U.S. patent application number 14/542192 was filed with the patent office on 2015-03-05 for led lamp using blue and cyan leds and a phosphor.
The applicant listed for this patent is Nthdegree Technologies Worldwide Inc.. Invention is credited to Reuben Rettke.
Application Number | 20150060907 14/542192 |
Document ID | / |
Family ID | 49234769 |
Filed Date | 2015-03-05 |
United States Patent
Application |
20150060907 |
Kind Code |
A1 |
Rettke; Reuben |
March 5, 2015 |
LED LAMP USING BLUE AND CYAN LEDS AND A PHOSPHOR
Abstract
Many thousands of micro-LEDs (e.g., 25 microns per side) are
deposited on a substrate. Some of the LEDs are formed to emit a
peak wavelength of 450 nm (blue), and some are formed to emit a
peak wavelength of 490 nm (cyan). A YAG (yellow) phosphor is then
deposited on the LEDs, or a remote YAG layer is used. YAG phosphor
is most efficiently excited at 450 nm and has a very weak emission
at 490 nm The two types of LEDs are GaN based and can be driven at
the same current. The ratio of the two types of LEDs is controlled
to achieve the desired overall color emission of the LED lamp. The
blue LEDs optimally excite the YAG phosphor to produce white light
having blue and yellow components, and the cyan LEDs broaden the
emission spectrum to increase the CRI of the lamp while improving
luminous efficiency. Other embodiments are described.
Inventors: |
Rettke; Reuben; (Phoenix,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nthdegree Technologies Worldwide Inc. |
Tempe |
AZ |
US |
|
|
Family ID: |
49234769 |
Appl. No.: |
14/542192 |
Filed: |
November 14, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13436613 |
Mar 30, 2012 |
8906713 |
|
|
14542192 |
|
|
|
|
Current U.S.
Class: |
257/89 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 33/502 20130101; H01L 2924/0002 20130101; H01L 2924/00
20130101; H01L 25/0753 20130101 |
Class at
Publication: |
257/89 |
International
Class: |
H01L 25/075 20060101
H01L025/075; H01L 33/50 20060101 H01L033/50 |
Claims
1. A lighting device comprising: a first light emitting diode (LED)
having a first peak emission wavelength and generating blue light;
a second LED having a second peak emission wavelength, longer than
the first peak emission wavelength, and generating a light greener
than the blue light; and a same wavelength-converting material over
the first LED and the second LED for converting at least the blue
light to longer wavelength light, wherein light generated by the
lighting device is a mixture of blue light from the first LED,
greener light from the second LED, and longer wavelengths of light
from the wavelength-converting material to create a white
light.
2. The device of claim 1 wherein the first LED emits blue light and
the second LED emits cyan light.
3. The device of claim 1 wherein the first peak emission wavelength
is about 450 nm and the second peak emission wavelength is about
490 nm.
4. The device of claim 1 wherein the first peak emission wavelength
and the second peak emission wavelength are between about 430-507
nm.
5. The device of claim 1 wherein the wavelength-converting material
comprises a YAG phosphor.
6. The device of claim 1 wherein the wavelength converting material
comprises a yellow phosphor.
7. The device of claim 1 further comprising: a first plurality of
LEDs emitting the first peak emission wavelength; a second
plurality of LEDs emitting the second peak emission wavelength; and
a third plurality of LEDs, including the first LED, emitting the
first peak emission wavelength, the third plurality of LEDs
energizing the wavelength-converting material, wherein a combined
emission from the wavelength-converting material and the third
plurality of LEDs produces white light.
8. The device of claim 7, wherein the first plurality of LEDs is
connected to a first current source, wherein the second plurality
of LEDs is connected to a second current source, wherein the third
plurality of LEDs is connected to a third current source, and
wherein the currents generated by the first current source, the
second current source, and the third current source are
controllable to create a desired emission from the device.
9. The device of claim 7 wherein the third plurality of LEDs
comprises a mixture of LEDs emitting the first peak emission
wavelength and the second peak emission wavelength, wherein the
mixture of LEDs are connected in parallel.
10. The device of claim 9, wherein the first plurality of LEDs is
connected to a first current source, wherein the second plurality
of LEDs is connected to a second current source, wherein the third
plurality of LEDs is connected to a third current source, and
wherein the currents generated by the first current source, the
second current source, and the third current source are
controllable to create a desired emission from the device.
11. The device of claim 1 further comprising: a first plurality of
LEDs, including the first LED, emitting the first peak emission
wavelength; and a second plurality of LEDs, including the second
LED, emitting the second peak emission wavelength, wherein the
first plurality of LEDs and the second plurality of LEDs are
distributed on a substrate and driven by at least one current
source, wherein a ratio of the first plurality of LEDs and the
second plurality of LEDs is selected to achieve a desired emission
from the device.
12. The device of claim 1 further comprising: a first plurality of
LEDs emitting the first peak emission wavelength; and a second
plurality of LEDs, including the second LED, emitting the second
peak emission wavelength, wherein the first plurality of LEDs and
the second plurality of LEDs are printed on a substrate as an LED
ink.
13. The device of claim 12 wherein the first plurality of LEDs
includes the first LED and energizes the wavelength-converting
material, and wherein the second plurality of LEDs also energizes
the wavelength-converting material.
14. The device of claim 1 further comprising: a first plurality of
LEDs emitting the first peak emission wavelength; and a second
plurality of LEDs, including the second LED, emitting the second
peak emission wavelength, wherein the first plurality of LEDs and
the second plurality of LEDs are printed on a substrate as an LED
ink, the LED ink comprising the first plurality of LEDs and second
plurality of LEDs mixed together in a solvent.
15. The device of claim 1 wherein the wavelength-converting
material is separated from the first LED and the second LED.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 13/436,613, filed on Mar. 30, 2012, incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] This invention relates to phosphor-converted (PC) light
emitting diode (LED) lamps that generate white light and, more
particularly, to a PC LED lamp that uses LEDs emitting at two
different peak wavelengths to improve the color rendering and
luminous efficacy of the lamp.
BACKGROUND
[0003] Commercially available LEDs typically emit a single color
(peak wavelength). Some experimental LED chips have been produced
that can produce multiple peak wavelengths. However, such LEDs are
fabricated using special techniques that add significant expense,
making them not yet commercially viable.
[0004] The most common white-light LED uses a 450 nm blue LED that
excites a relatively broad emission YAG:Ce phosphor deposited over
the LED. The peak emission wavelength of YAG is about 535-565 nm
(yellow-green). The blue light leaks through the YAG layer, and the
combination of the blue and yellow light (phosphor converted blue
light) creates white light. Such a di-chromatic light source
produces relatively poor color rendering, since the highest color
rendering is produced with a broad-spectrum source, such as the sun
or a radiating black body.
[0005] The color rendering index (CRI) is a quantitative measure of
the ability of a light source to faithfully reproduce the colors of
certain sample patches in comparison with an ideal or natural light
source, such as the sun, at the same color temperature. The sun has
a CRI of 100.
[0006] It is known to use a blue LED to excite a combination of
red, yellow, and green phosphors to produce a broader spectrum
having a high CRI. However, combining and depositing phosphors to
produce a consistent emitted color is difficult. Phosphors are also
relatively expensive and reduce the efficiency of the LED.
Additionally, the environmental impact of mining phosphors is
significant and growing due to the increased use of phosphors.
[0007] Another problem with phosphors is that the color cannot be
controlled once the phosphor is deposited over the LED.
[0008] It is known to combine red, green, and blue LEDs in a module
to produce white light, but each LED needs its own current source,
and the different LEDs react differently to heat. Accordingly, such
LED modules have better CRI but lack color stability and are
expensive.
[0009] What is needed is a technique for fabricating a white light
source that improves CRI, luminous efficiency, and color control
without the drawbacks discussed above.
SUMMARY
[0010] In one embodiment, thousands of micro-LEDs (e.g., 25 microns
per side) are deposited on a substrate, and electrical connections
are made using a transparent conductor layer. Some of the LEDs are
formed to emit a peak wavelength of 450 nm (blue), and some are
formed to emit a peak wavelength of 490 nm (cyan). The two types of
LEDs are GaN based and can be driven at the same current. The two
types of LEDs react similarly to heat. The ratio of the two types
of LEDs is controlled to achieve the desired overall color emission
of the LED lamp. In one embodiment, a yellow phosphor, such as YAG,
is then deposited on the LEDs, or a remote yellow phosphor layer is
used. YAG phosphor is most efficiently excited at 450 nm and has a
very weak emission at 490 nm. The blue LED efficiently excites the
yellow phosphor to produce white light having blue and yellow
components, and the cyan LED adds a green component to increase the
CRI of the light emission and luminous efficiency.
[0011] In one embodiment, a red phosphor is mixed with the YAG
phosphor or overlies the YAG phosphor to create a warmer white
light. If a red phosphor and yellow phosphor were used with
different peak excitation wavelengths, then controlling the
blue/cyan ratio or separately controlling the blue and cyan LEDs
with currents controls the relative red/yellow contributions of the
phosphors to the white light.
[0012] In another embodiment, the relatively broad emission YAG
phosphor is not used. Instead, a combination of red and non-YAG
yellow phosphors are deposited at least over the blue LEDs or used
remotely. The blue wavelength is optimized for the phosphor. The
cyan LEDs, which may not be covered by any phosphor, obviates the
need for a green phosphor to broaden the spectrum, improving the
efficiency of the lamp and improving color consistency.
[0013] This dual LED approach has been shown to improve luminous
efficiency and generate a higher CRI due to the broader spectrum
generated Improving the luminous efficiency is due to lower
phosphor usage and generating an emission that better conforms to
the luminous efficiency function of the human visual system.
[0014] In one embodiment, the micro-LEDs are combined together in a
liquid medium (as an LED ink) and screen printed, slot-die coated,
sprayed, or otherwise deposited onto a metal layer on a substrate.
The LED chips, in one embodiment, have a reflective metal (e.g.,
silver) electrode (an anode or cathode electrode) covering the
bottom surface of the chip and a distributed metal electrode on the
top surface of the chip to allow light to pass through the top
surface of the chip. The LEDs self-orient themselves as they sink
through the liquid medium so that the bottom electrode tends to
face the metal layer on the substrate. Since the large metal bottom
electrode has a density that is much greater than the GaN
semiconductor, the LEDs tend to orient with their bottom electrode
side down. Self-orientation may be accomplished in various other
ways.
[0015] The structure is then heated (or otherwise cured) to
evaporate the liquid medium (a solvent) and bond the silver
electrode to the metal layer on the substrate. In one example, the
metal layer on the substrate is an uncured aluminum layer,
deposited by, for example, slot-die coating, and the layer is
heated to cure the aluminum layer while bonding the LED electrodes
to the aluminum.
[0016] In another embodiment, the blue and cyan LEDs are separately
printed onto the substrate's metal layer in different areas or
zones so that they can be separately electrically contacted and
energized. The yellow phosphor may be printed over only the blue
LEDs or over a combination of the blue and cyan LEDs in a separate
zone. The YAG or yellow phosphor may be mixed with a red phosphor.
Some of the blue and cyan LEDs may not be covered by the phosphor.
The three groups of LEDs may then be separately controlled by
different current sources to select the amount of blue, cyan, and
phosphor wavelengths to generate a target color.
[0017] Correlated color temperature (CCT) is a specification of the
color appearance of the light emitted by a lamp, relating its color
to the color of light from a black body reference source (a
Planckian reference) when heated to a particular temperature,
measured in Kelvin (K).
[0018] Luminous efficacy of radiation (LER) is luminous flux
divided by radiant flux, where the luminous flux is a measure of
perceived brightness to a human eye (taking into account the
luminosity function of the eye), and the radiant flux is the total
flux output of the light source independent of wavelength. The SI
units for LER are lumens per Watt. The maximum LER value is 683
lm/w at 555 nm (green light), where the human eye is most
sensitive. The theoretical maximum for white light depends of the
color temperature and spectral power distribution (SPD).
[0019] The lamp is engineered for the warmest CCT, such as 2700K.
Then, by increasing the amount of blue and/or cyan, the color is
cooled to higher CCTs. For example, at one setting, the lamp may
produce a CCT of 2700K with a CRI of 94 and a LER of 288 lm/W.
Then, by increasing the current to the blue and/or cyan LEDs not
covered by the phosphor, the light can be cooled to 4000K or 6000K,
for example. All points on the Planckian locus may be possible.
Dynamically adjusting the light throughout the day for a circadian
rhythm is achievable.
[0020] The phosphor layer may be replaced by a quantum dot
layer.
[0021] In another embodiment, much larger and conventional 450 nm
and 490 nm LED chips (e.g., 0.5 mm per side) may be positioned on
the substrate using pick and place tools or using another method.
The usage of the phosphor and separate controlling of the blue and
cyan LEDs may be the same as described above to achieve a higher
CRI and improved LER.
[0022] The 490 nm LEDs may be replaced with GaN LEDs emitting a
longer peak wavelength (closer to green), such as 507 nm, to obtain
a high scotopic response, since the scotopic response typically
peaks at 507 nm. This would result in the lamp appearing brighter
than actually measured. In other words, it would increase the pupil
lumens.
[0023] Other peak emissions of the two types of LEDs may be used to
improve CRI and LER.
[0024] Thus, by using the inventive technique, there is less energy
loss due to phosphor conversion, the luminous efficacy is improved,
the CRI is increased, and the color can be controlled.
[0025] Other embodiments are described.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a cross-sectional view of a micro-LED chip,
emitting a peak wavelength of, for example, either 450 nm or 490
nm. The LED has a reflective metal bottom electrode and a
distributed top electrode that allows light to escape.
[0027] FIG. 2 illustrates LED ink (containing the micro-LED chips)
being printed over an uncured aluminum layer on a substrate.
[0028] FIG. 3 is a top down view illustrating the 450 nm and 490 nm
LED chips being randomly mixed over a small section of the
substrate.
[0029] FIG. 4 is a top down view illustrating how the 450 nm and
490 nm LED chips may be separately printed in zones, such as
strips, and a combination of 450 nm and 490 nm LEDs may be printed
in other zones, to allow the different zones to be separately
controlled by current sources to control the overall color.
[0030] FIG. 5 is a cross-sectional view along line 5-5 in FIG. 3
illustrating the LED chips having their bottom electrodes bonded to
an aluminum layer and their top electrodes contacted by a
transparent conductor overlying the LED chips, connecting the LED
chips in parallel.
[0031] FIG. 6 is a cross-sectional view along line 6-6 in FIG. 4
illustrating the LED chips with their top electrodes contacted by
separate strips of a transparent conductor for connecting the three
strips of LEDs to three different current sources for separately
controlling the strips of LED chips.
[0032] FIG. 7 illustrates a remote phosphor over the structure of
FIG. 6.
[0033] FIG. 8 illustrates a phosphor deposited directly on the LED
chips of FIG. 6. The LED chips may first be encapsulated for
protection and improved light extraction.
[0034] FIG. 9 is a top down view of a small portion of the
substrate illustrating mixed LED chips coated with a transparent
conductor, connecting the LED chips in parallel, and coated with a
phosphor.
[0035] FIG. 10 is a top down view of a small portion of the
substrate illustrating separately deposited groups of LED chips
connected to different current sources, where only some of the LED
chips are covered with a phosphor.
[0036] FIG. 11 is a flowchart identifying various steps used to
form an LED lamp in accordance with one embodiment of the
invention.
[0037] FIG. 12 illustrates a fabrication system used to form LED
lamp panels.
[0038] FIG. 13 illustrates multiple LED lamp panels being
interconnected to form a luminaire, such as for being mounted in a
ceiling for general illumination.
[0039] Elements that are similar or identical in the various
figures are labeled with the same numeral.
DETAILED DESCRIPTION
[0040] FIG. 1 is a cross-sectional view of a micro-LED chip 10
emitting a peak wavelength of either 450 nm or 490 nm, depending on
the structure of its active layer. The LED chip 10 is manufactured
as part of a wafer containing many thousands of LEDs. The wafer may
be broken along etched lines into small LED chips with sides
approximately 25 microns. Since the growth substrate is removed,
the chip 10 may be only a few microns thick, such as 3-10 microns
thick. Forming GaN-based LEDs to emit light with peak wavelengths
of 450 nm and 490 nm is well known.
[0041] Typically, the GaN-based LED semiconductor layers are grown
on a sapphire substrate or other suitable growth substrate. N-type
layers 12 are epitaxially grown, followed by an active layer 14,
followed by p-type layers 16. The growth substrate is then removed,
and ohmic contact to the n-type layers 12 is made by depositing a
large, reflective metal alloy (or multiple metal layers) on the
n-layers 12 to form a bottom electrode 18. Ohmic contact to the
p-type layers 16 is formed by depositing a metal alloy on the
p-layers 16 to form a top electrode 20. The top electrode 20 may be
patterned to be dots, lines, a radial pattern, or other pattern to
allow light to escape while distributing current to the p-layers
16. In an alternative embodiment, the large reflective electrode is
formed over the p-layers 16, and the patterned electrode is formed
over the n-type layers 12, where the light escapes through the
n-type layers 12.
[0042] By forming the active layer with different stoichiometries
of AlInGaN, the peak wavelength can be adjusted to be anywhere
between blue and green. In the present disclosure, the wavelength
of 450 nm will be referred to as blue, and the wavelength of 490 nm
will be referred to as cyan, even though 490 nm is near the
transition from blue to cyan. The 490 nm wavelength was chosen
since it fills in a wavelength gap when a blue LED is used to
excite a YAG phosphor (having a peak emission wavelength of about
575 nm). The 450 nm wavelength was chosen since the YAG phosphor is
most efficiently excited by 450 nm LED chips emitting other than
450 nm and 490 nm may be used, depending on the phosphor used or
other factors. Other types of phosphors can be used besides YAG.
For example, a non-YAG yellow phosphor may be used in combination
with a red phosphor. YAG will be used in the various examples.
[0043] Instead of phosphor, a quantum dots layer may be used in all
embodiments to create white light or any other color light.
[0044] In one embodiment, the blue and cyan micro-LED chips 10 are
mixed in a liquid medium (a solvent) to form an LED ink, whose
viscosity is relatively low to allow the LED chips 10 to easily
move through the medium. The ratio of the blue and cyan LED chips
10 in the medium is controlled to achieve the desired overall color
of the LED lamp.
[0045] FIG. 2 illustrates a substrate 24, which may be a thin,
flexible stainless steel sheet or a polymer sheet. The fabrication
process may be a roll-to-roll process performed at atmospheric
pressure. An uncured aluminum layer 26 is deposited over the
substrate 24 such as by slot-die coating, spraying, printing, etc.
The aluminum layer 26 is formed of particles of aluminum in a
solvent.
[0046] A printing or spraying mechanism 28, containing the LED ink
30, is moved relative to the substrate 24 to deposit a thin layer
of the LED ink 30 on the aluminum layer 26. The relatively high
mass of the bottom electrodes 18 naturally orient the LED chips so
that the bottom electrodes 18 face the aluminum layer 26. The
structure is then heated (cured) by an overhead heater or laser to
evaporate the solvents and fused the aluminum particles together to
form a high conductivity metal layer. This also creates an ohmic
bond between the bottom electrodes 18 and the aluminum layer
26.
[0047] There will typically be many thousands or millions of LED
chips 10 deposited over the aluminum layer 26.
[0048] FIG. 3 is a simplified top down view of a small portion of
the substrate 24 showing a random distribution of the blue LED
chips 32 and the cyan LED chips 34. In the example, the ratio of
blue LED chips 32 to cyan LED chips is two to one.
[0049] Although the peak wavelengths of LED chips varies somewhat
even within the same wafer, the average peak wavelength of the blue
LED chips 32 is about 450 nm, and the average peak wavelength of
the cyan LED chips 34 is about 490 nm.
[0050] In another embodiment, illustrated in FIG. 4, the blue LED
chips 32 are deposited separately, such as by slot-die coating or
screen printing, to be in specified zones on the substrate 24. The
zones may be narrow strips or other shapes that allow the light
from adjacent zones to mix to create a uniform color when viewed a
certain distance from the lamp. The cyan LED chips 34 are also
deposited separately to be in specified zones. There may also be a
mixture of blue and cyan LED chips deposited separately. In the
example of FIG. 4, the three groups of LED chips are deposited as
very narrow strips (shown oriented vertically). Each strip may have
many LED chips across its width, although the LED chips are shown
to be linearly oriented along the length of each strip for
simplicity.
[0051] FIG. 5 is a cross-sectional view along line 5-5 in FIG. 3.
FIG. 5 illustrates a dielectric layer 36 deposited over the LED
chips 32/34 by printing or spraying. The dielectric layer 36
partially encapsulates the chips and mechanically fixes the chips
in place. The top portion of the dielectric layer 36 is then etched
away, such as by a wet etchant, to expose the top electrodes of the
LED chips 32/34. A transparent conductor layer 38 is then deposited
over the LED chips 32/34 to connect them in parallel. The
transparent conductor layer 38 may be deposited in zones to
interconnect a group of LED chips 32/34 in parallel, and the
various transparent conductor layer zones are effectively
electrically isolated so they can be driven by different current
sources. This also increases reliability and reduces the current
needed to be delivered per current source. The transparent
conductor may be deposited by screen printing, where the screen is
patterned to print only through non-masked portions of the
screen.
[0052] In another embodiment, the substrate 24 is a dielectric
film, and the aluminum layer 26 is formed in electrically isolated
zones. By also forming the transparent conductor layer 38 as zones,
groups of parallel-connected LED chips can be connected in series
on the same substrate to obtain any voltage drop across the series
string.
[0053] Metal bus bars 40 are deposited at intervals to provide high
conductivity paths from the current sources to the transparent
conductor layer 38.
[0054] FIG. 6 is a cross-sectional view along line 6-6 in FIG. 4.
Instead of the transparent conductor layer 38 being blanket
deposited over the mixture of LED chips 32/34, as in FIG. 5, the
transparent conductor layer 38 is selectively deposited, such as by
screen printing, to only interconnect the LED chips in a single
vertical strip in parallel, where the LED chips were separately
deposited in the strips. This allows the blue LED chips 32 in the
rightmost strip to be separately controlled by a current source,
allows the cyan LED chips 34 in the leftmost strip to be separately
controlled by another current source, and allows the mixture of LED
chips 32/34 in the center strip to be separately controlled by
another current source.
[0055] FIGS. 7 and 8 illustrate the structure of FIG. 6 being used
with a phosphor layer. The structure of FIG. 5 may be used
instead.
[0056] In FIG. 7, in one embodiment, a phosphor layer 50 may be a
mixture of a YAG phosphor and a red phosphor, or a layer of a YAG
phosphor and a layer of a red phosphor. The phosphor layer 50 is
deposited on a transparent substrate 52, such as glass. The
phosphor layer 50 is separated from the LED chips 32/34, so is a
remote phosphor. The phosphor layer 50 may be formed as separate
pieces and affixed over the LED chips 32/34 using any technique,
such as by a transparent adhesive or a mechanical assembly.
Providing a remote phosphor layer 50 improves mixing of the LED
light, reduces the effects of light and heat on the phosphor, and
allows the manufacturer to select a pre-formed phosphor layer
having a desired characteristic to be used in the LED lamp to
achieve a color specified by the customer, while not changing the
LED chips 32/34.
[0057] The YAG/red phosphor will emit light having well defined
characteristics. The phosphor emission is shown by the light rays
Y/R. The blue LED chips 32 may be controlled by a current source to
be brighter to add more blue to the white light output to increase
the CCT. The blue emission is shown by the light rays B. The cyan
LED chips 34 may be controlled by a current source to be brighter
to add more green to the white light output. The cyan emission is
shown by the light rays C. Other phosphors may be used instead of
YAG to create white light.
[0058] If all the LED chips were connected in parallel, such as in
FIG. 5, the ratio of the cyan LED chips 34 to the blue LED chips 32
is selected to boost the 490 nm output of the LED lamp a certain
amount to create a broader spectrum of light to improve the CRI of
the lamp.
[0059] FIG. 8 illustrates the structure of FIG. 6 with the phosphor
layer 50 deposited directly over the LED chips 32/34, such as by
spraying or printing. The use of the cyan LED chips 34 improves the
CRI and LER of the LED lamp and allows the overall color to be
dynamically controlled by changing the currents supplied to the
various zones of LED chips.
[0060] Although the YAG phosphor is excited by the 490 nm emitted
by the cyan LED chips 34, the excitation efficiency is not as great
as with the 450 nm LED chips 32. Therefore, in one embodiment, the
phosphor does not cover the cyan LED chips 34 but only covers the
blue LED chips 32 to maximize efficiency. The cyan LED chips 34
then just supply the 490 nm wavelength that is substantially
lacking in the YAG phosphor emission.
[0061] In another embodiment, the phosphor layer 50 comprises a
yellow phosphor and a red phosphor that are energized by the blue
LED chips 32. The cyan LED chips 34 contribute a cyan component to
increase CRI, and the cyan LED chips 34 are optionally not covered
with a phosphor. The blue LED chip 32 peak wavelength may be tuned
to the peak excitation wavelength of the phosphors used without
compromising CRI.
[0062] Since the blue and cyan LED chips have very similar
structures, they can be driven by the same current, allowing them
to be connected in parallel.
[0063] The peak emission of the cyan LED chip 34 may be anywhere
within the cyan (blue-green) range while still achieving the
benefits of the invention. The peak wavelength of the blue LED chip
will typically be the most efficient energizing wavelength for the
phosphor. The optimum peak wavelengths of the two types of LED
chips are selected to improve CRI and LER and depend on the
particular phosphors used and CCT or chromaticity to be generated.
In one embodiment, the two LED type have a range between about 430
nm and 507 nm.
[0064] FIG. 9 is a top down view of a portion of a completed LED
lamp with either a remote phosphor layer 50 or a phosphor layer 50
deposited directly over the LED chips. All the LED chips in FIG. 9
are driven in parallel by the current source 54, since the
transparent conductor layer 38 connects together the top electrodes
of all the LED chips. CRI is increased by the addition of the cyan
LED chips 34 since some of the cyan emission passes through the
phosphor layer 50.
[0065] FIG. 10 is a top down view of a portion of another
embodiment of a completed LED lamp with either a remote phosphor
layer 50 or a phosphor layer 50 deposited directly over certain LED
chips. In FIG. 10, the transparent conductor layer 38 is deposited
in strips to only interconnect selected LED chips in parallel. If
the aluminum layer 26 were segmented, the LED chips connected to
the same segment may form a group of parallel LED chips that are
connected in series with other groups of parallel LED chips.
[0066] In the example of FIG. 10, the phosphor layer 50 is only
formed over the middle strip of LED chips. The leftmost strip of
cyan LED chips 34 and the rightmost strip of blue LED chips 32 are
not covered by a phosphor. Normally, there would be many more LED
chips covered by the phosphor than not covered by the phosphor to
generate a proper balance of wavelengths to produce white light.
Each of the three strips of LED chips is controlled by its own
current source 56, 57, 58 so that the color components of the white
light may be adjusted to achieve a wide range of CCT, such as 2700K
to 6500 K. All points on the Planckian locus may be possible. The
LED chips and phosphor are designed to provide the lowest target
CCT (e.g., 2700K) and, by increasing the currents to the separate
blue and cyan LED chips, the CCT can be raised.
[0067] In another embodiment, blue and cyan LED chips are mixed
together and connected in parallel for being controlled by the same
current source. The ratio of the LED chips controls the color. A
second group of either mixed LED chips or only blue LED chips is
covered by phosphor or overlain with a remote phosphor, and the
second group of LED chips is controlled by a separate current
source. By controlling the relative currents of the two current
sources, a wide range of CCTs is achievable, while increasing the
CRI and improving the LER.
[0068] The aluminum layer 26 and transparent conductor layer 38 may
be segmented in any manner to connect any number of LED chips in
series and parallel.
[0069] A quantum dot layer may be used instead of the phosphor
layer 50.
[0070] The current sources 56-58 may be automatically controlled
during the day to sweep the CCT between two temperatures to mimic
sunrise to sunset to correspond to the circadian rhythm.
[0071] In one embodiment, instead of using LED chips with a peak
wavelength of 490 nm, LED chips with a peak wavelength of 507 nm
are used since the scotopic response typically peaks at 507 nm.
This would result in the lamp appearing brighter than actually
measured by increasing the pupil lumens.
[0072] FIG. 11 is a flowchart summarizing various steps used for
form an LED lamp in accordance with one embodiment of the
invention. In step 61, blue and cyan LED chips are deposited on a
substrate. The LED chips may be mixed or separated on the
substrate. The ratio of the LED chips affects the resulting
color.
[0073] In step 62, the LED chips are electrically interconnected
and connected to current sources. The phosphor-converted LED chips
in a group may be connected to one current source. The blue LED
chips in a group may be connected to another current source. And,
the cyan LED chips in a group may be connected to a third current
source.
[0074] In step 63, a phosphor is selectively applied overlying all
the LED chips or only over a selected group of LED chips. The
phosphor may be a remote layer instead.
[0075] In step 64, the current sources are controlled to provide a
desired mix of the blue, cyan, and phosphor wavelengths to achieve
the desired CCT. CRI and LER may be optimized by the combination of
LED chips and phosphor.
[0076] FIG. 12 illustrates a fabrication system used to form LED
lamp panels. The substrate 24 may be moved relative to the various
stations. One or more depositing stations 82 are used for
depositing the LED chips and various layers in any pattern, such as
by using screen printing. A heating/curing station 84 cures the
various deposited materials. An etching station 86 removes
materials, as desired. All processes may be performed using a
roll-to-roll process at atmospheric pressures to fabricate
relatively large and inexpensive LED light panels. Each panel may
comprise many thousands of LED chips that are interconnected in
series and/or parallel.
[0077] FIG. 13 illustrates four LED lamp panels 100 being
interconnected to form a luminaire, such as for being mounted in a
ceiling for general illumination. There may be many more panels in
a single luminaire. Each panel 100 is made by any of the
above-described processes and each contains many thousands of LED
chips. A matrix of metal bus bars 114 is formed over the
transparent conductor layer to provide high conductivity paths
proximate to the LED chips. Electrical connectors 116 and 118
connect to the aluminum layer 26 (FIG. 2) and metal bus bars 114
for coupling a current to the various LED chips. There may be many
more connectors for each panel if there are multiple, electrically
isolated segments of LED chips that are to be separately
controlled.
[0078] The various panels 100 are then connected in any combination
of series and parallel by external conductors, such as wires or
part of a frame, to achieve the desired voltage and current. The
luminaire may replace a standard 2.times.4 foot fluorescent
troffer.
[0079] While particular embodiments of the present invention have
been shown and described, it will be obvious to those skilled in
the art that changes and modifications may be made without
departing from this invention in its broader aspects and,
therefore, the appended claims are to encompass within their scope
all such changes and modifications as fall within the true spirit
and scope of this invention.
* * * * *