The Challenge of
Dimming HB-LEDs in
Legacy Lighting Systems
By CUI Engineering and Digi-Key DSS
®
The Challenge of Dimming HB-LEDs
in Legacy Lighting Systems
Substituting a diode for a resistor in a circuit
powered by an AC line is typically an easy recipe
for disaster, but it’s the precise challenge faced by
designers seeking to replace incandescent lamps
with modern high-brightness LED technology. The
VLED15 series from CUI was specifically designed to
address the challenges of integrating LEDs in legacy
lighting installations.
returning to their high-impedance states, until the
process begins anew, which typically happens once
per every half-cycle of an AC waveform. Through this
process, the circuit of Figure 1 permits truncation
of the leading edge of each half-cycle of the AC
waveform, resulting in a reduction of the RMS voltage
applied to the load to a degree that corresponds to
the setting of R2.
High-brightness LEDs have come a long way in
recent years, and are regarded by many as the
technology most likely to replace incandescent
lamps. There are some very good reasons to take
such a view. Compared to competing technologies,
such as compact fluorescent lamps, HB-LEDs can
offer equivalent or greater luminous efficacies,
contain no disposal-complicating mercury, can
improve optical efficiencies by virtue of their
directionality, exhibit longer service life potentials,
and have greatly simplified drive requirements,
particularly in applications where a dimmable light
source is required.
This basic circuit, unlike many theoretical examples,
requires relatively few modifications or additions to be
The Compatibility Problem
One of the more significant factors supporting the
incumbency of incandescent lighting, however, is an
inherent incompatibility between LED technology and
most currently installed dimming controls.
The operational concept of the basic phase control
dimmer circuit as shown in Figure 1 is quite simple;
R1 and R2 compose a variable resistance of known
minimum value, which causes the voltage waveform
across C1 to lag behind the source voltage waveform
to a varying degree, as determined by the RC time
constant thus formed. When the voltage across diac
Q1 exceeds the device’s breakover voltage as a result
of this phase shift, the device enters its conduction
mode, allowing C1 to discharge through TRIAC Q2’s
gate terminal, thus triggering it into conduction.
When the current through Q1 and Q2 falls below their
respective holding currents, the devices commutate,
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Figure 1 : A basic phase-control dimming circuit.
a functional, practical product. The circuit in Figure 2,
developed through observation of a common dimmer
switch, shows a very strong resemblance to Figure1.
A trim potentiometer R2 (set and fixed at the factory)
is included in parallel with the main control pot R3
to allow compensation for component tolerances,
and a mechanical switch is added to provide a true
“off” position. Also added are L1 and C2, which limit
the slopes of the current and voltage waveforms
associated with the device in order to reduce electrical
noise emissions and ensure that TRIAC Q2 is operated
within its dynamic limits.
The phase control technique, implemented as
described in Figure 2 or in similar circuits, is an
effective, simple, and inexpensive way to control the
The Challenge of Dimming HB-LEDs
in Legacy Lighting Systems
amount of power applied to a load that is resistive or
nearly so—a prescription which incandescent lamps
fill quite nicely.
The problem arises because the control scheme of
these circuits, being connected in series with the load,
cannot function independently of the load. If the load is
Regardless of terminology, one of the first stages in
most modern electronic ballasts is a filtered diode
bridge such as that in Figure 3. Loads fed through
such a rectifier stage are invariably nonlinear, since
they permit no current flow in the AC line when the
instantaneous line voltage is less than the voltage
present across the filter capacitor.
When a load incorporating a diode bridge (Figure 3) is
supplied through a phase control dimmer (Figure 2),
the timing capacitor (C1) in Figure 2 no longer begins
charging at the zero-crossing of the AC line voltage,
but at whatever point the instantaneous line voltage
exceeds the voltage across the filter capacitor in
Figure 3 plus a few diode drops. Because this voltage
Figure 2: A commercial dimmer switch as installed.
Device boundary is indicated by dotted line.
more or less linear (i.e. it passes a current proportional
to applied voltage by some fixed constant), the
relationship between the potentiometer setting and
the firing angle of the main TRIAC will have a direct
correlation and the RMS voltage applied to the load
will have a well-defined, monotonic, and determinate
relationship with the potentiometer setting.
Unfortunately, incandescent lamps are the only
common lighting agents that naturally meet this
criterion. Because gas discharge tubes (fluorescent
lamps) and LEDs are nonlinear, there is no practical
way to power them from an AC line without benefit of
some mechanism to regulate current flow. Curiously,
the term “ballast” has been used for devices that
serve such a function for fluorescent lamps, while the
term “driver” seems to be preferred in the context of
LED-based devices.
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Figure 3: Filtered diode bridge.
is not necessarily consistent from cycle to cycle, the
result is variability in the dimmer’s turn-on point.
Additionally, since current flow through a Figure
3-style rectifier ceases once the instantaneous line
voltage falls below the voltage present across the
filter capacitor, use of such loads in conjunction with
a Figure 2-style phase control dimmer can cause
TRIAC Q2 to commutate early, prior to the end of the
AC half-cycle, and not necessarily with any degree
of cycle-to-cycle consistency. Commutation prior
to the completion of an AC half-cycle creates the
possibility of re-triggering during the same AC halfcycle. In addition to obscuring the measurement of
an overall conduction angle, multiple triggering during
a single half-cycle causes elevated power dissipation
The Challenge of Dimming HB-LEDs
in Legacy Lighting Systems
in the phase control circuit due to an increase in the
frequency of switching events.
In general, loads powered through a filtered diode
bridge render phase control circuits in the fashion of
Figure 2 more or less useless as a dimming control.
While the setting of the control potentiometer retains
some influence over the conduction angle in such
situations, the amount of voltage present on the filter
capacitor has an equal or greater influence. Since
the latter quantity can vary from cycle to cycle under
various influences outside the control of the dimmer
circuit, the conduction angle of the phase control
circuit varies along with it. Because the conduction
angle and dimming control signal are one and the
same, it becomes very difficult to design a nonincandescent lamp that is compatible with existing
dimming controllers without sacrificing dimming
performance. A limited dimming range and visible
“flicker” are among the most common problems
exhibited when HB-LEDs are retrofitted in existing
lighting applications that utilize TRIAC dimmers.
The Instability Problem
The current installation environment for HB-LEDs is
relatively undefined. There are numerous variables
that can affect LED performance if not properly
accounted for in the driver. The distance of the lighting
agent from the dimmer is a perfect example. In some
instances, the AC power cables may extend well over
100 feet from the source TRIAC. In other instances, the
source TRIAC dimmer may be positioned very close to
the LED modules. Another factor is the number of LEDs
connected to the TRIAC dimmer, which can also vary
greatly depending on the installation environment.
In certain applications, the TRIAC may only be asked
to drive a few modules, while in other applications,
30 modules or more may be connected to a
single TRIAC dimmer.
These varying installation scenarios inevitably lead
to line instability and line noise, causing many LED
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modules and their associated drivers to improperly
dim under TRIAC control or flicker during the dimming
cycle. The highest incidents of “flicker” normally
occur at either the highest current point or when the
conduction angle is approaching its lowest point.
The Regulatory Problem
The migration toward HB-LEDs as a commercial
and residential lighting solution has brought with it
concerns regarding the effect that this technology
may have on power distribution grids. As discussed,
basic implementations of these devices typically
incorporate a filtered rectification stage, resulting in a
device which draws current only near the peaks of the
AC waveform. The resulting current waveform has a
high harmonic content, which, if unmitigated, results
in elevated distribution losses and damage to the
power distribution infrastructure.
In response to these power quality concerns,
regulations regarding the minimum power factor of
common lamps are being issued in many localities.
Naturally these requirements vary by jurisdiction,
which results in a need to design to the most stringent
specification if a single product is to be globally
marketable. For LED-based products, that implies a
power factor exceeding 0.9.
The Challenge
The design of an LED driver module that allows for
seamless integration into legacy dimming applications
presents a number of challenges. Consider the
following requirements for an LED driver that
suits this purpose.
The LED driver must:
¬ be dimmable using currently-installed
TRIAC-based dimmers
¬ be dimmable over a wide range
¬ avoid perceptible flicker
The Challenge of Dimming HB-LEDs
in Legacy Lighting Systems
¬ meet the necessary safety standards
¬ comply with all applicable radiated and
conducted emissions standards
¬ last at least as long as the LEDs it is powering
¬ tolerate elevated operating temperatures
without premature failure
¬ exhibit a power factor >0.9
¬ exhibit high electrical efficiency
¬ be of modest physical size
¬ be able to operate in a wide variety of
installation environments
The potential for LEDs in replacing the installed base
of incandescent light bulbs is enormous. Government
regulations increasing the minimum efficiency levels
of lighting sources and in many cases, mandating
complete phase out of incandescent bulbs altogether,
has created a global opportunity for alternative
solutions. These solutions must be compatible with
the existing installed systems to accelerate adoption
around the world. One of the bigger hurdles that must
be considered relates to LEDs interfacing with TRIAC
dimmers. The inherent differences between operation
of an LED and incandescent creates a technical
challenge that CUI’s VLED15 series transparently
overcomes.
The VLED15 Series
The VLED15 LED driver from CUI Inc. is ideally suited
to expand LED lighting acceptance. The VLED15 series
has integrated TRIAC dimming support, operating
seamlessly with today’s industry standard dimmers.
This transparent functionality allows for immediate
use in retrofitting existing installations that utilize
TRIAC dimmers. The series is one of the first TRIAC
dimmable drivers to operate in most installation
environments without “flicker” related problems.
To address instability on the AC line due to varying
installation environments, the VLED15 series uses a
complex input filter to clean and isolate the AC line
from the internal operation of the driver. This results
in very stable dimming in most installations with
approved TRIAC dimmers.
These constant current devices incorporate active
power factor correction (>0.9) to meet regulatory
concerns and can drive up to 1500 mA of current.
The VLED15 drivers are rated for a 50K hour MTBF at
a 90°C case temperature, meet all applicable safety
and emissions standards, and include overvoltage,
overcurrent, and short-circuit protection.
www.cui.com
20050 SW 112th Ave.
Tualatin, Oregon 97062
page 5
04/2012
©CUI Inc 2012. All rights reserved.