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How to drive LEDs with power factor correction in a single stage

Wed, 07/16/2014 - 1:47pm
Brian King, Texas Instruments, www.ti.com

PFC-SEPIC LED Drivers - wow, that’s a lot of acronyms!

PFC stands for “power factor correction”

SEPIC stands for “single-ended primary inductor converter;”

And, of course, the LED stands for “light-emitting diode.”

Combining these three features into one converter can provide an efficient, single-stage solution for LED lighting with good power factor. Figure one shows the schematic for a PFC-SEPIC LED driver.

A PFC-SEPIC converter can be controlled using a traditional PFC-boost controller
PFC-SEPIC converter LED driver schematic
An important characteristic of the SEPIC is the lack of isolation.  This limits the use of this topology to applications that don’t require safety isolation.  Think about applications that are completely sealed with no user-accessible energized parts, like street lights and high-bay lighting.

Most PFC converters use a boost topology, which requires the output voltage to be higher than the peak line voltage.  The SEPIC topology allows the output voltage to be greater than or less than the input voltage.  This allows the controller to directly regulate the current in the LED string without needing a second power stage.

Any PFC-boost controller can be used to drive a PFC-SEPIC power stage.  This is because the power switch is located on the low-side, just like in the boost topology.

The PFC-boost controller provides good power factor by forcing the peak current in the switch to follow the input voltage sine wave. Unlike the boost topology, in the coupled-inductor SEPIC, when the switch is off, no input current is flowing.  As a result, the power factor is not quite as high as in a boost topology.

Yet, the power factor can easily be kept above 0.9, which is sufficient for most lighting applications.

Figure 2 shows an example of the line voltage and current, and the harmonic content of the current, which is kept below the requirements of EN61000-3-2 Class C.

Line voltage and current

 

 

 

 

 

 

 

 Very high efficiencies can be obtained using this topology.  This is due to a few things:

  1. Using a single-stage for PFC, power conversion, and regulation eliminates the penalty associated with double or triple conversion
  2. The clamping capacitor of the SEPIC recaptures leakage energy in the coupled-inductor, easily saving 2% or more efficiency compared to a flyback.


If this conversation has piqued your interest in the PFC-SEPIC topology, here are some PFC-SEPIC LED driver designs from the TI Designs PowerLab library that you can use to quick-start your project:

    PMP3976 – 150VAC-264VAC Input, 100W SEPIC-PFC LED
    PMP5112 – 95VAC-305VAC Input, 50W SEPIC-PFC LED Driver
    PMP5242 – PFC SEPIC 208V@0.110A Non-isolated LED String


This post originally published on TI’s Power House blog.

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