S. Y. Hui

General Steady-State Analysis and Control Principle of Electric Springs With Active and Reactive Power Compensations

This paper provides a general analysis on the steady-state behavior and control principles of a recently proposed concept of “electric springs” that can be integrated into electrical appliances to become a new generation of smart loads. The discussion here is focused on how different real and/or reactive load powers can be canceled or altered using the electric springs. Mathematical derivations supporting the theoretical framework of the concept are detailed in the paper. Experimental results validate the theoretical discussions and solutions proposed. It is demonstrated that the electric spring is capable of providing different types of power/voltage compensations to the load and the source.

Dynamic Prediction of Correlated Color Temperature and Color Rendering Index of Phosphor-Coated White Light-Emitting Diodes

Light-emitting diode (LED) technology is a multidisciplinary subject that involves photometry, electric power, heat, and chromaticity which are interdependent on one another. So far, the photoelectrothermal (PET) theory has linked up the first three aspects. This research includes chromaticity into the dynamic PET theory so that even the correlated color temperature (CCT) and color rendering index (CRI) of phosphor-coated white LEDs can be dynamically predicted, thus overcoming the low bandwidth problem of some light measurement equipment. This dynamic modeling of CCT and CRI has been verified with favorable agreements between theoretical predictions and measurements of several LED samples. The outcome of this project offers a new research and development tool for practicing LED system designers to predict the instantaneous variations of CCT and CRI when the power varies in a LED system.

Color Variation Reduction of GaN-Based White Light-Emitting Diodes Via Peak-Wavelength Stabilization

The color, electrical, and thermal properties of LED devices are highly dependent on one another. The peak wavelength of GaN-based white LED shifts in opposite directions under the influences of current and junction temperature change. This affects the correlated color temperature (CCT). Importantly, duty cycle control for LED dimming does not provide constant color (against conventional wisdom). An analysis model that links the peak wavelength, electrical, and thermal properties of LED devices is proposed. The color-shift trend of the LED with respect to the changes in its thermal and electrical operating conditions is described. The stabilized CCT performance of a dc or a bilevel-driven LED over a dimming range is found to be a result of the complex interactions between the selected current levels, duty cycle, thermal resistances of the heatsink and device, heat dissipation conversion ratio, and the physical parameters of the LED device. The predicted color variation is verified by experimental results, which demonstrate that the CCT stabilization of an LED with a dc drive requires less thermal energy than that with a bilevel drive. For a given thermal design, the reduction in CCT variation during light intensity change is possible via the combined adjustment of the current level and its duty cycle over the dimming operation.

Passive Radio-Frequency Repeater for Enhancing Signal Reception and Transmission in a Wireless Charging Platform

This paper reports new results of an emerging field that combines radio-frequency (RF) repeater design and wireless power technologies for charging applications. Power electronics-based wireless charging pads have recently been developed for charging portable electronic devices such as mobile phones. However, the electromagnetic shield underneath the pad could reduce transmission and reception signals. In this paper, a novel passive RF repeater for a wireless charging platform is proposed and investigated for mitigating the signal blocking problem. Since the effective shielding structure for the low-frequency charging signal is not RF friendly, the received RF signal of a wireless mobile device can be reduced when the phone is placed on a wireless charging platform. A dual-polarization free-positioning passive RF signal repeater has been successfully designed and implemented. It can improve the RF reception signal strength by at least 3.6 dB without noticeable reduction in energy efficiency in the charging process.

Chromatic, Photometric and Thermal Modeling of LED Systems With Nonidentical LED Devices

With the emergence of new color-mixing LED systems based on LED devices of different color temperatures, the need for a new modeling technique for LED systems with nonidentical LED devices becomes imminent. This paper presents a modeling technique for LED systems with LED arrays comprising nonidentical LED devices that have nonidentical optical-thermal-electrical properties. Based on a general 3-D photo-electro-thermal LED node model, LED devices of different kinds can be arranged in various array forms according to their system construction and design. By linking the system matrix to the correlated-color-temperature prediction, the proposed modeling technique provides an accurate prediction of the temperature distribution, luminous flux, and correlated color temperature of the LED systems. The temperature distribution and light output of the LED systems have been measured using an infrared imaging system and a spectrophoto-colorimeter with an integrating sphere. The modeling technique has been successfully demonstrated and experimentally verified on several LED systems comprising nonidentical LED devices. It is particularly useful as a modeling tool to study new color-mixing LED systems based on different types of LED devices.

Reduction of Thermal Resistance and Optical Power Loss Using Thin-Film Light-Emitting Diode (LED) Structure

In this paper, a GaN light-emitting diode (LED) with sapphire structure and a thin-film LED without sapphire structure are characterized in the photo electrothermal modeling framework for comparison. Starting from the analysis and modeling of internal quantum efficiency as a function of current and temperature of blue LED, this work develops the thin-film LED device model and derives its optical power and the heat dissipation coefficient. The device parameters of the two LED devices with different structural designs are then compared. Practical optical power measurements are compared with theoretical predictions based on the two types of fabricated devices. It is shown that the thin-film LED device has much lower thermal resistance and optical power loss.

Voltage and frequency control of electric spring based smart loads

With the increasing adoption of renewable energy sources into power grids, the issue of voltage and frequency fluctuation of the network becomes a more critical problem that needs to be resolved urgently. The so-called demand-side management (DSM) of power system provides the opportunity for the prospective grids to intelligently balance the power supply and consumption. As one of the crucial roles of DSM, smart loads are naturally committed to manage the power flow of the network. Other than previous communication-based smart loads, the recently proposed electric springs (ES) provide an opportunity to turn some conventional loads into smart loads without the need for communication. In this paper, a simple voltage and frequency (v&f) control method is applied to such a smart load for tackling the voltage and frequency fluctuations of the power grids. Simulation results reveal that with the proposed v&f control, the smart load can achieve better voltage and frequency regulation performance than that with the conventional control and smart load. Experimental results validating the feasibility of the proposed v&f controller on regulating the voltage amplitude and frequency of the common-coupling point of a power grid are provided.

Power Flow Analysis and Critical Design Issues of Retrofit Light-Emitting Diode (LED) Light Bulb

For retrofit applications, some high-brightness light-emitting diode (LED) products have the same form factor restrictions as existing incandescent light bulbs. Such form factor constraints may restrict the design and optimal performance of the LED technology. In this paper, some critical design issues for commercial LED bulbs designed for replacing E27 incandescent lamps are quantitatively analyzed. The analysis involves power audits on such densely packed LED systems so that the amounts of power consumption in: 1) the LED wafer; 2) the phosphor coating; and 3) the lamp translucent cover are quantified. The outcomes of such audits enable R&D engineers to identify the critical areas that need further improvements in a compact LED bulb design. The strong dependence of the luminous output of the compact LED bulb on ambient temperature is also highlighted.

Reset-sensing quasi-V 2 single-inductor multiple-output buck converter with reduced cross-regulation

This paper proposes a reset-sensing quasi-V2 single-inductor multiple-output (SIMO) converter with minimal cross-regulation. The conventional quasi-V2 sensing scheme in SIMO converters suffers from serious cross-regulation which is primarily induced by the load differentiation with unbalanced loads. It is shown that the proposed reset-sensing quasi-V2 control scheme can significantly reduce cross-regulation by completely discharging the feed-forward sensing node to zero volts during the idle phase in Discontinuous Conduction Mode (DCM). The cross-regulation with the conventional quasi- V2 single-inductor dual-output (SIDO) converter for a load current step of 150 mA is experimentally verified to be more than 1.25 mV/mA. By employing the proposed quasi- V2 control method, the experimental results demonstrate that the cross-regulation for a load current step of 150 mA is significantly reduced to within 0.087 mV/mA. Hence, with the proposed scheme, a load transient in one output will have a minimal effect on the DC operating point of another output. This enables separate current control at each individually-driven output of a SIMO converter.

Morphing switched-capacitor step-down DC-DC converters with variable conversion ratio

High-voltage-gain and wide-input-range DC-DC converters are widely used in various electronics and industrial products such as portable devices, telecommunication, automotive, and aerospace systems. The two-stage converter is a widely adopted architecture for such applications, and it is proven to have a higher efficiency as compared with that of the single-stage converter. This paper presents a modular-cell-based morphing switched-capacitor (SC) converter for application as a front-end converter of the two-stage converter. The conversion ratio of this converter is flexible and can be freely extended by increasing more SC modules. The varying conversion ratio is achieved through the morphing of the converters structure corresponding to the amplitude of the input voltage. This converter is light and compact, and is highly efficient over a very wide range of input voltage and load conditions. Experimental results show that the efficiency of a single SC module is higher than 98%.