U.S. patent application number 15/018375 was filed with the patent office on 2017-08-10 for solid state light fixtures having ultra-low dimming capabilities and related driver circuits and methods.
The applicant listed for this patent is Cree, Inc.. Invention is credited to Qingcong Hu, Yuequan Hu.
Application Number | 20170231045 15/018375 |
Document ID | / |
Family ID | 58018345 |
Filed Date | 2017-08-10 |
United States Patent
Application |
20170231045 |
Kind Code |
A1 |
Hu; Yuequan ; et
al. |
August 10, 2017 |
SOLID STATE LIGHT FIXTURES HAVING ULTRA-LOW DIMMING CAPABILITIES
AND RELATED DRIVER CIRCUITS AND METHODS
Abstract
A solid state light fixture includes a light emitting diode
(LED) load and a driver circuit that selectively provides one of a
switched-mode drive current and a linear-mode drive current to the
LED load in response to a dimming control signal.
Inventors: |
Hu; Yuequan; (Cary, NC)
; Hu; Qingcong; (Morrisville, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cree, Inc. |
Durham |
NC |
US |
|
|
Family ID: |
58018345 |
Appl. No.: |
15/018375 |
Filed: |
February 8, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H05B 45/395 20200101;
Y02B 20/30 20130101; Y02B 20/343 20130101; H05B 45/10 20200101;
H05B 45/37 20200101 |
International
Class: |
H05B 33/08 20060101
H05B033/08 |
Claims
1. A solid state light fixture, comprising: a light emitting diode
(LED) load; and a driver circuit that selectively provides one of a
switched-mode drive current and a linear-mode drive current to the
LED load in response to a dimming control signal.
2. The solid state light fixture of claim 1, wherein the driver
circuit includes: a first current regulation circuit that is
configured to provide the switched-mode drive current to the LED
load; a second current regulation circuit that is configured to
provide the linear-mode drive current to the LED load, wherein the
second current regulation circuit includes a first switch that the
linear-mode drive current continuously flows through when the
second current regulation circuit is activated.
3. The solid state light fixture of claim 1, wherein the first
switch is coupled in series between the LED load and a current
sensing resistor.
4. The solid state light fixture of claim 3, wherein the second
current regulation circuit includes an amplifier having an output
that controls an amount of current flowing through the first switch
to regulate the current flowing through the LED load.
5. The solid state light fixture of claim 4, wherein the second
current regulation circuit further includes a second switch that is
coupled in series between the output of the amplifier and
ground.
6. The solid state light fixture of claim 4, wherein the amplifier
has a first input that receives a signal that is proportional to a
current flowing through the LED load and a second signal that is
proportional to a reference current level for the LED load.
7. The solid state light fixture of claim 2, wherein a third switch
that is included in the first current regulation circuit is
configured to provide the switched mode drive current to the LED
load when the first current regulation circuit is activated and to
deactivate the first current regulation circuit when the second
current regulation circuit is activated.
8. The solid state light fixture of claim 2, wherein the driver
circuit further includes: a rectifier circuit that is configured to
rectify an input alternating current voltage; a boost power factor
correction converter that is coupled to an output of the rectifier;
and a buck converter that is coupled to an output of the boost
power factor correction converter, wherein the first current
regulation circuit is part of the buck converter.
9. The solid state light fixture of claim 2, wherein the second
current regulation circuit is coupled in series to the output of
the LED load.
10. The solid state light fixture of claim 2, wherein the driver
circuit includes a control circuit that is configured to disable
the first current regulation circuit and enable the second current
regulation circuit in response to the dimming control signal having
a value specifying dimming the solid state light fixture below a
preselected level.
11. The solid state light fixture of claim 7, wherein the first
current regulation circuit includes a first resistor that is
coupled in series with the third switch and the second current
regulation circuit includes a first switch and a second resistor
that are coupled in series to the LED load.
12. (canceled)
13. A solid state light fixture, comprising: a light emitting diode
(LED) load that includes a first LED; a boost power factor
correction converter that includes a first switch; a driver circuit
that supplies a drive current to the first LED, the driver circuit
including a first current regulation circuit that includes a second
switch and a second current regulation circuit that includes a
third switch; and a control circuit that is configured to set the
second and third switches to selectively enable one of the first
and second current regulation circuits to regulate the drive
current.
14. The solid state light fixture of claim 13, wherein the control
circuit is configured to selectively enable the second current
regulation circuit and disable the first current regulation circuit
in response to the dimming control signal having a value specifying
dimming the solid state light fixture below a preselected
level.
15. The solid state light fixture of claim 13, wherein the second
current regulation circuit comprises a linear current
regulator.
16. The solid state light fixture of claim 15, wherein the linear
current regulator includes a fourth switch that is coupled in
series between the LED load and a reference voltage.
17. The solid state light fixture of claim 16, wherein the linear
current regulator includes an amplifier having an output that
controls an amount of current flowing through the fourth switch to
regulate the current flowing through the LED load.
18-19. (canceled)
20. The solid state light fixture of claim 13, wherein the first
current regulation circuit comprises a switched-mode current
regulation circuit.
21. The solid state light fixture of claim 20, wherein the second
switch included in the first current regulation circuit is
configured to provide a switched-mode drive current to the LED load
when the first current regulation circuit is activated and to
deactivate the first current regulation circuit when the second
current regulation circuit is activated.
22-36. (canceled)
37. The solid state light fixture of claim 1, further comprising an
image sensor configured to detect an image of an object illuminated
by the LED load.
38. A solid state light fixture, comprising: a light emitting diode
(LED) load; a boost power factor correction converter that includes
a first switch and that supplies a drive current to the LED load; a
linear regulator that regulates a level of the drive current; and a
control circuit that is configured to cause the linear regulator to
regulate the drive current in response to a dimming control signal.
Description
FIELD OF INVENTION
[0001] The present application generally relates to solid state
lighting fixtures, and more particularly, to dimmable solid state
lighting fixtures and related driver circuits and methods.
BACKGROUND
[0002] As is well known to those of skill in the art, a
light-emitting diode (LED) is a solid state semiconductor device
that includes one or more p-n junctions. LEDs emit light when
current flows through the p-n junctions thereof. The brightness and
energy efficiency of the light emitted by an LED may be directly
related to the amount of operating or "drive" current flowing
through the p-n junction of the LED. Typically, an LED is designed
to operate at a drive current level that provides both high
brightness and high energy efficiency. LED-based solid state light
fixtures include driver circuits that provide the drive current to
the LEDs.
[0003] Initially, LEDs were primarily used in specialty lighting
applications such as providing back-lighting and/or indicator
lights in electronic devices. As the light output and energy
efficiency of LEDs has improved, LEDs have been used to form solid
state light fixtures such as LED-based light bulbs, downlights,
streetlights and the like. These LED-based solid state light
fixtures are increasingly replacing conventional incandescent and
fluorescent light bulbs in numerous applications including general
illumination applications such as lighting for homes and offices.
These solid state light fixtures generate less heat, are far more
energy efficient and have far longer lifetime as compared to
incandescent light bulbs. Solid state LED-based light fixtures also
exhibit numerous advantages over fluorescent light bulbs, including
better energy efficiency, faster turn-on, better dimmability and
longer lifetime. LED-based solid state light fixtures may also
generate more aesthetically pleasing light than fluorescent light
bulbs, and do not contain mercury.
SUMMARY
[0004] Pursuant to embodiments of the present invention, solid
state light fixtures are provided that include a light emitting
diode (LED) load and a driver circuit that selectively provides one
of a switched-mode drive current and a linear-mode drive current to
the LED load in response to a dimming control signal.
[0005] In some embodiments, the driver circuit may include a first
current regulation circuit that is configured to provide the
switched-mode drive current to the LED load and a second current
regulation circuit that is configured to provide the linear-mode
drive current to the LED load. The second current regulation
circuit may include a first switch that the linear-mode drive
current continuously flows through when the second current
regulation circuit is activated.
[0006] In some embodiments, the first switch may be coupled in
series between the LED load and a current sensing resistor.
[0007] In some embodiments, the second current regulation circuit
may include an amplifier having an output that controls an amount
of current flowing through the first switch to regulate the current
flowing through the LED load.
[0008] In some embodiments, the second current regulation circuit
may further include a second switch that is coupled in series
between the output of the amplifier and ground.
[0009] In some embodiments, the amplifier may have a first input
that receives a signal that is proportional to a current flowing
through the LED load and a second signal that is proportional to a
reference current level for the LED load.
[0010] In some embodiments, a third switch that is included in the
first current regulation circuit may be configured to provide the
switched mode drive current to the LED load when the first current
regulation circuit is activated and to deactivate the first current
regulation circuit when the second current regulation circuit is
activated.
[0011] In some embodiments, the driver circuit may further include
a rectifier circuit that is configured to rectify an input
alternating current voltage, a boost power factor correction
converter that is coupled to an output of the rectifier, and a buck
converter that is coupled to an output of the boost power factor
correction converter. In such embodiment, the first current
regulation circuit may be part of the buck converter.
[0012] In some embodiments, the second current regulation circuit
may be coupled in series to the output of the LED load.
[0013] In some embodiments, the driver circuit may include a
control circuit that is configured to disable the first current
regulation circuit and enable the second current regulation circuit
in response to the dimming control signal having a value specifying
dimming the solid state light fixture below a preselected
level.
[0014] In some embodiments, the first current regulation circuit
may include a first resistor that is coupled in series with the
third switch and the second current regulation circuit may include
a first switch and a second resistor that are coupled in series to
the LED load.
[0015] In some embodiments, the second resistor may be at least two
orders of magnitude larger than the first resistor.
[0016] Pursuant to further embodiments of the present invention,
solid state light fixtures are provided that include an LED load, a
boost power factor correction converter that includes a first
switch, a driver circuit that supplies a drive current to the LED
load, the driver circuit including a first current regulation
circuit that includes a second switch and a second current
regulation circuit that includes a third switch that is
electrically connected to the LED load, and a control circuit that
is configured to selectively enable one of the first and second
current regulation circuits to regulate the drive current.
[0017] In some embodiments, the control circuit may be configured
to selectively enable the second current regulation circuit and
disable the first current regulation circuit in response to the
dimming control signal having a value specifying dimming the solid
state light fixture below a preselected level.
[0018] In some embodiments, the second current regulation circuit
may comprise a linear current regulator. The linear current
regulator may include a first switch that is coupled in series
between the LED load and a reference voltage. The linear current
regulator may also include an amplifier having an output that
controls an amount of current flowing through the first switch to
regulate the current flowing through the LED load. The amplifier
may, for example, have a first input that receives a signal that is
proportional to a current flowing through the LED load and a second
signal that is proportional to a reference current level for the
LED load. The linear current regulator may further include a second
switch that is coupled in series between the output of the
amplifier and the reference voltage.
[0019] In some embodiments, the first current regulation circuit
may be a switched-mode current regulation circuit.
[0020] In some embodiments, a switch included in the first current
regulation circuit may be configured to provide a switched-mode
drive current to the LED load when the first current regulation
circuit is activated and to deactivate the first current regulation
circuit when the second current regulation circuit is
activated.
[0021] In some embodiments, the driver circuit may further include
a boost power factor correction converter that includes a first
switch, a driver circuit that supplies a drive current to the LED
load, the driver circuit including a first current regulation
circuit that includes a second switch and a second current
regulation circuit that includes a third switch that is
electrically connected to the LED load and a control circuit that
is configured to selectively enable one of the first and second
current regulation circuits to regulate the drive current.
[0022] In some embodiments, the control circuit may be configured
to selectively enable one of the first and second current
regulation circuits in response to a dimming control signal.
[0023] Pursuant to yet additional embodiments of the present
invention, methods of dimming a solid state light fixture having an
LED load are provided. Pursuant to these methods, a drive current
having a first average current level is supplied to the LED load
via a first switched-mode current regulation circuit. A drive
current having a second average current level that is less than the
first average current level is then supplied to the LED load using
a linear current regulator.
[0024] In some embodiments, the second average current level may be
at least 90% less than the first average current level. In some
embodiments, the second average current level may be at least 99%
less than the first average current level.
[0025] In some embodiments, the control circuit may supply the
drive current having the second average current level to the LED
load using the liner current regulator in response to a dimming
control signal that has a value specifying dimming the solid state
light fixture below a pre-selected level.
[0026] In some embodiments, the linear current regulator may
include a first switch that is coupled in series between the LED
load and a reference voltage. In some embodiments, the linear
current regulator may include an amplifier having an output that
controls an amount of current flowing through the first switch to
regulate the current flowing through the LED load. In some
embodiments, the linear current regulator may further include a
second switch that is coupled in series between the output of the
amplifier and the reference voltage.
[0027] Pursuant to still further embodiments of the present
invention, driver circuits are provided that include a first
current regulation circuit that is configured to provide a
switched-mode drive current to a load, a second current regulation
circuit that is configured to provide a linear-mode drive current
to the load, the second current regulation circuit including a
first switch that the linear-mode drive circuit continuously flows
through when the second current regulation circuit is activated,
and a control circuit that selectively provides one of the
switched-mode drive current and the linear-mode drive current to
the load in response to a control signal.
[0028] In some embodiments, the control signal may be a dimming
control signal.
[0029] In some embodiments, the solid state light fixture may
include an image sensor configured to detect an image of an object
illuminated by the LED load.
[0030] A solid state light fixture according to further embodiments
includes a light emitting diode (LED) load, a boost power factor
correction converter that includes a first switch and that supplies
a drive current to the LED load, a linear regulator that is coupled
to the LED load and is configured to regulate a level of the drive
current, and a control circuit that is configured to cause the
linear regulator to regulate the drive current in response to a
dimming control signal.
BRIEF DESCRIPTION OF THE FIGURES
[0031] The above and other aspects and features of the present
invention will become more apparent by the following description of
example embodiments thereof with reference to the attached
drawings. In the drawings and specification, like numbers refer to
like elements.
[0032] FIG. 1 is a circuit diagram of a conventional driver circuit
that has dimming capabilities.
[0033] FIG. 2 is a schematic block diagram of a solid state light
fixture according to embodiments of the present invention.
[0034] FIG. 3 is a schematic block diagram of a solid state light
fixture according to further embodiments of the present
invention.
[0035] FIG. 4 is a schematic block diagram of a solid state light
fixture according to additional embodiments of the present
invention.
[0036] FIG. 5 is a circuit diagram of a driver circuit according to
embodiments of the present invention.
[0037] FIG. 6 is a circuit diagram of a driver circuit according to
further embodiments of the present invention.
[0038] FIGS. 7A-7D are schematic perspective diagrams illustrating
example solid state light fixtures according to embodiments of the
present invention.
[0039] FIG. 8 is a flow chart illustrating a method of dimming a
solid state light fixture according to embodiments of the present
invention.
[0040] FIG. 9 is a schematic block diagram of a solid state light
fixture according to further embodiments of the present
invention.
[0041] FIG. 10 is a graph illustrating the effect of a control
voltage on the operation of the solid state light fixture of FIG.
9.
[0042] FIGS. 11A and 11B are schematic diagrams illustrating solid
state light fixtures in accordance with some embodiments that
include or that operate in conjunction with an image sensor.
DETAILED DESCRIPTION
[0043] Pursuant to embodiments of the present invention, solid
state light fixtures are provided that have ultra-low dimming
capabilities, along with related driver circuits that may be used
in, or in conjunction with, these solid state light fixtures. The
driver circuits according to various embodiments operate in one of
a switching mode to accommodate normal operation or moderate
dimming and a linear mode to accommodate ultra-low dimming. In
particular, a linear current regulator is provided for ultra-low
dimming operation. During normal or moderate dimming operation, the
linear current regulator may be decoupled from the LED load or
otherwise operated in a manner to reduce power loss in the linear
current regulator. In some embodiments, the linear current
regulator may be selectively activated to provide ultra-low level
dimming.
[0044] The driver circuits included in the solid state light
fixtures according to some embodiments of the present invention
have current regulation circuitry that operates in (at least) two
different operating modes. In particular, the current regulation
stage of the driver circuits according to embodiments of the
present invention may operate in either a switching mode to
accommodate normal operation and moderate dimming or in a linear
mode to accommodate ultra-low dimming. In the switching mode, a
control circuit of the driver circuit may control a first switch
that turns on and off at a high frequency (e.g., 20 kHz to a few
hundred kHz) to regulate the current supplied to the LED load
(which may comprise one or more LEDs). The drive current provided
by a current regulation stage that operates in the switching mode
is referred to herein as a switched-mode drive current. In the
linear mode, the control circuit may control a second switch that
is part of a linear current regulator to regulate the drive current
that is supplied to the LED load. The drive current provided by a
linear current regulator is referred to herein as a linear-mode
drive current The linear mode may be used when the solid state
light fixture is dimmed to low or ultra-low levels. As will be
explained in greater detail below, by configuring the driver
circuit for the solid state light fixture to selectively operate in
one of two different drive modes, high energy efficiency can be
realized during full power operations and during moderate dimming
when the current regulation stage is operated in a first mode. When
the current regulation stage is operated in a second mode, the
light fixtures may be dimmed to ultra-low levels while still
providing a smooth and stable light output. Ultra-low dimming may
be used, for example, when the solid state light fixtures according
to embodiments of the present invention operate in conjunction with
an image sensor, camera or other sensor device.
[0045] Conventional high power driver circuits for solid state
light fixtures have current regulation stages that are configured
to operate in a switching mode in order to reduce power loss and
improve efficiency. Unfortunately, because of the switching nature
of these circuits, the drive current to the LED load and hence the
light output by the LED load, may become unstable during low-level
dimming, which may be aesthetically unpleasing and/or causes
discomfort to human observers.
[0046] FIG. 1 is a circuit diagram of a conventional driver circuit
10 for a solid state light fixture. As shown in FIG. 1, the driver
circuit 10 includes an alternating current (AC) voltage source 12,
a boost power factor correction (PFC) controller 14, a buck
controller 16 and a dimming controller 18. The driver circuit 10
further includes an EMI filter 24, a bridge rectifier 30, a boost
PFC converter 40, and a buck converter 60. The driver circuit 10
supplies a drive current to an LED load 20, which is exemplarily
illustrated in FIG. 1 as comprising a pair of LEDs 22 that are
disposed in series. While the LED load 20 is illustrated in FIG. 1
to facilitate explanation of the operation of the driver circuit
10, it will be appreciated that the LEDs 22 that form the LED load
20 are not part of the driver circuit 10 but instead comprise the
load that is driven by the driver circuit 10.
[0047] The AC voltage source 12 may comprise, for example, a
standard 120 V wall outlet. It will be appreciated, however, that a
wide variety of AC voltage sources 12 may be used such as, for
example, AC voltage sources that output AC voltages in the range of
100 V to 277 V or higher. The driver circuit 10 converts the AC
voltage input from the AC voltage source 12 into a direct current
(DC) voltage that is suitable for powering the LED load 20.
[0048] The EMI filter 24 is used to filter out high frequency noise
that may be present on the AC power output from the AC voltage
source 12 and noise generated by the driver circuit 10. The EMI
filter 24 may, for example, divert high frequency noise components
that are carried on the conductors of the AC voltage source 12 to
ground. EMI filters are well known in the art and hence further
description thereof will be omitted here.
[0049] The bridge rectifier circuit 30 comprises a series of diodes
32, 34, 36, 38 that are arranged in a bridge configuration as shown
in FIG. 1. The bridge rectifier circuit 30 rectifies the AC output
from the AC voltage source 12 to provide a DC voltage at the output
of the bridge rectifier 30. Bridge rectifier circuits are also well
known in the art and hence further description thereof will be
omitted here.
[0050] The DC voltage output by the bridge rectifier circuit 30
(V.sub.REC) is the input to the boost PFC converter 40. The boost
PFC converter 40 includes an inductor 42, a switch 44, a diode 46,
a capacitor 48 and a resistor 50. The boost PFC converter 40
functions as a DC-to-DC power converter that converts a DC voltage
that is input from the bridge rectifier circuit 30 into a higher
voltage DC voltage V.sub.B that is output from the boost PFC
converter 40. As the boost PFC converter 40 steps up the voltage,
the current output by the boost PFC converter 40 is necessarily
reduced as compared to the input current as power (P=V*I) must be
conserved.
[0051] The switch 44 may comprise, for example, a MOSFET transistor
44. The boost PFC controller 14 provides a control signal to the
gate of the MOSFET 44 in order to turn the transistor on and off.
When the MOSFET 44 is turned on (i.e., the switch 44 is closed),
current flows through the inductor 42 in the clockwise direction
and the inductor 42 stores energy by generating a magnetic field.
When the MOSFET 44 is turned off (i.e., the switch 44 is opened),
the only path for the current is through the flyback diode 46, and
hence the capacitor 48 is charged when the MOSFET 44 is turned off.
There are two modes of PFC operations, namely a continuous
conduction mode (CCM) and a discontinuous conduction mode (DCM). In
the CCM mode of operation, the switch 44 is cycled between its on
and off states fast enough so that the inductor 42 does not fully
discharge during each time period when the switch 44 is turned off
(opened). In the DCM mode of operation, the inductor 42 current
always decreases to zero (i.e., the inductor 42 is fully
discharged) during each time period when the switch 44 is turned
off (opened). When the switch 44 is turned off, the DC voltage
output by the bridge rectifier circuit 30 and the inductor 42
appear as two voltage sources in series which allows the capacitor
48 to be charged to a voltage higher than the DC voltage V.sub.REC
that is output by the bridge rectifier circuit 30. When the switch
44 is closed by turning the MOSFET 44 on, the DC voltage output by
the bridge rectifier circuit 30 is applied across the inductor 42
and diode 46 is reverse biased. As such, current does not flow from
the bridge rectifier circuit 30 to the output of the boost PFC
converter 40, and the capacitor 48 provides the voltage and current
to the output of the boost PFC converter 40. The capacitor 48 is
recharged the next time the switch 44 is opened in the manner
described above. Thus, by controlling the frequency and/or duty
cycle at which the switch 44 is turned on and off, the boost PFC
controller 14 may regulate the output voltage of the boost PFC
converter 40 (i.e., the voltage across capacitor 48). The boost PFC
converter 40 also provides power factor correction by shaping the
input current so that it follows the shape of the input AC voltage
provided by the AC voltage source 12. The boost PFC converter 40
may achieve a high level of power factor correction (greater than
0.9) and low total harmonic distortion (less than 20%).
[0052] The boost PFC controller 14 may receive as inputs voltages
V.sub.1, V.sub.REC and V.sub.B. Voltage V.sub.1 is the voltage drop
across resistor 50, which may be used to sense the current flowing
through the switch 44. Voltage V.sub.REC is the voltage at the
output of the bridge rectifier circuit 30. Voltage V.sub.B is the
voltage across the output of the boost PFC converter 40. The PFC
controller 14 may use these voltages to adjust the frequency and/or
duty cycle at which the switch 44 is turned on and off to maintain
the output voltage V.sub.B at a desired level while also achieving
a high degree of power factor correction. A controller that is
commercially available from ST Microelectronics (part number L6564)
can be used to implement the boost PFC controller 14.
[0053] The buck converter 60 regulates the drive current supplied
to the LED load 20. The DC voltage output by the boost PFC
converter 40 is applied across the input to the buck converter 60.
The buck converter 60 includes a diode 62, a capacitor 64, a first
resistor 66, an inductor 68, a switch 70, a second resistor 72 and
a current monitor 74. The switch 70 is used to regulate the amount
of drive current that is supplied to the LED load 20, as will be
explained in more detail below. The switch 70 may comprise, for
example, a MOSFET transistor 70. The buck controller 16 provides a
control signal V.sub.GS to the gate of the MOSFET 70 in order to
turn the MOSFET 70 on and off. The first resistor 66 and the
inductor 68 are coupled in series with output of the LED load 20,
and the capacitor 64 is connected in parallel across the LED load
20.
[0054] The switch 70 is turned on and off to regulate the drive
current flowing through the LED load 20. In particular, the current
monitor 74 senses the current flowing through the LED load 20. The
capacitor 64 maintains the voltage across the LED load relatively
constant, thereby providing a relatively constant current to the
LED load 20, and filters out AC components in the drive current.
The diode 62 provides a current path that allows the energy stored
in the inductor 68 to be released to the LED load 20.
[0055] The current monitor 74 is connected across the first
resistor 66 and outputs a signal that reflects the current flowing
through the LED load 20. In the depicted embodiment, the current
monitor 74 outputs a voltage signal V.sub.3 that reflects the
voltage drop across the first resistor 66. Since the value of first
resistor 66 is known, the load current can be calculated directly
from the voltage drop V.sub.3 via Ohms Law. The second resistor 72
is coupled between the switch 70 and a reference voltage (e.g.,
ground). The voltage V.sub.2 at the connection between the switch
70 and the second resistor 72 reflects the voltage drop across the
second resistor 72, and thus may be used to measure the current
flowing through the switch 70.
[0056] The buck converter 60 may operate as follows. The DC signal
output by the boost PFC converter 40 provides a current to the LED
load 20. The buck controller 16 regulates the current flowing
through the LED load 20 by sensing the current through the LED load
20 (represented by voltage V.sub.3) and the current flowing through
the switch 70 (represented by voltage V.sub.2). The buck controller
16 outputs a signal V.sub.GS to the gate of the MOSFET 70 to
regulate the current through the LED load 20. When the signal
V.sub.GS that is applied to the gate of the MOSFET 70 is high, the
MOSFET 70 is turned on (i.e., the switch 70 is closed) and current
flows from the boost PFC converter 40 through the LED load 20,
through the first resistor 66 and the inductor 68 and then through
the switch 70. The inductor 68 stores energy by generating a
magnetic field during such time periods. When the signal V.sub.GS
that is applied to the gate of the MOSFET 70 is brought low, the
MOSFET 70 is turned off (i.e., the switch 70 is opened) and the
inductor 68 discharges through the diode 62 to maintain the current
flow through the LED load 20. In a fixed switching frequency CCM
mode of operation, a comparator in the buck controller 16 compares
the signal V.sub.2 to an error signal. The error signal may be the
result of a comparison of signal V.sub.3 to a reference voltage
that reflects a desired drive current level for the LED load 20.
When signal V.sub.2 reaches the error signal level, the control
signal V.sub.GS is pulled low and the switch 70 is turned off
(opened). When next switching cycle starts, the control signal
V.sub.GS is set to high and the switch 70 is turned back on again.
A controller that is commercially available from Texas Instruments
(part number UC3843) can be used to implement the buck controller
16 when operated in the CCM mode of operation. If the buck
controller 16 is operated in a CCM/DCM critical mode of operation,
i.e., the switch 70 is turned on at the moment the current of the
inductor 68 decreases to zero. When signal V.sub.2 reaches the
error signal level (a comparison of signal V.sub.3 to a reference
voltage that reflects a desired drive current level for the LED
load 20), the control signal V.sub.GS is pulled low and the switch
70 is turned off (opened). A controller that is commercially
available from Texas Instruments (part number UCC28811D) or a
controller commercially available from ST Microelectronics (part
number L6564) can be used to implement the buck controller 16. In
CCM/DCM critical mode operation, the switching frequency of the
switch 70 varies, i.e., it increases as the current through the LED
load 20 decreases and vice versa.
[0057] The ability to dim a solid state light fixture may be very
important in various applications, including general illumination
and specialty lighting applications within the home. Low and/or
ultra-low dimming may also be desired or necessary in some
applications, either for consumer preference or so that the solid
state light fixture can operate in conjunction with an internal or
external sensor such as an image sensor. As noted above, the driver
circuit 10 includes a dimming control circuit 18 that may be used
to accomplish such dimming. In particular, the dimming control
circuit 18 may generate one or more control signals that control
other elements of the driver circuit 10 to reduce the level of
drive current flowing through the LED load 20, thereby reducing the
amount of light output by the LEDs 22. The dimming control circuit
18 may operate in response to an external control signal. The
dimming control circuit 18 generates a dimming control signal
V.sub.DIM that is provided to the buck controller 16 to control the
dimming operations. The dimming control circuit 18 may also provide
a control signal to the boost PFC controller 14 that is used to
enable or disable the boost PFC operation.
[0058] The buck controller 16 may control the brightness of light
emitted by the LED load 20 in one of two ways in response to the
control signals provided by the dimming control circuit 18. In a
first approach for dimming, the buck controller 16 may reduce the
average current supplied to the LED load 20. This may be
accomplished, for example, by reducing a current reference that is
used by the buck controller 16 to determine the level of current
flowing through the load. The frequency at which the switch 70 is
toggled between its on and off states may be increased to reduce
the current flow through the LED load 20 when the buck converter 60
is operated in CCM/DCM critical mode.
[0059] During normal (i.e., non-dimmed) operation, the current
supplied to the LED load 20 can be very high, such as, for example,
a current of 1.5 A for power LEDs. Reducing the level of the drive
current supplied to the LED load 20 by the buck converter 60 will
not result in issues for moderate levels of dimming. However, when
very low dimming is required, issues may arise. For example, if the
same dimming, i.e., the buck converter 60 operates in a switched
mode with an adjustable current reference is used in the driver
circuit 10 of FIG. 1 and the light output must be dimmed to 0.1% of
the normal level, then assuming a nominal drive current of 1.5 A,
the drive current flowing to the LED load 20 must be regulated to
be at 1.5 mA to achieve the desired level of dimming. As discussed
above, the current monitor 74 is used to measure the drive current
flowing through the LED load 20, and hence must be able to
accurately measure this 1.5 mA drive current in order to maintain
the drive current at the value necessary to achieve the desired
level of dimming. Typically, the first and second resistors 66, 72
are implemented using resistors having low resistances in order to
reduce the power loss. Thus, in an example implementation, each of
the resistors 66, 72 might be implemented as a 0.25.OMEGA.
resistor. As an example, if the buck converter 60 is operating in
CCM/DCM critical mode (i.e., a mode where the switch 70 is turned
on each time the current flowing from the inductor 68 drops to
zero), the control signal V2 that reflects the current flowing
through the switch 70 and the control signal V3 that reflects the
current flowing through the LED load 20 will have values of 0.75 mV
and 0.375 mV, respectively. At these low voltage levels, noise
generated in the driver circuit 10 and/or external noise may be
close in magnitude to the control signals V2 and V3 and hence it
may be difficult or impossible for the buck controller 16 to
achieve stable regulation of the drive current supplied to the LED
load 20, as the noise will create significant instabilities in the
drive current levels. While resistors having higher resistances
could be used to implement the first and second resistors 66, 72 in
order to increase the magnitude of the control signals V2 and V3
during ultra-low dimming operation, the use of higher resistance
resistors increases the power loss and is not a practical solution
for applications where ultra-low dimming capabilities are
desired.
[0060] In the second approach, pulse width modulation dimming is
used. With pulse width modulation dimming, the drive current
flowing through the LED load 20 may be maintained at its normal
peak value (i.e., the value during non-dimming operations). A duty
cycle is applied so that the drive current is supplied to the LED
load 20 as a pulsed signal. During a first portion of each cycle,
the drive current is supplied to the LED load 20, and then the
drive current is cut off (by opening switch 70) from the LED load
20 (except for the current supplied by the inductor 68) during the
second portion of each cycle. In this fashion, the peak current
supplied to the LED load 20 may be maintained constant, but the
average current is reduced. The amount of dimming applied may be
controlled by varying the duty cycle of the pulses (i.e., the
percentages of each cycle during which the drive current is and is
not supplied to the LED load 20).
[0061] As pulse width modulation dimming maintains the drive
current supplied to the load at its peak level, the above-discussed
problems with reducing the peak current to achieve ultra-low
dimming will not be present when pulse width modulation dimming is
used, as the drive current would be maintained at 1.5 A in the
example above, but would only be applied to the LED load 20 for a
very small fraction ( 1/1000th) of each cycle. Unfortunately, due
to the long "off" periods in the duty cycle that are necessary to
achieve ultra-low dimming, undesired flickering or shimmering may
result when pulse width modulation dimming is used, which may cause
discomfort for a human observer. Such flickering and/or shimmering
may be reduced by using a very large electrolytic capacitor (e.g.,
470 microfarads) to implement the capacitor 64. However, the use of
such large electrolytic capacitors may be impractical in many
applications due to cost and/or size constraints, lifetime
requirements and/or because the response time of the capacitor may
be inadequate. Instead of using a large capacitor 64 to address the
problem of flicker during ultra-low dimming, the frequency of the
pulse width modulation of the drive current may be increased in
order to reduce the length of each "off" cycle. However, the use of
high frequency pulse width modulation may be undesirable because,
when ultra-low dimming is needed, the time for the buck converter
60 to operate will become too short for the LED current to be
regulated as desired due to the response time of the buck converter
60.
[0062] As discussed above, pursuant to embodiments of the present
invention, solid state light fixtures are provided that have
ultra-low dimming capabilities, along with related driver circuits
that may be used in these solid state light fixtures. The solid
state light fixtures according to embodiments of the present
invention have driver circuits with current regulation circuitry
that operates in (at least) two different operation modes. In
particular, the current regulation stage of the driver circuits
according to embodiments of the present invention may operate in
either a switching mode or in a linear mode. In the switching mode,
a control circuit that is part of the current regulation stage may
be used to control a first switch that turns on and off at a high
frequency (e.g., 20 kHz to a few hundred kHz) to regulate the
current supplied to the LED load (which may comprise one or more
LEDs). The switching mode may be used to provide the drive current
to the LED load during normal (non-dimmed) operation and during
dimming to moderate levels. Either amplitude dimming, i.e.,
changing the amplitude of the LED load current with switch 70
operating at high frequency, or pulse width modulation dimming may
be used when the current regulation circuitry operates in switching
mode.
[0063] When the solid state light fixture is to be dimmed to low
and/or ultra-low levels, the current regulation stage may switch to
a linear mode to provide a linear-mode drive current to the LED
load. In the linear mode, the control circuit of the current
regulation stage may control a second switch such as, for example,
a MOSFET or bipolar junction transistor (BJT) to regulate the
current through the LED using a linear current regulator. In
particular, the control circuit may adjust an amount of current
that continuously flows through the second switch to regulate the
drive current supplied to the LED load. Thus, the control circuits
of the current regulation stages of the driver circuits according
to embodiments of the present invention may selectively supply
either a switched-mode drive current or a linear-mode drive current
to the LED load. The type of drive current supplied to the LED load
may be based, for example, on an amount that the light output by
the solid state light fixture is to be dimmed. As will be explained
in greater detail below, by configuring the solid state light
fixture to selectively operate in one of two different drive modes,
high energy efficiency can be realized during full power operations
and during moderate dimming and, at the same time, the light
fixtures may be dimmed to low or ultra-low levels while still
providing a smooth and stable light output.
[0064] FIG. 2 is a schematic block diagram of a solid state light
fixture 100 according to embodiments of the present invention. As
shown in FIG. 2, solid state light fixture 100 includes a driver
circuit 110 and an LED load 150. The LED load 150 may comprise, for
example, one or more LEDs (not shown). When multiple LEDs are
provided, the LEDs may be arranged in series or in parallel or a
combination thereof.
[0065] The driver circuit includes a first current regulation
circuit 120, a second current regulation circuit 130 and a control
circuit 140. The first current regulation circuit 120 may, for
example, comprise a current regulation circuit that operates in a
switching mode to provide a switched-mode drive current to the LED
load 150. For example, the buck converter 60 of the drive circuit
of 10 of FIG. 1 may be used to implement the first current
regulation circuit 120 in some embodiments. The second current
regulation circuit 130 may, in some embodiments, be arranged in
series with the LED load 150 to regulate the drive current flowing
through the LED load 150. The second current regulation circuit 130
may comprise a linear current regulator in some embodiments that is
used to supply a linear-mode drive current to the LED load 150.
[0066] The control circuit 140 may be used to selectively activate
one or the other of the first current regulation circuit 120 and
the second current regulation circuit 130. While not shown in FIG.
2, one or more switches may be provided that are used to activate
the first and second current regulation circuits 120, 130. The
control circuit 140 may operate in response to an external dimming
control signal that indicates an amount that the solid state light
fixture 100 is to be dimmed.
[0067] FIG. 3 is a schematic block diagram of a solid state light
fixture 200 according to further embodiments of the present
invention. As shown in FIG. 3, solid state light fixture 200
includes a driver circuit 210 and an LED load 250. The LED load 250
may be the same as the LED load 150 that is discussed above and
hence further description thereof will be omitted here.
[0068] The driver circuit 210 may be configured to selectively
provide one of a switched-mode drive current and a linear-mode
drive current to the LED load 250 in response to a dimming control
signal. In some embodiments, the switched-mode drive current may be
provided to the LED load 250 when the solid state light fixture is
to operate at full power or under a moderate level of dimming,
while the linear-mode drive current is provided to the LED load 250
when the solid state light fixture is to be dimmed to ultra-low
levels.
[0069] FIG. 4 is a schematic block diagram of a solid state light
fixture 300 according to still further embodiments of the present
invention that includes a driver circuit 310 and an LED load 350.
The LED load 350 may be the same as the LED load 150 that is
discussed above and hence further description thereof will be
omitted here
[0070] The driver circuit 310 includes a rectifier 320, a boost PFC
converter 330, a buck converter 340, a control circuit 360 and a
linear current regulator 370. The rectifier 320 may receive an AC
voltage from an external source and may rectify this AC voltage.
Any suitable rectifier circuit may be used to implement rectifier
320. The boost PFC converter 330 may receive the rectified voltage
output by the rectifier 320 and may increase the voltage thereof.
The boost PFC converter 330 may operate under the control of the
control circuit 360. The boost PFC converter 330 may also provide
power factor correction in some embodiments. The buck converter 340
may also operate under the control of the control circuit 360 to
regulate the current supplied to the LED load 350 under normal
operating conditions and during dimming down to a certain
pre-selected level. The current regulation stage of the buck
converter 340 may operate in a switched mode to reduce power losses
and/or increase efficiency. When the buck converter 340 acts to
regulate the current supplied to the LED load 350, the linear
current regulator 370 may be turned off. When dimming to levels
below the pre-selected level is required, the control circuit 360
may activate the linear current regulator 370 and may disable the
buck converter 340. Under these conditions, the linear current
regulator 370 may maintain the drive current through the LED load
350 at a desired level.
[0071] FIG. 5 is a circuit diagram of a circuit driver 400
according to certain embodiments of the present invention. The
circuit driver 400 includes various circuit elements that are
included in the conventional LED driver circuit 10 that is
discussed above with reference to FIG. 1, and hence these circuit
elements are identified by the same reference numerals and further
description thereof will be omitted. Additionally, the driver
circuit 400 further includes a linear regulator circuit 480 (also
referred to herein as a linear current regulator) that is provided
in series with the LED load 20. While the LED load 20 is
illustrated in FIG. 5 to facilitate explanation of the operation of
the driver circuit 400, it will be appreciated that the LEDs 22 are
not part of the driver circuit 400 but instead comprise the load
that is driven by the driver circuit 400. The linear regulator
circuit 480 may be switched on when ultra-low dimming is
required.
[0072] As shown in FIG. 5, the linear regulator circuit 480
includes an error amplifier 482, a switch 484, a current sensing
resistor 486, a resistor 488 and a switch 490. The switches 484 and
490 may be implemented, for example as MOSFET transistor switches.
The error amplifier 482 outputs a signal through the resistor 488.
During normal (non-dimmed) operation and during dimming operations
where the amount the light output is to be dimmed is below a
pre-selected level, the switch 490 is turned on by applying a high
voltage signal V7 to the gate thereof. As the switch 490 is turned
on, the gate of switch 484 is pulled to a low value, thereby
turning off switch 484. Under these conditions, the linear current
regulator 480 is isolated from the LED load 20, and the current
regulation stage of the buck converter 60 (i.e., switch 70,
inductor 68, output capacitor 64, and diode 62) operates normally
in the manner described above with reference to FIG. 1 to regulate
the drive current provided to the LED load 20 using switched mode
current regulation.
[0073] When ultra-low dimming is required (or dimming to a level
that is below the pre-selected level), the control signal V.sub.GS
is set to a low level by the buck controller 16, thereby turning
off switch 70. As a return current path no longer exists through
the buck converter 460, the current regulation circuit of the buck
converter 60 is fully disabled once the inductor 68 discharges. At
the same time, the control signal V.sub.7 may be set to a low level
in order to turn off the switch 490, activating the linear
regulator circuit 480.
[0074] When the linear regulator circuit 480 is activated, the
error amplifier 482 operates to maintain the drive current through
the LED load 20 at a desired level. The linear regulator 480
accomplishes this by inputting a signal V.sub.4 that reflects the
actual current through the LED load 20 and a reference signal
V.sub.5 to the error amplifier 482. The signal V.sub.4 reflects the
voltage drop across the resistor 486 (a known value), and hence
directly corresponds to the drive current through the LED load 20.
The reference voltage V.sub.5 is a drive current reference voltage
that corresponds to a desired LED load current that will achieve
the specified amount of dimming requested. The voltage V.sub.5 can
be generated by any of the control circuits shown in FIG. 5 or by
another control circuit such as, for example, a wireless-controlled
module or a micro-controller within a communication bus.
[0075] When the linear current regulator 480 is activated, the gate
voltage of transistor 484 is adjusted based on the voltage output
by the error amplifier 482. In operation, if the drive current
through the LED load 20 becomes higher than a desired value (i.e.,
the value corresponding to reference voltage V.sub.5), V.sub.4
increases. Since voltage V.sub.4 is fed to the error amplifier 482,
the increase in the drive current through the LED load 20 acts to
decrease the output voltage of the error amplifier 482, which in
turn decreases the drive current through the LED load 20 until the
target drive current is achieved. If the drive current through the
LED load 20 drops below the desired value, V.sub.4 decreases, which
causes the error amplifier 482 to increase its output voltage,
resulting in an increase in the drive current through the LED load
20. Thus, the linear regulator circuit 480 may be used to set and
maintain the drive current through the LED load 20 at a desired
level. The linear current regulator 480 achieves this by allowing a
current to flow continuously through switch 484 and adjusting the
amount of current that flows by changing the voltage applied to the
gate of switch 484.
[0076] Since the linear regulator circuit 480 may only be used for
low or ultra-low dimming operations, the drive current through the
LED load 20 will be very low when the linear regulator circuit 480
is operating. Consequently, a significantly larger current sensing
resistance can be used to implement the resistor 486 (as compared
to, for example, resistors 66 and 72) with only a negligible impact
on power loss. For example, in one example embodiment, resistor 486
is implemented as a 124.OMEGA. resistor. Assuming an LED load drive
current of 1.5 mA, the sensed signal is 0.186V across 486, and the
power loss across resistor 486 is only 0.279 mW. Additional power
loss will occur across the switch 484. Assuming that V.sub.B=440V
and V.sub.LED=300V, the power loss across the switch 484 is 0.21 W,
which is manageable. Since a tightly-regulated and continuous
current flows through the LED load 20 during ultra-low dimming, no
flickering or shimmering may be present.
[0077] Thus, the driver circuit 400 supplies a switched-mode drive
current to the LED load 20 when the solid state light fixture is
not dimmed or when the solid state light fixture receives a dimming
control signal having a value specifying dimming the solid state
light fixture above a pre-selected level. In contrast, the driver
circuit 400 supplies a linear-mode drive current to the LED load 20
when the solid state light fixture receives a dimming control
signal having a value specifying dimming the solid state light
fixture below the pre-selected level. Herein, a switched-mode drive
current refers to a drive current that is supplied to the LED load
using a circuit having a switch that is turned on and off as part
of the normal delivery of the drive current to the LED load such
as, for example, the current regulation stage of the buck
converters discussed above. A linear-mode drive current refers to a
drive current that is supplied to the LED load in a continuous
fashion without toggling a switch on and off as part of the normal
delivery of the drive current to the LED load such as, for example,
the drive currents supplied to the LED loads using the linear
current regulator 480. A "drive current" refers to the operating
current that is supplied to the LED load to power the LEDs included
therein.
[0078] It should also be noted that references herein to dimming a
solid state light fixture below a pre-selected level means reducing
the output of the solid state light fixture so that the output is
below the pre-selected level. The pre-selected level may be
specified in various ways such as, for example, a fraction of the
average drive current during normal (non-dimmed) operations or a
fraction of the light output during normal (non-dimmed)
operations.
[0079] The pre-selected level of dimming where the driver circuit
400 switches from switched mode to linear mode may be set based on
a number of factors, including power loss, switching frequency, and
actual operating conditions like the output voltage of the PFC
boost converter 40 and the voltage of the LED load 20. If the
difference between the output voltage of the PFC converter 40 and
the voltage of the LED load 20 is relatively large, amplitude
dimming may be used at lower load current levels to avoid the power
loss of the switch 484 in the linear current regulator 480. In some
embodiments, the pre-selected amount of dimming may be selected to
encompass dimming in which the drive current through the LED load
is reduced to ten percent or less the drive current through the LED
load 20 under normal (non-dimmed) operating conditions. In other
embodiments, the pre-selected amount of dimming may be selected to
encompass dimming in which the drive current through the LED load
20 is reduced to five percent or less the drive current through the
LED load 20 under normal operating conditions. In still other
embodiments, the pre-selected amount of dimming may be selected to
encompass dimming in which the drive current through the LED load
20 is reduced to one percent or less the drive current through the
LED load 20 under normal operating conditions.
[0080] FIG. 6 is a circuit diagram of a driver circuit 500
according to further embodiments of the present invention. The
driver circuit 500 is similar to the driver circuit 400 that is
discussed above with reference to FIG. 5, except that in the driver
circuit 500 a micro-controller based control circuit 516 is used to
replace the analog buck controller 16 that is included in the
driver circuit 400. In the driver circuit 500, the inductor 68 of
driver circuit 400 is replaced with a first side 568-1 of a winding
568. The micro-controller based control circuit 516 may detect when
the current through the winding 568 falls to zero by sampling the
voltage across the second side 568-2 of the winding 568. When the
voltage across the second winding 568-2 drops to a level close to
zero from a high level after the switch 70 is turned off, the
switch 70 is then turned back on again. The micro-controller based
control circuit 516 may control the switch 70 based on the signals
V2, V3 and VDIM. While not shown in FIG. 5, a second winding on
inductor 68 may be used to sample the voltage across the inductor
68 in the driver circuit 400 of FIG. 5. When the voltage across
this second winding drops to a level close to zero from a high
level as the switch 70 is turned off, the switch 70 is turned back
on again.
[0081] It will be appreciated by those skilled in the art that the
front end of the buck converter 60 that is included in the current
driver circuits 400, 500 of FIGS. 5 and 6 can have any appropriate
topology including, for example, a flyback, a buck-boost or a buck
topology. Likewise, the switching current regulation circuitry that
is used to regulate the drive current to the LED load 20 during
normal operating conditions and moderate levels of dimming can be
any appropriate type of switching current regulation circuit. The
present invention may improve on conventional driver circuits by
providing a second current regulation circuit that is configured to
provide high performance during ultra-low dimming operation. This
second current regulation circuit may comprise a linear current
regulator that can stably maintain a low drive current through an
LED load. The driver circuits according to embodiments of the
present invention may switch from a power efficient switching
current regulation mode to a less efficient, but more stable,
linear current regulation mode to achieve stable LED current and
smooth dimming without flickering or shimmering when ultra-low
dimming is required.
[0082] The driver circuits according to embodiments of the present
invention may be incorporated into a solid state light fixture to
provide an energy efficient fixture that may stably dim to
ultra-low levels. These driver circuits may be incorporated into a
wide variety of different types of solid state light fixtures. For
example, FIG. 7A schematically illustrates a light bulb 600 that
may include a driver circuit according to embodiments of the
present invention. FIG. 7B schematically illustrates a downlight
610 that likewise may include one of the driver circuits according
to embodiments of the present invention. FIG. 7C illustrates a
"troffer" style solid state ceiling based light fixture 620 that
may include one of the driver circuits according to embodiments of
the present invention. FIG. 7D schematically illustrates a
streetlight 630 that likewise may include one of the driver
circuits according to embodiments of the present invention. It will
be appreciated that the driver circuits according to embodiments of
the present invention may be incorporated into any appropriate
solid state light fixture.
[0083] As noted above, the solid state light fixtures according to
embodiments of the present invention may be used in conjunction
with sensors such as, for example, image sensors. The driver
circuits disclosed herein may provide ultra low light levels
utilizing very low amounts of energy while still providing enough
light be utilized by sensor circuitry, such as image sensors or
cameras, to improve sensor functionality and/or to enable sensor
applications. The sensor may be internal to the solid state light
fixture, and thus it will be appreciated that any of the solid
state light fixtures 600, 610, 620, 630 illustrated in FIGS. 7A-7D
may include an image sensor or a camera, and the solid state light
fixture may be operated at ultra-low dimming levels (e.g., 1% or
less of the light output during normal, non-dimmed operation) to
maintain or improve sensor functionality. The driver circuits
disclosed herein may also be included in, for example, any of the
solid state light fixtures disclosed in U.S. patent application
Ser. No. ______, entitled "LED LUMINAIRE HAVING ENHANCED THERMAL
MANAGEMENT" (Attorney Docket P2532) and filed concurrently herewith
on Feb. 8, 2016 and assigned to the same Assignee as the present
application and/or U.S. patent application Ser. No. ______,
entitled "DISTRIBUTED LIGHTING NETWORK" (Attorney Docket P2592) and
filed concurrently herewith on Feb. 8, 2016 and assigned to the
same Assignee as the present application, the entire contents of
both of which are incorporated herein by reference.
[0084] FIG. 8 is a flow chart illustrating a method of dimming a
solid state light fixture according to embodiments of the present
invention. As shown in FIG. 8, operations may begin with a drive
current having a first average current level being supplied to an
LED load via a first switched-mode current regulation circuit
(Block 700). A dimming control signal may be received that request
ultra-low dimming (Block 710). In response to the dimming control
signal, a drive current having a second average current level that
is less than the first average current level may be supplied to the
LED load using a linear current regulator (Block 720).
[0085] It will be appreciated that a wide variety of changes may be
made to the example embodiments described above without departing
from the scope of the present invention. For example, in the
embodiments of the present invention shown in the circuit diagrams
of FIGS. 5 and 6, MOSFETs are used to implement various switches in
the PFC boost converter 40, the buck converter 60 and the linear
current regulator circuit 480. It will be appreciated that a wide
variety of different elements may be used to implement these
switches such as, for example, bipolar junction transistors,
thyristors, insulated gate bipolar junction transistors and the
like. As another example, while the driver circuits shown in the
examples herein have an AC voltage source, it will be appreciated
that a DC voltage source (e.g., a battery) may be used in other
embodiments. In such embodiments, the rectifier may be omitted.
[0086] FIG. 9 is a schematic block diagram of a solid state light
fixture according to further embodiments of the present invention,
and FIG. 10 is a graph illustrating the effect of a control voltage
on the operation of the solid state light fixture of FIG. 9.
[0087] In particular, some embodiments may omit the buck converter,
while retaining the use of a linear current regulator for low level
dimming. For example, a PFC circuit may supply a voltage to an LED
load, while a linear voltage regulator is provided for low-level
dimming.
[0088] Referring to FIG. 9, a boost PFC converter 40, which is a
switch-mode voltage converter, may receive an AC voltage from a
source 12 and supply a boost voltage V.sub.PFC across a capacitor
48, which provides an adjustable output voltage that drives the LED
load 22. A linear regulator circuit 580 includes an error amplifier
582, a switch 584, and a current sensing resistor 586 having a
resistance Rs. The switch 584 may be implemented, for example as
MOSFET transistor switch, although other types of switches can be
used.
[0089] A controller 516 senses a voltage V.sub.Linear at an output
of the load 22 and generates a control signal
V.sub.ctrl.sub._.sub.linear in response to the sensed voltage
V.sub.Linear and a dimming control signal output by a dimming
controller 518. The control signal V.sub.ctrl.sub._.sub.linear is
supplied to the non-inverting input of the error amplifier 582,
while a voltage across the current sensing resistor 586 (which is
proportional to the current flowing through the load 22) is
supplied to the inverting input of the error amplifier 518. The
value of the control signal V.sub.ctrl.sub._.sub.linear may be
selected to cause a predetermined level of current to flow through
the load during low dimming conditions. During high dimming
conditions, the value of the control signal
V.sub.ctrl.sub._.sub.linear may be set high, turning the switch 584
on and reducing the power dissipated in the switch 584.
[0090] That is, under high power conditions, the voltage drop on
the linear regulator should be kept low to reduce power loss. Under
low dimming conditions, the voltage drop across the switch 584
increases to enable the linear regulator 580 to regulate current
through the load even in the presence of ripple in the voltage
provided by the PFC boost converter.
[0091] In the embodiments of FIG. 9, a first stage boost PFC
converter 40 provides an output voltage V.sub.PFC with a high power
factor to the load 22. A second stage linear current regulator 580
regulates the voltage that is dropped across the switch 584 based
on a current sense signal output by the current sense resistor 586
and the control signal V.sub.ctrl.sub._.sub.linear. The linear
current regulator 580 controls the LED current to be:
I.sub.LED=V.sub.ctrl.sub._.sub.linear/Rs [1]
[0092] The control target for the PFC stage 40 is the voltage on
the linear regulator V.sub.Linear. V.sub.Linear should be high
enough under dimming condition so that the linear regulator can
effectively control the LED current. The minimum value for
V.sub.Linear can be estimated as:
V.sub.Linear.sub._.sub.min=V.sub.ctrl.sub._.sub.linear+V.sub.TH
[2]
where V.sub.TH is the threshold voltage of the switch 584. With
some headroom on V.sub.Linear, the linear regulator can function
properly even if there is some ripple on the output voltage
V.sub.PFC of the PFC converter 40.
[0093] In order to achieve ultra-low dimming, the LED current
should be regulated even if the PFC output voltage V.sub.PFC
contains ripple. The voltage ripple may become large when a phase
dimmer exists and the PFC controller enters burst mode operation at
light load. Increasing the voltage V.sub.Linear can provide more
headroom for the linear regulator 580 to handle ripple on the PFC
output voltage and reduce/prevent flashing or flickering of the
LEDs. Under high dimming conditions, a relatively high level of
V.sub.Linear may be tolerated, as the small LED current would limit
power loss. However, when the LED current is high, it is desirable
for the voltage across the linear regulator 580 to be as small as
possible to limit power loss and achieve good power efficiency for
the overall circuit.
[0094] FIG. 10 illustrates a control strategy that may be employed
in accordance with some embodiments. In particular, FIG. 10 is a
graph of V.sub.Linear vs. V.sub.ctrl.sub._.sub.linear.
V.sub.ctrl.sub._.sub.linear ranges from 0V to 1.1V. Under dimming
conditions, the linear regulator 580 regulates the current through
the LED load 22 so that the current through the LED load 22 is
proportional to V.sub.ctrl.sub._.sub.linear. When the load current
is large (i.e., when V.sub.ctrl.sub._.sub.linear is large),
V.sub.Linear remains low, at about 1.2V. As the voltage drop on the
transistor switch 584 is only 0.1V, power loss in the linear
regulator 580 may remain acceptably low. As
V.sub.ctrl.sub._.sub.linear near decreases, V.sub.Linear gradually
increases. When V.sub.ctrl.sub._.sub.linear is below 0.2V,
V.sub.Linear saturates at 12V, which should provide enough headroom
for the linear regulator to provide a stable output current even in
the presence of some ripple on the output voltage V.sub.PFC of the
PFC regulator 40.
[0095] The control strategy can be adjusted according to the
parameters of the particular circuit in which it is used. For
example, the control strategy can be adjusted such that the switch
584 remains fully conductive until the dimming signal output by the
dimming controller 518 passes a threshold level, with the linear
regulator only regulating current during ultra-low dimming
conditions and the PFC stage controlling LED current as the switch
584 is fully turned on under high power conditions.
[0096] FIGS. 11A and 11B are schematic diagrams illustrating
configurations of a solid state light fixture in accordance with
some embodiments that include or that operate in conjunction with
an image sensor. In particular, a solid state light fixture 640 in
accordance with some embodiments may provide an ultra-low dimmable
light source for illuminating an object 650. It will be appreciated
that a solid state lighting fixture in accordance with some
embodiments of the present invention need not include an image
sensor, and that an image sensor need not be used in conjunction
with a solid state lighting fixture in accordance with some
embodiments. Rather, a solid state lighting fixture in accordance
with embodiments of the invention may provide illumination with
ultra-low dimming for other purposes.
[0097] An image of the object 650 that is illuminated at a low
dimming level may be captured by an image sensor 660 (FIG. 11A) or
670 (FIG. 11B). In particular, the image sensor 660 shown in FIG.
11A is integral with the solid state lighting fixture 640, while
the image sensor 670 shown in FIG. 11B is separate from the solid
state lighting fixture 640. In either case, the image sensor 660,
670 may capture an image of the illuminated object 650 under
ultra-low dimming conditions provided by the solid state lighting
fixture 640.
[0098] The present invention is not limited to the illustrated
embodiments discussed above; rather, these embodiments are intended
to fully and completely disclose the invention to those skilled in
this art.
[0099] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises", "comprising", "includes" and/or
"including" when used in this specification, specify the presence
of stated features, integers, steps, operations, elements, and/or
components, but do not preclude the presence or addition of one or
more other features, integers, steps, operations, elements,
components, and/or groups thereof.
[0100] It will be understood that when an element is referred to as
being "connected" or "coupled" to another element, it can be
directly connected or coupled to the other element or intervening
elements may be present. In contrast, when an element is referred
to as being "directly connected" or "directly coupled" to another
element, there are no intervening elements present.
[0101] All of the above-described embodiments may be combined in
any way to provide a plurality of additional embodiments.
[0102] The foregoing is illustrative of the present invention and
is not to be construed as limiting thereof. Although exemplary
embodiments of this invention have been described, those skilled in
the art will readily appreciate that many modifications are
possible in the exemplary embodiments without materially departing
from the novel teachings and advantages of this invention.
Accordingly, all such modifications are intended to be included
within the scope of this invention as defined in the claims. The
invention is defined by the following claims, with equivalents of
the claims to be included therein.
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