The Challenges of Choosing Off-Line LED Driver Topologies

March 13, 2012 // By Brian Johnson, Fairchild Semiconductor
When considering LED driver topologies to convert AC input voltage to a constant regulated current source for the LED loads, it is helpful to view LED applications in three general power levels: low-power applications which require less than 20W input, mid-power up to 50W; and high-power requiring higher than 50 W input. The designer faces a different mixture of challenges from cost, space, efficiency, design complexity, power factor, and reliability - to name but a few. Basic topologies within these basic power ranges are recommended to satisfy the design challenges.

The low-power solutions targeting small lamp sizes require small design volumes to fit the LED driver, consistent light radiation by controlling the current through the LED, efficiency, and low cost. In order to be compliant with Energy Star Program Requirements for Luminaires, the light fixture must have a power factor ≥ 0.7 for residential applications and ≥ 0.9 for commercial applications for input power greater than 5 watts. [1] If isolation from the LED driver is not required, a buck regulator topology is the lowest BOM and therefore a low cost solution to consider. Figure 1 is a non-isolated buck topology example which also includes power factor correction and dimming capability. There is only one magnetic component which is a simple inductor and a single MOSFET/Diode pair for buck power conversion. This topology choice is best selected when the input voltage is larger than the output voltage required from the LED load.

Figure 1 Non-isolated Buck Converter with PFC

When isolation from the LED driver is required, a good topology choice is the primary-side regulated (PSR) flyback topology; Figure 2 is an example of a PSR flyback LED driver. Reduced cost is achieved by not requiring secondary side feedback so the topology has a smaller component count to achieving good constant current regulation. The MOSFET can be integrated into the controller to reduce BOM count and reduce the printed wiring board space. The reliability of the PSR flyback is improved without the use of an opto-isolater for secondary feedback.

Figure 2 Primary-Side Regulated Converter

Discontinuous Conduction Mode (DCM) is the preferred operating mode for the PSR flyback topology since it allows for better output regulation. The typical waveforms are shown in Figure 3.

Figure 3 Waveforms of DCM Flyback Converter

When operating a PSR LED driver in constant voltage regulation mode, during the inductor current discharge time TDIS, the sum of the output voltage and diode forward-voltage drop is reflected

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