Flicker-Free Single Switch Multi-String LED Driver With High Power Factor and Current Balancing

A flicker-free single switch multi-string LED driver with high power factor and current balancing is proposed and analyzed. It combines a pre-stage boost PFC converter and multiple second-stage buck dc-dc converters by a single active switch and multiple dc link capacitors. With the proposed circuit configuration of multiple dc link capacitors, the output current of each LED string can be balanced. Therefore, by controlling the output current of one LED string, the output currents of other LED strings can be controlled via current balancing of dc link capacitors. The effect of the ripple of dc link voltage on each output can be eliminated by fast voltage mode control loop. Therefore, flicker-free low ripple current for each LED string is achieved. With small voltage ripple across dc link capacitors, the turn on time of active switch is almost constant in a half line cycle. Therefore, power factor correction can be achieved with inductor current of pre-stage operating in discontinuous conduction mode (DCM). Finally, prototypes of dual-string LED driver with 90~135Vac input and 175~265Vac input for 47W output power are built to verify the studied results.