U.S. patent application number 13/980061 was filed with the patent office on 2013-11-21 for driver device and driving method for driving a load, in particular an led unit.
This patent application is currently assigned to KONINKLIJKE PHILIPS N.V.. The applicant listed for this patent is Carsten Deppe, Christian Hattrup, Georg Sauerlander. Invention is credited to Carsten Deppe, Christian Hattrup, Georg Sauerlander.
Application Number | 20130307428 13/980061 |
Document ID | / |
Family ID | 45558794 |
Filed Date | 2013-11-21 |
United States Patent
Application |
20130307428 |
Kind Code |
A1 |
Hattrup; Christian ; et
al. |
November 21, 2013 |
DRIVER DEVICE AND DRIVING METHOD FOR DRIVING A LOAD, IN PARTICULAR
AN LED UNIT
Abstract
The present invention relates to a driver device (40; 50; 60)
and a corresponding driving method for driving a load (12), in
particular an LED unit (12) including one or more LEDs, said driver
device comprising a power input unit (14) for receiving an input
voltage (V10) from an external power supply (18) and for providing
a rectified supply voltage (V12), a controllable resistor (48) for
providing a load current (I L) to power the load (12), a frequency
filter (42) connected to the power input unit (14) for providing a
voltage (V18) to the load (12), wherein the frequency filter (42)
is partially coupled in parallel to the load (12) and connected to
the controllable resistor (48) to provide a substantially constant
electrical power to the load (12).
Inventors: |
Hattrup; Christian;
(Wurselen, DE) ; Sauerlander; Georg; (Aachen,
DE) ; Deppe; Carsten; (Aachen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hattrup; Christian
Sauerlander; Georg
Deppe; Carsten |
Wurselen
Aachen
Aachen |
|
DE
DE
DE |
|
|
Assignee: |
KONINKLIJKE PHILIPS N.V.
EINDHOVEN
NL
|
Family ID: |
45558794 |
Appl. No.: |
13/980061 |
Filed: |
January 13, 2012 |
PCT Filed: |
January 13, 2012 |
PCT NO: |
PCT/IB2012/050173 |
371 Date: |
July 17, 2013 |
Current U.S.
Class: |
315/201 |
Current CPC
Class: |
H05B 45/395 20200101;
H05B 45/37 20200101; Y02B 20/343 20130101; Y02B 20/30 20130101 |
Class at
Publication: |
315/201 |
International
Class: |
H05B 33/08 20060101
H05B033/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 17, 2011 |
EP |
11151137.4 |
Claims
1. A driver device for driving an LED unit including one or more
LEDs, the driver comprising: a power input unit for receiving an
input voltage from an external power supply and for providing a
rectified supply voltage, a controllable resistor for providing a
load current to power the load, a frequency filter connected to the
power input unit for providing a voltage to the load, the frequency
filter being partially coupled in parallel to the load and
connected to the controllable resistor to provide a substantially
constant electrical power to the load, and a modulator coupled in
series to the LED unit for modulating the drive current and for
modulating the emitted light output, wherein a substantially
constant voltage is applied to the LED unit and the modulator by
means of the frequency filter.
2. The device as claimed in claim 1, wherein the frequency filter
comprises a capacitor coupled to the controllable resistor so that
a voltage provided by the capacitor is at least partially applied
to a control input of the controllable resistor as a control
voltage to drive the controllable resistor.
3. The device as claimed in claim 1, wherein the frequency filter
is connected to the power input unit to derive a voltage from the
rectified supply voltage, wherein the voltage is at least partially
applied to a series connection formed by the control input of the
controllable resistor and the load.
4. The driver device as claimed in claim 1, wherein the frequency
filter is a low-pass filter comprising a capacitor and a resistor,
wherein the resistor is coupled to the controllable resistor,
wherein a substantially constant voltage drops across the capacitor
and a second voltage including an AC-component of the rectified
supply voltage drops across the resistor.
5. The driver device as claimed in claim 4, wherein a control
contact of the controllable resistor is connected to a node between
the resistor and the capacitor.
6. The driver device as claimed in claim 4, wherein a voltage
limiting device is coupled to the controllable resistor to limit
the second voltage dropping across the resistor.
7. The driver device as claimed in claim 4, wherein the
controllable resistor comprises a transistor.
8. The driver device as claimed in claim 7, wherein the
controllable resistor comprises a Darlington stage.
9. The driver device as claimed in claim 8, wherein a resistor is
connected in parallel to the capacitor to decrease the voltage
dropping across the capacitor.
10. (canceled)
11. The driver device as claimed in claim 9, comprising a
modulator, which is coupled in series to an LED unit for modulating
a drive current driving the LED unit and for modulating the light
output emitted from the LED unit, the modulator including a
resistor and a controllable switch coupled in parallel to each
other to provide two different drive current levels.
12. The driver device as claimed in claim 11, wherein the
controllable switch comprises a transistor controlled by a control
unit.
13. The driver device as claimed in claim 11, wherein the modulator
comprises a second controllable switch controlled by the controller
and coupled in series to a second resistor, wherein the second
controllable switch and the second resistor are coupled in parallel
to the first controllable switch to provide at least three
different drive current levels.
14. Method of driving an LED unit including one or more LEDs,
comprising the steps of: receiving an input voltage from an
external power supply, rectifying the input voltage to a rectified
supply voltage and providing the rectified supply voltage to a
frequency filter, providing a modulated load current to the load by
means of a controllable resistor and a modulator, to power the
load, providing a voltage to the load by means of the frequency
filter, and providing a substantially constant electrical power to
the load by means of the frequency filter partially coupled in
parallel to the load and connected to the controllable
resistor.
15. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a driver device and a
corresponding driving method for driving a load, in particular an
LED unit comprising one or more LEDs. Further, the present
invention relates to a light apparatus. The present invention
relates further to a driver device comprising a modulator to
modulate the output of an LED unit.
BACKGROUND OF THE INVENTION
[0002] In the field of LED drivers for offline application,
solutions are demanded to achieve, inter alia, high efficiency,
high power density, long life time, high power factor or low cost.
The LED units known from the prior art are usually designed to be
connected to a DC-voltage supply. An LED unit including a driver
circuit designed to be connectable to a DC-voltage supply is e.g.
known from JP 5136461A. Practically, the LEDs and the driver
circuits should be designed to be connectable to the mains input
and should transform the mains energy into the form required by the
LEDs while keeping compliance with present and future power mains
regulations. It is of critical importance to guarantee a high
efficiency of the LEDs even if the supply voltage of the mains
input is a voltage having a variation of up to .+-.10%.
[0003] Usually, a rectifier bridge is used to connect the LEDs to
the mains input to provide a DC voltage, which is necessary to
drive the LED unit. The rectified voltage usually still comprises
an AC component and the efficiency normally decreases considerably
in the case of variations of the input voltage.
[0004] Further, it is known in the field of LEDs to modulate the
light output of a lamp and to embed information into the light in a
way not visible to the user. This modulated light can send data to
a receiver located beneath or in the proximity of the respective
lamp. A method of modulating light signals is described in detail
in WO 2009/010909.
[0005] The driving circuits to modulate the light output are
usually complex and include a large amount of components.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide a driver
device, which is adapted to connect a load, in particular an LED
unit, to the mains input, and which has an improved efficiency and
to provide a corresponding method. Further, it is an object of the
present invention to provide a corresponding light apparatus. It is
a still further object of the present invention to provide an
improved and simplified modulator to modulate the light output of
an LED unit.
[0007] According to an aspect of the present invention, a driver
device is provided for driving a load, in particular an LED unit
including one or more LEDs, comprising: [0008] a power input unit
for receiving an input voltage from an external power supply and
for providing a rectified supply voltage, [0009] a controllable
resistor for providing a load current to power the load, and [0010]
a frequency filter connected to the power input unit for providing
a voltage to the load, [0011] wherein the frequency filter is
partially coupled in parallel to the load and connected to the
controllable resistor to provide a substantially constant
electrical power to the load.
[0012] According to another aspect of the present invention, a
corresponding driving method is provided.
[0013] According to another aspect of the present invention, a
driver device is provided comprising a modulator, which is coupled
in series to an LED unit for modulating a drive current driving the
LED unit and for modulating the light output emitted from the LED
unit, the modulator including a resistor and a controllable switch
coupled in parallel to each other to provide at least two different
drive current levels.
[0014] According to still another aspect of the present invention,
a light apparatus is provided comprising a light assembly
comprising one or more light units, in particular an LED unit
comprising one or more LEDs, and a driver device for driving said
light assembly as provided according to the present invention.
[0015] Preferred embodiments of the invention are defined in the
dependent claims. It shall be understood that the claimed driving
method and the claimed light apparatus have similar and/or
identical preferred embodiments as the claimed driver device and as
defined in the dependent claims.
[0016] The present invention is based on the idea to provide a
driver circuit by which a substantially constant electrical power
is applied to the load by providing a DC component (constant
voltage or constant current) to the load, in particular to the LED
unit, and wherein an AC component of the rectified voltage is cut
off by means of the frequency filter and the controllable resistor.
This is achieved by deriving a nearly constant voltage from the
rectified voltage by means of the frequency filter, wherein the AC
component of the rectified voltage is applied to the controllable
resistor which is controlled by means of the frequency filter.
Thus, the AC component is not applied to the load; in particular it
is not applied to the LED unit. In this way, the electrical power
applied to the load, in particular to the LED unit, does not
comprise significant AC components, which would lead to a decreased
efficiency and unwanted AC components in the light output
(flicker). This driving circuit reduces the driver losses and
increases the system efficiency compared to state of the art
solutions. Further, it automatically adapts the LED power even if
the input voltage changes. According to the present invention, the
frequency filter is partially coupled in parallel to the load, i.e.
at least one component of the frequency filter is connected in
parallel to the load, wherein the voltage provided by the frequency
filter component drops at least partially across the load and, if
applicable, partially across additional devices connected in series
to the load.
[0017] The second aspect of the present invention relates to a
driver device comprising a modulator to modulate the drive current
and to modulate the light output emitted from an LED unit. This
modulator can be used in combination with the driver device
according to the first aspect of the present invention. Further,
the driver device according to the first aspect of the present
invention and the driver device comprising the modulator according
to the second aspect of the present invention can be used
independently of each other in different circuits. The driver
device including the modulator is based upon the idea that the
light output emitted from the LED unit corresponds to the current
driving the LED units. The modulator comprises a resistor and,
parallel thereto, a controllable switch, which is controlled, e.g.
by a controller. This provides a cheap and simple solution to
provide two drive current levels by switching the controllable
switch. If the switch is closed, the current is at a high level,
while the current is at a lower level if the switch is opened and
the current is passed through the resistor. This driving circuit
provides a simple possibility to modulate the light output up to
the MHz range. Thus, an easy and simple circuit to modulate the
drive current of the LED unit can be provided.
[0018] In an embodiment of the driver device, the frequency filter
comprises a capacitor coupled to the controllable resistor, so that
a voltage dropping across the capacitor is at least partially
applied as a control voltage to a control input of the controllable
resistor to drive the controllable resistor. That is to say, the
voltage is at least partially applied to the control input of the
controllable resistor, and, if applicable, partially applied to
additional devices connected in series to the control input of the
controllable resistor. This embodiment provides a load current to
the load without significant AC-components and with low technical
effort.
[0019] According to a further embodiment, the frequency filter is
connected to the power input unit to derive a voltage, in
particular a substantially constant voltage, from the rectified
supply voltage, wherein the voltage is at least partially applied
to a series connection of the control side of the controllable
resistor and the load. This embodiment provides a simple
possibility to apply a constant power to the load with low
technical effort.
[0020] In an embodiment of the driver device, the frequency filter
is a low-pass filter comprising a capacitor and a resistor, wherein
the resistor is coupled to the controllable resistor, in particular
connected to an input contact and a control contact of the
controllable resistor, wherein a substantially constant voltage
drops across the capacitor and a second voltage including an AC
component of the rectified supply voltage drops across the
resistor. This embodiment is a simple possibility to provide a
frequency filter and to provide a nearly constant voltage to the
load and to cut off the main AC components of the rectified
voltage.
[0021] According to a further embodiment, a control contact of the
controllable resistor is connected to a node between the resistor
and the capacitor. This provides a simple solution to drive the
controllable resistor by means of the frequency filter and to cut
off the main AC-components of the rectified voltage.
[0022] In a further embodiment, a voltage limiting device is
coupled to the controllable resistor, in particular to limit the
second voltage dropping across the resistor. This voltage limiting
device, which is preferably formed of a Zener diode, e.g. having a
forward voltage of 20 to 30 V, provides quick charging of the
capacitor of the frequency filter when the driver device is
connected to the mains. Further, the Zener diode provides a load
current while the capacitor of the frequency filter is charged, so
that electrical power is provided to the load very quickly after
connection to the mains. Therefore, the specified output of the
load, in particular the light emission of the LED unit, can be
reached more quickly.
[0023] In a preferred embodiment, the controllable resistor
comprises a transistor. This embodiment is a simple solution
enabling a controllable resistor to provide the load current and to
cut off the AC components of the rectified voltage. The transistor
can be formed of a bipolar transistor or a MOSFET.
[0024] In a further preferred embodiment of the present invention,
the controllable resistor comprises a Darlington stage. This
circuit provides a low current consumption at the control input
because of the typically high current gain of a Darlington
configuration. As a consequence, the value of the resistor of the
frequency filter can be rather large, reducing the losses in the
frequency filter.
[0025] In a further embodiment, the capacitor is coupled in
parallel to the load, in particular the LED unit, and the control
input of the controllable resistor. This embodiment provides the
possibility to define a precise and stable constant voltage to be
applied to the load, whereby the efficiency of the system is
increased.
[0026] In a further preferred embodiment of the present invention,
a resistor is connected in parallel to the capacitor to decrease a
voltage dropping across the capacitor. This increases the voltage
drop across the controllable resistor and decreases the voltage
drop across the capacitor. Thus, an offset is provided for the
voltage applied to the controllable resistor, so that a small
voltage drop of the supply voltage does not result in a drop of the
load current.
[0027] In a further preferred embodiment, the load is an LED unit
and a modulator is coupled in series to the LED unit, wherein a
substantially constant voltage is applied to the LED unit and the
modulator by means of the frequency filter to modulate the drive
current and to modulate the emitted light output. This embodiment
provides a possibility to modulate the light output and to transmit
signals by means of the LED unit.
[0028] In a preferred embodiment of the driver device comprising a
modulator, the controllable switch comprises a transistor
controlled by a control unit. This embodiment provides a simple
solution to modulate the load current and provides a high switching
speed of the controllable switch.
[0029] In a further preferred embodiment, the modulator comprises a
second controllable switch controlled by the controller and coupled
in series to a second resistor, wherein the second controllable
switch and the second resistor are coupled in parallel to the first
controllable switch to provide three different drive current
levels. This embodiment provides a possibility to provide three
different current levels symmetrically around the DC level which
does not provide visible flicker of the LED units and which does
not influence the efficiency of the LED unit. In an alternative
embodiment, more than three parallel paths are connected to the
load to provide more than three different current levels. Hence, a
high switching speed between the current levels can be
realized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] These and other aspects of the invention will be apparent
from and elucidated with reference to the embodiment(s) described
hereinafter. In the following drawings:
[0031] FIG. 1 shows a schematic block diagram of a known driver
circuit for an LED unit including a current source to modulate the
load current,
[0032] FIG. 2 shows a schematic diagram of a Manchester code,
[0033] FIG. 3 shows a schematic block diagram of a driver device
for driving an LED according to a first embodiment of the present
invention,
[0034] FIG. 4 shows a schematic diagram of a driver device to
modulate the load current of an LED unit and to modulate the light
output,
[0035] FIG. 5 shows a detailed schematic block diagram of an
embodiment of the driver device according to the invention,
[0036] FIG. 6 shows a diagram of the load current, the rectified
supply voltage and the constant voltage of the device according to
FIG. 5,
[0037] FIG. 7 shows a schematic block diagram of a further
embodiment of the present invention, and
[0038] FIG. 8 shows a diagram of the load current, the rectified
supply voltage and the constant voltage provided by the embodiment
according to FIG. 7.
DETAILED DESCRIPTION OF THE INVENTION
[0039] An embodiment of a known driver device 10 for driving an LED
unit is schematically shown in FIG. 1. The driver device 10
comprises an input unit 14 coupled to terminals 16 for connection
to mains power supply 18. The input unit 14 is connected in
parallel to the LED unit 12. The LED unit 12 is connected in series
to a modulator unit 20, which comprises a programmable current
source 22 to modulate a load current I.sub.L of the LED unit
12.
[0040] The light modulation according to this embodiment is
provided for LED units, wherein the light output of the LEDs
follows rather quickly the electric current I.sub.L driven by the
current source 22. The current source 22 provides in this
particular case a Manchester pulse of the load current I.sub.L, as
described below. The LED unit 12 comprises a plurality of LEDs
connected in series. These LEDs can be low or high voltage LEDs or
series-connected LEDs, wherein the forward voltage drop is less
than the minimum of the rectified and smoothened input voltage
supplied from the mains. The input unit 14 comprises a plurality of
components including capacitors, diodes and resistors. To adapt the
mains voltage to the LED voltage without power loss, the capacitor
C1 is provided in the input unit. The change of this capacitor C1
determines the average DC voltage of a capacitor C2, which is
provided in parallel to the output of the input unit 14.
[0041] The modulator 20 comprises the programmable current source
22. The current source 22 is connected to a resistance R4 and a
resistance R6 to control the load current I.sub.L of the LED unit.
A controller connected to the current source 22 is provided to
switch a series connection of a diode D8 and a resistance R7 to
ground or to a supply voltage and to switch a series connection of
a diode D9 and a resistance R8 to ground or a supply voltage. A
supply voltage V.sub.CC is provided to the controller and the
resistance R6, wherein a voltage source providing the supply
voltage V.sub.CC is not shown in FIG. 1. While switching takes
place of one of these diode-resistance combinations to the supply
voltage, no current is passed through this path, while if said
combination is connected to ground, a small additional current is
passed through this path. Thus, the controller is able to provide
three different levels of the load current I.sub.L to provide a
symmetrical modulation around the DC level of the load current
I.sub.L. If more different levels of the load current I.sub.L and
the respective light emission are required, more control paths need
to be implemented in the modulator 20.
[0042] FIG. 2 shows a clock signal 26, a data signal 28 of the data
to be transmitted, a first Manchester code 30 and an inverse
Manchester code 32 (according to IEEE 802.3). The Manchester codes
30, 32 are formed of a signal alteration around the DC level of the
respective value, e.g. a voltage or a current. The data signal 28
is transformed to the Manchester code, wherein a high level data
signal corresponds to a change in the Manchester code 30 from high
level to low level or in the inverse Manchester code 32 from low
level to high level. Thus, a modulation of the load current I.sub.L
is realized with three different levels, wherein the light signal
has a constant average value and no visible flicker of the light
output is produced if the modulation frequency is high enough.
[0043] A first embodiment of a driver device 40 according to the
present invention is schematically shown in FIG. 3. The driver
device 40 comprises the input unit 14 comprising the terminals 16
for connection to the mains 18. The input unit 14 preferably
comprises a rectifier bridge, such as a known full-bridge or
half-bridge rectifier, for rectifying an AC input voltage V10
provided from mains voltage supply 18 into a rectified voltage V12.
The input unit 14 is connected in parallel to a frequency filter 42
to apply or provide the rectified voltage V12 to the frequency
filter 42. According to this embodiment, the frequency filter 42 is
formed of a capacitor 44 and a resistor 46. The frequency filter is
designed in such a way that a substantially constant voltage V14
drops across the capacitor 44 and a filter voltage V16 drops across
the resistor 46. Thus, the rectified voltage V12 is divided into
the substantially constant voltage V14 and the filter voltage V16.
The filter voltage V16 comprises remaining AC components (ripple)
of a rectified voltage V12. The driver device 40 further comprises
a controllable resistor 48 having three electrical contacts 49. An
input contact 49a of the controllable resistor 48 is connected to
the power input unit 14. An output contact 49b of the controllable
resistor 48 is connected to the load 12. A control contact 49c is
connected to a node 45 between the resistor 46 and the capacitor
44. A control side or a control input of the controllable resistor
48 is formed between the output contact 49b and the control contact
49c. The resistor 46 of the frequency filter 42 is connected to the
input contact 49a and the control contact 49c of the controllable
resistor 48. The controllable resistor 48 provides the load current
I.sub.L to the LED unit 12. The constant voltage V14 is applied to
the LED unit 12 and the control side of the controllable resistor
48 connected in parallel to the capacitor 44. A load voltage V18
drops across the LED unit 12 and a control voltage V20 drops across
the control side of the controllable resistor 48, wherein
V14=V18+V20. The controllable resistor 48 is driven by a drive
current I.sub.D, which is controlled by the control voltage
V20.
[0044] Because of the parallel connection of the resistor 46 to the
controllable resistor 48, the remaining AC components of the
rectified voltage V12 are cut off, while the load current I.sub.L
provided to the LED unit 12 is a substantially constant DC current
depending on the load voltage V18 and the resistance of the load
12. Since the substantially constant supply voltage V14 dropping
across the capacitor 44 is supplied to the LED unit 12 and the
control side of the controllable resistor 48, the LED unit 12 is
powered by the substantially constant load voltage V18 and the load
current I.sub.L.
[0045] In a preferred embodiment of the driver device 40, a diode
(not shown in FIG. 3) is connected forward-biased in parallel to
the control side of the controllable resistor 48 to limit the
voltage V14, which is slightly larger than V18. Alternatively, a
diode e.g. a Zener diode, is connected reverse-biased in parallel
to the resistor 46 to limit the voltage V14.
[0046] The controllable resistor 48 is preferably formed by a
transistor, wherein the input contact 49a corresponds to the
collector or the source contact, the output contact 49b corresponds
to the emitter or drain contact and the control contact 49c
corresponds to the gate or base contact, respectively. In the case
that the controllable resistor 48 is a bipolar transistor, the
diode parallel to the control side is formed by the base-emitter
path. In the case that the controllable resistor 48 is a MOSFET,
the diode (not shown in FIG. 3) is preferably connected in parallel
to the gate-drain path.
[0047] A further alternative embodiment of a driver device 50 is
shown in FIG. 4. wherein identical elements are denoted by
identical reference numerals, and only the differences are
explained in detail.
[0048] The input unit 14 is connected in parallel to the LED unit
12, which is connected in series to a modulator 52. The modulator
52 comprises in parallel a resistor 54 and a controllable switch
56, which is preferably formed of a transistor. The controllable
switch 56 is controlled via a control input 58, which is preferably
connected to a controller unit. A modulator voltage V22 drops
across the modulator 52.
[0049] The load current I.sub.L is modulated by switching the
controllable switch 56. If the switch is closed, the LED unit 12 is
directly connected to ground and the rectified supply voltage V12
is directly applied to the LED unit 12 and the modulator 52,
wherein V12=V18+V22. If the controllable switch 56 is open, the
load current I.sub.L is passed through the resistor 54 such that
the load current 54 is reduced to a lower level. Thus, the load
current 54 is switchable to two different levels by switching the
controllable switch 56. Since the light output of the LED unit 12
corresponds to the load current I.sub.L, the light output can be
modulated by actuating the switch 56 via the control input 58.
[0050] As shown in FIGS. 3 and 4, these two embodiments can be used
separately from each other in different applications.
[0051] FIG. 5 shows schematically a driving device 60 according to
a further embodiment of the present invention, wherein identical
elements are denoted by identical reference numerals, and only the
differences are explained in detail. The rectified supply voltage
V12 is applied to the frequency filter 42. The frequency filter 42
comprises the resistor 46 and the capacitor 44. The substantially
constant voltage V14 drops across the capacitor 44 and the filter
voltage V16 drops across the resistor 46. The resistor 46 is
connected in parallel to the input contact 49a and the control
contact 49c of the controllable resistor 48, which is formed of a
Darlington stage 48. The Darlington stage 48 comprises a first
transistor 62 and a second transistor 64 connected to each other in
a Darlington configuration. The filter voltage V16 dropping across
the resistor 46 is applied to the Darlington stage 48 and forms in
this particular case the collector-base voltage of the first
transistor 62. The control voltage V20 is applied to the control
side or control input of the Darlington stage 48, and forms in this
particular case the base-emitter voltage of the Darlington stage
48, and controls the drive current I.sub.D, which drives the
Darlington stage 48. The Darlington stage 48 provides the load
current I.sub.L. A Zener diode 66 is connected in parallel to the
collector-base path of the Darlington stage 48 to limit the voltage
V16. Further, the Zener diode 66 is provided to charge the
capacitor 44 when the input unit 14 is initially connected to the
mains 18. An important advantage of the Zener diode 66 in this
configuration is that the load current I.sub.L rises quickly after
connection to the mains 18 while the capacitor 44 is charged.
Hence, the LED unit 12 is powered quickly after the driving device
60 is connected to the mains 18. Further, a resistor 68 is
connected to the control contact 49c of the Darlington stage 48 to
deliver the drive current I.sub.D to the control contact 49c of the
Darlington stage 48 from the voltage V14 and to limit the charging
current of the filter capacitor 44 when the system is initially
connected to the mains 18.
[0052] Thus, the constant voltage V14 is a smoothed representation
of the minimum rectified supply voltage V12. The substantially
constant voltage V14 is applied to the control input of the
Darlington stage 48, the LED unit 12 and the modulator 52, wherein
remaining AC components of the rectified supply voltage V12 are cut
off by means of the Darlington stage 48. The substantially constant
voltage V14 is applied to the control input, the LED unit 12 and
the modulator 52, wherein V14=V20+V18+V22. Thus, the LED unit 12 is
powered by the substantially constant load voltage V18 and the load
current I.sub.L. The load voltage V18 and the load current I.sub.L
form a substantially constant electrical power provided to the load
12. Therefore, the efficiency of the system is nearly independent
of variations of the mains voltage V10.
[0053] Further, the modulator 52 is connected in series to the LED
unit 12. The modulator voltage V22 drops across the modulator 52.
The modulator comprises the resistor 54 and the controllable switch
56 connected in parallel to each other. The modulator 52 further
comprises a second resistor 70 and a second controllable switch 72
connected in series to each other. The second resistor 70 and the
second switch 72 are connected in parallel to the switch 56 and the
resistor 54. The second switch 72 is controlled via a control input
74 preferably connected to a controller.
[0054] The modulator 52 can provide the load current I.sub.L at
three different levels by switching the switches 56, 72. If the
second switch 72 is closed, the load current I.sub.L passes through
the resistor 54 and the second resistor 70 and provides a
medium-load current level. If the switches 56, 72 are opened, the
current I.sub.L passes through the resistor 54, whereby a low level
of the load current I.sub.L is provided. If the switch 56 is
closed, the load current I.sub.L passes through the switch 56 to
ground, whereby a high level of the load current I.sub.L is
provided. In this case, the modulator voltage V22 is at the lowest
level, i.e. almost zero. Thus, the modulator 52 can provide three
different levels of the load current I.sub.L. In an embodiment of
the driving device 60, the modulator 52 comprises more than three
parallel switchable paths to provide more different current levels.
In that embodiment, the modulator 52 can provide as many current
levels as the number of parallel paths implemented.
[0055] Thus, the driving device 60 provides a high efficiency for
the LED unit and the possibility of modulating the light output of
the LED unit 12. FIG. 6 shows a diagram of the load current
I.sub.L, the constant supply voltage V14 and the rectified supply
voltage V12 provided by the driving device 60 according to FIG. 5.
The load current I.sub.L is rather constant and provided with a
modulated portion 76. Further, the load current I.sub.L shows
negative peaks or dips shown at 78. The constant supply voltage V14
shows a constant behavior over time. The rectified supply voltage
V12 comprising an AC component is also shown in FIG. 6. At certain
times, denoted by 80, the rectified supply voltage V12 equals or
falls below the constant voltage V14. At this time the voltage drop
across the controllable resistor 48 is at its minimum. Then, also
the load voltage V18 across the LED unit 12 drops, resulting in a
current dip. To avoid negative peaks in the load current I.sub.L
shown at 78, the filter voltage V16 needs to be increased such that
the filter voltage V16 does not fall to the level of the constant
supply voltage V14 or even below this voltage V14.
[0056] In FIG. 7, a further embodiment of the drive device 60 is
shown, wherein identical elements are denoted by identical
reference numerals, and only the differences are explained in
detail. In this embodiment, a further resistor 82 is connected in
parallel to the capacitor 44. This resistor 82 lowers the voltage
V14 dropping across the capacitor 44 and increases the filter
voltage V16 dropping across the resistance 46. Thus, the filter
voltage V16 applied to the Darlington stage 48 is increased and
does not fall to or below the constant supply voltage V14.
[0057] FIG. 8 shows a time diagram of the load current I.sub.L, the
constant supply voltage V14 and the rectified supply voltage V12
according to the drive device 60 of FIG. 7 including the resistance
82. The load current I.sub.L comprises the modulated portion 76.
The constant supply voltage V14 is a constant signal and the
rectified supply voltage V12 is identical to the signal shown in
FIG. 6 having an offset with respect to the constant supply voltage
V14. Thus, even if the rectified supply voltage V12 is decreased at
certain times shown at 84, the rectified supply voltage V16 does
not drop to or below the constant supply voltage V14. Thus, the
load current I.sub.L is constant over time without any negative
peaks or dips as shown at 78 in FIG. 6.
[0058] Thus, the driving device 60 provides a substantially
constant load voltage V18 applied to the LED unit 12 and a constant
load current I.sub.L passing through the LED units. Hence, the
efficiency of the LED unit 12 is increased even if the mains
voltage 18 varies.
[0059] Further, the driving device 60 provides a possibility to
modulate the light output by modulating the load current I.sub.L as
shown in FIG. 8 by means of the modulated portion 76 of the load
current I.sub.L.
[0060] While the invention has been illustrated and described in
detail in the drawings and foregoing description, such illustration
and description are to be considered illustrative or exemplary and
not restrictive; the invention is not limited to the disclosed
embodiments. Other variations to the disclosed embodiments can be
understood and effected by those skilled in the art in practicing
the claimed invention, from a study of the drawings, the
disclosure, and the appended claims.
[0061] In the claims, the word "comprising" does not exclude other
elements or steps, and the indefinite article "a" or "an" does not
exclude a plurality. A single element or other unit may fulfill the
functions of several items recited in the claims. The mere fact
that certain measures are recited in mutually different dependent
claims does not indicate that a combination of these measures
cannot be used to advantage.
[0062] Any reference signs in the claims should not be construed as
limiting the scope thereof.
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