Abstract:
In a ballast circuit for operating a discharge lamp, two different lamp power levels are adjustable. The ballast circuit is equipped with a switch to optimize the amount of power feedback for the lamp power level. This enables total harmonic distortion and power balance to be optimized for more than one power level.

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 which are coupled to the input terminals and which are used to rectify 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 which are connected in parallel with the first capacitive means and which serve to generate 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 connects a terminal N 1  of the inverter means to a terminal N 2  between the first unidirectional means and the second unidirectional means, which circuit device further comprises a second circuit connecting a terminal N 2  to terminal N 5  and comprising third capacitive means, and a third circuit connecting 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 and comprising a series arrangement of third unidirectional means and fourth unidirectional means, wherein neither the first circuit nor the third circuit include inductive means, and a fourth circuit connecting the third circuit to a terminal N 6 , which forms part of the load circuit. 
     Such a circuit device is disclosed in WO 97/19578. 
     The known circuit device is dimensioned such that the power feedback circuit is optimized for a certain power level with respect to total harmonic distortion and power balance. This means of course that at a power level which deviates from the predetermined power level, the total harmonic distortion and the power balance are sub-optimal; this is certainly the case, for example, at a difference of 40% in the case of different power levels of 100% and 60%. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to improve the known circuit device in such a manner that optimization with respect to total harmonic distortion and power balance is possible for more than one power level. 
     To achieve this, the invention provides a circuit device of the type mentioned in the opening paragraph, which is characterized in that the third circuit further comprises fifth unidirectional means interposed between the fourth unidirectional means and terminal N 4 , terminal N 4  is coupled to a terminal N 8  between the fourth and the fifth unidirectional means via a series arrangement of sixth unidirectional means and switching means, said sixth unidirectional means being oppositely directed relative to the third, fourth and fifth unidirectional means which are equally directed, and terminal N 8  is connected to terminal N 6  by means of a fifth circuit. 
     By virtue of said measures, the circuit device can be selectively adapted to different power levels while maintaining optimum total harmonic distortion and power balance. 
     It has been found that smooth operation of the circuit device can be achieved if the fifth circuit comprises fifth capacitive means. 
     In addition, capacitive voltage division enables optimum adaptation to discharge lamps to be used. 
     Furthermore, the invention enables optimization regarding total harmonic distortion and power balance to be achieved for more than two power levels by embodying a sub-circuit device comprising the fourth, fifth and sixth unidirectional means, the switching means and the fifth circuit so as to form a multiple sub-circuit device. The series connections of fourth and fifth unidirectional means of the multiple sub-circuit device are series-connected between terminals N 7  and N 4  of the circuit device and have a respective terminal N 8   i . The series connections of sixth unidirectional means and switching means of the multiple sub-circuit device are connected between the respective terminal N 8   i  and terminal N 4  of the circuit device. The fifth circuits of the multiple sub-circuit device are connected between the respective terminal N 8   i  and terminal N 6  of the circuit device. The fifth and the fourth unidirectional means may then be formed from common unidirectional means. 
     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 INVENTION 
     In the drawings: 
     FIG. 1 is a simplified diagram of the embodiment of a circuit device in accordance with the invention, wherein a discharge lamp LA is connected to the circuit device; and 
     FIG. 2 shows the multiple sub-circuit device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, K 1  and K 2  are input terminals for connecting the circuit device to a low-frequency supply-voltage source. L 2  is an inductor which forms an input filter in combination with a 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. A capacitor C 4  forms first capacitive means and, in combination with diodes D 5  and D 6 , a first circuit. Switching elements Q 1  and Q 2  form inverter means in combination with a drive circuit DC. The drive circuit DC is a circuit part for generating drive signals for rendering the switching elements Q 1  and Q 2  conducting and non-conducting. 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 connecting 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 third and fourth unidirectional means, respectively. The series arrangement of diodes D 7  and D 8  forms a third circuit. Capacitor CS forms fourth capacitive means as well as a fourth circuit. 
     Input terminals K 1  and K 2  are connected to each other by means of a series arrangement of inductor L 2  and capacitor C 3 . A first side of capacitor C 3  is connected to a first input terminal of the rectifier bridge, and a second side of capacitor C 3  is connected to a second input terminal of the rectifier bridge. A first output terminal N 3  of the rectifier bridge is connected 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 connected 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 connected to a first output terminal of drive circuit DC. A control electrode of switching element Q 2  is connected to a second output terminal of drive circuit DC. N 1  is a common terminal of switching element Q 1  and switching element Q 2 . Terminal N 1  is connected to terminal N 2  by means of a series arrangement 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 connected to terminal N 7  by means of capacitor C 5 . 
     If diode D 8  is directly connected to terminal N 4 , 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 present across capacitor C 4  serving as a buffer capacitor. The drive circuit DC renders the switching elements Q 1  and Q 2  alternately conducting and non-conducting, as a result of which a substantially square-wave voltage having an amplitude which is substantially 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 of the same frequency as the substantially square-wave voltage are present on terminal N 2  and terminal N 7 . These voltages on terminal N 2  and terminal N 7  make sure that a pulsating current is drawn from the supply-voltage source, also when the voltage across 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 not be noted that similar results were achieved for a configuration of the circuit device which differs somewhat from the configuration shown in FIG. 1 in that capacitor C 1  connects terminal N 2  to terminal N 5  instead of to terminal N 4 . In this slightly different configuration, capacitor C 1  forms third capacitive means and a second circuit. 
     So far, the circuit device and the operation thereof are conventional and known from WO 97/19578. 
     As opposed to the known circuit device, the inventive circuit device additionally comprises a diode D 9  in the third circuit, as shown in FIG. 1, which diode is interposed between diode D 8  and terminal N 4 . Diode D 9  forms fifth unidirectional means. Furthermore, terminal N 4  is connected to a terminal N 8  between the diodes D 8  and D 9  by means of a series arrangement of a diode D 10  and a switching element Q 3 . Diode D 10  and switching element Q 3  form, respectively, sixth unidirectional means and switching means. Diode D 10 , which is accommodated in the circuit, is oppositely poled relative to the equally poled diodes D 7 -D 9 . Finally, terminals N 6  and N 8  are connected to each other by means of capacitor C 6 . Capacitor C 6  forms a fifth circuit and fifth capacitive means. If necessary, or desirable, terminals N 4  and N 7  can be connected to each other by capacitor C 7 . Capacitor C 7  forms a sixth circuit and sixth capacitive means. Capacitors C 5  and C 7  form a capacitive voltage divider. As regards capacitor C 6 , a similar capacitive voltage division can be obtained by connecting terminals N 4  and N 8  to each other by means of a capacitor C 8 . Capacitor C 8  forms a seventh circuit and seventh capacitive means. 
     Switching element Q 3  and diode D 10  jointly short-circuit diode D 9  when the switching element Q 3  is in the conducting state, so that the circuit device of FIG. 1 works in the same conventional way as the known circuit device. 
     When the switching element Q 3  is not conducting, then diode D 9  is active. Diode D 9  co-operates with capacitor C 6  in the same conventional way as diode D 8  in combination with capacitor C 5 . The selective addition of diode D 9  and capacitor C 6  to the circuit device by means of circuit element Q 3  and diode D 10  enables the total harmonic distortion and power balance to be optimized for two power levels, for example for a 100% power level and a 60% power level when reduced lighting suffices, resulting, in the latter case, in a saving of energy at a satisfactory stability. In other words, two power feedback circuits are used, each of which is optimized with respect to the total harmonic distortion and power balance for one power level, and the appropriate power feedback path for optimum total harmonic distortion and power balance can be chosen. As a result, a ballast is obtained which meets the requirements in terms of a fixed output power at both output levels. Capacitive division can be applied for fine tuning the discharge lamp to be used, in particular to preclude vertical or horizontal power-supply thresholds (?) at high lamp voltages. 
     As shown in FIG. 2, use is made of a multiple sub-circuit device comprising the fourth, fifth and sixth unidirectional means (D 8 -D 10 , FIG.  1 ), the switching means (Q 3 , FIG. 1) and the fifth circuit (C 6 , in FIG.  1 ), the series connections of fourth and fifth unidirectional means D 8   1 , D 9   1 ; D 8   2 , D 9   2 ; . . . ; D 8   N , D 9   N  of the multiple sub-circuit device being series-connected between terminals N 7  and N 4  of the circuit device and comprising a respective terminal N 8   i , which is indicated in FIG. 2 by, respectively, N 8   1 , N 8   2 , . . . ,N 8   N . The series connections of sixth unidirectional means and switching means D 10   1 , Q 3   1 ; D 10   2 , Q 3   2 ; . . . ; D 10   N , Q 3   N  of the multiple sub-circuit device are connected between the respective terminals N 8   1 , N 8   2 , . . . , N 8   N  and terminal N 4  of the circuit device. The fifth circuits C 6   1 , C 6   2 , . . . ,C 6 N of the multiple sub-circuit device are connected between the respective terminals N 8   1 , N 8   2 , . . . , N 8   N  and terminal N 6  of the circuit device. 
     As is also shown in FIG. 2, the multiple sub-circuit device may also comprise the seventh circuit (C 8 , FIG.  1 ). The seventh circuits C 8   1 , C 8   2 , . . . , C 8   N , if present, are connected between the respective terminals N 8   1 , N 8   2 , . . . , N 8   N  and terminal N 4  of the circuit device. 
     The fifth and fourth unidirectional means of two successive sub-circuit devices, for example D 9   1 , D 8   2  and D 9   2 , D 8   3  in the case of a triple sub-circuit device, may be formed of common unidirectional means. For example, of two series-arranged diodes, one can be omitted.