Device for operating discharge lamps by means of a transformer with four windings, and a corresponding method

A device for operating a plurality of discharge lamps is to be fashioned cost-effectively. Consequently, two discharge lamps (71, 72) are operated with the aid of one ballast in whose load circuit the heating current for the individual incandescent filaments (711, 712, 721, 722) is transmitted via a heating transformer with three secondary windings (Lhs1, Lhs2, Lhs3). The associated primary winding (Lhp) is located in a coupling-out circuit (30) with the aid of which the required heating energy is coupled out via an inductor (Lres). The heating current can be controlled by a temperature-sensitive thermistor (PTC).

FIELD OF THE INVENTION

The present invention relates to a device for operating at least two discharge lamps. Moreover, the present invention relates to a corresponding method for operating two discharge lamps. In particular, the present invention relates to electronic ballasts in which such a device is integrated. Operating discharge lamps comprises in this case both their starting and their being alight.

BACKGROUND OF THE INVENTION

It is known to operate two discharge lamps with two load circuits. In this case, the term load circuit refers to the load of a bridge that is used as an inverter to operate a discharge lamp. Each load circuit has a dedicated preheating arrangement for the respective lamp. Furthermore, according to the internal prior art, it is possible to operate two lamps in one load circuit. Here, the primary coil of a heating transformer of a series circuit of two lamps is connected in parallel and the secondary coil of the heating transformer is connected between the two lamps. Furthermore, it is possible to heat all the filaments of the lamps by transformer via secondary windings, the primary winding being situated in a section of the bridge suitable for the application.

It is relatively complicated to implement the load circuits in terms of circuitry, since electronic control circuits with relay or transistor switches are required for a defined, sequential starting and subsequent joint operation of the lamps. By contrast, relatively favorable control circuits that use only passive components for controlling the preheating exist for the purpose of operating individual lamps. The essential constituent of such circuits is a heat-sensitive resistor with a positive temperature coefficient.

A bridge circuit with a relevant load circuit is illustrated in FIG.1. The bridge is implemented for the purpose of inversion as a half bridge with two switching elements1and2and two capacitors3and4. The load circuit5in the bridge comprises a coil6in series with a lamp7which is connected in parallel both with a resonance capacitor8and with a heat-sensitive resistor9.

The mode of operation of the circuit illustrated inFIG. 1may be explained as follows. By actuating the switches1and2suitably, an AC voltage for the load circuit5is generated in the center tap of the bridge from the DC voltage. The frequency of the AC voltage is advantageously in the region of the resonant frequency of the coil6and the capacitor8for the ignition process of the lamp. Before the ignition, as (PTC) thermistor the resistor9with a positive temperature coefficient (PTC) detunes the series resonant circuit6,8in such a way that the required ignition voltage across the lamp7or the capacitor8is not reached. However, the current is already flowing through the incandescent filaments10and11of the lamp7such that they are preheated for the ignition process. In the meantime, current is likewise flowing through the PTC thermistor9, which it heats in this preheating phase. Its resistance rises in the process, and so the detuning of the series resonant circuit,6,8is correspondingly reduced such that the ignition voltage across the lamp7can be reached. The PTC thermistor9is designed in this case such that it carries a sufficient quantity of current even after ignition in order to remain highly resistant so that the resonance can be maintained at an appropriate level of quality.

For the sake of clarity, the load circuit5is illustrated inFIG. 2awithout the coil6.FIG. 2bshows a variant of the load circuit ofFIG. 2a. Connected in series with the PTC thermistor9is a series capacitor12which has the effect that the detuning of the resonant circuit by the PTC thermistor9is not so marked as in the case of the circuit ofFIG. 2a. This means that in this case the ignition voltage is reached more quickly and the lamp is ignited more rapidly as a consequence thereof.

A further variant of the load circuits that are illustrated inFIGS. 2aand2bis reproduced in2c. In this case, the series capacitor12is chiefly active in the cold state of the PTC thermistor9, whereas the series circuit of the two capacitors8and9is only active in the warm state of the PTC thermistor9, that is to say during the operation and ignition of the lamp.

SUMMARY OF THE INVENTION

The object of the present invention consists in proposing a cost-effective preheating circuit for operating two lamps.

According to the invention, this object is achieved by means of a device for operating at least one first and one second discharge lamp having a coupling-out device for coupling out a heating current for the incandescent filaments of the discharge lamps from a supply branch of the device, the coupling-out device having a current control device for controlling the heating current, and a heating transformer unit, and respectively having a first contact device connected to the supply branch, and a second contact device for making contact with the first and second discharge lamp, a secondary coil unit of the heating transformer unit being connected to the first and second contact device for the purpose of supplying the incandescent filaments with heating current.

The advantage of the inventive circuit resides in that by contrast with the preheating circuit for one lamp the additional outlay for preheating a second lamp lies essentially in one component, specifically a transformer for transmitting the heating energy to the incandescent filaments of the two lamps.

The secondary coil unit preferably comprises three coils, specifically a first secondary coil for supplying a first incandescent filament of the first discharge lamp, a second secondary coil for supplying a second incandescent filament of the first discharge lamp and a first incandescent filament of the second discharge lamp, and a third secondary coil for supplying a second incandescent filament of the second discharge lamp. It is thereby possible for the individual incandescent filaments of the discharge lamps to be preheated in a targeted fashion by means of a transformer with four windings.

In one advantageous refinement of the inventive device, the supply branch comprises a resonance inductor and a resonance capacitor. The two lamps can thereby be operated with the aid of one resonant circuit. The resonance inductor can be used as an inductor. Furthermore, the resonance inductor can be at least a part of a coupling-out transformer for supplying the coupling-out device, or have an appropriate tap therefor.

The current control device advantageously comprises a PTC thermistor with a positive temperature coefficient. This component permits a relatively simple and cost-effective control of the preheating for the lamps. Instead of the PTC thermistor, the current control device can comprise a transistor. It is possible thereby to control the preheating in a more targeted but also more complicated way.

A sequential starting capacitor can be provided in parallel with the first and/or second contact device; it can be used advantageously to control the sequential starting sequence in the case of at least two lamps. Consequently, it is possible to achieve sequential starting in order to avoid very high ignition currents/voltages being reached, said starting permitting the use of components which cannot be so highly loaded and are therefore more cost-effective.

The inventive device is advantageously integrated in an electronic ballast for fluorescent lamps. It is thereby possible to operate two or more lamps with the aid of one ballast.

DETAILED DESCRIPTION OF THE INVENTION

The exemplary embodiments described below constitute only preferred embodiments of the present invention.

FIG. 3ashows an inventive load circuit in a ballast for two discharge lamps71and72. The discharge lamp71has two incandescent filaments711and712. Likewise, the second discharge lamp72has incandescent filaments721and722. The circuit has terminals20and21for the incandescent filament711of the first discharge lamp71, terminals22and23for the second incandescent filament712of the first discharge lamp71, terminals24and25for the first incandescent filament721of the second discharge lamp72, and terminals26and27for the second incandescent filament722of the second discharge lamp72.

The supply branch for the two discharge lamps71and72comprises a resonant circuit composed of a resonance capacitor Cresand a resonance inductor Lres. The resonance capacitor Cresis connected between the terminals20and26.

The coupling-out circuit30is driven via a coupling-out transformer that comprises, on the primary side, the inductor or resonance inductor Lresand, on the secondary side, a coil La. In addition to the secondary coil Laof the coupling-out transformer, this coupling-out circuit30comprises a temperature-dependent thermistor PTC and a primary coil Lhpof a heating transformer. The heating transformer has three coils on the secondary side. The first secondary-side heating coil Lhs1is connected between the terminals20and21for the first incandescent filament711of the first discharge lamp71. The second secondary coil Lhs2is connected to the terminals23and25for the second incandescent filament712of the first discharge lamp and the first incandescent filament721of the second discharge lamp72. The third secondary heating coil Lhs3is connected between the terminals26and27for the second incandescent filament722of the second discharge lamp72.

Moreover, the terminals22and24for the two incandescent filaments712and721are interconnected. Finally, a sequential starting capacitor Cseqis connected between the terminals24and26.

The mode of operation of the load circuit illustrated inFIG. 3amay be explained in more detail below. The supply branch with the resonant circuit Cresand Lresis very strongly damped at the beginning of operation. The reason for this is that at the start of operation the temperature-dependent thermistor PTC is still cool and therefore of low resistance. Consequently, a high energy component can be coupled out from the supply branch into the coupling-out circuit30via the coupling-out transformer Lres, La. The heating current flowing in the coupling-out circuit30is transmitted to the respective incandescent filaments via the heating transformer with the primary-side winding Lhpand the three secondary-side windings Lhs1, Lhs2and Lhs3. In this case, the incandescent filaments711and722are respectively supplied individually by means of the coils Lhs1, and Lhs3, and the two incandescent filaments712and721are supplied jointly by means of the coil Lhs2.

The two lamps71and72constitute a voltage divider at the resonance capacitor Cres. By virtue of the fact that the sequential starting capacitor Cseqis connected in parallel with the second discharge lamp72, a smaller voltage drops across the second discharge lamp72than across the first discharge lamp71. Consequently, the first discharge lamp71ignites before the second discharge lamp72.

At the end of the heating phase, the temperature-dependent thermistor PTC itself has been heated to such an extent that it has become of high resistance. Consequently, the damping of the resonant circuit CRes, LResdecreases, and the voltage across the discharge lamps71and72rises on the basis of the rise in the quality of the resonant circuit.

After the ignition, the current flows to the terminal26in the burning phase substantially from the terminal20via the incandescent filament711, the incandescent filament712, the terminal22, the terminal24, the incandescent filament721and the incandescent filament722.

Owing to the high resistance of the thermistor PTC, the current in the coupling-out circuit30, thus also the heating current for the incandescent filaments is greatly reduced in the burning phase. Consequently, all the filaments are subjected only to minimal heating during operation of the lamp in the burning phase.

A second embodiment of the present invention is illustrated inFIG. 3b. It differs from the first embodiment in accordance withFIG. 3aonly in that the resonance inductor is bipartite. It comprises the portions Lres1and Lres2, the second part Lres2constituting the primary coil of the coupling-out transformer. Owing to the bipartite nature of the resonance inductor, it is possible to use a standard transformer for coupling out, and to adapt the primary coil Lres2thereof to the resonance requirements of the supply branch by means of a separate inductor Lres1.

A further embodiment of the present invention is illustrated inFIG. 3c. Once again, the circuit design is virtually identical to that ofFIG. 3a. Instead of a coupling-out transformer, however, use is made here of a tap at the resonance inductor Lres. This means that the coupling-out circuit30is directly coupled to the resonance inductor Lres. The resonant circuit30therefore comprises the tapped part of the resonance inductor Lresin series with the PTC thermistor and the primary coil Lhpof the heating transformer.

The modes of operation of the embodiments illustrated inFIGS. 3band3care essentially identical to that ofFIG. 3a. The coupling-out circuit is driven by direct or inductive coupling to provide the heating current.