U.S. patent application number 14/509660 was filed with the patent office on 2016-04-14 for ripple reduction in light emitting diode (led)-based light bulb through increased ripple on an energy storage capacitor.
The applicant listed for this patent is Cirrus Logic, Inc.. Invention is credited to John L. Melanson.
Application Number | 20160105936 14/509660 |
Document ID | / |
Family ID | 54557466 |
Filed Date | 2016-04-14 |
United States Patent
Application |
20160105936 |
Kind Code |
A1 |
Melanson; John L. |
April 14, 2016 |
RIPPLE REDUCTION IN LIGHT EMITTING DIODE (LED)-BASED LIGHT BULB
THROUGH INCREASED RIPPLE ON AN ENERGY STORAGE CAPACITOR
Abstract
A buck stage configured as a second stage of a lamp circuit for
a light-emitting diode-based (LED-based) light bulb may reduce
manufacturing costs associated with the bulb by reducing a size of
a capacitor in the lamp circuit. A controller of the buck stage may
adjust a duty cycle of a switch to increase a ripple, such as a
voltage ripple, across the capacitor to increase accessibility of
energy stored in the capacitor and decrease a ripple, such as in
output current, to the LEDs.
Inventors: |
Melanson; John L.; (Austin,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cirrus Logic, Inc. |
Austin |
TX |
US |
|
|
Family ID: |
54557466 |
Appl. No.: |
14/509660 |
Filed: |
October 8, 2014 |
Current U.S.
Class: |
315/201 ;
315/200R; 315/246; 315/250; 363/15 |
Current CPC
Class: |
H05B 45/3725 20200101;
H05B 47/10 20200101; H05B 45/00 20200101; H05B 45/37 20200101; H02M
3/24 20130101 |
International
Class: |
H05B 33/08 20060101
H05B033/08; H02M 3/24 20060101 H02M003/24; H05B 37/02 20060101
H05B037/02 |
Claims
1. An apparatus, comprising: a line voltage input node configured
to receive a line voltage input; a rectifier coupled to the line
voltage input node; a first switching stage coupled to the
rectifier and configured to regulate an approximately average
current to an intermediate node; a capacitor coupled to the
intermediate node; a second switching stage coupled to the
intermediate node and configured to provide an approximately
constant ratio of input to output current, wherein the second
switching stage comprises a switch in series with a lighting load;
a first controller coupled to the first switching stage and
configured to regulate the first switching stage to maintain an
approximately average current out of the first switching stage and
into the intermediate node, the average current being averaged over
each half line cycle of the input line voltage; and a second
controller coupled to the switch, wherein the second controller is
configured to operate the switch to control a ripple on the
capacitor to reduce a ripple through the lighting load.
2. (canceled)
3. The apparatus of claim 2, wherein the first switching stage is
galvanically isolated from the second switching stage.
4. The apparatus of claim 3, wherein the second switching stage
operates at a higher operating frequency than the first switching
stage.
5. The apparatus of claim 2, wherein the lighting load comprises
one or more light emitting diodes (LEDs).
6. The apparatus of claim 5, wherein the first controller is
configured to regulate a current through the one or more light
emitting diodes (LEDs) as a value approximately equal to the
product of the average current from the first switching stage and
the ratio of the second switching stage.
7. The apparatus of claim 2, wherein the controller is integrated
into a printed circuit board (PCB) and wherein the first switching
stage is integrated into a second printed circuit board (PCB).
8. The apparatus of claim 2, wherein the second controller is
configured to operate the second switching stage at approximately a
constant average duty cycle over a period of at least one half of a
cycle of the line voltage input.
9. The apparatus of claim 8, wherein the second controller is
configured to: monitor a voltage across the capacitor; and reduce a
power usage of the lighting load during a peak of the monitored
voltage to allow the capacitor to charge.
10. The apparatus of claim 9, wherein the second controller is
configured to reduce a duty cycle of the switch to reduce the power
usage of the lighting load.
11. The apparatus of claim 8, wherein the second controller is
configured to: monitor a current through the lighting load; and
charge the capacitor when the current through the lighting load is
below a threshold.
12. (canceled)
13. The apparatus of claim 12, wherein the second controller is
configured to operate the switch to control a voltage ripple on the
capacitor to reduce a current ripple through the lighting load.
14. The apparatus of claim 1, wherein the first controller is
configured to maintain an approximately average current based, at
least in part, on a dimmer setting for the lighting load.
15. A method, comprising: regulating a current to an intermediate
node in a first switching stage of a lamp to maintain an
approximately average current for each half line cycle of an input
line voltage; delivering current from the intermediate node to a
second switching stage of the lamp; and regulating a ripple in an
energy storage capacitor of the second switching stage of the lamp
to provide an approximately constant ratio of input to output
current.
16. The method of claim 15, wherein the step of regulating
comprises regulating a voltage ripple in the energy storage
capacitor.
17. The method of claim 15, wherein the approximately constant
ratio is greater than one.
18. The method of claim 15, further comprising monitoring a voltage
across the energy storage capacitor.
19. The method of claim 18, further comprising reducing an output
current during a peak of the monitored voltage to allow the energy
storage capacitor to charge.
20. The method of claim 19, wherein the step of delivering current
from the intermediate node to the second switching stage comprises
galvanically isolating the second switching stage from the first
switching stage.
21. The method of claim 15, wherein the step of regulating a
voltage ripple comprises regulating an output current to have a
current ripple less than approximately 20% of an average voltage on
the energy storage capacitor.
22. The method of claim 15, wherein the step of regulating the
current to the intermediate node comprises maintaining an
approximately average current based, at least in part, on a dimmer
setting for the lamp.
23. The method of claim 15, further comprising regulating a current
through one or more light emitting diodes (LEDs) as a value
approximately equal to the product of the average current from the
first switching stage and the ratio of the second switching
stage.
24. An apparatus, comprising: a first printed circuit board (PCB):
a line voltage input node configured to receive a line voltage
input; a rectifier coupled to the line voltage input node; a first
switching stage coupled to the rectifier and configured to maintain
an approximately average current for each one half of a cycle of
the input live voltage; a transformer coupled to the first
switching stage; and a second printed circuit board (PCB)
galvanically isolated from the first printed circuit board (PCB),
the second printed circuit board (PCB) comprising: a lighting
output node configured to provide an output current; a second
switching stage coupled to the transformer to receive an input
current from the first printed circuit board (PCB) and configured
to provide an approximately constant ratio of input current to
output current; and a controller coupled to the second switching
stage and configured to operate the second switching stage at
approximately a constant average duty cycle over a period of at
least one half of a cycle of the line voltage input, wherein the
second switching stage of the second printed circuit board (PCB) is
coupled to the first switching stage of the first printed circuit
board (PCB) by two wires.
25. The apparatus of claim 24, further comprising one or more light
emitting diodes (LEDs) coupled to the lighting output node.
26. The apparatus of claim 25, wherein the second switching stage
of the second printed circuit board (PCB) comprises an energy
storage capacitor, and wherein the controller of the second
switching stage is configured to control a ripple on the energy
storage capacitor to reduce a ripple at the lighting output
node.
27. The apparatus of claim 26, wherein the controller of the second
switching stage is configured to control a voltage ripple on the
energy storage capacitor to reduce a current ripple at the lighting
output node.
Description
FIELD OF THE DISCLOSURE
[0001] The instant disclosure relates to lighting circuits. More
specifically, this disclosure relates to power supply circuitry for
lighting circuits.
BACKGROUND
[0002] Alternative lighting devices to replace incandescent light
bulbs differ from incandescent light bulbs in the manner that
energy is converted to light. Incandescent light bulbs include a
metal filament. When electricity is applied to the metal filament,
the metal filament heats and glows, radiating light into the
surrounding area. The metal filament of conventional incandescent
light bulbs generally has no specific power requirements. That is,
any voltage and any current may be applied to the metal filament
because the metal filament is a passive device. Although the
voltage and current need to be sufficient to heat the metal
filament to a glowing state, any other characteristics of the
delivered energy to the metal filament do not affect operation of
the incandescent light bulb. Thus, conventional line voltages in
most residences and commercial buildings are sufficient for
operation of the incandescent bulb. However, incandescent bulbs are
not very efficient in conversion of energy to light, and thus waste
energy.
[0003] One alternative lighting device with better efficiency is a
light-emitting diode based bulb (LED-based bulb). The LEDs in the
bulb consume energy from a line input source and convert the energy
to light through photoemission. However, the LEDs, unlike the metal
filament, are not a passive component. Whereas the metal filament
presents a nearly constant resistance to the line voltage source
and can operate from AC voltages, LEDs are DC devices that need to
have a controlled supply current. The controlled supply current is
conventionally supplied by one or more power stages placed between
the LEDs and the line voltage source. The power stages convert
energy from the line voltage source to an appropriate input for the
LEDs. The power stages also regulate the conversion of energy from
the line voltage to the LEDs by regulating current through the LEDs
because the emitted light is proportional to the current.
[0004] Line voltage sources are generally alternating current (AC)
waveforms. Because the voltage at the line source varies with time,
the energy available to the LED-based bulb also varies over time.
Without control over the conversion of energy within the LED-based
bulb, the light output of the LED-based bulb would ripple over time
along with the variations at the line voltage source. One
conventional lamp circuit for controlling a LED-based bulb to
reduce variations is shown in FIG. 1.
[0005] FIG. 1 is a circuit schematic illustrating a conventional
two-stage, line-operated lamp circuit. In circuit 100, a line
voltage input node 102 is coupled to rectifiers 110, which convert
alternating current (AC) at input node 102 to direct current (DC)
for output to a first stage DC-DC converter 112. First stage 112
delivers a peak power to capacitor 114 of approximately double the
average power consumed by light-emitting diodes (LEDs) 104. A
second stage DC-DC converter 116 consumes energy stored in the
capacitor 114 and generates a constant current to drive the LEDs
104. Although the circuit 100 provides a constant current with
little ripple to the LEDs 104, the circuit 100 includes two power
converters, which increases the final cost of an LED-based bulb
containing the circuit 100.
[0006] An alternate lamp circuit with only a single power converter
for performing a power factor correction (PFC) is shown in FIG. 2.
FIG. 2 is a circuit schematic illustrating another conventional
line-operated lamp circuit with a single stage. A circuit 200
receives line voltage input at node 102, which is provided to
rectifier 110. The DC output of the rectifier 110 is provided to
capacitor 114 in parallel with transformer 212 and switch 216. The
capacitor 114 is a relatively small capacitor, such as 10-500
nanoFarads. The transformer 212 delivers energy from the rectifier
110 to the capacitor 214 and isolates capacitor 214 from the
rectifier 110. The capacitor 214 stores energy during peaks in the
output of rectifier 110 and discharges energy during the troughs in
the output of rectifier 110 to LEDs 104. The LEDs 104 have small
resistances to improve the efficiency of conversion of energy to
light. Thus, to reduce ripple in the current at LEDs 104, the
capacitor 214 must have a large capacitance. The physical size of
capacitor 214 increases proportional to capacitance. Thus, the
circuit 200 can become costly to manufacture and occupy too much
space when capacitor 214 is large. To reduce cost, the capacitor
214 is generally decreased in size, but the size reduction results
in larger ripples in current through the LEDs 104, and consequently
ripples in the brightness of light output by the LEDs 104. Further,
the LEDs 104 have a minimum required voltage, the forward bias
voltage, in order to maintain the generation of light. Because the
capacitor 214 acts as the energy supply for maintaining the forward
bias voltage, the capacitor 214 must be a relatively large
capacitor 214. Additionally, other characteristics of the LEDs 104
place requirements on the size of the capacitor 214 in the design
of FIG. 2 based on, for example, the non-linearity of the LEDs 104.
Although the solution of FIG. 2 resolves the problem of too much
ripple in the output light, the circuit 100 is too costly to
implement in low-cost devices, such as low-cost consumer light
bulbs.
[0007] Shortcomings mentioned here are only representative and are
included simply to highlight that a need exists for improved lamp
circuits particularly for LED-based light bulbs. Embodiments
described here address certain shortcomings but not necessarily
each and every one described here or known in the art.
SUMMARY
[0008] Output ripple from a LED-based light bulb may be reduced
through the use of a two-stage power converter with reduced cost by
increasing a voltage ripple on a storage capacitor to decrease
current ripple in the LEDs. By allowing an increased ripple voltage
on the capacitor, more of the energy storage potential of the
capacitor may be utilized for generating light at the LEDs. For
example, the required forward bias voltage of the LEDs places a
required minimum stored charge on the capacitor, which
significantly reduces the available energy in the capacitor. In one
illustration of this restriction, when the required forward bias
voltage is 27 Volts and the capacitor is charged to 29 Volts, the
only energy available in the capacitor is that which when consumed
does not reduce the capacitor below 27 Volts. Increasing a ripple
on the capacitor above 29 Volts may unlock more energy storage
potential of the capacitor. Because a larger amount of the energy
storage potential of the capacitor may be utilized, a smaller
capacitor may be used in the lamp circuit of the light bulb. The
smaller capacitor allows for smaller lamp circuits and lower-cost
lamp circuit to be implemented in LED-based light bulbs. Further,
the second stage power converter may be manufactured at lower cost
than in conventional two-stage converters. The increased voltage
ripple on the capacitor may be obtained by a controller coupled to
a switch in series with the LEDs. The controller may adjust a duty
cycle of the switch, but maintain an approximately constant average
duty cycle over at least a half of a cycle of the line input
voltage.
[0009] According to one embodiment, an apparatus may include a line
voltage input node configured to receive a line voltage input; a
rectifier coupled to the line voltage input node; a first switching
stage coupled to the rectifier and configured to regulate a current
to an intermediate node; a capacitor coupled to the intermediate
node; a second switching stage coupled to the intermediate node and
configured to provide an approximately constant ratio of input to
output current; and/or a first controller coupled to the first
switching stage and configured to regulate the first switching
stage to maintain an approximately average current out of the first
switching stage and into the intermediate node, the average current
being averaged over each half line cycle of the input line
voltage.
[0010] In certain embodiments, the second switching stage may
include a switch configured in series with a lighting load; the
second switching stage may include a second controller coupled to
the switch; the lighting load may include one or more light
emitting diodes (LEDs); the controller may be integrated into a
printed circuit board (PCB); the first switching stage may be
integrated into a second printed circuit board (PCB); the second
controller may be configured to operate the second switching stage
at approximately a constant average duty cycle over a period of at
least one half of a cycle of the line voltage input; the second
controller may be configured to operate the switch to control a
ripple on the capacitor to reduce a ripple through the lighting
load; the second controller may be configured to operate the switch
to control a voltage ripple on the capacitor to reduce a current
ripple through the lighting load; the second controller may be
configured to monitor a voltage across the capacitor; the second
controller may be configured to reduce a power usage of the
lighting load during a peak of the monitored voltage to allow the
capacitor to charge; the second controller may be configured to
reduce a duty cycle of the switch to reduce the power usage of the
lighting load; the second controller may be configured to monitor a
current through the lighting load; the second controller may be
configured to charge the capacitor when the current through the
lighting load is below a threshold; the first switching stage may
be galvanically isolated from the second switching stage; the
second switching stage may operate at a higher operating frequency
than the first switching stage; the first controller may be
configured to maintain an approximately average current based, at
least in part, on a dimmer setting for the lighting load; and/or
the first controller may be configured to regulate a current
through the one or more light emitting diodes (LEDs) as a value
approximately equal to the product of the average current from the
first switching stage and the ratio of the second switching
stage.
[0011] According to another embodiment, a method may include
regulating a current to an intermediate node in a first switching
stage of a lamp to maintain an approximately average current for
each half line cycle of an input line voltage; delivering current
from the intermediate node to a second switching stage of the lamp;
and/or regulating a ripple in an energy storage capacitor of the
second switching stage of the lamp to provide an approximately
constant ratio of input to output current. In certain embodiments,
the approximately constant ratio may be greater than one; and/or
the regulated ripple may be a regulated voltage ripple.
[0012] In some embodiments, the method may also include monitoring
a voltage across the energy storage capacitor; reducing an output
current during a peak of the monitored voltage to allow the energy
storage capacitor to charge; the step of delivering current from
the intermediate node to the second switching stage may include
galvanically isolating the second switching stage from the first
switching stage; the step of regulating a voltage ripple may
include regulating an output current to have a current ripple less
than approximately 20%; rising regulating a current through one or
more light emitting diodes (LEDs) as a value approximately equal to
the product of the average current from the first switching stage
and the ratio of the second switching stage; and/or the step of
regulating the current may include maintaining an approximately
average current based, at least in part, on a dimmer setting for
the lamp.
[0013] According to a further embodiment, an apparatus may include
a first printed circuit board (PCB) and a second printed circuit
board (PCB) galvanically isolated from the first printed circuit
board (PCB). The first PCB may include a line voltage input node
configured to receive a line voltage input; a rectifier coupled to
the line voltage input node; a first switching stage coupled to the
rectifier and configured to maintain an approximately average
current for each one half of a cycle of the input live voltage;
and/or a transformer coupled to the first switching stage. The
second PCB may include a lighting output node configured to provide
an output current; a second switching stage coupled to the
transformer to receive an input current from the first printed
circuit board (PCB) and configured to provide an approximately
constant ratio of input current to output current; and/or a
controller coupled to the second switching stage and configured to
operate the second switching stage at approximately a constant
average duty cycle over a period of at least one half of a cycle of
the line voltage input. In some embodiments, the apparatus may also
include one or more light emitting diodes (LEDs) coupled to the
lighting output node.
[0014] In certain embodiments, the second switching stage of the
second printed circuit board (PCB) may include an energy storage
capacitor; the controller of the second switching stage may be
configured to control a voltage ripple on the energy storage
capacitor to reduce a current ripple at the lighting output node;
and/or the second switching stage of the second printed circuit
board (PCB) may be coupled to the first switching stage of the
first printed circuit board (PCB) by two wires.
[0015] The foregoing has outlined rather broadly certain features
and technical advantages of embodiments of the present invention in
order that the detailed description that follows may be better
understood. Additional features and advantages will be described
hereinafter that form the subject of the claims of the invention.
It should be appreciated by those having ordinary skill in the art
that the conception and specific embodiment disclosed may be
readily utilized as a basis for modifying or designing other
structures for carrying out the same or similar purposes. It should
also be realized by those having ordinary skill in the art that
such equivalent constructions do not depart from the spirit and
scope of the invention as set forth in the appended claims.
Additional features will be better understood from the following
description when considered in connection with the accompanying
figures. It is to be expressly understood, however, that each of
the figures is provided for the purpose of illustration and
description only and is not intended to limit the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For a more complete understanding of the disclosed system
and methods, reference is now made to the following descriptions
taken in conjunction with the accompanying drawings.
[0017] FIG. 1 is a circuit schematic illustrating a conventional
two-stage, line-operated lamp circuit.
[0018] FIG. 2 is a circuit schematic illustrating another
conventional line-operated lamp circuit with a single stage.
[0019] FIG. 3 is a circuit schematic illustrating a two-stage lamp
circuit with controllable voltage ripple on a storage capacitor
according to one embodiment of the disclosure.
[0020] FIG. 4 are graphs illustrating operation of the controller
maintaining an approximately constant average current according to
one embodiment of the disclosure.
[0021] FIG. 5 are graphs illustrating operation of a power
converter for a LED-based light bulb over a cycle of a line voltage
source according to one embodiment of the disclosure.
[0022] FIG. 6 is a flow chart illustrating a method of powering a
LED-based bulb with a two stage power converter with an increased
voltage ripple on a storage energy capacitor according to one
embodiment of the disclosure.
[0023] FIG. 7 is a circuit illustrating a second stage
configuration to reduce current ripple through LEDs according to
one embodiment of the disclosure.
[0024] FIG. 8 is a flow chart illustrating a method of controlling
a voltage ripple on an energy storage capacitor according to one
embodiment of the disclosure.
DETAILED DESCRIPTION
[0025] FIG. 3 is a circuit schematic illustrating a two-stage lamp
circuit with controllable voltage ripple on a storage capacitor
according to one embodiment of the disclosure. A circuit 300 may
include a line voltage input node 302 coupled to a rectifier 312.
The rectifier 312 converts an alternating current (AC) input from
line voltage V.sub.LINE to a direct current (DC) output for a first
stage power converter 300A. The capacitor 314 may filter the DC
output of the rectifier 312, such as to reduce electromagnetic
interference (EMI). A transformer 320 may couple the DC output of
the rectifier 312 to a second stage power converter 300B. The power
converter 300A may operate at a different switching frequency than
the power converter 300B. In one embodiment, the switching
frequency of 300B is higher than that of converter 300A.
[0026] A switch 316 in series with one winding of the transformer
320 may be operated by a controller 318 to control delivery of
current to an intermediate node 350. Adjusting a duty cycle of the
switch 316 from the controller 318 may change a level of average
current output by the transformer 320 to the intermediate node 350.
In one embodiment, the controller 318 may adjust the duty cycle of
the switch 316 to maintain an approximately average current for
each half line cycle of the line voltage at input node 302.
[0027] Current from the intermediate node 350 may power the second
stage power converter 300B. In converter 300B, current may flow
through a diode 332 to charge a capacitor 334. Current i.sub.1 352
may flow from the intermediate node 350 and from charge stored on
the capacitor 334. The current i.sub.1 352 continues as current
i.sub.2 354 to generate an output voltage V.sub.LED for LEDs 342.
The current through the LEDs 342 may also flow through inductor
336.
[0028] A switch 338 coupled to the inductor 336 may determine how
current flows from the inductor 336. When the switch 338 is closed,
some current may loop from the LEDs 342 to the capacitor 334
through the switch 338. When the switch 338 is open, all current
may loop from the LEDs 342, through the inductor 336 to the diode
340, and back through the LEDs 342. By operating the switch 338, a
controller 348 may adjust a ratio of input current into the
intermediate node 350 to output current through the LEDs 342. In
one embodiment, the controller 348 may operate the switch 338 to
provide an approximately constant ratio of input to output current.
The controller 348 may maintain the approximately constant ratio,
which may be greater than one, by monitoring a voltage at node 306
and/or a current at node 308. The controller 348 may generate a
control signal V.sub.CTRL,2 for operating the switch 338 based, in
part, on the sensed voltage at node 306 and current at node 308.
For example, the controller 348 may reduce a power usage of the
lighting load during a peak of the monitored voltage to allow the
capacitor 334 to charge. In another example, the controller 348 may
charge the capacitor 334 when the monitored current at node 308 is
below a threshold level.
[0029] FIG. 4 are graphs illustrating operation of the controller
maintaining an approximately constant average current according to
one embodiment of the disclosure. A graph 402 illustrates a current
i.sub.2 354 through the LEDs 342 and the inductor 336. A graph 404
illustrates the control signal V.sub.CTRL,2 for operating the
switch 338. When the switch 338 is closed at time 412, the current
i.sub.2 354 increases linearly as inductor 336 is charged. The
controller 348 monitors current at node 308. When the controller
348 determines the current i.sub.2 354 reaches a certain threshold
432, by detecting a proportionate amount of current at node 308,
the controller 348 may open the switch 338. At time 414, the switch
338 opens when the signal 404 goes to a low state. As a result,
current 402 decreases as the inductor 338 is discharged through the
LEDs 342. The average current for i.sub.1 352 over at least a half
line cycle may be approximately equal to the current i.sub.2 354
multiplied by a duty cycle, d, of the switch 338 (see "d" in FIG.
5).
[0030] The timing of operation of the switch 338 may change to
maintain an approximately constant average current through the LEDs
342. The average current may be set by a ratio of time the switch
is closed to time the switch is open. In FIG. 4, the time period
422 is a first time period during which the switch 338 is closed,
and the time period 424 is a second time period during which the
switch 338 is open. A ratio of the time period 422 to the time
period 424 may be defined as a duty cycle of the switch 338. The
ratio may be adjusted by the controller 348, which changes when the
switch 338 closes with the control signal V.sub.CTRL,2. One example
of this changing duty cycle over a full cycle of the line voltage
is shown in FIG. 5.
[0031] FIG. 5 are graphs illustrating operation of a power
converter for a LED-based light bulb over a cycle of a line voltage
source according to one embodiment of the disclosure. A graph 502
illustrates a rectified voltage V.sub.RECT output by the rectifier
312. The rectified voltage 502 follows the alternating current (AC)
input at input node 302. Graph 504 shows the power transfer from
the line voltage source to the lamp circuit. Graph 506 shows a
voltage V.sub.C3 across capacitor 334. Graph 508 shows a duty cycle
d of the switch 338 set by the controller 348. At time 512, the
line cycle begins and the rectified voltage V.sub.RECT of graph 502
increases with the line voltage. During this portion of the line
cycle, the duty cycle of graph 508 may be at a maximum, such as at
a duty cycle of 1.0. A duty cycle of 1.0 allows a maximum amount of
energy to be stored from the line voltage source in capacitor 334.
At time 514, the line voltage source, and consequently the
rectified voltage V.sub.RECT of graph 502, decreases into a trough.
Less energy is available during this decline and in the trough.
Thus, the controller 348 may decrease the duty cycle shown in graph
508.
[0032] The duty cycle may be adjusted by the controller 348 while
maintaining an average duty cycle. In one embodiment, the duty
cycle may vary from 0.9-1.0 and have an average value of 0.95. The
average current through the LEDs 342 may be determined as the
average current provided by transformer 320 divided by the average
duty cycle. When the average duty cycle is fixed by the controller
348, the controller 318 may be used to adjust a current through the
LEDs 342. For example, the controller 318 may adjust a duty cycle
of the switch 316 to adjust transfer of power from the first stage
300A to the second stage 300B.
[0033] Referring to FIG. 3, the controller 318 and the controller
348 may be independent controllers. Thus, for example, the
controller 348 and the second stage 300B may be added to a power
converter having the first stage 300A. The second stage 300B may
thus be connected through only two wires to transformer 320. In one
embodiment, a LED-based light bulb constructed with circuit 300 of
FIG. 3 may include diode 332, inductor 336, switch 338, and
controller 348 integrated into a light engine board along with LEDs
342. In another embodiment, the controller 318 and the controller
348 may be integrated into a single controller.
[0034] The circuit 300 of FIG. 3 may allow construction of a lamp
circuit for an LED-based bulb with a reduced current ripple in the
LEDs 342 similar to that achieved with prior art two-stage power
converters, but with a reduced cost similar to that achieved with
prior art one-stage power converters. In particular, the second
stage 300B of circuit 300 may be manufactured at lower cost than a
conventional second stage power converter. Current ripple through
the LEDs 342 may be reduced by controlling a duty cycle of the
switch 338 from the controller 348 to allow a higher voltage ripple
on capacitor 334. In one embodiment, current ripple may be reduced
below 20% for ideal light generation from LEDs 342. A higher
voltage ripple on the capacitor 334 may allow better utilization of
the energy storage capacity of the capacitor 334, and thus may
allow a smaller capacitor 334 to be used in the circuit 300. An
increase of approximately double the voltage ripple on the energy
storage capacitor may allow a reduction of size of the capacitor by
half. Smaller capacitors have a reduced cost and allow the
LED-based light bulb to be constructed in a smaller physical
volume.
[0035] Increasing the voltage ripple to allow use of a smaller
capacitor is illustrated in the example below. If the LEDs 342
operate at 27 Volts, then the capacitor 334 must be maintained
above 27 Volts for the LEDs to operate. Energy stored in a
capacitor, E, may be calculated as
E = CV 2 2 , ##EQU00001##
where C is a capacitance of capacitor 334 and V is a voltage across
the capacitor 334. A larger voltage ripple may increase the energy
stored by the capacitor 334 by increasing V. A larger energy store
within the circuit 300 may allow a lower ripple of current through
the LEDs 342. For example, when a voltage ripple of 2 Volts is
allowed across the capacitor 334, the available energy in the
capacitor 334 is the difference between the energy stored by the
capacitor at 29 Volts and the energy stored by the capacitor at 27
Volts. Energy stored in the capacitor when the voltage decreases
below 27 Volts is not available because the LEDs 342 do not operate
below 27 Volts. Thus, the energy accessible by the LEDs 342 in the
capacitor at a 2 Volt ripple is
E 29 V - E 27 V = C ( 29 ) 2 2 - C ( 27 ) 2 2 = 56 C ,
##EQU00002##
where C is the capacitance of capacitor 334. When a voltage ripple
of 4 Volts is allowed across the capacitor 334, the available
energy in the capacitor is
E 31 V - E 27 V = C ( 31 ) 2 2 - C ( 27 ) 2 2 = 116 C ,
##EQU00003##
where C is the capacitance of capacitor 334. In this example, a
ripple increase of 2 Volts on the capacitor 334 produces an
approximate doubling of the usable energy storage of the capacitor
334 for the same capacitance. Thus, the capacitance, and therefore
size, of the capacitor 334 may be reduced while maintaining a
certain level of energy storage in the circuit 300.
[0036] Further, reductions in size may be achieved with the
controller 348. For example, the controller 348 may operate the
switch 338 at a high switching frequency and a high duty cycle to
allow the size of inductor 336 to be reduced. Reducing the size of
the inductor 336 further allows a reduction in cost of the circuit
300 and a reduction in the physical volume of an LED-based light
bulb.
[0037] Operation of the circuit 300 to deliver a nearly constant
current to LEDs 342 is described in FIG. 6. FIG. 6 is a flow chart
illustrating a method of powering a LED-based bulb with a two stage
power converter with an increased voltage ripple on a storage
capacitor according to one embodiment of the disclosure. A method
600 begins at block 602 with regulating a current to an
intermediate node in a first switching stage to maintain an
approximately average current for each half-line cycle of an input
line voltage. At block 604, current may be delivered from the
intermediate node to a second switching stage, including an energy
storage capacitor. At block 606, a voltage ripple in the energy
storage capacitor may be regulated to provide an approximately
constant ratio of input to output current at the second switching
stage. In one embodiment, a controller may monitor a voltage across
the energy storage capacitor, such as capacitor 334, and operate a
switch, such as switch 338, to regulate a ripple on the capacitor.
The output current of the second switching stage may be provided to
LEDs. By regulating the voltage ripple on the energy storage
capacitor to increase accessibility of energy storage within the
circuit 300, the current ripple through the LEDs 342 may be reduced
and a more constant light output from the LEDs 342 may be
obtained.
[0038] A controller for regulating voltage ripple on a storage
capacitor may be implemented in other power converter
configurations. For example, a second stage may be modified to
include a capability to short some of the diodes during a trough of
the line voltage. FIG. 7 is a circuit illustrating a second stage
configuration to reduce current ripple through LEDs according to
one embodiment of the disclosure. A circuit 700 may include a
transformer 720 coupled by one winding to a PFC single stage (not
shown). A second winding of the transformer 720 may couple to
intermediate node 750 and to the capacitor 734. A controller 748
may be coupled to a switch 738, which is coupled to inductor 736.
The capacitor 734 may store energy when power transfer from a line
voltage source in the PFC single stage is high, such as during
peaks in the source waveform. When the power transfer is decreasing
or in a trough, the controller 748 may operate the switch 738 to
buck capacitor 734 to provide energy to the inductor 736, and thus
light emitting diodes (LEDs). Energy provided by the capacitor 734
may reduce current ripples through the light emitting diodes
(LEDs), and consequently reduce variations in light output from the
LED-based light bulb.
[0039] The controller 748 of FIG. 7 or the controller 348 of FIG. 3
may execute an algorithm for controlling a voltage ripple on the
energy storage capacitor 734 of FIG. 7 and the capacitor 334 of
FIG. 3, or link capacitor. FIG. 8 is a flow chart illustrating a
method of controlling a voltage ripple on an energy storage
capacitor according to one embodiment of the disclosure. A method
800 begins at block 802 with detecting a transient in the voltage
across the link capacitor. At block 804, a voltage ripple target is
set for the link capacitor to a first value, which may be a maximum
value. At block 806, the controller may monitor the link capacitor
voltage during at least a half of a line cycle. At block 808, it is
determined whether the voltage ripple is zero. If so, the method
800 may wait at block 808. When the voltage ripple exists, the
method 800 proceeds to block 810. At block 810, the link capacitor
maximum voltage V.sub.LINK,MAX may be compared to the result of a
target voltage V.sub.TGT with a scaling factor a, where the product
of a*V.sub.TGT is a predetermined threshold of maximum allowable
voltage on the capacitor that may be set to a value approximately
equal to a rated maximum voltage for the capacitor. If the
V.sub.LINK,MAX value is less than the result, then the ripple on
the link capacitor may be decreased at block 812. If the
V.sub.LINK,MAX value is not less than the result, then the method
800 proceeds to block 814. At block 814, the V.sub.LINK,MAX value
is compared to the result to determine if the V.sub.LINK,MAX value
is greater than the result. If so, then the voltage ripple on the
link capacitor is increased at block 816. If not, then the method
800 returns to block 808. The method 800 may continue through the
blocks 808, 810, and 814, for the duration of the half of a line
cycle. If the link capacitor voltage reaches V.sub.TGT, which is
larger than a*V.sub.TGT prior to a disconnect time of a dimmer
providing current to the LED-based bulb, then the output may be
immediately shut off until the disconnect time. The method 800 of
FIG. 8 may allow monitoring of a maximum voltage on the capacitor
over a half line cycle and increasing or decreasing the ripple on
the capacitor based on the monitored maximum voltage. This method
may further optimize the output ripple allowed on the capacitor and
further improve the amount of energy storage available in the
capacitor for use by the LEDs.
[0040] If implemented in firmware and/or software, the functions
described above, such as illustrated in FIG. 6 and FIG. 8, may be
stored as one or more instructions or code on a computer-readable
medium. Examples include non-transitory computer-readable media
encoded with a data structure and computer-readable media encoded
with a computer program. Computer-readable media includes physical
computer storage media. A storage medium may be any available
medium that can be accessed by a computer. By way of example, and
not limitation, such computer-readable media can comprise random
access memory (RAM), read-only memory (ROM),
electronically-erasable programmable read-only memory (EEPROM),
compact disc-read-only memory (CD-ROM) or other optical disk
storage, magnetic disk storage or other magnetic storage devices,
or any other medium that can be used to store desired program code
in the form of instructions or data structures and that can be
accessed by a computer. Disk and disc includes compact discs (CDs),
laser discs, optical discs, digital versatile discs (DVDs), floppy
disks and blu-ray discs. Generally, disks reproduce data
magnetically, and discs reproduce data optically. Combinations of
the above should also be included within the scope of
computer-readable media.
[0041] In addition to storage on computer readable medium,
instructions and/or data may be provided as signals on transmission
media included in a communication apparatus. For example, a
communication apparatus may include a transceiver having signals
indicative of instructions and data. The instructions and data are
configured to cause one or more processors to implement the
functions outlined in the claims.
[0042] Although the present disclosure and certain representative
advantages have been described in detail, it should be understood
that various changes, substitutions and alterations can be made
herein without departing from the spirit and scope of the
disclosure as defined by the appended claims. For example,
techniques described above may also be applied to linear
regulators, in which a drive current may equal an output current to
LEDs. Moreover, the scope of the present application is not
intended to be limited to the particular embodiments of the
process, machine, manufacture, composition of matter, means,
methods and steps described in the specification. As one of
ordinary skill in the art will readily appreciate from the present
disclosure, processes, machines, manufacture, compositions of
matter, means, methods, or steps, presently existing or later to be
developed that perform substantially the same function or achieve
substantially the same result as the corresponding embodiments
described herein may be utilized. Accordingly, the appended claims
are intended to include within their scope such processes,
machines, manufacture, compositions of matter, means, methods, or
steps.
* * * * *