for
Design Guide
Torch Application
rev1.2
(16. July. 2014)
Samsung Electronics
LH351A, LH351Y
1.Torch Application
1. Torch application
general in torch application, structure
2
2. Reflector
beam pattern, yellow ring effect
3
3. LED source
powerful, reliable, compatible
5
4. Driving
constant current driving from battery
8
5. Heat transfer
Cu heat slug PCB, 1st heat slug
9
1.1 General in Torch application
Thanks to compact size of battery having
high performance and powerful single LED
of spot lighting source, recently new
concept of torch with small size and light
body but having powerful performance
comes out. These combination makes
long lifetime, durability, advanced light
quality and portability. In this design guide,
some useful information will be
introduced briefly with respect to reflector,
LED source, driving and heat transfer
1.2 Torch structure
Battery
& Driver
(inside)
Body
PCB
HP LED
Ring
Reflector
[ Figure 1. Torch structure ]
Torch is composed of several function blocks – single high power LED, PCB board, reflector,
connection ring, battery and driving circuit. In torch application, high quality of light distribution
such as concentrated and clean intensity is combined with single high power LED and proper
reflectors.
2
2.Optic guide for reflector
2.1 Generate beam pattern
Normally LED emit light quite similar as
lambertian surface. So to get spot light as a
beam pattern, proper 2nd optic solution is
required. Generally there are two kinds of optic
ways. One is lens solution which is difficult to
get wanted beam pattern and have a lower
luminous transfer efficiency. Another is reflector
solution which has high efficiency and could
make excellent beam pattern.
Without reflector, beam angle is just changed
not making spot inner circle. Thus normally
output beam pattern is made by combination
between reflector and LED.
To get wanted hot spot light, width and length
of reflector is main design parameter.
Jig
[ With Jig ]
Body
Jig
[ With Body ]
The length of output beam and inner circle size – or length of focus - for torch is decided by the
luminous flux of LED and angle of reflector. And outer circle is decided through the width of
reflector
Reflector
L
3
L1 = L2
Heat
sink
Heat
sink
W1
W2
[ With Reflector ]
3
Heat
sink
W3
2.Optic guide for reflector
2.2 Control yellow ring effect
Yellow ring
Jig
Heat
sink
[ With Jig ]
[ With Reflector ]
Generally the density ratio between phosphor thickness and the strength of blue wavelength
from chip does not perfectly equal. This means color over angle does not same and at the side
area of LED light source, phosphor portion is slightly higher than center area. Therefore yellow
ring phenomenon might comes into sight at side angle. This yellow ring could be directly
presented to the output color of beam through the reflector.
yellowish
effect
less
Scattered
reflection
more
Specular
reflection
[ Comparison between scattered reflector and specular reflector ]
Even though the securing of LED with equivalent color characteristics over the angle does not so
easy way, some other simple techniques could be help to reduce yellow ring effect. One of
them is scattering way through the scattered reflector. From this way, yellow ring effect could
be reduced rather than that of specular reflector. And besides, various other ways exist in torch
application.
4
3.Optic guide for lens
3.1 Generate beam pattern
[ Consumer]
Beam
Intensity
Heat
sink
Heat
sink
Reflector
Lens
[ 2nd optic solution ]
[Military, Camping,
Bicycle, Dive]
In torch, there are two kinds of optic solutions. One is reflector type and another is lens type
solution and these different types make different beam patterns. At the first, reflector type
makes narrow bright center beam which makes possible to reflect spot area more brightly from
the long distance. And most of consumer torch adopted this reflector type solution. Secondly,
lens type is more proper solution to reflect large circle area uniformly at the near distance.
Military , camping and bicycle applications are more suitable for lens solution.
5
3.Optic guide for lens
3.2 Lens function & Symbol mark of LH351Y
Zoom out
Zoom in
[ Torch with lens solution]
LH351Y
distance
Heat
sink
Lens
(Short distance)
Heat
sink
Lens
(Long distance)
[ Zoom in/out function ]
Pure square
beam quality
[ Samsung LH351Y beam pattern]
Lens solution have some merits and one of them is variable zoom-in & out function. Long
distance between LED and Lens makes small center circle and concentrates beam emission to
spot area. When distance of body is setting to maximize, zoom in, chip pattern could be
appeared. Samsung high power LED has a unique pattern which has no any bonding pad and
any electrical metal pattern. These clean beam pattern makes pure quality for lens solution.
6
4.LED source
4.1 Powerful LED light source
In torch, the most important part is actually LED component which could make actual beam
and color performance. LH351A/Y has powerful performance optimized torch application.
[ LH351A ]
In LH351A, wide operating range could be possible up to 1.5A, 3W, 400lm for high-end torch
application. Various CCT - 5700K, 6500K, 7600K is ready for supply and from design support
tool (Circle-B), it is convenient for designer to estimate expected performance and range of
mass production.
[ LH351Y ]
LH351Y has almost same size of package. This light solution is optimized for economic torch
application. Up to 1A, 2W, almost 300lm could be possible. 6500K, 7000K and 7600K CCT is
ready for torch application.
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4.LED source
4.2 Reliable LED light source
High durability and reliability is required for torch due to its characteristics of mobility and bad
weather in outdoor. LH351A/Y has passed high level test item, especially thermal shock, on
going life test, temperature humidity cycle on/off test, EMI/ESD and temperature storage test,
etc,. Under LM80 test, expected lifetime could be calculated through TM21 method. And great
lifetime is expected like as following graph.
[ LH351A and expected life time ]
[ LH351Y and expected life time ]
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4.LED source
4.3 Compatible LED light source
[ Compact size of LH351A and LH351Y ]
In torch, One of the important requirement for designing and choosing LED, the size of LED,
form factor, is major selection point. Single LED light source is recommended for torch rather
than multi LED light sources as for quality of output beam shape. From these design issue, small
spot size and high performance of LED is optimal solution for torch. Thus LH351A/Y is quite
proper light source for torch.
[ Compatible reflector with LH351A and LH351Y ]
There are so many kinds of reflector for torch lamp. Most of all reflectors could be matched
with LH351A/Y for its compact size.
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5.Driving
5.1 Driving light source from battery
(+)
(+)
(+)
(+)
Changeable
VF
Battery
Constant
Voltage
Driving
Battery
(-)
(-)
[ Battery direct drive mode ]
(+)
Constant
VF
Constant
Current
Driving
Battery
(-)
(+)
Constant
IF
(-)
(-)
(-)
[ Constant voltage mode ]
[ Constant current mode ]
Usually torch is driven from battery. Therefore it doesn’t need AC/DC converter but DC/DC
converter and driver. Optimal circuit topology like as linear regulator, buck-down, boost-up
could be adopted considering battery supply voltage. For example if voltage of battery is 2.7V,
boost-up converter is required, but most of these case, two battery is connected in series. Then
voltage becomes as 5.4V, and thus more simple topology of buck converter or linear regulator
could be adoptable.
If LED is directly driven by battery like as battery direct drive mode, lighting output could be
unstable due to changeable voltage output quality from battery. Thus LED driving is
recommended that constant current mode which is more suitable for LED rather than constant
voltage mode. That reason could be known as follows.
1200
1000
800
600
400
200
25℃
50℃
75℃
100℃
0
1600
3.4
1400
3.3
1200
1000
800
600
25℃
400
50℃
75℃
200
100℃
0
2.7 2.8 2.9 3 3.1 3.2 3.3 3.4
Forward Voltage (V)
[ Forward current vs.
forward voltage ]
Forward Voltage (V)
1400
Forward Current (mA)
Forward Current (mA)
1600
3.2
3.1
3
25℃
50℃
75℃
100℃
2.9
2.8
2.7
0 1 2 3 4 5 6
Power Consumption (W)
[ Forward current vs.
power consumption ]
0
1 2 3 4 5 6
Power Consumption (W)
[ Forward voltage vs.
power consumption ]
Electrical current & voltage sweep curve is like as general diode properties. From these IV-curve,
some graph of current vs. power consumption and voltage vs. power consumption could be
drawn. If LED is driven by constant voltage, as temperature rise higher, output power could vary
with large variation range. But if LED is driven by constant current mode, stable output power
could be obtained even though LED solder temperature changes.
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6.Heat transfer
6.1 Heat transfer
LH351A/Y could make high output
performance. But excellent performance
including reliable lifetime is closely depends
on how thermal dissipation system is built in.
In most of all cases, substrate board is metal
PCB which has isolation layer between
thermal pad and Al-substrate.
But copper is easy material to manufacture
wanted shape and if copper heat slug PCB
which has non-isolation layer between
thermal paths is used for substrate, luminous
flux degradation is less than normal Al-metal
PCB due to low thermal resistance. Also the
robust reliability of LED could be acquired.
Cu
Luminous flux
Copper heat slug PCB
5~10% luminous
flux difference
Al
Normal metal PCB
Forward driving current
[ Flux improving effect
through Cu heat slug PCB]
Heat
Slug
Driver
Battery
1W ~
Power
consumption
Body
Body
Driver
Battery
~ 1W
Power
consumption
[ Connection of heat slug and body for heat transfer]
Also body is the major part of torch and anodizing is very important technique to the quality of
body. For maintain the heat transfer of LED, body also play role as a heat sink. Normally if power
consumption is operated over 1watt, then it would had been better small heat slug should be
inserted with metal PCB. Under 1watt power consumption, body itself might cover heat transfer
well.
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