Abstract:
In an electronic ballast for operating a discharge lamp at a high frequency, a single feedback and a double pump feedback are implemented in a combined way. The resulting electronic ballast combines the advantageous of a simple topology with favorable properties with respect to power factor, THD, efficacy, crest factor of the lamp current and EMI restrain.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a circuit arrangement for operating a lamp, comprising 
     supply input terminals for connection to a supply voltage source, 
     a rectifier coupled to the supply input terminals for rectifying an AC supply voltage supplied by the supply voltage source, 
     an inverter equipped with input terminals coupled to respective output terminals of the rectifier for generating a high frequency lamp current out of a DC voltage, 
     a first unidirectional element D 1  coupled between a first output terminal of the rectifier and an input terminal of the inverter, 
     a first branch that is coupled between a terminal N 1  between the first output terminal of the rectifier and the first unidirectional element and a terminal N 2  in the inverter on which during operation of the circuit arrangement a high frequency voltage is present. 
     The invention also relates to a compact lamp equipped with such a circuit arrangement. 
     A circuit arrangement as mentioned in the opening paragraph is known. The known circuit arrangement incorporates power feedback realized by the first unidirectional element and the first branch. Because of this power feedback the power factor of the known circuit arrangement is relatively high although the topology of the known circuit is relatively simple and therefore relatively cheap. Disadvantages of the known circuit arrangement are a relatively low efficacy and the fact that it is very difficult to restrain the EMI. 
     SUMMARY OF THE INVENTION 
     The invention aims to provide a circuit arrangement for operating a lamp that is relatively cheap and simple, has a good power factor combined with a relatively high efficacy and offers a good EMI restrain. 
     A circuit arrangement as mentioned in the opening paragraph is therefore according to the invention characterized in that the circuit arrangement is further provided with 
     a second branch comprising a series arrangement of a second (D 2 ), a third (D 3 ), a fourth (D 4 ) and a fifth unidirectional element (D 5 ) that connects the input terminals of the inverter, 
     a third branch comprising a first capacitive element C 1  and connecting a cathode of the second unidirectional element with an anode of the fourth unidirectional element, 
     a fourth branch comprising a second capacitive element C 2  connecting an anode of the fifth unidirectional element with a cathode of the third unidirectional element, 
     a fifth branch connecting a terminal N 5  between the third and the fourth unidirectional element with the terminal N 2 . 
     It was found that a circuit arrangement according to the invention has a good power factor. In addition it was found that the efficacy of a circuit arrangement according to the invention is relatively high and that the EMI could easily be restrained. 
     It is advantageous to equip a circuit arrangement according to the invention with a sixth branch comprising a third capacitive element (C 3 ) and coupling a cathode of the first unidirectional element to terminal N 5 . This third capacitive element controls the amplitude of the voltage present at the terminal N 5  and can be so dimensioned that a further improvement in the shape of the current supplied by the supply voltage source is realized. 
     Good results were obtained for a circuit arrangement according to the invention, in which the inverter comprises a bridge circuit that is equipped with a seventh branch comprising a series arrangement of two switching elements and in which the terminal N 2  is a terminal of the seventh branch between the two switching elements. 
     Good results have also been obtained for embodiments of a circuit arrangement according to the invention, in which the first branch comprises capacitive means. 
     Since a circuit arrangement according to the invention has a relatively simple topology, it can be manufactured so that it is very small. For this reason a circuit arrangement according to the invention is very suitable to be used in the ballast means of a compact lamp comprising 
     a light source provided with a vessel that is closed in a gastight manner and transmissive for visible radiation, 
     a housing connected to the light source and provided with a lamp cap, 
     ballast means electrically connected to the light source for operating the light source and positioned at least partly in a space surrounded by the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will be further explained making use of a drawing. 
     In the drawing 
     FIG. 1 shows a schematic representation of an embodiment of a circuit arrangement according to the invention with a lamp LA connected to it, and 
     FIG. 2 shows a compact lamp equipped with a circuit arrangement as shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 K 1  and K 2  are supply input terminals for connection to a supply voltage source. Supply input terminal K 1  is connected by means of inductive element L 1  to a first input terminal of a rectifier for rectifying an AC supply voltage supplied by the supply voltage source. Supply input terminal K 2  is connected to a second input terminal of the rectifier by means of inductive element L 2 . The first input terminal and the second input terminal of the rectifier are connected by means of a capacitor C 4 . Inductive elements L 1  and L 2  together with capacitor C 4  together form an input filter. The rectifier is formed by the diodes D 11 -D 14 . A first output terminal of the rectifier is connected to a second output terminal by means of a series arrangement of diode D 1 , capacitor C 1 , diode D 4 , diode D 3  and capacitor C 3 . Capacitor C 1  and diodes D 4  and D 3  are shunted by diode D 2 . Diodes D 4  and D 3  and capacitor C 2  is shunted by diode D 5 . Capacitor C 3  connects a cathode of diode D 1  to a common terminal of diodes D 3  and D 4 . Capacitor C 8  connects a terminal N 1  between the first output terminal of the rectifier to a common terminal N 5  of diodes D 3  and D 4 . Capacitor C 8  is shunted by resistor R 2 . The cathode of diode D 1  is connected to the second output terminal of the rectifier by means of a series arrangement of terminal N 3 , first switching element Q 1 , resistor R 14 , terminal N 2 , switching element Q 2 , resistor R 5  and terminal N 4 . First switching element Q 1  is shunted by diode D 9  second switching element Q 2  is shunted by diode D 10 . Terminal N 2  is connected to terminal N 5  by means of a series arrangement primary winding T 1 A, inductive element L 3  and lamp LA. The lamp LA is shunted by a capacitor C 6 . Capacitor C 6  is shunted by a series arrangement of PTC resistor PTC 1  and capacitor C 7 . Terminal N 2  is also connected to terminal N 5  by means of capacitor C 10 . Capacitor C 10  is shunted by resistor R 3 . A control electrode of switching element Q 1  is connected to terminal N 2  by means of resistor R 12 . Resistor R 12  is shunted by a series arrangement of resistor R 4 , inductive element L 4  and secondary winding T 1 B. A control electrode of switching element Q 2  is connected to terminal N 4  by means of resistor R 13 . Resistor R 13  is shunted by a series arrangement of resistor R 5 , inductive element L 5  and secondary winding T 1 C. Primary winding T 1 A and secondary windings T 1 B and T 1 C together form a control transformer T 1  for during operation generating control signals for rendering switching element Q 1  and switching element Q 2  alternately conducting and non-conducting. In this embodiment diodes D 1 -D 5  form a first to fifth unidirectional element respectively. Diodes D 2 -D 5  together form a second branch connecting the input terminals of the inverter. Capacitor C 1  forms a third branch connecting a cathode of D 2  with an anode of D 4 . Capacitor C 2  forms a fourth branch connecting an anode of D 5  with a cathode of D 3 . Capacitors C 8  and C 10  together with resistors R 2  and R 3  together form a first branch connecting terminal N 1  and terminal N 2 . Terminals N 3  and N 4  form input terminals of an inverter that is formed by terminals N 3  and N 4 , switching elements Q 1  and Q 2 , diodes D 9  and D 10 , the control transformer T 1 , resistors R 3 , R 4 , R 5 , R 12 , R 13 , R 14  and R 6 , inductive elements L 4 , L 5  and L 3 , capacitors C 6 , C 7  and C 10  and PTC 1 . Primary winding T 1 A, inductive element L 3 , capacitors C 6 , C 7  and C 10 , resistor R 3 , PTC 1  and the lamp LA in this embodiment form a fifth branch connecting terminals N 5  and terminal N 2 . C 3  forms a sixth branch coupling a cathode of diode D 1  to a common terminal N 5  of diodes D 3  and D 4 . The series arrangement of switching element Q 1 , resistor R 14 , terminal N 2 , switching element Q 2  and resistor R 6  forms a bridge circuit and a seventh branch that comprises a series arrangement of two switching elements. 
     The operation of the circuit arrangement shown in FIG. 1 is as follows. 
     When the input terminals are connected to a supply voltage source that supplies an AC supply voltage, the AC supply voltage is rectified by the rectifier and a DC voltage is present between the input terminals N 3  and N 4  of the inverter. By means of a circuit part well known in the art but not shown in FIG. 1, the inverter starts oscillating. Once oscillating the control transformer renders the switching elements Q 1  and Q 2  alternately conducting and non-conducting at a frequency f. As a result an AC current with frequency f flows through the branch formed by primary winding T 1 A, inductive element L 3 , capacitors C 6  and C 7 , PTC 1 . Immediately after the inverter has started oscillating the temperature of PTC 1  is low and therefore its impedance is also low. For this reason the AC current flows through the electrodes E 11  and E 12  of the lamp and preheats these electrodes. The current flowing through PTC 1  warms up PTC 1  thereby increasing its impedance. This increase in impedance causes the effective value of the capacity formed by PTC 1  and capacitors C 6  and C 7  to change and as a result the value of the frequency f at which the inverter oscillates changes as well. This frequency shift causes the lamp to ignite and the inverter from then on oscillates at a substantially constant frequency f stat . During operation a high frequency voltage with frequency f is present a terminal N 5 . This high frequency voltage causes diodes D 2 -D 5  and capacitors C 1 , C 2  and C 3  to function as a double pump feedback and also causes diode D 1  and capacitor C 8  to function as a single feedback. The double pump feedback and the single feedback both improve the power factor of the circuit arrangement considerably by making sure that high frequency current pulses are drawn from the supply voltage source at substantially any time during a period of the AC supply voltage. It was found that the circuit arrangement combined favorable properties with respect to power factor, THD, crest factor of the lamp current, efficacy, EMI restrain with a simple topology. For a practical circuit that had the topology as shown in FIG. 1, it was found that the power factor was 0.99, the THD was 13%, while the crest factor of the lamp current was 1.7. Additionally it was found that the efficacy of the practical circuit was very high and that the filter formed by inductive elements L 1  and L 2  and capacitor C 4  can be of relatively small dimensions and at the same time limit the EMI to acceptable proportions. 
     In FIG. 2, a light source  8  is provided with a discharge vessel which is closed in a gastight manner, is transmissive for visible radiation and is equipped with two electrodes (not shown). The light source contains a noble gas or a mixture of noble gases together with mercury. A housing  6  is connected to the light source and equipped with a lamp cap  3 . In this embodiment the part of the housing that is below the broken line A. The housing may for instance be formed out of a synthetic resin. Circuit arrangement B is a circuit arrangement for operating the light source. The topology of circuit arrangement B is as presented in FIG.  1 . Circuit arrangement B is electrically connected to electrodes in the light source. This connection is indicated by means of the broken lines  9 . The circuit arrangement B is placed in a space  7  which is surrounded by the housing  6 . E are conducting connectors between the circuit arrangement Band metal contacts  1  and  2  placed on the lamp cap. During operation the supply voltage is present between contacts  1  and  2 .