A high efficiency driver circuit is proposed for Light Emitting Diode (LED) lamps with dimming feature. The current regulation is accomplished by processing partial power of the power conversion circuit so that a high overall efficiency can be realized. The detailed description and analysis of circuit operation are provided. The dimming feature can be accomplished by means of linear current regulation, pulse-width modulation (PWM) or double pulse-width modulation (DPWM). Based on the circuit analyses and derived equations, a laboratory circuit is designed for an LED lamp which is composed of 40 high-brightness white LEDs in series. The performances with three dimming schemes are compared from the measured results. LEDs dimmed by DPWM have less color shift than those dimmed by linear current regulation and PWM. On the other hand, the dimming scheme with linear current regulation has the highest light efficiency over the entire dimming range. The circuit efficiency can be as high as 95.5% at the rated output and deteriorates slightly to 90.5% as the lamp is dimmed to 10% of the rated power.

This thesis proposes a dimming approach with Double Pulse Width Modulation for equalizing the light output of the back light with light emitted diodes (LEDs) for large scale outdoor liquid crystal displays (LCDs). The approach compensates the difference among the LED light bars by adjusting the power outputs of converters according to the feedback of light strength from light sensors. With the proposed Double Pulse Width Modulation method, local brightness adjustment on the light bars can be made to provide a uniform light output and the dimming function for LCD can be retained. Experiments results made on a 46¡¨ LCD with four LED light bars demonstrate that the double pulse-width- modulation can provide uniformly in the light bar output. The experimental results show the proposed Double Pulse Width Modulation (DPWM) method can alleviate the problem from divergence of the light bars and thus can generate more uniform light output on LCDs.

An efficient Light Emitting Diode (LED) lamp driver circuit is proposed for retrofitting the conventionally used incandescent lamps with existing TRIAC dimmer. The dimming feature in a wide range of firing angle from 30¢X to 130¢X can be accomplished by means of double pulse-width modulation (DPWM) and analog current regulation. The LED lamp driver adopts a flyback converter with an auxiliary active power MOSFET for synchronous switch and an associated inductor for zero voltage switching (ZVS), leading to lower switching loss and thus achieving a higher circuit efficiency. In the thesis, the mode operation of the driver circuit is analyzed and the design equations are derived accordingly. A laboratory circuit is designed for an 50 W LED lamp which is composed of 45 high-brightness white LEDs in series. Experiments are carried out to test the circuit performances with two dimming schemes. The experimental results indicate that the driver can achieve a circuit efficiency of 95 % at the rated output. When the LED lamp is dimmed, the circuit efficiency with DPWM is higher than that with the analog current regulation. On the other hand, the LED lamp dimmed by analog current regulation has a higher efficiency but a less color shift by DPWM.