The invention relates to a circuit arrangement for operating a lamp, provided with

- means I for generating a second DC voltage from a first DC voltage, comprising - input terminals for connection to a supply voltage source which delivers the first DC voltage,

- a switching element,

- a control circuit coupled to the switching element for rendering said switching element conducting and non-conducting with high frequency, - a unidirectional element,

- a transformer having a primary and a secondary winding,

- means II coupled to the secondary winding for generating a current through the lamp.

Such a circuit arrangement is known from German Patent DE 4141804 Cl. The means II in the known circuit arrangement are supplied with the second DC voltage during lamp operation. Since the means I comprise a transformer, it is possible to realize a second DC voltage with a comparatively high amplitude. As a result, the circuit arrangement renders it possible to operate lamps with a comparatively high burning voltage by means of a supply voltage source which delivers a first DC voltage of a comparatively low amplitude. A disadvantage of the known circuit arrangement is, however, that a considerable power dissipation occurs in the components with which the means I are realized during lamp operation, whereby the efficiency of the circuit arrangement is adversely affected.

The invention has for its object to provide a circuit arrangement with which it is possible to operate lamps with a comparatively high burning voltage by means of a supply voltage source delivering a first DC voltage of comparatively low amplitude such

that lower power losses occur in the circuit arrangement during lamp operation.

According to the invention, a circuit arrangement as described in the opening paragraph is for this purpose characterized in that the secondary winding, the input terminals, and the means II are coupled such that the means II are supplied during lamp operation with a voltage whose amplitude is equal to the sum of the first and the second DC voltage.

Owing to the measure according to the invention, part of the power is directly (i.e. not via the means I) supplied to the means II during lamp operation. Since only part of the power consumed by the means II is supplied through the means I, the power losses occurring in the means I are comparatively low. The part of the power supplied directly is equal to the product of the first DC voltage and the current passed by the means II.

Lamp operation by means of a circuit arrangement according to the invention is very efficient when the lamp is a high-pressure discharge lamp. A high-pressure discharge lamp exhibits a behaviour which is called the "run-up" of the high-pressure discharge lamp during a time interval after ignition, the duration of which is a function of the composition of the plasma of the high-pressure discharge lamp. The temperature of the high- pressure discharge lamp rises during this run-up, and as a result the composition of the plasma changes: the discharge vessel of the high-pressure discharge lamp may comprise, for example, substances which are solid at room temperature but which evaporate when the temperature rises so that they will form part of the plasma. As a result of this changing plasma composition, the burning voltage of the high-pressure discharge lamp gradually rises during the run-up from a comparatively low value to the value accompanying stationary lamp operation. The circuit arrangement is often so constructed that the supply voltage of the means II also gradually rises during the run-up. If the power consumed by the high-pressure discharge lamp during run-up is approximately equal to the power consumed during stationary lamp operation, the luminous flux of the high-pressure discharge lamp is considerably lower during run-up than during stationary lamp operation. Such a comparatively low luminous flux during run-up, however, is undesirable or even inadmissible in many applications. To render the luminous flux during the run-up approximately equal to the luminous flux during stationary lamp operation, or to limit the time duration of the run¬ up, it is necessary to have the high-pressure discharge lamp consume a higher power during the run-up than during stationary lamp operation. Owing to this higher power during run-up and to the fact that the burning voltage of the high-pressure discharge lamp is still

comparatively low during the run-up, the lamp current is comparatively high during the run¬ up. When a known circuit arrangement is used in which the full power consumed by the high-pressure discharge lamp is delivered via the means I, the power dissipation in the means I is comparatively great during the run-up. With the use of a circuit arrangement according to the invention, however, the portion of the power delivered directly to the means 2 is comparatively great because the first DC voltage represents a comparatively great fraction of the supply voltage of the means II (which is comparatively low during the run-up). A considerable improvement of the efficiency of the circuit arrangement is achieved thereby, especially during a phase in lamp operation in which a comparatively high power is supplied to the high-pressure discharge lamp. Thanks to this considerable improvement in efficiency, it has even become possible to construct the means I from components which satisfy less high requirements as to their power dissipation, so that the circuit arrangement can be made substantially more inexpensive.

In a preferred embodiment of a circuit arrangement according to the invention, the input terminal connected to the cathode of the supply voltage source during lamp operation is also connected to an end of the secondary winding. Since the potential of the cathode of the supply voltage source is often chosen to be ground potential, and the amplitude of the second DC voltage is often considerably higher than that of the first DC voltage, it is achieved thereby that the average potential in the plasma of a discharge lamp operated by the circuit arrangement is negative relative to ground. Diffusion of positively charged ions from the discharge vessel of a discharge lamp is counteracted by this negative average potential in the plasma of the discharge lamp. Such positively charged ions may be, for example, metal ions which form part of the plasma of certain types of high-pressure discharge lamps. Good results were found with a circuit arrangement wherein the means I comprise a DC-DC converter of the flyback type.

An embodiment of the invention will be explained in more detail with reference to a drawing, in which

Fig. 1 is a diagram of an embodiment of a circuit arrangement according to the invention, with a lamp connected thereto.

In the embodiment shown in Fig. 1, the means I are formed by input terminals Kl and K2 for connection to a supply voltage source which delivers a first DC voltage, capacitor Cl , switching element Tl , control circuit SCI , transformer Tr, capacitor C2, and diode D. Means II are formed by switching elements T2-T5, and control circuit SC2. A lamp La is connected to the means II.

Input terminals Kl and K2 are interconnected by a series arrangement of primary winding Pr of transformer Tr and switching element Tl . This series arrangement is shunted by capacitor Cl . A control electrode of switching element Tl is connected to an output of control circuit SC 1 for rendering the switching element conducting and non- conducting with high frequency. Secondary winding SE of transformer Tr is shunted by a series arrangement of a capacitor C2 and a diode D. Diode D in this embodiment forms a unidirectional element. Input terminal Kl , which is connected to the anode of the supply voltage source during lamp operation, is connected to a common junction point of capacitor C2 and diode D via a series arrangement of switching elements T2 and T3. The series arrangement of switching elements T2 and T3 is shunted by a series arrangement of switching elements T4 and T5. Control electrodes of switching elements T2-T5 are connected to outputs of the control circuit SC2. These connections are indicated in Fig. 1 with broken lines. A common junction point of switching element T2 and switching element T3 is connected to a first end of the lamp La, and a common junction point of switching elements T4 and T5 is connected to a further end of the lamp La.

The operation of the circuit arrangement shown in Fig. 1 is as follows. When the input terminals K l and K2 are connected to a supply voltage source delivering a first DC voltage, the control circuit SC renders the switching element Tl conducting and non-conducting with high frequency. The second DC voltage is present across capacitor C2 as a result of this. The supply voltage of the means II is present across the series arrangement of switching elements T2 and T3 and has an amplitude equal to the sum of the first and the second DC voltage. The control circuit SC2 renders the switching elements T2 and T5 and the switching elements T3 and T4 conducting and non-conducting alternately with low frequency. As a result, a low-frequency commutated direct current flows through the lamp whose amplitude can be controlled by means which are not shown in Fig. 1. The total power dissipated by the means II is equal to the supply voltage of means II ( = first DC voltage + second DC voltage) multiplied by the current passed by the means II, which is equal to the lamp current. Of this total power, a portion equal to the first DC voltage multiplied by the current passed by the means II is supplied directly to the means II,

so that the power dissipation in the means I is comparatively limited.

When the lamp La is a high-pressure discharge lamp, the lamp voltage is comparatively low during the run-up, after lamp ignition. The control circuit SCI controls the second DC voltage, and thus also the supply voltage of the means II at a comparatively low level adapted to the lamp voltage. At the same time, however, a comparatively strong current is passed by the means II during the run-up in order to limit the time duration of the run-up and/or render the luminous flux of the lamp during run-up substantially equal to the luminous flux during stationary lamp operation. A considerable power is directly supplied to the means II during the run-up, both in absolute terms (because of the comparatively strong current passed by the means II) and in relative terms (because of the comparatively low supply voltage of the means II). The power dissipation in means I is accordingly considerably reduced during the phase of lamp operation in which a comparatively high power is transmitted from the supply voltage source to the lamp.

Since the second DC voltage often has a considerably higher amplitude in practice than the first DC voltage, and the input terminal K2 is connected to ground potential, the average potential in the lamp plasma is negative relative to ground potential during lamp operation. Diffusion of positive ions from the discharge vessel of the lamp is prevented thereby.

The following results were found for a practical realization of a circuit arrangement as shown in Fig. 1 with which a high-pressure discharge lamp with a power rating of 35 W was operated by means of a battery delivering a supply voltage of 12 V. Lamp voltage during stationary lamp operation was 85 V. Immediately after ignition, however, the burning voltage of the lamp was 29 V. To accelerate the run-up and to make the luminous flux of the lamp comparatively high immediately after ignition, a total power of 75 W was supplied to the lamp immediately after ignition. Of this total power, approximately 31 W was supplied directly to the means II.