EP0793403B1 - Improvement in LED circuits, especially for vehicles, signalisation lights and control panels - Google Patents

Description

The present invention relates to an improvement to light-emitting diode illumination circuits, in particular for motor vehicles, as well as a device such as a signaling light or a control panel, incorporating it. Such circuits are known for example from EP-A-0 695 112.

It is more and more common to use as a light source in equipment for a motor vehicle a set of light emitting diodes to replace one or more filament lamps used conventionally. It may in particular be a complementary brake light, a side repeater or a signaling light, especially traditional rear.

In Figure 1, there is shown an electrical diagram of an illumination circuit according to the prior art. The circuit illumination includes an array of LED light emitting diodes, which is powered by battery voltage through a supply circuit, via two connection terminals 28 and 29.

The array of LED light-emitting diodes comprises in this case sixteen diodes connected in a circuit mixed parallel / series comprising, in series, four groups of four light-emitting diodes in parallel. We are will refer in the following to the “branches” of the network to designate the electrical branches within the meaning of the laws of Kirchoff (law of the meshes and law of the nodes) which each contain one of the said light-emitting diodes LED on the diagram of figure 1.

Such an assembly makes it possible to reduce the overall consumption of the circuit compared to an assembly which would include the sixteen diodes in parallel, and allows the illumination circuit to continue to function correctly in the event of failure of one of the light-emitting diodes, which would not be the case in an assembly which would include the sixteen diodes in series.

The supply circuit conventionally comprises a VDR varistor connected in parallel between the terminals of connection 28 and 29, a diode D and a resistor R in series, as well as a Zener diode DZ and a capacitor C in parallel.

However, unlike conventional filament lamps which have a current / voltage characteristic substantially straight and of relatively low slope in the range of variation of the voltage at the battery terminals, light emitting diodes are components that exhibit a large current excursion for low voltage excursions around a nominal conduction voltage.

However, in a vehicle, the voltage across the battery is likely to vary quite significantly in depending on its state of charge and the ambient temperature, conventionally between 11 and 16 volts.

In addition, the nominal conduction voltage of commercially available light-emitting diodes is present, by construction, a dispersion which can be significant over a given set of components, since it is not for example guaranteed by the manufacturer that between 1.9 and 2.7 volts.

From these two factors it follows that the current in the light-emitting diodes can vary appreciably depending on the light emitting diodes used and the operating conditions, which causes variations light intensity up to a ratio of 1 to 3.

The present invention aims to propose means for carrying out an assembly of light-emitting diodes of the type of a mixed parallel / series network with a slightly variable lighting characteristic as a function of the excursion of the voltage across the battery and of the dispersion over the nominal conduction voltage of the diodes electroluminescent used.

In order to clarify the description which follows, it is considered that the mixed parallel / series network 30 of FIG. 1 consists of "Rows" and "columns" of LED light emitting diodes, by analogy with the mathematical concept of matrix.

Thus, we denote i the number of lines of LED light-emitting diodes put in series in the network 30, and j the number of columns of LED light-emitting diodes placed in parallel in said array.

It is known to have a determined number N of light-emitting diodes in a mixed parallel / series arrangement. such as i.j = N. For example, we know arrays of sixteen light-emitting diodes with four lines and four columns (N = 16; i = 4; j = 4), arrays of twelve three-line light-emitting diodes and four columns (N = 12 ; i = 3; j = 4), or again arrays of twenty light-emitting diodes with four lines and five columns (N = 20 ; i = 4; j = 5).

In practice, the minimum number of light-emitting diodes to be used in the application is determined. envisaged as a function of the overall illumination required, and a number N of diodes is chosen which is just greater said minimum number and which is written as the product of two natural integers i and j. We generally limit ourselves the first number N which is suitable for reasons of cost of the components, of space requirement in the equipment, and the power consumption of the lighting circuit.

However, as will be explained in more detail in the description below, it has been found that some mixed parallel / series network configurations reduce the influence of terminal voltage variations of the battery and the dispersion of the conduction voltage of the diodes on the total current consumed by the circuit of illuminations, that is to say on the illumination produced by said circuit. These observations lead to choosing a number i determined lines of the mixed parallel / series network.

It is obvious that it is not necessarily possible to respect this new constraint without questioning the choices made according to the classic rules for building such a network.

Thus, if it has been calculated that a minimum of sixteen light-emitting diodes should be used to produce the illumination required in a given application, a person skilled in the art would have to arrange these sixteen diodes in a network with four lines and four columns (or incidentally a network with two lines and eight columns or vice versa). But if, as the invention teaches, it is found that a network comprising three lines produces better results in terms of excursion of the illumination as a function of the variations of the voltage across the terminals of the battery and of the dispersion of the characteristics of the diodes, then it will have to use a set of eighteen diodes because this number is the first which is both greater than sixteen and a multiple of three.

However, essentially for space constraints in a box, it may prove impossible to use eighteen diodes where it was possible to use sixteen.

The invention aims to overcome the drawbacks of the aforementioned state of the art by proposing means allowing to reconcile all of the building rules set out above.

Indeed, the invention relates to an illumination circuit with light-emitting diodes supplied by a voltage continues through a supply circuit, in particular for motor vehicles, which comprises a number N of light-emitting diodes arranged in a mixed parallel / series network comprising i rows and j columns, characterized in that the product i.j is equal to a number M which is greater than said number N, the M-N branches of said network which does not include a light-emitting diode comprising either an open circuit or a first dipole essentially resistive.

• le réseau comprend un nombre de lignes i égal à trois ;
• chaque diode électroluminescente est connectée en série avec un second dipôle essentiellement résistif comprenant une résistance ;
• le premier dipôle essentiellement résistif comprend au moins une résistance ;
• le premier dipôle essentiellement résistif comprend deux résistances de même valeur que la résistance dudit second dipôle essentiellement résistif ;
• le premier dipôle essentiellement résistif comprend une résistance et une diode en série ;
• le premier dipôle essentiellement résistif comprend une résistance et une diode Zener ;
• la résistance ci-dessus est de même valeur que la résistance du premier dipôle essentiellement résistif et la tension seuil de la diode Zener est de même valeur que la tension de conduction des diodes électroluminescentes ;
• le circuit d'alimentation comprend une diode en série, et une varistance en parallèle ;
• le circuit d'alimentation comprend également une diode Zener et un condensateur en parallèle.

According to other characteristics of the invention, taken individually or in combination:

• the network comprises a number of lines i equal to three;
• each light emitting diode is connected in series with a second essentially resistive dipole comprising a resistor;
• the first essentially resistive dipole comprises at least one resistor;
• the first essentially resistive dipole comprises two resistors of the same value as the resistance of said second essentially resistive dipole;
• the first essentially resistive dipole comprises a resistor and a diode in series;
• the first essentially resistive dipole comprises a resistor and a Zener diode;
• the above resistance is the same value as the resistance of the first essentially resistive dipole and the threshold voltage of the Zener diode is the same value as the conduction voltage of the light-emitting diodes;
• the supply circuit comprises a diode in series, and a varistor in parallel;
• the power supply circuit also includes a Zener diode and a capacitor in parallel.

The invention also relates to a signaling light for a motor vehicle, comprising an illumination circuit as described above.

Finally, it relates to a control panel, in particular for heating, ventilation and / or air conditioning installation. for a motor vehicle, comprising an illumination circuit as above.

• A la figure 1, déjà analysée, un circuit d'illumination selon l'art antérieur ;
• A la figure 2 : un circuit d'illumination selon la présente invention ;
• A la figure 3 : un diagramme de la caractéristique du courant total consommé par le circuit selon l'invention, en fonction de la tension de la batterie.

Other characteristics and advantages of the present invention will appear on reading the nonlimiting description which follows, with reference to the appended drawings which are:

• In FIG. 1, already analyzed, an illumination circuit according to the prior art;
• In Figure 2: an illumination circuit according to the present invention;
• In FIG. 3: a diagram of the characteristic of the total current consumed by the circuit according to the invention, as a function of the battery voltage.

If we look again at the circuit of Figure 1, we see that the resistance R is crossed by the current total Itot which is consumed by the lighting circuit. This current is the sum of the j currents (here j = 4) traversing the j branches of light-emitting diodes connected in parallel in the network 30.

The value of the resistance R is determined as j times the value of the nominal current In which must flow in the LED light emitting diodes. This resistance must be chosen so as to be able to dissipate by joule effect a relatively large power, of the order of 7 Watts (W).

Such resistance is a relatively expensive and bulky component, and, for reasons of space in for example the case of a light bar such as a complementary brake light for a vehicle, must often be mounted by insertion on a separate circuit from that on which the LED light-emitting diodes are mounted.

This is why, according to an advantage of the circuit according to the invention as shown in FIG. 2, it is replaced by resistors Ri of smaller value and having to dissipate much lower powers (of the order of 0.25W or 0.5W), which are connected in series with the LED light-emitting diodes in each branch of the network 30 having such a diode.

Advantageously, the resistors Ri are components to surface mount (CMS) which are very small and which exist at relatively low prices on the market. This presents in in addition to the advantage that the resistors can be deposited by a machine and welded to the wave at the same time as LED light-emitting diodes, on the same printed circuit.

In Figure 2, on which on the same elements as in Figure 1, have the same references, there is shown a mode of possible realization of an illumination circuit according to the invention.

This circuit is supplied via two terminals of connection 28 and 29. It includes a network 30 produced according to a mixed parallel / series mounting, some branches of which include a light emitting diode LED in series with a resistor Ri, and of which other branches essentially include a dipole resistive 22 or 23 whose exact nature will be explained later with particular reference to Figures 3a to 3d.

The supply circuit includes a diode D mounted in series for protection against reverse polarity between connection terminals 28 and 29, as well as a mounted VDR varistor in parallel and to protect the illumination circuit against variations in the battery voltage applied between connection terminals 28 and 29.

The supply circuit can also but not necessarily include a Zener DZ diode as well as a capacitor C connected in parallel with the diode network 30 LED light emitting lights.

Components D, VDR, DZ, and C above, existing on the market as surface mount components (CMS) it is possible, according to an advantage of the present invention, to deposit using a programmable automatic machine, with the LED light-emitting diodes and the resistors Ri on the same printed circuit, and solder them in the wave or by reflow in one operation.

Unlike the parallel / serial network in Figure 1, the parallel network / series 30 of figure 2 has three lines (i = 3) and six columns (j = 6) so that it includes a total of eighteen branches (M = 18).

We will explain later, with reference to Figure 4, the practical reasons for using a network comprising three lines while, for example being an application in which only sixteen LED light emitting diodes are required to achieve the desired light radiation and in which, for reasons of cost and space available in the light to receive the lighting circuit, we cannot allow to use eighteen light emitting diodes it would be easier to connect said sixteen diodes (N = 16) according a network with four lines and four columns.

According to the invention, the network therefore comprises N branches which include a light emitting diode LED optionally but not necessarily in series with a resistance Ri, and comprises M-N branches which include an essentially resistive dipole referenced 22 and 23 in Figure 2.

According to another embodiment, the M-N branches of the network that does not include a LED light emitting diode include an open circuit instead of any dipole, that is, they understand nothing. This produces a network which can be described as irregular since, necessarily, each line i does not include the same number j i of diodes in parallel, with the consequence that the diodes of two lines different are not necessarily crossed by the same current and therefore do not produce the same illumination.

This drawback may however be acceptable, in particular in case the lighting circuit is used in a fire comprising optical means such as prisms or lenses which attenuate the differences in diode illumination to produce uniform overall illumination.

In FIGS. 3a to 3d, different modes of possible, but not limiting, realization of dipole 22 or 23 which can be arranged in certain branches of network 30 to be substitute for the missing M-N light emitting diodes for complete this network with M branches.

In FIG. 3a, the dipole comprises a simple resistor 31.

In FIG. 3b, it includes two resistors 32 and 33 in series, which, in a preferred embodiment, each have the same value as the resistors Ri connected in series with the N LED light-emitting diodes in the N other branches of the network. So the programmable automatic machine store which places the components on the printed circuit during assembly in the factory advantageously only has a reference of resistances. This limits the multiplicity of references during supplies and allows to consume a larger quantity of the same components, therefore to obtain lower prices from suppliers.

In FIG. 3c, the dipole comprises a resistor 34 in series with a diode (a conventional junction diode).

In Figure 3d, the dipole includes a resistor 36 similarly value as the N resistors Ri, in series with a Zener diode 37 whose conduction voltage will advantageously be chosen close the conduction voltage of LED light-emitting diodes used in the N other branches of the network.

We are now interested in the graph of Figure 4 on which is shown, on the abscissa, the voltage Vbat across the terminals of the battery, and on the ordinate the total current Itot consumed by the illumination circuit of figure 2.

The equation resulting from Kirchoff's laws is written for a mesh comprising the diode D and a column of i light-emitting diodes LED in series with their i resistances Ri. Knowing that the current in these diodes is worth Itot / j with j the number of columns of the network, and eliminating the variable j by writing ij = M, it comes:

For a value of In which produces the desired illumination in the diodes equal to 55mA, for a voltage Vbat across the terminals of the battery having a fixed reference value Vbato (for example 13.5V) and for a conduction voltage Vd of the light-emitting diodes LED having a fixed reference value Vdo (for example 2.4V), we know that it is necessary to use resistors Ri having a value such that we will have Itot = j.In = M.In / i, hence it comes:

For example, for the values indicated above and with i = 3 and j = 6, we have Ri which is worth approximately 30 Ohms.

By rewriting the equation (1) of the mesh for any value Vbat of the voltage across the battery, and integrating into it the expression (2) of Ri, it comes: Itot = M · In · (Vbat - 0.9 - iVdo) i · [(Vbato - 0.9) - i · Vdo

We can isolate the factor α found before the term Vbat in the above expression, from which we derive: = M · In i · [(Vbato - 0.9) - i · Vdo

This term α translates the dependence of the total current Itot consumed by the circuit with respect to variations in voltage Vbat across the battery. We can show, for example by plotting the above function curve of α as a function of the variable i, that in the interval [1; 6], the coefficient α reaches a minimum for i close to three.

This is a demonstration of the fact that a network parallel / series comprising three lines (i = 3) presents a minimum dependence on variations in voltage at battery terminals.

Furthermore, by rewriting the equation (1) of the mesh for any value Vd of the conduction voltage of the light-emitting diodes LED, and by integrating with it the expression (2) of Ri, it comes: Itot = M · In · (Vbat - 0.9) i · [(Vbato - 0.9) - i · Vdo] - M · In · Vd (Vbato - 0.9) - i · Vdo

We can isolate the factor β found before the term Vd in the above expression, from which we draw: β = M · In (Vbato - 0.9 - iVdo)

It is clear that the smaller i is, the more the coefficient β is small. This means that the higher the number of lines in the network parallel / series, the lower the dependence on total current Itot consumed by the circuit compared to the dispersion of the diode conduction voltage is low, which is understandable moreover intuitively.

From the above considerations, it therefore appears that we have interest in using a parallel / serial network comprising three lines (i = 3) to minimize the influence on the total current consumed by the circuit, i.e. on the global illumination produced by this circuit, both with respect to the excursion of the voltage across the battery and across the conduction voltage of light emitting diodes used.

We have seen above that the invention proposes means for use a network with M branches, M decomposing like the product i.j with i = 3 and M being greater than the number N of diodes actually used in the network, these means consisting of substitute an open circuit or an essentially resistive dipole to the missing M-N diodes.

In FIG. 4, for example, the characteristics of an illumination circuit respectively referenced 41 for a first circuit using a network according to the prior art, i.e., such that M = N = 16 and i = j = 4 (Figure 1), and referenced 42 for a second circuit using a network according to the invention, i.e., such that M = 18, N = 16, i = 3 and j = 6 (Figure 2).

For a voltage Vbat across the battery terminals worth Vbato = 13.5V, the first circuit consumes a current I1 equal to about 220mA and the second circuit consumes an I2 current worth about 330mA due to the fact that it has six columns of diodes at instead of four.

We see that the line 42 is more flattened than the line 41 this which reflects the lower dependence of the total current Itot on screw of the variations of the voltage Vbat at the terminals of the battery.

In addition, the straight line 42 is liable to move parallel to itself depending on the dispersion of the conduction voltage Vd of light-emitting diodes with a excursion E2 lower than the corresponding excursion E1 of the right 41, which indicates the lower dependence of the current total Itot with respect to variations in said voltage Vd.

The total current Itot of the first circuit can vary between a minimum value Imin1 and a maximum value Imax1, while the second circuit can vary between a minimum value Imin2 greater than said value Imin1 and a maximum value Imax2 lower than said value Imax1.

The advantages of the invention can be summarized by saying that the Vbat / Itot operating point of the illumination circuit according to the invention can move depending on the voltage excursion across the battery and the voltage dispersion of conduction of light emitting diodes in a parallelogram 52 whose surface is much smaller than the surface corresponding to the parallelogram 51 of a circuit according to art prior.

It is obvious that the principle of the invention applies to an illumination circuit which can contain a number N any light emitting diode, and that the M-N circuits open or dipole completing the parallel / serial network at M can be arranged in any branches of said network.

Claims (12)

1. A lighting circuit having light-emitting diodes supplied with a unidirectional voltage via a power circuit, especially for motor vehicles, and having a number N of light-emitting diodes (LED) disposed in a mixed parallel/series network (30) that comprises i lines and j columns, characterised in that the product i.j is equal to a number M which is greater than the said number N, the M-N branches of the said network which do not include any light-emitting diode comprising either an open circuit or an essentially resistive first dipole (22, 23).
2. A circuit according to Claim 1, characterised in that the network comprises a number of lines i equal to three.
3. A circuit according to Claim 1, characterised in that each light-emitting diode is connected in series with an essentially resistive second dipole which includes at least one resistor (Ri).
4. A circuit according to Claim 1, characterised in that the first essentially resistive dipole (22, 23) includes at least one resistor (31).
5. A circuit according to Claim 4, characterised in that the first essentially resistive dipole (22, 23) includes two resistors (32 and 33) of the same value as the resistor (Ri) of the said second essentially resistive dipole.
6. A circuit according to Claim 1, characterised in that the first essentially resistive dipole includes a resistor (34) and a diode (35) in series.
7. A circuit according to Claim 1, characterised in that the first essentially resistive dipole includes a resistor (36) and a Zener diode (37).
8. A circuit according to Claim 7, characterised in that the said resistor (36) has the same value as the resistor (Ri) of the first essentially resistive dipole, and the threshold voltage of the Zener diode (37) has the same value as the conduction voltage (Vdo) of the light-emitting diodes (LED).
9. A circuit according to Claim 1, characterised in that the power circuit also includes a diode (D) in series and a variable resistance (VDR) in parallel.
10. A circuit according to Claim 9, characterised in that the power supply circuit further includes a Zener diode (DZ) and a capacitor (C) in parallel.
11. An indicating light for a motor vehicle, characterised in that it includes a lighting circuit according to one of Claims 1 to 10.
12. A control panel, especially for a heating, ventilating and/or air conditioning installation for a motor vehicle, characterised in that it includes a lighting circuit according to one of Claims 1 to 10.

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