U.S. patent application number 13/597000 was filed with the patent office on 2013-04-04 for feed forward controlled voltage to current source for led driver.
This patent application is currently assigned to Texas Instruments Incorporated. The applicant listed for this patent is Isaac Cohen. Invention is credited to Isaac Cohen.
Application Number | 20130082611 13/597000 |
Document ID | / |
Family ID | 47991914 |
Filed Date | 2013-04-04 |
United States Patent
Application |
20130082611 |
Kind Code |
A1 |
Cohen; Isaac |
April 4, 2013 |
FEED FORWARD CONTROLLED VOLTAGE TO CURRENT SOURCE FOR LED
DRIVER
Abstract
A DC to DC converter receives a DC input voltage and generates
an output DC voltage. A current sensor measures a DC input current.
A control circuit is coupled to the current sensor for controlling
the DC to DC converter to have a constant DC input current.
Inventors: |
Cohen; Isaac; (Dix Hills,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cohen; Isaac |
Dix Hills |
NY |
US |
|
|
Assignee: |
Texas Instruments
Incorporated
Dallas
TX
|
Family ID: |
47991914 |
Appl. No.: |
13/597000 |
Filed: |
August 28, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61528619 |
Aug 29, 2011 |
|
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|
61670002 |
Jul 10, 2012 |
|
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61546155 |
Oct 12, 2011 |
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61546159 |
Oct 12, 2011 |
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Current U.S.
Class: |
315/185R ;
315/307; 323/283; 363/95 |
Current CPC
Class: |
H05B 45/38 20200101;
H05B 47/10 20200101; G05F 1/468 20130101; H05B 45/37 20200101; H02M
2001/0009 20130101; Y02B 70/10 20130101; H02M 3/156 20130101; H02M
1/08 20130101; H02M 3/04 20130101; H02M 7/44 20130101; H02M 7/04
20130101; H05B 45/44 20200101; Y02B 20/30 20130101; H02M 1/4225
20130101; H02M 1/44 20130101 |
Class at
Publication: |
315/185.R ;
315/307; 323/283; 363/95 |
International
Class: |
G05F 1/46 20060101
G05F001/46; H02M 7/44 20060101 H02M007/44; H05B 37/02 20060101
H05B037/02 |
Claims
1. A DC to DC converter receiving DC input voltage and generating
an output DC voltage, comprising: a current sensor for measuring DC
input current; a control circuit coupled to the current sensor for
controlling the DC to DC converter to have a constant DC input
current.
2. The DC to DC converter of claim 1 wherein a load coupled to an
output is an LED.
3. The DC to DC converter of claim 2 wherein the LED is a string of
LEDs.
4. The DCD to DC converter of claim 1 wherein an output voltage of
the current sensor is compared to a fixed reference voltage for
generating a comparison voltage for controlling the DC input
current to the converter.
5. The DC to DC converter of claim 4 wherein a pulse width
modulation (PWM) circuit is coupled to receive the comparison
voltage.
6. The DC to DC converter of claim 1 wherein an output voltage of
the current sensor is compared with a voltage related to the output
voltage for generating a comparison voltage for controlling the DC
input current to the converter.
7. The DC to DC converter of claim 6 wherein a pulse width
modulation (PWM) circuit is coupled to receive the comparison
voltage.
8. The DC to DC converter of claim 6 wherein the load coupled to an
output is an LED.
9. The DC to DC converter of claim 1 wherein the output voltage of
the current sensor is compared with a pulsed reference voltage
generating a comparison voltage for controlling the DC input
current to the converter.
10. The DC to DC converter of claim 9 wherein a pulse width
modulation (PWM) circuit is coupled to receive the comparison
voltage.
11. The DC to DC converter of claim 10 wherein a load coupled to an
output is an LED.
12. The DC to DC converter of claim 1 wherein an output voltage of
the current sensor compared with absolute value of a sinusoidal
voltage reference for generating a comparison voltage controlling
the DC input current to the converter.
13. The DC to DC converter of claim 12 wherein a pulse width
modulation (PWM) circuit is coupled to receive the comparison
voltage.
14. The DC to DC converter of claim 13 wherein a load coupled to an
output is LED.
15. The DC to DC converter of claim 1 wherein an output voltage of
the current sensor is compared with the absolute value of a
sinusoidal reference voltage multiplied by value related to the
voltage on a load for generating a comparison voltage for
controlling DC input current's to the converter.
16. The DC to DC converter of claim 15 wherein a pulse width
modulation (PWM) circuit is coupled to receive the comparison
voltage.
17. The DC to DC converter of claim 16 wherein a load coupled to an
output is an LED.
18. A method of driving an LED comprising: measuring and input
current to it LED driver circuit; controlling the driver circuit so
that output power to drive the LED is proportional to input
voltage.
19. The method of claim 18 for driving LEDs from a voltage source
comprising: generating a current signal related to the input
current; comparing a signal related to the output voltage of the
driver to the current signal to generate a comparison signal;
controlling the driver with the comparison signal so that output
current is proportional to an input voltage.
20. The method of claim 19 wherein the comparison step compares the
current signal to a fixed reference voltage for generating a
comparison voltage for controlling the DC input current to the
driver.
21. The method of claim 19 wherein the comparison step compares the
current signal to a voltage related to the output voltage for
generating a comparison voltage for controlling the DC input
current to the driver.
22. The method of claim 19 wherein the comparison step compares the
current signal to a pulsed reference voltage for generating a
comparison voltage for controlling the DC input current to the
driver.
23. The method of claim 19 wherein the comparison step compares the
current signal to an absolute value of a sinusoidal voltage
reference for generating a comparison voltage for controlling the
DC input current to the driver.
24. The method of claim 19 wherein the comparison step compares the
current signal to an absolute value of a sinusoidal reference
voltage multiplied by a value related to the voltage on a load for
generating a comparison voltage for controlling the DC input
current to the driver.
25. A voltage to current converter receiving a DC input voltage
comprising: a current sensor for measuring DC input current; means
for generating an absolute value of a sinusoidal reference voltage;
means for multiplying a value related to an output voltage of the
converter by the absolute value of the sinusoidal reference voltage
to generate a first reference voltage; means for comparing an
output of the current sensor with the first reference voltage to
generate an error signal; modulation means responsive to the error
signal for operating the converter in order to maintain a constant
input current whereby an output current of the converter driving an
LED load is proportional to the DC input voltage.
26. A converter circuit comprising: a capacitor coupled across an
output of the converter; a controlled DC current source coupled
between the converter output and a load output; an error amplifier
having an inverting input coupled to one terminal of the capacitor
and a noninverting input coupled to another terminal of the
capacitor and an output coupled to a control input of the
controlled DC current source; wherein the error amplifier controls
the average voltage across the capacitor so that it is
constant.
27. A converter circuit comprising: a controlled DC current source
in series between an output of the converter and a load output; a
comparator coupled across the controlled DC current source to
measure the voltage across the controlled DC current source and
having an output coupled to a control input of the controlled DC
current source; wherein the comparator controls the controlled DC
current source so the average voltage drop across the controlled DC
current source is constant, whereby ripple across the load output
is reduced.
28. The converter circuit of claim 27 wherein the comparator
comprises a current measuring device coupled in series between
active switch of the controlled DC current source and a load
terminal to measure current going to a load at the terminal, the
current measuring device providing a current dependent reference
voltage for use by the comparator to control the controlled DC
current source.
29. The DC to DC converter of claim 10 wherein a load coupled to an
output is a DC to AC converter operating an AC load.
30. The DC to DC converter of claim 13 wherein a load coupled to an
output is a DC to AC converter operating an AC load.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims priority from U.S.
Provisional Application No. 61/528,619, filed Aug. 29, 2011 which
is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates to a voltage to current converter
having a constant input current and more specifically to a voltage
to current converter having a constant input current for driving an
LED.
BACKGROUND OF THE INVENTION
[0003] LEDs have become the technology of choice for lighting
products due to their high efficiency, low cost, compact size,
rugged design and very long life. Unlike more conventional lighting
technology, such as incandescent bulbs, fluorescent tubes and
compact fluorescent lamps (CFLs), LEDs are current driven devices.
Their light output is specified at a current through the LED 112.
The voltage drop across an LED or LED string may vary from batch to
batch by as much as 15 percent. Accordingly, it is common to drive
an LED or LED string from a constant current circuit. A well-known
circuit for this purpose is shown in FIG. 1, generally as 100. An
input voltage is provided from an AC source 104 to a full wave
rectifier 106. The output of the rectifier 106 is a DC voltage
which is provided to a DC to DC converter 108 of an LED driver 102.
An LED or LED string 112 is the load on the converter 108. A
current sensor 114 is placed in series with the LED 112 to measure
the current passing through the LED. The output of the current
sensor 114 is applied to the inverting input of an error amplifier
116. The noninverting input of the amplifier 116 is connected to a
current reference, such as voltage source 118. The output of the
error amplifier 116 is coupled to a modulator 110, the output of
which is coupled to the control input of the converter 108.
Modulator 110 controls the operation of the converter 108 so that a
predetermined current always flows through the LED 112 to produce a
premeasured light output. The advantage of this circuit is an
excellent line and load regulation of the output current. However,
the circuit suffers from disadvantages, including poor power factor
when operated from an AC input, a negative resistance input which
is prone to oscillations in the presence of EMI filters, and poor
compatibility with phase controlled dimmers. Phase controlled
dimmers operate by clipping a portion of the AC waveform. As the
voltage applied to the LED is reduced, the circuit attempts to
maintain the current through the LED constant, thus interfering
with the dimming. It is known to measure the angle of conduction,
read the angle, and convert the value to change the current in the
loop. This involves relatively complex circuitry.
[0004] A common problem in lighting is the variation in light
intensity caused by voltage changes. For example, incandescent
light bulbs suffer from a disadvantage that their light output
varies widely when their operating voltage changes. This is because
the power delivered to the bulb varies with the square of the
operating voltage: if the operating voltage changes by 10 percent,
the power delivered to the bulb changes by 21 percent. Furthermore,
as the power to the bulb is diminished, the filament runs
cooler--thus shifting more of the light into the invisible infrared
spectrum (and vice-versa). This further exacerbates the flicker
problem.
[0005] Therefore, there is a need for a simple and efficient device
for driving an LED which at the same time minimizes the flicker
problem.
SUMMARY OF THE INVENTION
[0006] It is a general object of the invention to provide for
efficiently driving an LED and minimizing the dimming problem.
[0007] An aspect of the invention a DC to DC converter receiving DC
input voltage and generating an output DC voltage, comprises a
current sensor for measuring DC input current; and a control
circuit coupled to the current sensor for controlling the DC to DC
converter to have a constant DC input current.
[0008] Another aspect of the invention includes a method of driving
an LED comprising: measuring and input current to it LED driver
circuit. Controlling the driver circuit so that output current to
drive the LED is proportional to input voltage.
[0009] Another aspect of the invention includes a voltage to
current converter receiving a DC input voltage comprising: a
current sensor for measuring DC input current; means for generating
an absolute value of a sinusoidal reference voltage. Means for
multiplying a value related to an output voltage of the converter
by the absolute value of the sinusoidal reference voltage to
generate a first reference voltage. Means for comparing an output
of the current sensor with the first reference voltage to generate
an error signal; and modulation means responsive to the error
signal for operating the converter in order to maintain a constant
input current whereby an output current of the converter driving an
LED load is proportional to the DC input voltage.
[0010] Another aspect of the invention includes a converter circuit
comprising a controlled DC current source in series between an
output of the converter and a load output. An error amplifier
coupled across the controlled DC current source to sense the output
voltage of the converter or the voltage across the controlled DC
current source and having an output coupled to a control input of
the controlled DC current source; wherein the error amplifier
controls the controlled DC current source so the average voltage
drop across the controlled DC current source is regulated to the
lowest value that will allow the controlled DC current source to
deliver an essentially DC current to the load, whereby an
essentially ripple-free current is delivered to the load.
[0011] A further aspect of the invention includes a converter
circuit comprising a capacitor coupled across an output of the
converter. A controlled DC current source coupled between the
converter output and a load output. An error amplifier having an
inverting input coupled to one terminal of the capacitor and a
noninverting input coupled to another terminal of the capacitor and
an output coupled to a control input of the controlled DC current
source, wherein the error amplifier controls the average voltage
across the capacitor so that it is constant.
BRIEF DESCRIPTION OF DRAWINGS
[0012] Further aspects of the invention will appear from the
appending claims and from the following detailed description given
with reference to the appending drawings.
[0013] Further aspects of the invention will appear from the
appending claims and from the following detailed description given
with reference to the appending drawings.
[0014] FIG. 1 is a block diagram of a prior art LED driver
circuit;
[0015] FIG. 2 is a block diagram of an LED driver circuit according
to the present invention;
[0016] FIG. 3 is a block diagram of a second embodiment of the
present invention;
[0017] FIG. 4 is a block diagram showing 2 of the circuits of FIG.
3 for explanatory purposes;
[0018] FIGS. 5A through 5C show the waveforms for the circuit of
FIG. 3;
[0019] FIG. 6 is a block diagram of another embodiment of the
invention;
[0020] FIG. 7 is a block diagram of a, the present invention;
[0021] FIG. 8 is a schematic diagram of a practical embodiment of
the circuit of FIG. 7;
[0022] FIGS. 9A through 9F illustrate the waveforms for the circuit
of FIG. 8;
[0023] FIG. 10 is a block diagram of a further embodiment of the
present invention;
[0024] FIG. 11 is a block diagram of another embodiment of the
present invention;
[0025] FIG. 12 is a schematic diagram of the circuit of FIG.
11;
[0026] FIG. 13 is a schematic diagram of another embodiment of the
present invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0027] An aspect of the present invention is shown generally in
FIG. 2 as 200. In FIG. 2, a DC input 204 is shown instead of of the
AC input 104, shown in FIG. 1. The DC input can be an AC source
coupled to a DC rectifier, such as a full wave rectifier 106, shown
in FIG. 1, for example. The DC input voltage 204 is applied to a DC
to DC converter 208 of LED driver circuit 200. The converter can
utilize any converter topology, including isolated and nonisolated
converters, and any control algorithm for controlling the input
current. A current sensor 214 is placed in series between the
negative input to the converter and the negative output of the
power source. The output of converter 208 is connected to a load
212, which may be an LED or LED string, for example. The load could
also be a DC to AC converter operating an AC load, such as a
flourescent tube, for example. The converter may be controlled by a
pulse width modulation (PWM) modulator circuit 210 which receives
the output of an error amplifier 216, here shown as a summing
point. The summing point 216 receives at its noninverting input an
current reference, here a DC voltage reference 218. The output of
the current sensor 214 is applied to the inverting input of error
amplifier 216. The input/output power balance, assuming near 100
percent efficiency, will cause the LED string current to be:
Po=Pin=VinIin=VinK.sub.1Vref Equation 1
Where Po=output power
[0028] Pin=input power
[0029] Vin=input voltage
[0030] Iin=input current
[0031] K.sub.1=constant
[0032] Vref=reference voltage [0033] Solving for the output current
Io:
[0033] Io = Po Vo = Vin K 1 Vref Vo Equation 2 ##EQU00001##
Where Io=output current [0034] Vo=output voltage
[0035] Here the LED current is proportional to the input voltage
and inversely proportional to the LED string voltage. By giving up
control of driving the LEDs with a constant current, they are
driven with a current that is proportional to the input voltage.
Although such circuit will be effected by changes in the input
voltage, such as caused by plugging in a high current draw
application, the effect will only be of the first order, instead of
more than the second order, as was an incandescent lamp. The
circuit will not fight reducing the input voltage to dim the LED
and all the LEDs and a fixture will dim together.
[0036] FIG. 3 shows another aspect of the present invention,
generally as 300. As with the circuit in FIG. 2, the circuit is
powered by a DC source 304 coupled to the input of a DC to DC
converter 308 of a voltage to current converter 302. The DC input
voltage from source 304 may be the rectified voltage of an AC
source such as the source 104 shown in FIG. 1. A current sensor 314
in series with the voltage source and the input to a converter 308.
The converter 308 may be operated under the control of a pulse
width modulation (PWM) circuit 310 which modulates the converter in
order to maintain a constant input currrent to the converter, as
shown in FIG. 2. The input of the pulse width modulator 310 is
connected to an error amplifier, indicated as a summing point 316.
The inverting input of the error amplifier 316 is connected to the
output of the current sensor 314. A load 312 placed across the
output terminals of the converter 308, receives an output current
Io. The load may be a LED or string of LEDs, for example. The load
312 could also be a DC to AC converter operating an AC load, such
as a flourescent tube, for example. The main difference between the
embodiment shown in FIG. 2 and that shown in FIG. 3, is that there
is no input 218 in the circuit shown in FIG. 3. Instead, the output
voltage, or a portion of the output voltage, is shown as applied to
the noninverting input of error amplifier 316.
[0037] Making the input current proportional to the LED string
voltage causes the LED string current to become independent of the
string voltage. The power delivered to the LED string is:
Po=Pin=VinIin=VinK.sub.2Vo Equation 3
Where K.sub.2=constant Solving for the output current:
Io = Po Vo = Vin K 2 Vo Vo = K 2 Vin Equation 4 ##EQU00002##
[0038] Thus, the LED string current is proportional to the input
voltage and fully regulated against variation in the string
voltage, resulting in a significantly better light stability than
that of an incandescent source.
[0039] LEDs are measured to determine their light output at a
predetermined current. Then they are binned according to their
light output at the predetermined current. Therefore, one trying to
build a light fixture would choose strings from the same bin, so
that the light outputs from each string would be the same. The
light output of LED string may vary as much as 15 percent from
batch to batch. Thus, the converter circuit must adjust its output
voltage to the voltage drop across the string at the specified
current.
[0040] In order to explain how the present invention automatically
compensates for differences in the voltage drop across the LED
string, two of the circuits 300 of the present invention have been
illustrated generally as 400 in FIG. 4. This should replicate the
situation in a lighting fixture comprising two strings of LEDs, for
example. The first voltage converter 302 corresponds to the voltage
to current converter shown in FIG. 3. It has a DC to DC converter
308 coupled to receive the voltage 304 and has a current sensor 314
in series with the voltage source 304, the load 312 is a LED
string, which for the present example, has a voltage drop of 10 V
across the string. A portion of the output voltage of the converter
308 is connected to the noninverting input of error amplifier 316.
The inverting input of error amplifier 316 receives the output of
current sensor 314. The output of the error amplifier 316 controls
a pulse width modulation circuit 310, which in turn modulates the
converter maintain a constant input current to the converter
308.
[0041] A second voltage to current converter 402 is essentially
identical to the voltage to current converter 302. It has a DC to
DC converter 408 the output voltage of which is connected to the
noninverting input of error amplifier 416. A load 412, here a
string of LEDs having a 20 V drop across the string, is connected
to the output of the converter. The input of the converter 408 is
connected to the same voltage source 304 for the converter 308.
This could be the situation in a lighting fixture, for example. A
current sensor 414 is in series with the parallel of connection of
the voltage 304 to the converter 408. The output of the current
sensor 414 is connected to the inverting input of error amplifier
416. The output of error amplifier 416 is connected to a pulse
width modulation circuit 410 which modulates the converter 408 so
that the input current to the converter is always constant.
[0042] It should be noted that the voltage difference between the
two LED strings 312, 412 is intentionally exaggerated both to
clarify the explanation of the principle behind the present
invention and to show its versatility. In addition, it should be
noted that, for the following analysis, the converters will be
assumed to be 100 percent efficient. In fact, converters utilized
in this situation are of very high efficiency, close to 100
percent, and close enough to be considered 100 percent for this
explanation. Those skilled in the art would recognize that since
the converters 308,408, are identical, if the actual conversion
efficiencies were utilized in the following analysis, the results
of the analysis would be the same.
[0043] For the purposes of explanation, we have assumed that the
input voltage is 100 V. We also assume that the first converter 302
is configured so that it will regulate the input current to 1 A.
The resulting input power is:
P=VI=1001=100 W
[0044] The converter 402 is configured the same as the converter
302. However, the voltage drop across the LED load is 20 V, instead
of 10 V. Because the input current is proportional to the output
voltage, the input current will be twice that of the converter 302,
or 2 A. The input power to converter 408 would thus be:
P=IV=1002=200 W
[0045] The output current through LED string 312 equals the 100 W
input divided by the 10 V drop across LED string 312. Thus, the
output current through the LED string will be 10 A. In the second
converter 402, the output current is 200 W power input divided by
the 20 V drop across LED string 412, which equals 10 A. Thus,
although there is a 2 to 1 difference in the voltage drop across
the LED strings to produce the identical amount of light, both
strings operate at the same current to yield the the same light
output. Thus, the circuit can easily handle a 15 percent normal
variation in LEDs from batch to batch. If the actual efficiencies
of the converters 308, 408 were utilized, the output currents will
be slightly different, but the currents through both LED strings
312, 412 would still be identical.
[0046] FIGS. 5A through 5C show the waveforms for a practical
implementation of the circuit shown in FIG. 3. FIG. 5A shows the
input voltage which is equal to 115 Vrms. The output voltage, is
equal to 200 V, the output current is 51.9 mA. The circuit shows a
power factor of 0.9. FIG. 5B shows the input current, which is a
square wave and FIG. 5C shows the output current.
[0047] Another aspect of the invention is shown in FIG. 6,
generally as 600. The circuit of FIG. 6 is similar to the previous
embodiment shown in FIG. 2, except that an AC input is utilized. AC
voltage source 604 is connected to the input of a full wave bridge
rectifier 606, the output of which is connected to DC to DC
converter 608 of voltage to current converter 602. A current sensor
614 is a series between the bridge rectifier 606 and the converter
608. The output is connected to a load 612, which may be a string
of LEDs, for example. The load 612 could also be a DC to AC
converter operating an AC load, such as a flourescent tube, for
example. A capacitor C is coupled across the output of the
converter. The converter may be modulated by a pulse width
modulation circuit 610 so that the current input to the converter
is always the same. An error amplifier, here shown as summation
point 616 is coupled to the input of pulse width modulation circuit
610. The inverting input of error amplifier 616 is coupled to the
output of current sensor 614. The noninverting input that error
amplifier 616, is coupled to input current reference 618, 620. In
the example illustrated in FIG. 6, the input reference 618 is a
sinusoidalal voltage Vref, which is an connected to an absolute
value circuit 620. The output of absolute value circuit 620 is
connected to the noninverting input of error amplifier 616. As
shown in FIG. 5B, if the input current is a square wave, the power
factor is 0.9. If the reference voltage Vref. 618 is a square wave
at twice the line frequency starting a conduction angle of
26.degree. and ending at a conduction angle of 180-26=154.degree.,
the power factor will be 0.968 with a TMD of 25-9 percent. Other
waveforms with an essentially constant current during the truncated
portion of the waveform could also be used. However, if Vref is the
more difficult to generate sinusoidal waveform,t he power factor
will be 1.0. This is a significant improvement over the circuit
shown in FIG. 3, for example.
[0048] Another aspect of the present invention is shown in FIG. 7,
generally as 700. The voltage to current converter circuit 702 is
similar to that shown in FIG. 6. The AC voltage source 704
connected in series with a full wave bridge rectifier 706, the
output of which is provided to DC to DC converter 708. A current
sensor 714 is in series with the voltage supplied to the converter
708. A pulse width modulator 710 maybe used to modulate the
converter 708 so that the input current is always the same. An
error amplifier, here illustrated as summation point 716, is
coupled to the input of pulse width modulation circuit 710. The
output of current sensor 714 is connected to the inverting input of
error amplifier 716. The noninverting input of error amplifier 716
is coupled to the output of multiplication circuit 722.
Multiplication circuit 722 is coupled to the output of absolute
value circuit 720 which takes the absolute value of the sinusoidal
reference Vref 718. This value is then multiplied by a value
related to the output voltage Vo across the load 712, which may be
an LED string, for example. The load could also be a DC to AC
converter operating an AC load, such as a flourescent tube, for
example. A capacitor C is coupled across the load 712. The circuit
combines the power factor advantage of the circuit in FIG. 6, with
the advantage of the circuit shown in FIG. 3 in which the current
through the LED string is independent of the voltage drop across
the LED string.
[0049] FIG. 8 shows an implementation of the present invention in a
practical circuit, generally as 800. AC voltage source 804 is
coupled to an optional EMI filter 805 to a full wave bridge
rectifier 806, the output of the full wave bridge rectifier 806 is
coupled to a DC to DC converter 808, here a boost converter. A
shunt 814 is used to measure the input current to the boost
converter 808. Converter 808 is modulated by control circuit 810 to
drive an LED light engine comprising 136 LEDs drawing approximately
60 mA of current at 400 V. In this embodiment, a boost converter
was chosen. In the event that a voltage surge occurs at voltage
source 804, boost converter 808 could not reduce the output voltage
to protect the LED string. Thus, it is possible the output voltage
will rise high enough to cause a failure of the LEDs. Accordingly a
circuit comprising resistor 824, NMOS transistor 826, NPN
transistor 828, shunt resistor 830 and LEDs 832 and 834 is used as
an overvoltage protection circuit. The resistor 824 is connected
between the gate of NMOS transistor 826 and the junction of LED 832
and the LED preceding LED 832 in the string. Thus, NMOS transistor
826 is normally on. In the event that too much current flows
through the LED string, enough voltage will be built up across
shunt resistor 830 to turn on NPN transistor 828 between the gate
of transistor 826 and the current reference 836. This will turn
transistor 826 off preventing damage to the LED string.
[0050] FIGS. 9A through 9F show the waveform of the circuit of FIG.
8. FIG. 9A shows the current through the LEDs. FIG. 9B shows the
voltage across the capacitor C. FIG. 9C shows the input current.
FIG. 9D shows the current in capacitor C. FIG. 9E shows the power
factor with the circuit and FIG. 9F shows the input current
spectrum showing a THD of 8.7 percent.
[0051] Because the input power varies with time, the output current
of the basic driver contains a substantial amount of AC ripple,
approximately 50% as shown in FIG. 5C. This is highly undesirable
because light output of the LEDs will vary with the ripple, which
can yield a stroboscopic effect. An output capacitor C, as shown in
FIG. 6, for example, is placed across the output voltage in order
to reduce the ripple. In order for the capacitor to filter out the
ripple, it must have an impedance at twice the line frequency (100
or 120 Hz) that is less than the series resistance of the LEDs.
This may require a large capacitor, which increases the size and
cost of the driver. Furthermore, if the voltage across the
capacitor is constant, all of the ripple will appear across the
LEDs and not flow through the capacitor.
[0052] A solution to this problem is shown in FIG. 10, generally as
1000. The circuit shown in FIG. 10 is similar to that shown in FIG.
3. A voltage source Vin 1004 is connected to the DC to DC converter
1008. A current sensor 1014 is series with the voltage input to the
converter. A pulse width modulator circuit 1010 may be used to
control the converter to maintain a constant input current. The
pulse width modulation circuit 1010 receives the output of an error
amplifier, here shown as summation point 1016, which receives the
output of the current sensor 1014 at its inverting input. The
noninverting input to error amplifier 1016 is the output voltage
across the load 1012, which may be a string of LEDs, for example.
The load 1012 could also be a DC to AC converter operating an AC
load, such as a flourescent tube, for example. The load 1012
receives the output voltage of Vo to generate across capacitor C by
converter 1008 and the load 1012. The controlled DC current source
1036 is placed in series between the output of converter 1008 and
controlled DC current source 1036 is controlled by error amplifier
1038, which has its noninverting input connected to Vo prior to the
controlled DC current source and its inverting input coupled to the
load ground through a voltage reference Vref1. The error amplifier
1038 controls the average voltage across capacitor C so that it is
constant. In order for the average voltage across capacitor C to be
constant, current flowing to load 1012 must exactly equal the
current output of the converter 1008. If this were not so, the
voltage across the capacitor would increase if the current flowing
to the load is less than the current output by the converter or
decrease if the current flowing to the load is greater than the
converter output. Therefore, the controlled DC current source
delivers to the load an average current exactly equal to the
average output current of the converter while substantially all the
AC ripple current flows into the capacitor C. The optimum voltage
for the reference Vref1 1040 is the voltage across the LEDs, plus
approximately 1/2 of the peak to peak ripple voltage. The voltage
across the LEDs must be the maximum voltage expected for the LED
string. As described above, the voltage drop across LEDs varies
from batch to batch. Therefore, in a manufacturing environment, the
converter would have to be built so that the output voltage chosen
for the voltage across the LEDs, when choosing Vref1, is the
maximum expected from any batch. This creates a problem in that for
all other LED strings, excess voltage will be generated which must
be absorbed by the controlled DC current source.
[0053] An improved solution is shown in FIG. 11, generally as 1100.
This circuit is similar to the circuit shown in FIG. 10. A DC
voltage source Vin 1104 is coupled to the input of converter 1108
with a current sensor 1114 connected in series between the voltage
source and the converter. The converter is modulated by a pulse
width modulation circuit 1110, which receives the output of error
amplifier 1116, here shown as a summing point. The error amplifier
1116 receives the output of the current sensor 1114 at its
inverting input. The error amplifier receives the output voltage
across the load at its noninverting input. A load 1112, which may
comprise a series string of LEDs, is placed across the output
voltage Vo from converter 1108. The load could also be a DC to AC
converter operating an AC load, such as a flourescent tube, for
example. A capacitor C is placed across the output voltage.
Controlled DC current source 1036 is in series between the
converter and the load. An error amplifier 1138 controls the
controlled DC current source and receives at its noninverting input
the voltage across the capacitor C. The inverting input of
comparator 1138 is coupled to the output voltage across the load
through a series reference Vref 1140, so instead of maintaining a
constant average voltage across the capacitor C, this circuit
configuration maintains a constant average voltage across the
controlled DC current source 1036. Thus, the voltage across the
controlled DC current source will absorb the ripple. The value of
the reference voltage Vref1 is selected to assure that the
controlled DC current source never saturates and is always able to
absorb the ripple voltage. Therefore, the minumum voltage required
to operate the load is always present and excessive voltage is not
generated.
[0054] FIG. 12 shows a practical implementation for the circuit
shown in FIG. 11. In FIG. 12, the voltage source 1204 is an AC
voltage source which is coupled to the voltage to current converter
1202 via full wave bridge rectifier 1206. The voltage to current
converter 1202 is similar to that shown in FIG. 8. The output
voltage is generated across the capacitor C. The LED string 1212 or
DC to AC converter operating an AC load, such as a flourescent
tube, for example, is in parallel with the capacitor C. A
controlled DC current source circuit comprising resistors R1,
capacitor C1, error amplifier 1238 and reference source 1240
controls an NMOS transistor 1242 in series between the capacitor C
and the LED string 1212. The capacitor Cl is coupled between the
capacitor C and the LED string 1212. The resistor R1, and a
capacitor Cl form a low pass filter which generates a signal equal
to the average voltage across the series transistor 1242. Converter
1202 also comprises a current sensor 1214, here shown as a shunt,
resistors R2, R3, R4, R5 and capacitor C2. An error amplifier 1216
has its noninverting input coupled to the junction of resistors R2,
R3 which is connected to the shunt 1214. The inverting input of a
amplifier 1216 is connected to ground through resistor R5 and to
its output to resistor R4 and capacitor C2. The output of error
amplifier 1216 is coupled to pulse width modulator circuit 1210
which modulates the converter so that the input current is always a
constant. The resistors R2, R3, divide down the output voltage and
compare it against the voltage developed across shunt 1214. The
operation of the regulator will be controlled so the input current
is a fraction of the output voltage.
[0055] FIG. 13 shows an improved circuit over the circuit 1200 in
FIG. 12 generally as 1300. An AC voltage source 1304 is coupled to
a full wave bridge rectifier 1306 which is coupled to a voltage to
current converter 1302. The voltage to current converter 1302 is
identical to that shown in FIG. 12. A capacitor C is placed across
the output of converter 1302. An NMOS transistor 1342 is coupled
between the capacitor C and the LED string 1312 which is placed
across the output voltage the converter. A resistor Rshunt1 is in
series with transistor 1342 and the LED string 1312. Resistor
Rshunt1 is coupled to the noninverting input of op amp 1344 via
offset reference 1340. The inverting input of an error amplifier
1344 is connected to the output voltage across the LED string or DC
to AC converter operating an AC load, such as a flourescent tube,
for example, via resistor Ry. A feedback resistor Rx is connected
between the inverting input of amplifier 1344 and the output
thereof A low pass filter comprising resistor Rf and capacitor Cf
is placed between the voltage across the capacitor C and the
voltage across the LED string 1312. The node between the resistor
and the capacitor is connected to the noninverting input of error
amplifier 1338, the inverting input of which is connected to the
output of op amp 1344. The output of error amplifier 1338 controls
the gate of transistor 1342.
[0056] In the circuit of FIG. 12, the voltage drop across the
transistor 1242 is controlled by a fixed voltage reference source
1240. The ripple voltage is dependent upon the current. If the
voltage is reduced, such as when the LEDs are dimmed, the ripple
voltage will be less. In the embodiment shown in FIG. 12, the
voltage drop is maintained at a constant by the reference voltage
source 1240 resulting in excess voltage which must be dissapated by
the controlled DC current source. In the embodiment shown in FIG.
13, the controlled DC current source circuit generates its own
variable reference voltage, which is proportional to the current
through the LEDs, so no fixed references are involved. Thus, in the
circuit of FIG. 13 reference voltage sources are eliminated and
excessive voltage across the controlled DC current source at
lighter loads is eliminated.
[0057] Although the invention has been described in detail, it
should be understood that various changes, substitutions and
alterations may be made thereto without departing from the spirit
and scope of the invention as defined by the appended claims. For
example, the present invention is not limited to a specific
converter's topology or input current control algorithm and any
converter topology or algorithm for controlling the input current
the converter may be utilized. In addition, although the load shown
in the present application are LEDs, although loads may be driven
by the present technique, such as batteries, which are charged at
constant current. The controlled DC current source of the present
application can be utilized separately with other converter
implementations.
* * * * *