A METHOD AND AN ARRANGEMENT FOR CONTROLLING GAS DISCHARGE LAMPS

Abstract: A method for controlling a drive unit for gas discharge lamps where applied power consists of a DC voltage applied to a converter stage which is adapted to convert the DC voltage to an AC voltage suitable for the lamp, wherein the AC voltage is applied to a lamp circuit that includes a lamp (2) and an inductance (3) connected in series with the lamp, and wherein there is provided a lamp ignition circuit (4). The invention is characterized in that there is connected in a current branch of the drive unit (1) common to both the AC voltage side and the DC voltage side a current measuring sensor (5) whose output signal is used to generate a first control signal (13) corresponding to the power supplied to the lamp circuit, and a second control signal (14) corresponding to the peak current of the lamp circuit or, alternatively, corresponding to the reactive power present in the lamp circuit; and in that a third control signal (17) which constitutes the dominant signal of the first (13) and the second (14) control signal or, alternatively, a function thereof, is caused to control the current supply to the lamp in a feedback system.

Description: A method and an arrangement for controlling gas discharge lamps The present invention relates to a method and to an arrange¬ ment for controlling a drive unit for gas discharge lamps. The drive unit can be used for a number of purposes, for instance to drive gas discharge lamps in vehicle headlamps. Gas discharge lamps have long been used and consequently the technique for powering and igniting such lamps is well known. An alternating voltage is applied to the lamp connected in series to an inductance. The purpose of the inductance is to stabilize the lamp current, since the characteristic of the lamp, particularly after ignition, would otherwise result in current surge through the lamp and therewith damage the lamp and the lamp drive means. Ignition of a lamp is achieved with the aid of induction or series resonance with the help of the inductance for generating the ignition voltage required by the lamp. This known method has drawbacks, however. A first drawback is that the start-up sequence after ignition is primarily determined by the series inductance connected to the lamp. This has been dimensioned with respect to a stable steady state of the lamp with a plasma heated to operating temperature. The possibility of quickly obtaining the light yield obtained in the steady state is therefore greatly limited. Another drawback is that differences in the properties of different lamps, for instance lamps from different manufac¬ turers, are not easily compensated with the aforedescribed most common drive arrangement. A third drawback is that the use of auxiliary components for measuring current and/or voltage in the lamp circuit is The arrangement also includes a signal processing circuit 6 having an input 11 to which the output signal 10 of the current sensor 5 is delivered; cf. Figure 5. The signal processing circuit 6 is adapted to form a first signal corresponding to the mean current value, which is a measure¬ ment of the power delivered to the lamp, and a second signal which corresponds to the peak current value or, alternative¬ ly, to the reactive power present in the lamp circuit. The signal corresponding to the mean signal value is formed in a mean value circuit 7, which may be an RC circuit. The signal corresponding to the peak value is formed in a peak value circuit 8. This signal may be a rectified mean value. Alternatively, the mean value of the negative half periods may be formed in a circuit 9 as a measurement of the reactive power in the lamp circuit. The signal processing circuit 6 also includes a comparison circuit 12 which functions to compare the first and the second signals 13, 14 respectively. The first signal 13 is amplified in an amplifier 15 by a factor k which is greater than 1, since the peak value is always greater than the mean value. If the AC voltage in the lamp circuit is fully sinusoidal, k would be equal to the square root of 2. The factor k may vary, although it will preferably lie in the range of 1.3 to 1.5. The comparison circuit is adapted to control a switch 16 which functions to connect the first signal 13 and the second signal 14 to the converter stage 1. It is the dominating signal of the two compared signals 13, 14 that is used as a third control signal and which is applied tc the converter stage via an input 18. The third control signal 17 is used as a control signal for controlling the supply of current to the lamp 2. The invention also relates to an arrangement of the kind defined in Claim 10 and having the main characteristic features set forth therein. The invention will now be described in more detail with reference to exemplifying embodiments of the invention and also with reference to the accompanying drawings, in which Figure 1 is a block schematic illustrating an inventive drive arrangement; Figure 2 illustrates an alternative embodiment; Figure 3 illustrates a first embodiment of a converter stage; Figure 4 illustrates a second embodiment of a converter stage; and Figure 5 illustrates a control circuit. Figure 1 illustrates an arrangement for controlling a drive unit for gas discharge lamps where supplied power is com- prised of a DC voltage applied to a converter stage 1 which functions to convert the DC voltage to an AC voltage suitable for the lamp 2, wherein an AC voltage is applied to a lamp circuit which includes the lamp 2 and an inductance 3 connected in series therewith, and wherein the arrangement also includes a known lamp ignition circuit 4. According to the invention, a current measuring sensor is connected in a current branch of the drive unit common to both the AC voltage side and the DC voltage side; cf. Figures 3 and 4. The current measuring sensor functions to produce an output signal on its output 10. In the illustrated embodiments, the current measuring sensor 5 is a resistor. However, other current measuring principles can be used, such as "sense-FET" transistors having current measuring outputs, for instance. frequency to which the converter stage 24 is caused to convert the applied DC voltage. This frequency is applied to the converter stage 24 via conductor 27. The control circuit 26 is of a suitable known kind, such as a DC/AC converter which is frequency-controlling in dependence on a control signal. In this embodiment, the start-up sequence can be further speeded-up by causing said frequency to be initially much lower than during the steady state, wherein the lamp current is initially caused to be much higher than in the steady state by virtue of the series choke. Figure 3 illustrates an embodiment of said second converter stage 23 and said converter stage 24 respectively. The converter stage 32, 24 includes a transformer 28 which converts the DC voltage 20 to a suitable AC voltage while using bridge-coupled semiconductor switches 29, 30 which are brought alternately to a conductive and a non-conductive state. Diodes 31, 32 are connected in parallel over the semiconductor switches, these diodes creating a current path for the reactive power that circulates in the lamp circuit due to the effect of the inductance. Figure 4 illustrates an alternative second embodiment of the converter stages 23, 24. This converter stage has, corre¬ spondingly, semiconductor switches 33, 34 over which diodes 35, 36 are connected. When the converter stage shown in Figure 3 or in Figure 4 is used in the Figure 1 embodiment, the semiconductor switches are controlled by means of an oscillator (not shown) which delivers a suitable fixed frequency at which the semiconduc¬ tor switches thus operate. When the converter stages in Figure 3 or Figure 4 are used in the Figure 2 embodiment, the frequency delivered via the In an alternative embodiment, the switch may be adapted so that the third signal is comprised of a function of the first and the second signal. In this regard, the comparison circuit and the switch will suitably be comprised of a microprocessor which is adapted to receive the first and the second signal and to deliver the third signal in accordance with said function. As an alternative to comparing the signals 13 and 14 from the circuits 7 and 8, the first signal 13 may instead be compared with an output signal 19 from the circuit 9 via a conductor indicated in broken lines in Figure 5. According to one preferred embodiment, the power supplied to the drive arrangement is comprised of rectified AC voltage delivered via a conductor 20. According to a first embodiment, shown in Figure 1, the converter stage 1 includes a first converter stage 21 adapted to convert an applied DC voltage 20 to a DC voltage 22 of another voltage level, and includes a second converter stage 23 which is adapted to convert this latter DC voltage to an AC voltage which is applied to the lamp circuit. The second converter stage is shown in Figures 3 and 4. The current measuring sensor 5 is coupled in the second converter stage. The aforesaid third control signal 17 is applied to the first converter stage 21 for converting the applied DC voltage to a DC voltage of desired level. The first converter stage includes to this end an appropriate known DC/DC converter. According to a second embodiment, shown in Figure 2, the converter stage includes a converter stage 24 which is adapted to convert an applied DC voltage 20 to an AC voltage 25 of desired frequency, which is applied to the lamp circuit. The current measuring sensor 5 is coupled in the converter stage 24. The third control signal 17 is applied to a control circuit 26 which is adapted to generate the 8 second signal 14, constitutes the control signal to the second converter stage, this stage will function to control the semiconductor switches 31, 32; 33, 34 so as to shorten the time in which the semiconductors are conductive and therewith introduce a current limitation in the lamp circuit. In addition to solely controlling the supply of current to the lamp during the start-up sequence and during the steady state, the inventive control arrangement is able to disclose a faulty state in the lamp circuit and the nature of the fault. According to one preferred embodiment, the arrangement includes a timing and detection circuit 37 which is activated when the lamp ignition circuit 4 is activated. The timing and detection circuit 37 is constructed to detect a fault state when said second control signal 14 is higher than said first control signal 13 after a predetermined time lapse, this time period corresponding to the lamp having reached a steady state. The fault is very probably caused by a short-circuit in the lamp or in the proximity thereof. The detection circuit thus includes a comparison circuit. According to a further preferred embodiment, the timing and detection circuit 37 is also constructed to detect a fault state after a predetermined timing period when said first and said second control signals are both lower than a predeter- mined value. This predetermined value is close to zero, i.e. when current flows in the lamp circuit solely because of circuit defects. This fault is most probably due to the lamp being unable to ignite. Alternatively, the circuit may have suffered an electric fault. The invention has been described in the aforegoing with reference to a number of exemplifying embodiments thereof. conductor 27 is applied to the semiconductor switches so that said switches operate at this frequency. The drive unit has been described in the aforegoing on the basis of conventional electronic components. For series production of the drive unit, the drive unit can instead be obtained with an arrangement that includes a microprocessor, a digital signal processor or an integrated circuit developed particularly to this end, and appropriate software. During the start-up sequence, when the lamp plasma has not reached a steady state, the lamp voltage is low and the effect of the inductance on the lamp current will thus dominate. In comparison with the steady state, this becomes high although the power delivered to the lamp will be low due to the low lamp voltage. Lamp tolerance is not limited in this stage of maximum permitted lamp power, however, but rather by the largest current that can be applied to the lamp without influencing its anticipated length of life. The first signal 13 is compared with the second signal 14. A heating state exists when the second signal is higher than the first signal, wherein the second signal 14 is used as a control signal to the converter stage 1. In this regard, the lamp is supplied with a higher current than that which corresponds to the steady state. This shortens the start-up sequence. The lamp voltage increases with increasing lamp temperature, wherein the first signal 13 rises and the second signal 14 falls due to the increasing impedance in the lamp circuit. When the first signal becomes higher than the second signal, the control signal to the converter stage 1 is switched to be the first signal 13, wherein the signal that corresponds to the lamp power becomes the control signal. According to one preferred embodiment, the converter stage 23; 24 is adapted so that when the control signal that corresponds to the peak current of the lamp circuit, i.e. the 10 CLAIMS 1. A method of controlling a drive unit for gas discharge lamps where applied power consists of DC voltage applied to a converter stage which functions to convert the DC voltage to an AC voltage suitable for the lamp, wherein the AC voltage is applied to a lamp circuit that includes a lamp (2) and an inductance (3) connected in series therewith, and wherein there is included a lamp ignition circuit (4), characterized in that there is connected in a current branch of the drive unit (1) common to both the AC voltage side and the DC voltage side a current measuring sensor (5) whose output signal is used to generate a first control signal (13) corresponding to the power supplied to the lamp circuit, and a second control signal (14) corresponding to the peak current of the lamp circuit or, alternatively, corresponding to the reactive power present in the lamp circuit; and in that a third control signal (17) which constitutes the dominant signal of the first (13) and the second (14) control signal or, alternatively, a function thereof- is caused to control the current supply to the lamp in a feedback system. 2. A method according to Claim 1, characterized in that the power delivered to the drive unit (1) is comprised of a rectified AC voltage. 3. A method according to Claims 1 or 2, characterized in that there is provided a signal processing circuit (6) to which the output signal of the current sensor (5) is deliv- ered; in that the signal processing circuit is caused to form a first control signal (13) corresponding to the mean value of the current, this value being a measurement of the power delivered to the lamp, and also a second control signal (14) which corresponds to the peak current value; and in that the signal processing circuit (6) includes a comparison circuit (15) in which the first and the second signal are compared, It will be obvious to the person skilled in this art that the various detail solutions can be modified and still retain the function of the invention. The present invention shall therefore not be considered to be limited to the aforedescribed embodiments, since modifica¬ tions can be made within the scope of the following Claims.

Claims: 12 converter stage (24) is caused to convert the delivered DC voltage. 7. A method according to any one of Claims 1-6, character- ized in that said converter stage includes semiconductor switches (31, 32; 33, 34); and in that when the control signal which corresponds to the peak current of the lamp circuit, i.e. the second signal (14), constitutes a control signal to said converter stage, the control signal (17) is caused to actuate the semiconductor switches (31, 32; 33, 34) so as to shorten the time in which the semiconductors are conductive and therewith obtain a current limitation in the lamp circuit. 8. A method according to any one of the preceding Claims, characterized in that there is provided a timing and detec¬ tion circuit (37) which is preferably activated when the lamp ignition circuit (4) is activated; and in that a fault state is detected after a predetermined time period when the second control signal (14) is higher than the first control signal (13). 9. A method according to any one of Claims 1-8, character¬ ized in that there is provided a timing and detection circuit (37) which is preferably activated when the lamp ignition circuit (4) is activated; and in that a fault state is detected after a predetermined time period when said first control signal (13) and said second control signal (14) are both lower than a predetermined value. 10. An arrangement for controlling a drive unit for gas discharge lamps where supplied power consists of a DC voltage applied to a converter stage which is adapted to convert the DC voltage to an AC voltage suitable for the lamp, wherein the AC voltage is applied to a lamp circuit which includes a lamp (2) and an inductance (3) connected in series there¬ with, and wherein the arrangement also includes a lamp wherein the dominating signal is used as said third control signal (17) . 4. A method according to Claims or 2, characterized in that there is provided a signal processing circuit (6) to which the output signal of the current sensor (5) is delivered; in that the signal processing circuit is caused to form a first control signal (13) corresponding to the mean current value, which is a measurement of the power delivered to the lamp, and also a second control signal (19) which corresponds to the reactive power present in the lamp circuit; and in that the signal processing circuit (6) includes a comparison circuit (15) in which the first control signal (13) and the second control signal (19) are compared, wherein the dominat- ing signal is used as the third control signal (17). 5. A method according to Claims 1, 2, 3 or 4, characterized in that the converter stage includes a first converter stage (21) which is caused to convert a delivered DC voltage to a DC voltage of another voltage level, and also includes a second converter stage (23) which is caused to convert this latter DC voltage to an AC voltage which is applied to the lamp circuit; and in that the current measuring sensor (5) is connected in the second converter stage (23); and in that the third control signal (17) is applied to the first converter stage (21) so as to thereby convert the delivered DC voltage to a DC voltage of desired level. 6. A method according to Claims 1, 2, 3 or 4, characterized in that the converter stage includes a converter stage (24) which is caused to convert a delivered DC voltage to an AC voltage of desired frequency which is applied to the lamp circuit; in that the current measuring sensor (5) is connect¬ ed in said converter stage (24); and in that the third control signal (17) is applied to a control circuit (26) which is adapted to generate the frequency to which the 14 that the current measuring sensor (5) is connected in the converter stage (24); and in that the third control signal (17) is applied to a control circuit (26) which is adapted to generate the frequency to which the converter stage (24) is adapted to convert the applied DC voltage. 13. An arrangement according to any one of Claims 10, 11 or 12, characterized in that the converter stages (23; 24) include semiconductor switches (31, 32; 33, 34); and in that when the control signal that corresponds to the peak current of the lamp circuit, i.e. the second signal (14), constitutes the control signal (17) to said converter stage, said signal functions to control the semiconductor switches (31, 32; 33, 34) such as to shorten the time in which the semiconductors are conductive and thereby obtain a current limitation in the lamp circuit. 14. An arrangement according to any one of the preceding Claims, characterized in that the arrangement includes a timing and detection circuit (37) which is adapted to be activated preferably when the lamp ignition circuit (4) is activated; and in that the timing and detection circuit is adapted to detect a fault state after a predetermined time period when said second control signal (14) is higher than said first control signal (13). 15. An arrangement according to any one of Claims 10-14, characterized in that the arrangement includes a timing and detection circuit (37) which is adapted to be activated preferably when the lamp ignition circuit (4) is activated; and in that the timing and detection circuit is adapted to detect a fault state after a predetermined time period when said first control signal (13) and said second control signal (14) are both lower than a predetermined value. ignition circuit (4) , characterized in that there is connect¬ ed in a current branch of the drive unit (1) common to both the AC voltage side and the DC voltage side a current measuring sensor (5) which is adapted to deliver an output signal; in that the arrangement also includes a signal processing circuit (6) to which the output signal of said current sensor is delivered; in that the signal processing circuit (6) is adapted to form a first signal (13) corre¬ sponding to the mean current value, which is a measurement of the power delivered to the lamp, and also a second signal (14) corresponding to the peak current value, or, alterna¬ tively, corresponding to the reactive power present in the lamp circuit; in that the signal processing circuit includes a comparison circuit (15) which functions to compare the first signal (13) and the second signal (14) , wherein the converter stage (21; 26) is adapted to receive the dominant signal of said two signals, or alternatively a function of said signals, this dominating signal constituting a third control signal (17) as a control signal for controlling the supply of current to the lamp. 11. An arrangement according to Claim 10, characterized in that the converter stage includes a first converter stage (21) which is adapted to convert applied DC voltage to a DC voltage of another voltage level, and also includes a second converter stage (23) which is adapted to convert this latter DC voltage to an AC voltage which is applied to the lamp circuit; in that the current measuring sensor (5) is connect¬ ed in the second converter stage (23) ; and in that the third control signal (17) is applied to the first converter stage (21) for converting the applied DC voltage to a DC voltage of desired level. 12. An arrangement according to Claim 10, characterized in that the converter stage includes a converter stage (24) adapted to convert applied DC voltage to an AC voltage of desired frequency which is applied to the lamp circuit; in

Inventors: HOEGBERG BO, HOEGBERG, BO
Applicants: HOEGBERG BO, LABINO AB, HOEGBERG, BO
Application year: 1996
Main classification: H05B41
IPC codes: H05B41