EP2416625A1 - Method for controlling light-emitting diodes - Google Patents

Abstract

The method involves starting a direct current-to-direct current converter to generate voltage pulses with a determined cyclic ratio. An established rate (VE) of each pulse is determined for a calibrated voltage pulse (IC) of a voltage level corresponding to a level of the established rate. Power is supplied to LEDs by applying each calibrated pulse to the LEDs by using a switching control circuit. The starting of the converter, the determination of the rate of the pulse and the supplying of the power are carried out successively before switching of the power supply to the LEDs. An independent claim is also included for a device for gradual controlling of a luminous lighting intensity of an LED, comprising a direct current-to-direct current converter.

Description

At present, light-emitting diodes, designated LEDs hereafter, are increasingly used for the implementation of lighting and / or signaling functions of motor vehicles.

In general, several LEDs are associated in an optical unit and supplied synchronously to perform the aforementioned functions.

When, in particular, a gradual control of the lighting intensity of the LEDs is to be executed, a gradual control known as "dimming" in English, the aforementioned groups of LEDs lend themselves satisfactorily to such a mode of operation. control, by means of a modulation of duration, called width, electrical current pulses feeding them. The width modulation of the pulses, pulses of electrical voltage transformed into electrical current pulses by the internal impedance of the aforementioned LEDs, is obtained for example from a DC-DC voltage converter, which is associated with a control circuit. switching controlled by a gradual control signal and to adjust accordingly the duty cycle and therefore the width of the power supply pulses LEDs, and finally the electrical energy supplied to them and thus the light intensity lighting groups of LEDs and powered.

The above procedure gives complete satisfaction, at least with regard to the high duty cycle pulses, greater than about 20%, either for average or high levels of illumination of the LEDs.

In particular, for pairs of DC / DC converter circuit / conventional type switching control circuit, the switching control circuit having a proportional and integral control loop of the voltage level of the pulses, the dynamic, that is, that is, the limited response time of the assembly thus formed generates steady-state settling times of the supply of the LEDs that are too slow.

When, in addition, a gradual control of the luminous intensity LED illumination is applied to obtain low illumination values, the duty cycle of the supply pulses being adjusted to a value less than a value of the order of 20%, the rise time too high. pulses degrades the average value of the LED supply current, which is no longer equal to the product of the value of the amplitude of the pulses in steady state and the value of the duty cycle selected for the application of the control gradual. This results in a non-linearity of the gradual control, source of inaccuracy of this command. In addition, the average value of the LED supply current depends on the supply voltage.

A solution to overcome the aforementioned drawbacks has been proposed by the patent DE 10236872 .

In the aforementioned solution, an analog capacitance circuit R2C2, as shown in FIG. figure 1 , taking up the figure 2 of the aforementioned patent, controlled by a switch T2 allows in fact to sample and store the maximum value of the amplitude of the pulses for the next pulse.

Such a system is technically satisfactory, but it implies a perfect synchronization of the switching of the switch T2 and the switch T3 in fact allowing the application of the supply pulses to the LEDs.

In addition, such a solution involves the implementation of a specific switching control circuit 4 to support the connection of the sampling circuit R2C2 to the reference voltage of the device, the mass, via the switch T2 .

This is not the case for the usual switching control circuits normally available commercially.

Accordingly, the present invention aims to overcome the disadvantages of the device of the prior art.

In particular, an object of the present invention is the implementation of a method of gradually controlling the LED illumination light intensity for supplying the LEDs conditionally to the determination of the steady state of the power supply pulses. the latter, which reduces substantially, if not remove, any inaccuracy of the gradual control of the lighting intensity finally obtained at low light values.

Another object of the present invention is the implementation of a device for gradually controlling the LED illumination light intensity using a conventional DC-DC converter, which is associated with a conventional switching control circuit in the same direction. absence of necessary electrical referencing of the latter to the ground voltage.

The method for gradually controlling the luminous intensity of illumination of light-emitting diodes by modulation of the pulse width of electrical current with a given duty cycle, which is the subject of the invention, is implemented from a DC voltage converter. -continu and a control circuit for switching the power supply current of the light emitting diodes.

It is remarkable in that it consists, at least in succession, before switching the power supply of the diodes, to start the DC-DC converter, to generate determined cyclic duty voltage pulses, to determine the established speed of each pulse, for generating a voltage level calibrated voltage pulse corresponding substantially at the steady state level, then supplying the light emitting diodes by applying each calibrated pulse to the light emitting diodes by means of the switching control circuit.

The method which is the subject of the invention is furthermore remarkable in that the step of determining the established speed of each pulse comprises detecting the amplitude level of each pulse with respect to a reference value.

The method which is the subject of the invention is also remarkable in that the step of determining the established speed of each pulse comprises the calculation of the establishment time of the established speed of each pulse.

The method which is the subject of the invention is finally remarkable in that for a switching control circuit comprising a proportional and integral servocontrol loop of the electrical voltage level of the pulses, this consists in calculating the establishment time of each pulse, and shifting the application of each calibrated pulse to the light emitting diodes with a delay time substantially equal to the establishment time.

The device for gradually controlling the light intensity of illumination of light-emitting diodes by modulation of the pulse width of a given duty cycle, which is the subject of the invention, comprises a DC-DC voltage converter and a DC switching control circuit. current supply pulses of the light-emitting diodes.

It is remarkable in that it furthermore comprises resources for determining the steady state of each pulse delivered by the DC-DC converter, for generating a calibrated pulse, and, upon reaching this established regime, control resources of supplying the light-emitting diodes by applying each calibrated pulse to the light-emitting diodes.

The device which is the subject of the invention is furthermore remarkable in that the resources for determining the established speed of each pulse comprise means for detecting the amplitude level of each pulse with respect to a reference value.

The device that is the subject of the invention is also remarkable in that the resources for determining the established speed of each pulse comprise means for calculating the establishment time of each pulse.

The device which is the subject of the invention is finally remarkable in that, for a switching control circuit comprising a proportional and integral control loop of the pulse voltage level, this comprises means for calculating the rise time of the pulse. each pulse, and a circuit for delaying the application of each pulse calibrated to the light-emitting diodes with a delay time substantially equal to the rise time of each pulse.

• la figure 2 représente, à titre d'exemple non limitatif, un chronogramme des étapes essentielles de mise en oeuvre du procédé objet de l'invention, étapes illustrées sur les différents signaux engendrés suite à l'exécution d'une commande graduelle par un utilisateur ;
• la figure 3 représente, à titre d'exemple non limitatif, un schéma de mise en oeuvre du dispositif objet de l'invention, dans un mode de réalisation préférentiel;
• la figure 4a représente, à titre d'exemple non limitatif, un chronogramme des différents signaux successifs engendrés par le dispositif objet de l'invention, tel que représenté en figure 3, pour une charge constituée par un groupe de LEDs comprenant 10LEDs, pour l'exécution d'une fonction d'éclairage d'un projecteur de véhicule automobile par exemple;
• la figure 4b représente, à titre d'exemple non limitatif, un chronogramme des différents signaux successifs engendrés par le dispositif objet de l'invention, tel que représenté en figure 3, pour une charge constituée par un groupe de LEDs comprenant 4LEDs, pour l'exécution d'une fonction de signalisation d'un feu de véhicule automobile par exemple.

A more detailed description of the method and the device for gradually controlling the light intensity of lighting of light-emitting diodes, in accordance with the subject of the present invention, will now be given in connection with with the following drawings in which, in addition to the figure 1 relating to the prior art and corresponding to the figure 2 patent DE 10236872 previously cited,

• the figure 2 represents, by way of non-limiting example, a chronogram of the essential steps of implementation of the method which is the subject of the invention, steps illustrated on the various signals generated following the execution of a gradual command by a user;
• the figure 3 represents, by way of nonlimiting example, an implementation scheme of the device according to the invention, in a preferred embodiment;
• the figure 4a represents, by way of nonlimiting example, a chronogram of the different successive signals generated by the device which is the subject of the invention, as represented in FIG. figure 3 for a load constituted by a group of LEDs comprising 10LEDs, for the execution of a lighting function of a motor vehicle headlamp for example;
• the figure 4b represents, by way of nonlimiting example, a chronogram of the different successive signals generated by the device which is the subject of the invention, as represented in FIG. figure 3 , for a load consisting of a group of LEDs comprising 4LEDs, for the execution of a signaling function of a motor vehicle fire, for example.

A more detailed description of the method which is the subject of the invention will now be given in connection with the figure 2 . On the figure 2 and the Figures 4a , 4b For timing diagrams of signals recorded on a multi-channel oscilloscope, the y-axis is graduated in relative amplitude AR, in fact representing the high and low levels of the different signals, and the x-axis is graduated in time, 1 cm representing 100 μs.

The aforementioned method preferably concerns, but is not limited to, the gradual control of the light intensity of lighting of light-emitting diodes by pulse width modulation of electric current, the duty cycle of which is determined, depending on the use, in particular when this duty cycle is less than 20% for example, from a DC-DC voltage converter and a control circuit of switching of the power supply current of the light-emitting diodes.

With reference to the figure 2 above, it consists at least, successively, to start, A, the DC-DC converter, and, of course, to apply the gradual control to generate determined duty cycle voltage pulses, corresponding to the demand for use of the user of the vehicle. This operation is represented by the curve A representing the gradual control voltage jump to the drawing.

The aforesaid step is followed by a step of determining the established regime VE of each pulse to generate a calibrated voltage pulse IC whose voltage level corresponds substantially to the level of steady state pulses. On the figure 2 B represents the establishment of the determined duty cycle pulse corresponding to the gradual control generated by the curve A, this establishment, noted as Establishment, being deemed to correspond to the establishment of the current of a resistance-capacitance circuit, as will be described later in the description.

When the set speed VE is reached, the method that is the subject of the invention consists in supplying the light-emitting diodes with the electroluminescent diodes by applying the calibrated pulse IC to the light-emitting diodes, by means of the switching control circuit. It is understood, in particular, that on reaching the set speed VE voltage switching to generate the calibrated pulse IC is performed without appreciable delay, the aforementioned voltage pulse for supplying the LEDs, according to the curve D shown in FIG. figure 2 which represents the current timing diagram of intensity IL delivered to the LEDs.

In a general manner, and in a first mode of non-limiting implementation, the method that is the subject of the invention, the step of determining the established speed of each pulse comprises detecting the amplitude level VE of each pulse. , vis-à-vis a reference value.

According to a second preferential non-limiting embodiment of the method which is the subject of the invention, the step of determining the established speed of each pulse comprises the calculation of the time of establishment, denoted t D on the figure 2 of the established regime of each pulse.

In particular, the method which is the subject of the invention can particularly advantageously be implemented for a switching control circuit comprising a proportional and integral servocontrol loop of the electrical voltage level of the pulses, this circuit being reduced to a circuit resistance-capacity, as will be described below in connection with the figure 3 .

In this hypothesis, the method according to the invention consists in calculating the establishment time tD of each pulse, and in shifting the power supply of the LEDs by applying each calibrated pulse IC to the latter delayed by a delay period substantially. equal to the establishment time tD.

So, with reference to the figure 3 mentioned above, the references 5 and 5 'designate the input terminals of the battery power supply of the vehicle of the device 1 for gradually controlling the light intensity of illumination of the light-emitting diodes by pulse width modulation determined cyclic object of the invention. On this figure 3 the reference 5 'designates, for example, the mass. This device comprises a DC-DC voltage converter 3 and a voltage breeder circuit, called a BOOST circuit, formed by an inductor L1, the switching transistor T1, the diode D and the capacitor C4. It should be noted, in particular, that any voltage breeder circuit of the BUCK, BUCK BOOST, SEPIC or CUK type, for example, and finally any type of DC / DC converter can however be implemented. The assembly thus formed must provide successive pulses supplying the load 2 constituted by the LEDs. This load is connected to the connection terminals 6 and 6 'and supplies the latter with the regulated current IL as a function of the gradual control applied. Other configurations of the aforementioned power supply circuit can be envisaged.

To the above-mentioned DC-DC converter 3 is furthermore associated a supply pulse switching control circuit 4.

The latter is powered by the supply voltage from the connection terminal 41. It comprises an internal reference voltage denoted Vref and a operational amplifier 40 receiving on its positive terminal the reference voltage Vref and whose negative terminal is connected to an input terminal 42 '. The switching transistor T1 is controlled by the output 44 of the supply pulse switching control circuit 4.

The input terminal 7 of the device 1 object of the invention is connected to the input 42 of the control circuit 4 and receives the gradual control signal representative of the pulse width modulation. This signal can be delivered either externally by the input terminal 7, or internally. The output terminal 45 of the control circuit 4 is connected to the control input of a switching transistor T3, which is connected in series on the one hand to the load 2 constituted by the LEDs and on the other hand to a resistor R3, itself connected to the 5 'supply terminal of the device 1 object of the invention. It is understood, in particular, that the resistor R3 has the function of measuring and giving an image of the value of the current IL passing through the load 2 constituted by the LEDs. The capacitor C4 of the voltage raising circuit is connected on the one hand to the power supply terminal 5 'of the device 1 and, on the other hand, to the load 2 constituted by the LEDs.

Finally, the output of the operational amplifier 40 is connected to the terminal 42 of the power pulse switching control circuit 4 and the terminals 42 and 42 'of the latter are connected by an RC series resistance-capacitor circuit. . The terminal 42 'is further connected by a resistor R' to the common terminal of the switching transistor T3 and the resistor R3.

The operational amplifier 40 is provided with a switch SW, which makes it possible to synchronize the integration carried out by the aforementioned operational amplifier, the RC circuit and the resistor R 'previously mentioned.

During a gradual command, the operational amplifier 40 controlled by the switch SW makes it possible to compare the voltage VR3 across the resistor R3, image of the current IL, with the internal reference voltage Vref, and then processes the error between the two voltage values by integration according to a proportional and integral loop of the voltage level of the pulses.

The integration constant is set by the value of the time constant

R'C. The output terminal 44 delivers a voltage value that adapts according to the result of the integration performed by the proportional and integral servocontrol loop.

The value of the voltage applied to the input terminal 42 'is given by the relation (1), for R being much greater than R': $$\mathrm{Vop}\left(\mathrm{t}\right)\mathrm{=}\left[\mathrm{1}\mathrm{/}\mathrm{R\text{'}C}\right]\mathrm{*}\left[\mathrm{\int }\left(\mathrm{Vref}\mathrm{-}\mathrm{VR}\mathrm{3}\right)\mathrm{d}\mathrm{t}\right]\mathrm{.}$$

In this relation, Vop (t) denotes the output voltage of the operational amplifier 40.

In the particular embodiment of the device which is the subject of the invention, as represented in figure 3 , the establishment time tD of the figure 2 is generated between the activation of the proportional and integral loop on switching of the switch SW and the delayed effective control of the switching transistor T3.

• soit on calcule tD, le temps d'établissement, car les valeurs de R ,R', C, Vref sont connues et VR3 = 0, car le transistor T3 n'est pas encore commuté, le courant IL étant égal à zéro, et la valeur cible de la tension de sortie de l'amplificateur opérationnel 40 est également connue et égale à la valeur VE, valeur du régime établi. Il est alors possible de calculer la valeur du temps d'établissement tD par la relation (2) : $$\mathrm{Vop}\left(\mathrm{tD}\right)=\left[1/\mathrm{RʹC}\right]*\left[\mathrm{Vref}*\mathrm{tD}\right]=\mathrm{VE},$$
  
  $$\mathrm{tD}=\mathrm{RʹC}\left[\mathrm{VE}/\mathrm{Vref}\right];$$
  
• soit on mesure et détecte l'atteinte de la tension d'établissement VE par la tension de sortie de l'amplificateur opérationnel Vop(t).

As mentioned previously in the description relative to the gradual control method that is the subject of the invention, the value of the output voltage of the operational amplifier can be obtained in two different ways:

• either tD, the settling time is calculated, since the values of R, R ', C, Vref are known and VR3 = 0, since the transistor T3 is not yet switched, the current IL being equal to zero, and the target value of the output voltage of the operational amplifier 40 is also known and equal to the value VE, value of the steady state. It is then possible to calculate the value of the establishment time tD by the relation (2): $$\mathrm{Vop}\left(\mathrm{tD}\right)=\left[1/\mathrm{R\text{'}C}\right]*\left[\mathrm{Vref}*\mathrm{tD}\right]=\mathrm{VE},$$
  
  $$\mathrm{tD}=\mathrm{R\text{'}C}\left[\mathrm{VE}/\mathrm{Vref}\right];$$
  
• or the attainment of the set-up voltage VE is measured and detected by the output voltage of the operational amplifier Vop (t).

This output voltage is linked by the gain M of the DC-DC converter to the cyclic ratio α of the latter which itself defines the gain or voltage rise ratio M = VLED / VBAT, in which relation VLED designates the voltage at the terminals of the load 2 and VBAT designate the supply voltage to the terminals 5 and 5 'of the device 1 object of the invention, that is to say the battery voltage of the vehicle, with M = [1 / (1- α)].

Thus, the steady state value VE of the voltage pulses can be calculated when VLED, VBAT and the gain M of the DC-DC converter are known.

• soit de fixer tD pour détecter la valeur correspondante de la tension de sortie de l'amplificateur opérationnel 40 Vop(t), ainsi que décrit précédemment, et finalement engendrer les impulsions calibrées IC successives ; des ressources de calcul de tD selon la relation (2) précédemment mentionnée peuvent alors être prévues.
• soit de mettre en oeuvre un système adaptatif ainsi que représenté à la figure 3, lequel peut ajuster la tension de sortie de l'amplificateur opérationnel 40 Vop(t) en mesurant par échantillonnage par exemple les valeurs VLED , VBAT , le gain du convertisseur continu-continu 3 étant connu.

According to a remarkable aspect of the device and method objects of the invention, it is therefore quite possible:

• either to set tD to detect the corresponding value of the output voltage of the operational amplifier 40 Vop (t), as described above, and finally to generate the successive calibrated pulses IC; calculation resources of tD according to the previously mentioned relation (2) can then be provided.
• to implement an adaptive system as well as represented in the figure 3 which can adjust the output voltage of the operational amplifier 40 Vop (t) by sampling for example the values VLED, VBAT, the gain of the DC-DC converter 3 being known.

In both cases, reaching the set speed VE makes it possible to control, by a control pulse delivered on the terminal 45 of the control circuit 4, the transistor T3 to apply the calibrated pulse IC and supply the LEDs constituting the load 2 .

The voltage at the terminals of the load 2, VLED, varies slowly as a function of the temperature but also in the case where the current in the LEDs is changed, by changing the reference voltage Vref. The voltage VLED is also changed from one mode of implementation to another, that is to say for different lighting or signaling functions, when of course we change the type and number of LEDs constituting the charge 2.

The most important change in the voltage across the load 2, VLED, occurs if the type and number of light-emitting diodes is changed. As a result, the measurement of the VLED value must only be performed when the current IL flowing in the load 2 is stabilized, ie for VR3 = Vref. For this purpose, it can be envisaged that at startup of the device 1 object of the invention the first phase at the high voltage value of the pulses generated by the gradual control is chosen longer, width of the larger pulse, in order to perform a measurement of the VLED value at the end of this phase. Fast electronics can be chosen accordingly.

On the Figures 4a and 4b the timing diagrams of the signals obtained by implementing the method according to the invention according to the embodiment of the device represented in FIG. figure 3 , for a load consisting of ten LEDs and four LEDs respectively.

The high and low levels of the signals are denoted 0A, 1A; 0B, 1B; 0C, 1C and 0D, 1D for the signals represented by the curves A, B, C, D and IC respectively. It can be seen that the establishment time tD2 for 4 LEDs is less than the establishment time tD1 for 10 LEDs, all things being equal, the successive switching times of the switches SW, T3 being represented by dotted lines identified by SW on, T3 on, T3 off. It is noted, in particular, that the switching control of T3, directly managed by the control circuit 4, causes the drop in intensity of the current IL delivered to the LEDs.

Claims (8)

Method for gradually controlling the light intensity of illumination of light-emitting diodes by modulating the pulse width of electrical current with a given duty cycle, from a DC-DC voltage converter and a switching control circuit supply current of the light-emitting diodes, characterized in that it consists, at least in succession, of the power supply of the diodes, to: - Start the DC-DC converter, to generate voltage pulses determined duty cycle; - Determine the established regime of each pulse, to generate a voltage level calibrated voltage pulse corresponding substantially to the level of the established regime; then, supplying the light-emitting diodes by applying each calibrated pulse to the light-emitting diodes by means of the switching control circuit. A method according to claim 1, characterized in that the step of determining the steady state of each pulse comprises detecting the amplitude level of each pulse with respect to a reference value. A method as claimed in claim 1 or 2, characterized in that the step of determining the steady state of each pulse comprises calculating the established rate of establishment of each pulse. Method according to Claim 3, characterized in that for a switching control circuit comprising a proportional and integral servocontrol of the electrical voltage level of the pulses, the latter consists of: - calculate the establishment time of each pulse; and, - Shift the application of each calibrated pulse LEDs with a delay time substantially equal to the time of establishment. Device for gradually controlling the light intensity of illumination of light-emitting diodes by pulse-width modulation of determined duty cycle, comprising a DC-DC voltage converter and a current-supply pulse switching control circuit electroluminescent diodes, characterized in that it further comprises: means for determining the established speed of each pulse delivered by the DC-DC converter, in order to generate a calibrated pulse; and, on reaching this established regime, means for controlling the supply of the light-emitting diodes by applying each calibrated pulse to the light-emitting diodes. Device according to claim 5, characterized in that the means for determining the steady state of each pulse comprise means for detecting the amplitude level of each pulse with respect to a reference value. Device according to claim 5 or 6, characterized in that the means for determining the steady state of each pulse comprise means for calculating the establishment time of each pulse. Device according to Claim 7, characterized in that , for a switching control circuit comprising a proportional and integral control loop of the voltage level of the pulses, the latter comprises: means for calculating the rise time of each pulse; and, a circuit for delaying the application of each calibrated pulse to the electroluminescent diodes with a delay time substantially equal to the rise time of each pulse.

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