Circuit arrangement

Abstract: The invention relates to a circuit arrangement (II) for igniting a high-pressure discharge lamp (L) provided with transformer means (2), switching means (3) and capacitive means (4) for a periodic charge inversion via the transformer means and switching means. The circuit arrangement is also provided with peak current limiting means (R s ) to protect the switching means. According to the invention, voltage limiting means (7, 8) are added to the transformer means. It is achieved in this way that lamp ignition is to a high degree independent of lamp connection line impedances.

Description: The invention relates to a circuit arrangement having means for forming a high voltage ignition pulse for igniting a high-pressure discharge lamp, provided with transformer means with a primary, breakdown element and capacitive means for periodic charge change via the primary of the transformer means and the breakdown element, as a result of which a voltage pulse is generated over the transformer primary and is transformed in the transformer means to the high voltage ignition pulse at a transformer secondary, and provided with peak current limiting means to protect the breakdown element. The invention also relates to a lamp operating circuit provided with such a circuit arrangement. A circuit arrangement of the kind mentioned in the opening paragraph is known from DE-OS 33 39 814. The peak current limiting means take the form of a coil there. Another possibility is to use a resistor as the peak current limiting means. Combinations of the said means are also possible. From EP-A-0 222 441 is known per se a circuit arrangement for operating a low pressure gas discharge lamp including a converter and means for starting the converter. The known arrangement means do not include a breakdown element as part of an oscillation circuit for generating a voltage pulse. The known circuit arrangement DE-OS 33 39 814 is highly suitable for igniting high-pressure discharge lamps, such as metal halide lamps and high-pressure sodium lamps. A primary part of the transformer means is then included in an electric circuit which comprises both the capacitive means and the switching means. A voltage pulse generated in this circuit as a result of the switching means becoming conducting and the ensuing charge change of the capacitive means is transformed in the transformer means to a high voltage pulse at a secondary part, to which the lamp to be ignited is connected. Although the peak current limiting means influence the level of an ignition pulse generated in the transformer means, this pulse level will be to a considerable degree dependent on the load which the connected lamp including the connection lines forms for the transformer means. In particular, the impedance formed by the connection lines is of major importance. The application possibilities of the known circuit arrangement are limited by this. Thus, if the circuit arrangement is dimensioned so as to ignite a lamp with comparatively long connection lines, so a comparatively great impedance, this circuit arrangement will not be suitable for igniting a similar lamp with comparatively short connection lines, since the ignition pulses will then become considerably higher than can be safely allowed. Conversely, a circuit arrangement dimensioned so as to ignite a lamp via comparatively short connection lines will generate ignition pulses which are insufficiently high for igniting a similar lamp with the use of comparatively long connection lines. The invention has for its object inter alia to provide a measure by which the described disadvantage can be eliminated. For this purpose, a circuit arrangement according to the invention is characterized in that voltage limiting means are placed as short-circuit across the transformer means. The measure according to the invention renders it possible to dimension the circuit arrangement in such a way that it is suitable for reliably igniting lamps which are connected through comparatively long connection lines. Owing to the presence of the voltage limiting means, the level of the ignition pulses will remain limited in the case of short connection lines, so that no unsafe situations are created. An additional advantage is that the transformer means become much less quickly saturated than in the case in which the known circuit arrangement is used. Thus a pulse width necessary for igniting the lamps is maintained as well. The voltage limiting means can be realised in a particularly simple, and thus advantageous manner in that at least two zener diodes are connected in series with opposite polarities as a voltage limiting short-circuit across the transformer means. The short-circuit may be placed across the secondary part of the transformer means. This has the advantage that the high voltage pulse is directly limited, but it involves the requirement that the zener diodes must stand up to high voltages. In practice this means a voltage of 3 to 4 kV. If the short-circuit is placed across the primary part of the transformer means, however, the zener diodes are loaded with a considerably lower voltage, so that less stringent requirements are imposed on the zener diodes in this respect. This renders it possible to use zener diodes of a comparatively inexpensive kind. In practice, supplementary capacitive means have been connected across the transformer means in the known circuit arrangement. This does reduce the influence of the connection line impedance on the ignition pulse level, but it does not eliminate the described disadvantage. In addition, the capacitance thus used forms an additional impedance both during lamp ignition and during stable lamp operation. An embodiment of a circuit arrangement according to the invention will be described in more detail with reference to a drawing, in which Fig. 1 is a diagram of a lamp operating circuit provided with a circuit arrangement according to the invention, and Fig. 2 shows a lamp operating circuit provided with a modification of a circuit arrangement according to the invention. In Fig. 1, a circuit arrangement II forms part of an electronic lamp operating circuit I provided with connection terminals I for connecting a supply source, for example, an AC voltage source of 220 V, 50 Hz and provided with lamp connection terminals A, B to which a lamp L is connected. The circuit arrangement II comprises a transformer 2 as the transformer means, a breakdown element 3 as the switching means, and a capacitor 4 as the capacitive means for periodic charge change in the form of a charge inversion via transformer 2 and breakdown element 3. The transformer 2 comprises a primary winding 21 as a primary part and a secondary winding 22 as a secondary part. An impedance R s is also included as a peak current limiting means to protect the switching means. Preferably, impedance R s is designed as a self-inductance, although alternatively a resistor or combinations thereof are possible. A resistor 6, which serves to ensure that the circuit arrangement will become operational after the supply source has been connected, is connected across breakdown element 3. Voltage limiting means in the form of zener diodes 7, 8 connected in series with opposite polarities shunt the transformer 2 as the voltage limiting short-circuit. The circuit arrangement II may also be provided with a capacitor C p , which safeguards a defined current path in the case of breakdown of the switching means. If the circuit arrangement is used in an electronic lamp operating circuit, the use of capacitor C p may be dispensed with, since such a lamp operating circuit contains sufficient capacitive means for safeguarding the desired current path in the case of breakdown of the breakdown element 3. The use of the separate capacitor C p , however, is desirable if the circuit arrangement II is used in a conventional lamp operating circuit. The embodiment described is particularly suitable for use in a lamp operating circuit in the form of a high-frequency switch mode power supply (SMPS) which supplies a square-wave voltage to the lamp. The configuration described realises substantially a doubling of the voltage across breakdown element 3 without requiring further auxiliary means. This renders it possible to choose the breakdown voltage of the breakdown element 3 to lie at such a level that it is certain that no breakdown can occur during stable lamp operation. A practical embodiment of the lamp operating circuit described was tested for the ignition of a metal halide lamp of a power rating of 70 W. The lamp operating circuit was realised in the form of a high-frequency switch mode power supply, comprising an up-converter for transforming the AC voltage of 220 V, 50 Hz which serves as the supply source to a DC voltage of 385 V, followed by a down-converter as the current source for operating the lamp included in a commutator network. The inclusion of the lamp in a commutator network serves to counteract the occurrence of cataphoresis during lamp operation. The circuit arrangement for igniting the lamp was included in the commutator network. The primary winding 21 of transformer 2 had 9 turns, and the secondary winding 22 had 270 turns. The zener diodes 7, 8 were of the BZT 03C 120 type with a breakdown voltage of 120 V. The breakdown element 3 was constructed as a series circuit of two sidacs, one of the K 1V 24 type and one of the K IV 16 type, with a joint breakdown voltage of 400 V. The K IV 24 type sidac was shunted by a resistor of 27 kΩ, and the K IV 16 type sidac was shunted by a resistor of 18 kΩ. Impedance R s was a self-inductance of 13,5 µH and capacitor 4 had a capacitance of 150 nF. Immediately after connection of the lamp operating circuit to the supply source, a square-wave voltage of 300 V is across the circuit arrangement, consecutive square waves having different polarities. The commutation frequency is approximately 100 Hz. The result is that the breakdown element 3 breaks down at the second square wave and that a charge inversion across capacitor 4 takes place via primary winding 21 and self-induction coil R s . This supplies a pulse level of 4 kV at the secondary winding 22. Fig. 2 shows a modification of circuit arrangement II in which parts corresponding to those of Fig. 1 have the same reference numerals. A charge change in the form of a discharge of capacitor 4 in this modification takes place in the separate closed electric circuit of primary winding 21, breakdown element 3 and self-inductance R s . Charging of capacitor 4 via resistor R takes place during the period before breakdown of the breakdown element 3. Substantially no voltage doubling occurs in this modification of circuit arrangement II. Therefore, the breakdown voltage level of the breakdown element 3 will have to be chosen below the maximum voltage supplied to the circuit arrangement II. Thus in this circuit arrangement II the breakdown element will have a lower breakdown voltage level, for example, 200 V.

Claims: A circuit arrangement (11) having means for forming a high voltage ignition pulse for igniting a high-pressure discharge lamp (L), provided with transformer means (2) with a primary (21), breakdown element (3) and capacitive means (4) for periodic charge change via the primary of the transformer means and the breakdown element, as a result of which a voltage pulse is generated over the transformer primary (21) and is transformed in the transformer means (2) to the high voltage ignition pulse at a transformer secondary (22), and provided with peak current limiting means (R s ) to protect the breakdown element, characterized in that voltage limiting means (7,8) are placed as short-circuit across the transformer means. A circuit arrangement as claimed in Claim 1, characterized in that the voltage limiting means (7,8) are placed as short-circuit across the primary of the transformer means (2). A lamp operating circuit (I) suitable for igniting and operating a high-pressure discharge lamp (L) and provided with the circuit arrangement (II) as claimed in any one of the preceding Claims.

Inventors: BLOM ANTON CORNELIS, BLOM, ANTON CORNELIS
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H05B41
IPC codes: H05B41, H01J61