MEAN WELL developed many power supplies series specifically for LED application. Single stage PFC was used in such developments due to low cost. This topology has the following restrictions:
- This topology does not use input bulk capacitor. For this reason, in areas with low AC quality, output voltage and current may become unstable causing variation in LED brightness. If the input AC voltage is stable, then this problem will not occur.
- Output ripple
This is also caused by lack of input bulk capacitor. As compared to power supplies using two-stage PFC, the ripple will be significantly larger (see Figure 4). There could be instances where the low end of the ripple may be too low for the driver IC to operate properly, and the LEDs will start to flicker. To solve this type of problem, the output voltage can be adjusted higher, so the low end is higher than the driver’s minimum working voltage. Or simply select a PSU with higher rated voltage.
- Current harmonics
Single stage PFC power supplies are optimized for constant current drive. Using these supplies as constant voltage sources (such as application including cascading a constant current driver IC), the harmonics might be worsening in this case. When operating in areas with unstable utility voltage or with driver IC, we highly recommend using general application types as found in table 1. Avoid using single stage PFC if possible and use a two-stage PFC power supply instead, or contact MEANWELL for more information.