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HOW TO CHOOSE A RIGHT DRIVER…
In today’s fast-paced and rapidly-changing LED arena, the complex and subtle choices involved
in choosing an LED or LED array can be difficult enough. Add to this the confusion of selecting
an LED driver in a solid state lighting (SSL) industry marked by obscure terminology where even
the experts still debate definitions and standards, and the task can be overwhelming. As part of
the LED driver selection process, it behooves a lighting product developer, or even a
professional lighting designer/specifier, to understand concepts such as the choice of currentand voltage-based drive, the process of matching drivers to LED circuit topologies, dimming and
flicker considerations, when and why to opt for multiple outputs, and other issues.
This article will serve as a guide to help you navigate and simplify the sophisticated process
of LED driver selection, understand and apply the appropriate features to the application
at hand, and ask the right questions of your LED driver suppliers.
Constant voltage or current?
The first question developers face is almost always the choice of constant-current or -voltage
supply. There are two fundamental types of LED arrays — those which run off a constant
voltage (CV) and those which run off a constant current (CC) — and they’ rematerially different
inside. A constant voltage array will contain devices to limit the current from going too high when
the LEDs get hot, such as resistors or constant current resistors (CCRs) or even a switching
DC-DC regulator of some kind. By contrast, a constant current LED array will have LEDs
connected in series and perhaps several of these strings connected in parallel. You’ll want to
choose a constant voltage array under two circumstances:
1) You have yet to determine your LED array and the application is one where you don’t know
exactly how many LED strings will be hung on that supply or what the current draw will be (i.e.,
cove lighting).
2) The LED array is of the constant voltage variety and therefore has a fixed range of current for
that fixed output voltage. In this instance, you’ll need to ensure that the driver you select is the
right voltage and that the allowable output current range is higher than the gross estimated
current draw of your LED loads. If you are aware of the current draw needed to match
application light-level requirements, you’d probably choose a constant current array because it’s
usually the most efficient arrangement. If your LED array requires a
constant current, then you’ll need a CC LED driver. This type of driver will only have a certain
range of voltages which it can drive; there will be a minimum voltage and a maximum voltage
permissible. You need to ensure that your LED array has a voltage requirement that falls inside
this permissible range. The table reviews AC-to-DC driver topologies and their pros, cons, and
applications. There is no wrong or right approach to driver selection. Instead, you need to match
topology to the application requirements. Now let’s address some specific issues. The
importance of dimming another early decision will consider the importance of dimming and that
can involve human visual perception and / or energy conservation issues. In order to select the
right features, it helps to understand the characteristics of human perception. The human eye
notices light changes on a scale which relates to what it’s already seeing and the light output of
an LED lamp is roughly proportional to the current going through it. As a result, dimming to 50%
is hardly noticeable to most people and 10% is perceived as just a few degrees dimmer than
that. Therefore, to have a discernable visual dimming effect, you need to be able to dim down to
1%. By comparison, movie theaters require dimming in the range of 0.1%.
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The lack of sensitivity doesn’t mean that dimming to above 1% is not useful. In fact, the situation
is quite the contrary. If you dim an LED light down to 10%, you’ve just saved 90% of its energy
consumption,
which is very significant. Dimming by any degree is worthwhile for energy-saving purposes, but
if you want to have a dimly-lit room or theater, you must have drivers capable of dimming down
below 1%, and down to 0.1% for full-range dimming. The bottom line is that while the benefits of
dimming in increments above the 1% range are often not perceived by the human eye, the
energy benefits generated by dimming even a few degrees can have a tremendous impact on
energy savings.
The flicker controversy with dimming come flicker issues and the question of how much flicker is
too much. In the lighting community, flicker is defined as the percentage fluctuation of the light
(or LED current) at twice the line frequency, expressed as a fraction of the steady light (or DC
current) through the LED.
Twenty years ago, magnetic ballasts drove fluorescent tubes, which produced an intense flash
near the peak of the power-line voltage cycle such that the entire light output consisted of a
series of light flashes at twice the frequency of the power line. It was found that even though
most people couldn’t detect this fluctuation, some developed headaches and other stress
symptoms when exposed to it. In response to the dissatisfaction with magnetic ballast flicker,
electronic ballasts were developed that were capable of less than 2% flicker, which proved
effective and quickly became the industry standard for good quality light.
The problem still being discussed is that f licker considerations aren’t simple.
Modern LED lights that have f licker are likely to have a 120-Hz fluctuation that has a smooth
alternating variation at twice the line power frequency. Even when this flicker approaches 100%,
it results in a significantly less-noticeable flicker than that of magnetic ballasts. We recommend
that the 120-Hz ripple in the output of a driver should be less than 10% for LED task and office
lighting, while for LED decorative lighting (e.g., cove lights, sconces, outdoor lighting, etc.), as
much as 100% flicker may be acceptable. Dimming methods and flicker Dimming
methodologies, however, can impact f licker. In the output of an LED driver, the percentage of
ripple at twice the line frequency is the parameter that corresponds to the flicker in the light
output. Many LED drivers produce dimming by switching the LED light on and off at a relatively
high frequency, a process called pulse-width modulation (PWM) dimming or digital dimming.
The human eye is completely oblivious to these high frequencies and simply perceives less
light. Dimmable LED drivers exist, however, that simply modulate the light on and off at twice
the line frequency; at low dim levels, the result can be a lot like the light output of old magnetic
ballasts where the flicker may be easily perceived. In addition, if used with a triac dimmer, which
doesn’t dim positive and negative half-cycles equally, it may introduce a line frequency
component to the PWM that will be perceptible to anyone.other LED drivers produce a uniform
DC current level, which is then adjusted downward to produce dimming. This methodology is
sometimes referred to as analog dimming. For task and office lighting, this approach is the most
trouble-free kind of dimming to use, though it’s likely to be more expensive than digital dimming.
There is also the issue of how dimming information is conveyed to drivers. In many cases,
drivers must work with legacy wall dimmers. The diagrams in Figs. 1 and 2 visually contrast two
legacy dimming methods used in today’s LED applications that are based on controlling the AC
line. Forward phase dimming typically operates on two wires and avoids the labor associated
with adding a third wire; it is commonly used in churches, auditoriums, and schools. Reverse
phase dimming is considerably more expensive but minimizes electromagnetic interference
(EMI-EMC) issues. The driver selected must have the ability to work with the dimmers deployed
in an application, especially in retrofit scenarios.
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LED driver lifetime Driver lifetime should also be a major concern. If the temperature of your
LED array is properly controlled, it should produce more than 70% of its initial light output after
50,000 hours. Obviously, you’d like your LED driver to last equally as long. The lifetime of an
LED driver is determined by the lifetime of the individual electronic components inside. The
weak link, in particular, is the electrolytic capacitors, which are like little batteries. The elec FIG.
2. Reverse-phase dimming cuts the trailing edge of each half-cycle. trolyte inside is typically a
gel that graduallyevaporates over the life of the component. The evaporation rate depends upon
the temperature inside the driver — which, in turn, correlates to the external temperature on the
driver case. Higher operating temperatures speed evaporation and hence shorten the life of the
capacitor.
On the label of most LED drivers there is a small circle called the hotspot or “Tc point.” This is
usually the hottest point on the can and is used to determine the can temperature. The
manufacturer will supply a temperature that must not be exceeded if the UL approval of the
product is to remain valid. However, be aware that if you use the driver close to this limiting
temperature,its operating lifetime will typically be shorter than if operated at a lower
temperature.The driver manufacturer is able to supply curves that correlate the lifetime of
the driver to its hotspot temperature. Fig. 3 provides an example of the curve for a typical LED
driver.
To ensure that the lifetime of the electrolytic capacitors exceeds the lifetime of your LED array at
the necessary temperature, you must ensure that the manufacturer has used long-life
electrolytic capacitors. Measures of power line quality There are additional power quality issues
that you should consider.
Datasheets for LED drivers can introduce bewildering terms such as THD (total harmonic
distortion), power factor FIG. 3. Driver lifetime varies with temperature. (PF), and universal input
voltage.
Understand these concepts when making a driver selection. THD. The distortion of the
sinusoidal waveform can lead to potentially dangerous consequences, such as the overheating
of electrical equipment and even fires in transformers and switching stations. Considering the
growth in electrical devices with non-linear loads, THD is becoming more and more important.
Today, THD below 20% is usually acceptable and a THD of less than 10% is exceptionally
good. PF. This is of major concern to utilities as it represents a difference between the power
actually delivered to a facility and the power detected by a meter that determines the bill to a
facility. Low power factor is costly to a utility. A conventional standard for PF is 0.9 or above;
anything lower may result in a penalty assessed by the utility in the form of a multiplier on your
electric bill. If PF isn’t mentioned in a driver specification, the default is referred to as normal
power factor and implies any PF below 0.9. The actual spec could be as low as 0.4 in some of
the least expensive lighting products. While PF is generally meaningless in a residential setting,
careful attention needs to be paid when installing large volumes of normal-power-factor
products in industrial or commercial applications.
Universal input voltage:
In India most commercial and industrial lighting runs off 240VAc, while consumer and retail
lighting is mostly selected in operating voltage range of 180-240VAc. An LED driver that can run
off either is said to have universal input voltage capability. It’s assumed that the adaptation is
completely automatic and reversible. Lighting distributors like to stock universal input voltage
products so they don’t have to worry about the voltage required.
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Multiple output ambiguity Now let’s discuss outputs in particular and the possible ambiguity
when a driver offers multiple outputs. The terms “outputs” and “channels” are often used
interchangeably, but professionals need to be aware that there’s a difference between multiple
outputs and multiple independent outputs. The
difference can have a significant impact on the luminaire’s reliability. The easiest way of
connecting a single driver to multiple outputs is to simply bring multiple wires out of the driver to
handle the current load. Inside the driver, the wires are all paralleled and provide multiple paths
for current to f low. While very simple, this creates some significant disadvantages. In this type
of driver, for example, the faults aren’t contained and there’s
poor balance of current through the multiple outputs, resulting in varied brightness of each
output. In addition, if one LED load shorts, it can bring down all of the other outputs; if the LED
load opens, it will cause the other loads to absorb the additional current and possibly drive them
to thermal overload and early life failure. Even under normal operation, without tightly matched
LED loads there will be very poor balance of current among the outputs. By contrast, a driver
with independent outputs (a true multi-output driver) will regulate each channel independently of
the other outputs. This architecture allows each channel to have its own current regulation or
fault protection and helps answer the question of “what happens on the remaining outputs when
one output (or light engine) fails?” Instead of regulating to a total output current of 1A, for
example, each of four theoretical outputs are regulated to 250 mA, allowing each output to have
its own control and ensuring that the brightness difference between each LED load is minimal.
To help illustrate this benefit, MELCON 54W driver is an example of a driver that goes one step
beyond and enables paralleling of those outputs to create different combinations. With its four
outputs of 350 mA each, not only can outputs be run separately, but it’s possible to run two 700mA outputs by tying the channels together. The E54W can also be connected in many other
combinations. Leaving a channel unconnected or having a channel inadvertently shorted will not
cause a change in performance within the unaffected channels.
Tradeoff considerations Last, you need to understand common tradeoffs in driver design and
how that might impact the selection process. Cost factors may require tradeoffs in features or
capabilities.
Following are five potential compromises to consider.
•
Output ripple: It’s straightforward to make an LED driver that has essentially no output
current ripple by building it with two power conversion stages; a first stage generates a
stable power supply and a second stage then generates the output current. A two-stage
design has two control chips and two lots of high-frequency transformers inside and is
more expensive. The cost of a driver can be significantly reduced by using just one
power conversion stage for both power factor correction (PFC) at the input and for
controlling the output current. The tradeoff is that now either the PFC is less perfect or
sometimes as much as a 50% ripple at twice the line frequency is introduced into the
output.
•
Startup time: A compromise between cost and efficiency is introduced with startup time.
A short startup time can be achieved by using high power to charge up all the capacitors
quickly. However, this same high power will still be there afterwards and will decrease
the efficiency of the lighting system. Components can be introduced to turn the excess
power off, but the tradeoff is additional system cost. It’s worth considering whether the
application requires fast startup at all for example, most HID street lights take a minute
or so to start up, so there’s no need for an LED street light to start in less than a second
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because it makes no difference to the users of the light and only gets switched on once
a day.
•
Dimming level and efficiency: Though advances are currently being witnessed in the
area of dimming, in general, to achieve lower dimming levels the tradeoff will be lower
efficiency.
•
Cost and efficiency: Generally, an LED driver can be made more efficient by using
oversize switching transistors/Mosfets (Typical Flyback Design topology) and highfrequency transformers, which adds expense.
•
Universal input voltage and cost: A universal input voltage product contains the
components and capability for both high input voltage operation and high input current
operation. You need higher current at the low voltage. The tradeoff is that you’re paying
for both and you can get better value by purchasing a single-voltage product. However,
fixture OEMs don’t often know what voltage the product will need, so it’s usually
worthwhile to pay for the more expensive universal input voltage feature.
Evaluating and selecting an LED driver for your project doesn’t have to be a daunting task. As
outlined in the previous overview, an understanding of such issues as current and voltage, LED
circuit topologies, dimming and flicker considerations, and multiple/ independent outputs will
help you ask the right questions of your driver supplier and help identify the important features
essential to optimizing your lighting system performance.
MELUX CONTROL GEARS P.LTD
PUNE, INDIA