Using a single-stage approach to power factor corrected LEDs

Power factor correction, commonly referred to as PFC by engineers, the SEPIC in PFC-SEPIC LED drivers stands for "single-ended primary inductor converter", and of course LED stands for "light- emitting diode ". So how do you combine these three features in one converter to provide a good power factor for LED lighting? Let's listen to the editor's explanation.

Figure 1 is a schematic diagram of a PFC-SEPIC LED driver.

Figure 1: The PFC-SEPIC converter can be controlled using a conventional PFC boost controller

An important characteristic of the SEPIC is the lack of isolation. This limits the use of the structure to applications that do not require safety isolation. Think of completely sealed applications where the user cannot access the powered parts, such as street lights and high-bay lights. 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 higher or lower than the input voltage. This allows the controller to directly regulate the current in the LED string without the need for a secondary power stage.

Any PFC boost controller can be used to drive the PFC-SEPIC power stage because the power switch is on the low side, just like in the boost topology. The PFC boost controller forces the peak current in the switch to follow the input voltage sine wave, thus providing a good power factor. Unlike the boost topology, in the coupled inductor SEPIC, no input current flows when the switch is off, so the power factor is not as high as in the boost topology. However, the power factor is easily maintained above 0.9, which is more than enough for most lighting applications.

Figure 2 is an example of line voltage and current and current harmonic content, where the harmonic content remains below the EN61000-3-2 Class C requirement.

Figure 2 This topology not only provides good power factor, but also complies with EN61000-3-2 Class C standards.

Very high efficiency can be achieved using this topology because:

(1) Using a single stage to implement PFC, power conversion and voltage regulation can eliminate the disadvantages of double or triple conversion;

(2) The clamping capacitor of SEPIC can recover the leakage energy in the coupled inductor, which can easily save 2% energy or improve efficiency compared with the flyback topology.