EP2757861A1

EP2757861A1 - Led lighting module with variable colour temperature.

Info

Publication number: EP2757861A1
Application number: EP13151395.4A
Authority: EP
Inventors: Nicola Trivellin; Matteo Meneghini; Marco Ferretti; Gaudenzio Meneghesso; Enrico Zanoni
Original assignee: Lightcube Srl
Current assignee: Lightcube Srl
Priority date: 2013-01-16
Filing date: 2013-01-16
Publication date: 2014-07-23
Anticipated expiration: 2033-01-16
Also published as: EP2757861B1

Abstract

The present invention refers to a LED lighting module that comprises a primary chain of LED devices and secondary chains of LED devices.

The primary chain of LED devices is connected in series to a current generator, adapted to provide a supply current to the lighting module.

The secondary chains of LED devices are connected in parallel to one another and in series to said current generator and to said primary chains of LED devices.

The primary chain and one the first and second secondary chains form different lighting units with different colour temperatures.

The lighting module comprises one or more electronic switches, through which it is possible to alternatively turn on/off one of the lighting units in order to change the colour temperature of the light emitted by the lighting module.

Description

The present invention refers to a lighting module, in particular a LED (Light Emitting Diode) lighting module.
As is known, the chromatic properties of a light source are generally described by using certain characteristic measuring parameters, including colour temperature, colour rendering index and chromatic quality.
The colour temperature of a light source is a quantity (measured in K) indicative of the temperature a black body should have in order that the light radiation emitted by the latter appears chromatically as close as possible to the light radiation emitted by the source itself. The colour rendering index of a light source is a quantity indicative of the capacity of said light source to illuminate a series of normalized colour samples in relation to the capacity of a black body to illuminate the same series of colour samples.
In general, the colour rendering index (Ra) is indicated by a maximum number of 100 (perfect colour rendering capacity). By way of example, the colour rendering capacity of a filament light source (incandescent bulb) is typically 98-99, therefore very close to that of a black body.
The chromatic quality of a light source is represented by the distance between the chromatic coordinates of a light source and the coordinates of a black body, at the same colour temperature, in the CIE 1931 chromaticity diagram. The greater the distance of the black body from the chromatic coordinates, the lower the chromatic quality of the light source.
In lighting systems for private and public use, lighting devices or bodies which comprise LED lighting modules able to emit white light are widely used.
The most traditional lighting modules use LED devices with phosphor conversion system, typically characterised by an emission spectrum with peaks in the blue and yellow wavelengths.
As is known, the colour rendering index (below 90) of these lighting modules is wholly unsatisfactory, in particular as regards their use in interior lighting devices/systems.
LED lighting modules have been developed that are able to emit white light by varying the colour temperature of the light radiation emitted.
Often these lighting modules use phosphor LED devices characterised by an emission spectrum with emission peaks in the wavelengths typical of blue/violet and yellow, coupled with LED devices, typically of AlInGaP (Aluminium Indium Gallium Phosphide) type able to emit red-amber light.
The colour temperature of the light radiation emitted can be varied by appropriately modifying the ratio between the light flux generated by the phosphor LED devices and the light flux generated by the red/amber LED devices.
The lighting modules of this type have some drawbacks.
In phosphor LED devices, the light flux generated varies with the drive current in a different manner from the red/amber LED devices.
The two types of LED devices (phosphor white LEDs and red/amber LEDs) furthermore behave differently when a variation in temperature occurs.
To remedy these difficulties, it is common practice to differentiate regulation of the drive current for the different types of LED devices.
Experience has shown that this solution, albeit effective, significantly increases the overall costs for the industrial production of said lighting modules.
A further drawback arises from the fact that these lighting modules are characterised by a high colour rendering index only for a limited colour temperature interval (approximately 50°K). Outside this colour temperature range, the chromatic qualities of the light emitted deteriorate significantly, turning green for the highest colour temperatures and red for the lowest colour temperatures (figure 8).
Other lighting modules of known type use different types of phosphor LED devices, able to emit white light with different colour temperatures.
Also in this case, the colour temperature of the light emitted can be varied by appropriately modifying the ratio between the light flux generated by LED devices of different types. Unfortunately also these lighting modules have a series of drawbacks.
The number of LED devices used is relatively high, generally double that of a single CCT solution, with consequent increase in costs and overall dimensions.
Furthermore, for colour temperature values that are intermediate with respect to those characteristic of the LED devices used, the light flux of the light radiation emitted significantly increases (in practice it doubles).
The main aim of the present invention is to provide a LED lighting module, which overcomes the drawbacks described above.
In the context of this aim, one object of the present invention is to provide a LED lighting module which allows variation of the chromatic characteristics of the light radiation emitted, in particular the colour temperature, in a relatively simple and effective manner.
A further object of the present invention is to provide a LED lighting module, in which the colour temperature of the light radiation emitted can be varied within a relatively wide range. A further object of the present invention is to provide a LED lighting module, in which the colour temperature of the light radiation emitted can be varied, maintaining the colour rendering index and the light flux substantially unchanged.
A further object of the present invention is to provide a LED lighting module, which has a relatively simple and compact overall structure.
A further object of the present invention is to provide a LED lighting module, which has chromatic characteristics (colour temperature, colour rendering and chromatic coordinates) that are stable when the operating temperature varies.
Last but not least, a further object of the present invention is to provide a LED lighting module which is easy to produce industrially, at competitive costs.
This aim and these objects, in addition to other objects which will appear evident from the following description and the accompanying drawings, are achieved, according to the invention, by a LED lighting module, as claimed in claim 1 and related dependent claims, proposed below.
In a further aspect, the present invention relates to a lighting body or device as claimed in claim 10.
Further characteristics and advantages of the lighting module, according to the invention, will become clearer by referring to the description given below and the accompanying figures, provided for purely illustrative non-limiting purposes, in which:

figure 1 schematically illustrates the structure of the lighting module, according to the invention; and
figure 2 schematically illustrates the structure of a preferred embodiment of the lighting module, according to the invention; and
figure 3 schematically illustrates the operation of the lighting module, according to the invention, in the embodiment of figure 2; and
figures 4-8 illustrate some experimental measurements carried out on an example of the lighting module, according to the invention, according to the embodiment of figure 2.

With reference to the cited figures, the present invention refers to a LED lighting module 1, particularly suitable for use in lighting bodies or lighting devices in general.
The lighting module 1 is operatively combined with a current generator 10 able to provide a power supply current I_UPPLY which is preferably constant over time.
Advantageously, the lighting module 1 comprises a pair of input terminals B₁, B₂ connected in parallel to the current generator 10.
The latter can be integrated with the lighting module 1 or, preferably, be comprised in a control stage or device (not illustrated) operatively combined with the lighting module.
The lighting module 1 comprises a primary chain M₁ of LED devices, which is electrically connected in series to the current generator 10.
The lighting module 1 comprises first and second secondary chains N₁, N₂, ..., N_H of LED devices, which are electrically connected in parallel to one another and are electrically connected in series to the current generator 10 and to the primary chain M₁.
The lighting module 1 may comprise any positive integer number H of first and second secondary chains N₁, N₂, ..., N_H.
The whole set of secondary chains of LED devices may be grouped according to the needs. For example, the first secondary chains may comprise the sole secondary chain N₁ while the second secondary chains may comprise the secondary chains N₂, ..., N_H.
A different grouping of the secondary chains N₁, N₂, ..., N_H is possible, according to the needs. For example, the group of the first secondary chains might comprise a plurality of chains of LED device while the group of the second secondary chains might comprise a single chain of LED devices.
Preferably, each of the secondary chains N₁, N₂, ..., N_H comprises a different number i₁, i₂, ..., i_H of LED devices.
However, in other embodiments of the present invention (not illustrated), all the secondary chains N₁, N₂, ..., N_H may comprise a same number of LED devices.
Preferably, the primary chain M₁ comprises a number of LED devices greater than that the number of LED devices of each of the first and second secondary chains N₁, N₂, ..., N_H.
The lighting module 1 furthermore comprises one or more switches S, each of which is electrically connected in series to the LED devices of one of the first and second secondary chains N₁, N₂, ..., N_H.
In practice, each switch S is inserted in one of the first and second secondary chains N₁, N₂, ..., N_H and is electrically connected in series with the LED devices thereof.
According to the invention, the switches S regulate the flow of the supply current I_UPPLY, provided by the current generator 10, along a first and second lighting unit C₁, C₂.
The first lighting unit C₁ comprises the LED devices of the primary chain M₁ and the LED devices of the (one or more) first secondary chains (e.g. the secondary chain N₁) while the second lighting unit C₂ comprises the LED devices of the primary chain M₁ and the LED devices of the (one or more) second secondary chains (e.g. the secondary chains N₂, ..., N_H). From the above, it is apparent that the actual number of LED devices of each lighting unit basically depends on the adopted grouping of the secondary chains of LED devices.
The switches S can advantageously regulate the passage of a current along the lighting units C₁, C₂, said current varying between zero and a maximum value (I_SUPPLY) set by the current generator 10.
In other words, the switches permit/prevent the flow of the supply current I_SUPPLY along the first lighting unit C₁ and along a second lighting unit C₂, in an alternate manner.
The switches S are thus capable to regulate the flow of the supply current I_UPPLY along alternative paths, which develop between the terminals B₁ and B₂ of the lighting module and which involve the LED devices of the first and second lighting units C₁, C₂, respectively. Each of said paths identifies a different lighting unit C₁, C₂ that is therefore able to emit its own characteristic light radiation when the supply current passes therethrough.
The number of switches S may be equal or smaller than the number of secondary chains N₁, N₂, ..., N_H. In other words, the switches S may be inserted in all or in some of the secondary chains of the lighting module 1.
The switches S can consist, for example, of a simple MOSFET or BJT transistors.
In alternative forms of the present invention, a more complex current switching circuits can be used.
Preferably, the electronic switches S are operatively combined with electronic control means (not illustrated) able to provide control signals in current or in voltage, for example a signal of the square wave type with duty-cycle adjustable by means of a PWM type adjustment system. In order to ensure the alternate activation of the lighting units C₁, C₂, the switches S electrically connected to the first secondary chains (first lighting unit C₁) are controlled by a common first control signal V_G1 while the switches S electrically connected to the second secondary chains (second lighting unit C₂) are controlled by a common second control signal V_G2.
Preferably, the LED devices of the primary chain M₁ and secondary chains N₁, N₂, ..., N_H are selected so that the LED devices, comprised in each of the above-mentioned lighting units, have complementary emission spectra, i.e. spectra having emission peaks at different sided wavelength ranges.
For example, the primary chain M₁ can be provided with LED devices with emission peaks in the blue and yellow wavelengths and the secondary chains of each of the lighting units C₁, C₂ can be provided with LED devices with emission peaks in the green and red wavelengths.
In this way, the spectrum of the radiation emitted by each lighting unit can be advantageously modulated so as to get as close as possible to the trend of the emission spectrum of a black body.
This allows the colour rendering index of the light radiation emitted by the lighting module 1 to be maintained constantly high (greater than 90 Ra), whichever lighting unit is active in a given operating condition.
Preferably, the LED devices of the primary chain M₁ and the first and second secondary chains N₁, N₂, ..., N_H are selected so that the lighting module 1 emits a light radiation with a different colour temperature when the supply current I_UPPLY passes along each of the above-mentioned lighting units C₁, C₂.
In this way, the switches S are able to modulate the colour temperature of the light radiation emitted by selectively regulating the flow of the supply current I_SUPPLY along the different lighting units C₁, C₂.
Preferably, the LED devices of the primary chain M₁ and the LED devices of the first and second secondary chains N₁, N₂, ..., N_H are selected so that the lighting module 1 emits a light radiation with constant light flux when the supply current I_SUPPLY passes along each of the above-mentioned lighting units C₁, C₂.
This ensures that, in all operating conditions, the lighting module 1 emits a substantially uniform light flux.
Preferably, the LED devices comprised in the lighting module 1 are solid-state devices based on properly chosen semiconductor materials or compounds, according to the needs.
In a preferred embodiment of the present invention, the lighting module 1 comprises a single primary chain M₁ of LED devices, a single first secondary chain N₁ and single second secondary chain N₂ (figures 2-3).
The primary chain M₁ is electrically connected in series to the current generator 10 while the first and second secondary chains N₁, N₂ are connected in parallel to one another and in series to the current generator 10 and to the primary chain M₁.
The secondary chains N₁, N₂ comprise a different number of LED devices with respect to the primary chain M₁, preferably a lower number of LED devices.
The lighting module 1 furthermore comprises a single electronic switch S, electrically connected in series with one of the secondary chains N₁, N₂ (e.g. the secondary chain N₂).
The electronic switch S advantageously allows/prevents, in an alternate manner, the flow of the supply current I_SUPPLY, provided by the current generator 10, along a first lighting unit C₁, which comprises the LED devices of the primary chain M₁ and of the first secondary chain N₁, and along a second lighting unit C₂, which comprises the LED devices of the primary chain M₁ and of the second secondary chain N₂.
In other words, the electronic switch S controls the flow of the supply current I_SUPPLY so that it flows, in an alternate manner, through the lighting units C₁ or C₂.
Preferably, the first secondary chain N₁ comprises P₁ LED devices and the second secondary chain N₂ comprises P₂ LED devices, where P₁ is a number greater than P₂.
To reduce the costs and overall dimensions of the lighting module 1, the number of LED devices in the secondary chains N₁, N₂ are selected to ensure the relation P₁ = P₂+1.
Said solution is furthermore expedient as it reduces the fluctuations in voltage perceived by the current generator 10, when the supply current I_SUPPLY is addressed from one lighting unit to another.
Preferably, the electronic switch S consists of a MOSFET transistor, having drain and source terminals connected in series with the LED devices of the second secondary chain N₂ and the gate terminal connected to appropriate control means (not illustrated).
The switch S is advantageously driven in a conducting state (ON) or in an interdicting state (OFF) by a voltage signal V_G of the square wave type, applied to the gate terminal. Advantageously, the drive signal V_G has an adjustable duty-cycle, for example by PWM type adjustment.
In this way, the switch S does not require a control circuit specifically designed for the purpose, since the switch S could be connected to PWM modulation (dimming) systems already present, as happens in numerous lighting plants and systems.
Thanks to the PWM regulation, furthermore, the switch S dissipates a negligible electrical power, not compromising the efficiency of the system.
Preferably, decoupling means 15 (such an opto-insulator) are electrically connected between the switch S and the control means thereof, so as to electrically decouple the ground terminal(s) of the switch S from the ground terminal(s) of said control means.
Preferably, in the embodiment of figure 2, the LED devices of the primary chain M₁ and secondary chains N₁, N₂ are selected so that the lighting module 1 emits a light radiation with a first colour temperature T₁ when the supply current I_SUPPLY passes along the first lighting unit C₁, and a light radiation with a second colour temperature T₂, different from T₁, when the supply current I_SUPPLY passes along the second lighting unit C₂.
In particular, the LED devices of the primary chain M₁ and secondary chains N₁, N₂ are selected so that the first colour temperature T₁ is below the second colour temperature T₂.
By properly controlling the switch S, it is thus possible to guarantee that the lighting module provides a colour temperature of the resulting light within the range T1-T2.
The LED devices of the primary chain M₁ and secondary chains N₁, N₂ are selected so that the colour rendering index is higher than 90 when the chains N₁ and N₂ are alternatively switched on.
This allows a colour rendering index greater than or equal to 90 to be guaranteed in all operating conditions.
Operation of the lighting module 1, in the embodiment of figures 2-3, is now described in further detail.
When the electronic switch is in the interdiction state (OFF), corresponding to a signal V_G at "low" logic level, all passage of current along the second secondary chain N₂ is prevented. The supply current I_SUPPLY is thus forced to pass along the first lighting unit C₁, i.e. along a path that comprises the LED devices of the primary chain M₁ and the first secondary chain N₁.
In this condition, the lighting module 1 emits the light radiation generated by the first lighting unit C₁.
When the electronic switch S is in the conducting state (ON), corresponding to a "high" logic level signal V_G, current flow is theoretically allowed along both secondary chains N₁, N₂.
Since the second secondary chain N₂ has a lower number of LED devices compared to the first secondary chain N₁, the voltage drop induced at its ends by the supply current I_SUPPLY is lower than the one induced at the ends of the first secondary chain N₁ and in any case not sufficient to set the LED devices of the latter to the conducting state.
The supply current I_SUPPLY, which runs through the primary chain M₁, is thus forced to flow through the second secondary chain N₂.
The supply current I_SUPPLY therefore passes along the second lighting unit C₂, i.e. along a path that comprises the LED devices of the primary chain M₁ and of the second secondary chain N₂.
In this condition, the lighting module 1 emits the light radiation generated by the second lighting unit C₂.
By modulating the operating status of the electronic switch S with an appropriate drive signal V_G, it is possible to obtain a mixture of the light emitted by the two lighting units C₁ and C₂ over time.
If the frequency of the signal V_G is high enough (>100 Hz), the human eye, due to the phenomenon of persistence of vision, perceives a light stimulus mediated between the light radiations emitted by the two lighting units C₁ and C₂ over time.
By modulating the status of the electronic switch S with an appropriate drive signal V_G, the colour temperature of the light radiation emitted by the lighting module 1 can be advantageously modulated over time.
For a certain duty-cycle value of the drive signal V_G, the radiation emitted by the lighting module 1 is thus characterised by a corresponding colour temperature value in the range T1-T2.
The colour temperature of the light radiation emitted by the lighting module 1 can thus be modulated in a simple effective manner, by adjustment of the duty-cycle of the signal V_G, for example using open chain control means.

EXAMPLE

An example of practical embodiment of the lighting module 1 is now described below, in the embodiment of figures 2-3.
In such an embodiment example, the primary chain M₁ of the lighting module 1 was produced by using 24 LED devices, connected in series.
In particular, 22 LED devices were used able to emit white light with colour temperature equal to 2700K, one LED device able to emit blue light, with emission peak at a wavelength between 460 nm and 480 nm, and one LED device able to emit green light, with emission peak at a wavelength between 520 nm and 530 nm.
The first secondary chain N₁ of the lighting module 1 comprises 8 LED devices, connected in series, with emission wavelength between 450 nm and 550 nm.
In particular, 1 LED device was used able to emit blue light, with emission peak at a wavelength between 460 nm and 480 nm, 2 LED devices able to emit royal blue light, with emission peak at a wavelength between 450 nm and 460 nm and 5 LED devices able to emit green light, with emission peak at a wavelength between 520 nm and 530 nm.
The secondary second chain N₂ of the lighting module 1 was produced using 5 LED devices, connected in series.
In particular, 4 LED devices capable to emit white light with colour temperature equal to 2700 K and 1 LED device capable to emit red light, with emission peak at a wavelength between 600 nm and 650 nm were used.
For the switch S, a type N MOSFET depletion transistor was used.

The experimental measurements performed on the light radiation emitted by the lighting module 1 during passage of the supply current I_SUPPLY along each of the lighting units C₁ and C₂ gave the following results (supply current I_SUPPLY = 350mA):

Lighting unit	Colour temperature (K)	Colour rendering index (Ra)	Flux (lumen)	Chromatic coordinate X	Chromatic coordinate Y
C₁	5329K	93	2474	0.34	0.345
C ₂	2796K	93	2255	0.45	0.40

The emission spectra of the light radiation emitted by the lighting module 1, during passage of the supply current I_SUPPLY along each of the lighting units C₁ and C₂, are illustrated in figure 4. Modulation of the chromatic characteristics of the light radiation emitted by the lighting module 1 was performed by controlling the electronic switch S with a drive signal V_G with amplitude of 10V, frequency 4kHz and duty-cycle varying between 0 and 100%.
During said modulation, the drive current I_SUPPLY was maintained at a constant value of 350mA.
The results of the photometric measurements performed on the light radiation emitted by the lighting module 1, during modulation, are shown in figures 5-8.
The following is highlighted:

the colour temperature of the light radiation emitted can vary throughout the range between 2800K and 5300K, showing a gradual variation of the colour temperature when the duty-cycle varies;
the colour rendering index is constantly greater than the value of 93. For a duty cycle values between 20% and 70%, the colour rendering index exceeds the value of 95;
the light flux undergoes relatively few variations when the duty-cycle varies (the light flux measured is equal to 2360 lumen +/- 5%);
the locus of the points of the light radiation emitted at the various colour temperatures, in the CIE 1931 chromaticity diagram, differs only very slightly from the Planckian black body emission curve. This indicates an optimal chromatic quality of the light radiation emitted.

It has been seen in practice that the connection device 1, according to the invention, solves the drawbacks described in the known art, achieving the set objects.
The lighting module 1 allows the chromatic characteristics of the light radiation emitted to be varied in a simple effective manner.
In particular, it allows variation of the colour temperature over a very wide range of values, while substantially maintaining the light flux and the colour rendering index constant, the latter at a relatively high level (always greater than 90 Ra).
The lighting module 1 therefore falls into class 1A for lighting devices, according to the classification of the UNI 10380 standard.
The lighting module 1 allows variation of the chromatic characteristics of the light radiation emitted using a relatively low number of LED devices and one single very cheap and compact current regulation device.
The lighting module 1 thus has a relatively simple compact overall structure, which is easy to produce at industrial level with relatively low industrial costs.

Claims

A LED lighting module (1) characterised in that it comprises:
- a primary chain (M₁) of LED devices, which is electrically connected in series to a current generator (10) that provides a supply current (I_SUPPLY);

- one or more first and second secondary chains (N₁, N₂, N_H) of LED devices electrically connected in parallel to one another and electrically connected in series to said current generator (10) and to said primary chain (M₁);

- one or more electronic switches (S), each of which is electrically connected in series with the LED devices of one of said first and second secondary chains (N₁, N₂, N_H), said electronic switches permitting/preventing, in an alternate manner, the flow of said supply current (I_SUPPLY) along a first lighting unit (C₁), which comprises the LED devices of said primary chain (M₁) and the LED devices of said first secondary chains (N₁), and along a second lighting unit (C₂), which comprises the LED devices of said primary chain (M₁) and the LED devices of said second secondary chains (N₂, N_H).
A lighting module, as claimed in claim 1, characterised in that the LED devices comprised in each of said first and second lighting units (C₁, C₂) have complementary emission spectra.
A lighting module, as claimed in one or more of the preceding claims, characterised in that the LED devices of said primary and secondary chains are selected so that said lighting module emits a light radiation with a different colour temperature (T₁, T₂) when said supply current (I_SUPPLY) passes along each of said first and second lighting units (C₁, C₂).
A lighting module, as claimed in one or more of the preceding claims, characterised in that the LED devices of said primary and secondary chains are selected so that said lighting module emits a light radiation with constant light flux when said supply current (I_SUPPLY) passes along each of said first and second lighting units (C₁, C₂).
A lighting module, as claimed in one or more of the preceding claims, characterised in that it comprises:
- a primary chain (M₁) of LED devices, connected in series to said current generator (10);

- a first secondary chain (N₁) and a second secondary chain (N₂) of LED devices, electrically connected in parallel to one another and electrically connected in series to said current generator (10) and to said primary chain (M₁), said first and second secondary chains (N₁, N₂) comprising a different number of LED devices;

- an electronic switch (S), electrically connected in series with the LED devices of one of said first and second secondary chains (N₁, N₂), said electronic switch permitting/preventing, in an alternate manner, the flow of said supply current (I_SUPPLY) along a first lighting unit (C₁), which comprises the LED devices of said primary chain (M₁) and the LED devices of said first secondary chain (N₁), and along a second lighting unit (C₂), which comprises the LED devices of said primary chain (M₁) and the LED devices of said second secondary chain (N₂).
A lighting module, as claimed in claim 5, characterised in that said first secondary chain (N₁) comprises P₁ LED devices and said second secondary chain (N₂) comprises P₂ LED devices, where P₁ is a positive number greater than P₂.
A lighting module, as claimed in claim 6, characterised in that said first secondary chain (N₁) comprises P₁ LED devices and said second secondary chain (N₂) comprises P₂ LED devices, with P₁=P₂ + 1.
A lighting module, as claimed in one or more of the claims from 5 to 7, characterised in that said electronic switch (S) is electrically connected in series to the LED devices of said second secondary chain (N₂).
A lighting module, as claimed in one or more of the claims from 5 to 8, characterised in that it emits a light radiation with a first colour temperature (T₁) when said supply current (I_SUPPLY) passes along said first lighting unit (C₁), and a light radiation with a second colour temperature (T₂) when said supply current (I_SUPPLY) passes along said second lighting unit (C₂), said first colour temperature (T₁) being higher than said second colour temperature (T₂).
A lighting body or device characterised in that it comprises a lighting module (1), as claimed in one or more of the preceding claims.