Abstract:
An electronic ballast for operating a lamp with a high-frequency current is equipped with a load branch comprising two LC combinations. The LC combinations are dimensioned such that the shape of the high-frequency current is in between a sine shape and a square-wave shape. The efficacy of a lamp operated by means of the electronic ballast is high, while the RFI generated by the lamp is low.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a switching device for energizing a lamp with a high-frequency lamp current at a frequency f, which switching device is provided with 
     a first circuit part for generating a periodical, substantially square-wave voltage at a frequency f, which first circuit part is provided with output terminals, 
     a load branch comprising 
     a first branch, which interconnects the output terminals, and which comprises a first series arrangement of a first inductive element and a first capacitate element, 
     a second branch, which shunts the first capacitate element, and which comprises a series arrangement of a second inductive element and a second capacitate element, and 
     a third branch, which comprises lamp terminals, and which shunts the second capacitate element during operation of the lamp. 
     Such a switching device is disclosed in U.S. Pat. No. 5,426,350. A high-frequency current is to be taken to mean a current with a frequency above 10 kHz. In the known switching device, both capacitate elements and both inductive elements are dimensioned such that, during stationary lamp operation, a substantially sinusoidal current flows through the lamp at a frequency f. In spite of the fact that the known switching device does not include a transformer, this sinusoidal current has a comparatively high amplitude as compared to the amplitude of the substantially square-wave voltage. By virtue thereof, it is possible to use the known switching device to supply a comparatively high burning voltage to a discharge lamp. 
     A drawback of the known switching device is, however, that the efficacy of the lamp, in other words the ratio between the luminous flux of the lamp and the power consumed by the lamp, is comparatively low. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the invention to provide a switching device for energizing a lamp, which transfers power to the lamp in such a way that the efficacy of the lamp is comparatively high. 
     To achieve this, a switching device as described in the opening paragraph is characterized in accordance with the invention in that the first branch and the second branch are dimensioned such that the following relation is met 
     
       
         0.14&lt;ΣA( n )/A( 1 )&lt;0.42, 
       
     
     wherein A( 1 ) denotes the amplitude of the first harmonic component of the high-frequency lamp current, and Σ A(n) is the sum of the amplitudes of the higher harmonic components of the lamp current. 
     Instead of being substantially sinusoidal, the high-frequency lamp current generated during lamp operation by a switching device in accordance with the invention has a shape in between that of a sine and a square-wave. It is known, for example from WO 96/19095, that a substantially square-wave lamp current enables a very high efficacy to be achieved. A drawback of such a substantially square-wave lamp current is the comparatively large amount of RFI generated by the lamp. As the current generated by a switching device in accordance with the invention is not substantially square-wave shaped, but rather has a shape in between that of a sine and a square wave, the amount of RFI generated by the lamp is comparatively small, while the efficacy is substantially higher than in the case of a substantially sine-shaped lamp current. In addition, a switching device in accordance with the invention has a comparatively simple structure and hence is comparatively inexpensive. 
     Very good results are achieved with embodiments of a switching device in accordance with the invention, wherein the first branch and the second branch are dimensioned such that the following relation is met 
     
       
         0.21&lt;ΣA( n )/A( 1 )&lt;0.35. 
       
     
     It proved advantageous to provide the first circuit part with 
     input terminals which are to be connected to a DC voltage source, 
     a fourth branch comprising a series arrangement of two switching elements, 
     a control circuit coupled to control electrodes of the switching elements for 
     rendering the switching elements alternately conducting and non-conducting. 
     In this manner, the first circuit part is obtained in a comparatively simple and reliable manner. 
     To render such an embodiment of a switching device in accordance with the invention suitable for being energized with an AC voltage, the first circuit part may additionally be provided with 
     mains input terminals which are to be connected to a supply-voltage source supplying an AC voltage, 
     rectifier means, coupled to the mains supply terminals and to the input terminals, which rectifier means are used to rectify the AC voltage. 
     To preclude that comparatively much power is dissipated in the two switching elements, the frequency f is generally chosen to be higher than the resonance frequency of the load branch during operation of the lamp. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 diagrammatically shows an example of a switching device in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, K 1  and K 2  are mains input terminals which are to be connected to a supply-voltage source supplying an AC voltage. The mains input terminals are connected to respective inputs of rectifier means GM which, in this example, are formed by a diode bridge. A first output of the rectifier means GM is connected to a second output by means of a buffer capacitor C. Respective sides of buffer capacitor C are connected to input terminal K 3  and input terminal K 4 . Input terminals K 3  and K 4  are connected to each other by means of a series arrangement of switching element S 1  and switching element S 2  which, in this example, forms a fourth branch. A control electrode of switching element S 1  is connected to a first output of control circuit Sc 1 . A control electrode of switching element S 2  is connected to a second output of control circuit Sc 1 . Control circuit Sc 1  is, in this example, a control circuit for rendering the switching elements S 1  and S 2  alternately conducting and non-conducting. Switching elements S 1  and S 2  and the control circuit Sc 1  jointly form a first circuit part for generating a periodical, substantially square-wave voltage at a frequency f. In this example, a junction point of the two switching elements S 1  and S 2  forms a first output terminal K 5  of the first circuit part. A second output terminal of the first circuit part is formed by input terminal K 4 . Switching element S 2  is shunted by a series arrangement of capacitor C 3 , coil L 1  and capacitor C 1 . Coil L 1  forms a first inductive element, and capacitor Cl forms a first capacitate element. Capacitor C 3  forms a DC-blocking capacitor, which is used to preclude that the lamp carries a direct current during operation of the lamp. Capacitor C 1  is shunted by a series arrangement of coil L 2  and capacitor C 2 . Coil L 2  forms, in this example, a second inductive element. Capacitor C 2  forms, in this example, a second capacitate element. Capacitor C 3  has a comparatively large capacitance as compared to the capacitance&#39;s of capacitor C 1  and capacitor C 2 . 
     The dimensions of the first branch and the second branch are such that the following relation is met 
     
       
         0.14&lt;ΣA( n )/A( 1 )&lt;0.42, 
       
     
     wherein A( 1 ) is the amplitude of the first harmonic component of the high-frequency lamp current and Σ A(n) is the sum of the amplitudes of the higher harmonic components of the lamp current. Capacitor C 2  is shunted by the discharge lamp LA. 
     The operation of the example shown in FIG. 1 is described hereinbelow. 
     If the mains input terminals K 1  and K 2  are connected to a supply-voltage source supplying an AC voltage, then the rectifier means GM rectify this AC voltage to a DC voltage present across the buffer capacitor C. The control circuit Sc 1  renders the switching elements S 1  and S 2  alternately conducting and non-conducting at a frequency f. As a result, between the output terminals K 4  and K 5  a symmetric substantially square-wave voltage is present at a frequency f. As a result of the dimensioning of the first and the second branch, the current through the lamp LA has a shape in between a square-wave shape and a sine shape. In this manner, it is achieved that the efficacy of the switching device shown in FIG. 1 is comparatively high, while the quantity of RFI generated by the lamp LA is comparatively low. 
     In a practical embodiment of the example shown in FIG. 1, the frequency f is chosen to be 50 kHz. The induction of coil L 1  is 1.15 mH and the induction of coil L 2  is 0.8 mH. The capacitances of the capacitors C 1 , C 2  and C 3  are, respectively, 3.3 nF, 3.9 nF and 100 nF. The lamp energized by means of the switching device is a low-pressure mercury vapor discharge lamp of the type T5 (Philips) having a rated power of 39 W. It has been found that the efficacy of the lamp is 2.4% higher than the efficacy achieved by using a sinusoidal lamp current. The quantity of RFI generated by the lamp is hardly higher than that generated in the case of a sinusoidal lamp current.