Device for operating a gas discharge lamp with reduced average lamp power

[0002] The invention is based on a device for operating a gas discharge lamp of the type having an energy supply circuit which provides an alternating-current voltage or an alternating current normally having a predetermined period in order to feed the gas discharge lamp with a predetermined power defined in the main claim. A circuit arrangement for operating a gas discharge lamp is known from DE-A 41 36 486 which includes a voltage transformer that converts the energy provided by a direct current source into an alternating-current voltage of suitable height and frequency. The voltage transformer includes a DC/DC converter and a downstream bridge circuit, in whose diagonal the gas discharge lamp is disposed. The DC/DC converter increases the voltage provided by a direct current source, which is configured as a battery, to a value sufficient to operate the gas discharge lamp. The bridge circuit operates as an inverse rectifier, and converts the direct-current voltage into a corresponding alternating-current voltage. The known circuit arrangement provides that, directly after ignition of the gas discharge lamp, the bridge circuit still does not operate as an inverse rectifier, and therefore acts upon the gas discharge lamp with the direct-current voltage generated by the DC/DC converter. Depending on a signal of a status determination device that emits a signal when the gas discharge lamp has ignited successfully, and depending on the time predetermined by a delay device, the inverse rectifier is controlled in such a way that the gas discharge lamp is acted upon by the provided alternating-current voltage. The previously observed extinguishing of the gas discharge lamp directly after ignition during the first zero crossings of the alternating current of the lamp is prevented by the known measure.

[0003] In subsequent operation with the alternating-current voltage, an instantaneous power reduction which can lead to a short-time reduction in the light arc, even in this operating state, occurs in the gas discharge lamp during each pole-reversal process. A sufficiently high operating frequency, an example of which would be a few hundred Hertz, ensures that the light arc is not completely extinguished during the brief zero crossing of the current. The voltage necessary to keep the light arc operating at normal rating after the pole-reversal process corresponds to the normal arc drop voltage of the gas discharge lamp. A reduction in the electrical power supplied to the gas discharge lamp leads to the occurrence of a complete breakdown of the light arc during the pole-reversal process. The energy supply circuit must therefore be capable of applying the necessary re-ignition voltage following the pole-reversal process. A reduction in the electrical power supplied to the gas discharge lamp is therefore possible to the extent that the energy supply circuit is capable of applying the re-ignition voltage.

[0004] It is therefore the object of the invention to provide a device for operating a gas discharge lamp that assures reliable operating of the gas discharge lamp in an operating phase with reduced power.

[0005] The object is accomplished by the features disclosed in the main claim.

[0006] The above object generally is achieved according to the present invention by a device for operating a gas discharge lamp, having an energy supply circuit which provides an alternating-current voltage or an alternating current normally having a predetermined period in order to feed the gas discharge lamp with a predetermined power, and circuit means responsive to a reduction of the average lamp power with respect to operation at normal rating, for causing an extension of the period of the lamp alternating-current voltage or the lamp alternating current.

[0007] The device according to the invention has the advantage that an increase in the output voltage is not necessary in order to achieve a higher re-ignition voltage. In accordance with the invention, it is provided that an extension of the period of the alternating-current lamp voltage or the alternating lamp current is provided when a reduction in the average lamp power occurs with respect to operation at normal rating. The reduction in re-ignition voltage through a lowering of the operating frequency could be attributed to the fact that a more intense heating of the lamp electrodes takes place, which leads to an increased electron emission.

[0008] Advantageous modifications and embodiments of the device of the invention ensue from the dependent claims.

[0009] An advantageous embodiment provides that the extension of the period is performed for each half-period. This measure results in a particularly simple embodiment of the device according to the invention, because only the operating frequency of the lamp is to be reduced. A change in a possibly predetermined mark-space ratio with an assumed rectangular voltage need not be undertaken.

[0010] Another advantageous embodiment provides that the amount of the extension of the period is a function of the amount of the provided reduction in power.

[0011] An advantageous modification relates to the energy supply circuit, which preferably includes a DC/DC converter that provides a direct-current voltage at its output which is converted into an alternating-current voltage in a subsequent bridge circuit in whose diagonal the gas discharge lamp is disposed.

[0012] An embodiment of this modification provides that the DC/DC converter is fed by a battery.

[0013] The modifications of the energy supply circuit provided in accordance with the invention permit the embodiment of a highly-efficient DC/DC converter that can have a significantly higher clock frequency internally that the alternating-current voltage acting upon the gas discharge lamp. The comparably low-frequency alternating-current voltage having a frequency of, for example, a few hundred Hertz is generated by corresponding actuation of the bridge circuit.

[0014] The device of the invention is particularly suited for operating a metal-doped, high-pressure gas discharge lamp, which is critical with respect to the consumption of a re-ignition voltage after the undesired extinguishing of the light arc. A preferred use of the high-pressure gas discharge lamp, particularly of the metal-doped, high-pressure gas discharge lamp, is in a headlight of a motor vehicle.

[0015] Further advantageous modifications and embodiments of the device of the invention for operating a gas discharge lamp ensue from further dependent claims and the following description.

[0016] FIG. 1 shows a block diagram of a device of the invention, and FIG. 2 shows, in a block diagram according to FIG. 1, a signal course occurring as a function of time.

[0017] FIG. 1 shows a voltage transformer 10 which converts an electrical power obtained from an energy source 11, which has a voltage U_B, into a corresponding output-side power having a predetermined output voltage U_Aor a predetermined output current I_A. The output voltage U_Ais supplied to a bridge circuit 12, at whose output a lamp voltage U_Lappears which is provided to feed a gas discharge lamp 13. A lamp current that flows through the gas discharge lamp 13 is indicated by I_L.

[0018] The voltage transformer 10 receives a control signal 14 that is provided by a power-rate-setting arrangement 15. The power-rate-setting arrangement 15 provides the control signal 14 as a function of a power rate setting 16, the detected output current I_Aand the detected output voltage U_A. The power-rate-setting arrangement 15, via unit 18, transmits a power reduction signal 17 to unit 20 to a switch-signal-generating element 18, which provides a switch signal 21 for the bridge circuit 12.

[0019] FIG. 2 shows the lamp current I_Las a function of the time t. The lamp current I_Lhas an at least approximately rectangular signal course; in operation at normal rating, the period is set at T_N. A reduced-power operation, in which a reduced lamp current I_Ris provided, exists in addition to operation at normal rating. FIG. 2 shows two discrete current levels I_R1, I_R2, which occur during reduced-power operation. Operation with reduced power occurs during the periods not indicated by T_N. Consequently, reduced-power operation takes place during the illustrated half-periods T_R12, T_R12, T_R21, T_R22.

[0020] The device shown in FIG. 1 is described in detail in conjunction with the course of the lamp current IL shown in FIG. 2, as a function of the time t:

[0021] The voltage transformer 10, the bridge circuit 12 as well as the power rate-setting arrangement 15 and the switch-signal-generating element 19 together form an energy supply circuit for the gas discharge lamp 13. The voltage transformer 10, which is preferably configured as a DC/DC converter, converts the power obtained from the energy source 11, preferably a battery, into a predetermined output voltage U_Aand/or a predetermined output current I_A. In the case of a battery, the voltage of the energy source 11 is set with U_B. The voltage transformer 10 converts the voltage U_Bof the energy source 11, which is generally low, into a generally higher output voltage U_A. The embodiment of the voltage transformer 10 as a DC/DC converter has the significant advantage that a highly-efficient conversion of the voltage can be effected with the clock frequency of the DC/DC converter being able to be set independently of other signal frequencies of the energy supply circuit.

[0022] The downstream bridge circuit 12 has the task of converting the direct-current voltage U_Ainto a lamp voltage U_Lsuitable for feeding the gas discharge lamp 13. An alternating-current voltage that prevents material transport effects in the gas discharge lamp and a non-uniform electrode wear is suitable. The bridge circuit 12 includes, for example, four semiconductor power elements that are connected as an H-bridge in whose diagonal the gas discharge lamp 13 is disposed. The bridge circuit 12 is controlled or switched by the switch signal the switch 21 of signal generating element 19 in such a way that the output voltage U_Aof the voltage transformer 10 is applied to the two electrodes of the gas discharge lamp 13 with alternating polarity. The switch signal 21 determines the frequency, which lies at, for example, a few hundred Hertz.

[0023] The voltage transformer 10 can either impress the output voltage U_Aor the output current I_Aof a gas discharge lamp 13 as a load. The respectively other value is determined by the internal resistance of the gas discharge lamp 13. The output current I_Ais preferably impressed, because the gas discharge lamp 13 is designed for specific currents. The output current I_Acorresponds in amount to the lamp current I_L. The output voltage U_Aand lamp voltage U_Llikewise correspond in amount.

[0024] The power-rate-setting arrangement 15, which transmits a corresponding control signal 14 to the voltage transformer 10, is provided for setting a specific power. For the power to be set, the power-rate-setting arrangement 15 receives a signal as a nominal value from the power-rate setting 16. The actual power value is determined from the detected output voltage U_Aand the detected output current I_A. The necessary sensors are not shown in FIG. 1.

[0025] During operation at normal rating of the gas discharge lamp 13, the amplitude of the lamp current I_Lis at, for example, the value of the nominal current I_Nshown in FIG. 2. The lamp current I_Lpreferably has an at least approximately rectangular signal course having a period T_Nwhich is predetermined by the switch signal 21.

[0026] During the pole-reversal process, in which the lamp current I_Land the lamp voltage U_Lhave a zero crossing, a more or less pronounced reduction in the light arc occurring in the gas discharge lamp 13 takes place as a function of the present operating data of the gas discharge lamp 13. In an extreme case, a complete extinguishing of the light arc can take place following a pole-reversal process. During operation at normal rating of the gas discharge lamp 13, in which the nominal current I_Nflows, for example, generally no notable increase in the re-ignition voltage takes place after the pole-reversal process. In order to assure reliable re-ignition of the lamp following a pole-reversal process, even in operation with reduced power of the gas discharge lamp 13, an extension of the period of the lamp voltage U_Lor the lamp current I_Lis provided in accordance with the invention. It was determined experimentally that, in comparison to normal operation, the re-ignition voltage does not increase, or only increases insignificantly, during operation with reduced power and an extension of the period. This can be attributed to the fact that the electrodes of the gas discharge lamp 13 reach a higher temperature during a longer period. A higher electrode temperature results in a higher electron emission.

[0027] The information that operation with reduced power is taking place is imparted to the switch-signal generating element 19 by way of the power-reduction signal 17. The power-reduction signal 17 is derived, for example, directly from the power-rate-setting 16, in the power-rate-setting arrangement 15. When the power is decreased, the switch-signal-generating element 19 increases the period of the switch signal 21. FIG. 2 shows a first current level I_R1, which can occur during a first reduction in power. The period during operation with the first reduced current level I_R1can be determined from the addition of the half-periods T_R11, T_R12in FIG. 2. An essential feature of the invention is that the half-periods T_R11, T_R12are extended with respect to the corresponding half-period T_N/2 of operation at normal rating. It can be seen from FIG. 2 that the half-periods T_R11, T_R12need not necessarily be of identical length. A change in the mark-space ratio can certainly be effected simultaneously with the extension of the periods T_R12, T_R12. The mark-space ratio is preferably kept constant, and only the period which results from the addition of the two half-periods T_R11, T_R12is increased. A short-time, variable mark-space ratio can result as a function of the constructional circuitry embodiment of the switch signal generating element 19 with transient procedures that occur during a change in power. FIG. 2 shows a further current level I_R2, which can occur in the event of a further power reduction. Corresponding to the further power reduction, the period which can be obtained from the addition of the two half-periods shown, T_R21, T_R22, is also extended. After the half-period T_R22, has ended, the operation initiated with the nominal current I_Nis set again with nominal power. Correspondingly, the period is again identical to the identical value of the period T_Nof operation at normal rating. Instead of the direct transition from reduced power to the higher power shown in FIG. 2, a gradual increase in power with a correspondingly gradual shortening of the period can be provided. The amount of the extension is preferably dependent on the extent of the power reduction.

[0028] A metal-doped, high-pressure gas discharge lamp is provided as an example of a gas discharge lamp 13. This type of gas discharge lamp is suited as a headlight lamp used, for example, in a motor vehicle.