This paper aims to present a modular dimmable high brightness (HB) light-emitting-diode (LED) driver for retrofitting general/residential illumination systems. An improved micro-controller based pulse-width modulation dimming technique is studied and implemented for selecting the brightness of LEDs. Under universal input voltage operation, high power factor and high efficiency can be achieved by a power-factor-correction converter followed by a coupled inductor fly-back converter. The operation principles of the proposed modular HB-LED dimmer and driver will be analyzed and discussed. A laboratory prototype is also designed and tested for verification.