Date of Graduation

12-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Electrical Engineering

Advisor

Simon S. Ang

Committee Member

Randy L. Brown

Second Committee Member

Scott C. Smith

Third Committee Member

H. A. Mantooth

Fourth Committee Member

Jia Di

Abstract

In this dissertation, light-emitting-diode (LED) drivers are investigated for efficiency issues related to driving Red-Green-Blue (RGB) pixels and multiple LED strings in parallel. A high-efficiency digitally controlled RGB LED driver was designed for driving a 3x3 RGB LED display panel. A multiplexer was used to sense the voltage drop across the current controllers. This driver maintained a minimum drive voltage across the RGB LED pixels required to keep it in regulation leading to a reduction of unwanted power losses in the RGB LED pixels by selecting the minimum drop across the current-controllers as the reference voltage of the digital controller. Additionally, analog dimming was implemented to dim each individual LED in a 3x3 RGB LED display panel. Efficiencies of 85.6 %, 93.3 %, and 91.1 % were experimentally obtained for red, green, and blue LEDs, respectively at the rated output current. For comparison, efficiencies of 38.3% for the red LED, 66.2 % for the green LED, and 64.5 % for the blue LED at the nominal current of 20mA using a 5 V supply were achieved in conventional LED drivers. For parallel connected LED strings, a current controller is required for each string to maintain the desired current with the drive voltage provided by a switch-mode power converter (SMPC). A novel analog/digital LED driver controller was designed for driving a two-string LED load with three white LEDs in each string for backlight application in liquid-crystal-displays (LCDs). In this implementation, an analog controller was used to control the output voltage of the SMPC while a digital controller was used to achieve the minimum drive voltage across the output LED strings, leading to an efficiency of 89 % at the rated output current. A phase shifted pulse width modulation (PSPWM) dimming was implemented to reduce load stresses, improve electromagnetic-interferences (EMI) and increase system efficiency. A mathematical model, based on multirate simulation technique, for merging the analog and digital controllers was proposed. Finally, a LED driver chip was designed and fabricated using a 0.5 micron CMOS process to reduce size of the overall LED driver system. The integrated circuit consisted of a 16-channel analog multiplexers, five current controllers, and buffer circuits. The LED driver chip function was experimentally verified using a SMPC to drive a five-string LED load in parallel with a single green LED in each string for display panel applications by maintaining a minimum drive voltage across the LED strings, thus leading to an efficiency of 75 % at the rated output current.

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