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EETE MAY 2013

POWER COMPONENTS Power factor and solid state lighting – implications, complications and resolutions By Hubie Notohamiprodjo Ligh ting comprises approxima tel y 17.5% of global electricity consumption. As the world transitions from incandescent to solid state lighting (SSL) technology, utilities and government regulatory agencies worldwide are concerned that, as this large segment of the consumption base switches to SSL, it will increase infrastructure costs. This is due to the reactive nature of LED-based solid state lighting, which results in higher distribution currents that adversely affect power factor (PF ) and, in turn create a larger demand on the power grid. Regulators have been working with utilities companies to enact rigid standards to control the impact of SSL technology on the power grid – see table 1. The move to LED-based solid state lighting promises a significant reduction in the carbon footprint of the electrical power grid simply due to the dramatic reduction in real power consumption. However, if power factor is not managed, the grid will still need to be able to provide a much higher power level than is actually needed at the load, eliminating a significant portion of the benefits of moving to solid state lighting. Historically, incandescent bulbs have had near-perfect power factor. Therefore, solid state lighting is being held to a much higher PF standard compared to legacy AC/DC power supplies. In most cases, power supplies are free from any form of power factor regulation for supplies rated up to 75W. However, for solid state lighting, PF regulations typically kick in as low as 5W or below. In order to effectively design an LED-based luminaire, designers need to understand power factor, the impact LED drivers have on it, and different techniques for integrating power factor correction cost-effectively in the LED driver design. Understanding power factor Power factor is a simple, unit-less ratio of real power to apparent power. Real power is the power used at the load measured in kilowatts (kW). Apparent power is a measurement of power in volt-amps (VA) that the grid supplies to a system load. In a highly reactive system, the current and voltage, both angular quantities, can be highly out of phase with each other. This results in the power grid needing to supply a much larger reactive power to be able to supply the actual real power at any given time – see figure 1. Fig. 1: The power factor is the ratio of real power (kW) to reactive power (kVA). The ratio of the reactive power to the real power is called power factor (PF ). This basically means that for an equivalent real power consumed by a highly reactive load, for example 5W, the actual current that the grid needs to supply to the load in order to provide the real power has to be higher than the real power by the power factor ratio. For the previous 5W example, for a load with a PF of 0.5, the grid needs to provide 2x the current actually required by the load at any given time. This adverse impact on the power grid does not apply to incandescent lighting, which is purely resistive and has a unity power factor. Solid state lighting that incorporates power factor correction can reduce the impact of the change from incandescent to LED based lighting by increasing the power factor to near unity by adding circuitry to the LED driver that corrects for the reactive input impedance. LED drivers and power factor correction LEDs have a non-linear impedance as do their drivers, causing the power factor to be inherently low. In order to combat this, the driver needs to compensate for power factor to increase that ratio as close to 1 as possible. When you take into consideration one LED lamp and its impact on the overall PF for an industrial warehouse or shopping mall, it is relatively insignificant, but the sum of all the lighting elements in a large commercial space can significantly impact the overall power factor and correction for either each individual bulb or for the ballasts that drive those bulbs needs to be implemented. Both active and passive methods exist for PF C. Passive PF C solutions typically consist of passive input filers and offer some cost benefits, but since passive PF C optimizes for a specific input voltage and current condition, when those conditions change, the power factor also decreases. In the case of dimmable luminaires, passive PF C is not acceptable as the power factor will vary broadly across the full operating brightness range of the bulb. Active PF C needs to be incorporated to adequately maintain high PF C across load and line conditions. With active PF C, there are approaches that use the main power conversion stage to compensate for the power factor (single-stage – figure 2a) and approaches that use an independent pre-regulator stage to provide the PF C (two-stage – figure 2b). Each approach has its benefits; the most obvious is that with a single-stage approach, the cost to implement is minimized since part of the PF C work is done in the main power conversion stage. Determining which topology best suits the end applications requires a deeper analysis of each type of converter. With a single-stage LED driver, the main power stage converts the input voltage to a usable DC voltage and current for driving the LEDs. Since there is only one power stage, the driving of the main power stage needs to be managed to increase the PF close to unity. Since the measure of power factor depends upon how linear the input of the driver looks to the mains input voltage, the modulation topology determines what the input impedance of the converter looks like to the mains. The best approach to maximize power factor is to use a constant-on time approach, which effectively creates a voltage- Hubie Notohamiprodjo is the director of marketing for Solid State Lighting Products at iWatt - www.iwatt.com 20 Electronic Engineering Times Europe May 2013 www.electronics-eetimes.com


EETE MAY 2013
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