Lighting-level driver design technology foundation (II)

3. Power Factor Correction

The U.S. Department of Energy (DOE) "ENERGYSTAR" solid-state lighting (SSL) specification stipulates that any power level must provide power factor correction (PFC). This standard applies to a series of specific products, such as recessed lights, cabinet lights and table lamps, among which the power factor of LED drivers for residential applications must be greater than 0.7, and greater than 0.9 in commercial applications; however, this standard is a voluntary standard. The EU's IEC61000-3-2 harmonic content standard stipulates the total harmonic distortion performance of lighting applications with a power greater than 25 W, and its maximum limit is equivalent to total harmonic distortion (THD) < 35%, and power factor (PF)> 0.94.

Although not all countries have an absolute mandate to improve power factor in lighting applications, some applications may have this requirement, such as utilities strongly promote the commercial application of products with high power factor in public facilities. In addition, when utilities purchase/maintain street lights, they can also decide whether to require high power factor (usually >0.95+) based on their wishes.

Figure 6. Schematic diagram of active PFC application circuit

PFC technology includes passive PFC and active PFC. Passive PFC solutions are larger in size and require additional components to better change the current waveform, which can achieve a power factor of about 0.8 or higher. Among them, in lower power applications of less than 5 W to 40 W, the flyback topology, which is almost the standard choice, only needs to use passive components and make slight circuit changes to achieve a power factor higher than 0.7.

Active PFC (see Figure 6) is usually added to the circuit as a dedicated power conversion stage to change the input current waveform. Active PFC usually provides boost, and the PFC output voltage range is up to 450 to 480 Vdc under a wide input range of 100 to 277 Vac. If the PFC stage is properly designed, it can provide high energy efficiency of 91% to 95%. However, with the addition of active PFC, a dedicated DC-DC conversion is still required to provide current regulation.

4. Energy efficiency issues

The energy efficiency of LED lighting applications needs to be considered in conjunction with power output. The US Energy Star solid-state lighting specification specifies the energy efficiency of lighting fixtures, but does not involve the energy efficiency requirements of individual LED drivers. As mentioned earlier, LED applications using AC-DC power supplies can use a two-stage distribution topology, so external AC-DC adapters may be used for power supply.

Energy Star does include specifications for single-output external power supplies. Version 2.0 of the external power supply specification came into effect in November 2008, requiring a minimum efficiency of 87% in standard operating mode and 86% in low-voltage operating mode. In this specification, PFC is only required when the power is greater than 100 W.

Figure 7: LED lighting energy efficiency research and development goals proposed by the U.S. Department of Energy in the fall of 2008

In LED applications using AC-DC power supplies, providing higher AC-DC conversion efficiency involves a trade-off between cost, size, performance specifications, and efficiency. For example, using higher quality components and lower on-resistance (RDSon) can reduce losses and improve efficiency; lowering the switching frequency generally improves efficiency, but increases system size. New topologies such as resonance provide higher efficiency, but also increase the complexity of design and components. If we limit the design to a narrow power and voltage range, it can help optimize efficiency.

5. Driver Standards

LED drivers themselves are also evolving, focusing on further improving energy efficiency, adding functionality and power density. The US Energy Star solid-state lighting specification proposes energy efficiency limits at the lighting fixture level, involving specific product requirements including power factor. The EU IEC 61347-2-13 (5/2006) standard has the following requirements for LED modules powered by DC or AC:

Maximum safety extra-low voltage (SELV) operating output voltage ≤ 25 Vrms (35.3? Vdc)

"Proper"/safe operation under different fault conditions?

No smoke or flammability when malfunctioning?

In addition, the ANSI C82.xxx LED driver specification is still under development. In terms of safety, it is necessary to comply with UL, CSA and other standards, such as UL1310 (Class 2), UL 60950, and UL1012. In addition, LED lighting design also involves product life cycle and reliability issues.