U.S. patent number 9,271,368 [Application Number 13/708,916] was granted by the patent office on 2016-02-23 for method and apparatus for providing a passive color control scheme using blue and red emitters.
This patent grant is currently assigned to Bridgelux, Inc.. The grantee listed for this patent is Tao Tong. Invention is credited to Tao Tong.
United States Patent |
9,271,368 |
Tong |
February 23, 2016 |
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.: |
13/708,916 |
Filed: |
December 7, 2012 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20140159612 A1 |
Jun 12, 2014 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
33/50 (20130101); H05B 45/10 (20200101); H01L
25/0753 (20130101); H05B 45/28 (20200101); F21K
9/64 (20160801); H05B 45/20 (20200101); H01L
2924/0002 (20130101); Y02B 20/30 (20130101); H01L
33/62 (20130101); H01L 2924/0002 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
H01L
27/15 (20060101); H05B 33/08 (20060101); H01L
25/075 (20060101); H01L 33/62 (20100101) |
Field of
Search: |
;315/297,309 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Owens; Douglas W
Assistant Examiner: Cho; James H
Attorney, Agent or Firm: Wu; James M. JW Law Group
Claims
What is claimed is:
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, the plurality of
first SLSs comprising multiple light emitters able to generate
light having a range of wavelength from 400 nm to 500 nm, the
plurality of first SLSs comprising light-emitting diodes ("LEDs")
with phosphor configured to convert a portion of blue photons into
photons in yellow or green spectral region; a plurality of second
SLSs configured to convert electrical energy to second optical
light, wherein the plurality of second SLSs is connected in series;
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 the first 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 first balancing resistor element; and a
second balancing resistor element coupled to the plurality of
second SLSs in series, wherein the second balancing resistor
element is capable of redistributing current passing through the
plurality of second SLSs to compensate differential voltage drop
between the plurality of first SLSs and the plurality of second
SLSs.
2. The device of claim 1, wherein the plurality of first SLSs is an
array of light-emitting diode ("LED") chips.
3. The device of claim 1, wherein the plurality of first SLSs
includes at least one light-emitting diode ("LED") chip having
multiple junctions.
4. The device of claim 1, wherein the phosphor is deposited in
proximity of the blue LED chips.
5. The device of claim 1, wherein the phosphor is located remotely
from the blue LED chips.
6. The device of claim 1, wherein a plurality of second SLSs
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.
7. The device of claim 1, 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.
8. 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; 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 the first
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 first balancing resistor element,
wherein the first balancing resistor element 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; and a second balancing resistor element
coupled to the plurality of second SLSs in series and capable of
redistributing current passing through the plurality of second SLSs
to compensate differential voltage drop between the plurality of
first SLSs and the plurality of second SLSs.
9. The device of claim 8, wherein the plurality of first SLSs is an
array of light-emitting diode ("LED") chips.
10. The device of claim 8, wherein the plurality of first SLSs
includes at least one light-emitting diode ("LED") chip having
multiple junctions.
11. The device of claim 8, wherein the plurality of first SLSs
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.
12. The device of claim 11, 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.
13. The device of claim 12, wherein the phosphor is deposited in
proximity of the blue LED chips.
14. The device of claim 12, wherein the phosphor is located
remotely from the blue LED chips.
15. The device of claim 11, wherein a plurality of second SLSs
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.
16. 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 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 first
balancing resistor element, wherein the first balancing resistor
element 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.
17. The device of claim 16, further comprising a second balancing
resistor element coupled to the plurality of second SLSs in series
and capable of redistributing current passing through the plurality
of second SLSs to compensate differential voltage drop between the
plurality of first SLSs and the plurality of second SLSs.
18. The device of claim 16, wherein the plurality of first SLSs is
an array of light-emitting diode ("LED") chips.
19. The device of claim 16, wherein the plurality of first SLSs
includes at least one light-emitting diode ("LED") chip having
multiple junctions.
20. The device of claim 16, wherein the plurality of first SLSs
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.
21. The device of claim 20, 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.
22. The device of claim 21, wherein the phosphor is deposited in
proximity of the blue LED chips.
23. The device of claim 22, wherein the phosphor is located
remotely from the blue LED chips.
24. The device of claim 20, wherein a plurality of second SLSs
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.
Description
FIELD
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
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.
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.
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.
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.
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
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.
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
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.
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;
FIGS. 2-3 are charts illustrating relationships between radiant
flux versus temperature in connection to red LED and blue LED;
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;
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;
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;
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;
FIG. 10 is a conceptual cross-sectional view illustrating an
exemplary fabrication process of an LED or LED devices;
FIG. 11 is a conceptual cross-sectional view illustrating an
example of an LED with a phosphor layer;
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;
FIG. 12B is a conceptual cross-sectional view of the LED array of
FIG. 12A;
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;
FIG. 13B is a conceptual cross-sectional view of the LED array of
FIG. 13A; and
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
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").
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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."
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