Abstract:
In an electronic ballast for operating a discharge lamp, the voltage across the heater windings during preheating the lamp electrodes is used to detect whether a lamp is present.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a circuit arrangement for feeding a discharge lamp, comprising 
     lamp clamps for holding the discharge lamp, 
     a main inverter coupled to the lamp clamps for generating, during stationary operation, a current which is fed to the discharge lamp, 
     an auxiliary inverter for preheating electrodes of the discharge lamp, provided with 
     an oscillator for generating an alternating voltage with a frequency f 1 , 
     a transformer provided with a primary winding coupled to the oscillator, and with a first and a second secondary winding which each shunt a lamp electrode during operation of the lamp, 
     a control circuit coupled to the main inverter and the auxiliary inverter for controlling the operating state of the circuit arrangement, 
     a first circuit part coupled to an input of the control circuit for generating a first signal which is a measure of the voltage difference between a first end of the first secondary winding and a first end of the second secondary winding. 
     Such a circuit arrangement is well-known. After putting the known circuit arrangement into operation, the control circuit ensures that, if a discharge lamp is connected to the lamp clamps, the circuit arrangement is successively brought into a number of operating states. In the first operating state, the lamp electrodes are preheated by means of the auxiliary inverter. Subsequently, in a second operating state, an ignition voltage is generated across the discharge lamp by means of the main inverter. If the discharge lamp ignites under the influence of this ignition voltage, the control circuit brings the circuit arrangement into a third operating state wherein the discharge lamp is fed so as to remain in the stationary mode of operation. The first signal, which is a measure of the voltage difference between a first end of the first secondary winding and a first end of the second secondary winding, represents the voltage across a discharge lamp connected to the circuit arrangement. The first signal is used by the control circuit to preclude that the voltage across the discharge lamp becomes too high during ignition, and to establish whether the discharge lamp has ignited. 
     As mentioned hereinabove, it is first checked whether a discharge lamp is present. For this purpose, the known circuit arrangement also comprises means for establishing whether a discharge lamp is connected to the lamp clamps. These means generally include a circuit part which generates a current which flows through one of the lamp electrodes and is subsequently detected. The detection, or non-detection, of this current affects the form of a lamp-presence signal which is present at an input of the control circuit. If said lamp-presence signal indicates that no discharge lamp is connected to the circuit arrangement, the control circuit keeps the circuit arrangement in a state of rest. A drawback of the known circuit arrangement resides in that the control circuit must be provided with an input where the lamp-presence signal is present and which input is used exclusively to determine whether a discharge lamp is connected to the circuit arrangement. Since the control circuit often comprises an IC, the total number of inputs and outputs of the control circuit is determined to a substantial degree by the number of pins of the IC. In the known circuit arrangement, the number of pins of the IC is relatively large in the control circuit. As a result, the control circuit is relatively expensive and difficult to manufacture. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a circuit arrangement for feeding a discharge lamp, wherein the means for determining whether a discharge lamp is connected to the lamp clamps are relatively simple, and the control circuit need only comprise a relatively small number of inputs. 
     To achieve this, a circuit arrangement of the type mentioned in the opening paragraph is characterized in that a second end of the first secondary winding and a second end of the second secondary winding are interconnected by a first conducting branch and in that, during operation of the circuit arrangement, the polarity of the voltage across the first secondary winding is equal to the polarity of the voltage across the second secondary winding. 
     An equal polarity of the voltages across the first and the second secondary winding can be readily obtained by suitably choosing the sense of winding of the first and the second secondary winding. If the oscillator in a circuit arrangement in accordance with the invention generates an alternating voltage with a frequency f 1 , then, consequently, a voltage is present across the first and the second secondary winding of the transformer. If a lamp is connected to the lamp clamps, the amplitudes of both said voltages are very small because substantially all of the electric power generated by the oscillator is dissipated in the lamp electrodes. As a result, also the voltage between the first end of the first secondary winding and the first end of the second secondary winding has a very low amplitude. If, however, no discharge lamp is connected to the lamp clamps, the amplitude of the voltage across the first secondary winding and the amplitude of the voltage across the second secondary winding are relatively high. As the voltages exhibit the same polarity, also the amplitude of the voltage between the first end of the first secondary winding and the first end of the second secondary winding is relatively high. Consequently, in a circuit arrangement in accordance with the invention, the presence of a lamp can be detected during the first operating state by means of the first signal. In a circuit arrangement in accordance with the invention, the first signal is used to determine whether a discharge lamp is connected to the lamp clamps as well as to monitor the voltage across the lamp. As a result, the number of inputs of the control circuit can be relatively low. 
     To preclude that, during stationary operation of the lamp, a relatively large amount of power is dissipated in the lamp electrodes, it is desirable that impedance is present in the first conductive branch. Satisfactory results have been obtained in examples wherein the impedance comprises a first capacitive element. 
     Preferably, the main inverter comprises a second conductive branch including a series arrangement of a first inductive element and a second capacitive element, and the second capacitive element forms part of a third conductive branch connecting the first end of the first secondary winding and the first end of the second secondary winding to one another. Such an embodiment of the main inverter enables the discharge lamp to be ignited in a relatively simple manner. However, in practice, the second capacitive element constitutes a relatively small impedance relative to the first signal generated by the auxiliary inverter. To preclude that this relatively small impedance causes a relatively small amplitude of the first signal, the value of f 1  is chosen to be close to the resonance frequency of the first inductive element and the second capacitive element. More particularly, satisfactory results have been obtained if f 1  is chosen in the range between 0.8*f 0  and 1.2*f 0 , wherein f 0  is the resonance frequency of the first inductive element and the second capacitive element. If the main inverter comprises a switching element which shunts the second conductive branch, then the control circuit is preferably provided with a circuit part for maintaining the switching element in the conducting state during preheating the electrodes of the discharge lamp. The switching element and the second conductive branch thus form a circuit of which the first inductive element and the second capacitive element form part. 
     To preclude that power is dissipated in the lamp electrodes during stationary operation, it is desirable that the first conductive branch exhibits an impedance which is at least hundred times the impedance of the second capacitive element. 
     It is noted that, dependent upon the construction of the circuit arrangement in accordance with the invention, the main inverter and the auxiliary inverter are built up, either entirely or partly, from the same components. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 diagrammatically shows an example of a circuit arrangement in accordance with the invention to which a discharge lamp is connected. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, K 3  and K 4  are the input terminals which are to be connected to a direct voltage source. Input terminal K 3  is connected to the terminal K 4  by means of a series arrangement of two switching elements T 1  and T 2 . Control electrodes of the switching elements T 1  and T 2  are connected to respective outputs of a control circuit SC 1  for rendering the switching elements T 1  and T 2  alternately conducting and non-conducting. The switching element T 2  is shunted by a series arrangement of a capacitor C 3 , coil L 1  and capacitor C 2 . In this example, this series arrangement forms a second conductive branch. Coil L 1  forms, in this example, a first inductive element. Capacitor C 2  forms a second capacitive element, in this example, and also a third conductive branch. Capacitor C 3  is a DC blocking capacitor. Capacitor C 2  is shunted by a series arrangement of a secondary winding L 2   a , capacitor C 1  and secondary winding L 2   b . In this example, capacitor C 1  forms first capacitive means. The secondary winding L 2   a  is coupled to the lamp clamp K 1 , and the secondary winding L 2   b  is coupled to the lamp clamp K 2 . A discharge lamp TL 1  is connected to the lamp clamps K 1  and K 2  in such a manner that a first lamp electrode El 1  is shunted by the first secondary winding L 2   a , and a second lamp electrode E 12  is shunted by the second secondary winding L 2   b . Switching elements T 1  and T 2 , control circuit SC 1 , capacitors C 3  and C 2  and coil L 1  jointly form a main inverter for generating a current with which the lamp TL 1  is fed. Input terminals K 3  and K 4  are also interconnected by means of a series arrangement of switching elements T 3  and T 4 . Control electrodes of switching element T 3  and switching element T 4  are connected to respective outputs of a control circuit SC 2  for rendering switching elements T 3  and T 4  alternately conducting and non-conducting. Switching element T 4  is shunted by a series arrangement of capacitor C 4  and primary winding L 2 . Primary winding L 2  is magnetically coupled to secondary windings L 2   a  and L 2   b . Switching elements T 3  and T 4 , control circuit: SC 2  and capacitor C 4  jointly form an oscillator for generating an alternating voltage of frequency f 1 . Primary winding L 2  and secondary windings L 2   a  and L 2   b  jointly form a transformer. The oscillator and the transformer jointly form an auxiliary inverter for preheating electrodes of the lamp TL 1 . CC is a control circuit for controlling the operating state of the circuit arrangement. A first output of control circuit CC is connected to an input of control circuit SC 1 . A second output of control circuit CC is connected to an input of control circuit SC 2 . A common point of capacitor C 2  and coil L 1  forms, in this example, a first circuit part and is connected to an input of control circuit CC. 
     The operation of the example shown in FIG. 1 is as follows. 
     Immediately after input terminals K 3  and K 4  have been connected to the poles of a direct voltage source, the control circuit activates a first operating state wherein the control circuit SC 2  renders the switching elements T 3  and T 4  alternately conducting and non-conducting with a frequency f 1 . In addition, during this first operating state, the control circuit CC renders the switching element T 2  conducting and the switching element T 1  non-conducting via the control circuit SC 1 . An alternating voltage with a frequency f 1  is present across the primary winding L 2 . As a result, voltages with a frequency f 1  are also present across secondary windings L 2   a  and L 2   b . Since the secondary windings are interconnected by means of capacitor C 1 , a voltage is present across capacitor C 2  the amplitude of which is equal to the sum of the voltages across both secondary windings L 2   a  and L 2   b  and the voltage across capacitor C 1 . This voltage across capacitor C 2  forms, in this example, a first signal. If the discharge lamp TL 1  is present, almost all the electric power generated by the auxiliary inverter is dissipated in the lamp electrodes E 11  and E 12 . As a result, the amplitudes of the voltages across the secondary windings are relatively low. For this reason, the amplitude of the first signal present at the input of the control circuit CC is also low, and the control circuit maintains the circuit arrangement in the first operating state. If, however, no discharge lamp is connected to the circuit arrangement, the amplitudes of the voltages across the secondary windings are relatively high. Since, as a result of a suitably chosen sense of winding of both the first and the second secondary winding, the polarity of the voltage across the first secondary winding is equal to the polarity of the voltage across the second secondary winding, also the amplitude of the first signal is relatively high. This can be contributed to the fact that in the absence of the discharge lamp, no power is dissipated in the lamp electrodes. This is partly caused by the fact that the frequency f 1  is chosen to be close to the resonance frequency of coil L 1  and capacitor C 2 . If the first signal present at the input of the control circuit CC is high, the control circuit CC brings the circuit arrangement into a state of rest, wherein the control circuits SCI and SC 2  maintain all switching elements in the non-conducting state. During ignition of the lamp, the voltage across capacitor C 2  is equal to the ignition voltage, and during stationary operation of the lamp, the voltage across capacitor C 2  is equal to the working voltage of the discharge lamp. For this reason, the first signal in a circuit arrangement in accordance with the invention can be used in different operating states of the circuit arrangement to monitor the operating state, and the control circuit CC requires relatively few inputs. This means that, if the control circuit CC comprises an IC, the number of pins of this IC can be relatively small.