Title: A method for producing a LED assembly and LED assembly produced by the method.

The invention relates to a method for producing a LED assembly having a desired illumination parameter and to a LED assembly produced by the method.

In recent years, light emitting diodes (LEDs) are applied more and more for illumination purposes. In the past years, many developments have taken place to produce efficient, high intensity LEDs which provide illumination at a certain color, e.g. white light. In order to produce white light LEDs, use has been made of LEDs which provide a blue radiation. A yellowish phosphor is applied which, excited by the blue light, provides for a yellow light emission. The human eye perceives a mixture of the blue and yellow light as white light.

A problem is that manufacturing tolerances will translate into a variation of the mutual intensities of the blue and yellow light provided by the white light LED. Production tolerances in the phosphor may for example affect a conversion of blue radiation into the yellow radiation, thereby affecting a balance between the blue and yellow radiation, which translates into a difference in color perceived by the human eye.

At present, this problem may be solved by a selection of the LEDs. The LEDs may be tested and classified in so called 'bins'. LEDs from a subset of the bins may be used for the specified purpose. Other LEDs may be discarded, or used for other applications. Although illumination having a desired color may be obtained by this selection method, many unusable LEDs remain, as in practice it appears that most applications seek to apply LEDs from roughly the same bins, which may thereby result in a low production yield. The invention intends to improve the above yield.

In order to achieve this or other goals, the method according to an aspect of the invention comprises:

- providing a plurality of LEDs, an illumination parameter of the LEDs exceeding a tolerance band of the desired illumination parameter; - selecting for the assembly LEDs from the plurality of LEDs, the illumination parameter of one or more of the individual LEDs as such exceeding the tolerance band of the desired illumination parameter; and

- providing a driving circuit for operating the selected LEDs, wherein a combination of illuminations from the selected LEDs to provide the desired illumination parameter.

Individual LEDs selected form the plurality of LEDs may thereby not meet the desired illumination parameter, however by grouping selected LEDs, an assembly thereof may combined be able to meet the desired illumination parameter. As an example, a white illumination color being desired, selecting a blueish and a yellowish LED, a combined operation of the blueish and yellowish LED may in combination, effectively provide for a perceived whitish color to the human eye. Effectively, thereby LEDs which according to the state of the art would have been discarded for e.g. providing a too blue or a too yellow color of illumination, may be used now in suitable combinations, which may increase a percentage of LEDs that are applicable for a given situation. The illumination parameter may comprise any suitable parameter, such as a color, color temperature, an area in a so called CCx, CCy color temperature diagram, etc.

The method may further comprise grouping, prior to the selection, the LEDs in groups, each group having an illumination parameter range, the selecting comprises selecting LEDs from at least two of the groups. Thereby, relevant combinations of LEDs which in combination result in the desired illumination parameter, may be obtained due to a purposeful selection. As an example a slightly yellowish LED may be combined with a slightly blueish LED to obtain a white color. According to another example, two slightly yellowish LEDs may be combined with one more or less blueish LED. In order to perform a relevant selection and combination, each group may be defined by an area in a two dimensional color space.

Alternatively, the selecting comprises randomly selecting LEDs from the plurality of LEDs. For each combination of LEDs thus obtained, it may then be determined if the desired illumination parameter can be met. Statistically, if two or more LEDs are applied per group, the chances that such group will not be able to meet the desired illumination parameter, are lower than when performing the selection on each LED individually, thereby increasing a yield. Thereto, it may be tested if the randomly selected LEDs of the assembly are able to provide the desired illumination parameter, and the assembly may be sorted based on an outcome of the testing. Thereby, selection is performed only after the assembly, and possibly after a controlling of the LEDs so as to drive the LEDs in order to obtain the desired illumination parameter. Statistically, a high percentage of the LED assemblies will be able to meet the desired illumination parameter if suitable driven (as will be outlined in some more detail below), thereby on the one hand possibly reducing a dropout, while on the other hand possibly reducing a burden of selection. In an embodiment, in order to provide an essentially white light, the plurality of LEDs are selected randomly from white LEDs having a characteristic outside the desired illumination characteristic. As an example, the desired illumination characteristic can be to produce an essentially white light having a specific color temperature, e.g. 3000 K. In order to provide such an LED assembly, in accordance with the invention, a random selection of LEDs is made that have a color

temperature that is different from 3000 K. By doing so, white LEDs unsuited for producing the desired characteristic may still be applied resulting in a higher yield of the production of assemblies employing white LEDs.

In order to provide further freedom in the selection of LEDs, the driving circuit may be arranged to control an intensity of each of the LEDs of the assembly so as to provide the desired illumination parameter. Thereby, in any of the above selections (randomly, in bins, etc), more different combinations of LEDs may provide for a desired result. As an example, a slightly blueish LED may be combined with a strongly yellowish LED, the slightly blueish LED being operated at a higher intensity than the strongly yellowish LED, to in combination obtain a desired white color illumination.

The driving circuit may comprise a light sensor feedback to automatically control the driving of the LEDs so as to provide the desired, combined illumination parameter. Alternatively, or in combination therewith, the driving circuit may comprise a LED forward operating voltage feedback: the forward voltage, at a given operating current, providing for an indication of the LED temperature, thereby e.g. providing a temperature feedback control of the LEDs.

In accordance with the invention, it can be noted that establishing the desired illumination parameter (e.g. to produce an essentially white light with a given color temperature) can be done in various ways. As an alternative to applying a light sensor feedback or in addition to, it can be established during manufacturing and assembly of a configuration of e.g. randomly selected LEDs which operating conditions should be applied to obtain the desired illumination parameter. Assuming, as an example, that an LED assembly comprises three or more white LEDs having a color temperature outside a desired operating point or range; by varying the duty cycles of the different LEDs, it can be established whether the combination of LEDs enables the generation of the desired illumination parameter or not. If so, the required duty cycles for providing the desired illumination parameter can be provided to the driving circuit (e.g. stored in a memory unit of the driving circuit). By establishing the required duty cycles during the manufacturing of the assembly, a light sensor could be omitted.

Although the combination of LEDs may be driven in any configuration, in an advantageous embodiment, at least two of the LEDs are series connectable as depicted in fig. 2, the driving circuit comprising switches parallel to each of the series connectable LEDs, so as to activate one or more of the series connectable LEDs by opening the respective parallel switch. A (constant or controllable) current source may be applied as a power supply.

Whether or not in a series connection of the LEDs, the driving circuit may be arranged to operate the LEDs in pulsed mode. Thereby, use may be made of any suitable pulse modulation, such as pulse width modulation, pulse repetition frequency modulation, and many others.

Operating the LEDs in a pulsed mode, intensities of the LEDs may be varied easily by e.g. duty cycle control. Furthermore, the driving circuit may be arranged to avoid an effect of a change in operating current on characteristics of the LED (e.g. color) which may allow for a more precise control of the LED illumination output over a wide operating range. According to a further aspect, the invention comprises a LED assembly provided by the method according to the invention.

Further advantages, effects and features of the invention will become clear from the below description of a preferred, non -limiting embodiment of the invention and enclosed drawings, wherein: Fig. 1 a depicts a color diagram and bins defined in the color diagram;

Fig. 1 b schematically depicts a color diagram and a first example of a randomly selected number of LEDs for producing a desired illumination parameter;

Fig. 1c schematically depicts a color diagram and a second example of a randomly selected number of LEDs for producing a desired illumination parameter; and Fig. 2 depicts a circuit diagram of (a part of) a driving circuit of a series connection of

LEDs

Below, an example will be described of a narrow white band in CIE color space (the Planckian curve) where each white-tint is denoted by a color temperature, from warm (low color temperature) to cool (high color temperature) white. A certain lighting application demands a single color temperature point over many fixtures. A previously known solution that a manufacturer of such fixtures will then order the LEDs of the color temperature he is interested in.

White LEDs are created by using a blue LED which lights up a yellowish phosphor that results in a white color. Due to variations in the phosphors and LEDs the actual resulting color temperature of an LED after production is not under perfect production control. Therefore the

LEDs are tested after production and put in the specific "bin" that indicates its position in the CIE, in a band around the Planckian curve.

A certain lighting application may demand a certain LED bin or small set of bins. Due to the popularity of a few number of bins (4 out of roughly 50) there is a large production overflow on non-popular bins. The non-popular bins are sometimes even dumped ... Even the price difference between popular and non-popular bins (up to a factor of 4) cannot change the actual white color temperature limited bin demand.

In effect, the white LED production yield may be lowered significantly by the process variations.

Non-popular white LED bins are available with up to a factor of 4 lower cost. A purpose of this invention may be to enable the use of non-popular white LED bins by color mixing multiple of these cost-effective LEDs, specifically selected by an algorithmic method (including random selection), at appropriate individual output levels in order to reach a demanded white color temperature. Fig. 1 a provides an example of bins around the Planckian curve, many of them may be not popular.

The non-popular and therefore more cost-effective bins can be put to use by applying multiple white color mixing. Mixing at least 3 - 4 LEDs may be preferred for proper output color temperature setting without losing too much light output. The 3 to 4+ LEDs (or more when LEDs from the same bin are grouped through series or parallel connections) will be non-popular bin selected in such a manner that setting them each to a preferably high duty-cycle the right mix is achieved for a particular colorpoint (or points when controllability is required). This requires LED characterization at the end of the production line in order to determine its bin and use its info to determine which LED bins to combine and which mix level to set.

Alternatively, no binning is determined at all. Instead the LEDs are purely random mixed into LED assemblies which contain several LEDs. After production these assemblies are tested and directly adjusted for operation at certain color temperatures and/or brightness levels. This results in a higher-usability factor of the LEDs and is therefore more cost-effective. Based on the outcome of the factory test, the LED assemblies can be grouped into LED assemblies that can be applied to realize certain operating conditions or application requirements i.e. provide a light source having a certain color temperature and/or brightness level. In order to determine whether an LED assembly can provide a certain output, e.g. indicated as a point or area in the CEI diagram, one can determine the position of the different LEDs of the assembly in the CEI diagram and determine whether a polygone can be determined, based on the positions, that encloses the required output characteristic. When a certain LED assembly that is compiled by a random mixing process of the applied LEDs is found to be capable of realizing a certain desired illumination parameter, e.g. as a combined illumination parameter of the different LEDs of the assembly, the different LEDs e.g operating at different duty cycles, a driver circuit for the assembly can thus be programmed to operate the LEDs at the given duty cycles.

Fig. 1 b schematically depicts a first example of a randomly selected set of LEDs 100, 110 and 120 having an illumination characteristic outside a tolerance band of a desired characteristic 130. In the example as shown, the desired illumination characteristic is to provide an essentially white light (as the desired characteristic 130 is substantially located on the Planckian curve 140. LEDs 100, 110 and 120 can be white LEDs (e.g. LEDs producing a blue or UV light arranged to

excite a yellow or yellowish phosphor) having a different color temperature. By operating the LEDs at the appropriate duty cycles, the desired illumination characteristic 130 can be established as, in the example shown in Fig. 1 b, the desired characteristic 130 is located inside a triangle 150 connecting the positions of the LEDs 100, 110 and 120 in the CEI diagram. Fig. 1 c schematically depicts a second example of the application of a randomly selected set of LEDs 200, 210, 220 and 230 having an illumination characteristic outside a tolerance band of a desired characteristic 240. In the example as shown, the desired illumination characteristic 240 can be established in different ways as the characteristic 240 is both located inside a triangle connecting the positions of the LEDs 200, 210 and 230 in the CEI diagram and a triangle connecting the positions of the LEDs 200, 220and 230 and the polygone formed by LEDs 200, 210, 220 and 230.

In such a situation, one can either opt to remove one of the LEDs and establish the appropriate duty cycles for providing the desired illumination parameters from the remaining LEDs or maintain the configuration as established by the random selection process. The latter case may be advantageous as it may enable to operate the LED assembly in a more efficient way and results in an LED assembly having a larger operational freedom (e.g. with respect to the achievable color set point). By operating all LEDs of the assembly, the 4 LEDs can be operated at a smaller duty cycle while providing the desired illumination characteristic compared to the situation where the desired illumination characteristic is established using only 3 LEDs. As a consequence, the operating temperature of the LEDs can be lower thereby obtaining a more efficient operation and a prolonged life expectancy of the LEDs.

With conventional driver electronics the 3 to 4 LED group mixing would require as many current sources as there are LED colors, this would not be cost effective versus single bin solutions, voluminous, and complex. However, by applying the principle of single current source mixing with pulsed modulation the LED cost savings more than outweigh the additional component cost for individually controlling each LED. In addition the following advantages may be obtained:

-graceful degradation: a damaged LED that interrupts the series connected chain can be bypassed in the series connection of LEDs by means of the parallel switch.. -network-ability by e.g. an optical, power-line communications or RF interface due to the availability of digital control. Networking may also allow for user changeable set-points

-duty-cycle dimming allows improved Planckian curve tracking by providing a suitable mix of the (possibly different) colors of the LEDs to obtain the desired white. Thermal, optical, and/or forward voltage feedback may be applied, as outlined below:

-thermal feedback maintaining color temperature is possible by measuring forward voltages of the LEDs.

-optical feedback maintaining color temperature by measuring each LED groups light output. -forward voltage feedback measuring a series voltage of a LED string for temperature feedback control, thereby making use of the knowledge that the forward voltage is a measure for the LED die temperature.

In addition to or instead of mixing white colors to reach a particular white color temperature, the above example can also be applied to any other required location in color space.

Using, 2 LEDs, each having a color in the fig. 1 color diagram, a combined color may be found along a line between the color points of the individual LEDs in the diagram. Depending on the intensities and driving of the LEDs, the combined output may be closer to the one or the other LED, and may be set by suitably balancing intensities. Applying 3 or more LEDs, a triangle, quadrangle, etc may be formed in the fig. 1 color diagram, an effectively obtained output illumination will be found in said triangle, quadrangle, etc, a location of the obtained illumination in the diagram depending on the intensities and driving of the LEDs.

The principle of a drive circuit providing pulsed modulation in series connection of LEDs as referred to above has been disclosed in International PCT Application No. PCT/NL2006/000182, and will be explained in some more detail with reference to figure 2.

Figure 2 shows a single power supply PS comprising N LED groups which are individually driven from a central processing unit (CPU). The number of LEDs that can be simultaneously ON is determined by the supply voltage divided by the maximum summed forward voltage of the LEDs. The CPU includes a clock which is sufficiently accurate to enable time control at adequate pulse width resolution (for example 10 bits at 1 kHz, i.e. 1 ms divided by 1024: ~ 1 μs. The CPU in this example controls the MOSFET switches by means of software, using a software-based pulse width generator, but a hardware-based generator is also an option. The power supply PS in this example has two fixed current settings; one for when at least one group is active, and a low current setting (or even OFF) if not a single group is active. The power supply PS can be an energy-inefficient linear (resistor or current-set transistor) or an energy-efficient switched version. A switched power supply PS consists of a current-feedback power supply which in principle consists of a pulse width-driven switch which is usually based on an integrated circuit and comprises a coil, flyback diode and a storage capacitor. In the case of a switched power supply PS it is necessary for the power supply PS to have a considerably higher regulating frequency than the pulse width modulation, to avoid undesirable oscillation interaction between the two

loops. In addition to this example comprising two fixed current settings, another option is to implement the CPU with a dynamic current drive arrangement.

The drive arrangement of the switches determines whether individual LED groups are active. In this example the switch is formed by a MOSFET, because of the low Rds-on (ON resistance) and actuation speed, but in principle, a transistor or even an (electronic) relay would be among the possible options. If a switch is ON, the current from the power supply will pass through the switch and not through the LEDs. If a switch is OFF all the current will pass through the LEDs, which will then light up. To avoid voltage and current peaks, the LED groups are driven in such a way, by means of a (hardware or software) algorithm that only one switch is actuated in each time interval. An example of an algorithm follows hereinafter. The drive protocol of the LEDS over time can be static or dynamic (light show). A dynamic drive protocol can comprise an autonomous software routine which drives a local lightshow. On the other hand, the drive instructions can also be driven by a communications interface. In yet another embodiment it is possible for a single light fitting among a set of such fittings to be appointed to coordinate a lightshow towards the other fittings (master/slave) by means of a bidirectional communications interface. The protocols for the communications interface can take many forms, for example direct control information for each color and each unit of time, or parameterized instructions. The communications interface can consist of a galvanic, optical or RF link for data transmission purposes. In the example, one LED group shown includes a single LED, one group includes two

LEDs connected in parallel, and one LED group shown includes M LEDs. In the LED group connected in parallel, the current will be split for each LED in this group, into two equal parts in the case of LEDs specially selected for this purpose (a customary principle in LED illumination). Allowing for component, current and voltage restrictions, the drive principle can be used in any combination of LED groups and LEDs in each group connected in parallel and in series.