Drive Circuits for Backlight Unit of Liquid Crystal Displays

Abstract

DoctorThis thesis presents the drive circuits for backlight unit (BLU) of liquid crystal displays (LCDs). It consists of two subjectsa drive circuit including an inverter and an equivalent circuit model of cold cathode fluorescent lamp (CCFL) for simulating a CCFL BLU, described in section III, and a direct-linked single-ended primary inductance converter for driving a light emitting diode (LED) BLU, described in section IV.The proposed drive circuit of CCFL BLU consists of an inverter as a driving circuit and an equivalent circuit model for CCFL as a load of the inverter. It can be used to simulate the behaviors of CCFL BLU for various input waveforms using the standard circuit simulators, thus make the design process of an inverter simpler and easier. The equivalent circuit model for CCFL consists of nonlinear resistors and parasitic circuit elements. The nonlinear resistors were modeled after the measured DC current-voltage (I？V) characteristics of CCFLs. The coupling effects between CCFLs and between CCFL and the BLU frame were represented using the parasitic capacitances and inductances. For both pulsed and sinusoidal inputs, the voltage error between electrical simulation and measurement was less than 1.3% and the current error was less than 5.8%, which demonstrates that the proposed drive circuit model can be used to simulate the electrical behavior of CCFLs in LCD BLU accurately. The direct-linked single-ended primary inductance converter (SEPIC) was proposed as a highly efficient step up and step down converter to drive LED. It consists of the original SEPIC components plus auxiliary diode and switch which form a new power delivery path between input and output. The Direct-linked SEPIC (DLSEPIC) can directly deliver much of the input power to the output with little losses, and reduce circulating current in circuit components. Therefore, the switching and conduction losses of switches and diodes, and winding resistive losses of inductors and equivalent series resistance losses of capacitors were reduced. The power delivery efficiency of the DLSEPIC (96.7%) is 5% greater than that of the original SEPIC (91.7%). This result confirms the validity of the proposed DLSEPIC as a highly efficient step up and step down converter to drive LED