Research On High Performance LED Backlight Driver

Thanks to the advantages of high luminous efficiency, good optical performance, environmentalprotection and long life, light-emitting diode (LED) has gradually become the mainstream of marketreplacing cold cathode fluorescent lamps (CCFL). Multiple LED strings in parallel is necessary inhigh power backlight system due to the power limitation of single LED. Directly connecting LEDstrings in parallel does not guarantee current balancing due to the LED’s negative temperaturecoefficient and the manufacturing tolerance in I-V characteristics. On the other hand, dimmingfunction is required in order to improve the picture quality. The dimming frequency in existingdimming schemes is lower than20kHz, which generates noise in the audible frequency range of thehuman ear. In high-end lighting system like machine vision or industrial inspection, the dimmingfrequency must be higher than20kHz, therefore it is necessary to research the solutions for currentsharing among multiple strings and wide range high frequency dimming for backlight driving system.Based on capacitor charge conservation, this paper proposes two capacitor crossed currentsharing schemes. They use capacitor charge conservation and staggered switches to achieve currentsharing and current sharing effect is independent on output voltage matching. Also, integrationtechnique can be used since the switches are open-loop control and the voltage stress is low. Thecurrent sharing principle and maximum current ripple expression are detailed derived and thedifference between the two proposed schemes is analyzed.Based on closed-loop transfer function Bode diagram, the high frequency parallel dimming underaverage current control is proposed in this paper. With the high frequency attenuation characteristicsin Bode diagram, the duty cycle of the converter does not response to the output voltage changeduring dimming process, which solves the response speed limitation to the dimming frequency. Byanalyzing the relationship between the dimming frequency and output inductor current ripple, thedimming frequency selection method is proposed.Based on the above analysis, this paper completes a60W backlight drivers; experimental resultsverify the effectiveness of the proposed schemes.