WO2011030246A2

WO2011030246A2 - Driving circuit for driving a plurality of light sources

Info

Publication number: WO2011030246A2
Application number: PCT/IB2010/053683
Authority: WO
Inventors: Martijn Henri Richard Lankhorst; Dennis Johannes Antonius Claessens
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-09-09
Filing date: 2010-08-16
Publication date: 2011-03-17
Also published as: WO2011030246A3

Abstract

The invention relates to a driving circuit (1) for driving a plurality of light sources (3,4) arranged in a series configuration, the circuit (1) comprising: at least two controllable switches (6,7,8), each switch configured to shunt one or more corresponding light sources (3,4,5); - a controller (9) for individually controlling operative states of the switches (6,7,8) in order to control a light output of the corresponding light sources (3,4,5); a secondary source (11) having input terminals (13,14) for connecting to a primary source and output terminals (18,19) for connecting to the series arrangement of light sources (3,4,5), wherein the secondary source (11) has the characteristics of a voltage source; - an electrical element (21,22,7,33,40) to provide an impedance between the secondary source (11) and the series arrangement of light sources (3,4,5) in series with the series arrangement of light sources (3,4,5).

Description

Driving circuit for driving a plurality of light sources

FIELD OF THE INVENTION

The invention relates to the field of driving circuits for driving a plurality of light sources arranged in a series configuration, specifically but not necessarily LEDs. The invention further relates to a lighting device comprising a plurality of light sources arranged in a series configuration and a driving circuit connected to said series arrangement of light sources for driving the plurality of light sources. The invention also relates to a method for driving a plurality of light sources arranged in a series configuration.

BACKGROUND OF THE INVENTION

In some cases a lighting device comprises a plurality of light sources. An example is the backlight of an LCD display, for use in a monitor, a TV, or the like. But lamps which are able to emit light with different colors are also gaining popularity.

Especially in case of LEDs as the light sources, which is not necessary, the light sources may be arranged in a series configuration and connected to a constant current source. At least two switches are provided to control the color-settings of the lighting device, wherein each switch shunts one or more corresponding light sources thereby being able to enable or disable said one or more corresponding light sources by respectively opening and closing the switch. Closing a switch short-circuits the one or more corresponding light sources and opening a switch will allow current to flow through the one or more

corresponding light sources.

A drawback of a driving circuit with constant current source is that the magnitude of the electrical current supplied by the current source is limited due to the maximal allowed heat generation. The maximal allowed heat generation is determined by the possible heat transfer to the ambient and the maximal allowed temperatures of the lighting device. The temperature should remain below a certain value to ensure sufficient lifetime and safety of the system. The heat generation is maximal when all light sources are enabled, i.e. on, so that the magnitude of the constant electrical current is chosen for this worst case scenario. However, in general the worst case scenario does not occur often as for a lot of different color settings not all light sources are enabled at the same time. With the above described driving circuit it is not possible to optimize the magnitude of the electrical current for each color setting while maintaining the heat generation below a certain desired value at the same time.

SUMMARY OF THE INVENTION

It would be desirable to provide an improved lighting device. It would also be desirable to provide a driving circuit that is able to optimize the magnitude of the electrical current provided to the light sources for each color setting. It would further be desirable to provide an improved method for driving a plurality of light sources.

To better address one or more of these concerns, in a first aspect of the invention a driving circuit is provided for driving a plurality of light sources arranged in a series configuration, the circuit comprising:

at least two controllable switches, each switch configured to shunt one or more corresponding light sources;

a controller for individually controlling operative states of the switches in order to control a light output of the corresponding light sources;

a secondary source having input terminals for connecting to a primary source and output terminals for connecting to the series arrangement of light sources, wherein the secondary source has the characteristics of a voltage source;

an electrical element to provide an impedance between the secondary source and the series arrangement of light sources in series with the series arrangement of light sources.

In an embodiment, the impedance is a substantially non-dissipative

impedance, for instance due to the use of an inductor and/or a capacitor as electrical element. This has the advantage that minimal power is dissipated by the impedance and thus provides a good efficiency of the driving circuit.

In another embodiment, the electrical element is part of the secondary source. As an example, the secondary source may comprise a switch-mode DC to DC convertor and a transformer, wherein the transformer may transfer the impedance of an electric element at the primary side of the transformer to the secondary side of the transformer to provide the impedance between the secondary source and the series arrangement of the light sources. Preferably, the impedance is constant in use. When a switch-mode DC to DC convertor is used in the secondary source, the magnitude of the impedance may be set by a switching frequency of the switch-mode DC to DC convertor.

According to another aspect of the invention a lighting device is provided, comprising a plurality of light sources arranged in a series configuration, and a driving circuit connected to said series arrangement of light sources for driving the plurality of light sources, said driving circuit comprising:

- a controller for individually controlling operative states of the switches in order to control a light output of the corresponding light sources;

- an electrical element to provide an impedance between the secondary source and the series arrangement of light sources in series with the series arrangement of light sources.

According to yet another aspect of the invention a method is provided for driving a plurality of light sources arranged in a series configuration, said method comprising the steps:

controlling a light output of one or more light sources by controlling operative states of respective switches shunting the one or more light sources;

providing a voltage source to the series arrangement of light sources;

providing an impedance between the secondary source and the series arrangement of light sources in series with the series arrangement of light sources.

In an embodiment, a current i provided by the voltage source has the characteristic i=(Vs-Vl)/Z, wherein Vs is the voltage provided by the voltage source to the series configuration of the impedance and the light sources, VI is the voltage drop across the series arrangement of light sources, and Z is the impedance.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a lighting device according to an embodiment of the invention;

Figure 2 depicts an equivalent circuit of the lighting device according to the embodiment of Fig. 1;

Figure 3 depicts a lighting device according to another embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts a lighting device according to an embodiment of the invention. The lighting device comprises a driving circuit 1 for driving a plurality of light sources 3,4 arranged in a series configuration. The light source 3,4 in this example are LEDs. The driving circuit 1 comprises a plurality of controllable switches 6,7, each light source 3,4 being shunted by a corresponding controllable switch 6,7. In an alternative embodiment, a controllable switch may also shunt more light sources at a time.

A controller 9 is provided to individually control the operative states of the switches 6,7 in order to control a light output of the corresponding light sources. Closing a switch 6,7 short circuits the corresponding light source 3,4, thereby disabling the light source, and opening a switch 6,7 allows a current to flow through the corresponding light source, thereby enabling the light source. Closing a switch refers to putting the switch in a conducting state, and opening a switch refers to putting the switch in a non-conducting state.

As the lighting device comprises two light sources 3,4, preferably each light source being of a different color, the lighting device is able to output four different color settings:

all light sources disabled;

all light sources enabled;

light source 3 enabled and light source 4 disabled; and

light source 3 disabled and light source 4 enabled.

The lighting device is also able to mix one or more of the four different color settings by rapidly switching between the different color settings. Preferably, switching between different color setting is done with a frequency high enough for human eyes to be unnoticed.

The series arrangement of light sources 3,4 is connected to a secondary source 11 via output terminals 18,19. The secondary source 11 has input terminals 13,14 for connecting to a primary source 16, which may be electric mains or a battery or any other kind of electrical power source. The secondary source 11 has the characteristics of a voltage source, and the specific configuration depends on the characteristics of the primary source and the desired electrical demands for the series arrangement of light sources.

In its simplest form, the primary source 16 is already a DC voltage source which output can directly be used to power the series arrangement of light sources. In that case, the secondary source comprises electrical connections which directly connect input terminal 13 to output terminal 18, and input terminal 14 to output terminal 19. In a more complex embodiment, the primary source is an AC voltage source, and the secondary source is a voltage regulator comprising at least a rectifier and possibly a DC to DC convertor.

The driving circuit comprises at least one electrical element. To show some possible configurations, two electrical elements 21,22 are shown in this embodiment, each of which are able to provide an impedance between the secondary source and the series arrangement of light sources in series with the series arrangement of light sources. Both electrical elements 21 and 22 are shown to clearly indicate that an electrical element can be placed anywhere in the driving circuitry as long as it is able to provide directly or indirectly the impedance between the secondary source and the series arrangement of light sources as described above. An electrical element may be part of the secondary source as electrical element 21 or may be located between the secondary source and the series arrangement of light sources as electrical element 22. As both electrical elements 21,22 are able to provide the impedance, one of them may be omitted, but a combination as shown in Fig. 1 is also possible. The electrical element may be any type of electrical component, including resistor, inductor, capacitor, transistor, diode, etc.

In an embodiment, nonlinear electrical elements can also be used to for instance make the impedance dependent of the current through the light sources.

By providing the secondary source with the characteristics of a voltage source, i.e. by providing a voltage source to the light sources, and the impedance, it is possible to optimize the current through the light sources while maintaining the amount of generated heat below a certain desired value. This is particularly advantageous for thermally limited lighting devices such as compact LED lighting devices/lamps which may be retrofitted for a halogen or incandescent bulb.

Fig. 2 shows a desired equivalent circuit of the lighting device of Fig. 1. The light sources 3,4 are connected in series with an impedance Z to a voltage source providing a voltage Vs. The earlier described electrical elements 21,22 form the impedance Z and the secondary source 11 is equivalent to the voltage source and provides the voltage Vs in use.

It is explicitly mentioned here that an internal resistance inherent to any non- ideal voltage source is not the electrical element providing the impedance Z. The internal resistance is not part of the driving circuitry, but part of the primary source. The advantage of a separate electrical element is that the current through the light sources can be optimized by choosing an appropriate value for the electrical element, as will be explained below with reference to the equivalent circuit of Fig. 2.

In Fig. 2, both switches 6,7 are shown in the non-conducting state, thereby enabling both light sources 3,4. Voltage VI is the sum of the voltage drop V2 across light source 4 and the voltage drop VI -V2 across light source 3, so that a current i through the light sources 3,4 is determined by (Vs-Vl)/Z. When one of the switches 6,7 is put in a conducting state, thereby disabling one of the light sources by short circuiting, the voltage VI will decrease, so that the difference between Vs and VI will increase resulting in an increased current i through the light source. When said switch is put back into the nonconducting state, the value of VI increases again and the current is decreased in a similar manner. Due to this configuration, the current i is automatically adjusted to the number of light sources that are enabled thereby keeping the amount of heat generation below a certain desired level, while making better use of the potential of a light source.

The above described working principle assumes a constant behavior of the impedance Z. It is also possible to have a variable impedance Z, the magnitude of which can be controlled by a control system, which can be part of the controller controlling the states of the switches, or at least uses information of the controller. By controlling the magnitude of impedance Z, the current can be further optimized for each color setting. Alternatively or additionally the impedance may also be the result of a nonlinear electrical element, which for instance provides an impedance in dependency of the current i.

The advantage of a constant impedance is the simplicity of the driving circuit, and the advantage of a variable impedance, e.g. including control system, are the extra optimization possibilities.

Fig. 3 shows a possible embodiment of a driving circuit 1 according to the invention. In this embodiment, the driving circuit 1 is configured for driving three light sources 3-5 arranged in a series configuration. The driving circuit 1 comprises a plurality of controllable switches 6-8, each light source 3-5 being bridged by a corresponding

controllable switch 6-8, and a controller 9 for individually controlling operative states of the switches in order to individually control the light output of the light sources. The color settings are thus controlled in a similar manner as in Fig. 1.

The driving circuit 1 further comprises a secondary source having input terminals 13,14 for connecting to a primary source indicated by a voltage Vp, and output terminals 18,19 for connecting to the series arrangement of light sources.

In this embodiment, the voltage Vp is assumed to be a DC voltage supplied by the primary source. It is however also possible that the primary source outputs an AC voltage, so that the driving circuit may further include a rectifier and/or filter to provide a DC voltage as voltage Vp.

In order to provide a voltage at terminal 18, i.e. such that secondary source 11 behaves like a voltage source, the secondary source comprises a switch-mode DC to DC converter which comprises two switches 35,36 in series parallel to two capacitors 30,31 in series. A midpoint between the two switches 35,36 is connected to a midpoint between the two capacitors by a series connection of an inductor 33 and a primary coil 42 of a transformer 40. Both sides of a secondary coil 44 of the transformer 40 are connected to output terminal 18 via respective diodes 46,47 to provide a rectified voltage at terminal 18. A control system 38 is provided to individually control the operative states of the switches 35,36. As is known to a person skilled in the art of DC to DC converters, the control system will alternately put the switches 35,36 in a conducting state and a non-conducting state, wherein when switch 35 is in the conducting state, switch 36 is in the non-conducting state and vice versa.

The required impedance is formed by the inductor 33, the transformer 42, and a high-frequency oscillation of the switches 35,36 by the control system 38. The imaginary impedance of the inductor 33 is transformed by the transformer 40 to the output terminal 18, so that the driving circuit has an equivalent circuit as shown in Fig. 2. The magnitude of the impedance can be set by the frequency with which the control system controls the switches 35,36.

The advantage of this configuration is that a non-dissipative impedance is created, thereby increasing the efficiency with respect to a dissipative impedance as a resistor. An advantage of the switch-mode DC to DC converter is the high efficiency, and the transformer provides an insulation between the light sources and the primary source.

Table 1 shows a result of an experiment with three light sources in a series arrangement using a prior art driving circuit with a constant current source providing a 450mA current. The three light sources are a red, green, and blue LED. Table 1 shows luminous flux output (in lumen) and power dissipation (in Watt) for four different color settings. In the red, green and blue color setting, only the respective red, green or blue LED is "on". The white color setting is a mix of the red, green and blue.

Table 1 : experiment results with prior art driving circuit

Color setting: Flux (lm): Power (W):

Red 42 2.9

Green 70 1.5

Blue 8 1.7

White 83 3.8

Table 2 shows a result of an experiment with the same three light sources of the experiment according to table 1, now using a driving circuit according to the invention. Again the luminous flux output and power dissipation are shown for the four different color settings. The driving circuit is set such that the maximum power dissipation for the white color setting is also 3.8W.

Table 2: experiment results with driving circuit according to invention

Color setting: Flux (lm): Power (W):

Red 44 3.4

Green 82 2.1

Blue 10 2.3

White 87 3.8

As shown by table 1 and 2, the luminous flux output for the green and blue color setting increases with 15-20%. For red and white, the increase is about 5%. The luminous flux output can thus be optimized with respect to a constant current source while maintaining the power dissipation below a certain desired value.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

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.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfil the functions of several items recited in the claims.

The terms program, software application, and the like as used herein, are defined as a sequence of instructions designed for execution on a computer system. A program, computer program, or software application may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.

A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

CLAIMS:

1. Driving circuit for driving a plurality of light sources arranged in a series configuration, the circuit comprising:

2. Driving circuit according to claim 1, wherein the impedance is a substantially non-dissipative impedance.

3. Driving circuit according to claim 1 or 2, wherein the electrical element is part of the secondary circuit.

4. Driving circuit according to any of the preceding claims, wherein the secondary circuit comprises a switch-mode DC to DC converter.

5. Driving circuit according to claim 4, wherein the secondary circuit comprises a transformer as the electrical element.

6. Driving circuit according to any of the preceding claims, wherein the impedance is constant.

7. Driving circuit according to claim 4, wherein a magnitude of the impedance is set by a switching frequency of the switch-mode DC to DC converter.

8. Driving circuit according to any of the preceding claims, wherein the light sources are LEDs.

9. Lighting device comprising a plurality of light sources arranged in a series configuration, and a driving circuit according to any of the preceding claims connected to said series arrangement of light sources for driving the plurality of light sources.

10. Method for driving a plurality of light sources arranged in a series

configuration, said method comprising the steps:

- providing a voltage source to the series arrangement of light sources;

providing an impedance between the voltage source and the series

arrangement of light sources in series with the series arrangement of light sources.

11. Method according to claim 10, wherein a current i provided by the voltage source has the characteristic i=(Vs-Vl)/Z, wherein Vs is the voltage provided by the voltage source to the series configuration of the impedance and the light sources, VI is the voltage drop across the series arrangement of light sources, and Z is the impedance.