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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 

Lower operating cost: For large-scale lighting applications, the higher efficiency enabled through PFC can translate to substantial savings on utility bills.

Industry momentum: Power factor correction has been mandatory in Europe, China, and Japan for over a decade. Despite its slow adoption in the United States, PFC continues to be mandated in an increasing number of applications, most notably lighting systems.

Clearly, PFC makes sense and will eventually be mandated for applications that today do not require PFC. Companies that anticipate PFC as a future requirement will be able to take advantage of PFC as a differentiating feature today. Manufacturers that fail to offer PFC will quickly find themselves unable to compete.

Inrush current
As the PFC capacitor initially charges, it pulls as much current as the system will allow. This brief, inrush current can be significantly higher than the system's operating current and, depending upon the lighting application, can damage other circuitry in the system. To prevent this damage requires a circuit to limit the in-rush current.

The core of an in-rush limiting circuit is high resistance. Placing a resistor in circuit limits how much current the capacitor can draw. However, once the capacitor is charged, if the resistor is left in the circuit, it will continue to cause heat losses and reduce overall efficiency. Typically, a switch is used to bypass the resistor once the in-rush current has been limited.

The most efficient way to handle inrush current is with a thermistor. A thermistor is a special type of variable resistor whose resistance is dependent upon temperature. A Negative Temperature Coefficient (NTC) thermistor, for example, exhibits a large, predictable decrease in resistance as its temperature rises.

To limit in-rush current, the NTC thermistor is placed between the power supply and both the PFC capacitor and inductive load (figure 2). Upon power up, the NTC thermistor has a low temperature and so provides high resistance. In addition to limiting the current to the capacitor, this high resistance generates heat that increases the temperature of the thermistor.

Figure 2: An NTC thermistor is added to limit the inrush current.

As the NTC self-heats, its resistance drops rapidly. By the time the in-rush current has leveled off, the NTC thermistor is warm enough to drop to a minimal resistance and pass current through without negatively impacting system operation or efficiency. In this way, an NTC thermistor effectively provides the resistance needed to limit in-rush current while eliminating the need for additional circuitry such as a bypass switch.

NTC thermistors are robust components with an effective operating range of -50 to 250°C. As an industry leader, Ametherm has been aware of the transition to 277 V and developed thermistors rated for use at this higher voltage for lighting applications. Ametherm also offers the industry's largest selection and highest rated UL and CSA certified thermistors, making it possible to choose optimal components that minimise power efficiency losses due to resistive heat.

NTC thermistors appropriate for lighting applications range in price from $0.15 to $0.90. Compare this to the $0.50 to >$1 price of a power resistor rated high enough to handle the large currents of light ballasts. The cost of the power resistor also needs to take into account the circuit needed to bypass the resistor after the in-rush current has been limited.

Power Factor Correction is incredibly simple and inexpensive to implement. For the boost in efficiency gained, PFC is a compelling addition to many inductive lighting applications, even those not mandated to have PFC. And with Negative Temperature Coefficient thermistors, OEMs can protect systems from PFC-related in-rush current without the need for a complicated and costlier bypass circuit.

About the author
Mehdi Samii is with Ametherm Inc.


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