The invention relates to a circuit arrangement for igniting and operating a discharge lamp, comprising input terminals for connection to the poles of a supply voltage source, means I coupled to the input terminals for generating a current through the discharge lamp from a supply voltage delivered by the supply voltage source, which means are provided with a control circuit for controlling the operational state of the circuit arrangement, inductive means, comprising a primary winding which carries a high- frequency current during ignition and during lamp operation and a secondary winding which is magnetically coupled to the primary winding and is electrically coupled to an impedance M for limiting the current carried by the secondary winding, and to an input of the control circuit via rectifying means.

Such a circuit arrangement is known. The means I may comprise, for example, a preconditioner such as an up-converter for generating a DC voltage from the supply voltage. Such a preconditioner is provided with a high-frequency operated switching element and with an inductive element across which a high-frequency AC voltage is present during operation of the means I. It is also possible for the means I to comprise a DC-AC converter for generating a high-frequency lamp current from a DC voltage. This DC-AC converter often uses one or several switching elements which are operated at high frequency and an inductive element across which a high-frequency voltage is present during operation. The control circuit of the known circuit arrangement comprises means for generating control signals for rendering the switching elements of the preconditioner and/or the DC-AC converter conducting and non-conducting and controlling the operational state of the circuit arrangement in this manner. The control circuit is supplied with a DC voltage of comparatively low amplitude when the circuit arrangement is in operation. This DC voltage

may be generated with the aid of the inductive element present in the preconditioner or of the inductive element present in the DC-AC converter. This inductive element then forms the inductive means mentioned in the opening paragraph and comprises a secondary winding. A high-frequency voltage is present across the primary winding during operation of the circuit arrangement. The magnetic coupling causes a high-frequency voltage to be present also across the secondary winding. The DC voltage is generated by means of the high-frequency voltage present across the secondary winding and the rectifying means and is applied to the input of the control circuit. If the inductive means form part of the DC-AC converter, however, the amplitude of the voltage across the primary winding will often be considerably lower during preheating of the electrodes of the lamp than during stationary lamp operation. As a result of this, the amplitude of the voltage across the secondary winding during preheating of the discharge lamp is also much lower than during stationary lamp operation. If a sufficient amount of power is to be supplied to the input of the control circuit also during preheating, it is necessary to choose the impedance value of the means M to be comparatively low. This has the result, however, that the current through the secondary winding is comparatively strong during stationary lamp operation as a result of the comparatively high voltage across the secondary winding, which means that the voltage at the input of the control circuit reaches a too high value. This latter effect may be counteracted through the use of a voltage limiter such as a zener diode. The use of such a voltage limiter does have the result that the voltage at the input of the control circuit is no longer too high, but the voltage limiter passes current continuously and accordingly dissipates power continuously, which means that the circuit arrangement functions comparatively inefficiently.

The invention has for its object to provide a circuit arrangement in which a DC voltage suitable for the control circuit is present at the input of the control circuit both before and during ignition of the discharge lamp and during stationary operation, while also comparatively little power is dissipated by the circuit arrangement during stationary lamp operation. According to the invention, a circuit arrangement as described in the opening paragraph is for this purpose characterized in that the circuit arrangement is provided with means X for increasing the impedance value of the impedance M after ignition of the lamp.

Since the means X increase the impedance value of the impedance M after

ignition of the discharge lamp, the current through the secondary winding is limited and it is prevented that the voltage at the input of the control circuit reaches a too high value. It is realized thereby that the amplitude of the DC voltage with which the control circuit is supplied is maintained at a level suitable for the control circuit both during ignition and during stationary lamp operation.

It was found to be advantageous in practice for the impedance M to comprise a first and a second impedance element, and for the means X to comprise a switching element SI. The impedance elements may be, for example, ohmic resistors which are connected in parallel by the switching element SI before and during ignition, while during stationary lamp operation one of the two resistors does not pass current because it is blocked by the switching element SI. It is also conceivable that during stationary lamp operation the two ohmic resistors are connected in series, while one of the resistors is shunted by the switching element during ignition. It is more advantageous, however, if the two impedance elements are capacitors, because the power dissipation in the impedance elements will be much lower in that case. These capacitors may be, for example, connected in parallel for ignition, whereas one of the capacitors passes no current during stationary lamp operation because it is blocked by the switching element SI. It is also possible to use the two capacitors in series during stationary lamp operation, one of the capacitors being shunted by the switching element before and during ignition. The amplitude of the DC voltage at the input of the control circuit may be maintained within comparatively narrow limits if the secondary winding is coupled to voltage-limiting means. These voltage-limiting means may comprise, for example, a zener diode.

It was found to be advantageous, furthermore, to couple the input of the control circuit to capacitive means. These capacitive means act as a buffer capacitance. A stabilization of the DC-voltage at the input of the control circuit is achieved by this.

In a preferred embodiment of a circuit arrangement according to the invention, the means comprise a timer circuit. This timer circuit increases the impedance value after a predetermined time interval has elapsed. It may be realized in this manner, for example, that the impedance M is increased after the preheating period of the lamp electrodes has passed.

Good results were obtained with circuit arrangements according to the invention in which the primary winding of the inductive means is connected in series with the discharge lamp during ignition and lamp operation.

An embodiment of a circuit arrangement according to the invention will be explained in more detail with reference to a drawing. In the drawing: Fig. 1 is a diagram of an embodiment of a circuit arrangement according to the invention.

Fig. 1 shows input terminals 4 and 5 for connection to the poles of a supply voltage source. All other components together form means I coupled to the input terminals for generating a current through a discharge lamp 1 connected to the circuit arrangement from a supply voltage delivered by the supply voltage source. SC is a control circuit for controlling the operational state of the circuit arrangement. Primary winding PRIM and secondary winding SEC together form inductive means. Primary winding PRIM and secondary winding SEC are magnetically coupled. Primary winding PRIM is connected in series with the discharge lamp 1 and passes a high-frequency current during ignition and during stationary lamp operation. Capacitor Cl forms a first impedance element and capacitor C3 forms a second impedance element. Together capacitor Cl and capacitor C3 form impedance M. Diode D forms rectifying means which couple secondary winding SEC to input II of the control circuit SC to which during operation a DC voltage of comparatively low amplitude is applied which serves as the supply voltage for the control circuit SC. Zener diode Z forms voltage-limiting means which are coupled to the secondary winding SEC. The means X for increasing the impedance value of impedance M are formed by ohmic resistor R and switching element SI. Capacitor C2 forms capacitive means to which the input of the control circuit is coupled.

Input terminals 4 and 5 are connected to respective inputs of diode bridge 7 via filter 6. Output terminals 8 and 9 of diode bridge 7 are interconnected by a capacitor 15. A DC voltage is present across the capacitor 15 during operation of the circuit arrangement. Reference numerals 16 and 17 denote input terminals of a DC-AC converter for generating a high-frequency current from the DC voltage present across capacitor 15. The DC- AC converter is formed by capacitors 18, 23 and 19, switching elements 24 and 25, diodes 25 and 26, and the control circuit SC. A first side of capacitor 15 is connected to input terminal 16. A further side of capacitor 15 is connected to input terminal 17. Input terminal 16 is connected to input terminal 17 via a series arrangement of capacitor 18,

capacitor 23, and ohmic resistor 22. The capacitors 18 and 23 are shunted by a series arrangement of switching element 24 and switching element 21 and by a series arrangement of diode 26 and diode 25. A common junction point of capacitor 18 and capacitor 23 is connected to a common junction point of switching element 24, switching element 21, diode 25, and diode 26 via a series circuit of capacitor 19 and primary winding PRIM. Discharge lamp 1 (provided with electrodes 2 and 3) shunts capacitor 19. Control electrodes of switching elements 24 and 21 are coupled to outputs of the control circuit SC. This coupling is indicated with a broken line in Fig. 1. Ends of ohmic resistor 22 are coupled to respective inputs of control circuit SC. Output terminal 9 is connected to input II of the control circuit SC via a series arrangement of capacitor C3, capacitor Cl, secondary winding SEC, and diode D. Input II is also connected to output terminal 9 via capacitor C2. A common junction point of secondary winding SEC and diode D is connected to output terminal 9 via a zener diode Z. An output of the control circuit SC is connected to a control electrode of switching element SI via ohmic resistor R. A first main electrode of switching element SI is connected to a common junction point of capacitor Cl and capacitor C3. A second main electrode of switching element SI is connected to output terminal 9.

The operation of the circuit arrangement shown in Fig. 1 is as follows. When the input terminals 4 and 5 are connected to a supply voltage source which delivers a sinusoidal AC voltage with a frequency of approximately 50 Hz, this sinusoidal voltage will be rectified by diode bridge 7. As a result of this, a DC voltage is present across capacitor 15. The control circuit SC renders the switching elements 24 and 21 alternately conducting and non-conducting with high frequency. As a result, a high-frequency current flows through the series circuit formed by primary winding PRIM, lamp electrode 3, capacitor 19, and lamp electrode 2 when the discharge lamp 1 has not yet ignited. This results in a high-frequency voltage across the primary winding PRIM. The high-frequency voltage across the primary winding PRIM induces a high-frequency voltage across the secondary winding SEC through the magnetic coupling between the primary winding PRIM and the secondary winding SEC. As long as the discharge lamp has not yet ignited, the amplitudes of the high-frequency voltages across primary winding PRIM and secondary winding SEC are comparatively low. At the same time, the output of control circuit SC connected to the switching element SI via ohmic resistor R is high immediately after switching-on of the circuit arrangement, i.e. during preheating of the lamp electrodes, so that the switching element SI short-circuits capacitor C3. The impedance M is formed by capacitor Cl before and during ignition, and the impedance value is comparatively low as a

result. Since the impedance value of the impedance M is comparatively low, a current flows through the series circuit formed by switching element SI, capacitor Cl, secondary winding SEC, and diode D which charges the capacitor C2 and whose amplitude is high enough for maintaining the voltage at input II of control circuit SC at the desired level, approximately 15 V in the present embodiment. A timer circuit forming part of the control circuit SC makes the output of control circuit SC low after a predetermined time interval, so that the switching element SI becomes non-conducting. The predetermined time interval is chosen such that the output of control circuit SC becomes low shortly after the lamp has ignited. The amplitude of the high-frequency voltage across primary PRIM rises after lamp ignition. As a result of this, the amplitude of the high-frequency voltage across secondary winding SEC also rises. When the switching element SI is non-conducting, however, the impedance M is formed by the series circuit of capacitor Cl and capacitor C3, and the impedance value of the impedance M is comparatively high. The amplitude of the current which flows through the series circuit formed by capacitor C3, capacitor Cl, secondary winding SEC, and diode D as a result of the high-frequency voltage across secondary winding SEC after the switching element SI has become non-conducting is thus limited. The voltage at the input II of control circuit SC remains limited to the level of approximately 15 V suitable for the control circuit as a result of this, while at the same time the zener diode Z does not dissipate a comparatively large amount of power. It is achieved thereby that the voltage at input II is kept at a level suitable for the control circuit SC both before and after a discharge has been generated in the discharge lamp 1.

During stationary lamp operation, the control circuit SC generates a signal which is a measure for the average value of the current through resistor 22. This signal is compared with a reference value. Depending on the outcome of this comparison, the control circuit SC adjusts the frequency and/or the duty cycle of the signals with which switching elements 24 and 21 are rendered conducting and non-conducting. The average value of the current through resistor 22 is maintained at a substantially constant value in this manner.