# 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.

So what is a PFC circuit, and what should I look for in a power supply?

Equation 4.

Figure 5. Geometric Definition of Power Factor

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.

Figure 7. Litepanels Astra power adapter complies with DOE “Level 6” Standards (as of 2016 production).

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