Current Balancing Circuit Design for Parallel LED Strings in QDOT High Bright TV with Dual-Transformer LLC Resonant Topology

Abstract

New LED technologies demand slim designs with increased backlight power. When the parallel-connected LED strings are driven by multiple transformers, the magnetic coupling difference and the LED string voltage difference cause backlight current mismatch problem. This imbalance causes the line with low voltage to draw more current hence to create heat spots on the backlight causing reduced lifetime. This problem is observed as backlight fluctuation at the end-user side. In this study, a cost-effective balancing circuit is simulated and implemented for eight parallel-connected LED strings using two transformers in an LLC half-bridge resonant topology while meeting the power demand in a relatively slim design. For the 145W backlight power, the current mismatch was 57.59% when there was an 8% deviation in transformer leakage inductance and a 10% variation between LED string voltages. The temperature difference between the strings was measured as 31. 8°C taken at room temperature with the open back cover. With the balancing circuit, the LED current difference being reduced to 9.99%. When the string voltages are the same, the maximum deviation in the current is measured as 2.59%.