Yuk Ming Lai

An Alternative Approach to LED Driver Design Based on High-Voltage Driving

This paper discusses an alternative approach to designing LED drivers based on high-voltage operation. Compared with the conventional approach of driving LEDs with low voltage and high current, the alternative approach aims to deliver the same power to LED load with high voltage and low current so that small and nonelectrolytic capacitor can be used. High-voltage operation also facilitates the use of simple PFC preregulators such as boost or SEPIC converters as single-stage LED drivers. This paper begins with the derivation of the relationship between the nominal output voltage and output capacitors size of a PFC preregulator for achieving a given output voltage ripples size, followed by a review of the recently emerged high-voltage LED packages suitable for the proposed application. This paper also discusses the flicker and colorimetric performances of LED light sources under the influence of large current ripple when nonelectrolytic capacitor is used. The method of third-harmonic current injection to the input current of LED drivers is also incorporated as an option for further reducing the output capacitors size while meeting a specified flicker limit. Finally, a design example of high-voltage LED driver based on a boost PFC preregulator with third-harmonic current injection is presented and verified experimentally.

Boundary Control With Ripple-Derived Switching Surface for DC–AC Inverters

Boundary control (BC) has the advantages of fast dynamical response and accurate steady-state regulation when applied to the control of power converter systems. However, it has the drawback of wide variation in switching frequency, and usually requires an oversized filter to remove unwanted signals. This situation becomes more severe when BC is applied to DC-AC inverters that have a time-varying reference. In this paper, we provide a thorough study into this problem, and propose an enhanced BC method to restrict the variation of the switching frequency of DC-AC inverters. Based on the ripple information of the state variables, a BC method with ripple-derived switching surface is developed and implemented in the inverter. Experimental results show that the proposed control method is capable of limiting the switching frequency variation while maintaining the characteristics of fast dynamical response and accurate steady-state regulation.

Accurate Capacitive Current Balancing in Multistring LED Lighting Systems Based on Switched-Capacitor-Controlled

The combination of high-frequency ac drive and capacitors offers an attractive passive current balancing solution for multistring light-emitting diodes (LED) lighting systems due to the advantages of compactness, high efficiency, and ease of modularity. Given that frequency-controlled resonant converters are commonly used as high-frequency ac drivers, the conventionally used switching frequency modulation could impair the effectiveness of capacitive balancing as the impedance of the balancing capacitors is a sensitive function of the switching frequency which is constantly varied for the purpose of output voltage regulation. In this paper, a constant-frequency capacitive impedance current balancing method is proposed. Compared to the conventional method, output voltage regulation of resonant converter is achieved by modulating the resonant frequency of resonant tank by switch-controlled capacitor (SCC) technique, which gives rise to constant balancing capacitors impedance and constant LED current over a wide range of load conditions. Its implementation in an LCC resonant network is studied in this paper and a systematic design procedure of a SCC-based constant-frequency LCC resonant converter is presented. The effectiveness of the proposed method is experimentally verified by a 70-W experimental prototype.

LCC

A modified LCL resonant tank incorporating switch-controlled inductor (SCI) is introduced to realize a tunable resonant tank that ensures ZVS in primary side and ZCS in secondary side switches of an isolated bidirectional dual-active-bridge dc-dc converter (DAB-IBDC) under a wide range of output voltage and charging current of super-capacitor. The nominal operating point is carefully chosen to reduce conduction loss arising from circulating current. Moreover, the firing-angle of SCI is selected in such a way that it keeps ZCS at secondary side to avoid excessive ringing in voltage and current because of parasitic inductances in switches. Simulation and experimental results are shown to validate the effectiveness of the proposed resonant tank and control strategy on DAB-IBDC, which is designed to interface a 400 V dc bus to super-capacitor with voltage variation in the range of 10 V to 48 V, at maximum rated current of 10 A and rated power of 480 W. Finally, an efficiency of up to 94.23 % is achieved using proposed topology and modulation scheme.

Resonant Network

This chapter describes the power supplies that are commonly used in electric equipment, namely, linear regulators and switching regulators. Their properties, performances, operating principles, and characteristics of these regulators will also be given in this chapter.

A switched-inductor-augmented resonant DAB converter for achieving wide-range zero voltage switching

Both conduction loss and switching loss can contribute significantly to the overall power loss of an isolated bidirectional dual-active-bridge series-resonant dc–dc converter (DABSRC) operating at high frequency. To achieve soft switching and minimum-tank-current operation under wide-range variations in output voltage and current, a switched-impedance-based DABSRC is proposed. Minimum-tank-current operation aims to reduce conduction loss arising from circulating current at the low voltage, high-current side of DABSRC. Full-range soft switching is achieved in all switches, thus, switching loss is significantly reduced. With this new topology, power control is achieved by controlling a switch-controlled capacitor in the series resonant tank while ensuring minimum-tank-current operation and soft switching in all switches. The proposed topology and modulation scheme are validated by means of a 1-kW experimental prototype of DABSRC operating at 100 kHz designed to interface a 250-V dc bus to a supercapacitor with a rated output voltage of 48 V. The effectiveness of the proposed topology for charging/discharging a supercapacitor at a maximum rated power of 1 kW is verified by simulations and experimental results with a maximum efficiency of 97.5%.