Principles of Powering LED Lighting - Part 2
In Part 1 of this two-part series on powering LED lighting, we looked at switching power supply basics, dimmers and system efficiency. In this concluding article, we examine Power Factors, Energy Efficiency Standards and LED fixture selection.
Power Supplies (part 2)
Power Factor Rating and Line Harmonics
We have already mentioned the Power Factor Controller (PFC) Circuit and the increasing requirements for an SMPS to utilize this circuit to pass compliance agency requirements.
So what is a PFC circuit, and what should I look for in a power supply?
A PFC circuit is typically a separate “Boost” DC-DC Converter that forces the AC-Line-supplied voltage and AC-Line current to align in phase. This circuit is placed before the DC-DC converter in an SMPS.
Equations 4 and Figure 5 show that non-resistive, “reactive” loads to the power line cause a phase angle between the voltage and current from the power line. This phase angle creates problems for the utility companies in that their meters do not read accurately, and the line itself becomes contaminated with high-RMS current waveforms that dissipate excess power.
Figure 5. Geometric Definition of Power Factor
These definitions are understood by Electrical Engineers, but the take-away is seen by examples shown here:
Click to enlarge image. Fig. 6a: A purely resistive load, such as a filament bulb, draws current in-phase with the line voltage, and, it does not distort the line current or voltage. It is the perfect load on the line. When the phase angle is zero degrees, the cosine of zero degrees is equal to 1.00 (or otherwise put, PF = 100%). Fig. 6b: For Linear Power Supplies, the power factor refers to the phase matching of the line voltage and line current. If the linear supply is drawing current off of the line, the voltage will rise with a delay relative to the current. PFC is needed to re-align. Fig. 6c: For SMPS, there will be a voltage lag plus an overall distorted current waveform due to a low “conduction phase angle”. The limits of these distortions are quantified in the EU by the IEC 62000-3-2 standard.
For SMPS with no PFC circuit, because of phase shift and distortions (quantified by Total Harmonic Distortion, or THD), the defining equation for Power Factor becomes Equation 5.
Efficiency Standards and Energy Star
There are a myriad of governing bodies with Efficiency standards that cover Power supplies, LED Light Fixtures, and just about anything else you can add power to. Driving organizations include Energy Star, the California Energy Commission (CEC), the US Department of Energy (DOE), and the European Union via European conformity (CE) regulations.
Energy Star does not qualify LED lighting products intended for use in open-plan offices, warehouses, or industrial use.1 A link to DOE additional information on "Eligible Commercial Fixtures" can be found here. The DOE seems to be leading the call for standardizing LED lighting efficacies at Level 5 and now Level 6 efficacy for external power adapters shipped after Feb. 10th, 2016.
Figure 7. Litepanels Astra power adapter complies with DOE “Level 6” Standards (as of 2016 production).
For Level VI DOE compliance, an external power adapter for a LED lighting system must exhibit an efficiency of better than 87% for an average between no-load conditions to full-load condition, with power supply sizes ranging between 49-Watt and 250-Watt output power. As an example, the Litepanels Astra supply averages > 90% average efficiency. Higher levels of efficiency can be obtained, but they are generally available only on smaller input voltage range supplies, not 90VAC to 264VAC “Universal Input” supplies.
Product Power Specification—what to look for when evaluating LED light fixtures
In order to sell a product in a country, the said product must pass a host of compliance specifications for Safety, Electromagnetic Emissions, Immunity to ESD and Power Line Events, and power line usage. A per-country listing of these requirements would be far beyond the scope of this paper. The buyer should be aware of the agency compliance regulations in their country or region.
If the product “wears” a CE badge, UL logo, or equivalent agency approval, the buyer can be satisfied that a rigorous evaluation of the product has been made and that one can typically use the product with confidence. Where this strategy may not be sufficient is in the areas of Power Factor Correction.
PFC requirements vary by region and country. They also vary depending upon the power usage of the light. For example, CE approval requirements, per EN61000-3-2:2006 Class-C (LED Lighting) standard, allow for any one piece of equipment using less than 75 Watts from the power line to be effectively excluded from needing a PFC circuit. However, if a large volume of this equipment is used in a broadcast studio where, say, 40 of these lights are used, the resultant combined power factor of all of these lights, when added up, can exceed a utility company’s limits for Power Factor and Line Harmonics. Special corrective equipment or exemption by the local utility company would be required to run that many non-PFC lights off of one service.
EN61000-3-2 also has a clause that allows for an exception to be made for any lights that are run off of a dimmer rack. CE may forgive the usage, but the utility company may object. It is advisable to work with your local utility company while planning the power scheme for your lights.
Evaluating a light for its overall Efficiency and Efficacy is important as well. Making the move to LED lighting typically buys nearly an order of magnitude of energy-efficiency relative to incandescent lighting, considering the cooling of the studio. Also consider that the type of LED light in use also plays into the overall efficacy. For instance, Fresnels operated in spot mode don’t usually have the same efficacy as a lensed panel light. The level of diffusion on a “soft-light” will reduce the overall efficacy of the light, as will the use of soft boxes.
LED lights designed only five years ago will have likely used less efficient and efficacious components than those found in current lights.
With so many suppliers of LED lighting, it is often difficult to discern reputable products that reflect best-design practices and that provide conformity to the latest industry standards for energy efficiency and full compliance to the latest governmental standards in development. Armed with knowledge of the operation of the product, however, one can confidently look past the many misunderstandings that exist about LED lights, and make a more informed choice for regarding lighting solutions.
Part 1 of this two-part series can be found here.
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