EP0793403A1 - Improvement in LED circuits, especially for vehicles, signalisation lights and control panels - Google Patents

Abstract

The illumination circuit includes three parallel branches of LEDs forming "i" lines and "j" columns. The network is connected with each branch connected in series and individual LEDs paralleled up, forming a total of "ij" LEDs. Some of the branches have resistors in parallel (22,23). Each LED circuit has a series resistance and zener diode (Ri) placed in series with it.

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.

It is more and more common to use as a light source in a motor vehicle equipment 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, in particular a traditional rear light.

In Figure 1, there is shown an electrical diagram of an illumination circuit according to the prior art. The illumination circuit comprises an array of LED light-emitting diodes, which is supplied by the 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 mixed parallel / series arrangement comprising, in series, four groups of four light-emitting diodes in parallel. Reference will be made hereinafter to the “branches” of the network to designate the electrical branches within the meaning of Kirchoff's laws (law of meshes and law of nodes) which each contain one of said light-emitting diodes LED in the diagram of FIG. 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 connection terminals 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 substantially rectilinear current / voltage characteristic and a relatively small slope in the range of variation of the voltage at the battery terminals, light-emitting diodes are components which exhibit a high current excursion for small voltage excursions around a nominal conduction voltage.

However, in a vehicle, the voltage at the terminals of the battery is liable to vary quite considerably as a function of its state of charge and of the ambient temperature, conventionally between 11 and 16 volts.

In addition, the nominal conduction voltage of commercially available light-emitting diodes presents, by construction, a dispersion which can be significant over a given set of components, since it is for example guaranteed by the manufacturer only between 1, 9 and 2.7 volts.

From these two factors it follows that the current in the light-emitting diodes can vary appreciably according to the light-emitting diodes used and according to the operating conditions, which generates variations in light intensity which can go 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 having an illumination characteristic that is slightly variable as a function of the excursion of the voltage across the terminals of the battery and of the dispersion on the nominal conduction voltage of the light-emitting diodes used.

In order to clarify the description which follows, it is considered that the mixed parallel / series network 30 of FIG. 1 is made up of "rows" and "columns" of light-emitting diodes LED, 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.

. Par exemple, on connaît des réseaux de seize diodes électroluminescentes à quatre lignes et quatre colonnes (N=16 ; i=4 ; j=4), des réseaux de douze diodes électroluminescentes à trois lignes et quatre colonnes(N=12 ; i=3 ; j=4), ou encore des réseaux de vingt diodes électroluminescentes à quatre lignes et cinq colonnes(N=20 ; i=4 ; j=5).It is known to have a determined number N of light-emitting diodes according to a mixed parallel / series arrangement such that $\text{ij = N}$

. For example, arrays of sixteen four-row and four-column light-emitting diodes (N = 16; i = 4; j = 4) are known, arrays of twelve three-row and four-column light-emitting diodes (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 which must be used in the application envisaged is determined as a function of the overall illumination required, and a number N of diodes is chosen which is just greater than said minimum number and which s' write as the product of two natural integers i and j. It is in fact generally limited to the first number N which is suitable for reasons of cost of the components, of spatial space in the equipment, and of electrical consumption of the illumination circuit.

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

It is obvious that it is not necessarily possible to comply with 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, those skilled in the art would be led 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 at the terminals of the battery and the dispersion of the characteristics diodes, then it will need to use a set of eighteen diodes because this number is the first which is both greater than sixteen and which is a multiple of three.

However, essentially for space constraints in a housing, 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 making it possible to reconcile all of the construction rules set out above.

In fact, the invention relates to an illumination circuit with light-emitting diodes supplied by a direct voltage through a supply circuit, in particular for motor vehicles, which is characterized in that it comprises a number N of light-emitting diodes arranged in a mixed parallel / series network comprising i rows and j columns, the product ij being equal to a number M which is greater than said number N, the MN branches of said network which do not comprise a light-emitting diode comprise 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 a 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 total current Itot which is consumed by the illumination circuit. This current is the sum of the j currents (here j = 4) flowing through 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 light-emitting diodes LED. This resistance must be chosen so as to be able to dissipate by a Joule effect a relatively large power, of the order of 7 Watts (W).

Such a resistor is a relatively expensive and bulky component, and, for reasons of space in the case for example of a light bar like a complementary brake light for a vehicle, must often be mounted by insertion on a separate printed circuit from the one 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 comprising such a diode.

Advantageously, the resistors Ri are components for surface mounting (SMD) which are very small and which exist at relatively low prices on the market. This also has the advantage that the resistors can be deposited by a machine and wave soldered at the same time as the LED light-emitting diodes, on the same printed circuit.

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

This circuit is supplied via two connection terminals 28 and 29. It comprises a network 30 produced according to a mixed parallel / series arrangement, some branches of which comprise a light-emitting diode LED in series with a resistance Ri, and of which d other branches include an essentially resistive dipole 22 or 23, the exact nature of which will be explained below with reference in particular to FIGS. 3a to 3d.

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

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

The 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 them using '' a programmable automatic machine, with LED light-emitting diodes and Ri resistors on the same printed circuit, and to solder them to the wave or by remelting in a single operation.

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

The practical reasons for using a network comprising three lines will be explained below, with reference to FIG. 4, when, for example, it is an application in which only sixteen light-emitting diodes LED are required to reach the light radiation. desired and in which, for reasons of cost and space available in the light to receive the illumination circuit, we can not afford to use eighteen light emitting diodes, it would be simpler to connect said sixteen diodes (N = 16) in a network of four lines and four columns.

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

According to another embodiment, the M-N branches of the network which do not include a light-emitting diode LED comprise an open circuit instead of any dipole, that is to say that they do not understand anything. This produces a network which can be described as irregular since, necessarily, each line i does not include the same number j of diodes in parallel, with the consequence that the diodes of two different lines are not necessarily traversed by the same current and therefore do not produce the same illumination.

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

In FIGS. 3a to 3d, various possible, but nonlimiting, embodiments have been shown of the dipole 22 or 23 which can be arranged in certain branches of the network 30 to replace the MN light-emitting diodes missing to complete this network with M branches .

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

In FIG. 3b, it comprises 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 light-emitting diodes LED 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 includes a reference of resistors. This limits the multiplicity of references during procurement and allows consumption of a larger quantity of the same components, therefore obtaining lower prices from suppliers.

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

In FIG. 3d, the dipole comprises a resistor 36 of the same value as the N resistors Ri, in series with a Zener diode 37 whose conduction voltage will advantageously be chosen close to the conduction voltage of the light emitting diodes LED used in the N others branches of the network.

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

, il vient : $$\text{Itot =}\frac{\text{M}}{{\text{i}}^{\text{2}}}\text{·}\left(\frac{\text{Vbat ― 0,9}}{\text{Ri}}\right)\text{―}\frac{\text{M}}{\text{i}}\text{·}\frac{\text{Vd}}{\text{Ri}}$$

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 $\text{ij = M}$

, he comes : $$\text{Itot =}\frac{\text{<figure-callout id="2" label="M i" filenames="imgf0002.png" state="{{state}}">M </figure-callout>}}{{\text{i}}^{}}\text{·}\left(\frac{\text{Vbat - 0.9}}{\text{Ri}}\right)\text{-}\frac{\text{M}}{\text{i}}\text{·}\frac{\text{Vd}}{\text{Ri}}$$

, d'où il vient : $$\text{Ri =}\frac{\text{1}}{\text{In}}\text{·}\left(\frac{\text{Vbato ― 0,9}}{\text{i}}\text{― Vdo}\right)$$

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 resistors Ri must be used having such a value that we will have $\text{Itot = j.In = M.In / i}$

, where it comes from : $$\text{Ri =}\frac{\text{1}}{\text{In}}\text{·}\left(\frac{\text{Vbato - 0.9}}{\text{i}}\text{- Vdo}\right)$$

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: $$\text{Itot = M · In ·}\frac{\text{(Vbat - 0.9 - iVdo)}}{\text{i · [(Vbato - 0.9) - i · Vdo}}$$

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

This term α translates the dependence of the total current Itot consumed by the circuit with respect to variations in the voltage Vbat at the terminals of the battery. It can be shown, for example by plotting the curve of the above function 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 parallel / serial network comprising three lines (i = 3) has a minimum dependence on variations in the voltage across the terminals of the battery.

Furthermore, by rewriting equation (1) of the mesh for any value Vd of the conduction voltage of the diodes electroluminescent LEDs, and by integrating the expression (2) of Ri, it comes: $$\text{Itot =}\frac{\text{M · In · (Vbat - 0.9)}}{\text{i · [(Vbato - 0.9) - i · Vdo]}}\text{-}\frac{\text{M · In · Vd}}{\text{(Vbato - 0.9) - i · Vdo}}$$

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

It is clear that the smaller i is, the smaller the coefficient β. This means that the lower the number of lines of the parallel / series network, the lower the dependence of the total current Itot consumed by the circuit on the dispersion of the conduction voltage of the diodes, which is intuitively understood. .

From the above considerations, it therefore appears that it is advantageous to use a parallel / series network comprising three lines (i = 3) to minimize the influence on the total current consumed by the circuit, that is to say on the 'global illumination produced by this circuit, both with respect to the excursion of the voltage across the battery and the dispersion on the conduction voltage of light emitting diodes used.

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

et $\text{i=j=4}$

( figure 1), et référencée 42 pour un second circuit utilisant un réseau selon l'invention, i.e., tel que M=18, N= 16, i=3 et j=6 (figure 2)In FIG. 4, for example, the characteristics of an illumination circuit respectively referenced 41 have been represented for a first circuit using a network according to prior art, ie, such as $\text{M = N = 16}$

and $\text{i = j = 4}$

(Figure 1), and referenced 42 for a second circuit using a network according to the invention, ie, such that M = 18, N = 16, i = 3 and j = 6 (Figure 2)

For a voltage Vbat at the terminals of the battery equal to Vbato = 13.5V, the first circuit consumes a current I1 equal to approximately 220mA and the second circuit consumes a current I2 equivalent to approximately 330mA because it has six columns of diodes instead of four.

It can be seen that the straight line 42 is more flattened than the straight line 41, which reflects the lower dependence of the total current Itot on variations in the voltage Vbat at the terminals of the battery.

In addition, the straight line 42 is capable of moving parallel to itself as a function of the dispersion of the conduction voltage Vd of the light-emitting diodes with an excursion E2 smaller than the corresponding excursion E1 of the straight line 41, which translates the lower dependence of the total current Itot on 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 less than said value Imax1.

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

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

Claims (12)

Illumination circuit with light-emitting diodes supplied by a direct voltage through a supply circuit, in particular for motor vehicles, which is characterized in that it comprises a number N of light-emitting diodes (LED) arranged in an array (30) mixed parallel / series comprising i rows and j columns, the product ij being equal to a number M which is greater than said number N, the MN branches of said network which do not comprise a light-emitting diode comprise either an open circuit or a first essentially resistive dipole (22.23). Circuit according to Claim 1, characterized in that the network comprises a number of lines i equal to three. Circuit according to claim 1, characterized in that each light-emitting diode is connected in series with a second essentially resistive dipole comprising a resistor (Ri). Circuit according to claim 1, characterized in that the first essentially resistive dipole (22,23) comprises at least one resistor (31); Circuit according to claim 4, characterized in that the first essentially resistive dipole (22,23) comprises two resistors (32 and 33) of the same value as the resistance (Ri) of said second essentially resistive dipole. Circuit according to claim 1, characterized in that the first essentially resistive dipole comprises a resistor (34) and a diode (35) in series. Circuit according to claim 1, characterized in that the first essentially resistive dipole comprises a resistor (36) and a Zener diode (37). Circuit according to Claim 7, characterized in that the said resistance (36) is of the same value as the resistance (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). Circuit according to Claim 1, characterized in that the supply circuit comprises a diode (D) in series, and a varistor (VDR) in parallel. Circuit according to Claim 9, characterized in that the supply circuit also comprises a Zener diode (DZ) and a capacitor (C) in parallel. Signaling light for a motor vehicle, characterized in that it comprises an illumination circuit according to one of claims 1 to 10. Control panel, in particular for a heating, ventilation and / or air conditioning installation for a motor vehicle, characterized in that it comprises an illumination circuit according to one of claims 1 to 10.

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