Research On Current Balancing Methods For Multi-channel LEDs

The light-emitting diode (LED) VI characteristics of temperature dependence and devices variation make LED currents imbalanced when parallel-connected LEDs are powered by the common voltage source. Current imbalance will cause fast degradation or even failure in some LEDs that should be avoided in LED applications.To solve it, a current sharing circuit based on the pulse width modulation (PWM) under a common bus voltage can realize the required averaged current. If in balance, a small duty cycle will necessitate a large LED on-time current, which may cause temporal overheat and low luminous efficacy of LEDs. Hence, the minimum on-time current amplitude control scheme is proposed to maximize the duty cycles and improve the LED luminous performance. Moreover, the individual dimming control in each LED string can readily be incorporated into the current sharing circuit without additional circuit and cost. The analysis, implementation and verification are detailed in this paper.The capacitive balancing scheme is widely used to mitigate current imbalance in driving multiple LED strings due to simplicity and reliability. To drive LEDs with the required current, variable frequency control will bring the high reactive power and increase VA ratings of the main switches. Fixed frequency control does not permit zero-voltage switching (ZVS) under load variations. To solve it, this paper proposes a multi-string LED driver with constant-current output and automatic balancing. The driver based on an LCLC resonant tank decouples the effect of load variation to output current, and has the resistive input impedance. So fixed frequency control can be implemented to avoid the reactive power. Also, it combines the existing capacitive balancing scheme to realize multi-string current balance with the smallest additional components. Moreover, it has short-circuit protection and the PWM dimming can be incorporated easily.