This reference document describes a built-and-tested, GreenPoint solution for an isolated 8 W constant current LED driver which is intended to support the residential power factor requirements of the DoE ENERGY STAR Standard for Solid State Lighting Luminaires (Version 1.1− 12/19/08). Some of the typical products in this category include portable desk lamps, under-cabinet lights, and outdoor porch lights.
One of the most common power supply topologies for low power offline LED drivers is an isolated flyback topology. Unfortunately standard design techniques used for these supplies typically result in a power factor in the range of 0.5−0.6. This design note describes why the power factor islow and discusses techniques to improve the power factor. Finally it illustrates how an existing design was modified to substantially improve the power factor and easily comply with the residential power factor requirements
The bulk capacitor fills in the missing power providing a more constant input to the switching regulator maintaining power flow to the load.This configuration comes at the expense of poor utilization or power factor of the input line waveform. Line current is drawn in high amplitude narrow pulses near the peaks of the voltage waveform introducing disruptive highfrequency harmonics. Passive solutions are well documented but typically introduce many additional components. One approach is the valley-fill type rectifier where a collection of electrolytic capacitors and diodesincreases the line frequency conduction angle resulting in improved power factor. In effect, this process charges the series-connected capacitors from the high line voltage at low current and discharges them to the switching regulator at a lower voltage with higher current. A typical application usestwo capacitors and three diodes or, for enhanced power factor performance, three capacitors and six diodes.