U.S. patent application number 14/829639 was filed with the patent office on 2015-12-10 for using an led die to measure temperature inside silicone that encapsulates an led array.
The applicant listed for this patent is Bridgelux, Inc.. Invention is credited to Michael Neal Gershowitz, Babak Imangholi, R. Scott West.
Application Number | 20150355032 14/829639 |
Document ID | / |
Family ID | 52114926 |
Filed Date | 2015-12-10 |
United States Patent
Application |
20150355032 |
Kind Code |
A1 |
Imangholi; Babak ; et
al. |
December 10, 2015 |
Using An LED Die To Measure Temperature Inside Silicone That
Encapsulates An LED Array
Abstract
A light-emitting diode (LED) device includes first and second
LED dies with the same structure and that are both encapsulated by
the same silicone layer. The first LED is supplied with sufficient
drive current to illuminate the LED. Control circuitry supplies the
second LED with a constant current, determines the voltage across
the second LED, and calculates the temperature of the second LED
based on the voltage across the second LED. The constant current
has a maximum magnitude that never exceeds the maximum magnitude of
the drive current. The LED device is able to calculate the
temperature of a diode with a gallium-nitride layer (GaN or GaInN)
that is receiving a large drive current and emitting blue light by
determining the voltage across an adjacent similar diode with a
gallium-nitride layer through which a small constant current is
flowing. Preferably, the band gap of the LEDs exceeds two electron
volts.
Inventors: |
Imangholi; Babak;
(Livermore, CA) ; Gershowitz; Michael Neal; (San
Jose, CA) ; West; R. Scott; (Pleasanton, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bridgelux, Inc. |
Livermore |
CA |
US |
|
|
Family ID: |
52114926 |
Appl. No.: |
14/829639 |
Filed: |
August 19, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13930672 |
Jun 28, 2013 |
9164001 |
|
|
14829639 |
|
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|
|
Current U.S.
Class: |
374/163 |
Current CPC
Class: |
H05B 45/10 20200101;
H01L 2924/181 20130101; H01L 2224/8592 20130101; H01L 33/32
20130101; H01L 2224/48091 20130101; H05B 47/10 20200101; G01K 7/00
20130101; H01L 2224/49107 20130101; H01L 33/502 20130101; H01L
27/156 20130101; H01L 2224/48137 20130101; H05B 45/18 20200101;
G01K 7/01 20130101; H01L 2224/73265 20130101; H01L 2224/48091
20130101; H01L 2924/00014 20130101; H01L 2924/181 20130101; H01L
2924/00012 20130101 |
International
Class: |
G01K 7/00 20060101
G01K007/00; H01L 33/50 20060101 H01L033/50; H01L 27/15 20060101
H01L027/15; H01L 33/32 20060101 H01L033/32 |
Claims
1-20. (canceled)
21. A device comprising: a diode that has a temperature; an
encapsulant that covers the diode, wherein the encapsulant contains
phosphor; and circuitry adapted to determine the temperature of the
diode based on a voltage across the diode.
22. The device of claim 21, wherein the diode has a gallium nitride
(GaN) layer.
23. The device of claim 21, wherein the diode has a band gap that
exceeds two electron volts.
24. The device of claim 21, further comprising: a second diode,
wherein the encapsulant covers the second diode, and wherein the
second diode emits light.
25. The device of claim 24, wherein each of the diode and the
second diode has a gallium nitride (GaN) layer.
26. The device of claim 21, wherein the circuitry determines the
temperature of the diode by supplying a constant current to the
diode.
27. The device of claim 26, wherein the constant current never
exceeds ten milliamps.
28. A device comprising: a first diode; an encapsulant through
which light is emitted, wherein the encapsulant covers the first
diode, and wherein the encapsulant has a temperature; and circuitry
adapted to use the first diode to determine the temperature of the
encapsulant.
29. The device of claim 28, wherein the first diode has a gallium
nitride (GaN) layer.
30. The device of claim 28, wherein the first diode has a band gap
that exceeds two electron volts.
31. The device of claim 28, wherein the encapsulant contains
phosphor.
32. The device of claim 28, further comprising: a second diode,
wherein the encapsulant covers the second diode, and wherein the
second diode emits the light.
33. The device of claim 32, wherein each of the first diode and the
second diode has a gallium nitride (GaN) layer.
34. The device of claim 32, wherein the first diode and the second
diode have the same structure, and wherein the second diode emits
light with a wavelength between 445 and 455 nanometers.
35. The device of claim 28, wherein the circuitry determines the
temperature by supplying a constant current to the first diode and
by sensing a voltage across the first diode.
36. The device of claim 28, wherein the first diode is supplied
with a current that never exceeds ten milliamps.
38. A device comprising: a first diode that has a temperature; a
second diode, wherein each of the first diode and the second diode
has a gallium nitride (GaN) layer; an encapsulant that covers the
first diode and the second diode; and circuitry adapted to
determine the temperature of the first diode based on a voltage
across the first diode.
39. The device of claim 38, wherein the circuitry determines the
temperature of the first diode by supplying a constant current to
the first diode.
40. The device of claim 39, wherein the constant current never
exceeds ten milliamps.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of, and claims priority
under 35 U.S.C. .sctn.120 from, nonprovisional U.S. patent
application Ser. No. 13/930,672 entitled "Using An LED Die To
Measure Temperature Inside Silicone That Encapsulates An LED
Array," now U.S. Pat. No. ______, filed on Jun. 28, 2013, the
subject matter of which is incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates generally to packaging for
light-emitting diodes, and more particularly, to a method of
determining the temperature inside a silicone layer that
encapsulates an array of LED dies.
BACKGROUND INFORMATION
[0003] Light emitting diodes (LEDs) are an important class of
solid-state devices that convert electric energy into light.
Improvements in these devices have resulted in their use as light
sources replacing conventional incandescent and fluorescent light
fixtures. The energy conversion efficiency of LEDs now approaches
the level attained by fluorescent light fixtures and promises to
exceed even these efficiencies. Moreover, LEDs have significantly
longer lifetimes than both incandescent bulbs and fluorescent
tubes. However, the useful lifetime of LEDs is significantly
reduced if the operating temperature exceeds certain limits.
[0004] The operating environment of an LED light source is
typically hot, and overheating must be controlled in order to
extend the operating life of the light source. The high operating
temperatures of commercial white LED light sources result primarily
from two factors. First, the phosphor that converts blue light from
the LED dies into longer wavelength light generates heat. Thin
layers of Group III nitrides, such as gallium nitrides (GaN or
gallium indium nitride GaInN), are used to produce LEDs for general
commercial lighting applications. For example, thin epitaxial
layers of gallium nitrides are grown on sapphire substrates
(Al.sub.2O.sub.3). Light is emitted from the epitaxial layers
sandwiched between oppositely doped layers when a voltage is
applied across the doped layers. Gallium-nitride LED dies (GaN or
GaInN) emit blue light having a wavelength in a range from 430
nanometers to 460 nanometers. A phosphor coating then absorbs some
of the emitted blue light and fluoresces to emit light with longer
wavelengths so that the overall LED device emits light with a wider
range of wavelengths, which is perceived as "white" light by a
human observer. The phosphor does not convert all of the blue light
to longer wavelength light, but rather converts much of the blue
light to heat.
[0005] Second, a single LED die produces too little light to be
used as a replacement for a conventional light source in most
applications. Hence, a replacement light source must include a
large number of individual LED dies. The large number of LED dies
that are packaged in close proximity to one another under a
transparent carrier material that contains phosphor particles
results in a large amount of heat generated within a small volume.
The temperature under the transparent carrier material rises when
the large amount of heat generated by the many LED dies cannot be
conducted fast enough away from the LED device due to inadequate
heat conduction of the luminaire housing, which may be exacerbated
in a hot environment.
[0006] Although LED package designs include heat carriers and heat
sinks that conduct heat away from the LED device, it is
nevertheless advantageous to determine the temperature of the LED
device in order to take corrective measures if heat is not
dissipated sufficiently to maintain the temperature of the LED
device below a critical level. A conventional way to determine the
temperature of the LED device is to place a thermistor or
thermocouple on the LED package near the LED device. However, this
method does not measure the temperature directly at the LED dies
covered by the transparent carrier material. Depending on how the
heat propagates away from the LED dies, the temperature at the
thermistor does not reflect the actual temperature under the
transparent carrier material. Moreover, this manner of measuring
temperature provides a relatively slow feedback and can lead to
oscillation in the temperature control. Because the source of the
heat is the LED dies and the phosphor particles under the
transparent carrier material, the temperature at the thermistor or
thermocouple outside the transparent carrier material is indicative
of the heat that was produced earlier within the transparent
carrier material. By the time the thermistor or thermocouple
measures a temperature that exceeds a threshold and LED drive
current is reduced in order to reduce the heat generated by the LED
device, the temperature within the transparent carrier material may
already have fallen because the temperature measured at the
thermistor or thermocouple resulted from earlier produced heat that
later reached the thermistor or thermocouple. The delayed feedback
will cause the current control to overcompensate both after the
measured temperature exceeds an upper threshold and after the
measured temperature falls below a lower threshold. An oscillating
LED device temperature results.
[0007] Thermistors and thermocouples are typically not placed near
the LED dies under the transparent carrier material, however,
because they absorb light and would result in a non-uniform pattern
of light generation from the LED device. Moreover, placing a
thermistor or thermocouple within the LED array would add an
additional manufacturing step and would require additional
machinery. So the cost of the resulting LED device would increase
significantly. A inexpensive method is sought for determining the
temperature of LED dies covered by a transparent carrier material
that includes phosphor without causing the light emitted from the
LED device to be non-uniform.
SUMMARY
[0008] A light-emitting diode (LED) device includes first and
second LED dies that both have the same structure and that are both
encapsulated by a silicone layer. Driver circuitry supplies the
first LED die with sufficient drive current to illuminate the first
LED die. Control circuitry supplies the second LED die with a
constant current, determines the voltage across the second LED die,
and calculates the temperature of the second LED die based on the
voltage across the second LED die. The LED drive current has a
maximum magnitude that exceeds ten milliamps, and the constant
current that is supplied to the second LED die never exceeds ten
milliamps. Thus, the maximum magnitude of the constant current
never exceeds the maximum magnitude of the drive current. The LED
device is able to calculate the temperature of a diode with a
gallium-nitride layer (GaN or GaInN) that is receiving a large
drive current and emitting blue light by determining the voltage
across an adjacent similar diode with a gallium-nitride layer
through which a small constant current is flowing.
[0009] A method for determining the temperature of an LED die of an
array of LED dies covered by a silicone layer involves determining
the voltage drop across a single LED die. Both a first LED die and
a second LED die are encapsulated by the same silicone layer in
which phosphor particles are suspended. And both the first LED die
and the second LED die have the same structure. In one
implementation, the band gap of the LED dies exceeds two electron
volts. The first LED die is illuminated by supplying a drive
current to the first LED die. While the second LED die is being
supplied with a small constant bias current, the voltage across the
second LED die is determined. The temperature of the second LED die
is determined based on the voltage across the second LED die.
[0010] Further details and embodiments and techniques are described
in the detailed description below. This summary does not purport to
define the invention. The invention is defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, where like numerals indicate like
components, illustrate embodiments of the invention.
[0012] FIG. 1 is a top view of a device with an LED die used to
determine the temperature under a silicone layer that covers an
array of other LED dies.
[0013] FIG. 2 is a cross-sectional view of a device that uses an
LED die to determine the temperature within a silicone layer and
roughly corresponds to a cross section of the device of FIG. 1.
[0014] FIG. 3 is another cross-sectional view of a device that uses
an LED die to determine the temperature within a silicone layer and
roughly corresponds to another cross section of the device of FIG.
1.
[0015] FIG. 4 is a graph of the temperature-voltage relationship of
a gallium-nitride diode under a constant current of five
milliamps.
[0016] FIG. 5 is a graph of the temperature-current relationship of
a gallium-nitride diode under a constant voltage of 2.6 volts.
[0017] FIG. 6 is a simplified schematic block diagram of control
circuitry that supplies a sensor LED die with a constant current
and that determines a voltage across the LED die.
[0018] FIG. 7 shows an embodiment of the device of FIG. 1 in which
the control circuitry of FIG. 6 has been incorporated into the
ceramic package.
[0019] FIG. 8 is a flowchart illustrating a method for determining
the temperature of an LED die of an array of LED dies covered by a
silicone layer.
DETAILED DESCRIPTION
[0020] Reference will now be made in detail to some embodiments of
the invention, examples of which are illustrated in the
accompanying drawings.
[0021] FIG. 1 shows a light-emitting diode (LED) device 10 with an
LED die 11 used to determine the temperature under a transparent
carrier material that includes phosphor and that covers an array of
other LED dies. LED device 10 includes an array of sixty-one
structurally identical LED dies (chips). Each of the LED dies
includes epitaxial layers of GaN or GaInN grown on a sapphire
substrate. In other embodiments, the gallium-nitride layer is grown
on a substrate of crystalline silicon. Each of the sixty-one LED
dies is mounted on an aluminum substrate 12 that is housed in a
ceramic package 13. The gallium-nitride LED dies emit blue light
with a wavelength of about 452 nanometers when a sufficient drive
current is passed through the diodes. For example, a string of ten
LED dies 14-23 are connected in series such that a drive current
can flow from a positive supply terminal 24 through the LED dies
14-23 to a negative supply terminal 25. The LED dies are connected
to each other and to the supply terminals by bond wires 24. For
example, a bond wire 26 connects LED die 14 to a landing pad 27 of
positive supply terminal 24.
[0022] The array of sixty-one LED dies is covered by a transparent
carrier material, such as a layer of silicone or epoxy. Particles
of phosphor are suspended in the transparent carrier material. The
phosphor converts a portion of the blue light generated by the LED
dies into light in the yellow region of the optical spectrum. The
combination of the blue and yellow light is perceived as "white"
light by a human observer. A slurry of phosphor suspended in
silicone is dispensed into a ring or dam 28 around the array of LED
dies. The silicone layer inside dam 28 forms an optical surface,
such as a lens or a structured surface. The blue light from the LED
dies and the yellow light from the phosphor particles is more
likely to exit the silicone layer if the surface is structured as
opposed to smooth because the scattered light is more likely to
strike the surface at a normal angle that exhibits a lower total
internal reflection (TIR). For example, the structured surface of
the silicone layer can have a small sinusoidal wave structures,
"rectified" wave structures (hemispheres) or saw-tooth
structures.
[0023] Drive current is not passed through LED die 11, and LED die
11 is not connected to the power supply terminals or other driver
circuitry. Instead, sensor LED die 11 is connected to control
circuitry that supplies LED die 11 with a constant current and that
determines the voltage across LED die 11. At a constant current
flowing through LED die 11, the voltage across LED die 11 depends
on the temperature of LED die 11. Because the silicon layer
encapsulates LED die 11 as well as the other LED dies and all of
the dies are in the same environment, the temperature of LED die 11
is approximately the same as the temperature of the other dies in
its vicinity, such as dies 14-15. In fact, the temperature of all
of the sixty-one LED dies under the silicone layer is nearly the
same and does not vary by more than a few degrees. Certainly, the
temperature of LED die 11 is much closer to the temperature of the
other LED dies than would be the temperature of a thermistor that
is not covered by the silicone layer and that is placed outside dam
19.
[0024] FIG. 2 is a cross-sectional view of an LED device 30 that
uses LED die 11 to determine the temperature within a silicone
layer 31. The cross-sectional view of LED device 30 in FIG. 2
roughly corresponds to the cross section of LED device 10 through
the string of LED dies 14-23 in FIG. 1. Sensor LED die 11 and the
other LED dies are mounted directly to aluminum substrate 12 in a
chip-on-board (COB) manner. The top of aluminum substrate 12
includes various layers of dielectric and metal to form contacts
and isolations. In some embodiments, the illuminated LED dies 14-19
are electrically connected to power and driver circuitry, and
sensor LED die 11 is electrically connected to control circuitry,
through conductive layers at the top of aluminum substrate 12 with
bond wires connecting to tap points on the conductive layers. In
the embodiment of FIG. 2, however, the illuminated LED dies 14-19
are electrically connected to power through bond wires 26, 32. FIG.
2 shows the silicone layer 33 that contains phosphor particles 34.
Silicone or another transparent carrier material is poured into dam
28 and hardens forming a conformal covering over the LED dies. The
silicone layer 33 also protects the bond wires 32.
[0025] FIG. 3 is another cross-sectional view of LED device 30
passing through LED die 11. The cross-sectional view of LED device
30 in FIG. 3 roughly corresponds to the cross section of LED device
10 through LED die 11 in FIG. 1. Sensor LED die 11 is adjacent to
illuminated LED dies 14-15 in LED device 30. Whereas the
illuminated LED dies 14-19 are connected to driver circuitry and
are provided with a drive current, sensor LED die 11 is connected
to control circuitry that supplies LED die 11 with a small constant
bias current. The maximum magnitude of the constant bias current
never exceeds the maximum magnitude of the drive current that
illuminates LED dies 14-19. In fact, the constant bias current is
very small and never exceeds ten milliamps. In a preferred
embodiment, the constant current supplied by the control circuitry
to sensor LED die 11 is about one milliamp. However, the constant
bias current can even be on the order of microamps. LED die 11 is
connected to the control circuitry by bond wires 35-36. The control
circuitry determines the voltage across sensor LED die 11 and then
determines the temperature of LED die 11 based on that voltage.
[0026] FIG. 4 is a graph of the temperature-voltage relationship of
a gallium-nitride diode under a constant current of five milliamps.
The voltage across gallium-nitride LED die 11 while a constant
current is flowing varies linearly with the temperature of LED die
11. The voltage across LED die 11 decreases as the temperature
increases while the constant current is flowing. For example, the
control circuitry can determine that the temperature of LED die 11
is about 105.degree. C. if the voltage across LED die 11 is 2.45
volts while a constant current of 5 mA is flowing through LED die
11.
[0027] Although all of the LED dies under silicone layer 33 are
capable of generating light, they are nevertheless diodes and
exhibit the standard characteristics of a diode. A diode is created
by joining a p-type semiconductor with an n-type semiconductor to
form a pn junction. The p-type semiconductor is doped with a
trivalent atom such as indium or aluminum. The three valence
electrons covalently bond with the semiconducting material and
leave a "hole" in the fourth bond. The n-type semiconductor is
doped with a donor atom such as arsenic. Four of the donor atom's
electrons bind covalently with the semiconducting material while
the fifth electron is free to move into the conduction band if the
diode receives the appropriate amount of energy. The amount of
energy required to move electrons into the conduction band is the
band gap energy. The band gap of a standard silicon diode is 1.1
electron volts, and the band gap of a red diode is about 1.4
electron volts. The band gap of the gallium-nitride, light-emitting
diodes in LED device 30, however, is much higher. Gallium-nitride
LED dies that emit blue light at about 452 nanometers have a band
gap of 2.7-2.8 electron volts.
[0028] The voltage across a diode through which a constant current
is flowing varies with temperature according to the relationship
V=C-T/B, where C is indicative of the constant current, and B is
indicative of the band gap energy of the diode. For a constant
current of 5 mA, C equals 2.5873. For a gallium-nitride diode that
emits light at 452 nanometers, B equals 769.231. Thus, the
temperature-voltage relationship shown in FIG. 4 can be expressed
as V=2.5873-T/769.231. Using the voltage V across sensor LED die 11
as an input, the control circuitry calculates the temperature of
LED die 11 using the formula T=769.231.times.(2.5873-V) for a
constant current of 5 mA flowing through LED die 11. The
calibration factor C must be adjusted when a different constant
bias current is used.
[0029] Using a smaller constant bias current has the advantage that
less heat is produced as current flows through sensor die 11. Any
heat produced by the bias current results in a higher temperature
around sensor die 11 that around the other LED dies. In addition,
some 452-nm light is generated even by a small bias current. The
blue light emitted by sensor LED die 11 even with a small bias
current results in a color over position inhomogeneity of the
overall light emitted from LED device 30 and should be minimized.
However, a smaller constant bias current also results in a lower
signal-to-noise ratio of the voltage detection signal from sensor
die 11. A good compromise between reducing heat and color
inhomogeneity and reducing noise in the temperature signal is a
constant bias current of between 0.1 mA and 1 mA.
[0030] Silicon diodes and red diodes would be unsuitable for
sensing the temperature inside silicone layer 33 because these
diodes would absorb the longer wavelength light emitted by the
phosphor particles 34 and would produce a current. Just as
light-emitting diodes produce light when a current is passed
through the diodes, the diodes produce a current when light with
the appropriate amount of energy (the band gap energy) is absorbed
by the diodes. The current produced when light with a band gap
energy of 1.1 or 1.4 eV for silicon or red diodes is absorbed would
add to the constant bias current, would effect the voltage
detection signal and would thus interfere with the temperature
measurement. On the other hand, white light and the light emitted
by the phosphor particles 34 does not have sufficient energy to
bridge the band gap of the gallium-nitride, light-emitting diodes
of LED device 30. Whereas diodes with a band gap energy of 1.1 or
1.4 eV would absorb almost 100% of the light emitted by LED dies
14-19, gallium-nitride LED die 11 with a band gap energy of 2.7-2.8
eV absorbs only a fraction of 1% of the light that strikes it
within LED device 30.
[0031] The low light absorption of gallium-nitride LED dies
compared to silicon or red diodes has another advantage besides not
interfering with the temperature measurement. The low light
absorption of gallium-nitride LED dies allows one of the dies to be
used to sense temperature within the silicone layer 33 without
decreasing the lumen output of LED device 30. Because a silicon or
red diode would absorb almost 100% of the generated light that
strikes it, such a diode would have to be covered by a reflective
material to prevent absorption. The sapphire substrate of LED die
11, however, is substantially transparent to the white light.
Placing a gallium-nitride LED die under silicone layer 33 and using
the die to sense temperature will not create a dark spot on LED
device 30.
[0032] Other advantages of using a gallium-nitride LED die instead
of a silicon or red die to sense temperature under silicone layer
33 are cost and performance. Using diodes in LED device 30 that are
all of the same type is less expensive than sourcing and placing a
second type of diode next to the LED dies on substrate 12. The cost
of placing an additional LED die on substrate 12 to be used to
sense temperature is minimal because the same processes and
equipment is used. The performance of LED die 11 that is used as a
temperature sensor is also superior to that of a silicon or red
diode. The LED die 11 will last as long as the other LED dies on
LED device 30 that are of the same type. Moreover, the LED dies
have been designed and tested to last for 50,000 hours and to
resist spikes in temperature of up to 200.degree. C.
[0033] FIG. 5 is a graph of the temperature-current relationship of
a GaN diode under a constant voltage. The temperature of LED die 11
can also be determined by monitoring the current through LED die 11
while the voltage across LED die 11 is maintained constant. The
curve of FIG. 5 shows how the current flowing through LED die 11
varies with the temperature of LED die 11 as a constant voltage of
2.6 volts is maintained across LED die 11. However, this method is
not preferred over sensing the voltage with constant current
because the current flowing through a diode with a constant voltage
does not vary linearly with temperature. Consequently, each sensor
LED die would have to be calibrated to determine the best-fitting
curve through the calibration points. For the particular GaN LED
die that generated the calibration points of FIG. 5, an approximate
relationship between temperature and current for a constant voltage
of 2.6V across the diode can be expressed as I=7.3794
e.sup.0.02.GAMMA.. The non-linear relationship of temperature to
current also requires a more complex control circuitry to calculate
than does the linear relationship of temperature to voltage.
[0034] FIG. 6 is a simplified schematic block diagram of control
circuitry 40 that supplies sensor LED die 11 with a constant
current 41 and that determines a voltage across LED die 11. Control
circuitry 40 includes a voltage detector 42 that determines the
voltage across LED die 11 using a voltage detection signal 43.
Voltage detector 42 includes a differential amplifier, a band pass
filter, an analog-to-digital converter and a microcontroller to
read out the voltage value. Control circuitry 40 generates a
constant current using constant current generator 44. Constant
current generator 44 includes an N MOSFET 45, an operational
amplifier 46 and a shunt resistor 47. In one embodiment, constant
current generator 44 sinks a constant 1 mA current 41 through LED
die 11. The 1 mA current 41 is monitored by operational amplifier
46 using resistor 47. Operational amplifier 46 compares the voltage
across resistor 47 to a reference voltage and opens transistor 45
to the extent required to maintain the voltage across resistor 47
at the reference voltage. The current flowing through transistor 45
that is required to maintain the voltage across resistor 47 at the
reference voltage is the 1 mA current 41.
[0035] FIG. 7 shows an embodiment of LED device 10 in which the
control circuitry 40 of FIG. 6 has been incorporated into ceramic
package 13. LED device 10 includes a first LED die 14 and a second
LED die 11 which both have the same structure. Both dies 11 and 14
are covered by transparent carrier material 33 that encapsulates
the dies. First LED die 14 is used to emit blue light, and second
LED die 11 is used as a thermocouple to sense temperature. An LED
drive current with a magnitude sufficient to illuminate the first
LED die 14 is supplied to the first LED die 14 through landing pad
27 of positive supply terminal 24. Landing pad 27 and supply
terminal 24 are part of the driver circuitry that supplies the
first LED die 14 with the drive current. The driver circuitry
includes a driver that is not incorporated into ceramic package 13.
Control circuitry 40 supplies the second LED die 11 with a constant
current and determines the voltage across the second LED die
11.
[0036] LED device 10 can be used to illuminate first diode 14 by
supplying a drive current to first diode 14. At the same time, LED
device 10 with control circuitry 40 is used to supply second diode
11 with a constant current whose maximum magnitude never exceeds
ten milliamps and to determine the temperature of second diode 11
based on the voltage across second diode 11. The temperature of
both first diode 14 and second diode 11 is the same because both
are encapsulated by silicone layer 33 with suspended phosphor
particles 34. In one implementation, the maximum magnitude of the
constant current flowing through second diode 11 is five milliamp.
The control circuitry calculates the temperature in degrees Celsius
of second diode 11 based on the voltage across second diode 11
using the formula T=769.231.times.(2.5873-V). The voltage detector
42 of control circuitry 40 outputs a temperature signal 48 that
provides a real-time indication of the temperature of the LED dies
in LED device 10. Temperature signal 48 is provided to an
integrated control module 49 that can take action in the event that
the temperature of the LED dies exceeds a predetermined threshold.
For example, the integrated control module can reduce the drive
current to the LED dies to reduce the temperature within the
silicone layer 33. Or the integrate control module can send a
message via a wireless interface indicating that LED device 10 has
exceeded the predetermined threshold for a measured amount of
time.
[0037] FIG. 8 is a flowchart illustrating steps 50-52 of a method
for determining the temperature of an LED die of an array of LED
dies covered by a silicone layer. In a step 50, a first LED die
with a gallium-nitride layer is supplied with a small constant bias
current. All of the LED dies of the array have the same
semiconductor structure as the first LED die. In one
implementation, the maximum magnitude of the constant bias current
never exceeds ten milliamps. In step 51, the temperature of the
first LED die is calculated by determining the voltage across the
first LED die. In step 52, a second LED die with a gallium-nitride
layer is illuminated by supplying the second LED die with a drive
current. Both the first LED die and the second LED die are
encapsulated by a silicone layer. The method involves measuring a
voltage drop across a single LED die that is covered by a silicone
layer along with other LED dies of the same structure.
[0038] Although certain specific embodiments are described above
for instructional purposes, the teachings of this patent document
have general applicability and are not limited to the specific
embodiments described above. Although the sensor LED die 11 is
described above as being a GaN diode that emits light at 452
nanometers, an LED that emits light at other wavelengths can also
be used to sense the temperature under the silicone layer 33. For
example, a GaInN diode or a diode that does not contain gallium can
be used. But the value for B in the formula V=C-T/B must correspond
to the band gap energy of the other diode instead of that of the
452-nm GaN LED. Accordingly, various modifications, adaptations,
and combinations of various features of the described embodiments
can be practiced without departing from the scope of the invention
as set forth in the claims.
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