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Improve phase-cut dimming performance in LED lights

30-12-2013

One of the remaining barriers to widespread adoption of LED lights is the historically poor performance when used with the installed base of phase-cut dimmers. The phase-cut dimmer found in more than 150 million residential installations alone in North America, according to NEMA (National Electrical Manufacturers Association), was designed for use with an incandescent light source. The inherent thermal inertia of the incandescent bulb masks many of the undesirable features of these dimmers. Conversely, LED lights, and in particular their electronic power supply or driver circuitry, have struggled to deal with the variations and unstable output of the phase-cut dimmers in the market. The resulting wide variation from unit to unit — and from dimmer to dimmer — is not acceptable to end users, although adaptive driver designs can improve the dimming performance.

 

The phase-cut dimmer was designed as a simple, efficient, and inexpensive method to dim incandescent light sources. It operates by limiting the power delivered to the load, only conducting a certain percentage of the AC power source each half-cycle. Varying the dimmer position varies the conduction period and hence the power delivered to the load, resulting in a change in light output.

 

FIG. 1. Typical waveforms produced by (a) leading-edge and (b) trailing-edge phase-cut dimmers.
FIG. 1.

Phase-cut dimmers

Two different types of phase-cut dimmer exist (Fig. 1). Leading-edge dimmers delay the firing angle after the zero crossing, cutting out the initial portion of the AC half-cycle and conducting the end portion. Trailing-edge dimmers operate in the opposite manner, conducting the initial portion of the AC half-cycle and cutting out the end portion. Leading-edge dimmers can be built with only a single active component — the triac — making them very inexpensive, thus their dominance in the North American market.

 

One of the parameters of the phase-cut dimmer that contributes to the variation in LED lights performance is the wide range of minimum and maximum firing angles the dimmers produce. The angles vary significantly from factory to factory and from model to model. Consequently, the range of conduction periods and power delivered to the load varies. Indeed, in surveying 64 dimmers on the market from various vendors, we found the minimum firing angle ranged from 17° to 72° and the maximum firing angle ranged from 104° to 179° (Fig. 2) — a significant span on each end of the spectrum.

FIG. 2. Range of firing angles produced by 64 phase-cut dimmers from several manufacturers.
FIG. 2.

The table shows the specific minimum and maximum firing angles produced for phase-cut dimmers from two commonly installed manufacturers. For all dimmable LED power supplies, the conduction period of the dimmer directly relates to the LED current and therefore to the amount of light produced. If the LED driver has a fixed dimming curve, as all drivers on the market today currently do, the driver will deliver different performance with different dimmers. Further, any nonlinearity in the dimming curve can exacerbate the difference in performance from dimmer to dimmer.

 

TABLE. Dimmer characteristics
TABLE.

Firing angle variation

When designing a phase-cut dimmable power supply, manufacturers have to decide at what firing angles their power supply will output minimum and maximum LED current. If the power supply has a fixed dimming curve, this forces them to make one or more compromises in terms of performance with certain dimmers.

 

Consider the case of a power supply that has a minimum dimming level of 1% output current at a firing angle of 30°, a maximum dimming level of 100% at a firing angle of 158°, and a dimming curve as shown in Fig. 3. If this power supply were operated with the dimmers in the table, it would match perfectly with the Leviton dimmer, reaching its minimum dimming level when the dimmer was in the minimum physical position and maximum dimming level when the dimmer was in the maximum physical position. However, if it were operated with the Lutron dimmer, it would not reach its maximum dimming level of 100%. It would only reach a dimming level of 49% at most because the Lutron dimmer cannot produce a firing angle as wide as 158°. Furthermore, it would also not reach its minimum dimming level of 1% because the Lutron dimmer cannot produce a firing angle as narrow as 30°. In this case, it would only reach 1.7% output.

 

FIG. 3. A fixed dimming curve will not match the firing angles of all commercial dimmers.
FIG. 3.

The alternative solution is to set the minimum and maximum dimming levels to match the Lutron dimmer at 45° and 138°, respectively, as shown in Fig. 4. In this scenario, the driver would function perfectly with the Lutron dimmer, but there would be issues if it were operated with the Leviton dimmer. Using these limits for the dimming curve, the driver would reach its desired minimum dimming level of 1% and maximum of 100%; however, there would be 12% dead travel at the bottom of the dimmer and 16% dead travel at the top of the dimmer where the dimmer could be moved but the dimming level would not change.

 

Industry guidelines, such as developed in the Lighting Research Center (LRC) ASSIST (Alliance for Solid-State Illumination Systems and Technologies) program, recommend that dead travel be kept below 10% anywhere in the dimming range, with further LRC research showing that people who notice dead travel describe it as somewhat to very distracting. The example given previously would fail to meet the recommendation at both the top and bottom ends of the dimmer travel, and the problem can be even greater with different dimmers. Given the range of phase angles produced by dimmers on the market, it makes it impossible to support all of them with a fixed dimming curve and avoid both variation in output levels and dead travel at the dimmer.

 

FIG. 4. One unacceptable result of a dimmer and driver mismatch is dead travel in the dimmer.
FIG. 4.

Adaptive dimming solution

To eliminate this variation in behavior with different dimmers, a driver must dynamically adjust the dimming curve to match the specific characteristics of the dimmer being used. An intelligent driver can employ a software-based learning algorithm to adapt to the phase-angle variation.

 

To support all the dimmers on the market, the intelligent driver must start with a default dimming curve that can be adjusted depending on the values observed. The default dimming curve should have a minimum and maximum phase angle limit that falls within the worst-case limits of all the dimmers available on the market. Setting the default limits so the curve reaches maximum dimming output at 95° and minimum dimming output at 75° would meet this requirement (Fig. 5a). Then as the driver is operated with a phase-cut dimmer, the learning algorithm can monitor the phase angles produced by the dimmer and adjust the limits and curve if values are detected that exceed the current limits.

 

In the case of the Lutron dimmer defined in the table, when the dimmer is moved into its maximum position it produces wider phase angles up to 138° . The drivers learning algorithm detects that the angle is greater than its previously stored maximum limit, and updates both the limit and curve to match. When the dimmer is moved into its minimum position it produces narrower phase angles down to 45° , and the drivers learning algorithm detects that the angle is lower than the drivers previously stored minimum limit and updates both the stored limit and curve to match.

FIG. 5. Adaptive drivers must start with a default dimming curve (a). The adaptive driver shifts it dimming curve to match the Lutron dimmer (b); the curve can also be adapted to match the Leviton dimmer (c).
FIG. 5.

Fig. 5b shows the curve and limits once the algorithm has adapted to the Lutron dimmer, and clearly shows that the driver reaches 100% dimming output at the dimmer maximum without any dead travel, and conversely reaches 1% dimming output at the dimmer minimum, again without any dead travel. In this case, the driver has perfectly matched the Lutron dimmer with which it is being operated.

 

Continuous adaptation

There are two occasions when a driver might ideally adapt to manage different phase-angle limits — at installation time and when a dimmer is replaced. To simplify the installation process and not force a complicated or time-consuming learning routine on the user, the drivers adaptive dimming algorithm can be configured to be always active. By continuously monitoring the incoming phase angles, it can determine if it needs to update its limits and dimming curve. Any time it detects a difference that requires either the maximum or the minimum limit to change, it stores the new value in nonvolatile memory and recalculates the dimming curve. As a result, the end user can operate the dimmer as they see fit and the driver will adapt seamlessly to the phase-angle inputs it receives.

 

If the Lutron dimmer is replaced with the Leviton dimmer, the learning process continues. When the Leviton dimmer is moved into its maximum and minimum positions, it creates phase angles 158° and 30°, respectively. The drivers adaptive dimming algorithm detects the new limits and adjusts its values and curve to match. Fig. 5c shows the updated limits and curve after operation with the Leviton dimmer. The curve has again been perfectly matched to the dimmer characteristics without any dead travel, while maintaining the same minimum and maximum dimming levels.

 

Light-Based Technologies uses proprietary software in its line of Ultra Compatible LED drivers to implement the adaptive dimming algorithm described here. These drivers are guaranteed to eliminate any variation in behavior between dimmers, consistently delivering the dimming performance the user expects.

 
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