EP1349434B1 - Lighting circuit, especially for cars - Google Patents

Description

The invention relates to a lighting circuit, in particular for motor vehicles. Such lighting circuits are increasingly constructed with light-emitting diodes as lamps. Depending on the application, a different number of LEDs is required. This raises the question of the circuit arrangement in which the LEDs are arranged. A series connection of all LEDs would have the disadvantage that all LEDs would fail if one LED fails. A pure parallel connection of all LEDs would, especially in motor vehicles, the disadvantage of high power loss, since the vehicle electrical system voltage, which is typically 12 volts, is substantially higher than the forward voltage of an LED, which is typically between 2V and 3V, and therefore a correspondingly high voltage drop on a series resistor to adapt to the LED flux voltage is necessary, which in turn causes a high power dissipation on the series resistor. A compromise between these two extremes is a combined series / parallel connection in which the parallel-connected rows are networked together to form a light-emitting diode matrix. Such a light-emitting diode matrix consists of n parallel-connected rows, each with m strings connected in series, wherein a light-emitting diode is arranged in each strand and runs between the strands of each row an electrically conductive cross-connection to the respective adjacent rows. The cross connections ensure that if one LED fails, the remaining LEDs can continue to be supplied with power and can continue to illuminate. Such a light-emitting diode matrix for motor vehicles is known, for example, from EP 0896 899A2. The cross connections between the rows are realized by cables or printed conductors whose ohmic resistance is negligibly small.

In most applications there is a requirement that the LEDs shine uniformly bright, so that a homogeneous lighting impression is produced. For this purpose, preferably only LEDs are installed in such a light-emitting diode, which lie in the same voltage class, the voltage class refers to the range of permissible Fußspannungen. However, the LEDs of a voltage class also differ in terms of their diode characteristic within a tolerance range. As a result, in unfavorable constellations, the currents through the different strands of the light-emitting diode matrix can be very different; in extreme cases, they can turn around differentiate the factor 2 and more. This in turn undesirably causes different brightnesses. In addition, this can lead to the rated currents of certain LEDs in the light-emitting diode matrix being significantly exceeded, which has a negative effect on the life of the LEDs.

In order to counter these disadvantages, it is proposed in EP 0 793 402 B1 to assign a resistor in series to each LED of a string. As a result, a similar linearization of different LED characteristics is achieved and thus a total of a uniform current distribution on the different strands / LEDs. The large number of resistors that is needed, however, is problematic because it involves high component and assembly costs. In addition, additional space is required for the large number of resistors. Another problem is that across the series-connected resistors in each case a voltage drops, which generates a thermal power loss. On the one hand, the thermal power loss reduces the efficiency of light generation and, on the other hand, the high temperatures generated by the thermal power dissipation cause an increased degradation of the LEDs.

The object of the invention is to provide a lighting circuit for an LED matrix, which overcomes the disadvantages listed above.

This object is achieved in that in at least one of the conductive cross-connections, an ohmic resistance is arranged. On resistances in the strands, which are each connected in series to an LED (see EP 0793 402 B1), can be dispensed with. The cross resistances allow potential separation between strings of adjacent parallel rows and at the same time the flow of a compensation current if the LED forward voltages are different. As a result, in the case of different forward voltages a more uniform current distribution through the different LED strands is achieved than in the prior art according to EP 0896 899A2. In contrast to EP 0793 402 B1 flows through the cross resistances only a thermal power dissipation generating compensation current when the LED forward voltages are actually different, while always according to EP 0793 402 B1 on the series resistors in each LED strand, even if the forward voltages are the same, the LED current flows and generates power loss there. In addition, by the solution according to the invention the Number of components significantly reduced. For example, in the case of an LED matrix consisting of 2 rows each having 3 strands, a maximum of 2 transverse resistors are required according to the invention in contrast to 6 series resistors according to EP 0 793 402 A1.

Figur 1

eine LED-Matrix gemäß dem Stand der Technik nach EP 0896 899A2,

Figur 2

eine LED-Matrix gemäß dem Stand der Technik nach EP 0793 402 B1,

Figur 3

eine erfindungsgemäße LED-Matrix gemäß einer ersten Ausführungsform,

Figur 4

eine erfindungsgemäße LED-Matrix gemäß einer zweiten Ausführungsform,

Figur 5

eine erfindungsgemäße LED-Matrix gemäß einer dritten Ausführungsform,

Figur 6

eine Widerstandssternschaltung, wie sie gemäß Ausführungsform von Figur 5 verwendet wird,

Figur 7

den Kennlinienverlauf zweier LEDs mit unterschiedlichen Flußspannungen,

Figur 8

den Einfluß des Querwiderstands auf eine gleichmäßige Stromverteilung.

Reference to the accompanying drawings, the invention will be explained in more detail below. It shows:

FIG. 1

an LED matrix according to the prior art according to EP 0896 899A2,

FIG. 2

an LED matrix according to the prior art according to EP 0793 402 B1,

FIG. 3

an LED matrix according to the invention according to a first embodiment,

FIG. 4

an LED matrix according to the invention according to a second embodiment,

FIG. 5

an LED matrix according to the invention according to a third embodiment,

FIG. 6

a resistance star circuit as used according to the embodiment of FIG. 5,

FIG. 7

the characteristic curve of two LEDs with different forward voltages,

FIG. 8

the influence of the transverse resistance on a uniform current distribution.

FIG. 1 shows an LED matrix according to the prior art according to EP 0896 899A2. It consists of a combined series / parallel circuit with 2 rows, each having 3 strands. In each strand an LED is arranged. Between the strands of each row an electrically conductive connection is arranged, the ohmic resistance is virtually equal to zero. The LED matrix is connected to the supply voltage via a series resistor (Rv). A protective diode serves as reverse polarity protection.

FIG. 2 shows an LED matrix according to the prior art according to EP 0793 402 B1. There, a series resistor of a LED is connected in series in each strand.

FIG. 3 shows a lighting circuit with a 6-LED matrix, as in FIG. 1, but according to the invention, a transverse resistance is incorporated in the cross-connections between the strands. The effect achieved according to the invention, which is achieved with the transverse resistances, will be explained with reference to FIGS. 7 and 8. FIG. 7 shows the characteristics for the LED with the number 4 and the LED with the number 3. The LED with the No. 4 should have a forward voltage at the lower edge of the tolerance range of a voltage class while the forward voltage of the # 3 LED is at the top. The forward voltages of the remaining LEDs (1, 2, 5, 6) are the same and lie in the middle of the tolerance band. The rated current of the LEDs is 50 m amperes. Without the transverse resistance according to the invention, ie with a short-circuited cross connection (Rq = 0) according to the prior art (see FIG. 1), the LED currents through the LEDs 3 and 4 are very different. At Rq = 0, the current through the # 3 LED is only 39m amperes, while the current through the # 4 LED at 61m amps is above the rated current. This current difference causes a different brightness of the LEDs and permanently causes a degradation of the LEDs. With increasing transverse resistance then decreases the current difference, the currents approach the rated current. From approx. 80 Ω, the currents have then approximated to a maximum of the nominal current. If the forward voltages of the LEDs are identical, no current flows through the cross resistances, thus no power dissipation is generated. The LED matrix is connected via series resistors to the supply voltage (Vcc). In this case, the number of LEDs connected in series in accordance with the forward voltage (approximately 2 to 3V) can be selected such that the voltage drop across all LEDs connected in series (functional voltage limit) approaches as close as possible to the lower limit of the supply voltage due to fluctuation. In a motor vehicle, the supply voltage can drop in the short term from normally 14 volts to 9 volts. The series resistor is used to adjust the current flowing into the LED matrix. In order to evenly distribute the power loss, two or more series-connected series resistors (Rv) are preferably provided.

In contrast to the lighting circuit according to EP 0 793 402 B1, where the series resistors are also arranged in the LED matrix in series with the LEDs, in the lighting circuit according to the invention, the LED matrix with the thermally low-loaded transverse resistances in the housing of a filament (eg headlights or tail lamp), while the or the series resistors can be arranged outside the housing. Thus, the disturbing heat loss is kept away from the interior of the housing. In the prior art according to EP 0 793 402 B1, the series resistors are included as part of the LED matrix in the housing. In addition, according to the invention, in contrast to EP 0 793 402 B1, the equalizing currents can be adjusted independently of the load current through the LEDs.

In an embodiment not shown, the LED matrix is powered by a constant current source that provides a constant current independent of voltage fluctuations. In this case, a low, optimally adapted to the functional voltage limit supply voltage can be selected because the voltage drop across the series resistors in the strands according to EP 0 793 402 B1 is omitted.

FIG. 4 shows the embodiment of a lighting circuit with a 9-LED matrix consisting of 3 rows connected in parallel, each having three strings. In this case, a transverse resistance is arranged between the first and the second strand of the first row and between the first and the second strand of the second row. Likewise, a transverse resistance is arranged between the first and the second strand of the second row and between the first and the second strand of the third row. Corresponding transverse resistances can also be found between the second and third strands of the respective adjacent rows. In this respect, the lighting circuit according to Figure 4 is analogous to the lighting circuit of Figure 3. In contrast to Figure 3, however, is added that also between the first and second strand of the first row and between the first and second strand of the third row and between the second and third strand of the first row and between the second and third strand of the third row, a transverse resistance is arranged. Thus, not only the adjacent rows are connected via transverse resistances, but also the outer rows. Due to the quasi-annular networking and the outer rows by cross resistances a better symmetry of the LED matrix is achieved, which ensures that even if one LED fails in an "outer" row as uniform as possible power distribution. Due to the annular cross-linking, it makes no difference whether an LED fails in an inner or an outer row or has a different characteristic. A distinction between inner rows and outer rows is thus actually no longer present.

In the embodiment according to FIG. 5, the transverse resistances are each arranged in a star connection of m (= row number of the LED matrix) transverse resistances. In this case, the transverse resistances are each with their one end with a common Sternkontenpunkt connected and connected at their other end in each case with a row in the area between two strands. In FIG. 6, this star connection is represented once more for the sake of clarity. The use of such a star connection allows greater freedom in the layout of the printed circuit board on which the LED matrix is arranged.

Claims (5)

1. Lighting circuit, specifically for motor vehicles, featuring a matrix of light-emitting diodes (LED matrix) in a combined series/parallel connection that direct current and/or a constant current is supplied to by a power supply circuit, where the LED matrix is made up of n rows connected in parallel, each row containing m bead chains connected in series, where every chain has a light emitting diode (bead) and where the beads on every row have a conductive cross connection to each of the neighboring rows,
   wherein
   at least one shunt resistor is contained in at least one of the aforementioned cross connections.
2. Lighting circuit as in claim 1,
   wherein,
   if three or more rows are connected in parallel, there is a cross connection containing at least one resistor between at least one first chain and a second chain of the first row and between the same first and second chains of the last row.
3. Lighting circuit as in claim 2,
   wherein
   the design contains at least one star connection between m shunt resistors, where one end of each shunt resistor connects to a shared star node and each other end separately connects to a row somewhere along the stretch between two chains.
4. Lighting circuit as in one of the above claims,
   wherein
   at least one ballast resistor in connected in series with the LED matrix.
5. Lighting circuit as in claim 4,
   wherein
   the LED matrix with the shunt resistors resides in the housing of a luminous element whereas the at least one ballast resistor is located outside the housing.

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