Abstract:
There is provided an electronic ballast for supplying a load, for example a lamp, in particular a gas discharge lamp, with an a.c. voltage (Un) the frequency of which is higher than the mains frequency. The ballast has a rectifier (GL) to be connected to the a.c. mains, an inverter connected downstream thereof, having two switches (S2, S1), which inverter generates the higher frequency a.c. voltage for the load (Z), and an electronic harmonics filter arranged between the rectifier (GL) and the inverter, in the form of a clocked switching controller. The control switch (S1) of the clocked switching controller at the same time forms one of the two switches (S1) of the inverter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an electronic ballast for supplying a load, for example a lamp, in particular a gas discharge lamp, with an a.c. voltage the frequency of which is higher than the mains frequency, having a rectifier to be connected to the a.c. mains, an inverter comprising two switches connected downstream of the rectifier, which inverter generates the higher frequency a.c. voltage for the load, and a harmonics filter arranged between the rectifier and the inverter, in the form of a clocked switching controller. 
     In particular the invention relates to an electronic ballast, the electronic harmonics filter of which consists of a clocked switching controller in the form of an inverting transformer (inverse transformer). 
     2. Description of the Related Art 
     Gas discharge lamps have an unsatisfactory light efficiency and tend to flicker when operated with mains frequency. For this reason they are usually operated with electronic ballasts which work at mains voltage and generate a higher frequency operating voltage for the gas discharge lamps therefrom, the frequency of which lies in the region from 20 to 50 kHz. The electronic ballasts have a rectifier, which rectifies the mains voltage and an inverter, which generates the higher frequency operating voltage from the rectified mains voltage. Through the utilization of electronic ballasts the light efficiency of the gas discharge lamps may be considerably increased and flickering may be avoided. At the same time, however, the problem occurs that as a result of the rectification harmonic waves result which cause unacceptable disturbances when fed back into the mains. To avoid this, a passive filter can be connected to the rectifier and the mains which filter must however comprise a relatively large and expensive choke. This choke can be avoided if an electronic harmonics filter is connected between rectifier and inverter instead of the passive filter. The electronic harmonics filter, which is usually constructed in the form of a clocked switching controller, manifests itself in that it removes pulse-like current from the rectifier, whereby the envelope curve of the current pulses is approximated to a sinusoidal half-wave form with mains frequency. 
     Such a harmonics filter is described for example in EP 268 043 A1 or in the book by C. H. STURM/E. KLEIN, &#34;Betriebsgerate und Schaltungen fur Lampen&#34; (&#34;Operating devices and circuits for lamps&#34;), Publisher Siemens AG, 6th Edition, 1992, pages 127/128. The electronic harmonics filter consists of a clocked switching controller; which can for example be a step-up transformer (booster) or an inverting transformer. The principle of such a clocked switching controller is also described in the book by U. STIETZE, CH. SCHENK, &#34;Halbleiterschaltungstechnik&#34; (&#34;Semiconductor circuitry techniques&#34;), Publisher Springer, 9th Edition, pp. 563 to 570. With the known harmonics filter the output voltage of the switching controller is constantly monitored and compared with a fixed desired-value voltage. From the control difference, the controller generates a control signal of variable switching frequency for the switch of the switching controller. 
     The combination of the known component circuits provides for the electronic ballast a circuitry configuration consisting, in terms of function, of a rectifier, an electronic harmonics filter and an inverter. Hereby an electronic harmonics filter structured in such a way proves to be, in terms of circuitry configuration, too complicated and to result in unacceptable manufacturing costs, in particular when used for individual lamps. 
     There were therefore suggested electronic ballasts with harmonics filters in the form of step-up transformers (boosters) in which the control switch of the clocked switching controller forms at the same time one of the two inverter switches. Such circuits are known for example from U.S. Pat. No. 5,224,025 or EP-A-O 435 628. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to so modify an electronic ballast of the kind mentioned in the introduction, having a harmonics filter connected between the rectifier and the inverter, which filter is in particular configured in the form of an inverting transformer (inverse transformer), that it may be manufactured with reduced circuitry outlay. 
     The object according to the invention is achieved in that the control switch of the clocked switching controller, formed as an inverting transformer (inverse transformer); at the same time forms one of the two switches of the inverter. As known per se, the switching controller formed as an inverting transformer (inverse transformer) has a first impedance in the form of a coil and a control switch, which form a series circuit connected in parallel to the rectifier, and whereby a second impedance is connected in parallel with the first impedance, which second impedance is decoupled from the rectifier by means of at least one diode. Whenever the control switch is opened, the energy stored in the first impedance with the control switch closed is transferred to the second impedance. Therewith a load can be operated at the second impedance via an inverter, whereby one of the current paths of the usual inverter can be provided via the control switch of the harmonics filter. In this way not only the second switch for the inverter is saved, but also the driver circuitry required therefor, whereby the electronic ballast can be manufactured more compactly and more economically. 
     According to a preferred embodiment of the invention the other of the two inverter switches is connected in parallel to the second impedance. 
     Further advantageous developments of the invention are set out in the subsidiary claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be now be described in detail with reference to exemplary embodiments and with reference to the drawings, which show: 
     FIG. 1 a first exemplary embodiment for the illustration of functional principles, 
     FIG. 2 a second, further simplified exemplary embodiment, with reversed polarity in comparison to FIG. 1 and 
     FIG. 3 a further exemplary embodiment for use for gas discharge lamps. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the first exemplary embodiment, which is illustrated in FIG. 1, a series circuit of the coil L1 and the switch S1 is connected to a d.c. voltage source (not shown) at the terminals 3 and 4. As soon as the switch S1 is closed, a current flows through the coil L1. There thus results a current flow which is designated with SI. Parallel to the coil L1 there is connected a series circuit which is formed by a diode D1 and a capacitor C1&#39;. The diode is poled in such a way that it blocks the d.c. voltage UG1 present at the points 3 and 4. As soon as the switch S1 opens, a voltage at the coil L1 falls, in accordance with the equation: U(t)=-L·di/dt, which has a polarity opposite to that of the voltage at the points 3 and 4. On account of the coil voltage the electric energy stored in the coil with the switch S1 closed is now transferred to the capacitor C1&#39; via the diode D1, whereby the voltage at the capacitor C1&#39; has a reversed polarity as compared to the d.c. voltage UG1. In parallel with the capacitor C1&#39; there is connected a series circuit of two capacitors C1 and C2, which are of the same size and therefore symmetrically divide the voltage at the capacitor C1&#39;. At the nodal point connecting the capacitors C1 and C2 a terminal of a load Z is connected. At the other end of the load Z a switch S2 is connected, the other terminal of which is connected to that side of the capacitor CI&#39; which is connected with the diode D1. The switch S2 has the function of one of two switches of an inverter, whilst the switch S1 assumes the function of the second switch of the inverter. If the capacitors C1 and C2 or C1&#39; are charged, the switch S2 can be closed, whereby the capacitor C1 discharges via the load Z and forms a current flow which is designated with LII. If the switch S2 opens, the switch S1 is closed, so that the capacitor C2 discharges via the load Z, a diode D3 and the switch S1. The diode D3 is connected with the load Z and the terminal 4 and is poled in such a way that it blocks the voltage present at the terminal 4 with the switch S2 closed in the direction towards the nodal point connecting the diode D1 and the capacitor C1&#39;, yet allows the flow of a load current, which load current is designated with LI. The switch S1 thus carries at the same time the current which provides for the storage of energy in the coil L1, and the load current LI. When the switch S1 is opened and the switch S2 is closed, the charging of the capacitors C1&#39;, C1 and C2 and the operation with the load current LII occur at the same time. The diodes D4 and D2 shown in FIG. 1 form respective freewheeling diodes for the switches S2 and S1, which protect the switches from overvoltage with the wrong polarity, in a way which is known per se, if these switches are for example semiconductor switching transistors. 
     FIG. 2 shows an exemplary embodiment, which largely corresponds with the exemplary embodiment shown in FIG. 1. The difference is that the polarity of the d.c. voltage connected to the terminals 3 and 4 is reversed as compared to FIG. 1. Thus, at the same time there results a reversed poling of the diodes D1 to D4. The arrangement illustrated in FIG. 1, consisting of the capacitors C1&#39;, C1 and C2 can, as illustrated in FIG. 2, be replaced by two capacitors C1 and C2 whereby these then--with appropriate dimensioning--take over the energy-storing function of the capacitor C1&#39; according to FIG. 1. Apart from saving a capacitor, the exemplary embodiment illustrated in FIG. 2 is further advantageous in that at the nodal point connecting the coil L1, the switch S1 and the capacitor C2, an earthing point is formed which is also suitable for a control circuit (not shown) for controlling the switches S1 and S2. 
     FIG. 3 shows an exemplary embodiment which is similar to that shown in FIG. 2, whereby it is adapted for use in an electronic ballast for gas discharge lamps. At the terminals 3 and 4 a bridge rectifier is connected which rectifies an a.c. mains voltage Un at the terminals 6 and 7. A coil Ln and a capacitor Cn are provided as a mains filter. Further there is a safety-fuse Si also provided at the terminal 7. Therefore, there is provided at the terminals 3 and 4 a d.c. voltage pulsing at mains frequency. In this exemplary embodiment the load Z shown in FIGS. 1 and 2 is replaced by a gas discharge lamp, whereby the coil L2 and the capacitor C3 form the oscillation circuit necessary for the operation of gas discharge lamps. The capacitor C3 carries the pre-heating current flowing through the heating coil of the gas discharge lamp, whereby a transformer T having three coils is connected in such a way that the magnetomotive force which is generated by the heating current eliminates itself and only the magnetomotive force resulting from the lamp current remains effective. Consequently a current which is proportional to the lamp current IL flows in the coil T1-C and a corresponding voltage value can be taken at the coil T1-C. This voltage value is fed as a parameter IL to a central control circuit 1 provided for the control of the switches S1 and S2. At the same time, the lamp voltage is fed to the control circuit 1 as parameter UL, the voltage at the points 3 and 4 is fed as parameter UE, the voltage dropped by the resistor RS is fed as parameter Is, which corresponds to the current through the switch S1, and the output voltage which is dropped over the series connection of the two capacitors C1 and C2 is fed as parameter UA. Hereby the voltage UA is transformed into a value range which is suitable for the central control circuit by means of a voltage divider which consists of the series connection of the resistors R4 and R3. In this way all parameters can be detected which are required for the switching of switch S1 and switch S2 in such a way that, in dependence upon the operating state of the lamp, the pulse current loading the rectifier GL always has the envelope curve of a sinusoidal half wave. 
     At the same time the parameters serve for monitoring the switching operation secured, so that faulty operation can be recognised quickly. 
     Apart from the use for gas discharge lamps, the circuits according to FIGS. 1 and 2 can also be used for the operation of low voltage halogen lamps. Hereby, the primary side of a transformer generally used for the operation of low voltage halogen lamps is employed in place of the load Z.