U.S. patent application number 13/708916 was filed with the patent office on 2014-06-12 for method and apparatus for providing a passive color control scheme using blue and red emitters.
This patent application is currently assigned to BRIDGELUX, INC.. The applicant listed for this patent is TAO TONG. Invention is credited to TAO TONG.
Application Number | 20140159612 13/708916 |
Document ID | / |
Family ID | 50880217 |
Filed Date | 2014-06-12 |
United States Patent
Application |
20140159612 |
Kind Code |
A1 |
TONG; TAO |
June 12, 2014 |
Method and Apparatus for Providing a Passive Color Control Scheme
Using Blue and Red Emitters
Abstract
A lighting device capable of generating warm or neutral white
light using blue light-emitting diodes ("LEDs"), red LEDs, and/or
luminescent material that responds to blue LED emission is
disclosed. The lighting device includes multiple first solid-state
light-emitting structures ("SLSs"), second SLSs, and balancing
resistor element. The first SLS such as a string of blue LED dies
connected in series is able to convert electrical energy to blue
optical light, which is partially turned into longer wavelength
emission by the luminescent material. The second SLS such as a red
LED die is configured to convert electrical energy to red optical
light, wherein the second SLSs are connected in series. While the
first SLSs and second SLSs are coupled in parallel, the balancing
resistor element provides load balance for current redistribution
between the first and second SLSs in response to fluctuation of
operating temperature.
Inventors: |
TONG; TAO; (FREMONT,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TONG; TAO |
FREMONT |
CA |
US |
|
|
Assignee: |
BRIDGELUX, INC.
LIVERMORE
CA
|
Family ID: |
50880217 |
Appl. No.: |
13/708916 |
Filed: |
December 7, 2012 |
Current U.S.
Class: |
315/297 ; 257/89;
315/309 |
Current CPC
Class: |
F21K 9/64 20160801; Y02B
20/30 20130101; H01L 2924/0002 20130101; H05B 45/10 20200101; H05B
45/20 20200101; H01L 33/62 20130101; H01L 25/0753 20130101; Y02B
20/341 20130101; H01L 33/50 20130101; H01L 2924/0002 20130101; H01L
2924/00 20130101 |
Class at
Publication: |
315/297 ; 257/89;
315/309 |
International
Class: |
H05B 33/08 20060101
H05B033/08; H01L 27/15 20060101 H01L027/15; H01L 33/50 20060101
H01L033/50 |
Claims
1. A light-emitting device, comprising: a plurality of first
solid-state light-emitting structures ("SLSs") configured to
convert electrical energy to first optical light, wherein the
plurality of first SLSs is connected in series; a plurality of
second SLSs configured to convert electrical energy to second
optical light, wherein the plurality of second SLSs is connected in
series; and a first balancing resistor element coupled to the
plurality of first SLSs in series and configured to provide load
balance between the plurality of first SLSs and the plurality of
second SLSs, wherein the plurality of first SLSs including a
balancing resistor element and the plurality of second SLSs are
coupled in parallel allowing current redistribution in response to
fluctuation of operating temperature due to the temperature
dependent characteristics of the balancing resistor element.
2. The device of claim 1, further comprising a second balancing
resistor element coupled to the plurality of second SLS in series,
wherein the second balancing resistor element is capable of
redistributing current passing through the plurality of second SLS
to compensate differential voltage drop between the plurality of
first SLS and the plurality of second SLS.
3. The device of claim 2, wherein the plurality of first SLSs is an
array of light-emitting diode ("LED") chips.
4. The device of claim 2, wherein the plurality of first SLSs
includes at least one light-emitting diode ("LED") chip having
multiple junctions.
5. The device of claim 1, wherein a plurality of first SLS
configured to convert electrical energy to first optical light
includes multiple light emitters able to generate blue light having
a range of wavelength from 400 nm to 500 nm.
6. The device of claim 5, wherein the plurality of first SLSs
includes blue light-emitting diodes ("LEDs") having phosphor
conversion able to convert a portion of blue photons into photons
in yellow or green spectral region.
7. The device of claim 6, wherein the phosphor is deposited in
proximity of the blue LED chips;
8. The device of claim 6, wherein the phosphor is located remotely
from the blue LED chips.
9. The device of claim 5, wherein a plurality of second SLS
configured to convert electrical energy to second optical light
includes multiple light emitters capable of generating light in the
red or orange wavelength region having a range from 580 nm to 700
nm.
10. The device of claim 2, wherein the first balancing resistor
element coupled to the plurality of first SLSs and configured to
provide load balance between the plurality of first SLSs and the
plurality of second SLSs further includes a passive resistor
element having a predefined value based on physical properties of
blue light-emitting diodes ("LEDs") and red LEDs wherein combined
blue to red light flux generated by blue LEDs and red LEDs together
with phosphor conversion generate a predefined desirable color
point.
11. A light-emitting device, comprising: a string of blue
light-emitting diodes ("LEDs") having multiple junctions able to
convert electrical energy to blue/cyan light (400-500 nm), wherein
the blue LEDs is connected in series; and a string of red LEDs
having multiple junctions configured to convert electrical energy
to red/orange (580-700 nm) light, wherein the red LEDs are
connected in series, wherein the string of blue LEDs and the string
of red LEDs are coupled in parallel allowing current redistribution
in response to fluctuation of junction temperature for compensating
differential voltage drop between the string of blue LEDs and the
string of red LEDs.
12. The device of claim 11, further comprising a first balancing
resistor element coupled to the string of blue LEDs for load
balance and current redistribution.
13. The device of claim 12, further comprising a second balancing
resistor element coupled to the string of red LEDs for load balance
and current redistribution.
14. The device of claim 11, further comprising a first balancing
resistor element coupled to the string of blue LEDs for load
balance and current redistribution.
15. The device of claim 11, wherein the string of blue LEDs is able
to convert electrical energy to blue light having a range of
wavelength from 400 nm to 500 nm.
16. The device of claim 15, wherein the blue LEDs have phosphor
conversion to convert a portion of the blue photons into photons in
yellow or green spectral region; wherein the phosphor is deposited
in proximity of the blue LED chips; and wherein the phosphor is
located remotely from the LED chips.
17. The device of claim 15, wherein the string of red LEDs is able
to convert electrical energy to red light having a range of
wavelength between 580 nm and 700 nm.
18. The device of claim 12, wherein the first balancing resistor
element includes a passive resistor having a predefined value based
on physical properties of blue LEDs and red LEDs wherein combined
blue to red light flux ratio together with phosphor conversion
generate a redefined desirable color point.
19. A light-emitting device, comprising: a first string of blue
light-emitting diodes ("LEDs") having multiple junctions able to
convert electrical energy to blue light, wherein the blue LEDs are
connected in series; and a second string of red LEDs having
multiple junctions configured to convert electrical energy to red
light, wherein the red LEDs are connected in series, wherein the
string of blue LEDs and the string of red LEDs are coupled in
series; a third string of red LEDs coupled to a shunt control
device in parallel, wherein the first string, the second string,
and the third string including a shunt control are coupled in
series, wherein the shunt control device is configured to control
current passing through the third red LED string in response to
fluctuation of junction temperature to maintain an overall blue to
red light ratio within a predefined range of color target.
20. The device of claim 19, wherein the blue LEDs are able to
convert electrical energy to blue light having a range of
wavelength from 400 nm to 500 nm.
21. The device of claim 20, wherein the blue LEDs have phosphor
conversion to convert a portion of blue photons to photons in
yellow or green spectral region; wherein the phosphor is deposited
in proximity of blue LED chips; and wherein the phosphor is located
remotely from the LED chips.
22. The device of claim 19, the shunt control allows a higher
current flowing through the third LED string at higher operating
temperature and allows a smaller current flowing through the third
red LED string at lower operating temperature.
23. A method for generating light from a solid-state light emitting
device, comprising: facilitating a first current traveling from a
first junction of a blue light-emitting diode ("LED") to a second
junction of the blue LED; converting the first current to blue
optical photons by the first junction and the second junction of
the blue LED; and adjusting magnitude of the first current
automatically when voltage across each junction of the blue LED
changes in response to temperature of the junctions of the blue
LED.
24. The method of claim 23, further comprising: facilitating a
second electric current traveling from a first junction of a red
LED to a second junction of the red LED; converting the second
electric current to red optical photons by the first junction and
the second junction of the red LED; and adjusting magnitude of the
second current automatically when voltage across each junction of
the red LED changes in response to temperature of the junctions of
the red LED.
25. The method of claim 24, wherein adjusting magnitude of the
second current automatically when voltage across each junction of
the red LED changes in response to temperature of the junctions of
the red LED includes redistributing a portion of the first electric
current to the second electric current from a parallel connection
of the blue LED and the red LED in response to voltage drop across
the first junction and the second junction of the red LED.
Description
FIELD
[0001] The exemplary aspect(s) of the present invention relates to
solid-state lighting devices. More specifically, the aspect(s) of
the present invention relates to light radiation emitted by a
solid-state light apparatus using light-emitting diode ("LED")
device.
BACKGROUND
[0002] With decades of technical advancements and breakthroughs in
the areas of semiconductor based solid-state light emitting
devices, Edison's incandescent light bulbs, which typically have
30% or less light efficiency, will soon be replaced with
energy-efficient light-emitting diodes ("LEDs"). A conventional LED
is small and energy efficient with good lifetime. Various
commercial applications of LEDs, such as homes, buildings, traffic
lights, and electronic billboards, have already placed in
service.
[0003] An LED is a semiconductor diode with a biased p-n junction
capable of emitting narrow-spectrum light or electroluminescence.
Color of emitted light typically depends on the composition of
material used in the device. Color variations for visible light are
usually defined by electromagnetic radiation or optical wavelengths
from approximately 400 nm (nanometer) to 700 nm.
[0004] To generate white light, a conventional approach is to
combine multiple emission wavelengths of LED sources (e.g., red,
green, and blue LEDs) to produce desirable white light with various
correlated color temperature (CCT). Alternatively, LEDs may be
combined and/or added with luminescent material such as phosphors
to convert at least a portion of LED emitted light to longer
wavelength emissions to achieve a combined emission spectrum with
white light of various CCT.
[0005] The quality of a white light source on color appearance of
objects is usually measured by the Color Rendering Index ("CRI"),
wherein the highest CRI rating is 100 when the alternative light
source closely mimics the radiation spectral distribution of
incandescent object at the same color temperature. Typical cool
white fluorescent lamps, for example, have a CRI of 62. On the
other hand, lamps having multiple component rare-earth phosphors
can be constructed with CRI of 80 or better.
[0006] To enhance color appearance, a conventional approach is to
mix blue LED(s), red LED(s) and some luminescent material that
responds to blue emission in a package to generate warm or neutral
white light. A problem associated with placing blue LED(s) and red
(or orange) LED(s) in a same package is the color shift which is
due to different temperature-dependent radiant (or luminous) flux
drop as temperature drops. A factor that causes color shift is that
the radiant (or luminous) flux drop between blue LED and red LED is
different as operating temperature rises. For example, conventional
indium gallium nitride ("InGaN") based blue LED chip and aluminum
gallium indium phosphide ("AlInGaP") based red LED chip typically
have different rate for flux drop in response to temperature
change.
SUMMARY
[0007] Aspect(s) of present invention discloses a solid-state
lighting device ("SLD") capable of generating warm white light
using red solid-state light emitter and passive color control
scheme. The SLD includes first solid-state light-emitting
structures ("SLSs"), second SLSs, and balancing resistors. In one
example, the first SLSs are blue light-emitting diodes ("LEDs") and
the second SLSs are red LEDs. The blue LED, for example, is able to
convert electrical energy to blue optical light and the red LED is
capable of converting electrical energy to red optical light. To
convert blue light into the light with longer wavelength such as in
the yellow or red region(s), luminescent materials such as phosphor
material can be added. The first SLSs are connected in series as a
string and the second SLSs are connected in series as a second
string. The string of blue LEDs and string of red LEDs, in one
aspect, are coupled in parallel. A balancing resistor coupled to a
string is configured to provide load balance and current
redistribution between the strings in response to temperature
fluctuation.
[0008] It is understood that other aspects of the present invention
will become readily apparent to those skilled in the art from the
following detailed description, wherein it is shown and described
only exemplary configurations of an LED by way of illustration. As
will be realized, the present invention includes other and
different aspects and its several details are able to be modified
in various other respects, all without departing from the spirit
and scope of the present invention. Accordingly, the drawings and
the detailed description are to be regarded as illustrative in
nature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The exemplary aspect(s) of the present invention will be
understood more fully from the detailed description given below and
from the accompanying drawings of various aspects of the invention,
which, however, should not be taken to limit the invention to the
specific aspects, but are for explanation and understanding
only.
[0010] FIG. 1 is a diagram illustrating a solid-state lighting
device ("SLD") having a string of blue LEDs, a string of red LEDs,
and/or luminescent material capable of adjusting current flow in
response to temperature change in accordance with one aspect of the
present invention;
[0011] FIGS. 2-3 are charts illustrating relationships between
radiant flux versus temperature in connection to red LED and blue
LED;
[0012] FIGS. 4-6 illustrate a self-adjustment process using
electrical characteristics and/or relationship between voltage,
current, and resistance to passively redistribute current in
response to temperature variation in accordance with one aspect of
the present invention;
[0013] FIG. 7 is a diagram with a chart showing an exemplary layout
of SLD capable of adjusting current flow based on operating
temperature in accordance with one aspect of the present
invention;
[0014] FIG. 8 is a diagram illustrating an alternative
configuration of a SLD capable of outputting a warm white light
having a color temperature within a predefined range in accordance
with one aspect of the present invention;
[0015] FIG. 9 is a flowchart illustrating a process of generating
warm light using blue and red LEDs and redistributing current to
compensate flux loss based on temperature variation in accordance
with one aspect of the present invention;
[0016] FIG. 10 is a conceptual cross-sectional view illustrating an
exemplary fabrication process of an LED or LED devices;
[0017] FIG. 11 is a conceptual cross-sectional view illustrating an
example of an LED with a phosphor layer;
[0018] FIG. 12A is a conceptual top view illustrating an example of
an LED array using a combination of blue and red LEDs in accordance
with one aspect of the present invention;
[0019] FIG. 12B is a conceptual cross-sectional view of the LED
array of FIG. 12A;
[0020] FIG. 13A is a conceptual top view illustrating an example of
an alternative configuration of an LED array that can be used with
flexible LED connections in accordance with one aspect of the
present invention;
[0021] FIG. 13B is a conceptual cross-sectional view of the LED
array of FIG. 13A; and
[0022] FIG. 14 shows exemplary lighting devices including LED
devices using blue and red LEDs in accordance with one aspect of
the present invention.
DETAILED DESCRIPTION
[0023] Aspects of the present invention are described herein in the
context of a method, device, and apparatus of solid-state lighting
device capable of generating warm white light using a set of blue
and red light-emitting diodes ("LEDs").
[0024] The present invention is described more fully hereinafter
with reference to the accompanying drawings, in which various
aspects of the present invention are shown. This invention,
however, may be embodied in many different forms and should not be
construed as limited to the various aspects of the present
invention presented throughout this disclosure. Rather, these
aspects are provided so that this disclosure is thorough and
complete, and fully conveys the scope of the present invention to
those skilled in the art. The various aspects of the present
invention illustrated in the drawings may not be drawn to scale.
Rather, the dimensions of the various features may be expanded or
reduced for clarity. In addition, some of the drawings may be
simplified for clarity. Thus, the drawings may not depict all of
the components of a given apparatus (e.g., device) or method.
[0025] It will be understood that when an element such as a region,
layer, section, substrate, or the like, is referred to as being
"on" another element, it can be directly on the other element or
intervening elements may also be present. In contrast, when an
element is referred to as being "directly on" another element,
there are no intervening elements present. It will be further
understood that when an element is referred to as being "formed" on
another element, it can be grown, deposited, etched, attached,
connected, coupled, or otherwise prepared or fabricated on the
other element or an intervening element.
[0026] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skills in the art to which this
invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and this disclosure.
[0027] Various aspects of an LED luminaire will be presented.
However, as those skilled in the art will readily understand, these
aspects of invention may be extended to aspects of LED luminaries
without departing from the invention. The LED luminaire may be
configured as a direct replacement for conventional luminaries,
including, by way of example, recessed lights, surface-mounted
lights, pendant lights, sconces, cove lights, track lighting,
under-cabinet lights, landscape or outdoor lights, flood lights,
search lights, street lights, strobe lights, bay lights, strip
lights, industrial lights, emergency lights, balanced arm lamps,
accent lights, background lights, and other light fixtures.
[0028] As used herein, the term "light fixture" shell mean the
outer shell or housing of a luminaire. The term "luminaire" shell
mean a light fixture complete with a light source and other
components (e.g., a fan for cooling the light source, a reflector
for directing the light, etc.), if required. The term "LED
luminaire" shall mean a luminaire with a light source comprising
one or more LEDs. LEDs are well known in the art, and therefore,
will only briefly be discussed to provide a complete description of
the invention.
[0029] An aspect of present invention discloses a solid-state
lighting device ("SLD") having solid-state light-emitting
structures ("SLSs") and balancing resistor(s) capable of generating
warm white light using passive color control scheme. The SLS, for
example, can also be referred to as an LED, LED die, LED chip, LED
junction, or the like. The terms "LED," "LED die," "LED chip,"
and/or "LED junction" are herein used interchangeable. The SLD
includes a string of blue LEDs connected in series, a string of red
LEDs connected in series, and at least one balancing resistor. The
string of blue LEDs and string of red LEDs, in one aspect, are
coupled in parallel. The balancing resistor is configured to
provide load balancing between the strings. SLD is able to maintain
warm white light using automatic current redistribution between the
string of blue LEDs and string of red LEDs in response to
temperature fluctuation.
[0030] FIG. 1 is a diagram illustrating a SLD 100 having two
strings of LEDs capable of adjusting magnitude of current in
response to temperature change in accordance with one aspect of the
present invention. SLD 100 includes a blue string 102 having
multiple blue LEDs, a red string 104 having multiple red LEDs, and
two balancing resistors 110-112 (or R1 and R2). R1 is coupled to
blue string 102 in series and R2 is coupled to red string 104 in
series. Strings 102-104 together with R1 and R2 are connected in
parallel as shown in FIG. 1. SLD 100 is capable of maintaining warm
white light within a predefined range of color point using a
passive color control scheme able to facilitate redistributing
current based on temperature fluctuation. In one example, the
predefined range of color point may have a color temperature
ranging between CRI of 80 and CRI of 90. It should be noted that
the underlying concept of the exemplary aspect(s) of the present
invention would not change if one or more elements (or devices)
were added to or removed from SLD 100.
[0031] String of blue LEDs or blue string 102, in one aspect,
includes multiple LED dies such as LEDs 106 and 116 connected in
series. LED dies such as LEDs 106 and 116 may be fabricated by
InGaN material that allows LED dies to emit blue light. It should
be noted that the underlying concept does not change if LED dies in
blue string 102 are fabricated by other chemical compounds as long
as they emit blue light efficiently. Each LED die such as LED 106
contains a positive terminal and a negative terminal. The positive
terminal of LED 106, for example, is coupled to the negative
terminal of LED 126 and the negative terminal of LED 106 is coupled
to the positive terminal of LED 116. When all LED dies in blue
string 102 are linked in series as a string, blue string 102
includes a positive end and a negative end. When a current such as
IB 122 travels from the positive end of blue string 102 to the
negative end of blue string 102, LED dies in string 102 such as LED
dies 106 and 116 emit blue light.
[0032] String of red LEDs or red string 104 includes multiple LED
dies such as LEDs 108 and 118 connected in series. LED dies such as
LEDs 108 and 118 may be fabricated by AlInGaP material that allows
the LEDs to emit red light. It should be noted that the underlying
concept does not change if LED dies in red string 104 are
fabricated materials other than AlInGaP as long as they emit red
light efficiently. Each LED die such as LED die 108 contains a
positive terminal and a negative terminal. The positive terminal of
LED die 108, for example, is coupled to the negative terminal of
LED 128 and the negative terminal of LED 108 is coupled to the
positive terminal of LED 118. When all LED dies in red string 104
are linked in series as a string, string of red LEDs or red string
104 has a positive end and a negative end. When a current such as
IR 120 travels from the positive end of string 104 to the negative
end of string 104, LED dies in string 104 such as LED dies 108 and
118 emit red light or red flux.
[0033] Blue string 102, in an alternative aspect, contains n number
of blue LED junctions and red string 104 contains m number of red
LED junctions, where n and m are integers. The blue LED junctions
are connected in series and the red LED junctions are also coupled
in series. Strings 102-104 are coupled in parallel as shown in FIG.
1. The total number of blue LEDs (n) and red LEDs (m) are arranged
so that the voltages across both strings 102-104 should be
approximately the same.
[0034] Balancing resistors 110-112 or R1-R2, in one aspect, are
coupled to strings 102-104 in series and are used to provide load
balancing for strings 102-104. Depending on characteristics of the
red and blue LED dies, R1 and R2 may or may not have the same
resistance. In other examples, one or both R1-R2 may be removed if
blue string 102 and red string 104 are substantially or
approximately balanced.
[0035] During operation, when current 130 enters SLD 100, it splits
into IR 120 as a first current flow and IB 122 as a second current
flow wherein IR 120 and IB 122 travel through strings 104 and 102,
respectively. When IB 122 passes through string 102, blue LEDs such
as LED 106 emits blue light or flux. Similarly, when IR 120 travels
through string 104, red LEDs such as LED 108 emits red light or
flux. After IB 122 and IR 120 pass through R1 and R2, they merge
into current 132 before exiting SLD 100. It should be noted that
current 130 and 132, in one aspect, should have substantially the
same magnitude. In one example, yellow or green phosphor such as
yttrium aluminum garnet ("YAG") is used with blue string 102 to
produce cool or white light. Depending on numbers of blue LEDs, red
LEDs, and amount of phosphor are used, a warm white light or a
predefined range of color point may be maintained.
[0036] To adjust or maintain a predefined range of color point, SLD
100, in one aspect, is capable of redistribute current between
strings 102-104 based on differential voltage drop ("Vf") across
strings due to fluctuations of operating temperature. The operating
temperature, in one example, is referred to as device physical
temperature during operation. For example, when the operating
temperature rises from room temperature such as 20 Celsius
(".degree. C.") to 80.degree. C., the red radiant flux generated by
red LEDs drops more rapidly than the blue radiant flux generated by
blue LEDs. To maintain voltage across strings 102-104, the current
flowing through strings 102-104 may be automatically adjusted which
effectively compensates the loss of red flux. For example, when Vf
at red string 104 drops more than Vf at blue string 102, IR 120 is
automatically adjusted by increasing the magnitude of IR 120. The
red LEDs such LED 108 illuminates more red flux in response to IR
120 with large magnitude. Because of the additional red flux
generated by red string 104, the predefined range of color point is
maintained.
[0037] It should be noted that SLD 100 as a passive system is
capable of packaging one or more strings of multi-junction blue
LEDs connected with one or more strings of multi-junction red LEDs
in parallel. The blue and red strings 102-104 are driven in
parallel with current flows. With differential Vf drops (or
reductions) between the blue LED chips and red LED chips, SLD 100
is able to implement a passive color control scheme based on Vf
drops redistribute current between blue and red strings 102-104 to
mitigate overall color shift.
[0038] An advantage of using SLD 100 is that the lighting device is
able to maintain warm white light within a predefined range of
color point using electrical characteristics of circuits. Another
advantage of using red LEDs in SLD 100 is to minimize photon
conversion loss (or Stotes loss).
[0039] FIG. 2 is a chart 200 illustrating a relationship between
radiant flux and stage temperature for different LEDs such as red
LEDs and blue LEDs. Chart 200 shows the X-axis and Y-axis wherein
X-axis represents relative radiant flux 202 and Y-axis represents
stage temperature 204. Chart 200 illustrates a blue curve 206 and a
red curve 208 with respect to X-axis and Y-axis. Blue curve 206, in
one example, shows behavior of blue flux generated by a blue LED
based on temperature fluctuations. Similarly, red curve 208 shows
behavior of red flux generated by a red LED based on changes in
temperature changes.
[0040] Blue curve 206 and red curve 208 illustrate that the
emission of red flux is more sensitive to temperature change than
the emission of blue flux. For example, when the blue LED and red
LED are initially activated at the room temperature around
20.degree. C., both red flux and blue flux are emitted at their
predefined calibrated setting. When blue and red LEDs are gradually
heating up, the amount of blue and red flux begins to drop as shown
in Chart 200. When, for example, the operating temperature reaches
80.degree. C. for both red and blue LEDs, the blue flux could drop
10 to 13% of the original setting while the red flux can drop up to
30% of the original setting. In other words, the red flux could
drop or reduce 17% more flux than the blue flux whereby color shift
of combined light emitted can easily occur.
[0041] Both blue flux and red flux are dropped or reduced as a
function of temperature, but the amount the reduction of flux
between the blue color and red color is different. Since the red
LED is more sensitive to temperature change, a self correction or
passive color control scheme is needed. It should be noted that the
difference of flux reduction between red flux and blue flux could
be even greater if the operating temperature rises above
100.degree. C.
[0042] FIG. 3 is a graph 300 showing an envelope of chromaticity
diagram 302 with a blackbody curve 304 illustrating a spectrum of
light color in connection to red LED and blue LED. Envelope of
chromaticity diagram 302 illustrates a region of radiant wavelength
region that is visible to human eyes. Graph 300 includes a green
phosphor line 306, yellow phosphor line 308, and red flux 310.
Depending on the specific amount of green phosphor, yellow
phosphor, and red flux, a white light can be generated around a
warm white region 312 which is generally referred to as blackbody
radiation at 3000 Kelvin (K). Since green or yellow color is
supplied by blue LEDs and red flux is supplied by red LEDs, the
color shift can happen when the red flux drops due to the rising
temperature.
[0043] When red and blue LEDs are placed or fabricated in the same
package, the overall color shift of light emission in response to
temperature change can occur because the performance of red LED
will fluctuate more than the blue LED. For example, as operating
temperature rises, the overall light color may gradually change
cool white light because of less red flux. As such, to maintain a
range of color point from shifting, a passive scheme of self
compensation, self correction, and/or self calibrating using
electrical property and/or device characteristics to automatically
increase the red flux.
[0044] FIGS. 4-6 illustrate a self-adjustment process using
electrical characteristics and/or relationship between voltage,
current, and resistance (Ohm's law) to passively redistribute
current in response to temperature variation in accordance with one
aspect of the present invention. FIG. 4, which is similar to FIG.
1, illustrates a SLD 400 having a string of blue LEDs 102, a string
of red LEDs 104, and two balancing resistors 110-112 (or R1 and
R2). R1 is coupled to string of blue LEDs 102 in series and R2 is
coupled to string of red LED 104 in series. Strings of blue and red
LEDs together with R1 and R2 are connected in parallel as shown in
FIG. 4. SLD 400 is capable of maintaining a warm white light within
a predefined range of color point (or color temperature) using a
passive flux control scheme by redistributing current between
strings 102-104 based on temperature fluctuation.
[0045] During operation, when current 416 enters SLD 400 which has
just been activated with an operating temperature at room
temperature such as 20.degree. C., current 416 subsequently splits
into IR 420 and IB 418 wherein IR 420 and IB 418 travel through
strings 104 and 102, respectively. String 102 emits blue light as
IB 418 passes through each blue LED die while string 104 emits red
light as IR 420 travels through each red LED dies. After IB 418 and
IR 420 pass through R1 and R2, they merge into current I 422 before
exiting SLD 400. In one example, yellow or green phosphor such as
YAG may be used with blue string 102 to produce cool or white
light. Depending on the number of blue LEDs, red LEDs, and amount
of phosphor used, a warm white light within a predefined range of
color temperature may be achieved and maintained.
[0046] As SLD 400 continues to generate warm white light, the
operating temperature for every LED die begins to rise. Since the
LED dies are generally sensitive to temperature fluctuation, the
blue flux emitted by blue LEDs in string 102 and red flux emitted
by red LEDs in string 104 begin to change. The loss of red flux is
generally greater than the loss of blue flux since red LEDs are
more temperature sensitivity than blue LEDs. As such, additional
red flux is needed if the warm white light is to be maintained.
[0047] When, for example, the operating temperature reaches
60.degree. C. as shown in FIG. 5, SLD 550 incrementally
redistributes current to increase magnitude of IR 520 whereby red
LEDs in string 104 can emit more red flux in response to the larger
current of IR 520. To maintain a predefined range of color point,
SLD 550 increases magnitude of IR 520 to obtain additional red flux
whereby the loss of red flux due to rising temperature can be
properly compensated if a corresponding amount of IR 520 is
supplied.
[0048] When operating temperature reaches 80.degree. C. as shown in
FIG. 6, SLD 650, in one example, has reached a steady state of
operating temperature. It should be noted that overall resistance R
across strings 102-104 generally has minimal change due to
temperature rising. To maintain overall voltage V across strings
102-104 within a predefined voltage range, the current such as IB
618 and IR 620 is redistributed, recalibrated, and/or self-adjusted
based on electrical characteristics of electrical components such
as red LEDs and blue LEDs. Since Vf across each red LED die drops
more than Vf across each blue LED die, a larger current or IR 620
can mitigate Vf drops across string 104. With a larger magnitude of
IR 620, additional red flux is generated accordingly. As such, a
passive scheme of color control between blue LEDs and red LEDs can
reduce overall color shift.
[0049] FIG. 7 is a diagram with a chart showing an exemplary layout
of a SLD 750 capable of adjusting current flow based on operating
temperature in accordance with one aspect of the present invention.
SLD 750 includes two strings of blue LEDs 752, one string of red
LEDs 754, and two balancing resistors R1 and R2. R1 is coupled to
both strings of blue LEDs 752 in series and R2 is coupled to string
of red LED 754 in series. Strings of blue and red LEDs are
connected in parallel as shown in FIG. 7. SLD 750 is able to
maintain an output of warm white light within a predefined range of
color temperature using a passive color control scheme. The passive
color control scheme, in one aspect, is configured in accordance
with electric characteristics between voltage, resistance, and
current to passively redistribute current between strings 752-754
based on temperature fluctuation.
[0050] Chart 760 illustrates various exemplary calculations
associated with SLD 750. For instance, chart 760 shows 2.14 volt
("V") across each red LED junction and 3.15 V across each blue LED
junction as indicated by numeral 762. Chart 760 also illustrates
that SLD 750 include two blue dies and one red die wherein each
blue die includes 16 blue LED junctions while each red die includes
24 LED junctions as indicated by numeral 764. At the room
temperature, blue string of LED junctions has total Vf of 50.4 V
(16.times.3.15) while red string of LED junctions has total Vf of
51.36 V (24.times.2.14) as indicated by numeral 766. As can be
seen, the voltages across blue and red string of LED junctions are
approximately same or same. Resistor heating 768 of chart 760 shows
that change of resistances due to temperature fluctuation is
minimal.
[0051] SLD 750, in one example, is able to provide a warm white
light by mixing red flux, blue flux, and yellow flux wherein the
blue and yellow/green flux are supplied by the blue LED junctions
while the red flux is supplied by the red LED junctions. With
implementation of passive color control scheme, SLD 750 is able to
output a range of warm white light by redistributing current
passing through strings of red and blue LED junctions 752-754. From
the data indicated in chart 760, the implementation of passive
color control scheme can be readily achieved.
[0052] FIG. 8 is a diagram illustrating an alternative
configuration of a SLD 850 capable of outputting a warm white light
having a color temperature within a predefined range in accordance
with one aspect of the present invention. SLD 850 includes a string
of blue LED 852, a string of red LED 854, a shunt controller 856,
and a group of shunt red LED 858. Note that the group of shunt red
LED 858 contains at least one red LED. It should be noted that the
underlying concept of the exemplary aspect(s) of the present
invention would not change if one or more elements (or devices)
were added to or removed from SLD 850.
[0053] During operation, when a current I begins to flow from
strings 850-854 to shunt controller 865 bypassing shunt red LED
dies 858 via current 860, string 852 emits blue flux by various
blue LED and string 854 emits red flux by various red LED. It
should be noted that no current flows through shunt red LED 585
when the operating temperature is at room temperature. If no
current flows through shunt red LED 585, it does not emit any
flux.
[0054] As the operating temperature of LEDs gradually rises, the
blue flux may drop approximately 10-15% while the red flux may drop
around 25-30% since the red LED is more sensitive to temperature
fluctuation. When shunt controller 856 detects that the loss of red
flux is more than the loss of blue flux, shunt controller 856
begins to redistribute current by redirecting a portion of current
860 to current 862. When current 862 passes through shunt red LED
858, it begins to emit red flux which attempt to compensate the
loss of red flux due to temperature fluctuation. When the operating
temperature reaches at a steady state such as 80.degree. C., shunt
controller 856, in one aspect, redirects all current from current
860 to current 862 whereby shunt red LED 858 is fully activated or
illuminated.
[0055] FIG. 9 is a flowchart 950 illustrating a process of
generating warm white light using a set of blue and red LEDs and is
capable of passively compensating the loss of red flux based on
temperature variation in accordance with one aspect of the present
invention. The process, at block 952, is able to facilitate a first
current traveling from a first junction of a blue LED to a second
junction of the blue LED. After converting the first current to
blue optical photons by the first and second junctions of blue LED
at block 954, the process, at block 956, automatically adjusts
magnitude of first current when voltage across junction of blue LED
changes in response to operating temperature of blue LED.
[0056] At block 958, after facilitating a second current traveling
from first junction of red LED to second junction of red LED, the
second current, at block 960, is converted to red optical photons
by the first and second junctions of red LED. The process, at block
962, is able to automatically adjust magnitude of second current
when voltage across each junction of red LED changes in response to
operating temperature of red LED. The process, in one aspect, is
able to redistribute at least a portion of the first current to the
second current via a parallel connection in response to voltage
drop across the first and second junctions of red LED.
[0057] Having briefly described aspects of SLD capable of
generating a warm white light using a combination of blue and red
LEDs in which the aspect of present invention operates, the
following figures illustrate exemplary process and/or method to
fabricate and package LED dies, chips, device, and/or fixtures.
[0058] FIG. 10 is a conceptual cross-sectional view illustrating an
exemplary fabrication process of an LED, LED die, or LED device. An
LED is a semiconductor material impregnated, or doped, with
impurities. These impurities add "electrons" or "holes" to the
semiconductor, which can move in the material relatively freely.
Depending on the kind of impurity, a doped region of the
semiconductor can have predominantly electrons or holes, and is
referred respectively as n-type or p-type semiconductor regions.
Referring to FIG. 10, LED 500 includes an n-type semiconductor
region 504 and a p-type semiconductor region 508. A reverse
electric field is created at the junction between the two regions,
which cause the electrons and holes to move away from the junction
to form an active region 506. When a forward voltage sufficient to
overcome the reverse electric field is applied across the p-n
junction through a pair of electrodes 510, 512, electrons and holes
are forced into the active region 506 and recombine. When electrons
recombine with holes, they fall to lower energy levels and release
energy in the form of light.
[0059] In this example, the n-type semiconductor region 504 is
formed on a substrate 502 and the p-type semiconductor region 508
is formed on the active layer 506, however, the regions may be
reversed. That is, the p-type semiconductor region 508 may be
formed on the substrate 502 and the n-type semiconductor region 504
may formed on the active layer 506. As those skilled in the art
will readily appreciate, the various concepts described throughout
this disclosure may be extended to any suitable layered structure.
Additional layers or regions (not shown) may also be included in
the LED 500, including but not limited to buffer, nucleation,
contact and current spreading layers or regions, as well as light
extraction layers.
[0060] The p-type semiconductor region 508 is exposed at the top
surface, and therefore, the p-type electrode 512 may be readily
formed thereon. However, the n-type semiconductor region 504 is
buried beneath the p-type semiconductor layer 508 and the active
layer 506. Accordingly, to form the n-type electrode 510 on the
n-type semiconductor region 504, a cutout area or "mesa" is formed
by removing a portion of the active layer 506 and the p-type
semiconductor region 508 by means well known in the art to expose
the n-type semiconductor layer 504 there beneath. After this
portion is removed, the n-type electrode 510 may be formed.
[0061] FIG. 11 is a conceptual cross-sectional view illustrating an
example of an LED with a phosphor layer. In this example, a
phosphor layer 602 is formed on the top surface of the LED 500 by
means well known in the art. The phosphor layer 602 converts a
portion of the light emitted by the LED 500 to light having a
different spectrum. A white LED light source can be constructed by
using an LED that emits light in the blue region of the spectrum
and a phosphor that converts blue light to yellow light. A white
light source is well suited as a replacement lamp for conventional
luminaries; however, the invention may be practiced with other LED
and phosphor combinations to produce different color lights. The
phosphor layer 602 may include, by way of example, phosphor
particles suspended in a carrier or be constructed from a soluble
phosphor that is dissolved in the carrier.
[0062] In a configuration of LED luminaries, an LED array may be
used to provide increased luminance. FIG. 12A is a conceptual top
view illustrating an example of an LED array, and FIG. 12B is a
conceptual cross-sectional view of the LED array of FIG. 12A. In
this example, a number of phosphor-coated LEDs 600 may be formed on
a substrate 702. The bond wires (not shown) extending from the LEDs
600 may be connected to traces (not shown) on the surface of the
substrate 702, which connect the LEDs 600 in a parallel and/or
series fashion. In some aspects, the LEDs 600 may be connected in
parallel streams of series LEDs with a current limiting resistor
(not shown) in each stream. The substrate 702 may be any suitable
material that can provide support to the LEDs 600 and can be
mounted within a light fixture (not shown).
[0063] FIG. 13A is a conceptual top view illustrating an example of
an alternative configuration of an LED array, and FIG. 13B is a
conceptual cross-sectional view of the LED array of FIG. 13A. In a
manner similar to that described in connection with FIGS. 12A and
12B, a substrate 702 designed for mounting in a light fixture (not
shown) may be used to support an array of LEDs 500. However, in
this configuration, a phosphor layer is not formed on each
individual LED. Instead, phosphor 806 is deposited within a cavity
802 bounded by an annular ring 804 that extends circumferentially
around the outer surface of the substrate 702. The annular ring 804
may be formed by boring a cylindrical hole in a material that forms
the substrate 702. Alternatively, the substrate 702 and the annular
ring 804 may be formed with a suitable mold, or the annular ring
804 may be formed separately from the substrate 702 and attached to
the substrate using an adhesive or other suitable means. In the
latter configuration, the annular ring 804 is generally attached to
the substrate 702 before the LEDs 500, however, in some
configurations, the LEDs may be attached first. Once the LEDs 500
and the annular ring 804 are attached to the substrate 702, a
suspension of phosphor particles in a carrier may be introduced
into the cavity 802. The carrier material may be an epoxy or
silicone; however, carriers based on other materials may also be
used. The carrier material may be cured to produce a solid material
in which the phosphor particles are immobilized.
[0064] FIG. 14 shows exemplary devices including blue and red LEDs
capable of providing a warm white light in accordance with aspects
of the present invention. The devices 900 include a lamp 902, an
illumination device 904, and a street light 906. Each of the
devices shown in FIG. 14 includes LEDs having blue LEDs and red
LEDs capable of providing passive color control scheme as described
herein. For example, lamp 902 includes a package 916 and an LED
908, in which LED 908 employs one or more metal traces to provide
flexible connections. Lamp 902 may be used for any type of general
illumination. For example, lamp 902 may be used in an automobile
headlamp, street light, overhead light, or in any other general
illumination application. Illumination device 904 includes a power
source 910 that is electrically coupled to a lamp 912, which may be
configured as lamp 902. In one aspect, power source 910 may be
batteries or any other suitable type of power source, such as a
solar cell. Street light 906 includes a power source connected to a
lamp 914, which may be configured as lamp 902. It should be noted
that aspects of the LED described herein are suitable for use with
virtually any type of LED assembly, which in turn may be used in
any type of illumination device and are not limited to the devices
shown in FIG. 14.
[0065] The various aspects of this disclosure are provided to
enable one of ordinary skills in the art to practice the present
invention. Various modifications to aspects presented throughout
this disclosure will be readily apparent to those skilled in the
art, and the concepts disclosed herein may be extended to other LED
lamp configurations regardless of the shape or diameter of the
glass enclosure and the base and the arrangement of electrical
contacts on the lamp. Thus, the claims are not intended to be
limited to the various aspects of this disclosure, but are to be
accorded the full scope consistent with the language of the claims.
All structural and functional equivalents to the elements of the
various aspects described throughout this disclosure that are known
or later come to be known to those of ordinary skills in the art
are expressly incorporated herein by reference and are intended to
be encompassed by the claims. Moreover, nothing disclosed herein is
intended to be dedicated to the public regardless of whether such
disclosure is explicitly recited in the claims. No claim element is
to be construed under the provisions of 35 U.S.C. .sctn.112, sixth
paragraph, unless the element is expressly recited using the phrase
"means for" or, in the case of a method claim, the element is
recited using the phrase "step for."
* * * * *