Abstract:
A circuit arrangement for operating a lamp has two power feedback loops, and a transformer is incorporated in one of the feedback loops. The transformer is used to adapt the amount of power fed back to the amplitude of the mains supply voltage for different lamp voltages.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a circuit device for operating a discharge lamp by means of a high-frequency current, which circuit device comprises input terminals for connecting it to a low-frequency supply voltage source, rectifier means coupled to the input terminals for rectifying the low-frequency supply voltage, a first circuit coupled to a first output terminal N 3  of the rectifier means and to a second output terminal N 5  of the rectifier means, which first circuit comprises a series arrangement of first unidirectional means, second unidirectional means and first capacitive means, which circuit device further comprises inverter means connected in parallel with the first capacitive means for generating the high-frequency current, a load circuit comprising a series arrangement of inductive means, second capacitive means and means for applying a voltage to the discharge lamp, which load circuit couples a terminal N 1  of the inverter means to a terminal N 2  between the first unidirectional means and the second unidirectional means, and a second circuit coupling a terminal N 2  to a terminal N 4  and comprising third capacitive means, a third circuit coupling the first output terminal N 3  of the rectifier means to a terminal N 4  between the second unidirectional means and the first capacitive means, which third circuit comprises a series arrangement of third unidirectional means and fourth unidirectional means, neither said first circuit nor said third circuit comprising inductive means, and a fourth circuit coupling the third circuit to a terminal N 6 , which forms part of the load circuit. 
     Such a circuit device is known from WO 97/19578. 
     The known circuit device has an optimum operating point at a load voltage which is approximately half the low-frequency supply voltage. In the case of sub-optimal discharge lamps, the arc voltage must be adapted. For this purpose, use can be made of a capacitive voltage divider; in addition to a capacitor coupling the terminal N 4  to a terminal N 7  between the third unidirectional means and the fourth unidirectional means, a capacitor coupling together the terminals N 4  and N 7 . The use of capacitive voltage division, however, causes high currents in the inductive means of the load circuit and in the inverter means, particularly the switching elements of the latter. This is problematic, in particular, in the case of circuits having an electric power in excess of 100 W, since it requires additional cooling of the switching elements. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a more efficient method of adapting the power feedback source to the low-frequency supply voltage. 
     To achieve this, the invention provides a circuit device of the type mentioned in the opening paragraph, which is characterized in that the circuit device comprises a fifth circuit by means of which the fourth circuit couples the third circuit to a terminal N 6 , said fifth circuit comprising transformer means which couple the terminal N 4  to the fourth circuit, and a branch of which is coupled to a terminal N 7  between the third unidirectional means and the fourth unidirectional means. 
     In accordance with the present invention, preferably the fifth circuit comprises fifth capacitive means coupling the branch of the transformer means to terminal N 7  to make sure that a direct current leading to saturation of the transformer means does not flow through the transformer means and the fourth unidirectional means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     An embodiment of the invention will be explained in more detail with reference to the drawing wherein the sole FIGURE shows a simplified diagrammatic view of the embodiment of a circuit device in accordance with the present invention, wherein a discharge lamp LA is coupled to the circuit device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, K 1  and K 2  represent input terminals for making a connection with a low-frequency supply voltage source. L 2  is an inductor which forms an input filter jointly with capacitor C 3 . Diodes D 1 -D 4  are rectifier means for rectifying the low-frequency supply voltage. Diodes D 5  and D 6  form, respectively, first and second unidirectional means. Capacitor C 4  forms first capacitive means and, jointly with diodes D 5  and D 6 , a first circuit. Switching elements Q 1  and Q 2  and control circuit DC jointly form inverter means. The control circuit DC is a circuit part which is used to generate control signals for making switching elements Q 1  and Q 2  conductive and non-conductive. Inductor L 1 , capacitor C 2  and terminals K 3  and K 4  for connecting a discharge lamp jointly form a load circuit. Inductor L 1  forms inductive means, capacitor C 2  forms second capacitive means, and terminals K 3  and K 4  for establishing a connection with a discharge lamp form means for applying a voltage to the discharge lamp. Capacitor C 1  forms third capacitive means. Capacitor C 1  and capacitor C 4  jointly form a second circuit. Diodes D 7  and D 8  form, respectively, third and fourth unidirectional means. The series arrangement of diodes D 7  and D 8  forms a third circuit. Capacitor C 5  forms fourth capacitive means as well as a fourth circuit. 
     Input terminals K 1  and K 2  are interconnected by means of a series arrangement of inductor L 2  and capacitor C 3 . A first side of capacitor C 3  is coupled to a first input terminal of the rectifier bridge, and a second side of capacitor C 3  is coupled to a second input terminal of the rectifier bridge. A first output terminal N 3  of the rectifier bridge is coupled to a second output terminal N 5  of the rectifier bridge by means of a series arrangement of diode D 5 , diode D 6  and capacitor C 4 . N 2  is a common terminal of diode D 5  and diode D 6 . N 4  is a common terminal of diode D 6  and capacitor C 4 . Terminal N 2  is coupled to terminal N 4  by means of capacitor C 1 . A series arrangement of diodes D 7  and D 8  is connected in parallel with the series arrangement of diodes D 5  and D 6 . N 7  is a common terminal of diodes D 7  and D 8 . A series arrangement of switching elements Q 1  and Q 2  is connected in parallel with capacitor C 4 . A control electrode of switching element Q 1  is coupled to a first output terminal of control circuit DC. A control electrode of switching element Q 2  is coupled to a second output terminal of control circuit DC. N 1  is a common terminal of switching element Q 1  and switching element Q 2 . Terminal N 1  is coupled to terminal N 2  by means of a series connection of, respectively, capacitor C 2 , inductor L 1 , terminal K 3 , discharge lamp LA and terminal K 4 . N 6  is a common terminal of inductor L 1  and terminal K 3 . Terminal N 6  is coupled to terminal N 7  by means of capacitor C 5 . 
     If capacitor C 5  directly connects terminal N 6  to terminal N 7 , then the operation of the hitherto described part of the circuit device shown in FIG. 1 is as follows. 
     If input terminals K 1  and K 2  are connected to the poles of a low-frequency supply voltage source, then the rectifier bridge rectifies the low-frequency supply voltage supplied by this source, so that a DC voltage is applied across a capacitor C 4  which serves as a buffer capacitor. The control circuit DC renders the switching elements Q 1  and Q 2  alternately conducting and non-conducting, and, as a result thereof, a substantially square-wave voltage having an amplitude which is approximately equal to the amplitude of the DC voltage across capacitor C 4  is present on terminal N 1 . The substantially square-wave voltage present on terminal N 1  causes an alternating current to flow through inductor L 1  and capacitor C 2 . A first part of this alternating current flows through terminals K 3  and K 4 , the discharge lamp LA and terminal N 2 . The remaining part of this alternating current flows through capacitor C 5  and terminal N 7 . As a result, voltages having the same frequency as the substantially square-wave voltage are applied to terminal N 2  as well as terminal N 7 . These voltages applied to terminal N 2  and terminal N 7  make sure that a pulsating current is drawn from the supply voltage source, also if the current across the capacitor C 4  is higher than the instantaneous amplitude of the rectified low-frequency supply voltage. For this reason, the power factor of the circuit device has a comparatively high value and the total harmonic distortion of the supply current is comparatively low. 
     It is to be noted that similar results were achieved with a configuration of the circuit device which slightly differs from the configuration shown in FIG. 1 in that capacitor C 1  couples terminal N 2  to terminal N 5  instead of to terminal N 4 . In this slightly different configuration, the capacitor C 1  forms third capacitive means and a second circuit. 
     Hitherto, the circuit device and the operation thereof are conventional and known from WO 97/19578. 
     Instead of being directly coupled to terminal N 7 , as in the conventional circuit device, capacitor C 5  is coupled, as is shown in FIG. 1, to terminal N 7  via a transformer which, as shown, is preferably an autotransformer L 3 , L 4  and, preferably, capacitor C 6 . Autotransformer L 3 , L 4  forms transformer means and, in this case in combination with capacitor  6 , a fifth circuit. Capacitor C 6  forms fifth capacitive means. The fourth circuit couples the third circuit to terminal N 6  be means of the fifth circuit. Autotransformer L 3 , L 4  couples terminal N 4  to the fourth circuit, and a branch of the autotransformer L 3 , L 4  is coupled to terminal N 7 , preferably by means of capacitor C 6 . The fifth circuit then comprises fifth capacitive means coupling the branch of the transformer means to terminal N 7 . 
     Said circuit device has an optimum operating point at a load voltage which is approximately equal to half the low-frequency supply voltage. The autotransformer L 3 , L 4  is used for adapting to the arc voltage of sub-optimal discharge lamps. This measure enables the power feedback source to be more efficiently adapted to the low-frequency supply voltage than, for example, a capacitive voltage divider whose main drawback resides in a high current in inductor L 2  and in the switching elements Q 1  and Q 2 , particularly, in circuits operating at an electric power above 100 W, which require additional cooling of the circuit elements. 
     Capacitor C 6  is preferably used to block the flow of direct current in order to preclude saturation of the autotransformer L 3 , L 4 .