1. Dimmer compatibility per Watt of
LED drivers: What to look for?
Victor Zwanenberg, Sr. Application Engineer, NXP
1
www.nxp.com/GreenChip
Victor Zwanenberg, Sr. Application Engineer, NXP,
explains which aspects of phase cut dimmable LED
lamp electronics should be considered to evaluate
the performance in terms of dimmer compatibility.
Not only stability, but also controllability is
introduced. The trade-off between electronics
dissipation and dimming curve is emphasized.
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. 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. For
example, a 40 Watt frosted incandescent lamp produces about 440 Lumen, and a 60
Watt clear incandescent lamp makes about 750 Lumen. Today, commercially available
LED lamps need about 7 Watt to replace the 40 Watt lamp and 8.7 Watt 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 0 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 smarter way to energy efficiency

2. 2
www.nxp.com/GreenChip
Dimmer compatibility per watt of LED drivers:
What to look for?
The smarter way to energy efficiency
the threshold at which things become difficult is below 10 Watt input power. At this
moment, solutions that offer acceptable compatibility under 4.4 Watt input power
are not offered, though with the existing trend in lower lamp powers this will be the
application hot spot in the 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%.
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). Steps in light output, dead bands, flash at startup, settling
time and hysteresis at turn on and turn off are also taken into account.
Weighing factors are arbitrarily given (2:1) for the above two aspects, from which a total
absolute performance score can be derived between 0 and 333. The 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 Watts power. A formula where the lamp

3. www.nxp.com/GreenChip
Dimmer compatibility per watt of LED drivers:
What to look for?
The smarter way to energy efficiency
absolute performance is divided by lamp input power minus hypothetical minimum
lamp power (set at 2 Watt) will give an indication to what stage the driver and IC
is capable to utilize 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.
3
Figure 1:
Lamp power versus lamp dimmer compatibility
2 * Stability + Controllability
Plamp
– Pmin
Power Merit =

4. www.nxp.com/GreenChip
Dimmer compatibility per watt of LED drivers:
What to look for?
The smarter way to energy efficiency
4
Main conclusions from this graph are that the products currently sold by NXP can be
more than competitive, and a substantial improvement in achieving compatibility has
been achieved using the same ICs since first release in 2008. Dimmer compatibility only
partially depends on the IC features, but mostly on the way the IC has been designed
in. Application knowledge is of crucial importance in designing a mains dimmable
LED driver.
Parameters not taken into account to this point are internal electronics dissipation,
peak dissipation during dimming and power factor. Though it is possible to create a
higher level of dimmer compatibility using a design with extreme low power factor
and high mains harmonics at dimmed position, the current trend is not to exploit this
loophole because it does not comply with future and current quality standards, and is
misleading in terms of energy consumption. It is recommended that the reactive power
of a dimmable LED lamp remains below two times the undimmed real power over the
complete dimming range. For internal electronics dissipation, most customers specify
only the driver efficiency without use of a dimmer. There are, however, more parameters
to consider:
• Total peak dissipation of the lamp – This determines the thermal
design of the lamp, and thus the size of the heat sink and lamp size.
Total peak dissipation occurs mostly with leading edge dimmers at large
opening angles. Additional losses required to maintain a stable mains
line signal add to the total lamp dissipation, and depending on the
dimming curve, the lamp power can exceed nominal power.
• Peak dissipation of the internal electronics – Electronics construction
and components must be designed such that peak dissipation can
be handled. Though provisions can be made to distribute additional
dissipation during dimming over a larger number of components, all
dimmable LED lamps have additional circuitry to provide additional load
to keep dimmers operational and stable.

5. www.nxp.com/GreenChip
Dimmer compatibility per watt of LED drivers:
What to look for?
The smarter way to energy efficiency
5
• Lamp power at lowest dimming angle – In order to achieve energy
savings, the user expects the lamp to draw substantial less power when
dimmed. However, this power cannot be zero, because the lamp still has
to provide sufficient load to keep the dimmer operational and stable. A
lamp power less than 1 Watt is recommended.
Figure 2 shows peak electronics dissipation powers between 4.5 and 1.5 Watts. The
lamp output power can be utilized as complementary load to the electronics in the
dimmer. As such, a trade-off exists between dimming curve, dimming range and
internal electronics dissipation. Some manufacturers utilize this in an active way: The IC
senses if unstable operation is reached at a certain dimming angle, and provided the
electronics dissipation bandwidth is fully used, the lamp output power, and thus light
Figure 2:
Electronics dissipation versus dimming angle of a number of dimmable 230V led lamps.

6. www.nxp.com/GreenChip
Dimmer compatibility per watt of LED drivers:
What to look for?
The smarter way to energy efficiency
6
amount, is increased again until stability is reached. Future quality standards already
anticipate on this method, defining minimum dimming level below 50% as compliant.
From end user perspective, the lamp operates stable, but the dimming range is
dissatisfying. Also, the dimming curve can be shifted towards small opening angles.
This way, electronics dissipation at large opening angles can be reduced, but
a dead band will occur when controlling the dimmer at large opening angles. It is
evident that these aspects and trade-offs become more critical at lower lamp powers.
At higher wattage lamps, one could utilize the headroom to improve dimming curve
and controllability again.
About the Author: Victor Zwanenberg, Sr. Application Engineer, NXP
Victor has over 25 years experience in various fields of electronics design and
production. Since 2003, Victor has grown expertise in the AC/DC LED driver
electronics field and retrofit LED lamp design.