DESIGNfeature
JEFF PERRY
Senior Development Manager, National Semiconductor
Optimizing LED Lighting Systems
for Efficiency, Size and Cost
An LED lighting system can
be optimized for efficacy,
footprint, lifetime and cost
by varying the LED’s current
and controlling its temperature with a heat sink. To get
accurate results, however,
the dynamic nature of LEDs
requires modeling their
behavior across current and
temperature. The appropriate
driver topology depends on
the system’s input voltage.
W
ith the rising costs of energy and increasing governmental
requirements for high efficiency lighting, the high brightness LED market is expanding rapidly. LEDs promise high
efficiency, but the cost can be high, so how can a designer
get more light out of an LED to reduce the number of LEDs
required in an array? The most direct way is to increase the
current, but that in turn increases temperature, which can
degrade the light output, decrease efficiency and reduce lifetime. Thus, heat sinking
is necessary. A proper LED lighting design will take into account all these factors
and also the LED driver and LED string sizing. New tools have been developed to
ease the LED lighting design process and allow users to make tradeoffs between high
efficiency, small footprint and low cost.
LED BEHAVIOR IS DYNAMIC
The first goal a lighting designer needs to set is the light output of the system. This
is typically specified using luminous flux, in units of lumens, which is a measure of
visible light from a given source. Traditionally, the designer would go to an LED
datasheet and look at the specifications for luminous flux and use those parameters to
choose the number of LEDs required for the system. But as anyone who has studied
an LED datasheet knows, LED behavior
Luminous Flux Vs Current
is dynamic based on the current being
Luminous Flux vs Temperature
105%
330%
used to drive the LEDs and also the
300%
100%
250%
LED temperature.
95%
200%
90%
Typically, the luminous flux is speci150%
85%
fied
at a constant 25ºC temperature
100%
80%
using
a short burst of current in the lab.
50%
72%
0%
But
in
reality, LEDs run hot and the
0.35A 0.5
1
0
1.5A
25
51C
75
100
125 150C
1a
1c
temperature is considerably above the
Fig. 1A: Luminous flux vs LED current. Can go
Fig. 1C: Luminous flux vs LED temperature.
ambient. The best production LEDs on
above nominal current rating to raise the light
Shows lower light output with increasing temthe market today are only about 25% to
output and reduce the number of LEDs to lower perature.
30% efficient, which may be considered
the cost.
surprising given the energy efficiency
LED Temperature vs Current
Luminous Efficacy vs Current
150
175
hoopla surrounding them. In fact, this
140
150C
is great compared to the efficiency of a
128L/W
125
100
tungsten filament bulb, which may run
110
100
75
around 2.2% for a 100W bulb giving off
90
51C
80
1500 lumens. But the question is where
25
69L/W
does that 70% power loss in an LED go?
60
0
0
.35A 0.5
1
1.5A
.35A 0.5
1
1.5A
1b 0
1d
Unlike a tungsten bulb which emits a
Fig. 1B: LED temperature vs LED current for
Fig. 1D: Luminous efficacy vs LED current. May
significant amount of infrared radiation
fixed heat sink size. Higher temperatures mean
lose environmental rating by running at high
to give off heat, LEDs must get rid of
reduced lifetime.
current
heat through conduction. And that
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January 2011 | Power Electronics Technology
29
LED DRIVERdesign
Small
footprint
13
0.70
131
35
92
$47.10
Low Cost
8
1.35
140
81
74
$30.07
Balanced
9
1.00
109
119
88
$34.27
Higher
Efficacy
12
0.65
78
175
108
$46.08
current and decreases with temperature which may affect
the driver design. For example, in a series string of LEDs,
the total forward voltage must be kept below the minimum
input voltage for a simple buck design, otherwise a boost or
buck-boost topology may be required. Thus, we see that
the lighting designer must make compromises between cost,
footprint, reliability and efficiency when designing an LED
system. It’s not as simple as just raising the current.
Highest
Efficacy
19
0.35
48
837
128
$88.75
LED DESIGN TRADEOFFS
# Current Temperature Area Efficacy
Optimization LEDs (A)
(cm2) (lumens/W)
(c)
Cost
160
900
140
800
700
120
0111Perry-Fig. 2a
600
100
500
80
400
60
Area
Temperature Efficacy, Cost
Fig. 2A: Variety of optimizations for 2500 lumen design
300
40
200
20
100
Highest Efficacy
Higher Efficacy
Balanced
Low Cost
Small footprint
0
0
Fig. 2B: Tradeoffs for different optimizations of 2500 lumen design
means heat sinks and temperature control are a must. What
specific parameters should a designer be concerned with
which vary with the LED current and temperature? The
important ones include luminous flux, Vf (LED forward
voltage drop) and luminous efficacy (luminous flux divided
by the power consumed in units of lumens/watt) which is a
measure of the efficiency of the LED.
The luminous flux of LEDs goes up with LED current,
which can be useful if a designer wants to reduce the number of LEDs in the array to lower the cost (Fig. 1A). In fact,
LEDs can often be driven with up to 2x or 3x the nominal
current (check the datasheet for maximum current) to get
more light output. But the tradeoff is high temperature
which increases with increasing current for a fixed heat
sink size (Fig. 1B). Higher temperatures mean decreased
lifetime and reliability for the LEDs. This also lowers the
light output of the LED, perhaps significantly (Fig. 1C). To
lower the temperature, a larger heat sink can be used, but
this will increase the cost and footprint of the design.
In contrast to the luminous flux, the luminous efficacy
goes down with increasing current (Fig. 1D). This drop
in efficacy may cause the loss of a governmental efficiency
standard approval and certainly make the product less
appealing from an energy conservation standpoint. In
addition, the forward voltage of the LEDs increases with
30
Power Electronics Technology | January 2011
Let’s take a look at several scenarios using a high efficacy LED for a design targeting 2500 lumens. This can
be done using conventional manual methods, or by using a
design tool like National Semiconductor’s WEBENCH LED
Architect. This new tool draws from an extensive library of
LEDs, drivers and heat sinks to design complete systems.
WEBENCH LED Architect enables designers to perform
real-time comparisons and optimize complex lighting systems
for performance, size and cost in minutes using graphical
visualizations of the critical parameters. To start a design, the
user enters the desired light output in lumens and is presented
with a listing of suitable LED and heat sink solutions in both
table and chart form. The user can tune the design with the
unique WEBENCH Optimizer Dial, prioritizing size, efficacy
and cost trade-offs. After selecting the LED and heat sink,
different driver options are shown allowing the user to choose
between driver topologies and LED string configurations.
Graphical charts are utilized to visualize the compromises
between footprint, efficiency and price.
After choosing the driver, all the components for the system are calculated along with the schematic and operating
values such as duty cycle, currents and power dissipation.
Electrical simulation is available for analyzing transient
behavior. The designer can fine tune the bill of materials
(BOM) by choosing different components from a library
of over 20,000 passive components or by entering custom
component values if desired. Lastly, the designer can order
components for prototyping, share the complete system
with others, or easily print a complete project report including schematics, BOM and performance characteristics.
Using the tool, the variables will be the heat sink thermal
resistance (θSA in ºC/W), the LED current, the LED operating temperature and the number of LEDs. The heat sink
areas are calculated based on typical extruded aluminum
profiles but other solutions, including high thermal conductivity board material, may be used. The scenarios are
shown in table form in Fig. 2A and in graphical form in Fig.
2B. The first example is the smallest footprint case. With
a small heat sink area we are limited in cooling capability,
thus the LED current will need to be kept moderate and
the operating temperature will need to be high. We end up
with an array of 13 LEDs requiring a heat sink θSA of about
4ºC/W, giving an area of 35cm2. The LED temperature is
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LED DRIVERdesign
Footprint of HS+ driver (cm2)
on the high side at 131ºC and the price
144
for the LEDs and heat sink is $47.10. The
142
LED efficacy is medium at 92 lumens/W.
Buck-Boost:
With the relatively high temperature, the
140
2x5
LED lifetime will be reduced.
138
The second case is the lowest cost
136
Buck: 3 x 3
scenario. To do this, we will raise the
134
LED current to the maximum to reduce
Boost:
132
1x9
the number of LEDs required and use
a low cost heat sink, with the tradeoff
130
that the operating temperature will go
128
to a maximum. In this case, eight LEDs
are required, requiring a heat sink θSA
73
74
75
76
77
78
79
80
81
82
System efficacy (lumens/watt)
of 2.4ºC/watt and area of 81cm2. The
operating LED temperature is very high
at 140ºC. Efficacy is the lowest at 74 Fig. 3: LED string array and driver options for 15V to 25V input voltage range for a 2500 lumen system.
lumens/W due to the combination of Circle size is the cost.
high LED current and high temperature.
monitor the marketplace on a regular basis to keep up with
However, the cost for the LEDs and heat sink is the lowest
the latest releases.
of all the scenarios at $30.07. The sacrifice here with the
high temperature is reduced LED lifetime.
The third scenario is a balance between small size, low
DRIVING THE LED
cost and high efficacy. Here the current is relatively high,
In the LED system, after the number of LEDs, the heat sink
but the heat sink is allowed to increase in size to an area of
and the LED current are determined, a suitable driver must
119cm2 with a θSA of 2.1ºC/W. This lowers the temperabe found to power the LEDs. To achieve the high efficiencies that go along with LEDs, this typically means a switching
ture to 109ºC, thus giving an efficacy of 88 lumens/W with
regulator is required. Two interacting issues arise: The topolsomewhat increased lifetime, but the temperature is still on
ogy of the driver must be decided upon and the LED array
the high side. The cost is moderate at $34.27.
must be determined.
We go to the fourth example to get higher efficacy and
lower temperature. This utilizes moderate LED current,
At this point, the relationship of the LED array voltage to
but the heat sink is larger with an area of 175cm2 and a
the input voltage range becomes a critical parameter. If the
θSA of 1.8ºC/W. The temperature is reduced to 78ºC, thus
total voltage of the LED array is less than the minimum Vin
(plus a bit extra to account for losses across the switch), then
increasing LED lifetime and yielding an efficacy of 108
a buck topology can be used. This is the simplest topology
lumens/W. The $46.08 cost is on the high side.
to implement and it carries the advantages of high efficiency
The last scenario targets the highest efficacy. It does this
and low input current requirement.
by lowering the LED current to the nominal datasheet value
If the total LED voltage is above the maximum input
and using a large heat sink. This keeps the temperature at a
voltage, then a boost topology is called for. This is also a
minimum of 48ºC, which will result in the longest lifetime
proven topology but has the disadvantage of requiring a
of the group. The results also show the highest efficacy of all
the scenarios at128 lumens/W but the required heat sink θSA
is very low at 0.7ºC/W giving a very large area of 837cm2.
Driver
Vin
Vin
Driver
Driver
Driver
With the low LED current, there are 19 LEDs required, so
Area
IC
min max
Topology
Efficiency
Cost
2
(cm )
the cost is also very high at $88.75.
Thus, we see that the LED performance can vary over a
3.5
35
40
Buck
93%
LM3414 $1.90
very large range depending on the current and temperature
of the LED. Also, there is no single best solution to the
6.3
20
25
Boost
93%
LM3429 $3.02
problem. High efficacy and long lifetime are achieved only
by sacrificing cost and area. On the other hand, going for
low cost and small footprint requires a compromise between
8.0
25
35
Buck Boost LM3429 $4.04
88%
efficacy and lifetime, which are the two main selling points
for LEDs. The good news is that LED performance is
improving at a breakneck pace with better efficacy bins and
Fig. 4: Driver options for different input voltages with total LED string voltage
new LED models appearing often. So it is important to
of 28.6V.
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January 2011 | Power Electronics Technology
31
LED DRIVERdesign
ILED sim:1
32
Power Electronics Technology | January 2011
Vin sim:1
high-voltage, high-current FET dependTaking the balanced optimization
Area
Cost
Footprint
(cm2)
ing on how much the voltage must be
from the previous 2500 lumen example
increased. This may result in higher cost
which in a series configuration has nine
LEDs +
119 $34.27
and larger footprint.
LEDs for a total string voltage of 28.6V,
Heat Sink
Lastly, if the LED array voltage is
we now examine several driver scebetween the maximum and minimum
narios using an input voltage range of
3.5 $1.90
Driver
input voltage, then a buck-boost topol15V-25V. Fig. 3 is a chart of the total
ogy is required. This allows the most
system footprint vs the system luminous
flexibility with the LED array voltage,
efficacy including the driver losses. The
but the driver design is the most com- Fig. 5: LED design is dominated by the LEDs.
circle size is proportional to the cost.
plicated and expensive to implement.
The lower right of the plot shows the
Also, like boost, it has the disadvantage of requiring high
highest efficacy, lowest footprint solutions. These use a
current if the input voltage goes below the LED voltage.
single series string of nine LEDs. The total LED voltage of
28.6V is above the 25V maximum Vin, so this requires a
boost driver topology.
LED ARRAY CONFIGURATION
The LED array configuration can be arranged to allow for a
The middle of the plot shows buck driver solutions which
desired driver topology. If a buck topology is desired, the
have the LEDs broken into three separate strings of 9.5V
LED array can be broken down into parallel strings such that
each, so they are below the minimum input voltage of 15V.
the LED string voltages are less than the minimum input
The chart shows three separate drivers being used, but they
voltage. However, if parallel strings are combined on the
could also be combined to use one driver with one current
same single output driver with one current sense resistor, it
sense resistor to reduce the cost, but at the risk of uneven
has the disadvantage that the current in each string may be
current sharing. The last group in the upper left uses two
different due to the variations in LED forward voltage. This
strings of 5 LEDs, each of which results in an LED string
may lead to differences in brightness and temperature and
voltage of 15.9V.
eventually variations in LED lifetime between the strings.
Another scenario, shown in Fig. 4, is to vary the input
This can be solved by using a driver with multiple outputs
voltages in order to get different driver topologies using one
and current sense resistors, or by using multiple single outseries string of LEDs. This table zeros in on the driver perput drivers.
formance only and does not include the LED and heat sink
contributions. In the first case, we target a buck topology so
To avoid the current sharing problem, the LEDs can be
we use an input voltage range of 35V to 40V, which is above
arranged in series. However, the total LED voltage may be
the 28.6V LED string voltage. The driver performance,
quite high. If the LED voltage exceeds 60V, additional safety
excluding the LEDs, produces an efficiency of 93% with a
features and certifications may be required to meet governcomponent area of 3.5 cm2 and a cost of $1.90.
mental standards.
To get a boost topology, we lower the input voltage range
26
1.05
to 20V to 25V so it is below the LED voltage. The efficiency in this case is the same at 93%, but the footprint is larger
24
1
at 6.3 cm2 and the cost about a dollar higher at $3.02.
In the last case, we use an input voltage of 25V to 35V
22
0.95
which results in a buck-boost topology since the LED voltage is between the maximum and minimum input voltage.
20
0.9
This gives a lower efficiency of 88%, a higher component
footprint of 8 cm2 and a cost of $4.04.
18
0.85
Thus, we can see that the LED driver comprises just 5%
to 15% of the total system cost and the LED driver efficiency
16
0.8
is high at 93% vs 24% for the total system (see Fig. 5 ).
The last step in the driver design process is creating the
14
0.75
actual design. Driver design tools automatically generate a
bill of materials and allow the user to change passive compo12
0.7
0 50 100 150 200 250 300 350 400 450
nents and run simulations to verify system performance. Fig.
t e-6 secs
iled
vin
6 displays the results of an input transient simulation run in
the WEBENCH LED (http://www.national.com/analog/
Fig. 6. Results of an input transient simulation run in National Semiconductor’s
led#software) Architect design tool, showing the effects of
WEBENCH LED Architect driver design tool showing the effects of changing the
changing the input voltage on the LED current.
input voltage on the LED current.
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