Abstract:
The invention relates to a circuit arrangement for operating lamps. The circuit arrangement essentially comprises a full bridge. According to the invention, at least one inductor is connected in series with a full-bridge branch for switching load relief purposes. The circuit arrangement is preferably suitable for operation using a charge pump. PFC is thus achieved in a single-stage arrangement.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to circuit arrangements for operating lamps. In the present patent application, the term lamp encompasses apparatuses which are suitable for producing electromagnetic radiation having a wavelength of between 50 nanometers and 50 000 nanometers from electrical energy. Examples of such lamps are incandescent lamps, gas discharge lamps or light-emitting diodes.  
         [0002]     The circuit arrangement is, in particular, a full bridge, whose switches are relieved of load. Furthermore, the circuit arrangement is also suitable for keeping line current harmonics low.  
       BACKGROUND OF THE INVENTION  
       [0003]     Specification U.S. Pat. No. 4,864,479 (Steigerwald) discloses a DC-to-DC converter which is in the form of a full bridge. Parasitic capacitances and diodes of the switches in the full bridge provide load relief for the switches. The switches only switch on when the voltage which is applied to the switches is zero. This is referred to in the literature as “zero voltage switching” (ZVS). The functional principle described is based on a resonant process between the parasitic capacitances and the inductive component of a load resistance. Load relief for the switches can thus only be realized in a restricted range of the operating frequency. Since lamps often need to be operated in a frequency range which is prescribed by the lamp technology, and, in addition, the inductive component of the load resistance is prescribed by inductors for current limitation purposes, switching load relief is only rarely possible in the case of full bridges from the prior art during lamp operation.  
       SUMMARY OF THE INVENTION  
       [0004]     It is the object of the present invention to provide a circuit arrangement having a full bridge for operating lamps which provides switching load relief at an operating frequency which can be selected. This object is achieved by a circuit arrangement having a full bridge which is made up of a first and a second full-bridge branch, a first inductor being connected between the full-bridge branches, and a second inductor being connected in series with one of the full-bridge branches. In the present patent application, the operating frequency is understood to be the frequency of a clock at which the switches of a full-bridge branch open and close.  
         [0005]     Owing to the circuit arrangement according to the invention, the potentials of the full-bridge branches oscillate with respect to one another such that ZVS results. This takes place at an operating frequency which can be set by the first and the second inductors and does not depend on the inductive component of the load resistance.  
         [0006]     One advantageous development of the circuit arrangement according to the invention consists in the fact that a charge pump is supplied which brings about a reduction in the line current harmonic.  
         [0007]     Furthermore, a phase shift in the driving of the two full-bridge branches as is described in U.S. Pat. No. 4,864,479 (Steigerwald) can also be used in a circuit arrangement according to the invention. It is thus possible with the aid of the phase shift for a desired lamp current to be set.  
         [0008]     The invention therefore makes possible a single-stage circuit arrangement for operating a lamp having low circuit complexity, in the case of which the lamp current can be set via the above-described phase shift, and the energy balance in the charge pump can be set via the operating frequency, all of the switches provided being relieved of load. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention will be explained in more detail below using exemplary embodiments with reference to drawings, in which:  
         [0010]      FIG. 1  shows an exemplary embodiment of a circuit arrangement according to the invention,  
         [0011]      FIG. 2  shows an exemplary embodiment of a circuit arrangement according to the invention having a charge pump for the purpose of reducing the line current harmonics,  
         [0012]      FIG. 3  shows an exemplary embodiment of a circuit arrangement according to the invention having an alternative variant of a charge pump for the purpose of reducing the line current harmonics,  
         [0013]      FIG. 4  shows an exemplary embodiment as shown in  FIG. 2  having an additional capacitor for switching load relief purposes, and  
         [0014]      FIG. 5  shows an exemplary embodiment as shown in  FIG. 3  having additional capacitors for switching load relief purposes. 
     
    
       [0015]     In the text which follows, switches are given the letter S, diodes the letter D, capacitors the letter C, nodes the letter N and inductors the letter L, in each case followed by a number. The same references are also used throughout in the text which follows for identical and functionally identical elements of the different exemplary embodiments.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0016]      FIG. 1  shows a circuit arrangement according to the invention. In the text which follows, the topology of this circuit arrangement is described.  
         [0017]     A first full-bridge branch comprises the series circuit comprising a first and a third electronic switch S 1 , S 3  which are connected at a first center point N 1 . A second full-bridge branch comprises the series circuit comprising a second and a fourth electronic switch S 2 , S 4  which are connected at a second center point N 2 .  
         [0018]     The first and the second full-bridge branch are each connected with one connection to a positive node N 3  and with the other connection to a negative node N 4 . An energy source which feeds in a DC voltage between the positive node N 3  and the negative node N 4  is not illustrated. A storage capacitor C 6  is connected between the positive node N 3  and the negative node N 4 .  
         [0019]     The first and the second center points N 1 , N 2  are connected via a first inductor L 1 . According to the invention, at least one second inductor L 2  is connected in the second full-bridge branch in series with the second and the fourth electronic switches S 2 , S 4 .  
         [0020]     In the exemplary embodiment, the second inductor L 2  is connected between the third node N 3  and the second electronic switch S 2 . A fourth inductor L 4  is connected between the fourth node N 4  and the fourth electronic switch S 4 . In the exemplary embodiment, the second and the fourth inductors L 2 , L 4  are magnetically coupled. This coupling makes the currents in the switches symmetrical, which results in reduced radio interference.  
         [0021]     The series circuit comprising a first and a second capacitor C 1 , C 2 , which are connected at a fifth node N 5 , is connected in parallel with the series circuit comprising the second and the fourth electronic switches S 2 , S 4 . The values for C 1  and C 2  are selected such that the voltage across C 1  and C 2  can be assumed to be constant during one cycle of the operating frequency. A connection for a load circuit is thus created at N 5 .  
         [0022]     In the exemplary embodiment, the load circuit is connected between the fifth node N 5  and the first center point N 1 . Lamps Lp can be coupled to the load circuit.  
         [0023]     In the exemplary embodiment, the load circuit comprises a low-pass filter comprising the series circuit comprising a third inductor L 3  and a third capacitor C 3 . A lamp Lp is connected in parallel with the third capacitor C 3 . The limit frequency of the low-pass filter is preferably below the operating frequency of the full bridge. A sinusoidal lamp current is thus achieved.  
         [0024]     In order to make the circuit arrangement symmetrical, the parallel circuit comprising a fourth capacitor C 4  and a first diode D 1  is connected, in the exemplary embodiment, between the second and the third nodes N 2 , N 3 , and the parallel circuit comprising a fifth capacitor C 5  and a second diode D 2  is connected between the second and the fourth nodes N 2 , N 4 .  
         [0025]     In each case a diode and a capacitor are connected in parallel with each electronic switch S 1 , S 2 , S 3 , S 4 . Said diode and capacitor are parasitic elements which are incorporated in the practical design of an electronic switch. The diodes are in principle not provided in the case of bipolar transistors and IGBTs, but are often integrated in the switches as freewheeling diodes. MOSFETs are often used as the electronic switches, in the case of which a so-called body diode is in principle always incorporated. According to the invention, the parasitic capacitors of the electronic switches form, together with the inductors L 1 , L 2  and L 4 , a resonant circuit which brings about switching load relief.  
         [0026]     In each full-bridge branch the electronic switches S 1 , S 3  and S 2 , S 4 , respectively, are alternately opened and closed at a clock, the frequency of the clock being the same in each full-bridge branch. Drive devices which bring about the opening and closing of the electronic switches S 1 , S 3 , S 2 , S 4  are not illustrated in  FIG. 1 . The current through the lamp can be set by means of a phase shift between the first and the second clocks.  
         [0027]     An exemplary embodiment of a circuit arrangement according to the invention having a charge pump for the purpose of reducing the line current harmonics is illustrated in  FIG. 2 . In  FIG. 2 , as an addition to  FIG. 1 , a pump capacitor C 7  is connected between the second node N 2  and a sixth node N 6 , and a pump diode D 7  is connected between the sixth node N 6  and the third node N 3 . The energy is now no longer fed into the positive node N 3  and the negative node N 4 , as in the previous exemplary embodiment, but is fed into the sixth node N 6  and the negative node N 4 . The energy originates from a system voltage source UN which is connected to system voltage connections J 1 , J 2 . J 1  and J 2  are connected to the AC voltage input of a bridge rectifier comprising the diodes D 3 , D 4 , D 5 , D 6 . The positive output of the bridge rectifier is connected to the sixth node N 6 . The negative output of the bridge rectifier is connected to the fourth node N 4 . The diodes D 3  and D 4  are colored in, whereas D 5  and D 6  are not. This indicates that D 3  and D 4  may be slow diodes which switch at the system frequency, whereas D 5  and D 6  must be fast diodes which operate at the operating frequency of the full bridge.  
         [0028]     In each case a capacitor C 8 , C 9 , which is used for interference suppression purposes, is connected in parallel with D 3  and D 4 . D 1 , D 2  and C 4 , C 5  are illustrated using dashed lines since they can be dispensed with.  
         [0029]     The topology described realizes a charge pump for the purpose of reducing line current harmonics, as is known from the literature, for example from U.S. Pat. No. 6,259,213 (Rudolph). The circuit arrangement according to the invention shown in  FIG. 1  may be equipped with a charge pump with very little circuitry complexity. The energy balance of the charge pump can be set with the aid of the operating frequency.  
         [0030]     In  FIG. 3 , a charge pump in a circuit according to the invention is realized, as is described in specification U.S. Pat. No. 6,208,085 (Lehnert). A system voltage source UN is again connected to the system voltage connections J 1 , J 2 .  
         [0031]     The AC voltage input of a full-bridge rectifier D 8 , D 9 , D 10 , D 11  is connected to the system voltage connections J 1 , J 2 . The positive output of the full-bridge rectifier D 8 , D 9 , D 10 , D 11  is connected to the positive node N 3 , and the negative output of the full-bridge rectifier D 8 , D 9 , D 10 , D 11  is connected to the negative node N 4 .  
         [0032]     At least one system voltage connection J 1 , J 2  is connected to the second node N 2  via a capacitor C 8 , C 9 . In the exemplary embodiment, the two capacitors are provided. This is necessary in the case of lamps having a high power in order to provide sufficient pump energy. The capacitors C 4  and C 5  are illustrated using dashed lines in order to indicate that it is also only these which can be dispensed with, whereas D 1  and D 2  are provided.  
         [0033]      FIG. 4  differs from  FIG. 2  in the parallel circuit comprising a capacitor C 10  and the pump diode D 7 . Load relief for the electronic switches S 1 , S 2 , S 3  and S 4  is thus further improved.  
         [0034]      FIG. 5  differs from  FIG. 3  in that an eleventh capacitor C 11  is connected between the first system voltage connection J 1  and the third node N 3 , and/or in that a twelfth capacitor C 12  is connected between the second system voltage connection J 2  and the fourth node N 4 . Load relief for the electronic switches S 1 , S 2 , S 3  and S 4  is thus further improved. In the exemplary embodiment in  FIG. 5 , C 11  and C 12  are provided. This is necessary if it is desired to make the circuit arrangement symmetrical.