Abstract:
A ballast circuit includes an equalizer transformer for equalizing current between two or more lamps operated in parallel, combined on one core with a choke. In a 2-lamp embodiment, four windings are provided on one main core. The first and second are closely coupled and have N turns each, each connected separately to one of the lamps. A third and fourth winding are loosely coupled with respect to each other, and are arranged symmetrically about the first and second windings on the main core. These latter windings may be connected in series or in parallel to be an inductive impedance in the ballast circuit.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a circuit arrangement for operating two or more lamps in parallel, equipped with 
     an equalizer transformer comprising a main core of magnetizable material around which a first winding and a second winding are provided, the first and second winding each comprising a number of turns substantially equal to N, said first and second winding during operation being connected in series with respective lamps, 
     inductive ballast means. 
     The invention also relates to an inductive component. 
     A circuit arrangement as mentioned in the first paragraph is known from EP 0766500-A1. The known circuit arrangement is widely used for supplying low-pressure mercury discharge lamps. In the known circuit arrangement the equalizer transformer fulfils two functions. First of all, during ignition, after the first lamp has ignited, the equalizer transformer makes sure that the voltage over the lamp that has not yet ignited is high enough to realize the ignition of that lamp. In the second place, during steady state operation the equalizer transformer makes sure that the currents flowing through the lamps are substantially equal. The inductive ballast means is normally formed by a single choke that is in series with both lamps. The function of the inductive ballast means is to limit the current flowing through the lamps. A disadvantage of the known circuit arrangement is that the total number of components comprised in the circuit arrangement is comparatively high so that it is relatively complicated and therefore expensive to manufacture the circuit arrangement. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention aims to provide a circuit arrangement that comprises only a comparatively small number of components. 
     A circuit arrangement as mentioned in the opening paragraph is therefor according to the invention characterized in that the inductive ballast means comprise a third winding and a fourth winding around the core of magnetizable material, said third and said fourth winding each comprising a number of turns substantially equal to M, and in that the third and the fourth winding are arranged symmetrically with respect to the first and second winding. 
     The equalizer transformer and the inductive ballast means have been integrated into a single inductive component in a circuit arrangement according to the invention. The process of assembling a circuit arrangement according to the invention and its integration in a compact lamp are comparatively simple as a result of this. 
     Since the third and fourth winding comprise a substantially equal number of turns and they are arranged symmetrically with respect to the first and second winding, the magnetic fluxes through the third and fourth winding hardly interfere with the functioning of the first and the second winding during operation. In other words there is hardly any interaction between the inductive ballast means and the equalizer transformer. The third and the fourth winding are so arranged that during operation there is only a relatively small magnetic coupling between them; that is, they are loosely coupled magnetically. Because of this relatively small magnetic coupling the third and fourth winding can together form an inductor. The first and the second winding, however, are so arranged that during operation, there exists a good magnetic coupling between them; that is, they are loosely coupled magnetically. This good coupling is necessary for the first and second winding to form an equalizer transformer. The magnetic coupling between the first and the second winding is good when the two windings are close together on the main core. The magnetic coupling can be further improved by forming the first and second winding as a bifilar winding. In this case the first and the second winding are not just close to each other but occupy the same space. 
     Preferably the first winding and the second winding are arranged between the third and the fourth winding. In such an arrangement the coupling between the first and second winding can be good since they can be close together. At the same time the third and fourth winding are separated from each other by the first and second winding so that the coupling between the third and fourth winding is relatively small. 
     A circuit arrangement according to the invention preferably comprises a first E-core and a second E-core formed out of magnetizable material, wherein the end surfaces of the transverse legs of the first E-core are facing the end surfaces of the second E-core, and wherein the first winding and the third winding surround the middle transverse leg of the first E-core, while the second and the fourth winding surround the middle transverse leg of the second E-core. 
     In another preferred embodiment of a circuit arrangement according to the invention, the main core comprises a first U-core and a second U-core formed out of magnetizable material, wherein the end surfaces of the transverse legs of the first U-core are facing the end surfaces of the transverse legs of the second U-core and wherein the first winding and the third winding surround a first transverse leg of the first U-core, while the second and the fourth winding surround a first transverse leg of the second U-core, the end surface of the first transverse leg of the first U-core facing the end surface of the first transverse leg of the second U-core. In practice it has been found that the performance of a main core comprising two U-cores is not as good as the performance of a main core comprising two E-cores, but in the first case less magnetizable material needs to be used, so that the circuit arrangement can be smaller and cheaper. 
     It be mentioned that, for instance in U.S. Pat. No. 5,313,176, an inductive component is disclosed in which the functions of two chokes and a transformer are integrated. This inductive component comprises only two windings and is very suitable for use e.g. in an EMI/RFI filter. However, since both functions of the component strongly influence each other, the chokes and the transformer cannot be used independently, or more particularly conduct different currents. For these reasons the inductive component disclosed in U.S. Pat. No. 5,313,176 is not suitable for use as an integrated equalizer transformer plus inductive ballast means in a circuit arrangement according to the present invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     Embodiments of the invention will be explained in detail with reference to a drawing, in which 
     FIG. 1A shows an embodiment of an inductive component according to the invention; and 
     FIG. 1B shows an embodiment of an inductive component according to the invention; and 
     FIG. 1C shows an embodiment of an inductive component according to the invention; and 
     FIG. 1D shows an embodiment of an inductive component according to the invention; and 
     FIG. 1E shows an embodiment of an inductive component according to the invention; and 
     FIG. 2 shows an embodiment of a circuit arrangement according to the invention comprising such an inductive component, together with two lamps. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1A, E 1  and E 2  are two E-cores formed out of magnetizable material together forming a main core. The end surfaces of the transverse legs of the first E-core E 1  are facing the end surfaces of the second E-core E 2 .  1  and  2  are first and second windings surrounding the transverse middle leg of the first E-core E 1  and the middle transverse leg of the second E-core E 2  respectively. Since the first and second winding are very close together there is a good magnetic coupling between them. The first winding  1  and the second winding  2  together with the two E-cores E 1  and E 2  form an equalizer transformer. A third winding  3  surrounds the middle transverse leg of the first E-core E 1 , while a fourth winding  4  surrounds the middle transverse leg of the second E-core E 2 . Because the third and the fourth winding are separated from each other by the other windings the magnetic coupling between the first and the second winding is relatively small. For this reason the third winding  3  and the fourth winding  4  together with the two E-cores form an inductor that can be used as an inductive ballast means. The first winding  1  and the second winding  2  each comprise a number of turns that is substantially equal to N, while the third winding  3  and the fourth winding  4  each comprise a number of turns substantially equal to M. The first winding  1  and the second winding  2  are arranged between the third winding  3  and the fourth winding  4 . During operation of the inductive component shown in FIG. 1 the first winding and the second winding are so connected that each of them carries substantially the same amount of current and that the magnetic fluxes generated by them substantially cancel each other. Similarly the third and fourth winding are so connected that during operation they carry substantially the same amount of current and that the fluxes generated by them substantially cancel each other in the part of the main core that is surrounded by the first and the second winding. Consequently the part of the inductive component that forms the inductive ballast means does not interact with the functioning of the part of the inductive component that forms the equalizer transformer. 
     In the other embodiments shown in FIG. 1, similar parts are indicated by means of the same reference numerals. The embodiment shown in FIG. 1B only differs from the embodiment in FIG. 1A in that the first winding and the second winding are implemented as a bifilar winding. This feature further improves the coupling between the first and the second winding so that the performance of this embodiment is slightly better than that of the embodiment shown in FIG.  1 A. 
     The embodiment in FIG. 1C differs from the one in FIG. 1B in that the bifilar winding is not round the middle transverse legs of the E-cores but round two of the side transverse legs of the E-cores. To decrease the magnetic coupling between the third and the fourth winding, an air gap is present between the end surfaces of the middle transverse legs of both E-cores. In this embodiment the equalizer transformer and the ballast inductor are even more magnetically decoupled than in the embodiment in FIG. 1B., so that the performance is slightly improved. 
     In the embodiment in FIG. 1D, the first and second windings are formed by a bifilar winding around two of the side transverse legs of the E-cores, while the third and the fourth winding are each around a further side transverse leg of an E-core. In this embodiment the magnetic coupling between the equalizer and the ballast inductor is further reduced with respect to the embodiment in FIG.  1 C. An air gap is present between the third and the fourth winding to reduce the magnetic coupling between them. The overall performance of this component is very similar to that of the embodiment in FIG.  1 A. 
     In the embodiment in FIG. 1E use is made of two U-cores, U 1  and U 2 , instead of two E-cores. The bifilar winding that forms the first and the second winding surrounds a first transverse leg of the first U-core U 1  and a first transverse leg of the second U-core U 2 . The third winding surrounds the first transverse leg of the first U-core U 1  and the fourth winding surrounds the first transverse leg of the second U-core U 2 . In this embodiment less magnetizable material is used so that this embodiments is relatively cheap. In practice, however, the performance of this embodiment is found to be not as good as that of the embodiments comprising two E-cores. 
     In FIG. 2, K 1  and K 2  are input terminals suitable for connection to a DC voltage supply source. Input terminals K 1  and K 2  are connected by means of a series arrangement of switching elements S 1  and S 2 . Control electrodes of switching elements S 1  and S 2  are connected to respective output terminals of a control circuit SC for rendering the switching elements alternately conductive and non-conductive. Switching element S 2  is shunted by series arrangement of third winding  3 , fourth winding  4  and capacitor C 1 , K 3 , K 3 ′, K 4  and K 4 ′ are terminals for lamp connection. Capacitor C 1  is shunted by a first series arrangement of terminal K 3 , lamp LA 1 , terminal K 3 ′ and first winding  1 . Capacitor C 1  is also shunted by a second series arrangement of terminal K 4 , lamp LA 2 , terminal K 4 ′ and second winding  2 . 
     The operation of the circuit arrangement shown in FIG. 2 is as follows. 
     In case a DC voltage source is connected to input terminals K 1  and K 2 , the control circuit SC renders the switching elements S 1  and S 2  alternately conductive and non-conductive. As a result an AC-voltage is present over capacitor C 1 . In case for example lamp LA 1  ignites as a result of the presence of this AC voltage, the first winding  1  will carry a current. Because of the good magnetic coupling between the first and the second winding, the current through the first winding induces a voltage over the second winding. The voltage present over the second lamp is the sum of the voltage over capacitor C 1  and the voltage over the second winding  2 . Although the voltage over capacitor C 1  has decreased after the ignition of lamp LA 1 , this sum voltage is high enough to ignite the lamp LA 2 . 
     After both lamps have ignited both the first and the second winding carry a current that is equal to the current through lamp LA 1  and the current through lamp LA 2  respectively. If the currents through both lamps are substantially equal the magnetic coupling between the first and the second winding causes an induced voltage over each of the windings that substantially cancels the voltage that is present over each of the windings because of the current it carries. However, if for instance the lamp current through lamp LA 1  is larger than that through lamp LA 2 , the voltage over the second winding that is induced by the magnetic coupling between the first and the second winding is higher than the voltage that is present over the second winding because of the current this second winding carries. As a result both voltages present over the second winding do not completely cancel each other and a rest voltage is present over the second winding. For similar reasons a rest voltage with a substantially equal amplitude but of opposite polarity is present over the first winding. The rest voltage present over the first winding forces the current through lamp LA 1  to decrease while the rest voltage present over the second winding forces the current through lamp LA 2  to increase. As a result the currents through each of the lamps are maintained substantially equal during steady-state operation. 
     In the circuit shown in FIG. 2 the third winding and the fourth winding are arranged in series and each carry a current that is twice as large as the current through each of the first and the second windings. Alternatively the third winding can be arranged in parallel with the fourth winding. An important advantage of the topology shown in FIG. 2, however, is the fact that the third and the fourth winding always carry the same current.