presented. Experimental results from a 1.5-kW prototype are provided to validate the proposed concept.

Advanced Dynamic Simulation of Supercapacitors Considering Parameter Variation and Self-Discharge

In this paper, dynamic simulation of the equivalent circuit model of the supercapacitor, taking into account the parameter variations and self-discharge, is discussed. Self-discharge is modeled with equivalent impedance including a constant phase element (CPE), and the parameter variations depending on the voltage are reflected. Since it is difficult to directly simulate the ZARC element (R-CPE parallel circuit) with a circuit simulation tool such as the professional simulation program with integrated circuit emphasis (PSPICE), equivalent transformation to three R-C parallel circuits is introduced in the simulation. The accuracy of simulation with the model is then verified through a comparison with results of an experiment. The comparison shows that the model using a CPE is effective in representing the dynamic characteristics and self-discharge of supercapacitors. Accordingly, it proves that the method proposed in this study can be useful in developing systems that include supercapacitors, and can be applied in an integrated simulation of a supercapacitor and a power electronic system.

Development of the intelligent charger with battery State-Of-Health estimation using online impedance spectroscopy

In this research, a novel intelligent charger with battery diagnosis function is proposed. The diagnosis function is implemented by way of impedance spectroscopy achieved by controlling the charger to create a frequency swept excitation voltage at the battery terminals with no additional hardware. The impedance variation of battery according to the degradation over the life is measured and used for evaluating the State-of-Health (SOH) of the battery. The voltage perturbation and the current response of the battery are measured by the digital lock-in amplifier embedded in the digital signal processor (DSP) in order to calculate the impedance of the battery. The parameters of the equivalent circuit model for the lead-acid battery are extracted by using the complex non-linear least square method and compared to the reference values to estimate the SOH of the battery. The design procedure of the proposed charger is detailed and the feasibility of the system is verified by the experiments.

A Novel Soft-Switching Battery Charge/Discharge Converter With the Zero Voltage Discharge Function

One important test for the formation and grading of a lithium-ion battery is to confirm the performance of the battery while discharging it down to zero volts. In this paper, a novel soft-switching charge/discharge converter with the zero voltage discharge function is proposed. The proposed converter is able to discharge a battery until its voltage reaches zero volts. The phase-shift full-bridge method is used to charge the battery and the current-fed push-pull method with bidirectional switches is used for the discharge. Zero voltage switching (ZVS) of the switches is achieved at turn-on during the charge operation. ZVS turn-on and zero current switching turn-off of the switches are achieved during the discharge operation. The charge/discharge controllers are designed taking into account the internal impedance of the battery. The performance of the system is verified by the experiments using lithium-ion batteries.

Design of a Nonisolated Fuel Cell Boost Charger for Lithium Polymer Batteries With a Low Output Ripple

In the design of a fuel cell charger, it is important to find a suitable topology and to design the converter to guarantee the performance of both the fuel cell and the battery. Most of the chargers developed so far have used step-down converters. However, since a small fuel cell stack can only generate a low voltage, it is necessary to use a step-up converter to charge batteries. In this paper, a nonisolated boost charger topology with an additional output inductor for a proton exchange membrane fuel cell is presented to meet the strict ripple requirements for the battery charge, and its control method using a proportional-integral (PI) controller is detailed. The feasibility of the boost charger and its control method is then verified through the experimental results.

Selection Criteria for Supercapacitors Based on Performance Evaluations

In this paper, criteria for better selection of a supercapacitor through EIS (Electrochemical Impedance Spectroscopy) experiments are presented. The performance characteristics of a supercapacitor are thoroughly analyzed in terms of losses and available energy to select the optimal product. The validity of the proposed criteria is demonstrated through the computer simulations and experiments on a fuel cell vehicle using a supercapacitor module with the FTP-72 urban dynamometer driving schedule.

Analysis of the Output Ripple of the DC-DC Boost Charger for Li-Ion Batteries

In the design of battery chargers, limiting the output ripple current according to the manufacturer’s recommendation is important for reliable service and extended battery life. Ripple components can cause internal heating of the battery and thus reduce the service life of the battery. Care must be exerted in the design of the switching converter for the charge application through the accurate estimation of the output current ripple value. This study proposes a method to reduce the output current ripple of the converter and presents a detailed analysis of the output current ripple of the DC–DC boost converter to provide a guideline for the design of the battery charger.

Novel Time Division Multiple Control Method for Multiple Output Battery Charger

Multiple output converters (MOCs) are widely used in electronic equipment for industrial, commercial, and military application such as the voltage regulator modules in portable electronic devices, multiple output power supplies, and multiple output charger due to their advantages in cost, volume, and efficiency. However, most of the MOCs developed so far have limitations in terms of the number of outputs, the tight regulation of all the outputs, the simplicity of the structure, and the ease of control. In this letter, a novel time division multiple control (TDMC) method which can regulate all of the outputs with high precision is proposed. The proposed method is simple in its control and structure. In addition, it provides an even degree of tight regulation for all the outputs. The validity and the feasibility of the proposed method are verified by applying it to a multiple output battery charger based on the double ended forward converter.

Novel Estimation Technique for the State-of-Charge of the Lead-Acid Battery by using EKF Considering Diffusion and Hysteresis Phenomenon

State-of-charge (SOC) is one of the significant indicators to estimate the driving range of the electric vehicle and to control the alternator of the conventional engine vehicles as well. Therefore its precise estimation is crucial not only for utilizing the energy effectively but also preventing critical situations happening to the power train and lengthening the lifetime of the battery. However, lead-acid battery is time-variant, highly nonlinear, and the hysteresis phenomenon causes large errors in estimation SOC of the battery especially under the frequent discharge/charge. This paper proposes a novel estimation technique for the SOC of the Lead-Acid battery by using a well-known Extended Kalman Filter (EKF) and an electrical equivalent circuit model of the Lead-Acid battery considering diffusion and hysteresis characteristics. The diffusion is considered by the reconstruction of the open circuit voltage decay depending on the rest time and the hysteresis effect is modeled by calculating the normalized integration of the charge throughput during the partial cycle. The validity of the proposed algorithm is verified through the experiments.