Yuk Ming Lai

A new method for online estimation of power-stage parameters and autotuning of feedback controller for point-of-load (POL) converter is presented in this paper. The autotuning controller presented has been specifically developed to handle wide variations of the resonant frequency of L_oC_o output filter and the equivalent series resistance (ESR) zero of output capacitor. The resonant frequency of L_oC_o output filter and the time constant of output capacitor's ESR zero are estimated by examining the start-up transient response of converter and online measurement of steady-state output voltage ripple, respectively, hence the proposed method is non-intrusive and does not affect converter's normal operation. Based on these results, digital compensator is automatically tuned based on some user defined phase margin and crossover frequency to provide the desired transient response and output voltage regulation over wide variations of power-stage parameters. Simulation and experimental results from a 12 V-to-1.5 V, 22.5 W, 500 kHz digitally controlled POL buck converter are provided to verify the effectiveness of the proposed method.

This paper presents a unified approach to controlling the white color point of the red/green/blue (RGB) light-emitting diode (LED) existing in all aging states. In contrast to conventional color control systems where the average driving currents of the primary-color LEDs can become saturated when the LEDs have undergone prolonged aging, which causes the resulting white color point to go out of regulation, the proposed method avoids this problem by adjusting the color set points when a predefined threshold current is reached by one or more of the primary-color LEDs. It is shown that the method can effectively maintain the white color point of RGB LED at the desired value when the primary-color LEDs are subjected to an accelerated aging through repetitive current stress cycles.

Multistring arrays of LED are increasingly used for high-luminance lighting applications. The parallel connection of multiple LED strings, however, gives rise to the issue of current balancing between the strings, as a common voltage applied to them does not guarantee an equal current sharing due to the manufacturing spread in electrical properties. In relation to the need of current balancing, the emergence of AC-LED has enabled a direct driving of LED with ac voltage/current, and capacitors, whose impedance are designed to dominate over the equivalent resistance of LED string, are employed to achieve quasi-lossless current balancing. Although high-efficiency resonant inverters represent an ideal choice for driving AC-LED, their properties tend to be changed significantly by the addition of current-balancing capacitors, which necessitates a reformulation of the conventional design approach. In this paper, the characteristics of an LCC resonant network, which is based on the conventional parallel resonant network and modified by the addition of current-balancing capacitors, are analyzed in depth, and a systematic design procedure for developing an LCC-based resonant inverter with current balancing is formulated. A 100-W prototype is constructed and its performance is tested to verify the proposed design procedure. It is also shown that, with the proposed LCC resonant network, the inverter is capable of reconfiguring itself and operating stably in the case of LED failures.

In this paper, a power management control scheme based on triple-droop control strategy is proposed for low-voltage microgrid system. The proposed control method is based on traditional P/f and Q/V droop control, but adds two other levels of droop control in response to the DC bus voltage conditions. One of them regulates the output power of micro-source (MS), while the other adjusts the output parameters of Droop Control 1. With this control scheme, the output power of MS can be adjusted automatically without the need for communication among the MS systems. The proposed control scheme is proven as capable of operating in both autonomous mode and grid-connected mode. In addition, the droop-controlled VSI systems ensure a smooth transition from grid-connected mode to autonomous mode without the need for mode-switching or communication. The design methodology of the triple-droop control scheme is presented in this paper. The effectiveness of the proposed scheme and design methodology are verified by simulation results on PSIM.

This paper presents a synthesis method for two-state ladder-structured DC-DC power converters by duality transformation. The proposed method can uncover all known converters and also some configurations that are never mentioned in literature. The voltage-to-current configurations are most suitable to be used in PV and LED applications which require constant current sources.

This paper presents a new LCC resonant inverter that is compatible with capacitor current balancing in multi-string AC-LED array. A thorough theoretical analysis of the voltage transfer function and input impedance characteristics of the LCC network is presented. Based on the analysis, a guided selection of the desirable operating frequency for achieving zero-voltage switching (ZVS) is developed and a systematic design procedure for choosing the reactive component values is formulated. A 100-W prototype is constructed to verify the proposed design.

A decoupled structure for multi-input bi-directional converter based on the concept of paralleled current sources is proposed in this paper. In order to minimize the interaction between the multiple control loops associated with different energy sources or storages, a decoupled control strategy is implemented by dividing the load power demand into several frequency ranges according to the frequency characteristics of the energy sources or storages. The operating principles of the proposed topology and its control system design are analyzed in detail and verified by simulation results. It is shown that the proposed topology and control method can be adopted for systems containing multiple energy sources or storages, such as micro-grid and hybrid electric vehicles.

Direct methanol fuel cells (DMFC) have been widely researched for applications in portable electronics due to their use of liquid fuel for easy storage and transportation compared to gaseous hydrogen. However, DMFC's performance is strongly affected by methanol crossover that significantly degrades the energy efficiency at low output power, and is characterized by an increasing efficiency at increasing output power. The maximum efficiency point (MEP) is inherently difficult to track due to the commonly unknown methanol crossover rate, but since it is experimentally found to be located very close to the maximum power point (MPP), it is suggested that the MPP can be practically viewed as the quasi-MEP and an alternative tracking approach based on the MPP is proposed for the purpose of MEP tracking. In this paper, a fuel-cell-oriented MPP tracking algorithm based on resistance matching is developed, implemented, and tested in the context of a DMFC/supercapacitor hybrid energy system. To account for the generally slow fuel cell dynamics, the DMFC is constantly tracked at the MPP, while any surplus or deficit power is absorbed or delivered by the supercapacitor bank. The detailed formulation of the algorithm and the power flow design and realization are also discussed.