EP3426007A1 - Vehicle lighting device with power control - Google Patents

Abstract

In a motor vehicle lighting device, with a voltage source supplying a desired voltage (U _S ), and an LED which can be connected to the voltage source, as well as with a series resistor upstream of the LED, the invention proposes that the LED be an electronic overvoltage protection circuit, referred to as a circuit for short upstream of the power control by means of intermittent voltage modulation, which is designed interrupting the power supply at intervals in such a way that the LED is alternately switched on and off, each a turn-on and a turn-off define an interval, the circuit is configured such that under Maintaining an actual voltage applied to the circuit (U _i ), the actual duty cycle (D _i ) is shortened within an interval in comparison to a predetermined desired duty cycle (D _S ).

Description

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

Generic automotive lighting devices are known in practice.

Outside the automotive sector, it is known to operate LEDs on a constant current source, or at a constant voltage source of, for example, 12V. Power supplies ensure compliance with the desired current or voltage values In the automotive sector, when installing well-known, generic lighting fixtures, it is often assumed that a specific setpoint voltage of, for example, 12V is applied with a sufficiently small fluctuation range, one in the lighting fixture intended to operate LED provided in the constant voltage operation. The statement of 12V, to which reference is also made in the following description, is purely exemplary and represents the respectively provided on-board voltage, the z. B. also 24V, 48V or other voltage values may be. For cost reasons, the LEDs used in the automotive sector are provided with correspondingly adapted to the intended on-board voltage series resistors, as is customary for operation on constant voltage power supplies.

With this usual practice in practice of LED-based lighting devices in the automotive sector, however, there are surprising disadvantages: In fact, one can in the automotive sector do not assume that the nominal voltage of 12V is maintained with a sufficiently small fluctuation range, as with a constant voltage power supply. On closer inspection, it turns out that compared to the setpoint voltage of 12 V, for example, the voltage supply for the LEDs is often above this setpoint voltage. Namely, the charging end voltage is for example 14.4 V, so that in practice the LEDs are often operated with an actual voltage of above 12 V, namely up to an actual voltage of 14.4 V, which is an overvoltage of 20% equivalent.

Since the voltage also increases the current, due to the quadratic influence of the voltage for the power calculation, the lighting devices are operated at an overvoltage of 25% with effective powers that can be three times the rated connection value of the LED. For commercial LED strips, which are equipped with LEDs and Vorwiderständen and operated at an applied voltage of 12V with a power of 100%, increases when the end of charge voltage of 14.4V and thus a surge of 20%, the performance more than 200%. In particular, the series resistors are exposed to stresses that lead to their destruction.

The effects of this significantly increased effective output consist first of all in an increased heat output which, depending on the installation situation, must be avoided from the point of view of the risk of fire. Second, the life of the affected LED is significantly reduced. Third, in addition to the reduced life of the LED, the practical life of the lighting device, which is much shorter than the lifetime, can be significantly reduced. In this case, the period of use during which the illumination device emits a light which can be used by the user is referred to as a practical service life. The practical life depends therefore for example, from the state of charge of a battery with which the lighting device is operated. While in motorized vehicles, which are operated with an internal combustion engine, together with the internal combustion engine usually also a generator is operated, in electric vehicles, the drive motors of the vehicles are just as much consumers as the lighting device of the vehicle. And in motorless vehicles, such as trailers, a self-sufficient electrical power supply in the form of a voltage source is provided, for example, a battery, for example, in the camping area. The next charging process, for example, only again when the trailer is connected to a towing vehicle and its electrical system. Up to this next charging process, the practical life of the lighting devices can be undesirably shortened by the increased power consumption.

The invention has for its object to improve a generic automotive lighting device to the extent that this allows operation with the desired performance even with an applied overvoltage.

This object is achieved by a motor vehicle lighting device according to claim 1. Advantageous embodiments are described in the subclaims.

In other words, the invention proposes to precede the LED with an electronic overvoltage protection circuit, briefly referred to as a circuit, for power regulation. By means of the circuit, the effective power of the lighting device is controlled by an undesirable entry of power into the LED lamp prevents and rather the power input is 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 illumination device, is not changed, so that a particularly economical circuit can be used.

Random influences such. As large cable lengths or small cable cross-sections that cause a sufficiently large drop in the supply voltage to the voltage applied to the LED light voltage can cause accidental, unscheduled protection of the LED light before the aforementioned overvoltage, while another LED light due to a different installation position and correspondingly shorter cable length, the overvoltage may be exposed to the same vehicle and operated at too high a capacity. By means of the proposed circuit according to the voltage applied there can be determined and taken into account for each LED light.

The circuit effects the power control by means of an intermittent modulation. The power supply for the LED is thus alternately switched on and off at intervals, so that, accordingly, the LED is turned on and off alternately. As is known from the phase angle or section for the dimming of conventional 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, but the optical sensation of a constantly illuminated illumination device is maintained can occur when the on and off cycles with a physiologically high enough frequency. Since such an intermittent modulation for dimming the LED light is provided anyway, so to influence their brightness control, the present proposal is constructive with relatively simple means and economically feasible.

By means of the proposed circuit provided initially the actually applied actual voltage is detected. In the event of an overvoltage compared to the target voltage, for example, the provided 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 continuous voltage, this voltage supply can theoretically be divided into seamlessly arranged, equally long intervals of a specific period of time, for example. B. 50 or 100 intervals per second. Each of these intervals can be mentally divided into two different switching states, namely a switch-on phase and a switch-off phase. In the switch-on phase, the actual voltage is unchanged, and in the switch-off phase, the voltage has the value zero. If the voltage is applied constantly, the switch-off phase has a duration of zero accordingly. If an overvoltage occurs, the switch-off phase is extended by the circuit according to the proposal to a value greater than zero, and the switch-on phase is accordingly shortened.

For a human observer remains at a sufficiently high switching frequency of the visual impression of a constantly lit LED also exist in their intermittent operation. Therefore, the circuit can advantageously take into account correspondingly many intervals per second, that is, correspondingly switch many on and off phases per second. From the field of the screening of films is known that even at frequencies below 30 Hz, the impression of an uninterrupted, fluid movement can be achieved. Advantageously, therefore, the switching frequency also be correspondingly high. In order to reliably exclude the unpleasant perception of a flicker, in particular more than 100 intervals per second may be provided, for example 200 to 700 intervals.

Not only in the mentioned continuous operation of the LED with a constantly applied supply voltage, the proposed Circuit use, but also with a dimmer. In a manner known per se, a frequency-based dimmer can be used which alternately generates switching on and off phases. By means of the proposed circuit, the duration of the switch-on is reduced when overvoltages occur, so that in this case, based on a target duty cycle in operation with the target voltage, the actual voltage actually occurring in comparison to a correspondingly shorter actual duty cycle leads.

The quadratic influence of the voltage for the power calculation is offset by the merely linear influence of the shortened switch-on phase. However, the effects of an overvoltage on the one hand, the LEDs and on the other hand, the ballast resistors of the lighting device are also different, so that an exact calculation would require a comparatively complicated design of the circuit to ensure the exact identical power consumption of the lighting device under the actual conditions by means of the circuit as they would otherwise be given under target conditions. In particular, depending on the type, the manufacturer, and possibly the batches of products, the LEDs and the series resistors of the lighting devices may behave differently, so that an exact adaptation of the circuit would be required to ensure the mentioned identical power consumption under the actual conditions.

In particular, if not the absolute values of overvoltage and the excess of power are taken into account, but if relative or percentage values are calculated, the range in which the overvoltages occur in practice for vehicles, irrespective of the actual setpoint value, results. Board voltage an approximately linear relationship. Accordingly, with a sufficiently good Accuracy according to the relative overvoltage the shortening of the so-called "duty cycles" or duty cycle in the on and off cycles are controlled. Empirically, a relatively linear relationship between overvoltage and resulting increase in power could be determined in initial experiments. Regardless of the power, voltage and type of LED lamp, in practice often in the form of an LED strip, it was in fact a linear relationship between the relative overvoltage and the relative increase in relative power to observe, with the LED lights in the Practice often common, simple Vorwiderstände were installed, and no active regulation.

In an advantageous embodiment, it is therefore provided that the circuit shortens the switch-on phase within an interval with an applied overvoltage by a specified factor as a function of the determined, actually applied actual voltage. This embodiment enables a simple, economical construction of the circuit. In this way, although the illumination device is not supplied with the same power as with applied nominal voltage, it can be ensured that the negative effects of the overvoltage are considerably reduced or even avoided to a practically relevant extent.

und $${\mathrm{D}}_{\mathrm{s}}-\left({\mathrm{D}}_{\mathrm{s}}*\mathrm{Ü}*\mathrm{F}\right)={\mathrm{D}}_{\mathrm{i}}$$

The circuit can thus be designed in such a way that it determines the overvoltage and depending on the turn-off phase according to the following formulas: $${\mathrm{U}}_{\mathrm{i}}/{\mathrm{U}}_{\mathrm{s}}-\mathrm{1}=\mathrm{Ü}.$$

and $${\mathrm{D}}_{\mathrm{s}}-\left({\mathrm{D}}_{\mathrm{s}}*\mathrm{Ü}*\mathrm{F}\right)={\mathrm{D}}_{\mathrm{i}}$$

• U_i die tatsächlich anliegende Ist-Spannung,
• U_S die Soll-Spannung (z. B. des Fahrzeug-Bordnetzes),
• Ü das Maß der Überspannung als dimensionsloser Relativwert, bezogen auf die Soll-Spannung,
• D_S die Soll-Dauer der Einschaltphase,
• D_i die Ist-Dauer der Einschaltphase, und
• F den erwähnten Faktor

Denoted therein

• U _{i is} the actually applied actual voltage,
• U _{S is} the nominal voltage (eg of the vehicle electrical system),
• Ü the measure of the overvoltage as dimensionless relative value, based on the nominal voltage,
• D _{S is} the target duration of the switch-on phase,
• D _{i is} 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 nominal voltage provided per se. With an overvoltage of 10%, consequently, the switch-on phase is shortened by 10% in comparison with a nominal switch-on duration of the LED which is provided per se, so that the product of voltage and switch-on duration yields the same value as the product of the non-reduced switch-on duration and the desired value -Tension. In order to achieve a greater reduction in power consumption compared to the aforementioned factor 1 and to further compensate for the quadratic influence of the voltage, another factor in the circuit can be taken into account. For example, a factor of 3 results in a 15% reduction of the power-up phase when a 5% overvoltage is applied, and a 60% overvoltage reduces the power-up phase by 60% to only 40% of its target duration.
The present proposal is explained in more detail below with reference to the purely schematic representation.

In this case, a diagram is shown in the drawing, in which an electrical voltage U is plotted over the time t, that is, the voltage applied to a LED voltage U in the vertical direction and the time t is plotted in the horizontal direction.

In the diagram, the theoretically provided desired voltage is designated by Us, which has a value of 12 V, but is not present in the illustrated example. For this reason, the course of the desired voltage U _{S is shown in} dashed lines.

In addition, no constant, but an interval-like operation of the LED is provided, namely by means of a dimmer, the intermittent brightness of the LED in the manner of a Voltage modulation allows. In order to operate the LED with a certain power - and a resulting, perceptible perceived brightness - the switch-on phase at setpoint voltage U _{S is} a period of time which is identified as the set duration D _S and from a switch-on time t _{1 of} FIG first interval until an end time t _S lasts.

After expiration of this desired duration D _S a switch-off is provided. This lasts until a time t ₂ , at which the first interval ends and the second interval begins by the connected LED is turned on again.

In actuality, however, deviating from the desired values described, there is no desired voltage U _S of 12 V, but rather an actual voltage U _i which has a value greater than 12 V, that is to say an overvoltage. The course of the actual voltage U _i is shown in solid lines.

A circuit, which may be referred to as an overvoltage protection circuit for power regulation, serves to detect the actual voltage U _i and to modulate the switch-on phases in such a way that nevertheless the connected LED is to be operated at approximately the desired power, and In any case, the operation is avoided with an unhealthy high performance. 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 set value D _S to an actual duration D _i , so that the switch-on phase from the time t ₁ at the beginning of the interval only to the actual end time t _i instead of the actual intended end Time t _S takes.

In the graphical representation of the diagram, the same area contents result for the rectangle with the nominal values, ie in dashed lines, as well as 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 setpoint voltage U _S as overvoltage.

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

In contrast to the described, occurring in practice overvoltages can not be compensated by the proposed circuit undervoltages that also occur in practice. However, they do not cause the disadvantages described above for the temperature development and the life of the LEDs and for the practical life of the lighting device, so that the undervoltages can be put up with rather than overvoltages.

With reference to the example shown in the drawing, it was explained that by means of the proposed circuit and the factor 1 considered therein, the product of voltage and on-time in practical operation is exactly the same size as the product of the intended target voltage and the provided target Power-time. By way of derogation, the circuit provided according to the proposal may also be designed in such a way that another factor is applied, 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 is not linear 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, by means of such correction factors the actual duration D _{i be} further shortened, as described with reference to the embodiment, so that the positive effects of the circuit come particularly to bear.

Standard LED strips available on the market are designed to operate at a constant voltage. Since the amount of light emitted by an LED depends exclusively on the current, it is necessary to keep this current constant. If a constant supply voltage is present, ensure a constant current through a series resistor. When operating with power supplies, e.g. in building installations, that's to be ensured. Due to the cable resistance caused by longer building cabling, the required terminal voltage is often even undershot and the maximum permissible LED power or brightness is not reached. But the situation looks different in the car. Here board voltages of 10-20% above the rated voltage of 12V are even the rule. What hardly anyone has taken into account so far is that this overvoltage in the case of LEDs with series resistor can drastically increase the performance of the system. Depending on the LED quality, this can lead to up to 200% of the nominal power at 20% overvoltage. Not even the brightness is doubled, but the series resistor contributes significantly to the warming.

The luminous flux increases linearly with the current, so that the power consumption of the LED increases almost linearly. But Ohm's law describes the power consumption at the resistor with P = R * I ² . Thus, the essential power requirement is simply transferred to heat, without contributing to the clarification. Of course, the LED brightness is still dependent on the on-board voltage. Remains to mention that the life or lifetime of an LED essentially depends on its operating temperature. This can easily be reduced by the wrong temperature working range of 40,000h to a fraction eg on average 4,000h.

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

Das bekannteste Verfahren um solche negativen Auswirkungen zu verhindern, ist die Nutzung von Konstantstromquellen. Diese werden exakt auf die verwendeten LEDs bzw. auf die Anwendung abgestimmt und sind entweder extern vorgeschaltet oder mit der LED gemeinsam aufgebaut. Letzteres führt zu erhöhten Kosten. Ersteres ist aufgrund der niedrigen Bordnennspannung von 12V nur für max. 3 (weiße) LED in Reihe geeignet. In jedem Fall muss aber die angeschlossene LED-Leistung bzw. der LED-Strom vorab bekannt sein, damit auch die richtige LED-Leistung eingestellt wird.So you're dealing 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 to prevent such negative effects is the use of constant current sources. These are matched exactly to the LEDs used or to the application and are either connected upstream or built together with the LED. The latter leads to increased costs. The former is due to the low board voltage of 12V only for max. 3 (white) LED in series suitable. In any case, however, the connected LED power or the LED current must be known in advance, so that the correct LED power is set.

An inventively designed automotive lighting device aims to avoid overheating of the LED lights with changing and increased on-board voltages. The first desired side effect is the constant brightness. This is achieved because the human eye integrates the brightness of a light source via perception, similar to a television picture. Everyone knows that a television picture shows frames in a row, yet they are perceived as flowing movements. In the present case, a technique is used as it is also used in the dimming of LEDs. Here, the functional relationship between on-board voltage and the resulting connected load has been determined, so that their influence can be compensated.

The limitation of the output power at excessive voltages is indeed feasible with a constant current source, but this always requires the knowledge or determination of a certain nominal power. According to the invention, however, no such determination is required here and it will automatically over the entire power range on the currently connected LED power limited. Even different parallel connected constant voltage LED lights can be limited in their respective individual performance. In addition, an overvoltage protection is integrated automatically, which completely switches off the LEDs above, for example, an adjustable 16V onboard (over) voltage, thus providing reliable protection.

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

A permanently wrong connection to a too high voltage can damage the control nevertheless. In order for even self-powered LEDs to operate on such a controller, this feature can be selectively turned off for one or all channels without affecting the general protection circuitry (e.g., 16V). In principle, all LEDs with series resistors can be so limited in their performance and protect against overheating.

1st case

In order to keep the operating state of LEDs within defined limits, either a constant voltage or a constant current is necessary. At constant voltage, the desired operating state can be established by suitable selection of a (constant) series resistor.

Disadvantage: If the actual voltage even slightly exceeds the rated voltage by 10-20%, the power consumption of such a concept immediately increases up to twice the rated power and may lead to rapid failure due to overheating. or drastically reduce the lifetime of the LED due to overheating.

2nd case

If the voltage is not really constant, a defined operation by means of a constant current source is possible. This regulates occurring overvoltages within a certain range.
Disadvantage: The current determines the power, so it must be known beforehand, so that a defined work area can be produced. A universal use is not possible because the power source must be matched to the LEDs.

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.

These two disadvantages avoid the inventive concept, because it works regardless of the actual connected power. Just the measurement of the difference between the actual and nominal voltage is sufficient to prevent a 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 an LED control according to the invention to keep the power consumption of the LEDs with series resistance in spite of overvoltage approximately constant.
For this purpose, only a few and easily determinable or known quantities must be taken into account as parameters of the function. These are:

1. 1. Target voltage, eg 12V or 24V
2. 2. Actual tension = target tension + 10-20%
3. 3. Breakthrough 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 thereby keeps the power and perceived brightness at a constant level. $$P_{\mathit{rel}}=-\frac{U_{\mathit{LED}}{U}_{\mathit{is}}}{U_{\mathit{call}}\left(-U_{\mathit{LED}}+U_{\mathit{is}}\right)}+\frac{U_{\mathit{LED}}{U_{\mathit{is}}}^2}{{U_{\mathit{call}}}^2\left(-U_{\mathit{LED}}+U_{\mathit{is}}\right)}+\frac{{U_{\mathit{is}}}^2}{{U_{\mathit{call}}}^2}$$

• U _is = 14.4V
• U _nenn = 12V
• U _LED = 9.6V
• U _rel = 1.2
• P _rel = 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 reduced by this factor and thus remains constant. It is thus possible to calculate from the voltage relationships alone how the power increases at a certain 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 at different nominal voltages. Here alone is the LED voltage for LED group (3, 6, ..) adapt.

The formula for P _rel could be supplemented by a term that takes into account how U _LED also increases with increasing current (assumed to be constant here). However, since this physical influence on the power is negligible and the equation would thus become a differential equation, this has been omitted here.

The theoretically expected quadratic dependence is barely perceptible in empirical practice, since the observed range from 0% to approximately 30% is still approximately linear. Therefore, one could use a linear equation with sufficient accuracy. The terms in the equation can be identified as follows.
The linear term is the LED, the power increases linearly, while the (loss) increases power at the series resistor due to P = I ² _before XR _before square.

Reference numerals:

U

Tension t time

U _S

Target voltage

D _S

Target duration

t ₁

Time: Start of the first interval

t _s

Time: Target end of the first interval

t ₂

Time: Start of the second interval

U _i

Actual voltage

D _i

Actual duration

T _i

actual end time

Claims (7)

Automotive lighting device,
with a voltage source supplying a desired voltage (Us),
and an LED which can be connected to the voltage source, as well as a series resistor connected upstream of the LED, characterized
that the LED is connected in a short-called electronic circuit overvoltage protection circuit for power control by means of intermittent voltage modulation,
which the power supply is intermittently designed such that the LED is alternately switched on and off, wherein in each case one switch-on phase and one switch-off phase define an interval,
wherein the circuit is configured such that, while maintaining an actual voltage applied to the circuit (U _i ), the actual duty cycle (D _i ) within an interval compared to a predetermined desired duty cycle (D _S ) is shortened. Motor vehicle lighting device according to claim 1,
characterized,
in that the illumination device has a dimmer, which is connected upstream of the circuit and switches on and off phases. Automotive lighting device according to claim 1 or 2,
characterized,
that the circuit is designed such that it switches 25 or more switching on and off per second. Motor vehicle lighting device according to claim 3,
characterized,
that the circuit is designed such that it switches 100 or more switching on and off per second. Automotive lighting device according to one of the preceding claims,
characterized,
that the circuit is designed such that in the calculation of the actual duty (D _i) flows in a factor (F), in such a way that, for a determined according to the formula U _i / U _S -1 = U overvoltage Ü the actual duty cycle (D _i ) after the calculation
D _S - (D _S * T * F) = D _{i is} fixed. Automotive lighting device according to one of the preceding claims,
characterized,
that the circuit is designed in such a way that, while maintaining an actual voltage (U _i ) applied to the circuit, the product of actual on-time (D _i ) and actual voltage (U _i ) is the same size as the product of a predetermined setpoint Duty cycle (D _S ) and the setpoint voltage (Us). Automotive lighting device according to one of the preceding claims,
characterized,
that the circuit is configured such that in the calculation of the actual duty cycle (D _i ), a correction factor takes into account the brightness curve of the LED in the region of the overvoltages.

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