FIELD OF THE INVENTION

The present invention relates to a lighting device, and more specifically to a lighting device including a discharge lamp, as well as to a method of operating a discharge lamp. In particular, the present invention relates to discharge lamps used in automotive front lighting.

BACKGROUND OF THE INVENTION

Discharge lamps, specifically high intensity discharge (HID) lamps are widely used today for different lighting purposes, and specifically for automotive front lighting. In such discharge lamps, light is generated from an electrical arc ignited between two electrodes by applying a high voltage. After ignition, and before the lamp enters a steady-state mode of operation, in which thermal conditions are stable and electrical power is supplied at a substantially constant level, the lamp is operated in a run-up interval, during which the lamp heats up and halides contained within the discharge vessel are evaporated.

In lighting devices including discharge lamps used for automotive front lighting, it is required to achieve a fast run-up, where high luminous flux is rapidly achieved. In order to meet this requirement, automotive HID lamps are usually supplied during start up with an electrical power which is higher than the nominal power.

A problem occurring in many lamps during run-up is electromagnetic interfe- rence (EMI). Due to rapid variations of voltage and/or current of the lamp, electromagnetic radiation is emitted which may cause interference, e. g. to other electronic devices on board of a vehicle.

US 2007/0296349 Al describes a method and a device for operation of a high pressure discharge lamp. For ignition, an operating device supplies high- voltage pulses to the lamp. Once a gas discharge has been ignited, the lamp is operated in a starting phase at a multiple of its rated power in order to vaporize metal halides in the discharge vessel and to induce light emission in a short period of time. During the starting phase and thereafter, a substantially square-wave current of alternating polarity is applied to the lamp by a voltage converter, where the lamp current flows through the secondary winding of a transformer. In order to ensure good electromagnetic compatibility, the secondary winding has an inductance of 0.7 mH. As a result, the commutation time is only 25 μβ, and only a small amount of electromagnetic radiation is emitted.

SUMMARY OF THE INVENTION

It may be considered an object of the present invention to provide a lighting device with a discharge lamp, as well as a method of operating a discharge lamp, where the amount of electromagnetic interference is reduced.

This object is solved by a lighting device according to claim 1 and by an operating method according to claim 12. Dependent claims refer to preferred embodiments of the invention.

The present inventor has studied EMI emitted from different discharge lamps, and has discovered that EMI during the run-up phase does not occur entirely randomly.

While a first type of variations occurs closely related in time to the commutation (zero- crossing) of the alternating lamp current, a second type responsible for a large portion of EMI occurs independent in time of the zero-crossings. This second type of EMI generating variations have been found by the present inventor to be dependent on the electrical parameters of the lamp operation in the run-up phase, so that a range of values of the electrical parameters (voltage, current) applied to the lamp may be identified, where this type of EMI occurs re- producibly. Thus, in a diagram of values for the lamp current and the lamp voltage, an EMI region may be identified, where if the lamp during run-up is supplied with current and voltage values within this EMI region, electromagnetic emissions from the lamp may be expected to be significantly higher than for operation outside of the EMI region.

Based on this surprising discovery, the invention proposes to reduce the electromagnetic interference emitted from a discharge lamp during the run-up interval by choos- ing a run-up curve for the electrical parameters that either completely avoids the EMI region, or which at least reduces operation within the EMI region.

The lighting device according to the invention includes a discharge lamp and a driver circuit. The driver circuit may be integrated in the lamp base, or may be separate therefrom. The discharge lamp, as conventional, will usually comprise a burner with an enclosed discharge space surrounded by a discharge vessel, e. g. of quartz glass or ceramics. A filling within the discharge space will typically comprise a rare gas, in particular xenon, and a metal halide composition. The driver circuit delivers electrical power to operate the discharge lamp, in which by applying a high voltage an arc discharge is generated between electrodes within the discharge vessel. Within a run-up interval of duration of e. g. 10 - 100 s, preferably 30 - 80 s, the driver circuit applies electrical power to the lamp according to a run-up curve for the lamp current and voltage. It will be appreciated by a skilled person that the mentioned run-up relates to a cold start of a discharge lamp, and that the conditions for re-start of a lamp ("hot restrike") will be different. Detailed examples of run-up curves will be discussed be- low. After the run-up curve is completed and the lamp has reached a balanced and stable operating state, where the operating conditions are thermally stable, the lamp is driven in steady-state operation with at least substantially constant power (nominal power).

According to the invention, the run-up curve comprises an overshoot curve chosen to avoid at least a part of the EMI region. The provision of this overshoot curve is based on the discovery of the inventors that the amount of EMI generated during run-up may be substantially reduced by avoiding at least a part of the EMI region. In the method and device according to the invention the electrical power within at least a part of the run-up interval, here referred to as an overshoot interval, is not only higher than the nominal power of the lamp, but also higher than the increased power level which has been applied to discharge lamps during run-up in the prior art. However, while prior operating methods and lighting devices only apply electrical power during run-up to achieve a luminous flux which is at most equal to the luminous flux generated during steady-state operation, the overshoot curve proposed according to the invention provides operating values where the lamp emits light with a luminous flux which is higher than during steady-state operation.

The increased electrical power applied during the overshoot interval is effective to avoid the EMI region completely or at least in part, thereby substantially reducing electromagnetic interference.

Experiments with different lamp types have shown that the method and device according to the invention succeed in reducing or even avoiding EMI during run-up. While the exact position of the EMI region will differ for different types of lamps, it can be clearly identified for a specific lamp or a specific type of lamps (where e. g. a certain security margin may account for tolerances), and, once identified, may be avoided by choosing an appropriate run-up curve including an overshoot curve.

It should be understood that in the context of the present invention it is not required that each and every one of the run-up intervals of starting the lamp includes the overshoot curve and overshoot interval. The present invention may be realized by a driver circuit and a method of operation which provide the mentioned overshoot only in some, but not necessarily in all run-up intervals. For example, a run-up interval may include an overshoot curve only depending on a parameter, which may be measured or pre-determined, such as e. g. the lamp lifetime, which will be discussed below. Likewise, the duration of the overshoot interval and the amount of light overshoot may vary for different run-up intervals.

It is clear, that operation at increased power during the overshoot interval may result in increased stress for the lamp. As will be discussed below, it is possible to limit both the duration of the overshoot interval and the magnitude of the electrical values applied according to the overshoot curve, thus reducing possible detrimental effects.

As the EMI region will vary for different types of lamps or with lamp age, the required light overshoot during the overshoot interval may vary as well. Generally, it is preferred that the luminous flux during the overshoot interval mounts up to a maximum of at least 110 % of the luminous flux emitted from the lamp in steady- state mode of operation. According to a preferred embodiment, the luminous flux during the overshoot interval reaches a maximum of 125 - 210 % of the steady-state luminous flux. Particularly preferred are values of 130 - 200 %.

The overshoot interval, i. e. the interval during which the luminous flux is higher than during steady-state operation of the lamp, may have a duration of e. g. 1 - 50 s, depending on the lamp type, lamp age and identified EMI region. Typically, the duration of the overshoot interval may be 5 - 30 s, preferably 10 - 20 s, so that the duration of the higher power applied is quite limited.

The run-up curve will generally comprise multiple curve parts, where opera- tion of the lamp is effected according to different conditions. Generally, the run-up curve may comprise a current limit part (where duration of the lamp is effected limiting the lamp current to a predetermined maximum current value), a power limit part (where the electrical power supplied to the lamp is limited to a predetermined maximum power value), and/or an iso lumen part (where the run-up curve is chosen such that the lamp emits light at a predeter- mined, constant luminous flux level, at least essentially equal to the luminous flux emitted e. g. during steady state operation). Preferably, the run-up curve comprises first a current limit part (to warm up the lamp and increase the lamp voltage), then a power limit part (to increase the emitted luminous flux, typically up to the luminous flux emitted during steady state operation), and after that an iso lumen part (for keeping the luminous flux at least essentially con- stant while reducing the operating power up to the nominal power level).

These different parts of the run-up curve may be implemented by suitable control of the lamp current, while the lamp voltage rises over time after ignition.

According to a preferred embodiment of the invention, the run-up curve comprises the overshoot curve after the start of the iso lumen curve, especially preferred during the iso lumen curve (the run-up then follows a first part of the iso lumen curve, then the overshoot curve, then a second part of the iso lumen curve again).

Generally, it is preferred to choose the overshoot curve such as to avoid the EMI region fully or in part by supplying a lamp current at a current value above the EMI region. However, in order to limit the additional stress, it is further preferred to choose the overshoot curve to avoid the EMI region by supplying a lamp current at a current value not more than 25 % above the EMI region.

The run-up curve of the invention may be implemented in different ways. In particular values of voltage and/or current corresponding to the EMI region, or corresponding to the overshoot curve determined to at least partly avoid the EMI region, may be predetermined for the specific lamp. Such values may be stored and used to determine the appropriate operation during run-up. For example, values may be arranged in one or more lookup tables stored e. g. in a memory which is part of, or accessible to, the driver circuit.

The present inventors have found that the EMI region will differ for lamps of different age, i. e. the position and/or size of the EMI region within the current/voltage diagram will typically shift and expand in dependence on an age parameter of the lamp. Such an age parameter of the lamp may be e. g. the total number of hours of operation. Alternatively, the steady state lamp voltage during operation, which, as the skilled person knows, will gradually increase over the lamp lifetime, may serve as an age parameter. According to a preferred embodiment, values for the EMI region or for the overshoot curve in dependence on the age parameter may be stored in the storage, so that control may be effected differently depending on the lamp age. Typically, the duration of the overshoot interval will increase with increasing lamp lifetime. For example, it is possible that a new lamp may first be operated in a run-up curve without overshoot, and that operation with overshoot starts at a predetermined lamp age.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter and shown in the drawings.

In the drawings,

Fig. 1 shows a symbolic representation of a lighting device including a discharge lamp;

Fig. 2a shows a diagram of the lamp power over time during startup; Fig. 2b shows a diagram of a lamp current and lamp voltage, including an EMI region;

Fig. 3a - 3c show diagrams of the lamp voltage, the lamp power and the

luminous flux of a lamp during a run-up interval;

Fig. 4a, 4b show a diagram of a lamp current and lamp voltage, indicating an EMI region for a new discharge lamp (4a) and an aged discharge lamp (4b).

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows an automotive headlamp 10 as an embodiment of a lighting device. A discharge vessel 12 of a discharge lamp 14 is arranged within a reflector 16. The lamp 14 comprises a lamp base 18, show symbolically in fig. 1, which comprises a fully integrated operating circuit 19 for the discharge lamp 14, including a driver circuit 20, an igniter 21 and a controller 22.

As conventional for discharge lamps to be used in automotive headlights, the discharge lamp 14 comprises electrodes 24 projecting into the discharge space enclosed by the discharge vessel 12, which comprises a filling of xenon and metal halides.

The lamp 14 is operated by the operating circuit 19 within the lamp base 18, which is connected to the electrical board net 26 of an automobile. Controlled by the controller 22, the igniter 21 provides a high voltage pulse to the electrodes 24 of the lamp 14 to ignite an arc discharge within the discharge vessel 12. In subsequent operation of the lamp 14, a driver circuit 20 supplies, under control of the controller 22, lamp current II to the lamp. The lamp current II is supplied as a direct current, however with polarity alternating according to a switching frequency of e. g. 300 Hz. The driver circuit 20 may e. g. be comprised of a DC/DC converter to deliver a voltage/current level as requested by the controller 22, and a full bridge switching circuit to obtain the desired rectangular current waveform.

For a cold start of the lamp 14, the lamp after ignition is first driven in a runup interval of a duration T _mn _ _up of e. g. 60 s according to a run-up sequence illustrated symbolically in figures 2a, 2b.

Fig. 2a shows the electrical power P supplied to the lamp 14 over time t. Fig. 2b shows the lamp voltage VL over the lamp current II. In both fig. 2a, 2b, a conventional run-up curve or run-up sequence is shown by a dashed line, whereas the run-up according to the present embodiment of the invention is shown as a solid line. After ignition of the lamp 14 (indicated in fig. 2a, 2b as A), the controller 22 controls the driver 20 first in a current limit interval 32, where the lamp current II is limited to a maximum current value. As shown in fig. 2b, the initial lamp voltage VL after ignition is relatively low and then rises over time as the lamp 14 heats up, so that the power increases.

The controller 22 monitors the lamp voltage VL and lamp current II, and thus the electrical power P supplied to the lamp 14. As the power reaches a predetermined maximum power value P _max stored in a storage 25 of the operating circuit 19 (indicated as B in fig. 2a, 2b), the run-up curve enters a second, power limit part 34, in which the controller 22 controls the driver circuit 20 to supply a lamp current II at a magnitude suited for keeping the power constant despite the still rising lamp voltage VL.

The power limit interval 34 in fig. 2a (or the power limit part 34 of a run-up curve in fig. 2b) continues until the luminous flux of the light generated by the lamp 14 reaches a predetermined value (indicated at C in fig. 2a, 2b). The luminous flux from the lamp 14 may be measured, but will usually be calculated within the controller 22 according to available lighting data stored in storage 25. The predetermined value of luminous flux generated at the location C of the run-up curve is in the presently described embodiment equal to 100 % of the luminous flux generated during the later steady-state mode of operation 38.

In a following iso lumen part or interval 36 of the run-up, the controller 22 con- trols the driver circuit 20 to supply, for the still rising voltage VL of the lamp, a lamp current II at a value suited to continuously achieve this constant luminous flux.

In conventional operation of a discharge lamp, electromagnetic interference (EMI) may occur during run-up. The present inventors have found that in the V/I diagram of fig. 2b an EMI region 50 may be identified, i. e. a range of values for the lamp current II and the lamp voltage VL, where for operation in the indicated range the level of EMI emitted from the lamp 14 is significantly higher than outside of the EMI region 50.

The exact size and position of the EMI region 50 will differ for lamps of different type, and - due to tolerances - even for different lamps of the same type. However, for a given lamp, the EMI region 50 may be identified by operating the lamp in run-up sequences with different values for lamp current and lamp voltage, and by observing the amount of EMI generated. By choosing a suitable threshold value for the generated EMI, the boundary of the EMI region 50 may thus be identified for any given lamp.

As an example, fig. 4a shows measurements conducted a discharge lamp of 25 W nominal power. The lamp was operated in a number of different run-up sequences and EMI emissions where measured. Emissions above an arbitrarily defined threshold value are marked in fig. 4a as X, whereas electromagnetic emissions below the chosen threshold value are shown as o. By evaluating these experiments, it is possible to define the boundary of the EMI region 50. Lamps of the same type were found to show only minor deviation, such that it is possible to define an EMI region 50 for a series of lamps which have, beside tolerances, the same lamp parameters, such as size of the discharge vessel and electrodes, filling of the discharge vessel etc.

The conventional run-up curve, shown as a dashed line in fig. 2a, 2b will in many cases traverse the EMI region 50, such that an increased level of EMI is generated. In the example of fig. 2b, an iso lumen curve 36 enters the EMI region approximately at the location indicated D and traverses it approximately up to the location indicated E.

In order to avoid the generation of EMI, control of the run-up according to the present embodiment of the invention provides to avoid the EMI region 50 by effecting control in an overshoot interval 40 according to an overshoot curve 40 between locations D and E, where an increased current I _L is supplied such that the EMI region 50 is avoided.

Fig. 2b shows how the overshoot curve 40 avoids the EMI region 50, however increases the current II only by a small amount as compared to the boundary of the EMI region 50, so that the additional stress caused at the lamp is limited. For example, the control program of controller 22 for the overshoot curve 40 may provide to control the current II to a value of no more than 10 % above the boundary for the EMI region.

As shown in fig. 2a, 2b, the overshoot curve and interval 40 provide an increased electrical power supplied to the lamp 14 as compared to the iso lumen curve and interval 36 (dashed line). In consequence, the luminous flux emitted from the lamp 14 will be above the luminous flux value of 100 % during the overshoot interval 40. Thus, the control strategy proposed according to the present embodiment of the invention may be detected by observing luminous flux generated the lamp during the run-up. Within the overshoot interval 40, the luminous flux emitted will be noticeably above 100 % of the luminous flux generated in the later steady state 38.

The duration of the overshoot interval 40 will depend on the lamp type, and on size and position of the EMI region 50. Since the EMI region, as will be explained below is age-dependent and typically expands over the lamp lifetime, the necessary duration of the overshoot interval will also vary. For example, a new lamp with a small EMI region may not require an overshoot interval or only a short duration thereof. An aged lamp will typically require a longer overshoot interval. For a total duration T _mn _ _up of the run-up of about 60 s, the overshoot interval may e. g. have a duration of 1 - 30 s. Typically durations for an aged lamp, e. g. of more than 1500 h of operation, or of a lamp which has a relatively large EMI region already in the new state, may be in the range of e. g. 10 - 30 s. The amount of light overshoot will also be dependent on the lamp type and EMI region 50, and will typically be in the range of 30 - 100 % light overshoot (i. e. 130 - 200 % luminous flux as compared to steady state).

Fig. 3a - 3c show values for the lamp voltage VL, lamp power P _L and for the light output from a sample lamp of 25 W nominal power. Conventional run-up is shown as a dashed line, whereas the run-up according to the present embodiment (including the over- shoot interval 40), is shown as a solid line.

Fig. 3a shows the rise of the lamp voltage VL, which is due to the heating of the lamp over time and is otherwise not significantly influenced by the driver circuit 20.

The lamp power PL depicted in fig. 3b shows in an overshoot interval from approximately t = 10 s up to t = 20 s a substantially increased value. Consequently the lumin- ous flux emitted from the lamp in the overshoot interval shows a substantial light overshoot 42 up to a maximum of about 180 % of the level of luminous flux generated during later operation in steady state 38 at the nominal operating power of 25 W.

In the lighting device 10 the data for the specific type of lamp 14 is stored within storage 25 in the form of a look-up table of values for II, VL. Either the values of the boundary for the EMI region 50, or the overshoot curve 40 avoiding the EMI region 50 may be stored in such look-up tables. The data may be obtained beforehand by experimentation with a number of lamps of the same type as lamp 14 to obtain the EMI region 50 and statistical data about the influence of possible tolerances.

During control of the run-up of the lamp 14, the controller 22, which may be a programmable microcontroller executing a control program to follow the above described control strategy, will then continuously evaluate the measured values of VL and II according to the different parts 32, 34, 36 of the run-up curve. If control according to the iso lumen curve 36 would traverse the EMI region 50 or come too close to its boundary, the control program will provide an increased lamp current II according to the overshoot curve 40 to avoid the EMI region 50.

While in the first embodiment the storage 25 provides in a look-up table data on the EMI region 50, a second embodiment provides as further modification a three- dimensional look-up table representing an EMI region 50 varying over the lifetime of the lamp. As the skilled person will appreciate, over the lifetime of the lamp 14, which may be measured in hours of operation on a scale of hundreds of hours, the electrical parameters of the lamp change. Due to different effects such as electrode burn-back, the lamp voltage V _L in steady-state 38 operation will gradually increase. The present inventors have found that also the size and position of the EMI region for each cold restart of an aged lamp will vary over time. As an example, while fig. 4a shows EMI measurements conducted at a newly produced lamp, fig. 4b shows measurements of the same lamp after 3000 hours of operation. As shown, the EMI region 50 has shifted and expanded. This is why aged lamps generally show worse EMI behavior as compared to new lamps.

According to a second embodiment of the invention, data on the EMI re- gion 50 after different intervals of lamp lifetime, e. g. after 500 h, 1000 h, 1500 h, 2000 h, 2500 h and 3000 h are obtained in experiments beforehand and a three-dimensional lookup table is stored in storage 25, so that the microcontroller 22 may obtain data on the EMI region 50 in dependence on an age parameter (which may be the number of hours of operation of the lamp 14 or alternatively the lamp voltage during steady state 38).

According to the second embodiment, control of the lamp 14 during the run-up is effected in the same way as described above, only that data on the EMI region 50 is obtained in dependence on the age parameter stored or determined by the microcontroller 22. In this way, run-up control is effected to avoid EMI, however the amount of overshoot and thus additional stress to the lamp 14 is limited. For example, a new lamp may not require light overshoot at all, whereas the same lamp after many hours of operation requires overshoot to avoid EMI.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodi- ments.

The example values for lamp current, lamp voltage, luminous flux and time durations have been found for automotive HID xenon lamps of 25 W and 35 W nominal power. As the skilled person will readily appreciate, appropriate values for different lamp types may be found by the described experiments.

Further, while in the above embodiments a run-up control is described fully avoiding the EMI region 50, it may be an option to follow a run-up curve including an overshoot curve 40, however providing a reduced amount of overshoot so that the EMI region 50 is not fully avoided, but traversed only for a shorter period of time. Such a control strategy will be effective to limit the amount of EMI generated during run-up, while avoiding exces- sive amounts of additional stress to the lamp. In particular for aged lamps, the control strategy may provide to limit the amount of increased lamp current during the overshoot interval 40 to acceptable values, such that avoidance of the EMI region may not be entirely possible for all lamp ages.

The invention mainly deals with a cold start of a lamp. However, for hot re- strike it will also be advantageous to use light overshoot in order to avoid operating conditions leading to adverse EMI behavior, although the specific values will differ from those described above.

Other variations to the disclosed embodiment can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a", or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims, or in mutually different embodiments, does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.