Study on Passive Self-Equalization of Parallel-Connected LED Strings

Parallel connection of several LED strings is a practical way to reduce the overall lamp voltage when a large number of emitters are needed. However, this connection might lead to high current mismatches between strings due to the dispersion of characteristics that even binned devices feature. Several techniques have been proposed in order to eliminate or, at least, reduce the forward current mismatch between paralleled strings. Provided that a cost-effective design for commercial lighting is pursued, passive self-equalization of LED strings will be studied in this paper, where the dynamic resistance will be considered as the equalizing resistor. Thus, a statistical study will first be developed on four commercial devices in order to determine the forward voltage deviation in both binned and unbinned LEDs. Then, the dynamic resistance will be accurately determined for the operating current in a wide temperature range by the procedure followed in previous works, since this parasitic effect will be employed as the equalizing resistor. The I-V and R_D-T_j curves will thus be experimentally obtained for a large sample. Once the thermal and statistical characterizations are done, the study of the LED linear model will be performed in order to determine the impact on forward current mismatches that forward voltage and dynamic resistance variations imply in both binned and unbinned LEDs. Finally, experimental results will show that binned LEDs are suitable for self-equalization, even for a tight current-mismatch tolerance, particularly for a significant number of series-connected LEDs in each string.