DESCRIPTION

Field of the invention

The description refers in general to solid-state lighting devices based on LED technology, and methods for controlling them.

Description of the prior art

Lighting applications that make use of light-emitting diode or LED technology take advantage of the efficiency, availability of different colors, and small dimensions of these devices. However, these applications suffer from a drop-off in light emission due to LED ageing. In addition, the conditions of use, such as ambient temperature or the levels of current, influence the luminous flux of the LEDs. Lastly, devices that emit at different wavelengths exhibit different coefficients of temperature and current dependency, making it expensive in terms of control hardware and processing time to estimate the actual flux.

Given this starting point, it is preferable to detect LED luminous flux in real time, giving a more accurate estimate of the decay of the LEDs together with an assessment of the effect of the conditions of use on the total flux. Moreover, in applications that require modulation of the color, such as RGB LED systems or systems with adjustable white, optical feedback enables the colors to be modulated correctly.

In order to compensate for ageing and thermal drift and control the quality of the color of the various LED systems, two approaches are considered.

One approach involves control by thermal feedback only, together with assessment of characteristics of the LEDs such as the ageing curve, luminosity as a function of temperature and current. Although such an approach makes it possible to reduce costs in terms of components, its drawback is that it requires dedicated hardware and software to operate the control. It also means that all the characteristics of the LEDs, including the decay curve, must be known.

A second approach involves control by means of thermal feedback and optical feedback, using a dedicated optical sensor. In this case, the optical sensor can offer a good degree of accuracy in the light detection. However, this approach has at least two main drawbacks, viz. the cost of the additional component and the need to obtain an optical coupling between the sensor and the emission area, which can be difficult to achieve because of the presence of other optical elements, electronic components and mechanical constraints.

Object and summary of the invention

It is an object of the invention to overcome the drawbacks outlined above.

According to the invention, this object is achieved with a lighting device having the features claimed in the appended claims. The invention also relates to a method for controlling a lighting device having the features claimed in claim 10.

The claims are an integral part of the technical teaching given here in relation to the invention. Brief description of the accompanying illustrations

The invention will now be described, purely by way of non-restrictive example, with reference to the appended illustrations, in which:

figures 1 and 2 are circuit diagrams illustrating an embodiment of a lighting device in two operating conditions. Detailed description of embodiments

The following description sets out various specific details aimed at giving an in-depth understanding of the embodiments. The embodiments may be realized without one or more of the specific details, or with other methods, components, materials etc. In other cases, known structures, materials or operations are not shown or described in detail so as not to obscure various aspects of the embodiments.

Reference to "an embodiment" in this description means that a particular configuration, structure or feature described in relation to the embodiment is included in at least one embodiment. Hence, phrases such as "in an embodiment", which may occur at various points in this description, do not necessarily refer to the same embodiment. Moreover, particular shapes, structures or features can be combined in a suitable way in one or more embodiments. The references used here are purely for convenience and do not therefore define the scope of protection or range of the embodiments .

The proposed solution offers economical and fully integrated light detection using an LED as a light sensor.

The solution exploits the dual nature of LEDs, which work as light emitters when a forward voltage is applied to them, but as sensors when a reverse voltage is applied to them, because of the photoconductivity caused by generation of electron-hole pairs assisted by photons at the junction in the LED. Because an LED that emits at a certain wavelength is sensitive to light radiation at shorter wavelengths, it can be used as a sensor for an LED emitting at shorter wavelengths and, more generally, for light emitted at shorter wavelengths. In particular, an LED emitting in the red is sensitive to the entire visible spectrum with the exception of red light itself. The relevance of this is that the red LED is included in solid-state lighting solutions of adjustable white type, RGB type for generating colors, and warm-white type for replacing incandescent bulbs. More generally, in systems using LEDs emitting at different wavelengths, the LED that emits at the longest wavelength can be used as a light sensor.

Figures 1 and 2 show the circuit diagram of a lighting device, indicated as a whole by reference 10, comprising an LED 16, in two different operating configurations.

The lighting device 10 comprises a drive source 11 as a supply circuit, in this example a current source, which is connected to the LED 16 by a first switch 12 located between the current source 11 and the anode of the LED 16 and by a second switch 13 located between the cathode of the LED 16 and ground. The lighting device 10 comprises another supply circuit represented by a voltage source 17 connected via a positive clamp on itself to the cathode of the LED 16, through a fourth switch 15. The anode of the LED 16 is also connected, by a third switch 14, to the input of a sense amplifier 18 which comprises a shunt resistor 19 to sense the current. The switches 12, 13, 14, 15 identify a switching circuit and are controlled by a control circuit, which is implemented by the circuit which controls the lighting system. Figure 1 shows a first working configuration of the lighting device 10, corresponding to a light-emitting state in which the first switch 12 and the second switch 13 are closed, allowing the drive source 11 to control the LED 16, by feeding it with a drive current I _p and applying a forward bias . The third switch 14 and the fourth switch 15 are open, isolating the voltage source 17 and the amplifier 18. The LED 16 in this first configuration emits light.

Figure 2 shows a second working configuration of the lighting device 10, corresponding to a light detecting state, in which the first switch 12 and the second switch 13 are open, isolating the drive source 11 from the rest of the circuit. The fourth switch 15 is closed, so that this fourth switch 15 connects the voltage source 17 to the cathode of the LED 16, while the third switch 14 in the closed state connects the anode of the LED 16 to the amplifier 18 and to the shunt resistor 19 which is connected at the other end to a ground node. The LED 16 is thus reverse-biased and therefore a reverse current flows through the LED 16, corresponding to a photocurrent I _F toward the shunt resistor 19, causing a voltage drop across this resistor 19 which is amplified by the amplifier 18. An output signal 20 from the amplifier 18 is proportional to the luminous flux which illuminates the surface of the LED 16 and generates the photocurrent I _F .

Other embodiments can be made by modifying the control logic of the circuit shown in figure 1 and 2, reducing the number of switches or including other LEDs biased by the same drive source.

The lighting device according to the invention can be used as a sensor for compensating for LED ageing. In applications such as spotlights or downlights, which comprise LEDs emitting at different wavelengths, the LEDs are mounted close together. The LEDs which emit at longer wavelengths can be used as sensors to monitor the ageing of the other LEDs. This function is useful in LED systems for emitting adjustable white and warm white, because these systems include a red LED which can be used to monitor the luminous flux of the main emitter, e.g. the white LED. In this way the ageing of the LED can be compensated for by adjusting the current in order to keep the flux constant. Alternatively, it is possible to operate by reducing the current in order to extend the life of the LED. Optical coupling of the sensor LED with the other, emitting LEDs, is ensured by the proximity of the LEDs.

The lighting device according to the invention can also be used as a sensor for controlling the color mixing of the LEDs. In RGB applications and for emitting adjustable white, where the LEDs are arranged close together, it is also possible to use the red LED to detect the light emitted by the other LEDs, monitoring the light by turning the emitting LEDs on one by one, that is in sequential mode, in order to calculate the color co-ordinates of all the LEDs by capturing their photocurrent values. Because the flux of the red LED cannot be detected, it can be derived from the data, stored in the controller, of the characteristics of the LEDs. Calibration is performed at predetermined time intervals. Optical coupling of the sensor LED with the other, emitting LEDs is ensured by the proximity of the LEDs.

The lighting device according to the invention can also be used as a sensor for monitoring and compensating for thermal drifts which influence the performance of the LEDs. Using the same technique as explained for controlling the color mix described above, it is possible to monitor the intensity of each LED during heating of the system and the current in the various channels can be controlled in order to keep the color temperature or color coordinates constant. Once again, the optical coupling of the sensor LED with the other, emitting LEDs is ensured by the proximity of the LEDs.

The lighting device according to the invention can also be used as an ambient light sensor. In spot- and downlight-type applications and for road lighting, the LEDs are coupled to an optical element such as a reflector and/or lens. Ambient light is thus focused onto the LEDs. If the lighting unit is equipped with a red LED, this ambient light can be detected and used to control luminosity. For example, in road lighting applications, the LEDs can be damped as a function of the ambient light until the light is extinguished. In this way it is possible to improve the efficiency of the system by avoiding energy consumption when no luminous flux is required.

The solution set out in this description makes it possible to obtain a lighting device that is advantageous in that it removes the necessity of an optical sensor and its associated coupling system. This also means that the unit can be made more compact. The use of an LED belonging to the system has the advantage of allowing a good coupling to be obtained between the LEDs for emission and detection. The resulting lighting system is highly reliable because of the compensation for ageing. It is also possible to achieve highly accurate chromatic properties in color mixing applications. It is possible to achieve high efficiency of the system in ambient light detection and smart control of luminosity.

Clearly, without departing from the principle of the invention, the details of construction and embodiments may vary, even significantly, from those illustrated purely by way of non-restrictive example without thereby departing from the scope of the invention as defined in the appended claims.