EP3426007B1 - Vehicle lighting device with power control - Google Patents

Description

The invention relates to a motor vehicle lighting device according to the preamble of claim 1.

Generic motor vehicle lighting devices are known from practice.

Outside the automotive field, it is known to operate LEDs from a constant current source or from a constant voltage source of 12V, for example. Power supplies ensure compliance with the desired current or voltage values. In the automotive sector, when known, generic lighting devices are installed, it is apparently often assumed that a specific target voltage of, for example, 12V is present with a sufficiently small fluctuation range in order to be able to operate an LED provided in the lighting device in constant voltage mode. The specification of 12V, which is also referred to in the rest of this description, is purely an example and represents the respective on-board voltage that is provided, e.g. B. can also be 24V, 48V or other voltage values. For cost reasons, the LEDs used in the automotive sector are provided with series resistors that are adapted to the on-board voltage provided, as is also usual for operation with constant-voltage power supplies.

However, surprising disadvantages are associated with this operation of LED-based lighting devices in the motor vehicle sector, which is common in practice: In fact, in the motor vehicle sector, one can do not assume that the target voltage of 12V will be maintained with a sufficiently small fluctuation range, as with a constant voltage power supply. A closer look reveals that compared to the target voltage of 12 V, for example, the power supply for the LEDs is often above this target voltage. The end-of-charge voltage is 14.4 V, for example, so that in practice the LEDs are often operated with an actual voltage of more than 12 V, namely up to an actual voltage of 14.4 V, which corresponds to an overvoltage of 20%. is equivalent to.

Since the current also increases with the voltage, the quadratic influence of the voltage for the power calculation means that with an overvoltage of 25%, the lighting equipment is operated with an effective power that can be three times the nominal connected load of the LED. With commercially available LED strips, which are equipped with LEDs and series resistors and are operated with an applied voltage of 12V with an output of 100%, the output increases when the end-of-charge voltage of 14.4V and thus an overvoltage of 20% is applied more than 200%. In particular, the series resistors are exposed to loads that lead to their destruction.

The effects of this significantly increased effective output consist firstly in increased heat emission, which, depending on the installation situation, should be avoided from the point of view of the risk of fire. Secondly, the service life of the affected LED is significantly reduced. Third, in addition to the reduced lifetime of the LED, the practical useful life of the lighting device, which is much shorter than the lifetime, can be significantly reduced. The length of time during which the lighting device emits a light that can be used by the user is referred to as the practical service life. The practical service life therefore depends for example, from the state of charge of a battery, with which the lighting device is operated. While in motorized vehicles that are operated with an internal combustion engine, a generator is usually also operated together with the internal combustion engine, in electric vehicles the drive motors of the vehicles are to be regarded as consumers, just like the lighting equipment of the vehicle. And in the case of motorless vehicles, such as trailers, a self-sufficient electrical energy supply in the form of a voltage source, for example a rechargeable battery, is provided, for example in the camping area. The next charging process only takes place, for example, when the trailer is connected to a towing vehicle and its on-board network. Until this next charging process, the practical service life of the lighting devices can be undesirably shortened due to the increased power consumption.

From the U.S. 2017 048 935 A1 , which represents the closest prior art, a motor vehicle lighting device is known in which protection against overvoltage and compliance with a constant amount of light emitted by the LED is sought. In this case, a "current control circuit" is provided so that operation on an (board) voltage source with a fluctuating supply voltage is made possible by using the "current control circuit" to create a constant current source.

In practice, however, the problem often arises that it is not the LEDs themselves that are to be operated, but so-called LED illuminants. LEDs on the one hand and LED illuminants on the other hand for operation with constant voltages differ in that the LED illuminants have an integrated constant current source. In most cases, this is realized by a simple series resistor and the formation of LED groups in a series circuit. However, the constant current generated in this way is only maintained if the current-limiting series resistor and the LED group are operated at a constant voltage. In the normal case, i.e. in the case of an application as a fixed installation that is connected to the public electricity or voltage network, a constant voltage power supply ensures the supply with a constant voltage. However, this is precisely what happens with a mobile installation such as e.g. This is not the case, for example, in a motor vehicle, so that, as already explained, the power consumption in the LED light source can increase to 200% or more, depending on the overvoltage applied in each case.

From the U.S. 2014 354 169 A1 a lighting device is known which can also be used in a motor vehicle. This publication also provides that the LED is connected directly and not, for example, an independent LED light source with its own current limitation - ie an LED with a series resistor - is operated. This publication specifies an efficiency of the LEDs in such a way that about 40 to 80% of the energy supplied is converted into heat, with the heat being able to permanently damage the LED.

From the U.S. 2013 141 635 A1 a lighting device for a flashlight is known. Since the flashlight is powered by a battery, if necessary with a rechargeable battery or accumulator, there is no problem of overvoltage occurring during operation, as can occur in the automotive sector if the vehicle electrical system is charged with a higher than the Nominal voltage of 12V is operated, for example with the end-of-charge voltage of 14.4 V, which is provided for charging the starter battery.

From the DE 10 2004 042 676 A1 a method for controlling an electric light source by pulse width modulation is known, which can be applied to a motor vehicle lighting device and is used to prevent brightness fluctuations, so that z. B. a brake light with constant brightness radiates even if there is a voltage drop in the vehicle's electrical system while the brake is being applied.

The invention is based on the object of improving a generic motor vehicle lighting device in such a way that it enables operation with the desired power even when an overvoltage is present.

This object is achieved by a motor vehicle lighting device according to patent claim 1.

Advantageous configurations are described in the dependent claims.

In other words, the invention proposes connecting an electronic overvoltage protection circuit, referred to for short as a circuit, upstream of the LED for power regulation. The effective power of the lighting device is regulated by means of the circuit in that an undesired power input into the LED light is prevented and the power input is rather limited to the desired target value. At the same time, this reduces the energy requirement of the lighting device compared to operation without the proposed circuit. The actual voltage applied to the lighting device is not changed in the process, so that a particularly economical circuit can be used.

Random influences such as B. long cable lengths or small cable cross-sections, which cause a sufficiently large drop in the supply voltage to the voltage applied to the LED lamp, can cause accidental, unplanned protection of the LED lamp from the mentioned overvoltage while another LED lamp may be exposed to overvoltage and operated with excessive power on the same vehicle due to a different installation position and correspondingly shorter cable length. by means of circuit provided according to the proposal, the voltage present there can be determined and taken into account for each LED light.

The circuit effects the power regulation by means of an intermittent modulation. The power supply for the LED is therefore alternately switched on and off at intervals, so that the LED is also switched on and off alternately accordingly. Similar to what is known from the leading edge or trailing edge for dimming classic light bulbs, the human observer has a certain perception of brightness depending on the amplitude of the voltage and the duty cycle of each pulse, although the optical perception of a constantly shining lighting device is retained can if the on and off cycles take place with a physiologically sufficiently high frequency. Since such an intermittent modulation for dimming the LED light is provided anyway, ie for influencing its brightness control, the present proposal can be implemented with comparatively simple structural means and economically.

The actual voltage actually present is first detected by means of the circuit provided according to the proposal. In the event of an overvoltage compared to the target voltage, for example the intended on-board voltage of 12V, the duty cycle of the LED is reduced.

If, for example, the LED is to be operated with a constantly applied voltage, this voltage supply can be mentally divided into seamlessly lined up, equally long intervals of a certain duration, e.g. B. 50 or 100 intervals per second. Each of these intervals can be divided into two different switching states, namely a switch-on phase and a switch-off phase. In the switch-on phase, the actual voltage remains unchanged, and in the switch-off phase the voltage has the value zero. Accordingly, if the voltage is constant, the switch-off phase has a duration of zero. If an overvoltage occurs, the switch-off phase is lengthened to a value greater than zero and the switch-on phase is shortened accordingly by means of the circuit.

With a sufficiently high switching frequency, the visual impression of a constantly lit LED remains for the human observer, even during intermittent operation. The circuit can therefore advantageously take into account a correspondingly large number of intervals per second, that is to say switch a correspondingly large number of on and off phases per second. It is known from the field of film projection that the impression of uninterrupted, fluid movement can already be achieved at frequencies below 30 Hz. The switching frequency can therefore advantageously also be correspondingly high. In order to reliably exclude the unpleasant perception of flickering, in particular more than 100 intervals per second can be provided, for example 200 to 700 intervals.

The proposed circuit can be used not only in the above-mentioned continuous operation of the LED with a constantly applied supply voltage, but also together with a dimmer. A frequency-based dimmer can be used in a manner known per se, which alternately generates on and off phases. By means of the proposed circuit, the duration of the switch-on phases is reduced when overvoltages occur, so that in this case too, based on a target switch-on period in operation with the target voltage, the actual voltage that actually occurs results in a correspondingly shorter actual switch-on period leads.

The quadratic influence of the voltage for the power calculation is opposed to the purely linear influence of the shortened switch-on phase. However, the effects of an overvoltage on the LEDs on the one hand and the ballast resistors of the lighting device on the other are also different, so that an exact calculation would require a comparatively complex design of the circuit in order to ensure exactly identical power consumption of the lighting device under the actual conditions by means of the circuit , as would otherwise be the case under target conditions. In particular, depending on the design, the manufacturer and possibly the product batches, the LEDs and the series resistors of the lighting devices can behave differently, so that an exact adjustment of the circuit would be necessary to ensure the identical power consumption mentioned under the actual conditions.

In particular, if the absolute values of overvoltage and excess power are not taken into account, but if calculations are made with relative or percentage values, the range in which overvoltages occur in vehicles in practice, regardless of the actual target On-board voltage an almost linear relationship. Accordingly, a first alternative for protecting the LEDs can be the use of a protective circuit in the motor vehicle lighting device by controlling the shortening of the so-called "duty cycles" or switch-on duration in the switch-on and switch-off cycles with sufficiently good accuracy according to the relative overvoltage will. A relatively linear correlation between overvoltage and the resulting power increase could be determined empirically in the first tests. Regardless of the power, voltage and type of LED light, in practice often in the form of an LED strip, there was actually a linear relationship between the relative overvoltage and the occurring A relative increase in output was observed, with the simple series resistors that are often common in practice being installed in the LED lights and no active control.

To protect the LEDs, it is provided that the circuit shortens the switch-on phase by a fixed factor within an interval when an overvoltage is present, depending on the determined actual voltage that is actually present. This enables a simple, economical construction of the circuit. In this way, the lighting device is not supplied with the same power as when the reference voltage is applied, but it can be ensured that the negative effects of the overvoltage are significantly reduced or even avoided to an extent that is relevant in practice.

The circuit is designed in such a way that it determines the overvoltage and, depending on this, the switch-off phase according to the following formulas: $$\begin{array}{ccc}{\mathrm{u}}_{\mathrm{i}}/{\mathrm{u}}_{\mathrm{S}}-1=\mathrm{Ü},& \mathrm{and}& {\mathrm{D}}_{\mathrm{S}}-\left({\mathrm{D}}_{\mathrm{S}}*\mathrm{Ü}*\mathrm{f}\right)={\mathrm{D}}_{\mathrm{i}}\end{array}$$

Ui

die tatsächlich anliegende Ist-Spannung,

US

die Soll-Spannung (z. B. des Fahrzeug-Bordnetzes),

Ü

das Maß der Überspannung als dimensionsloser Relativwert, bezogen auf die Soll-Spannung,

DS

die Soll-Dauer der Einschaltphase,

Di

die Ist-Dauer der Einschaltphase, und

F

den erwähnten Faktor

referred to therein

wow

the actually present actual voltage,

U.S

the target voltage (e.g. of the vehicle electrical system),

Ü

the degree of overvoltage as a dimensionless relative value, related to the target voltage,

DS

the target duration of the switch-on phase,

Tue

the actual duration of the switch-on phase, and

f

the mentioned factor

If this factor F is 1, for example, the switch-on phase is shortened by the same amount by which the overvoltage exceeds the intended voltage. At a With an overvoltage of 10%, the switch-on phase is shortened by 10% compared to the intended target switch-on time of the LED, so that the product of voltage and switch-on time results in the same value as the product of the non-reduced switch-on time and the target voltage . In order to achieve a greater reduction in power consumption compared to the factor 1 mentioned and to further compensate for the quadratic influence of the voltage, another factor can be taken into account in the circuit. For example, a factor of 3 shortens the switch-on phase by 15% when there is a 5% overvoltage, and shortens the switch-on phase by 60% when there is a 20% overvoltage, to just 40% of its target duration.

The present proposal is explained in more detail below on the basis of the purely schematic representation.

The drawing shows a diagram in which an electrical voltage U is plotted against time t, ie the voltage U that can be applied to an LED is plotted in the vertical direction and the time t is plotted in the horizontal direction.

In the diagram, the theoretically envisaged setpoint voltage is marked with U _S , which has a value of 12 V but is not present in the example shown. For this reason, the course of the target voltage U _S is drawn in as a broken line.

In addition, the LED is not operated constantly, but rather at intervals, namely by means of a dimmer, which enables the brightness of the LED to be influenced intermittently in the manner of voltage modulation. In order to operate the LED with a certain power - and a resulting, certain perceptible brightness - the switch-on phase at target voltage U _S is a period of time that is identified as target duration D _S and runs from a switch-on time t ₁ des first interval to an end time t _s lasts.

A switch-off phase is provided after this target duration D _S has elapsed. This lasts up to a point in time t ₂ , at which the first interval ends and the second interval begins, in which the connected LED is switched on again.

In fact, however, deviating from the described setpoint values, there is no setpoint voltage U _S of 12 V, but rather an actual voltage U _i that has a higher value than 12 V, ie represents an overvoltage. The course of the actual voltage U _i is shown in solid lines.

A circuit that can be referred to as an overvoltage protection circuit for power control is used to detect the actual voltage U _i and to modulate the switch-on phases in such a way that the connected LED can still be operated with approximately the desired power, and that in any case, operation with an unacceptably high power is avoided. For this purpose, the actual voltage U _i is not changed, but the LED is operated with this overvoltage. However, the switch-on phase is reduced within an interval from the target value D _S to an actual duration D _i , so that the switch-on phase from time t ₁ at the beginning of the interval only ends at the actual end time t _i instead of up to the actually intended end Time t _S lasts.

In the graphical representation of the diagram, the same areas result both for the rectangle with the target values, ie in dashed lines, and for the rectangle in solid lines, which represents the actual values. These make it clear that the circuit shortens the switch-on phase by the same amount - ie by a factor of 1 - by which the actual voltage U _i exceeds the target voltage U _S as overvoltage.

Compared to operating the LED on a constant current source, it can be ensured in an economically advantageous manner by means of the proposed circuit that the LEDs are operated with an acceptable power and the disadvantages that would be associated with an undesired operation with an increased effective power are reliably avoided can become.

In contrast to the explained overvoltages that occur in practice, undervoltages that also occur in practice cannot be compensated for by means of the proposed circuit. However, they do not cause the disadvantages described at the outset in terms of temperature development and the service life of the LEDs or the practical service life of the lighting device, so that undervoltages are more acceptable than overvoltages.

Using the example shown in the drawing, it was explained that using the proposed circuit and the factor 1 taken into account therein, the product of the voltage and the switch-on time in practical operation is exactly the same as the product of the intended target voltage and the intended target Power On Duration. Deviating from this, the circuit provided according to the proposal can also be designed in such a way that another factor is used, or that an additional correction factor is included in the calculation of the actual duration D _i : for example if the brightness of the LED does not change linearly with the applied voltage developed, the mentioned correction factor can ensure that the LED is operated with the desired brightness. In particular, when the brightness increases disproportionately, the actual duration D _i can be further shortened by means of such correction factors than was described with reference to the exemplary embodiment, so that the positive effects of the circuit are particularly noticeable.

Standard LED strips available on the market are designed to be operated at a constant voltage. Because the amount of light emitted by an LED depends solely on the current, it is necessary to keep this current constant. If the connection voltage is constant, a constant current must be ensured using a series resistor. This must be ensured when operating with power packs, e.g. in building installations. Here, due to the line resistance caused by longer building cabling, the desired connection voltage is often undercut and the maximum permissible LED output or brightness is not achieved. But the operation in the car looks different. On-board voltages of 10-20% above the nominal voltage of 12V are even the norm here. What hardly anyone has taken into account is that this overvoltage drastically increases the performance of the system in LEDs with a series resistor. Depending on the LED quality, this can lead to up to 200% of the nominal power at 20% overvoltage. The brightness is not even doubled, but the series resistor contributes significantly to the heating.

The luminous flux increases linearly with the current, so the power consumption of the LED also increases almost linearly. But Ohm's law describes the power consumption at the resistor with P = R * I ² . This means that the essential power requirement simply turns into heat without contributing to the illumination. Of course, the LED brightness still depends on the on-board voltage. It remains to be mentioned that the service life or lifespan (lifetime) of an LED essentially depends on its operating temperature. This can easily be reduced from 40,000 hours to a fraction, for example 4,000 hours on average, by using the wrong temperature working range.

1. 1. Akkukapazität wird verschwendet,
2. 2. die Helligkeit der LED schwankt mit der Bordspannung,
3. 3. die LED-lifetime sinkt dramatisch.

So you have to deal with several negative effects:

1. 1. Battery capacity is wasted,
2. 2. the brightness of the LED varies with the on-board voltage,
3. 3. the LED lifetime drops dramatically.

The best-known method of preventing such negative effects is the use of constant current sources. These are precisely matched to the LEDs used or to the application and are either connected externally or built together with the LED. The latter leads to increased costs. Due to the low board voltage of 12V, the former is only suitable for a maximum of 3 (white) LEDs in series. In any case, the connected LED power or the LED current must be known in advance so that the correct LED power can be set.

A motor vehicle lighting device designed according to the invention aims to avoid overheating of the LED lights in the event of changing and increased on-board voltages. The first desired side effect is keeping the brightness constant. This is achieved because the human eye integrates the brightness of a light source over perception, much like a television picture. Everyone knows that a television picture shows frames in succession, but these are still perceived as flowing movements. In the present case, a technique is used that is also used for dimming LEDs. The functional connection between on-board voltage and the resulting connected load has been determined here so that their influence can be compensated.

Limiting the output power in the case of excessive voltages can also be done with a constant current source, but this always requires knowledge or a commitment to a specific nominal power. According to the invention, on the other hand, no such definition is required here and it is automatically limited to the currently connected LED power over the entire power range. Even different constant voltage LED lights connected in parallel can be limited in their respective individual output. In addition, overvoltage protection is automatically integrated LEDs above e.g. adjustable 16V on-board (over) voltage completely switched off and thus safely protected.

The controller can be used on 12V or 24V systems and can be operated with an input voltage of up to 30V. A separate protection circuit becomes active, which protects the controller itself. However, only for a limited time of a few minutes. The system is designed to be self-resetting so that no user intervention on the control is necessary. If the voltage falls below an adjustable threshold, the function is automatically reactivated.

A permanently incorrect connection to a voltage that is too high can still damage the controller. So that LEDs with their own power source can also be operated on such a controller, this function can be switched off selectively for individual or all channels without affecting the general protective circuit (e.g. 16V). In principle, all LEDs with a series resistor can be limited in their performance and protected from overheating.

1st case

In order to keep the operating status of LEDs within defined limits, either a constant voltage or a constant current is required. If the voltage is constant, the desired operating state can be established by suitably selecting a (constant) series resistor.

Disadvantage: If the actual voltage exceeds the nominal voltage by just 10-20%, the power consumption of such a concept immediately increases to twice the nominal power and may lead to rapid failure due to overheating, or drastically reduces the lifetime of the LED due to overheating sink.

2nd case

If the voltage is not really constant, a defined operation is possible using a constant current source. This regulates overvoltages that occur within a certain range.

Disadvantage: The current determines the performance, which is why this must be known in advance so that a defined working area can be established. Universal use is not possible because the power source has to be matched to the LEDs.

The concept according to the invention avoids these two disadvantages because it works independently of the actual connected power. Simply measuring the difference between the actual and target voltage is sufficient to prevent load-independent power limitation and thus excessive heating of the LEDs. For this purpose, a functional relationship between overvoltage and power consumption was determined. This functional relationship is used in an LED controller according to the invention in order to keep the power consumption of the LEDs with a series resistor approximately constant despite overvoltage.

1. 1. Sollspannung, z.B. 12V oder 24V
2. 2. Istspannung = Sollspannung + 10-20%
3. 3. Durchbruch-Spannung der verwendeten LEDs, bei weiß ca. 2.8V bis 3.2V pro LED
4. 4. Anzahl der LED pro Gruppe, 3 Stück bei 12V, 6 Stück bei 24V usw.

To do this, only a few variables that are easy to determine or known need to be taken into account as parameters of the function. These are:

1. 1. Target voltage, eg 12V or 24V
2. 2. Actual voltage = target voltage + 10-20%
3. 3. Breakdown voltage of the LEDs used, with white approx. 2.8V to 3.2V per LED
4. 4. Number of LEDs per group, 3 pieces at 12V, 6 pieces at 24V etc.

• U_ist = 14.4 V
• U_nenn = 12 V
• U_LED = 9.6 V
• U_rel = 1.2
• P_rel = 1.92

From these parameters alone, a factor can be determined using the following formula, which, for example, superimposes a set PWM (pulse width modulation) and thus keeps the performance and perceived brightness at a constant level. $$P_{\mathit{rel}}=-\frac{u_{\mathit{LEDs}}{u}_{\mathit{is}}}{u_{\mathit{name}}\left(-u_{\mathit{LEDs}}+u_{\mathit{is}}\right)}+\frac{u_{\mathit{LEDs}}{u_{\mathit{is}}}^2}{{u_{\mathit{name}}}^2\left(-u_{\mathit{LEDs}}+u_{\mathit{is}}\right)}+\frac{{u_{\mathit{is}}}^2}{{u_{\mathit{name}}}^2}$$

• U _is = 14.4 V
• U _rated = 12 V
• U _LED = 9.6V
• _Urel = 1.2
• _Prel = 1.92

U _rel and P _rel are dimensionless factors. U _LED stands for the LED breakdown voltage (assumption: 3 x 3.2V = 9.6V).

U _rel = 1.2 stands for 20% overvoltage and P _rel = 1.92 stands for 1.92 times the power consumption. If, for example, the PWM is reduced by this factor, the power consumption is also reduced by this factor and thus remains constant. This means that the voltage ratio alone can be used to calculate how the power increases for a specific overvoltage. Since neither the current nor a connected load is included in the formula, this method is very universal and can therefore be used on any LED circuit with series resistors, even with different nominal voltages. Only the LED voltage for the LED group (3, 6, ..) needs to be adjusted here.

The formula for P _rel could be supplemented with a term that takes into account how U _LED also increases with increasing current (here assumed to be a constant). However, since this physical influence on the performance is negligible and the equation would become a differential equation as a result, it was not used here.

The theoretically expected quadratic dependency is hardly perceptible in empirical practice, since the range from 0% to approx. 30% is still almost linear. Therefore one could also with sufficient accuracy a use linear equation. The terms in the equation can be identified as follows.

The linear term stands for the LED whose power increases linearly, while the power (loss) at the series resistor increases quadratically because of P _vor = I ² x R _vor .

References:

u

tension

t

time

U.S

target voltage

DS

target duration

t1

Time: Beginning of the first interval

tS

Time: Target end of the first interval

t2

Time: beginning of the second interval

wow

actual voltage

Tue

actual duration

Ti

actual end time

Claims (5)

1. Vehicle lighting device having a power source supplying an actual voltage U_i that possibly deviates from a nominal voltage U_s and having an LED that can be connected to the power source and having a series resistance connected in series with the LED, where an overvoltage protection circuit, referred to as "circuit" for short, is connected in series with the LED for power control by means of intermittent voltage modulation, which is designed to interrupt the voltage supply at intervals so as to switch the LED alternately on and off, where the switching-on phase and a following switching-off phase define an interval, where the circuit is designed so that, while maintaining the actual voltage U_i applied to the circuit, the actual switching-on period D_i is shortened within an interval by comparison with a nominal pre-defined switching-on period D_s, if the actual voltage U_i of the power source exceeds its nominal voltage U_s, characterised in that the circuit is designed so that a factor F is included in the calculation of the actual switching-on period D_i so that with an overvoltage U determined by using the formula U_i/U_s - 1 = U the actual switch-on period is determined by means of the calculation $${\mathrm{D}}_{\mathrm{s}}-\left({\mathrm{D}}_{\mathrm{s}}*\mathrm{Ü}*\mathrm{F}\right)={\mathrm{D}}_{\mathrm{i}}.$$

   
2. Vehicle lighting device in accordance with claim 1, characterised in that the lighting device incorporates a dimmer connected in series with the circuit to generate switching-on and switching-off phases.
3. Vehicle lighting device in accordance with claim 1 or 2, characterised in that the circuit is designed to activate 25 or more switching-on and switching-off operations a second.
4. Vehicle lighting device in accordance with claim 3, characterised in that the circuit is designed to activate 100 or more switching-on and switching-off operations a second.
5. Vehicle lighting device in accordance with any one of the foregoing claims, characterised in that the circuit is designed so that a correction factor that takes into consideration the brightness variation of the LED within the overvoltage range is included in the calculation of the actual switch-on time D_i.

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