Metric for evaluating dimmable LEDs' performance
Recent developments in SSL technology have resulted in a large number of LED driver ICs for retrofit lamps that offer phase-cut dimmer compatibility. The manufacturers of these devices claim "dimmer compatibility" without mentioning constraints. However, when observed in more detail, one will invariably notice limits on the applicable range of these solutions – as well as a wide range of what can be defined as "compatible."
The absorbed mains input power is the most important factor that dictates the level of compatibility. Other factors are power factor, THD, electronics dissipation, dimming range and electronics dissipation under dimmed conditions. Because of steadily increasing LED efficacy (lumen/watt), the input power of retrofit LED lamps intended to replace incandescent 40- or 60-watt bulbs, is going down. As an example, a 40 watt frosted incandescent lamp produces about 440 lumen, and a 60-watt clear incandescent lamp about makes 750 lumen. Today, commercially available LED lamps need about 7 W to replace the 40-watt lamp and 8.7 W to replace the 60-watt lamp (corresponding to 63 respectively 86 lumen/watt), and this input power keeps on decreasing.
Now imagine one would create a LED lamp with 60-watt input power and a power factor of 1 and a cosines ? of 1. Perfect compatibility with all dimmers requiring a minimum load of 60 watt or less is almost certain, since the load now is equal to the original load for which the dimmer was designed. In fact, the threshold at which things become difficult is below 10 W input power. At this moment, solutions that offer acceptable compatibility under 4.4 W input power are not offered, though with the existing trend in lower lamp powers, this will be the application hot spot in near future.
"Compatibility per watt" is a more meaningful parameter to express the performance of dimmable IC solutions than compatibility alone, and comparison of solutions can only be valid if the same lamp power level and other characteristics are present. Unfortunately, this is never the case. Further confusion can be created by the term compatibility. Some manufacturers are considering stable light output (no flicker, shimmer or other visible variation in light output) as the only relevant parameter to determine compatibility, and sacrifice other parameters like dimming range and continuous dimming control to achieve this goal. However, the end user who replaces an incandescent bulb not only looks at stability but also expects a performance on other parameters to be equal to the original light source. Looking at incandescent lamps, the typical dimming range in light output is about 1:1000, or 0.1 per cent.
NXP determines lamp performance from end user perspective on a combination of these criteria, divided into two aspects:
Stability: The level to which the lamp is able to avoid visual disturbances like flicker, shimmer and flashing
Controllability: The level to which the user can control and reduce the lamp light output. This not only includes dimming range, but also the ability of the lamp to deliver full light output when a dimmer is inserted, and the range over which the lamp is sensitive to change in dimmer position ( control span ). Also, steps in light output, dead bands, flash at start-up, settling time and hysteresis at turn on and turn off are taken into account.
Weighing factors are arbitrarily given (2:1) for above two aspects, from which a total absolute performance score can be derived between 0 and 333. Last rating applies to the 60-watt incandescent lamp. Though this score may be meaningful, it is even more informative to include lamp power. No products are offered yet that provide 1 to 1 compatibility at lamp powers below 4.4 W power. A formula where the lamp absolute performance is divided by lamp input power minus hypothetical minimum lamp power (set at 2 W) will give an indication to what stage the driver and IC is capable to utilise the available lamp power to create dimmer compatibility. This quotient we named power merit factor.
From this score, we can compare our own and competitors progress. Figure 1 plots the results when the dimmer compatibility score is set against input lamp power.
Figure 1: Lamp power versus lamp dimmer compatibility.
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