EP2422583A1

EP2422583A1 - Driver for an led lamp

Info

Publication number: EP2422583A1
Application number: EP10717797A
Authority: EP
Inventors: Markus Cornelius Vermeulen
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2009-04-23
Filing date: 2010-04-16
Publication date: 2012-02-29
Also published as: RU2011147477A; WO2010122463A1; CA2759448A1; JP2012524961A; CN102415213A; KR20120018773A

Abstract

An illumination device (100), comprises: a current source (1) having output lines (5; 6); a main string (101) of LEDs (LED1,i) arranged in series between the current source output lines; at least one secondary string (102) of LEDs (LED2,i) arranged in series between the current source output lines; and a controllable current limiter (120) arranged in series with the LEDs (LED2,i) of the secondary string (102). In a preferred embodiment, the controllable current limiter comprises: - a transistor (T2) having its collector-emitter path in series with the LEDs (LED2,i) of the secondary string (102); a current sense resistor (R2x) arranged in series with said collector-emitter path; an opamp (121) having an output connected to the transistor's base, having its inverting input connected to said current sense resistor (R2x), and having its non-inverting input connected to a reference voltage level.

Description

Driver for an LED lamp

FIELD OF THE INVENTION

The present invention relates in general to an illumination device comprising a plurality of LEDs and a single current source.

BACKGROUND OF THE INVENTION

In the recent years, illumination devices comprising LEDs as light sources have seen great development. Likewise, drivers for such devices have been developed. One straight forward way of powering LEDs is to connect a string of series-connected LEDs between the output terminals of a current source. Figure 1 is a block diagram schematically illustrating such design, wherein the current source 1 is implemented as a conventional copper-iron ballast having input terminals 2 for connection to mains, an inductive impedance 3 and a rectifier 4 arranged in series. A first string 11 of LEDs is connected between output terminals 5 and 6 of the rectifier 4.

It should be clear to a person skilled in the art that the number of LEDs to be connected in series depends on the voltage drop over each LED and of the voltage available between the output terminals 5 and 6. If the number of LEDs is too large, the LEDs will not produce light. If the number of LEDs is too low, a series resistance should accomodate the voltage difference between source and LEDs; such resistance is not shown for sake of simplicity.

If it is desirable that the number of LEDs is increased, it is possible to connect a second string 12 of LEDs between the output terminals 5 and 6 of the rectifier 4, in parallel to the first string 11 , as also shown in figure 1. In order to accommodate for differences in characteristics, each string should be provided with its own series resistor, but that it not shown for sake of simplicity. The current I_TOTAL provided by the source 1 will split up in two currents Il and 12 for the two strings 11 and 12, respectively, so that Il + 12 = I_TOTAL- It is further possible to connect a third, fourth, etc string in parallel, depending on the amount of current the source is capable to provide.

Although the design of figure 1 has an advantage of simplicity, it has a disadvantage that it is not possible to vary the power ratio between the different strings, in other words to vary the ratio Il .12. This may not pose a problem if all LEDs are of the same type (colour). However, in a more general design, the LEDs have different colour, and it may be desirable to vary the string current ratio(s) in order to vary the overall colour output or, conversely, it may be desirable to vary the string current ratio(s) in order to keep the overall colour output constant in case of varying temperature, taking into account that LEDs of different type (colour) have different temperature dependencies.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a simple ballast capable of powering two or more strings of LEDs, in which it is possible to vary the power ratio between said strings.

According to an important aspect of the present invention, an illumination device comprises a main string of LEDs arranged between power output lines of a current source. A series arrangement of a secondary string of LEDs and a controllable current limiter is arranged in parallel to said main string. A controller for controlling the current limiter receives power from a node of the main string.

Further advantageous elaborations are mentioned in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:

Figure 1 is a block diagram schematically illustrating a conventional way of powering LEDs, having a string of series-connected LEDs connected between the output terminals of a current source;

Figure 2 is a block diagram schematically illustrating an illumination device according to the present invention;

Figure 3 is a graph illustrating current as a function of time in the illumination device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 2 is a block diagram schematically illustrating an illumination device 100 according to the present invention. The device has power lines 5, 6, which typically are identical to or connected to the current output terminals of a current source 1, which current source may in principle have any suitable design and may be equal to the current dource of figure 1. It will be assumed that the power lines 5, 6 carry rectified AC current, the upper line 5 being positive with respect to the lower line 6, but the gist of the present invention will also function if the power lines 5, 6 carry DC current. The current received from the power source will be indicated as ITOTAL-

The device 100 comprises a main string 101 of LEDs arranged in series with each other and in series with a series resistor Rl, connected between the power lines 5, 6. The current in this string is indicated as II. The LEDs in this string are indicated as LEDl,i with i ranging from 1 to n, n being the number of LEDs in the main string 101.

The device 100 comprises a secondary string 102 of LEDs arranged in series with each other and in series with a series resistor R2, connected between the power lines 5, 6. The current in this secondary string is indicated as 12. The LEDs in this string are indicated as LED2,i with i ranging from 1 to n, n being the number of LEDs in the secondary string 102. It is noted that the number of LEDs in the secondary string 102 may differ from the number of LEDs in the main string 101.

The device 100 further comprises a controllable current limiter 120 arranged in series with the secondary string 102, more particularly arranged between the lowest power line 6 and the last LED LED2,n. The controllable current limiter 120 is here by way of example implemented as an NPN transistor T2, having its collector connected to the last LED LED2,n, having its emitter connected to the lowest power line 6 via a current sense resistor R2x, and having its base connected to the output of an operational amplifier (opamp) 121. An inverting input of the opamp 121 is connected to the transistor's emitter. A non- inverting of the opamp 121 is connected to the lowest power line 6 via a capacitor 122, and is connected to an output of a microcontroller 124 via a resistor 123.

Power for the microcontroller 124 is derived from the main string 101. In a typical example, each LED in this string has a forward voltage of approximately 3.5 - 4 V. Thus, at the node A between the second and third LEDs LEDl,(n-2) and LEDl,(n-l) counted from the end, a voltage of about 7-8 V is available. This voltage is used to supply the microcontroller 124, after rectifying in a stage comprising a diode 125, capacitor 126 and voltage stabilizer 127, typically a 78L05 or similar component to provide 5 V DC. If the operating voltage of the microcontroller is in the same range as the forward voltage of one of more LEDs, the voltage regulator may be omitted. Before explaining the operation, it is noted that the current drawn by the microprocessor 124, indicated as I₁, _μc, is so small as to be negligible, so that the effect on the current in and the light emitted by the last two LEDs LEDl,(n-l) and LEDl,n is negligible. It is further noted that it is possible to have a second secondary string 103 in parallel to the main string 101, similar to the secondary string 102, as illustated. More secondary strings can be added. Each further secondary string 103 will be provided with its own controllable current limiter 220. In figure 2, it is assumed that the second secondary string 103 is provided with its own microprocessor 224 and associated supply circuit 225, 226, 227 connected to node A. However, it is possible that this supply circuit 225, 226, 227 is omitted, and that the microprocessor 224 is supplied from the supply circuit 125, 126, 127 of the first secondary string 102. It is even possible that the second microprocessor 224 is omitted, if the first microprocessor 124 has a second output terminal for controlling the second controllable current limiter 220; in other words, the microprocessors 124 and 224 may be integrated.

For explaining the operation, it is first assumed that the transistor T2 is conducting fully. The current magnitudes Il and 12 will set themselves on the basis of the dynamic resistance of the respective LEDs and the number of LEDs in each string. This situation is illustrated in the lefthand graph of figure 3, showing current (vertical axis, mA) as a function of time (horizontal axis, ms). The current Il in the main string 101 has an amplitude of 700 mA and the current 12 in the secondary string 102 has an amplitude of 300 mA in this example. Thus, I_TOTAL has an amplitude of 1000 mA.

The microcontroller 124 has a user input 128 for inputting a signal indicating a ceiling for the current 12 in the secondary string 102. On the basis of this input signal, the microcontroller 124 generates an output signal setting a reference level at the non- inverting input of the opamp 121. In the embodiment shown, the microcontroller 124 produces current pulses of variable duration (PWM), resulting in a voltage level at the non- inverting input of the opamp 121 of which the level depends on the pulse width. As long as this voltage level is higher than the voltage level at the opamp's inverting input, the opamp produces a HIGH output signal keeping the transistor T2 conductive. The voltage level at the opamp's inverting input is always proportional to the current 12.

Assume that the user changes the setting of the user input 128, reducing the pulse width of the opamp's output pulses and thus reducing the voltage level at the non- inverting input of the opamp 121 to the equivalent of 250 mA, lower than the amplitude of the voltage signal provided by the measuring resistor R2x. This situation is illustrated in the righthand graph of figure 3. At first, current 12 rises from 0, as usual, until at about 8 ms 12 becomes equal to 250 niA, sufficient to make the opamp 121 switch off the transistor T2 partly. Particularly, the transistor is kept conductive just sufficiently to keep the current 12 equal to 250 mA (flat part of the curve in figure 3). Thus, as compared to the situation of the lefthand side of this figure, current 12 in the seccondary string 102 has reduced, as far as peak current is concerned and as far as average current is concerned. In the peak at 10 ms, the total current I_TOTAL is still 1000 mA, so the peak current of Il is now 750 mA in this example.

Thus, it can be seen that the ratio between 12 and Il can be changed.

It is noted that the control input signal for the microcontroller 124 at its input 128 can be generated in may ways. For instance, the device may be provided with a temperature sensor for sensing the temperature of the LED assembly, and providing a temperature signal to a microcontroller's input, in which case the microcontroller may be programmed to change the current ratio in order to counteract temperature effects. Further, the device may be provided with a light sensor for measuring the light output of both strings and for providing a light signal to a microcontroller's input, in which case the microcontroller may be programmed to keep the light output constant. Further, the device may be provided with a user input device such as a potentiometer connected to a microcontroller's input. Further, the microcontroller may be responsive to a remote control for wireless user control. The above options (and others) may be implemented in combination.

Summarizing, the present invention provides an illumination device 100, comprising: a current source having output lines 5; 6; a main string 101 of LEDs arranged in series between the current source output lines; at least one secondary string 102 of LEDs arranged in series between the current source output lines; and a controllable current limiter 120 arranged in series with the LEDs of the secondary string 102.

In a preferred embodiment, the controllable current limiter comprises: a transistor T2 having its collector-emitter path in series with the LEDs of the secondary string 102; a current sense resistor R2x arranged in series with said collector-emitter path; an opamp 121 having an output connected to the transistor's base, having its inverting input connected to said current sense resistor R2x, and having its non- inverting input connected to a reference voltage level. While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, the diode 125 may be omitted, or may be replaced by a series arrangement of two diodes. Further, the current limiter may be implemented in a different way.

In the above, it has been mentioned that it is possible to have two or more secondary strings. When deciding on the number of LED strings, there are several design considerations to take into account. On the one hand, when all secondary strings are switched ON, they all require a certain current magnitude, and the current source should be capable of providing this current. On the other hand, when all secondary strings are switched OFF, the full total current I_TOTAL will flow through the main string 101 and this should not lead to overpowering of that string.

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. 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 processor 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. Any reference signs in the claims should not be construed as limiting the scope.

In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.

Claims

CLAIMS:

1. Illumination device (100), comprising: a current source (1) having output lines (5; 6); a main string (101) of LEDs (LEDl,i) arranged in series between the current source output lines (5, 6); at least one secondary string (102) of LEDs (LED2,i) arranged in series between the current source output lines (5, 6); and a controllable current limiter (120) arranged in series with the LEDs (LED2,i) of the secondary string (102).

2. Illumination device according to claim 1, wherein the controllable current limiter (120) comprises: a transistor (T2) having its collector-emitter path in series with the LEDs (LED2,i) of the secondary string (102); a current sense resistor (R2x) arranged in series with said collector-emitter path, coupled between the transistor's emitter and one of said current source output lines (6); an opamp (121) having an output connected to the transistor's base, having its inverting input connected to the node between the transistor's emitter and said current sense resistor (R2x), and having its non-inverting input connected to a reference voltage level.

3. Illumination device according to claim 2, wherein the reference voltage level is provided by a controllable pulse generator (124) and a pulse integrator stage (123, 122).

4. Illumination device according to claim 3, wherein the pulse generator (124) is implemented as a microprocessor.

5. Illumination device according to claim 3, wherein the pulse generator (124) has a control input (128) for receiving control input signals.

6. Illumination device according to claim 5, further comprising a temperature sensor for sensing the temperature of the LED assembly, and wherein the pulse generator (124) is responsive to the sensor's temperature signal to change the ratio between the currents (II; 12) in the main and secondary strings in order to counteract temperature effects.

7. Illumination device according to claim 5, further comprising a light sensor for measuring the light output of both strings, and wherein the pulse generator (124) is responsive to the sensor's light signal to keep the light output constant.

8. Illumination device according to claim 5, further comprising a user input device such as a potentiometer connected to the control input (128).

9. Illumination device according to claim 3, further comprising a low power voltage source (125, 126, 127) for the controllable pulse generator (124), said low power voltage source (125, 126, 127) having a power input connected to a node (A) between two adjacent LEDs (LEDl,i; LEDl,i+l) of the main string (101).

10. Illumination device according to claim 9, wherein said node (A) is between the second and third LEDs (LEDl,(n-2); LEDl,(n-l)) counted from the end of the main string (101).

11. Illumination device according to claim 9, wherein said low power voltage source (125, 126, 127) comprises a voltage stabilizer (127), preferably having a diode (125) arranged between its input terminal and said node (A), and preferably having a capacitor (126) connected in parallel to its input terminal.