NL1036907C2 - Device for driving a light source. - Google Patents

Description

TITLE: Device for driving a light source

FIELD OF THE INVENTION

The present invention relates in general to apparatus for generating light.

5 BACKGROUND OF THE INVENTION

The incandescent lamp is a commonly known lamp, where light is generated on the basis of heating a filament. Such lamp has a rather low efficiency, and there is a desire to replace such lamps by lamps of higher efficiency. LEDs are 10 very promising candidates for such replacement. However, LEDs can not be supplied from mains directly: a LED driver is reguired, which receives mains AC voltage at its input and which is capable of generating LED current at the voltage drop as set by the LED. In order to be actually suitable to replace 15 the common incandescent lamp, such driver should be very small.

An interesting feature of an incandescent lamp is that it can easily be dimmed by placing a resistor in series with the lamp: this will reduce the lamp current. This is a 20 disadvantageous solution, at least with a view to efficiency, because excess power is dissipated in the resistor. Further, if a LED current is reduced in this way, the colour of the light emitted by the LED will also change. This is especially s disadvantageous in situations where the colour of the 25 generated light is of key importance.

On the other hand, it may be desirable to have a light source capable of generating light with variable colour. It is generally known that such light source can be designed on the basis of multiple LEDs of mutually different colours, for 30 instance three LEDs of colours Red, Green, Blue (RGB) and perhaps a fourth White LED. In such light sources, it is important that the current magnitude can be varied (for 1 0 3 6 9 0 7 2 varying the light intensity) without changing the colour of the light output.

Figure 1 illustrates a generally known method for driving an LED that meets the above requirements. A light source 1 5 comprises an LED 2 connected in series with a controllable switch 7 and a resistor 8, arranged between output terminals 4 and 5 of a voltage source 3. The light source 1 further comprises a controller 6 for controlling the status of the switch 7, so that it is either ON (conductive) or OFF (non-10 conductive). When the switch is OFF, no current flows through the LED, and no light is generated. When the switch is ON, current flows through the LED, and the LED generates light.

The resulting current magnitude and voltage drop are determined by the properties of the LED; the difference 15 between supply voltage and LED voltage drop is developed over the resistor 8, and results in power being dissipated. The switch is repeatedly switched ON and OFF at a certain switching frequency, wherein a duty cycle (defined as the ratio of ON time to switching period) may be varied. Since at 20 all times the current has the same nominal magnitude, the light colour is always the same. Varying the duty cycle will result in varied average current magnitude and varied average light intensity. Hence, the light intensity can be varied while the colour remains the same.

25 A disadvantage of this known approach is that the resistor has to be designed for dissipating quite a lot of power and therefore has to be relatively bulky. Further, the fact that power is lost is in itself already a disadvantage.

Figure 2 is a block diagram of a supply assembly 11 for 30 an LED 2 as disclosed in US-7.071.762, wherein basically the resistor of figure 1 is replaced by an inductor 18. The switch 17 in series with the inductor 18 and LED 2 is controlled by a controller 16. A freewheel diode 19 is connected anti-parallel to the series arrangement of inductor 18 and LED 2. The 35 operation of this known device is as follows.

Figure 3 is a graph illustrating the operation; the horizontal axis represents time in arbitrary units, the vertical axis represents current in arbitrary units. When switch 17 is closed (era (I) in figure 3), a current with 3 increasing magnitude flows from the voltage supply though the inductor 18 and the LED 2; the energy contents of the inductor increases. When switch 17 is opened (era (II) in figure 3), a current with decreasing magnitude flows from the inductor 18 5 through the LED 2 and the freewheel diode 19; the energy contents of the inductor decreases. The switch 17 is opened/closed with a relatively high switching frequency, resulting in a more or less triangular current through the LED, equivalent to a DC current with a triangular ripple 10 superimposed thereon, as illustrated in figure 3. This mode of operation is interrupted by a period during which the switch 17 is maintained opened, so that no current can flow. The device regularly changes from a high frequency switching mode to a non-conductive mode, with a repetition frequency much 15 lower that the switching frequency. On a time scale larger than the repetition period, the average current can be changed (dimming of the light output) by changing the relative time duration of the high frequency switching mode (duty cycle control).

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SUMMARY OF THE INVENTION

The present invention proposes a different control of such device, so that the device can be implemented with very 25 small components. Further, the present invention aims to provide a device that can operate with one single frequency, in contrast to the prior art device which uses a dual frequency. According to an important aspect of the invention, the inductor in series with the LED is very small, so that it 30 can contain a little amount of energy only. Charging of this inductor is done during a very short time, in order to avoid saturation, and likewise discharging is done during a very short time. To this end, the switch is closed only very briefly. Average current is controlled by varying the 35 repetition frequency of the current pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following 4 description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: figure 1 schematically shows a block diagram of a driver for 5 an LED; figure 2 schematically shows a block diagram of a driver for an LED;.

figure 3 is a graph schematically showing the current signal as generated according to the prior art; 10 figure 4 schematically shows a block diagram of a driver according to the present invention for driving an LED; figure 5 is a graph illustrating the control of a driver according to the present invention; figure 6 schematically shows a block diagram illustrating an 15 illumination system implemented according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to figure 4, a driver 41 according to the 20 present invention can have the same basic design as the design shown in figure 2, therefore the same reference numerals are used where possible and the corresponding explanation will not be repeated here. Output terminals are indicated at 41c, 41d, while input terminals for receiving an input voltage are 25 indicated at 41a, 41b. In figure 4, the controller 16 of figure 2 has been replaced by a controller 46, which comprises a clock signal generator 42 and a pulse generator 43. The clock signal generator 42 generates a clock signal Scl, typically a block signal, as illustrated in figure 5. The 30 pulse generator 43 has an input coupled to the output of the clock signal generator 42 for receiving the clock signal Scl, that acts as a trigger signal for the pulse generator 43. The clock signal Scl has a repetition period Tel. On a fixed phase of the clock signal Scl, the pulse generator 43 provides an 35 output pulse signal Sp having a predetermined, fixed duration tp much smaller than the clock repetition period Tel. In an exemplary embodiment, the output pulse duration is in the order of about a few microseconds. In the example illustrated 5 in figure 5, the pulse generator 43 is triggered at the rising edge of the clock signal Scl, but that is not essential.

The output pulse signal Sp from the pulse generator 43 is applied to the switch 17 as control signal. Thus, during the 5 duration of the output pulse Sp, the supply voltage is applied to the inductor 18 and the inductor current I rises from zero. In the figure 5, it is for sake of convenience assumed that the time derivative of the inductor current I is constant. After termination of the output pulse, the switch 17 returns 10 back to its non-conductive state, and the inductor current I drops back to zero, in which case the time derivative of the inductor current I is again assumed to be constant. The current stops when reaching zero: the current can not change its direction.

15 It is noted that the time derivative of the inductor current I during charging of the inductor is normally not equal to the time derivative of the inductor current I during discharging of the inductor. This is mainly caused by the fact that the voltage over the inductor during charging is equal to 20 the supply voltage minus the voltage drop over the LED, whereas the voltage over the inductor during discharging is equal to the voltage drop over the freewheel diode 19 plus the voltage drop over the LED.

In a variation, the pulse generator 43 is designed to 25 generate two (or even more) output pulses in response to one trigger event, as shown at B by interrupted lines.

It is noted that for each pulse signal generated by the pulse generator 43, a pulse-shaped current is received by the LED 2, i.e. the current is not constant but rises quickly from 30 zero to a maximum value and the drops quickly to zero again. Consequently, seen at the time scale of such current pulse, the light intensity and the colour of the light output are not constant. However, in view of the very short duration of the current pulses, these variations are not visible to the human 35 eye. On a time scale larger than the duration of one current pulse, the human eye only perceives an average light intensity and an average colour. Further, whatever the precise characteristic of the current as a function of time, this characteristic will be the same for every current pulse, at 6 least in first approximation, therefore the perceived average light intensity and perceived average colour will be the same for every current pulse.

The controller 46 has an input 47 for receiving a user 5 input signal defining a dimming level between a certain minimum value and 100%. In response to the user input signal, the clock signal is adapted such as to increase or decrease the clock repetition period Tel. This results in a decrease or increase, respectively, of the current pulse frequency and 10 hence of the average light intensity, while the average colour impression remains constant. The light intensity is at maximum if the clock repetition period Tel is equal to the LED current pulse duration. The light intensity can in theory be reduced to zero, by making the clock repetition period Tel infinitely 15 large, but this is not practical. In general, the clock repetition period Tel should not be made so large that a user perceives flickering light. This will practically mean that the clock repetition frequency should not be lower than 100 Hz: if the current pulses have a duration of 1 ps, this 20 would correspond to a dimming level of 0.01%.

In a practical embodiment, the driver can be designed for maximum output and maximum efficiency, starting from the properties . of the LED(s) to be driven. Such LED has a maximum 25 effective current, indicated as IMec· The inductor 18 should be designed to be capable of handling a current at least twice as high before reaching saturation; the inductor 18 will have an inductance L. A maximum current level Imax can be selected, approximately equal to 2xIMec- Using the formula I max = {V/L) · tP, 30, the duration tP of the pulses produced by the pulse generator 43 can be calculated, in which V indicates the voltage over the inductor 18, thus being equal to the supply voltage minus the voltage drop over the LED. The pulse repetition frequency would then be equal to l/2tP.

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It is noted that the functions of triggerable pulse generator 43 and controllable clock generator 42 may be implemented by separate devices, and the clock generator may even be controlled by a separate controller. However, it is 7 preferred that all these functions are integrated in one single device, either in hardware on one single chip or in software as a suitably programmed processor.

In the embodiment as described so far, the control of the 5 LED output is open loop: on the basis of the user input, the pulse repetition period or frequency is set, typically on the basis of a look-up table or formula stored in a memory (not shown) associated with the controller 46. It may however be, for instance caused by manufacturing tolerances and/or ageing, 10 that the light output of the LED does not conform to the model as stored in the memory. Therefore, a feedback of the actual light output is preferred, for instance provided by a light sensor sensing the light output from the LED, or for instance provided by a current sensor sensing the LED current. The 15 sensor signal is received by the controller 46, which adapts its control of the clock generator accordingly.

Figure 6 illustrates an illumination system 100 implemented according to the present invention. The system 100 20 comprises a plurality of LEDs; in figure 6, three LEDs 102, 103, 104 are shown, which may be Red, Green and Blue. The system, however, is not restricted to three LEDs: the number of LEDs may be equal to two or to four or more. For instance, the system may also comprise a White LED.

25 The system comprises a common converter 110, having an input 111 for connection to mains (i.e. 230 V AC 50 Hz in Europe) and having an output 112 for providing a DC voltage of for instance 6 V. For driving each LED 102, 103, 104, corresponding dedicated drivers 120, 130, 140 are provided, 30 each having input terminals 121, 131, 141 connected to the output 112 of the converter 110, and each having output terminals 122, 132, 142 connected to the corresponding LED 102, 103, 104. Each driver is implemented as the driver 41 of figure 4, in that it comprises an arrangement of controllable 35 switch 17, inductor 18 and freewheel diode 19 (or second switch switched in counter phase); these details are not shown in figure 6. Each controllable switch of each driver is controlled by a pulse generator 43 which in turn is triggered by a clock signal of a clock generator 42. Each individual 8 clock generator of each individual driver 120, 130, 140 is controlled by a central controller 150, having output terminals 152, 153, 154 coupled to the respective drivers 120, 130, 140. At each output terminal 152, 153, 154, the central 5 controller 150 generates respective control signals for controlling the clock rate of the respective clock generators to thus control the timing of the respective switches and to thus, ultimately, control the individual light outputs of the respective LEDs.

10 It is noted that it is also possible that the respective clock generators are integrated with the central controller 150, so that the signals communicated to the drivers 120, 130, 140 are the respective pulse trigger signals.

It is noted that it is also possible that the respective 15 clock generators as well as the respective pulse generators are integrated with the central controller 150, so that the signals communicated to the drivers 120, 130, 140 are the respective pulse signals for the respective switches.

The central controller 150 has a user input 161, for 20 allowing a user to input a signal defining a desired output colour and a desired output light intensity. Based on this input signal, the central controller 150 sets the control signals for the individual drivers so that the overall output of all LEDs together matches the user's command.

25 Preferably, the central controller 150 is capable of taking account of the amount of time that the system has been operating, by adapting its control signals for the individual drivers to compensate for any ageing effects. The central controller 150 may itself contain a time keeping 30 functionality. It is also possible that the central controller 150 has a time input 165 for receiving a time keeping signal from an external clock device 175.

Preferably, the central controller 150 is capable of taking account of the ambient temperature by adapting its 35 control signals for the individual drivers to compensate for any temperature effects. The central controller 150 may itself contain a temperature measuring functionality. It is also possible that the central controller 150 has a temperature 9 input 166 for receiving a temperature measurement signal from an external temperature sensor 176.

Preferably, the central controller 150 is capable of compensating for individual LED deviations by adapting its 5 control signals for the individual drivers on the basis of the actually produced light output. To this end, the central controller 150 is provided with feedback inputs 162, 163, 164 for receiving measurement signals from respective light sensors 172, 173, 174 arranged for sensing the light from the 10 respective LEDs 102, 103, 104.

It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and 15 modifications are possible within the protective scope of the invention as defined in the appending claims.

For instance, instead of a freewheel diode 19, it is possible to use a second switch (MOSFET) switched in opposite phase with the first switch 17. Although this may slightly 20 increase the component costs, an advantage is achieved in that the voltage drop over such switch is lower than the voltage drop over a diode, thus the efficiency is increased.

In the above, the present invention has been explained with reference to block diagrams, which illustrate functional 25 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 30 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.

1036907

Claims (11)

A driver (41) for driving a light source (2), with input terminals (41a, 41b) for coupling to a voltage source and with output terminals (41c, 41d) for coupling to the light source; which driver comprises: an inductor (18) connected in series with one of the output terminals (41c); a controllable switch (17) connected in series with one of the input terminals (41a); a controller (46) for controlling the controllable switch; wherein the controller is designed to repeatedly generate a control signal for the controllable switch, wherein the control signal comprises a train of at least one short control pulse (Sp) having a short duration (tp), each control pulse being such that the switch is ON is (conductive) during said pulse duration (tp) while the switch is OFF (non-conductive) outside said pulse duration; wherein the control signal has a signal repetition period; 20 and wherein the controller is designed to vary the signal repetition period to vary the light output of the light source. 2. Driver according to claim 1, wherein the light source comprises at least one LED. The driver according to claim 1 or 2, wherein the pulse duration (tp) is less than the time it takes for the inductor (18) to increase from zero to saturation. 30 Driver according to any one of the preceding claims, wherein the distance between two consecutive signal pulses is greater than the time required for the inductor current to fall back to zero. 35 1036907 Driver according to any of the preceding claims, wherein the controller comprises a triggerable pulse generator (43) with an output terminal coupled to a control input terminal of the switch (17), the pulse generator 5 being responsive to a trigger signal to generate at least one output pulse . 6. Driver according to claim 5, wherein the pulse generator is responsive to the trigger signal to generate exactly one output pulse. The driver of claim 5, wherein the pulse generator is responsive to the trigger signal to generate a train with a predetermined number of pulses. 15 8. Driver as claimed in any of the foregoing claims 5-7, wherein the controller furthermore comprises a clock signal generator (42) for generating a clock signal (Sc1), the clock signal being applied to the pulse generator as a trigger signal. 9. Driver according to claim 8, wherein the clock signal is a square wave signal, and wherein the pulse generator is triggered by a rising or falling edge of the clock signal. Driver according to claim 8 or 9, wherein the controller is capable of varying the clock frequency of the clock signal. 30 An illumination system (100) comprising: a plurality of light sources (102; 103; 104); a plurality of drivers (120; 130; 140) for driving the respective light sources (102; 103; 104); Wherein each driver is implemented according to any of the preceding claims 1-10; and wherein the system comprises a central controller (150) for controlling the respective drivers. 1036907