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Building better, more efficient lighting systems

Posted: 18 Apr 2016     Print Version  Bookmark and Share

Keywords:Power Factor Correction  PFC  inductive load  Negative Temperature Coefficient  NTC 

There are various ways to build a lighting system, and good design leads directly to better power efficiency and a lower bill of materials cost. Currently, the lighting industry is transitioning from 240 V to 277 V for greater efficiency. This makes it an excellent time to introduce Power Factor Correction (PFC) to existing product lines. As these systems need to be updated anyway, OEMs can take advantage of the many benefits of PFC at the same time.

The need for PFC arises from the move to inductive loads. Traditional lighting applications used resistive loads such as incandescent lights. However, resistive loads have the disadvantage that the resistance they introduce to the system generates heat. This heat represents power losses and decreases efficiency. To eliminate these losses, the lighting industry continues to move to inductive loads such as florescent lights that offer greater efficiency. Figure 1 shows a lighting system based on an inductive load.

Figure 1: A parallel capacitor is added across the inductive load.

Power Factor Correction
The unfortunate truth is that many OEMs implement inductive loads in a manner that severely curtails their efficiency. In many cases, they are simply unaware that these issues can be easily and inexpensively resolved with Power Factor Correction.

By its nature, an inductive load shifts the voltage and current out of phase to each other. Specifically, the inductive reactance introduced is out of phase with the system's resistance. This phase difference reduces the efficiency of the system.

The Power Factor (PF) is the ratio of the real power of the system to its apparent power, where apparent power is what you expect from the system and real power is what you actually get. Depending upon the application, an out of phase system can drop to as low as 60% efficiency.

The goal of Power Factor Correction is to minimise the phase difference between voltage and current. Capacitive reactance can be used to bring the inductive reactance back into phase with the system's resistance. A capacitor with the right characteristics (i.e., a high enough power rating and 180° out of phase with the inductive reactance) is all that is required (figure 1). For a more detailed explanation of the mathematics behind PFC, click here.

There are numerous benefits gained from implementing PFC in lighting systems:

Increased efficiency: Adding PFC to a system can increase efficiency up to 80 to 95%, again depending upon the application. With rising utility costs, this makes PFC-based lighting systems highly attractive to end customers.

Simple to implement: PFC can be introduced to a lighting system with a single capacitor. Note that an in-rush current limiter is also required to prevent the initial draw of the capacitor during power up from damaging the system (see below, inrush current).

Lower power supply cost: A system with a high power factor can perform the same work as a system with a low power factor using a smaller power supply. The need to carry less current means smaller and less expensive generators, conductors, transformers, and switches, in turn resulting in more compact form factors and bill of material cost savings.

Higher reliability: More efficient systems have to dissipate less heat, making it easier to keep systems operating reliably within an acceptable temperature range.

Differentiating feature: Whether your design is a stand-alone product or integrated as part of a larger system, higher power efficiency will always drive a higher premium than an otherwise comparable system with lower efficiency.

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