Method of controlling external electrode fluorescent lamps for local dimming of liquid crystal displays

This paper proposes a method of controlling external electrode lamps (EEFLs) for local dimming of liquid crystal displays (LCDs). In the proposed method, each EEFL is divided into four partitions using external electrodes formed on the outer surface of the lamp. The luminance is controlled by varying the duration of ac power applied to the center electrode of each partition. The shape of the external circuit for the center electrode is designed to have a reliable on/off operation. Balance circuits connected to the grounded electrodes are used to reduce the luminance difference between the left and right parts of each partition. The proposed method was tested on a 42-in. LCD using an EEFL backlight unit (BLU) after dividing the BLU into 24 blocks. The EEFL BLU could operate reliably up to an ac peak voltage of 1.4 kV, while controlling the luminance of each block independently. When one block at the corner was turned on, and the others were turned off, the luminance ratio was ∼11.1:1 between the block and the nearest one, and ∼9000:1 between the block and the farthest one. These experimental results demonstrate that the proposed method is an effective approach for local dimming of LCDs using EEFL BLUs.

Research highlights
► EEFLs are used to implement local dimming of LCDs.
► An external power circuit suppresses erroneous discharge in the OFF partition.
► A ground balance circuit reduces luminance difference in the ON partition.
► Luminance ratio between the ON and farthest OFF blocks was ∼9000:1.