CIRCUIT ARRANGEMENT FOR OPERATING A HIGH-PRESSURE DESCHARGE LAMP

Abstract: The invention relates to a circuit arrangement for operating a discharge lamp (1) by means of a direct voltage source, which circuit arrangement is provided with - direct voltage terminals (A,B) for connection of the di­ rect voltage source, - alternating voltage output terminals (K,L) for connection of the discharge lamp, - a direct voltage/alternating voltage converter (3), and - a current limiter (5) for limiting the current through the lamp during the operating condition. According to the invention, the direct voltage/­ alternating voltage converter (3) is a sine converter and a controllable direct voltage converter (2) is arranged between the sine converter and the direct voltage input terminals (A,B).

Description: The invention relates to a circuit arrangement for operating at least one high pressure discharge lamp by means of a DC voltage source provided in a transport means, this circuit arrangement being provided with input terminals for connection of the DC voltage source, alternating voltage output terminals for connection of the discharge lamp, a controllable DC/DC voltage converter, a direct voltage/alternating voltage sine converter, and a current limiter for limiting the current through the lamp in the operating condition. The term sine converter is to be understood to mean in this description and in the claims a converter which supplies during operation a substantially sinusoidal alternating voltage. A circuit arrangement of the kind mentioned in the opening paragraph is known from GB-A-2024544. In the known arrangement the input teminals are intended to be connected to an ac mains as voltage source such as 120 V, 60 Hz mains supply. The supply voltage is in the arrangement unfiltered rectified and fed to the controllable voltage converter, being a down converter to convert the input voltage to a dc voltage of 35 to 50 V. The dc voltage forms the input for the sine converter to which the lamp is connected as part of a load circuit. With the known circuit arrangement it is possible to operate a discharge lamp on an ac mains with high frequency pulsed current with low or nearly none mains current distortion and with a near unity power factor. "High frequency" is to be understood herein to mean a frequency of 500 Hz or higher. The known circuit arrangement is provided with a battery which is intended to directly supply the sine converter in case of a short mains failure and which causes the operated high pressure discharge lamp not to extinguish and therefore makes it possible that the high pressure discharge lamp will come back on to its intensity when the power from the mains is restored. In literature high pressure discharge lamps are proposed for use as head light of transport means. For operating a high pressure discharge lamp as a transport means head lamp the known circuit arrangement is completely unsuited as a transport means, like a motor car is namely not provided with a mains voltage source. Thus for operating a lamp on motor cars and the like use must be made of the motor car accumulator as the only kind power source, being in general a source of relative low voltage in the range of 9 to 15 dc. A problem to be solved is however the relative large spreading in accumulator voltage values. Another problem is the difference between the level of input voltage and the usual level of lamp voltage of about 100 V. Thus the invention has for its object to provide a circuit arrangement which is suitable for operating a high pressure discharge lamp as a head light of a transport means. For this purpose, a circuit arrangement of the kind mentioned in the opening paragraph is characterized in that the input terminals are direct voltage input terminals which during lamp operation are connected to a transport means accumulator and that the controllable voltage converter is an up-converter being connected between the direct voltage input terminals and the sine converter. An up-converter, also designated as boost converter or step-up converter is known in general sense inter alia from a book entitled "Switch Mode Power Supply, Basic Theory and Design" of K. Kitsum, New York 1984. By use of an up-converter between accumulator input voltage and sine converter it has appeared that the circuit arrangement can be highly insensitive to spread in or variation of the value of the direct voltage of the accumulator. A further advantage of the invention is that an up-converter derives during operation a continuous current from the direct voltage accumulator in contrast to the input current of a down convertor. Yet another advantage is that the voltage as supplied by the up-converter has a higher value than the accumulator voltage as a result of which transformation in the sine converter can be limited. From German Patent Specification 1.149.111 it is known to operate discharge lamps on a transport means accumulator. However in the known art low pressure lamps are thus operated and the lamps are operated on a rectangular alternating voltage. In case of operating high pressure discharge lamps such rectangular alternating voltage may give rise to unstable operation of the lamp due to acoustic resonances. A highly sinusoidal alternating voltage signal is then of importance because for many types of high-pressure discharge lamps it holds that the operation of the relevant lamps free from unstabilities due to the acoustic resonances is limited to comparatively narrow frequency ranges. In an advantageous embodiment of the circuit arrangement the controllable up-converter is also provided with a power regulation as to regulate the output voltage of the sine converter in accordance with power consumed by the lamp. In a further embodiment of a circuit arrangement, the current limiter is arranged between the sine converter and the alternating voltage output terminals, which has the advantage that passive elements, such as capacitors and self-inductances, can be used as current limiter, as a result of which, whilst maintaining the sinusoidal form, the power dissipation in the current limiter can remain limited. Preferably, the current limiter comprises a series arrangement of a self-inductance shuntable by a switch and a saturable self-inductance. The shuntable self-inductance serves for current limitation during a keep-alive state. During starting and in the operating condition, this shuntable self-inductance is short-circuited by a shunt. The saturable self-inductance serves as a current limiter in the operating condition, but permits of choosing the current during the transient phase considerably larger in comparison with the current in the operating condition whilst using a comparatively small increase of the alternating voltage produced in the sine converter. A large transient current is conductive to rapidly reaching the stable operating condition and hence to rapidly emitting light by the lamp. A keep-alive state is a state in which a small current conduction is maintained through the lamp, but in which no or substantially no light is produced. The keep-alive state is of importance because instantaneous light production in a high-pressure discharge lamp is possible from this state. An embodiment of a circuit arrangement according to the invention will be described more fully with reference to a drawing. The circuit arrangement comprises two direct voltage input terminals A, B for connection of a direct voltage source and two alternating voltage output terminals K, L for connection of a high-pressure discharge lamp 1. Direct voltage input terminals A, B are connected to input terminals C, D of a direct voltage converter 2, of which output terminals E, F serve at the same time as input terminals of a direct voltage/alternating voltage converter 3 constructed as a sine converter. The sine converter has output terminals G, H. A series-combination of a shuntable self-inductance 4, a saturable self-inductance 5 and a capacitor 6 is arranged as a current limiter between the output terminal G and the alternating voltage output terminal K. The shuntable self-inductance 4 is shunted by a switch 40 a . A branch 5 a of the saturable self-inductance 5 is connected to a starting circuit 8, which is connected to the alternating voltage output terminal L. The direct voltage converter 2 is constructed as an up-converter consisting of a self-inductance 21, a diode 22, a capacitor 24 and an electronic switch 23. The control electrode 25 of the electronic switch is connected to a control circuit 26. A first input terminal M of the control circuit 26 is connected to a junction 11 of a voltage divider circuit comprising resistors 28 and 29, which is connected between the terminals E and F. A second input terminal N of the control circuit 26 is connected to the terminal F. The terminal F is further connected via a measuring resistor 27 to the input terminal D. A third input terminal F of the control circuit 26 is connected to the input terminal D. By means of the control circuit 26, the operation of rendering the electronic switch 23 conducting and non-conducting and hence the voltage at the terminals E, F can be controlled. The voltage at the terminals G, H is again dependent upon the voltage at the terminals E, F. A broken line indicates that the input terminal M of the control circuit 26 can be connected via a resistor 201 and a switch 40 b to an auxiliary direct voltage source V+. In a practical embodiment of a circuit arrangement, this arrangement is suitable for operating a high-pressure metal halide lamp of 35 W having a stable work-point at a voltage across the lamp of 100 V and a current through the lamp of 0.35 A. A lamp of this kind is known in general sense from NL-PA 8204653 (PHN 10513). The circuit arrangement is then connected to an accumulator as a direct voltage supply source having a voltage of about 13 V. The sine converter 3 is proportioned in this case for supplying an alternating voltage having a value lying between 200 and 300 V at a frequency of 10 kHz corresponding to a direct voltage at the input terminals E, F between 16 and 24 V. The electronic switch 23 is constructed as a MOSFET and its control is proportioned so that the voltage at the terminals E, F is 24 V as far as the current through the resistor 27 is not more than 1.7 A. With current intensities through the resistor 27 of more than 1.7 A, the control 26 ensures that the voltage at the terminals E, F satisfies the relation 9.31 I + 1.05 V = 41 , where: I is the current through the resistor 27, and V is the voltage at the terminals E, F. The saturable self-inductance is proportioned so that in the case of a closed switch 40 a and a voltage of 200 V at the terminals G, H the current is limited to 0.35 A and in the case of a voltage of 300 V at the terminals G, H the current is limited to 1 A. The operation of the circuit arrangement is as follows. The switch 40 a is closed when the lamp is started and remains closed until the keep-alive state is adjusted. After the lamp has been started by means of starting pulses produced in the starter 8, the transient phase of the lamp begins. Immediately after starting, the voltage across the lamp is about 20 V and the power consumed by the lamp is very low. During starting and initially in the transient phase, the current thorugh the resistor 27 is also low and the control of the up-converter ensures that the voltage at the terminals E, F is a maximum, i . e . is 24 V. The voltage at the terminals G, H will therefore be 300 V, as a result of which at least during the transient phase the maximum current is 1 A will flow through the lamp. The lamp voltage will gradually increase, as a result of which the power consumed by the lamp increases. This results in an increase of the current through the resistor 27 until it has reached a value of 1.7 A. The power extracted by the circuit arrangement and the lamp from the supply source then amounts to 24 x 1.7 = 41 W . With an efficiency of the circuit arrangement of 0.85, this corresponds to a power consumed by the lamp of 35 W. With a further increase of the current through the resistor 27, the control of the up-converter passes to the relation 9.31 I + 1.05 V = 41 and the voltage at the terminals E, F will decrease, as a result of which the voltage at the terminals G, H will also decrease. At the same time, the voltage across the lamp further increases to the voltage associated with the stable work-point, i . e . 100 V. The saturable self-inductance thus comes out of its saturation and limits the current through the lamp. This course is continued until the lamp has reached its stable work-point corresponding to a current through the lamp of 0.35 A and a voltage across the lamp of 100 V. The voltage at the terminals G, H is then 200 V and that at the terminals E, F is 16 V, while the current through the resistor 27 is about 2.6 A. With the operation described, it has been found that in practice a regulation for operating the lamp with a constant power is realized on close approximation, although the relation 9.31 I + 1.05 V = 41 deviates from an exact constant power relation ( I x V = constant ). Due to the combination of the sine converter, an acceleration of the transient phase is also obtained, which has the advantage that the lamp more rapidly produces light. In case the lamp is brought into the keep-alive state, this is effected after the lamp has traversed the transient phase. When the switch 40 a is opened, the self-inductance 4 is connected in series with the lamp, as a result of which the lamp current descreases. The value of the self-inductance 4 in the example described is 42 m H and the lamp current then becomes 0.12 A. A decrease of the lamp current results in that ionization in the lamp is maintained, but that no or sunstantially no light is produced. The power consumed by the lamp in combination with the self-inductances 4 and 5 also decreases, as a result of which the control of the up-converter will cause the voltage at the terminals E, F and hence at the terminals G, H to increase. In order to avoid that a high voltage of long duration at the terminals G, H will form an overload for the sine converter, an auxiliary voltage V+ is applied via the resistor 201 and the switch 40 b to the junction 11 of the voltage divider circuit 28, 29. The application of the auxiliary voltage V+ will result in that the voltage at the terminals E, F and hence the voltage at the terminals G, H decreases. In the example described, the auxiliary voltage V+ and the resistor 201 are chosen so that in the keep-alive state a voltage of 262 V is applied to the terminals G, H. The switch 40 b will be closed when the switch 40 a is opened and will be opened when the switch 40 a is closed.

Claims: A circuit arrangement suitable for operating at least one high pressure discharge lamp by means of a DC voltage source provided in a transport means, this circuit arrangement being provided with input terminals for connection of the DC voltage source, alternating voltage output terminals for connection of the discharge lamp, a controllable DC/DC voltage converter a direct voltage/alternating voltage sine converter, and a current limiter for limiting the current through the lamp in operating condition, characterized in that the input terminals are direct voltage input terminals which during lamp operation are connected to a transport means accumulator and in that the controllable DC/DC voltage converter is an up-converter being connected between the direct voltage input terminals and the sine converter. A circuit arrangement according to claim 1, characterized in that the current limiter comprises a series arrangement of a self-inductance shuntable by a switch and a saturable self-inductance.

Inventors: LUURSEMA, MEERTEN
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H05B41
IPC codes: H05B41, B60Q1