Patent Description:
Illumination systems for generating light are commonly known, and the same applies to the use of LEDs as light source in such illumination systems. Therefore, a detailed explanation thereof will be omitted here.

Generally speaking, one may define several operational requirements for an illumination system. An obvious requirement is that the system can be switched ON and OFF. A second requirement is dimmability: it is desirable that the intensity of the light output can be varied. A third requirement is color variability: it is desirable that the color of the light output can be varied.

With respect to color, it is commonly known that colors as perceived by the human eye can be described in a two-dimensional color space. In this space, pure or monochromatic colors, i.e. electromagnetic radiation having one frequency within the visible spectrum, are located on a curved line having two end points, corresponding to the boundaries of the visible spectrum. This curve, together with a straight line connecting said end points, forms the well-known color triangle. Points within this triangle correspond to so-called mixed colors. An important feature of colors is that, when the human eye receives light originating from two light sources with different color points, the human eye does not distinguish two different colors but perceives a mixed color, wherein the color point of this mixed color is located on a straight line connecting the two color points of the two light sources, while the exact position on this line depends on the ratio between the respective light intensities. The overall intensity of the mixed color corresponds to the respective light intensities added together. Thus, it is possible to generate light having a color point corresponding to any desired point of said line with, within limits, any desired intensity. Similarly, with three light sources, it is possible to render any color point within the triangle defined by the three respective color points.

In the field of illumination, there is a general desire to be able to generate light of which the color can be controlled. Depending on the type of application, the desired characteristics of the illumination system may be different. A specific type of illumination system is a daylight lamp capable of generating white light and/or capable of simulating the change in light color of daylight from sunrise to sunset. Another specific type of illumination system is a replacement for an incandescent lamp, having the same "warm" light output.

While the above basically applies to any type of light source, a light source particularly suitable in color systems is the LED, in view of its size and cost, and considering the fact that an LED produces monochromatic light. Thus, illumination systems have been developed comprising <NUM> or <NUM> (or even more) different LED types. By way of example, the RGBW system is mentioned, comprising RED, GREEN, BLUE and WHITE LEDs.

In order to be able to achieve dimmability in an LED system, it is known to apply pulse width modulation: instead of a constant current, the LED receives current pulses of a certain duration at a certain repetition frequency, selected to be sufficiently high such as not to lead to perceivable flicker.

For driving an LED, an LED driver is used, capable of generating the required LED current at the corresponding drive voltage.

In order to be able to set and/or vary a desired color point of the light output, it is necessary to be able to individually vary the intensities of the different colors. While a simple system may comprise one LED per color, practical systems usually have a plurality of LEDs per color. It is possible to drive an array of LEDs by one common driver, and the LEDs may be connected in parallel or in series, or both. Nevertheless, the prior art requires that there be at least one driver per color. This makes such a system relatively costly. Further, between driver system and LED system at least <NUM> wires are needed, even <NUM> wires if it is undesirable to have a common ground.

Document <CIT>describes in figure 1A a variable intensity LED illumination system <NUM> according to the present invention. LED illumination system <NUM> is designed for small, low voltage DC, variable intensity lighting applications, such as the illumination of a vehicle instrument panel or dashboard, or for providing internal lighting for avionics, flight instruments, indicators, and the like. LED illumination system <NUM> includes a positive input node <NUM> and a ground node <NUM> for connection to a variable voltage direct current source <NUM>.

<CIT> discloses a lighting device where sets of LEDs are employed using the natural characteristics of the LEDs to resemble incandescent lamp behavior when dimmed, thereby obviating the need for sophisticated controls. A first set of at least one LED produces light with a first color temperature, and a second set of at least one LED produces light with a second color temperature. The first set and the second set are connected in series, or the first set and the second set are connected in parallel, possibly with a resistive element in series with the first or the second set. The first set and the second set differ in temperature behavior or have different dynamic electrical resistance. The light device produces light with a color point parallel and close to a blackbody curve.

As an example, an illumination system comprising <NUM> different LED groups driven by one common driver is provided, in which dimmability and color variability are possible. <NUM> or <NUM>.

In state of the art technology, an LED driver is typically implemented as a current source. As commonly known by persons skilled in the art, an LED, like any other type of diode, has as a characteristic an almost constant voltage when in its forward conductive state, indicated as forward voltage. Thus, while the driver output current is determined by the driver, the driver output voltage is determined by the LED. According to the present invention, an illumination system comprises a controllable current distribution means having one input receiving the driver current and having a plurality of outputs coupled to the respective LED groups for providing the respective LED currents. Further, the driver actively sets its output voltage, which is used as a control signal for the current distribution means. Depending on this control signal, the current distribution means sets a specific ratio of the respective LED currents.

Further advantageous elaborations are mentioned in the dependent claims.

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:.

<FIG> shows a block diagram schematically illustrating a prior art design of an illumination system <NUM> comprising driver means <NUM> and an LED system <NUM>, wherein in this example the LED system <NUM> comprises four LEDs <NUM>, <NUM>, <NUM>, <NUM>. In the prior art design, the driver means <NUM> actually comprises individual drivers <NUM>, <NUM>, <NUM>, <NUM> dedicated to driving a corresponding one of the LEDs <NUM>, <NUM>, <NUM>, <NUM>. In order to be able to set or vary the output color of the LED system <NUM> as a whole, for instance by a user action, the illumination system <NUM> comprises a control device <NUM> receiving a user input signal Sui and calculating individual driver control signals for the individual drivers <NUM>, <NUM>, <NUM>, <NUM>. The figure clearly shows that eight wires are needed to connect the driver means <NUM> to the LED system <NUM>.

<FIG> is a graph schematically illustrating the electrical behavior of a diode, particularly an LED. The horizontal axis represents voltage (arbitrary units), the vertical axis represents current (arbitrary units). A diode has two terminals, one being indicated as anode and the other being indicated as cathode. Assuming that a DC voltage is applied across the diode terminals, with the anode being positive and the cathode being negative; this will be indicated as positive bias (righthand side of the graph). As long as the voltage magnitude is below a certain threshold value Vth, the current may be considered to be zero and the diode is said to be non-conductive (it is noted that in reality a very small current may flow, but this is neglected here). If the voltage magnitude is above said threshold value Vth, the current rises very steeply as a function of voltage and the diode is said to be forwardly conductive.

When the polarity of the DC voltage is reversed, this will be indicated as negative bias or reverse bias (left-hand side of the graph). In practical conditions relevant to the present invention, the current is zero. In extreme conditions, when the voltage magnitude becomes very high, the diode does show conduction, as illustrated in the graph, but this will typically involve damaging the diode and is not considered to be a normal operative condition.

Thus, for explaining the present invention, three situations will be distinguished:.

It is noted that the threshold voltage Vth may be considered to be constant for a single diode specimen, although the value may be different for different types of diode. For instance, for a standard germanium diode, Vth is about <NUM> V, for a standard silicium diode, Vth is about <NUM> V, and for power LEDs, Vth may be in the range of <NUM> V to <NUM> V.

In principle, it is possible that a driver <NUM>, <NUM>, <NUM>, <NUM> has the characteristics of a voltage source: the load determines the current, and by precisely controlling the voltage, it is possible to set the current. However, slight variations in the voltage result in large variations in the LED current, while the LED output intensity may be considered to be substantially proportional to the LED current, so that visible intensity variations may result. Therefore, it is typically preferred that a driver has the characteristics of a current source. If this is the case, the load determines the output voltage of the driver. Thus, in both cases, the driver output power is determined by the load.

<FIG> is a block diagram schematically illustrating the design of an illumination system <NUM> according to an example. Again, this system has driver means <NUM> and an LED system <NUM> comprising four LEDs <NUM>, <NUM>, <NUM>, <NUM>. Unlike the prior art, the driver means <NUM> comprises just one driver <NUM> having output terminals <NUM>, <NUM>, and the LED system <NUM> having input terminals <NUM>, <NUM> comprises controllable current distribution means <NUM>. The figure shows that the driver <NUM> is powered from the mains M, but it is noted that this, although typical, is not essential. A control device <NUM> may receive a user input signal Sui, and may control the driver <NUM>. It is noted that this control device and driver may be integrated.

When implementing the present invention, it is again possible that the driver <NUM> has the characteristic of a current source. However, it is now preferred that the driver <NUM> has the characteristic of a voltage source. For defining the protective scope and hence the wording of the claims, the precise characteristic of the driver should not be interpreted as being a limiting factor. While an ideal voltage source has a vertical characteristic and an ideal current source has a horizontal characteristic, a realistic power source typically has a sloping characteristic intersecting both the current axis and the voltage axis. Nevertheless, in all cases, an LED driven by the driver may have the same working point (a point in the graph of <FIG> defined by the combination of actual voltage and actual current). Since this working point establishes itself on the basis of the LED's characteristic, while the precise location on that characteristic is determined and varied by the driver output, the general phrase used in the claims will be that the driver provides working power. Nevertheless, in the following explanation it will be assumed that the driver <NUM> does have the characteristic of a voltage source, since such a characteristic is preferred as it allows the working voltage to be set easier.

As mentioned in the following explanation, it will be assumed that the driver <NUM> has the characteristic of a voltage source, and that the control device <NUM> is capable of setting the driver output voltage. It is noted that LED drivers having a controllable output voltage are known per se, so that a detailed explanation thereof is not needed here. According to the principles proposed by the present invention, the output voltage of the driver <NUM>, i.e. the input voltage received by the current distribution means <NUM>, is considered to be a control parameter for the distribution of the current among the LEDs <NUM>, <NUM>, <NUM>, <NUM>.

In an example, the current distribution means <NUM> comprises an active processor and a memory containing information defining relationships between the control parameter "input voltage" Vi and the individual currents of the individual LEDs. With the number of individual LEDs equal to N, and an index i ranging from <NUM> to N, these relationships can be expressed as: Ii = fi(Vi) with the functions fi typically being mutually different such that together they define, for the color point of the overall light output, a certain predefined path in the color space. Preferably, for at least one LED or group of LEDs, the current (function fi) is only non-zero within a certain range of input voltages, while this range overlaps with a range of input voltages where all other LEDs have zero current, so that in this overlap range the light output has the pure color of said one LED or group of LEDs. It is to be noted that the driver <NUM> supplies the summation of all LED currents.

In an embodiment which is preferred in view of its simplicity and low costs, the current distribution means <NUM> does not comprise active processor means but consists of the hardware configuration of the LED system <NUM>. In the following, some exemplary embodiments will be discussed.

<FIG> is a block diagram schematically illustrating an example of an LED system, indicated in general by the reference numeral <NUM>. The input terminals are indicated by reference numerals <NUM>, <NUM>. The LED system <NUM> comprises two groups of LEDs <NUM>, <NUM>. These groups are connected in parallel to the input terminals <NUM>, <NUM>. An impedance <NUM> is connected in series with the first group <NUM> of LEDs. An impedance <NUM> is connected in series with the second group <NUM> of LEDs. In the following explanation, it will be assumed that this impedance is resistive, for instance a resistor.

In <FIG>, the first group <NUM> is shown by the symbol of a single LED, but this does not mean that there is only one LED in the first group. The group may actually comprise a plurality of LEDs arranged in series and/or in parallel with each other. These LEDs may be mutually identical, but the group may also comprise LEDs of mutually different colors. Apart from the LEDs, other electrical components may be connected in series and/or in parallel to the LEDs, for instance common diodes. While each individual LED or diode has its individual threshold voltage, as explained with reference to <FIG>, the group <NUM> as a whole has a group threshold voltage VT1 which typically corresponds to the summation of the threshold voltages of LEDs arranged in series. Thus, if the group <NUM> consists of a series arrangement of three identical LEDs each having an individual threshold voltage Vth, the group threshold voltage VT1 of the group is equal to 3Vth.

The same applies to the second group <NUM>. When comparing the second group <NUM> with the first group <NUM>, there is one important difference: the second group <NUM> has a group threshold voltage VT2, hereinafter simply indicated as second threshold voltage, larger than the group threshold voltage VT1 of the first group <NUM>, hereinafter simply indicated as first threshold voltage.

Further, the impedance value of the second impedance <NUM> in series with the second LED group <NUM> may differ from the impedance value of the first impedance <NUM> in series with the first LED group <NUM>. The impedance value of the second impedance <NUM> may be smaller than the impedance value of the first impedance <NUM>, and the second impedance <NUM> may even be omitted, in which case the function of second impendance will be performed by the series wiring of the second LED group <NUM>.

The operation of the LED system <NUM> will now be explained with reference to <FIG>, which is a graph showing the light output L1 of the first group of LEDs <NUM> and the light output L2 of the second group of LEDs <NUM> as a function of the input voltage Vi received at the input terminals <NUM>, <NUM> of the LED system <NUM>.

As long as Vi is smaller than VT1, all LEDs are off.

When Vi is higher than VT1 but still smaller than VT2, the second group of LEDs are still off. Current will flow through the first group of LEDs <NUM>, with a voltage drop developing across the first group of LEDs <NUM>; this voltage drop will be almost equal to VT1. While in practice this voltage drop will increase slightly with increasing current (see <FIG>), in the following explanation it will be assumed for the sake of convenience that the voltage drop is equal to VT1. The difference VR1 = Vi - VT1 will be the voltage across the resistor <NUM>, so that the current magnitude will be equal to (Vi - VT1)/R1, with R1 indicating the resistance of the resistor <NUM>. This current is proportional (in reality: almost linearly proportional) to the input voltage Vi, and hence the first light output L1 is proportional to the input voltage Vi. The light output of the LED system <NUM> as a whole has the first color point.

It is noted that the above applies when R1 is sufficiently large. When R1 is too low, the current will be determined by the LED characteristics of the first group <NUM>: the current cannot become higher than the current of the diode characteristic.

Similarly, when Vi is higher than VT2, current will also flow through the second group of LEDs <NUM>, with a voltage drop taken to be equal to VT2 developing across the second group of LEDs <NUM>. The difference VR2 = Vi - VT2 will be the voltage across the second resistor <NUM>, so that the current magnitude will be equal to (Vi - VT2)/R2, with R2 indicating the resistance of the second resistor <NUM>. This current is proportional to the input voltage Vi, and hence the second light output L2 is proportional to the input voltage Vi. It should be clear that the first light output L1 is still proportional to the input voltage Vi.

The ratio between R1 and R2 determines the ratio between the proportionality of L1 and L2 versus Vi, respectively. Typically, it will be advantageous if R2 is smaller than R1, so that the current in the second group <NUM> rises faster as a function of Vi as compared to the current in the first group <NUM>, and it will be advantageous if the number of LEDs in the second group <NUM> is larger than the number of LEDs in the first group <NUM>, such that all in all the second light output L2 rises faster than the first light output L1, as illustrated.

In the above explanation, for understanding the electrical behavior of the circuit, the color points of the LEDs do not play any role. All individual LEDs may even be mutually identical. In a particularly preferred embodiment, the group color point of the light output of all LEDs of the second group combined, hereinafter simply indicated as second color point, differs from the group color point of the light output of all LEDs of the first group combined, hereinafter simply indicated as first color point. When all LED groups are placed relatively closely together, a human observer will perceive the overall light output as a blend having one blend color point. When increasing the input voltage Vi, this blend color point travels in a straight line from the first color point towards the second color point. In the embodiment where the first color point is red and the second color point is white, increasing the input voltage causes a change from red light to warm white light, which corresponds to the dimming of an incandescent lamp.

<FIG> illustrates an embodiment <NUM>, in which the second group of LEDs <NUM> is connected to a node of a voltage divider <NUM> formed by two resistors <NUM>, <NUM> connected in series between the input terminals <NUM>, <NUM>. Thus, this node provides a voltage derived from the input voltage Vi. Even if the second group threshold voltage VT2 is lower than the first group threshold voltage, the second group <NUM> can only start to conduct if the input voltage Vi is equal to or higher than (R432 + R431)/R432 times VT2.

<FIG> illustrates an embodiment <NUM>. <FIG> is a graph comparable to <FIG>, illustrating the behavior of this third embodiment <NUM>. As compared to the example <NUM>, the second resistor <NUM> is replaced by a resistor <NUM> in series with the parallel arrangement of first group <NUM> and second group <NUM>. For Vi smaller than VT2, the operation is the same as the operation of the example <NUM>, with this difference that the current magnitude will be equal to (Vi - VT1)/(R1+R3), with R3 indicating the resistance of the common series resistor <NUM>.

When Vi is higher than VT2, current will also flow through the second group of LEDs <NUM>, with a voltage drop VT2 developing across the second group of LEDs <NUM>. The difference VR3 = Vi - VT2 will be the voltage across the second resistor <NUM>, and the voltage across the first group of LEDs <NUM> plus series resistor <NUM> will be clamped to VT2, as a result of which the first current L1 will remain constant.

In the embodiments as described above, where the LEDs are mounted closely together and the groups have mutually differing color points, varying the driver output voltage will result in the LED system <NUM>; <NUM> as a whole generating a blend light output of which the color point travels in a straight line from the first color point towards the second color point. In an illustrative embodiment, the first color point is substantially red and the second color point is substantially white. In the simplest embodiment, the first group <NUM> consists of precisely one red LED and the second group <NUM> consists of precisely two white LEDs arranged in series.

However, the blend color point will not quite reach the second color point, because the first group <NUM> is on at all times when the second group <NUM> is on.

Claim 1:
An illumination system (<NUM>) comprising:
- an LED driver (<NUM>) for generating a controllable output voltage (Vi), the LED driver (<NUM>) having two output terminals (<NUM>, <NUM>) coupled to two input terminals (<NUM>, <NUM>) of an LED system (<NUM>), wherein the LED driver (<NUM>) is a voltage source or a current source; and
- the LED system (<NUM>) comprising:
- a first group of LEDs (<NUM>) having a first group threshold voltage (VT1);
- a second group of LEDs (<NUM>) having a second group threshold voltage (VT2);
- passive current distribution means (<NUM>) in the form of a passive impedance (<NUM>), coupled only in series with the first LED group (<NUM>); and
- two input terminals (<NUM>, <NUM>), wherein the first group of LEDs (<NUM>) in series with the passive current distribution means (<NUM>) is coupled between the two input terminals (<NUM>, <NUM>);
wherein the first group threshold voltage (VT1) is smaller than the second group threshold voltage (VT2), and
wherein the passive impedance (<NUM>) is sufficiently large such that a current of the first group of LEDs (<NUM>) is determined by the passive impedance (<NUM>),
wherein the second group of LEDs (<NUM>) is coupled to a node of a voltage divider (<NUM>) formed by two resistors (<NUM>, <NUM>) connected in series between the input terminals (<NUM>, <NUM>).