Abstract:
An electrical circuit arrangement for producing pulsed-voltage sequences for the operation of discharges which are impeded dielectrically comprises a series circuit formed from a tuned circuit inductance (TR 2 -A) and a controlled switch (T 1 ), a pulse generator (OS) which drives the switch (T 1 ), an electrical valve (D 1 ) which is connected in parallel with the switch (T 1 ), a tuned circuit capacitance (C 2 ) which is likewise connected in parallel with the switch (T 1 ), a means (TR 2 -B, a″, b″) for coupling a lamp (La 1 ) to at least one electrode which is impeded dielectrically, and, optionally, a buffer and feedback capacitor (C 1 ) which is connected in parallel with the series circuit formed by the tuned circuit inductance (TR 2 -A) and the switch (T 1 ). The means for coupling a lamp comprises in particular two connections (a″, b″) and the secondary winding (TR 2 -B) of an autotransformer (TR 2 ), which is connected between a first pole of the switch (T 1 ) and the corresponding connection (a″), the primary winding (TR 2 -A) of the autotransformer (TR 2 ) acting as the tuned circuit inductance. The second connection (b′) is connected to the second pole of the switch (T 1 ). In operation, the switch (T 1 ) opens and closes alternately in time with the drive signal of the pulse generator (OS), as a result of which a sequence of voltage pulses, which are separated by pauses, is produced at the electrodes, which are impeded dielectrically, of a lamp (La 1 ) which is connected to the connections (a″, b″).

Description:
TECHNICAL FIELD 
     The invention relates to an electrical circuit arrangement for producing pulsed-voltage sequences for the operation of discharge lamps. The invention further relates to the method in accordance with which the circuit arrangement produces the pulsed-voltage sequences. 
     To be more precise, the circuit arrangement according to the invention is used to operate discharge lamps or radiators in which at least the electrodes of one polarity are impeded dielectrically, by means of unipolar or at least substantially unipolar voltage pulses, such as those described in WO 94/23442, for example. This method of operation uses a sequence, which is in principle unlimited, of voltage pulses which are separated from one another by pauses. The critical factors for the efficiency of the wanted radiation production are, essentially, the pulse shape as well as the time durations of the pulse and pause times. Typical duty ratios are in the range between about 1:5 to 1:10. The peak value of the high-voltage pulses depends on the design of the respective lamp, for example the number of electrodes, the flashover distance and the nature and thickness of the dielectric, and is typically between 1 kV and 5 kV. The pulse repetition frequency is also dependent on the geometry of the lamp and is in the range from about 25 kHz to about 80 kHz. Conventional methods of operation for such lamps in contrast use sinusoidal AC voltages. 
     In contrast to conventional discharges, as are normally used for discharge lamps, discharges which are impeded dielectrically have a dielectric which is arranged between the interior of the discharge space and the electrode or electrodes of one polarity (impeded dielectrically on one side) or else all the electrodes, that is to say the electrodes of both polarities (impeded dielectrically on both sides). Such electrodes are also called electrodes which are impeded dielectrically. The charge carrier transportation from an electrode which is impeded dielectrically to the ionized gas in the discharge path thus takes place by means of a displacement current rather than by means of a conduction current. This results in a capacitive component in the electrical equivalent circuit for such a discharge. In consequence, the circuit arrangement has to be suitable for injecting the energy capacitively into the lamp. 
     PRIOR ART 
     DE 195 48 003 A1 (Huber et al., U.S. Pat No. 5,581,394) discloses an electrical circuit arrangement for producing pulsed-voltage sequences, in particular for the operation of discharges which are impeded dielectrically. This circuit arrangement has a charge circuit which is fed from an input voltage and has a charge capacitor, a discharge and pulse circuit having a fast controllable switch which is connected to a pulsed drive circuit, and a pulse transformer with a load connected to it, as well as a feedback circuit with a feedback electrical valve and a buffer capacitor which is connected in parallel with the input of the charge circuit. During the phases when the switch is switched on, the electrical energy stored in the charge capacitor is always transmitted to the load via the pulse transformer. The oscillating energy returning from the load and the pulse transformer passes through the feedback circuit, is fed into the feedback point, and is absorbed by the buffer capacitor. Thus, during the reverse oscillation phases, the potential of the secondary winding is clamped to the potential of the input voltage. In addition, the energy fed back is in this way also used for the charging phase of the charge capacitor. The disadvantages of this solution are the high pulsed load on the pulse transformer and on the switch, the relatively poor efficiency, as well as the not inconsiderable component complexity. In addition, the specific design of the pulse transformer has a critical effect on the operation of the circuit. Furthermore, the optimum design of the pulse transformer can be determined only by experiment. 
     DESCRIPTION OF THE INVENTION 
     The object of the present invention is to provide a circuit arrangement with whose aid largely unipolar pulsed-voltage sequences can be produced, with low circuit losses. In addition, it is intended to be possible to produce pulsed-voltage sequences with pulse shapes that are as smooth as possible on loads which act in a predominantly capacitive manner. A further aspect of the invention is to provide a relatively simple circuit with as few components as possible. 
     A further object of the invention is to provide a method of producing the above mentioned pulsed-voltage sequences. 
     The basic idea of the invention is explained in the following text with reference to a simplified block diagram in FIG.  1 . Fed from an energy supply source  1 , an inductive energy reservoir  3  is first of all cyclically charged up during the switched-on phase of a controllable switch  2 . After the charging-up phase, that is to say as soon as the switch  2  switches off, the magnetic energy stored in the inductive energy reservoir  3  is transmitted to a capacitive energy reservoir  4 . In consequence, a first voltage half-cycle of a roughly sinusoidal oscillation is produced on the inductive energy reservoir  3 , while a similar voltage half-cycle, but in antiphase, is produced on the capacitive energy reservoir  4 . This first voltage half-cycle is used as a voltage pulse for the lamp  5 —which is coupled either to the inductive energy reservoir  3  or to the capacitive energy reservoir  4 . After this, the energy is fed back from the capacitive energy reservoir  4 , via the inductive energy reservoir  3 , into the energy supply source  1 , which advantageously contains an additional feedback reservoir (not illustrated). In this case, the voltage on the capacitive energy reservoir  4  is clamped to the voltage which is dropped across the open electrical valve  6 . In consequence, during this process, the voltage on the inductive energy reservoir  3  is equal to the supply voltage. This process is repeated cyclically after a time which can be predetermined. The timing is controlled via a signal transmitter  7  which is connected to the controllable switch  2 . 
     In this way, a sequence of essentially half-sinusoidal voltage pulses in the same phase is produced at the lamp electrodes, the individual voltage pulses being separated from one another by pauses, that is to say times during which the voltage at the electrodes is largely constant and is considerably less than the peak value of the voltage pulses, preferably being close to zero. 
     This idea of the invention is in essence achieved by the series circuit formed by a controllable switch and an inductance which is used, inter alia, as an inductive energy reservoir and is also referred to in the following text, for short, as a tuned circuit inductance, the switch having connected in parallel with it the electrical valve and a capacitance which is used as the capacitive energy reservoir—also referred to as a tuned circuit capacitance in the following text, for short. 
     The width of the voltage pulses, inter alia, can be influenced by the specific values of the tuned circuit inductance and the tuned circuit capacitance. Typical values for the operation of radiation sources of the type mentioned in the introduction are in the range between about 500 μH and 10 μH for the tuned circuit inductance, and about 100 pF and 1 μF for the tuned circuit capacitance. 
     A capacitor may be used, for example, as the tuned circuit capacitance, or alternatively the actual intrinsic capacitance of a discharge arrangement which is provided with electrodes which are impeded dielectrically. If the switch is provided by a controllable semiconductor switch, for example by a bipolar transistor, IGBT (Insulated Gate Bipolar Transistor) or MOSFET (Metal Oxide Semiconductor Field Effect Transistor), the depletion layer capacitance of the semiconductor switch can also be used as the tuned circuit capacitance, since the tuned circuit capacitance—as will be shown later—is significant to the operation of the circuit arrangement only during the phase when the switch is switched off. In fact, an additional capacitor offers the advantage of in this way being able to influence the width of the voltage pulse. The value of an additional capacitor is thus chosen depending on the desired pulse width. A capacitor connected in parallel with the input terminals of the arrangement may be used as a feedback reservoir. The feedback reservoir may also be a component of an energy supply which can be fed back. In the latter case, it is possible to dispense with a specific feedback reservoir in the input of the circuit arrangement. 
     In the simplest case, the discharge arrangement or lamp is coupled directly to the capacitor or to the controllable semiconductor switch. To do this, lamp supply leads are connected to the connections of the capacitor or of the semiconductor switch. This simple solution is particularly suitable for lamps having a relatively low maximum pulsed voltage (pulsed voltages of less than about 1500 V), since, in this case, the maximum voltage across the semiconductor switch when it is switched off limits the maximum pulsed voltage that can be produced. 
     In a preferred variant for lamps having a higher maximum pulsed voltage, the secondary winding of an autotransformer is connected in one of the lamp supply leads. The tuned circuit inductance, which is used as an inductive energy reservoir, is in this case provided by the primary winding of this autotransformer. 
     Finally, in a further variant, the lamp is coupled via a high-voltage transformer. The primary winding of the high-voltage transformer acts as the inductive energy reservoir. The lamp supply leads are in this case connected to the connections of the secondary winding. Higher maximum pulsed voltages can likewise be produced with this variant. In fact, this solution is more complex, and therefore also more expensive than the abovementioned solution. The disadvantages in comparison with the autotransformer variant are also the higher losses and the less favourable turns ratio. In order, for example, to triple the voltage on the lamp supply leads in comparison with the voltage on the primary winding, a transformation ratio of three is likewise required. In contrast, a transformation ratio of two is sufficient for the autotransformer variant, owing to the electrical circuitry of the primary and secondary windings, and the fact that they are wound in the same sense. 
     The circuit arrangement according to the invention is also particularly suitable for operation at low voltages, for example for battery operation when used in a motor vehicle or the like. For operation at mains voltage, the circuit arrangement has a voltage-matching converter connected upstream of it, by means of which it is also possible, at the same time, to ensure that the current drawn from the mains is sinusoidal. 
     In addition, protection is claimed for a radiation system which comprises the abovementioned novel pulsed-voltage source and a discharge lamp or a discharge radiator of the type mentioned initially. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following text with reference to a plurality of exemplary embodiments. In the figures: 
     FIG. 1 shows a block diagram to illustrate the principle of the invention, 
     FIG. 2 shows a first cost-effective exemplary embodiment for relatively low pulsed voltages, 
     FIG. 3 shows a further exemplary embodiment having a high-voltage transformer for higher pulsed voltages, 
     FIG. 4 shows yet another exemplary embodiment having an autotransformer for higher pulsed voltages, 
     FIG. 5 shows measured value curves relating to the time response of the circuit from FIG. 4, 
     FIG. 6 shows an exemplary embodiment using a MOSFET as the switch. 
    
    
     FIG. 2 shows a simplified illustration of a preferred circuit arrangement for lamps with a relatively low maximum required pulsed voltage. Since, in this circuit arrangement, the lamp is connected in parallel with the semiconductor switch, the maximum pulsed voltage which can be produced is, namely, limited by the maximum voltage across the semiconductor switch when it is switched off. 
     The circuit arrangement comprises a buffer capacitor C 1  which is supplied by a DC voltage +U o , for example the output voltage of a rectifier circuit or of a battery, and which is in addition also used as a feedback reservoir, a series circuit formed from the tuned circuit inductance L 1  and a bipolar transistor T 1  with a freewheeling diode D 1 , which series circuit is connected to the negative lead of the buffer capacitor C 1 , as well as a tuned circuit capacitor C 2  which is connected in parallel with the transistor T 1 . A lamp La 1  with electrodes which are impeded dielectrically is connected in parallel with the tuned circuit capacitor C 2 , by means of the connections a,b. 
     The transistor T 1  is driven by an asymmetric square-wave oscillator OS, for example a pulse generator IC which is known per se. As long as the transistor T 1  is switched on, a linearly rising current flows through it and through the tuned circuit inductance L 1 . At the end of the switched-on time t 1 , the current reaches the peak value I s . At this time, the magnetic energy stored in the tuned circuit inductance is: 
     
       
           W   m =0.5· L   1   ·I   s   2   (1) 
       
     
     At the time t=t 1 , the transistor T 1  is switched off, and a free sinusoidal oscillation now takes place whose period is 
     
       
           T   s =2 ·π·{square root over (L 1   ·C   2 +L ·)}   (2) 
       
     
     In the process, the magnetic energy W m  charges the tuned circuit capacitor C 2  to a voltage U C2 , which results from the energy W m  in accordance with: 
     
       
           W   m =0.5· L   1   ·I   s   2 =0.5· C   2   ·U   c2   2 ,  (3) 
       
     
     that is to say                U   c2     =           2   ·     W   m         C   2         .             (   4   )                                
     In this assessment for the series tuned circuit C 1 , L 1 , C 2 , the capacitance of the buffer capacitor C 1 , typically a few μF, has become negligible in comparison with the capacitance of the tuned circuit capacitor C 2 , typically several 100 pF. 
     The positive half-cycle of the sinusoidal oscillation at the tuned circuit capacitor C 2  and having the amplitude U C2  is in parallel with the transistor T 1  and reverse-biases the back-to-back connected diode D 1 . The negative half-cycle is clamped by the diode D 1 , and the tuned circuit inductance L 1  feeds energy back into the feedback capacitor C 1 . In this way, a roughly half-sinusoidal pulsed voltage U i  is produced, with an amplitude which is much higher than the supply voltage U o . In this case, the width of the voltage pulse is 
     
       
           t   i   =π·{square root over (L+L  1   ·C +L  2 .)}   (5) 
       
     
     FIG. 3 shows a variant of the circuit from FIG. 2, which is also suitable for lamps with higher maximum pulsed voltages. Equivalent components are given the same reference symbols. In this case, the tuned circuit inductance L 1  from FIG. 2 is replaced by a transformer TR 1 . The lamp La 1  is connected by means of the connections a′, b′ to the secondary winding TR 1 -B of the transformer TR 1 . In consequence, it is possible to operate even lamps whose maximum pulsed voltage is considerably above the maximum voltage across the semiconductor switch T 1  when it is switched off. 
     As in the case of the circuit in FIG. 2, the transistor T 1  is driven by an asymmetric square-wave oscillator OS. When the transistor T 1  is switched on, a linearly rising current flows through the primary winding TR 1 -A of the transformer TR 1  with the inductance L P , and through the transistor T 1 . At the end of the switched-on time t, the current reaches the peak value I s . At this time, the energy stored in the primary inductance L P  is 
     
       
           W   m =0.5 ·L   P   ·I   s   2   (6) 
       
     
     At the time t=t 1 , the transistor T 1  is switched off, and this is followed by a free sinusoidal oscillation with the period 
     
       
           T   s =2 ·π·{square root over (L P   ·C   2 +L .)}   (7) 
       
     
     At the same time, the magnetic energy W m  charges the tuned circuit capacitor C 2  to a voltage U C2  which corresponds to the energy W m : 
     
       
           W   m =0.5· L   P   ·I   s   2 =0.5 ·C   2   ·U   C2   2 ,  (8) 
       
     
     that is to say                U   c2     =           2   ·     W   m         C   2         .             (   9   )                                
     The voltage on the primary winding TR 1 -A of the transformer TR 1  is thus 
     
       
           U   LP   =U   C2   −U   o .  (10) 
       
     
     This voltage U LP  is transformed in accordance with the transformation ratio          u   ¨     =       w   s       w   p                              
     of the transformer TR 1  onto the secondary winding TR 1 -B and, in consequence, is applied to the lamp La 1  connected there. In equation (12), w S  is the number of turns on the secondary winding and w P  is the number of turns on the primary winding. The voltage U LS  on the secondary winding TR 1 -B of the transformer TR 1  is thus 
     
       
           U   LS   =U   LP ·ü.  (12) 
       
     
     The positive half-cycle of the sinusoidal oscillation on the tuned circuit capacitor C 2  having the amplitude U C2  is in parallel with the transistor T 1 , and thus switches off the back-to-back connected diode D 1 . The negative half-cycle is, in contrast, clamped by the diode D 1 , while energy is fed back into the feedback capacitor C 1  via the primary inductance L P  of the transformer TR 1 . 
     During this feedback process, the voltage on the primary inductance L P  is 
     
       
           U   LP   =U   o ,  (13) 
       
     
     which is likewise transformed in accordance with the transformation ratio ü of the transformer TR 1  onto the secondary winding TR 1 -B. In consequence, during the pulse pauses, that is to say during the times between the pulses, there is an offset voltage on the lamp La 1 . In order to prevent any adverse effect on lamp operation, the circuit is designed such that the offset voltage is very much less than the pulsed voltage. 
     FIG. 4 schematically illustrates a preferred variant of the circuit from FIG. 3, which is likewise suitable for lamps with higher maximum pulsed voltages. 
     In this variant, the transformer TR 1  is replaced by an autotransformer TR 2  whose primary winding TR 2 -A is used as the inductive energy reservoir, and whose secondary winding TR 2 -B is connected between the tuned circuit capacitor C 2  and the corresponding connection a″ of the lamp La 1 . In consequence, the lamp La 1  is connected by means of the connections a″,b″ in parallel with the series circuit formed by the secondary winding TR 2 -B and the tuned circuit capacitor C 2 . One advantage of this solution that results from this over the solution in FIG. 3 is the more favourable turns ratio. For example, in order to triple the voltage from the lamp supply leads in comparison with the voltage on the primary winding TR 2 -A, a transformation ratio of just two is sufficient for the autotransformer variant TR 2 , owing to the electrical circuitry of the primary TR 2 -A and secondary TR 2 -B windings, and the fact that they are wound in the same sense. In contrast, a transformation ratio of three is required for this purpose in the solution from FIG.  3 . Apart from this, the other function of the variant in FIG. 4 corresponds to that function which has already been described in the explanation relating to FIG.  3 . Further advantages over the solution from FIG. 3 are a lower parasitic inductance and lower losses, as well as a lower winding capacitance. Owing to the lower winding capacitance, voltage pulses with steeper pulse flanks are possible, which is advantageous for efficient operation of radiators with a discharge which is operated in a pulsed manner and is impeded dielectrically. 
     FIG. 5 shows measurement curves for the drive signal for the transistor T 1  (CH 1 ) of the current through the primary winding TR 2 -A (CH 2 ) and the voltage on the lamp La 1  (CH 3 ). The time t (one unit corresponds to 2 μs) is plotted on the x-axis, and the respective signal strength is plotted, in arbitrary units, on the y-axis. As can be seen from FIG. 5, the pause time between two voltage pulses can be influenced by the length of the period T of the control signal. It is likewise possible to see from FIG. 5 the requirement that, on the one hand, the duration of the switched-off time t 2  of the transistor T 1  must be longer than the duration t i  of the voltage pulse since, otherwise, the falling flank of the voltage pulse is cut off. On the other hand, the switched-off time t 2  must have ended before the zero crossing of the current through the tuned circuit inductance L 1  since, otherwise, interference oscillations will normally occur. The pause duration a between the individual voltage pulses can be influenced by the duration of the switched-on time t 1  of the transistor T 1 . The parameters switched-on time t 1  and switched-off time t 2  can also be used for dimming a connected radiation source. 
     FIG. 6 illustrates schematically a variant of the autotransformer circuit from FIG.  4 . The tuned circuit capacitor is in this case formed by the intrinsic capacitance of the lamp La 1  (not illustrated) or by the lamp capacitance transformed by the autotransformer and, in addition, by the depletion layer capacitance (not illustrated) of the MOSFET T 2 . A supplementary tuned circuit capacitance in the form of a discrete capacitor, for example as in FIG. 4, is dispensed with here. In addition, a discrete freewheeling diode is dispensed with since its task is carried out by the diode inherent in the MOSFET T 2  (not illustrated). The rest of the circuit and the principle of the method of operation correspond to those in FIG.  4 . The lamp La 1  is in consequence connected by means of the connections a″, b″ in parallel with the series circuit formed by the secondary winding TR 2 -B and the MOSFET T 2 . This circuit variant thus manages with an extremely small number of components.