Younghoon Cho

Digital Plug-In Repetitive Controller for Single-Phase Bridgeless PFC Converters

This paper investigates a plug-in repetitive control scheme for bridgeless power factor correction (PFC) converters to mitigate input current distortions under continuous conduction mode and discontinuous conduction mode operating conditions. From the PFC converter model and the fact that a type-II compensator is used, a design methodology to maximize the bandwidth of the feedback controller is suggested. After that, the error transfer function including the feedback controller is derived, and the stability of the repetitive control scheme is evaluated using the error transfer function. The implementation of the digital repetitive controller is also discussed. The simulation and experimental results show that the input current THD is significantly improved by using the proposed control scheme for a 1-kW single-phase bridgeless PFC converter prototype.

A Carrier-Based Neutral Voltage Modulation Strategy for Multilevel Cascaded Inverters Under Unbalanced DC Sources

This paper proposes a pulsewidth-modulation strategy to achieve balanced line-to-line output voltages and to maximize the modulation index in the linear modulation range where the output voltage can be linearly adjusted in the multilevel cascaded inverter (MLCI) operating under unbalanced dc-link conditions. In these conditions, the linear modulation range is reduced, and a significant output voltage imbalance may occur as voltage references increase. In order to analyze these effects, the voltage vector space for MLCI is evaluated in detail. From this analysis, the theory behind the output voltage imbalance is explained, and the maximum linear modulation range considering an unbalanced dc-link condition is evaluated. After that, a neutral voltage modulation strategy is proposed to achieve output voltage balancing as well as to extend the linear modulation range up to the maximum reachable point in theory. In the proposed method, too large of a dc-link imbalance precludes the balancing of the output voltages. This limitation is also discussed. Both the simulations and the experiments for a seven-level phase-shifted modulated MLCI for electric vehicle traction motor drive show that the proposed method is able to balance line-to-line output voltages as well as to maximize the linear modulation range under the unbalanced dc-link conditions.

A Three-Phase Current Reconstruction Strategy With Online Current Offset Compensation Using a Single Current Sensor

This paper proposes a three-phase current reconstruction technique with an online current offset compensation function for three-phase inverter applications utilizing only a single current sensor. In the proposed current sensing method, a phase current and a branch current are simultaneously measured twice in a switching period by using a single current sensor. After that, the current reconstruction algorithm is applied to obtain the three-phase current information. Compared to previous single current sensor strategies, in the proposed method, the sensor output is regularly sampled, and the dead zone is located near the boundary of the voltage vector space instead of near the origin and the borders of each sector. This boundary-neighbored dead zone makes the proposed method more attractive in extremely low modulation index cases because it avoids periodical dead zones which have been an issue in the existing methods. Moreover, the online compensation method for a current measurement offset makes it possible to achieve purely balanced three-phase current control without an offset component. The effectiveness of the proposed method has been verified through simulations and experiments by measuring and reconstructing three-phase currents under various conditions.

An Active Current Reconstruction and Balancing Strategy With DC-Link Current Sensing for a Multi-phase Coupled-Inductor Converter

This paper proposes an active technique to reconstruct and balance the phase currents in multi-phase dc-dc converters implementing current-mode control. Only one current sensor at the dc-link is required to measure and reconstruct the phase currents. Multiple current samples in the PWM switching period are synchronized with the carrier waveforms to actively reconstruct the phase currents using a simple active balancing algorithm that is implemented in a digital controller. The proposed technique improves system performance, stability, efficiency, and prevents inductor saturation. A 5 kW four-phase dc-dc converter prototype for battery charging application is built to experimentally verify the proposed algorithm.

Controller design for dynamic voltage restorer with harmonics compensation function

This paper describes the design of the control loop of a enhanced dynamic voltage restorer (DVR) controller which can handle power quality problems from not only voltage sag/swell but also harmonics. A novel controller, which has a feed-forward loop and two feed-back loops, is proposed. A feed-back loop is used to actively increase the damping component of LC filter system to keep the output voltage overshoot within allowable range, while a feed-forward loop is used to compensate the voltage drop due to the feedback loop. Another feed-back loop based on the digital repetitive control algorithm is implemented to compensate the periodic harmonics of load voltage, which may come from the source, power converter, and nonlinear load. The feasibility of the proposed DVR controller has been verified by the computer simulation results and also by experimental results on 5 kVA DVR system under the various source and load conditions. The both results reveal satisfactory performance not only against sag/swell but also against harmonics.

High-Efficiency Multiphase DC–DC Converter for Fuel-Cell-Powered Truck Auxiliary Power Unit

This paper presents the design and control of a high-efficiency multiphase dc-dc converter for a truck auxiliary power unit (APU) fed by a fuel-cell stack. The power-stage design methodology and the operation of the designed power stage used to achieve high efficiency are described in detail. To operate the designed converter as a truck APU, the mode transfer control scheme, where smooth transitions of the voltage and current control modes are necessary, is proposed, and the current controller design procedure is described. A 5-kW four-phase dc-dc converter prototype is built with the proposed power-stage configuration, and the proposed control strategy is applied to control the dc-dc converter. The efficiency of the converter is more than 99.5% at near 10% load and above 98% from 10% to 100% load range. Simulations and experimental results are provided to verify the feasibilities of dc-dc converter operation and the proposed control strategy in the truck APU application.

Dual-Mode Double-Carrier-Based Sinusoidal Pulse Width Modulation Inverter With Adaptive Smooth Transition Control Between Modes

Three dual-mode boost-buck-derived inverters with smooth transition between modes have been proposed: 1) boost-full bridge inverter; 2) boost-H5 inverter; 3) boost-dual buck inverter. For the same topologies, in conventional control method, the first stage is needed to boost input voltage to a constant high voltage, and the second stage is for sinusoidal inverting. However, in the proposed three inverters, either buck or boost mode works at a single time, thus, only one switch works at a high switching frequency. In this way, a lot switching loss can be reduced, and moreover, since the voltage across the middle capacitor is smaller than the conventional constant high voltage, the further switching loss mitigation is possible. In order to achieve smooth transition between the two modes, a double-carrier-based sinusoidal pulse width modulation (SPWM) is proposed. Other than this, three advanced modulation methods are proposed: 1) double-carrier with different frequencies; 2) double-carrier with different magnitudes; and 3) double-carrier with different frequencies and magnitudes. Following that, the loss distribution in every component is provided and the California Energy Commission efficiency of these inverters under different input voltage conditions is compared. Finally, the experimental results show the dual-mode double-carrier-based SPWM inverter can improve the efficiency by 2% than the traditional constant dc bus voltage solutions.

Derivation of boost-buck converter based high-efficiency robust PV inverter

A single-phase grid connected transformerless PV inverter for residential application is presented. The inverter is derived from a boost cascaded with buck converter along with a line frequency unfolding circuit. High efficiency can be achieved because the converter allows the use of power MOSFET and ultra fast reverse recovery diode, which also features a robust structure because the phase-leg does not have shoot-through issue. By interleaving multiple phases, the ripple can be reduced, and thus the inductor size can also be reduced. This paper begins with theoretical analysis and modeling of this boost-buck converter based inverter to define the operation limit and to design the closed-loop controller. A two-phase interleaved inverter is then designed accordingly. Finally, the simulation and experiment results are shown to verify the concept.

Torque Ripple Reduction of a PM Synchronous Motor for Electric Power Steering using a Low Resolution Position Sensor

MDPS (motor driven power steering) systems have been widely used in vehicles due to their improved fuel efficiency and steering performance when compared to conventional hydraulic steering. However, the reduction of torque ripples and material cost are important issues. A low resolution position sensor for MDPS is one of the candidates for reducing the material costs. However, it may increases the torque ripple due to the current harmonics caused by low resolution encoder signals. In this paper, the torque ripple caused by the quantized rotor position of the low resolution encoder is analyzed. To reduce the torque ripples caused by the quantization of the encoder signals, the rotor position and the speed are estimated by measuring the frequency of the encoder signals. In addition, the compensating q-axis current is added to the current command so that the 6th order torque harmonic is attenuated. The reduction of torque ripples by applying the estimated rotor position and the compensated q-axis current is verified through experimental results.

High-efficiency design of multiphase synchronous mode soft-switching converter for wide input and load range

A four-phase interleaved bidirectional dc-dc converter with coupled inductor is adopted in both boost- and buck-mode operations. The circuit allows synchronous mode operation to achieve zero-voltage switching. Together with the use of low Rds-on power MOSFET as the switching device, ultra high efficiency can be obtained. This paper shows a high-voltage example system operating completely in synchronous mode with 99.2% efficiency at the full load, but only 93% at 10% load. A low-voltage system example is then proposed to operate in synchronous mode under light-load condition and in continuous conducting mode under heavy load condition to achieve high efficiency over a wide load range. The peak efficiency of the second example system reaches 99.5% in between 10 and 20% load and maintains above 98% throughout the rest of load range.