EP0489747B1 - Circuit arrangement for operating a gas discharge lamp - Google Patents

Abstract

The proposal is for a circuit arrangement for operating a gas discharge lamp (1) in which the high voltage to strike the gas discharge lamp is generated via the energy stored in a transformer (7) or capacitor. To ensure a steep rise in the ignition voltage, it is fed to the gas discharge lamp via a voltage-controlled switch (5). It is proposed that the voltage-controlled switch (5) takes the form of a tumbler diode which has very short switching times. The ignition circuit is switched off after ignition. This circuit arrangement is especially suitable for striking gas discharge lamps which are still warm from operation. It is therefore particularly suitable for motor vehicle headlamps.

Description

The invention relates to a circuit arrangement for operating a gas discharge lamp according to the preamble of the independent claim. From DE-A 31 08 548 an ignition circuit for a high pressure metal vapor discharge lamp is known, in which an LC series resonant circuit acts on the switchable primary winding of a transformer. The high voltage induced in the secondary winding ignites the gas discharge lamp. In the known ignition circuit, the secondary winding of the transformer remains part of the lighting circuit after the gas discharge lamp has been ignited. The ignition circuit is constantly thermally stressed by the additional losses that occur. The rate of rise of the ignition voltage, which should have high values in particular for igniting warm, high-pressure gas discharge lamps, is limited in the known ignition circuit by the components contained in the lighting circuit of the gas discharge lamp.

From DE-A 20 60 472 and WO-A 82/01113 circuits for operating gas discharge lamps are known which have an ignition circuit. A separation of the ignition circuit from the gas discharge lamp as a function of the voltage generated in the ignition circuit is not provided in the previously known circuit arrangements. The connection, shown in WO-A 82/01113, of a secondary winding of an ignition transformer by means of a diode to the gas discharge lamp causes decoupling only under certain potential conditions. Accordingly, a direct voltage operation of the gas discharge lamp is in the Known circuit arrangement provided. In the event of an electrical vibration occurring in the secondary winding, only negative signal components are kept away from the gas discharge lamp.

From EP-A 0 331 840 a circuit arrangement for operating a high-performance gas discharge lamp is known, which contains components which perform predetermined functions both during the burning operation and during the ignition process. After the gas discharge lamp has been ignited, those components which are only required during the ignition process are separated from the circuit arrangement by means of relays.

Advantages of the invention

The circuit arrangement according to the invention with the characterizing features of the main claim has the advantage over the fact that the transformer is only actively connected to the gas discharge lamp during the ignition phase, so that the power loss of the circuit arrangement is very low.

It is also particularly advantageous that the rate of rise of the ignition voltage pulses is very high. As a result, the ignition energy stored in the transformer can be used for igniting the gas discharge lamp with almost no loss, so that in particular a warm gas discharge lamp can also be ignited safely.

Another advantage can be seen in the decoupling of the ignition circuit from the firing circuit, which is achieved by the voltage-controlled switch. This prevents undesired crosstalk of the ignition voltage pulses on the firing circuit. It is also particularly advantageous that no resonant circuit is required to increase the ignition voltage. As a result, the circuit structure can be produced inexpensively.

Advantageous further developments and improvements of the circuit arrangement specified in the main claim are possible through the measures listed in the subclaims. It is particularly advantageous that the ignition part is switched off in the burning mode, since the high ignition voltage is not required in the burning mode.

It is also advantageous if a breakover diode is used as the semiconductor switch, which has a high reverse voltage and only becomes conductive when the high ignition voltage is reached. Such breakover diodes have a low residual current in the blocked state and are very low-resistance when they break into the conductive state.

Another advantage is that the ignition is monitored by means of a light sensor. As long as the lamp is not on, the light sensor controls the control circuit to generate new ignition pulses. After the gas discharge lamp has been ignited, the light sensor suppresses the generation of further ignition pulses.

Since the lamp does not continue to burn in the event of a defect, for example as a result of a short circuit or an interruption, the signal from the light sensor can advantageously be used to monitor the lamp circuit, regulate the lamp power or the light intensity or to switch off the high voltage. This is particularly advantageous for safety reasons when changing the lamp.

By using an autotransformer, the primary winding of which can be charged by a capacitor, a particularly cost-effective construction of the circuit arrangement is achieved for the generation of the ignition voltage.

Another advantage is the fact that the ignition coil of the motor vehicle engine is used instead of the transformer. As a result, existing components are used several times, so that the construction of the circuit is very inexpensive.

The use of a gas discharge lamp in a motor vehicle headlight is also favorable. Since the ignition is guaranteed even when the lamp is warm, the lamp can also be used as a flasher.

It has also proven to be favorable if a capacitor is connected upstream of the ballast to support the ignition process, the high voltage of which is decoupled from the generator by a diode.

A particularly simple circuit arrangement results if a second generator is used instead of the transformer.

Further advantages and improvements of the invention can be found in the description.

drawing

Embodiments of the invention are shown in the drawing and explained in more detail in the following description. 1 shows a first embodiment, FIG. 2 shows a second embodiment, FIG. 3 shows a third embodiment and FIG. 4 shows a fourth embodiment.

Description of the embodiments

1 shows a first exemplary embodiment, in which a gas discharge lamp 1 is connected to the voltage output of an AC voltage generator 2 via the series connection of a ballast 4 and a capacitor 3, the firing circuit 10 of the circuit arrangement. The alternating voltage generator 2 is connected with its second output to the second electrode of the gas discharge lamp 1, which is continued as a ground line. A series circuit is arranged in parallel with the gas discharge lamp 1 and has a voltage-controlled switch 5, the secondary winding 12 of a transformer 7 and a controlled switch 9. The primary winding 11 of the transformer 7 is connected by its one connection to the switched side of the secondary winding and by its second connection to a first output 13 of a control circuit 6. The control circuit 6 has a second output 14, which is led to the control input of the controlled switch 9, which is expediently designed as a semiconductor switch is. The control circuit 6 is controlled by a light sensor 8, the arrangement of which is selected such that it can detect the ignition of the gas discharge lamp 1. This circuit part is referred to as the pulse ignition part.

The voltage supply to the light sensor 8, the control circuit 6 and the generator 2 takes place via common lines which are formed by the connections 15 and 16.

The mode of operation of this circuit is described below.

As can be seen from the circuit diagram shown in FIG. 1 as a block diagram, the circuit arrangement essentially has two circuits for the operation of the gas discharge lamp. The first circuit is formed by the generator 2, the capacitor 3 and the ballast 4 in connection with the gas discharge lamp 1. In this circuit, the generator 2 supplies the operating voltage necessary to maintain the burning operation of the gas discharge lamp 1. Depending on the lamp type, the necessary operating voltage is e.g. 60 to 120 volts. The operating frequency of the generator 2 is advantageously in the kilo-Hz range, because this makes the components and the generator smaller and less expensive to manufacture and, in addition, the luminous efficacy of the gas discharge lamp is particularly high in this frequency range.

However, since several kilovolts (for example 5 to 15 KV) are required to ignite the lamp, the lamp is connected in parallel to an ignition circuit which essentially has the voltage-controlled switch 5, the transformer 7 and the controlled switch 9 Monitoring of the burning function is provided by the control circuit 6, which can be activated via the light sensor 8. The control circuit 6 contains switching elements, clock generators, the voltage supply for the transformer and monitoring elements for monitoring the function of the gas discharge lamp and safety functions.

To ignite the gas discharge lamp 1, the switch 9 is first closed, so that the primary winding 11 of the transformer 7 can charge via the first output of the control circuit 6. After charging, the switch 9 is opened so that a voltage is induced in the secondary winding 12. With a suitable transmission ratio of the two windings, the induced voltage for igniting the gas discharge lamp 1 is sufficiently high. The voltage-controlled switch 5 is initially blocked when the voltage is low. If the induced voltage rises above the threshold value of the switch 2, then it becomes practically abruptly conductive, so that a high voltage amplitude is available at the combustion electrodes for igniting the gas discharge lamp. In this exemplary embodiment, a breakover diode was used as the voltage-controlled switch 5. Toggle diodes have the advantage that they are very high-resistance below their threshold voltage, which is adapted to the ignition voltage requirement of the lamp and is, for example, 20,000 volts, so that the losses caused by leakage currents are very low. If the threshold voltage is exceeded, the breakover diode becomes very low-resistance so that it can transmit the energy stored in the transformer 7 to the gas discharge lamp 1. Here, the gas discharge lamp 1 is decoupled from the AC generator 2 and capacitor 3 by the ballast 4.

In parallel to the ignition circuit, the capacitor 3 is provided to support the ignition process in the combustion circuit. Since the capacitor 3 is further charged with each unsuccessful attempt to ignite, it can also transfer its energy to the hot electrode of the gas discharge lamp via the ballast during the ignition process and support the successful last attempt to ignite. In particular in the case of a warm gas discharge lamp, this also supports safe ignition and promotes the coupling of the lighting circuit.

The generator 2 can also be followed by a diode 42, of which a capacitor 41 can be connected to a connecting terminal 15, 16, preferably to ground (see FIG. 4).

In the exemplary embodiment, the ignition circuit was dimensioned in such a way that a single ignition pulse has about a few microseconds. If this pulse was not sufficient to ignite the lamp, then further ignition pulses are generated, with the voltage at the capacitor 3 and at the gas discharge lamp 1 continuously increasing with each ignition pulse. The charging voltage of the capacitor 3 is determined by the ignition energy transmitted by the voltage-controlled switch 5 during the ignition process. The charging voltage of the capacitor 3 is increased continuously since the capacitor was not discharged if the ignition attempt was unsuccessful.

After the ignition of the gas discharge lamp 1, which is monitored by the light sensor 8, the control circuit 6 keeps the switch 9 open, so that the primary winding 11 of the transformer 7 is prevented from being recharged. The voltage-controlled switch 5 returns, as after each ignition pulse, to its high-resistance state and thus decouples the ignition circuit from the gas discharge lamp 1. The combustion mode is now maintained by the generator 2. The ballast 4, in conjunction with the capacitor 3, limits the current, since the gas discharge lamp 1 has a low resistance during the burning process. A resonance tuning between the capacitor 3 and the ballast 4, which is necessary in known ignition circuits, is not necessary here. Instead of the transformer 7, ignition coils or corresponding transformers of a motor vehicle engine can also be provided.

Photo transistors or photo resistors can be used as light sensor 8. They recognize the optical radiation of the gas discharge lamp 1 and emit a corresponding electrical signal which can be used to control the control circuit 6. The control circuit 6 also has switching devices, for example comparators for current measurement, which prevent unwanted ignition in the event of a missing gas discharge lamp 1 or a short circuit in the lines. This is for security reasons required if this circuit arrangement is used to operate the headlight of a motor vehicle.

With a controller, not shown, the control circuit 6 can regulate the combustion current of the gas discharge lamp in a light-dependent manner.

A thyristor or a spark gap can also be used as a voltage-controlled switch. However, spark gaps have the disadvantage that they emit high levels of interference, which may be undesirable, and they are also subject to wear. In contrast, the use of a breakover diode has the advantage that when the breakover voltage is reached, the breakover diode is sufficiently fast, i.e. changes to the conductive state within a few nanoseconds. Such breakover diodes have already been proposed in patent application EP 88/00456.

A second exemplary embodiment of the circuit arrangement is shown in FIG. 2. Compared to the circuit arrangement shown in FIG. 1, the control of the transformer 7 is changed here. The primary winding 11 of the transformer 7 is preceded by a second capacitor 20 which is charged via a diode 21 from the output 13 of the control circuit 6. A switch 9 is connected to ground between the capacitor 20 and the diode 21. After the capacitor 20 has been charged, this switch 9 is closed via the output 14 of the control circuit 6, as a result of which the ignition voltage is induced in the secondary winding 12 of the transformer 7. After reaching the breakover voltage, the voltage-controlled switch 5 becomes conductive and very quickly transmits the energy stored in the transformer 7 to the gas discharge lamp 1. As in the first exemplary embodiment, the circuit arrangement is dimensioned such that the capacitor 3 can receive several charging pulses until the ignition voltage for the gas discharge lamp 1 is reached. After the ignition, the generator 2 takes over the energy supply for the Gas discharge lamp 1 while the transformer 7 remains switched off.

In a further embodiment of the invention, it is provided according to FIG. 3 to use a high-voltage generator 30, for example an ignition transformer, which charges a capacitor 32 which can be connected in parallel to the gas discharge lamp 1 via the voltage-controlled switch 5 via a diode 31. The energy stored in the capacitor 32 switches through the controllable switch 5 for igniting the gas discharge lamp 1 when the breakover voltage is reached. Otherwise, this circuit arrangement works like the previously described exemplary embodiments.

The proposed exemplary embodiments can also be operated on a DC voltage network if the available DC voltage, for example from the battery of a motor vehicle, is clocked to generate a pulsed high voltage. Such chopper circuits are known and need not be described in detail.

If gas discharge lamps 1 with a low operating voltage, for example for 12 volts, are used, the generator 2 can be replaced by a battery. In this case, the capacitor 3 is to be replaced by a capacitor connected to ground.

Claims (9)

1. Circuit arrangement for operating a gas-discharge lamp (1) on a voltage generator (2) having an inductor (4) as a ballast, with a control circuit (6) for the starting process of the gas-discharge lamp (1), characterised in that the control circuit (6) can be connected via a voltage-controlled switch (5) to the gas-discharge lamp (1).
2. Circuit arrangement according to Claim 1, characterised in that an ignition transformer (7), which is assigned to the control circuit (6), is provided with a primary winding (11) and a secondary winding (12) and in that the secondary winding (12) can be connected by means of the voltage-controlled switch (5) to the gas-discharge lamp (1).
3. Circuit arrangement according to Claim 1, characterised in that the control circuit (6) contains a generator (30) which charges a capacitor (32), and in that the capacitor (32) can be connected by means of the voltage-controlled switch (5) to the gas-discharge lamp (1).
4. Circuit arrangement according to Claim 2, characterised in that the transformer (7) is connected as an economy transformer and in that its primary winding (11) is connected via a second capacitor (20).
5. Circuit arrangement according to Claim 2, characterised in that the transformer (7) is an ignition coil.
6. Circuit arrangement according to Claim 1, characterised in that the voltage-controlled switch (5) is a semiconductor switch, preferably a trigger diode.
7. Circuit arrangement according to one of the preceding claims, characterised in that a light sensor (8) is provided which records the light of the gas-discharge lamp (1) and acts on the control circuit (6) to end the charging process when the gas-discharge lamp (1) starts.
8. Circuit arrangement according to one of the preceding claims, characterised in that the control circuit (6) interrupts the charging process when the gas-discharge lamp (1) is missing or has a short-circuit.
9. Circuit arrangement according to one of the preceding claims, characterised by the use in a motor vehicle headlight.

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