The invention relates to a circuit arrangement for operating a lamp comprising -circuit input terminals for connection to a supply voltage source, -an inverter coupled to said circuit input terminals for generating an AC voltage with a frequency f out of a supply voltage supplied by the supply voltage source and equipped with inverter output terminals, -a piezotransformer comprising transformer input terminals, coupled to the inverter output terminals, and transformer output terminals, -terminals for lamp connection coupled to the inverter output terminals -a detector for detecting whether the lamp has ignited.

Such a circuit arrangement is known from Jp H6-89789. The known circuit arrangement is very suitable for operating a discharge lamp and comprises a switching circuit part that connects the inverter output to the transformer input during the ignition of the lamp. The lamp is directly connected to the transformer output terminals. The frequency f of the AC voltage generated by the inverter is very close to one of the resonance frequencies of the piezotransformer. Therefore the AC voltage is transformed by the piezotransformer to an ignition voltage with the same frequency f but a much higher amplitude that is present over the lamp. After the lamp has ignited under the influence of this ignition voltage, the detector that is part of the circuit arrangement detects a lamp current and generates a signal that triggers the switching circuit part to disconnect the inverter output and the transformer input. After the transformer input has been disconnected from the inverter output, the piezotransformer no longer generates the ignition voltage and the lamp is operated by means of the AC voltage with frequency f that is generated by the inverter. An important avantage of the known circuit arrangement is that the inverter is used both in the generation of the voltage that is used to operate the lamp during stationary conditions as well as in the generation of the ignition voltage. For this reason the known circuit arrangement comprises a relatively small amount of components and is therefore relatively inexpensive

and compact. A disadvantage of the known circuit arrangement, however, is that although a separate oscillator to drive the piezotransformer during ignition can be dispensed with, the switching circuit part comprised in the known circuit arrangement is relatively expensive and complex.

The invention aims to provide a circuit arrangement for operating a lamp in which a separate oscillator for driving the piezotransformer can be dispensed with and that is equipped with relatively simple and inexpensive means to change the operation of the circuit arrangement after the lamp has ignited.

A circuit arrangement as described in the opening paragraph is therefore characterized in that the inverter comprises frequency control means coupled to the detector for changing the frequency f in response to the ignition of the lamp.

The frequency f of the AC voltage that is generated during ignition by the inverter comprised in a circuit arrangement according to the invention is very close to a resonance frequency of the piezotransformer. For this reason the piezotransformer effectively transforms the AC voltage into an ignition voltage with a relatively high amplitude. After ignition the frequency control means for changing the frequency f in response to the ignition of the lamp change the frequency to a value that differs substantially from all the resonance frequencies of the piezotransformer so that the transformation factor (Vout/Vin) of the piezotransformer is very small. As a result the operating voltage that is present over the lamp is substantially determined by the AC voltage generated by the inverter so that the frequency change has effectively switched the piezotransformer off. A change in the frequency f of the AC voltage generated by the inverter can be accomplished making use of relatively simple and inexpensive means. Since it is unnecessary to disconnect the transformer input terminals and the inverter output terminals, the connection between them can be realized using only passive components.

Preferably, the frequency control means change the frequency from a first fixed value to a second fixed value in response to the ignition of the lamp.

Good results have been obtained with a circuit arrangement according to

the invention, wherein the transformer output terminals are coupled to the terminals for lamp connection and a filter is coupled between the inverter output terminals and terminals for lamp connection. In this topology the lamp is ignited by generating an ignition voltage between the terminals for lamp connection. However, since the inverter output terminals are also coupled to the terminals for lamp connection it is often necessary to protect the inverter against the occurrence of a relatively high voltage between the inverter output terminals. This protection can be realized by means of a filter. Alternatively it is possible to couple a first transformer output terminal to one of the terminals for lamp connection and couple a second transformer output terminal to an auxiliary electrode. During ignition this auxiliary electrode is present outside of the lamp in the direct vicinity of the lamp vessel. The ignition voltage is present between the auxiliary electrode and a first electrode of the lamp connected to the terminal for lamp connection that is coupled to the first transformer output terminal. Since the distance between the first electrode of the lamp and the auxiliary electrode can be adjusted to be much smaller than the distance between the electrodes of the lamp, the use of an auxiliary electrode renders a very effective ignition of the lamp possible. The auxiliary electrode can be part of the lamp but can also be part of the circuit arrangement.

The detector may comprise a current sensor that directly or indirectly measures a current through the lamp. In this way the detector is realized in a relatively simple and effective way.

Good results have been obtained for a circuit arrangement according to the invention, wherein the inverter comprises a bridge circuit.

A relatively simple and inexpensive embodiment of a circuit arrangement according to the invention is obtained in case the transformer input terminals are connected directly to the inverter output terminals.

Embodiments of a circuit arrangement according to the invention will be illustrated making use of a drawing.

In the drawing Figure 1 shows a schematic representation of a first embodiment of a circuit arrangement according to the invention with a lamp connected to it, and Figure 2 shows a schematic representation of a second embodiment of a circuit arrangement according to the invention with a lamp connected to it.

In Figure 1 K1 and K2 are circuit input terminals for connection to a supply voltage source. K1 and K2 are connected to respective input terminals of rectifier bridge RB. Output terminals of rectifier bridge RB are connected by means of a capacitor Cl functioning as a buffer capacitor. One end of capacitor Cl is connected to ground potential.

Respective ends of capacitor Cl are connected to input terminals of a DC-DC-converter DDC comprising inductive means, unidirectional means and at least one switching element.

A control electrode of the switching element is connected to a first control circuit CC1 for generating a control signal for rendering the switching element conductive and non- conductive at a high frequency. This connection is represented schematically in Figure 1 by means of a dotted line connecting an output terminal of the first control circuit CC 1 with an input terminal of the DC-DC-converter DDC. Output terminals of the DC-DC-converter are connected to respective input terminals of a full bridge circuit that is formed by the series arrangement of switching elements Sl and S2, the series arrangement of switching elements S3 and S4 and the second control circuit CCII for rendering the switching elements S1-S4 conducting and non-conducting with frequency f. Respective output terminals of second control circuit CCII are connected to the respective control electrodes of the switching elements S1-S4. These connections are shown schematically by means of the dotted line that connects second control circuit CCII to the full bridge circuit. The DC-DC-converter DDC together with the first control circuit CC1 and the full bridge circuit together form an inverter for generating an AC voltage with frequency f out of a supply voltage. The second control circuit CCII comprises a circuit part I. Circuit part I forms means for changing the frequency f in response to the ignition of the lamp. For this purpose circuit part I is coupled to resistor R. This coupling is indicated by means of a dotted line. K3 and K4 are inverter output terminals. K5 and K6 are input terminals of piezotransformer PT. K7 and K8 are terminals for lamp connection. Output terminal K3 is connected to input terminal K5 and output terminal K4 is connected to both input terminal K6 and lamp connection terminal K8 via ohmic resistor R. Input terminal K5 is connected to terminal K7 for lamp connection by means of choke L. Input terminal K5 is also connected to ground potential by means of capacitor C2. Choke L and capacitor C2 together form a filter. Ohmic resistor R forms a detector for detecting whether the lamp has ignited. Output terminal K9 of piezotransformer PT is connected to lamp connection terminal K7. In the embodiment shown in Figure 1 K6 forms another output terminal of piezotransformer PT. A high pressure discharge lamp La comprising electrodes Ell and E12 is connected to the terminals for lamp connection.

The operation of the circuit arrangement shown in Figure 1 is as follows.

When the circuit input terminals are connected to the poles of a supply voltage source supplying a low frequency AC supply voltage, the low frequency AC supply voltage is rectified by means of the rectifier bridge RB, so that a DC voltage is present over capacitor C 1. The first control circuit CCI renders the switching element comprised in the DC-DC-converter DDC conducting and non-conducting at a high frequency. As a result the DC voltage present over capacitor Cl is converted by means of DC-DC-converter DDC into a substantially constant DC voltage present between the input terminals of the full bridge circuit. The second control circuit CCII renders on the one hand switching elements Sl and S4 and on the other hand switching elements S2 and S3 alternately conducting and non- conducting with a frequency f. Out of the substantially constant DC voltage that is present between its input terminals, the full bridge circuit generates a substantially square wave shaped AC voltage with frequency f that is present between the inverter output terminals K3 and K4. This substantially square wave shaped AC voltage is also present between the transformer input terminals K5 and K6. During lamp ignition the frequency f is chosen so, that it is very close to one of the resonance frequencies of the piezotransformer PT. The piezotransformer transforms the substantially square wave shaped AC voltage with frequency f to a sinusoidal ignition voltage with frequency f and a relatively high amplitude that is present between transformer output terminals K6 and K9 and between terminals K7 and K8 for lamp connection. The filter that is formed by choke L and capacitor C2 protects the full bridge circuit against the ignition voltage present over the lamp La. When the lamp ignites under the influence of the ignition voltage, the lamp and also resistor R start conducting a current. In reaction to the occurrence of a voltage drop over resistor R, the circuit part I comprised in the second control circuit CCII changes the frequency f at which the switching elements comprised in the full bridge circuit are rendered conducting and non-conducting to a value that corresponds to the stationary operation of the lamp. The frequency f is changed to a value that differs substantially from all the resonance frequencies of the piezotransformer so that at that frequency its voltage transformation ratio is very low and the voltage over the lamp is therefore almost completely determined by the inverter.

The embodiment of a circuit arrangement according to the invention that is shown in Figure 2 is to a large extent identical to the embodiment shown in Figure 1.

Components and/or circuit parts of the embodiment shown in Figure 2 that are similar to components and/or circuit parts in the embodiment shown in Figure 1 are labelled similarly.

An important difference between the embodiments shown in Figure 1 and Figure 2 is that in the latter the transformer output terminal K9 of piezotransformer PT is connected to an

auxiliary electrode AEL instead of to terminal K7 for lamp connection. The auxiliary electrode is a body of electrically conducting material such as for instance Ni that is present outside the lamp vessel in the vicinity of one of the lamp electrodes, in this case Ell. In the embodiment shown in Figure 2, K5 functions both as a transformer input terminal as well as a transformer output terminal. Therefore the ignition voltage is present between lamp electrode Ell and the auxiliary electrode AEL during ignition of the lamp. Because of the use of the auxiliary electrode AEL the filter that is present in the embodiment shown in Figure 1 can be dispensed with. Furthermore, since the distance between lamp electrode Ell and auxiliary electrode AEL can be chosen relatively small, the ignition behaviour of the lamp is strongly improved. During ignition of the lamp the ignition voltage establishes a discharge between lamp electrode Ell and auxiliary electrode AEL. This discharge introduces so many charge carriers in the plasma of the lamp that another discharge between lamp electrodes Ell and El2 is established almost immediately after the discharge between lamp electrode Ell and auxiliary electrode AEL.

After the lamp has ignited the frequency f is changed to a value corresponding to stationary operation of the lamp during which the lamp is operated directly by the inverter.