Abstract:
A method for use in a lamp ballast includes obtaining a measurement signal representative of a voltage at an output of a half-bridge circuit. The half-bridge circuit includes first and second semiconductor switching elements, a resonant circuit connected to the half-bridge circuit, and a snubber capacitance connected in parallel with one of the semiconductor switching elements. The method also includes providing a comparison sinal by comparing heeasurem ment signal with a reference value. The method further includes detecting one of a first type of non-zero-voltage switching operation and a second type of non-zero-voltage switching operation based on evaluations of the comparison signal, wherein the evaluations of the comparison signal occurs in each case before the first semiconductor element is switched on and in each case before the second semiconductor element is switched on.

Description:
[0001]    This application is a division of co-pending U.S. Ser. No. 12/381,648, filed Mar. 13, 2009, which in turn, is a division of co-pending U.S. Pat. No. 7,560,873, filed Aug. 2, 2005, which claims the benefit of German Application No. DE 102004037388.4-54, filed Aug. 2, 2004. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for detection of the operating state, in particular of non-zero-voltage switching operation, of a ballast for florescent lamps, and to a ballast. 
       BACKGROUND 
       [0003]    In order to assist understanding of the invention as explained in the following text, the fundamental design of an electronic ballast, which is used to drive a florescent lamp, and its method of operation will first of all be explained with reference to  FIGS. 1 and 2 . A ballast such as this is described, by way of example, in EP 1 066 739 B1, U.S. Pat. No. 6,617,805 B2 or in the Data Sheet No. PD 60182-I for the IR2156(S) integrated module produced by International Rectifier, Calif., USA. 
         [0004]    An electronic ballast has a half-bridge with two semiconductor switching elements Q 1 , Q 2 , whose load paths are connected in series between supply terminals K 1 , K 2 , between which a DC voltage Vb is applied. These two semiconductor switching elements S 1 , S 2  are driven via a drive circuit  20  which drives each of the two semiconductor switching elements S 1 , S 2  in a clocked form. The two semiconductor switches Q 1 , Q 2  are in this case driven alternately in order to ensure that the two semiconductor switches are never switched on at the same time. A voltage V 2 , which has an essential square waveform, is produced at an output K 3  of the half-bridge, which is formed by a node that is common to the load paths of the semiconductor switching elements. 
         [0005]    This voltage V 2  feeds a resonant tuned circuit with a resonant inductance L 1  and a resonant capacitor C 1 , with a florescent lamp being connected in parallel with the resonant capacitor C 1  in the example. A further capacitor C 2 , which is connected in series with the resonant inductance L 1  and upstream of the parallel circuit formed by the florescent lamp  10  and the resonant capacitor C 1 , is used as a blocking capacitor, and blocks direct-current components. 
         [0006]    A snubber capacitor C 3  is connected in parallel with the load path of the second semiconductor switching element Q 2 , with the object of reducing the switching losses during zero-voltage switching operation (ZVS) of the two semiconductor switching elements Q 1 , Q 2 . 
         [0007]    The illustration does not show the normally provided measurement connections of the drive circuit  20 , via which by way of example a voltage across the florescent lamp  10  or a current through the half-bridge Q 1 , Q 2  is determined, and supply connections via which a voltage supply is provided for the drive circuit  20 . The DC voltage Vb for the ballast is provided, for example, by a switched-mode converter with a power factor correction function (power factor controller, PFC). In this context, reference is made, for example, to EP 1 066 739 B1, U.S. Pat. No. 6,617,805 B2 or U.S. Pat. No. 6,400,095 B1, as cited above. 
         [0008]      FIG. 2  shows the waveform of the output voltage V 2 , which is produced between the output terminal K 3  of the half-bridge Q 1 , Q 2  and the reference ground potential GND, of the half-bridge circuit Q 1 , Q 2 , of the current Iq 2  through the second semiconductor switching element Q 2 , the current I 1  into the load that is connected to the half-bridge circuit Q 1 , Q 2 , and the drive signals S 1 , S 2  for the semiconductor switching elements S 1 , S 2  for a disturbance-free operating state after starting of the florescent lamp. 
         [0009]    The semiconductor switching elements Q 1 , Q 2  are switched on by the drive circuit  20  via the drive signals S 1 , S 2 , with a respective phase shift, for switched-on durations Ton 1 , Ton 2 , with the drive periods Tp for the two semiconductor switches S 1 , S 2  each being the same. The drive is provided, for example, in such a way that there is a minimum switched-off time toff between one of the two semiconductor switching elements being switched off and the other being switched on. The switched-on durations Ton 1 , Ton 2  are normally each of equal length, the duty cycle, that is to say the ratio of the switched-on duration to the period duration is, for example, about 45%. 
         [0010]    When the first semiconductor switch S 1  is switched on and the second semiconductor switch S 2  is switched off, the output voltage V 2  from the half-bridge circuit Q 1 , Q 2  corresponds approximately to the DC voltage Vb between the terminals K 1 , K 2 , ignoring the switched-on resistance of the first semiconductor switching element Q 1 . This voltage results in a lamp current I 1 , which flows in the opposite direction to that shown in  FIG. 1  and whose magnitude increases as the time for which the first semiconductor switching element S 1  is switched on increases. Once the first semiconductor switching element S 1  has been switched off, this current is first of all still maintained by virtue of the inductance L 1  of the series tuned circuit L 1 , C 1  and thus discharges the snubber capacitor C 3 , which is connected in parallel with the second semiconductor switching element Q 2 , as a result of which the voltage across the load path of this second semiconductor switching element Q 2  tends to zero. Once this capacitor C 3  has been discharged, the body diode of the second semiconductor switching element Q 2 , which is in the form of an n-channel MOSFET, carries the lamp current I 1 , in this case acting as a freewheeling diode. This lamp current I 1  changes its polarity in the time period after the second semiconductor switching element S 2  has been switched on, and flows in the direction shown in  FIG. 1  before the second semiconductor switching element S 2  is switched off. Once the second semiconductor switching element Q 2  has been switched off, the snubber capacitor C 3  is charged via the current flowing through the inductance L 1  to the value of the DC voltage Vb, with any further voltage rise being limited by an integrated body diode in the first semiconductor switching element, which is formed by an n-channel MOSFET. In this case, the first semiconductor switching element Q 1  is not switched on until the voltage at the output K 3  has risen to the value of the DC voltage Vb, and the voltage across the load path of the first semiconductor switching element, Q 1  is thus zero. 
         [0011]    The snubber capacitor C 3  assists zero-voltage switching of the first and second semiconductor switching elements Q 1 , Q 2 , that is to say switching of these semiconductor switching elements Q 1 , Q 2  when the voltage across their load path is equal to zero. The switches Q 1 , Q 2  can admittedly also be switched on at zero voltage without the snubber capacitor C 3 . The only precondition for this is that the current through the load path continues to flow with the same polarity until the corresponding switch Q 1 , Q 2  is switched on. Without any snubber capacitor C 3 , the voltage would, however, rise very quickly after switching off a switch Q 1 , Q 2 , leading to corresponding switching-off losses. The snubber capacitor C 3  limits this rate of voltage rise, and thus reduces the switching losses. 
         [0012]    However, situations in which such zero-voltage switching operation cannot be achieved may occur during operation of a florescent lamp. In this case, the snubber capacitor C 3  charge is not changed by means of the current that is induced in the resonant inductance L 1  but by means of the currents flowing through the semiconductor switching elements on switching on, and this is associated with considerable losses. Operating states such as these may occur, for example, when the lamp has been removed from the socket or is damaged, or when the DC voltage Vb falls for a lengthy time period during normal operation. 
         [0013]    In order to avoid overloading of the semiconductor switching elements which are designed to be continuously loaded only for zero-voltage switching operation during non-zero-voltage switching operation, it is necessary to identify an operating state such as this and, if necessary, to switch off the florescent lamp by interrupting the drive to the half-bridge if this operating state lasts for longer than a predetermined time period. 
         [0014]    In order to detect such non-zero-voltage switching operation, it is known from U.S. Pat. Nos. 6,331,755 B1 and 5,973,943 for a current to be detected by the low-side switch in the half-bridge and to be assessed against a reference value at the time at which the switch is switched on and off. U.S. Pat. No. 6,400,095 B1 and EP 1 066 739 B1 propose that the current through the lamp be detected by means of a shunt resistance, and be assessed against a reference value. 
       SUMMARY 
       [0015]    One aim of the present invention is to provide a method for detection of non-zero-voltage switching operation of a lamp ballast, and to provide a ballast having a detector circuit for detection of non-zero-voltage switching operation. 
         [0016]    This aim is achieved by methods and apparatus disclosed herein. 
         [0017]    In the method according to a first embodiment of the invention for detection of non-zero-voltage switching operation of a lamp ballast, which has a half-bridge circuit with a first and a second semiconductor switching element, a resonant tuned circuit connected to one output of the half-bridge circuit, and a snubber capacitance connected in parallel with one of the semiconductor switching elements, provision is made for a voltage measurement signal which is dependent on a voltage at the output of the half-bridge to be produced, and for the voltage measurement signal to be evaluated by comparison of the voltage measurement signal with a reference value, in each case before the switching-on times of at least one of the first and second semiconductor switching elements. 
         [0018]    In the case of this method, non-zero-voltage switching operation is detected when the voltage measurement signal falls below the level of the reference signal before the switching-on time of the first semiconductor switching element, and/or when the voltage measurement signal exceeds the reference value before the switching-on time of the second semiconductor switching element. 
         [0019]    The voltage measurement signal is preferably compared with the reference value in each case before the switching-on times of the first semiconductor switching element and before the switching-on times of the second semiconductor switching element, thus making it possible to distinguish between individual different non-zero-voltage switching operating modes. 
         [0020]    The voltage measurement signal in one embodiment of the method is produced by means of a resistive voltage divider from the voltage at the output of the half-bridge, and in another embodiment is produced by means of a capacitive voltage divider from the voltage at the output of the half-bridge. 
         [0021]    Another embodiment of the invention is a lamp ballast having a half-bridge circuit with a first and a second semiconductor switching element, which are driven on the basis of first and second drive signals, and having an output at which a half-bridge voltage is produced, and has a resonant tuned circuit which is connected to the output of the half-bridge circuit. The lamp ballast also has a detector circuit for detection of non-zero-voltage switching operation, having the following features: 
         [0022]    a voltage measurement arrangement which is connected to the output of the half-bridge circuit and provides a voltage measurement signal based on the half-bridge circuit, 
         [0023]    an evaluation circuit to which the voltage measurement signal is supplied and which is designed to evaluate the voltage measurement signal by comparison of the voltage measurement signal with a reference value, in each case before the switching-on times of at least one of the first and second semiconductor elements, and to produce at least one evaluation signal on the basis of this comparison. 
         [0024]    In order to preset the evaluation times, the lamp ballast is supplied, for example, with at least one of the first and second drive signals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The present invention will be explained in more detail in the following text with reference to figures, in which: 
           [0026]      FIG. 1  shows the fundamental design of a lamp ballast with a florescent lamp inserted (prior art). 
           [0027]      FIG. 2  shows the waveform of selected signals in the lamp ballast as shown in  FIG. 1 , during zero-voltage switching operation (prior art). 
           [0028]      FIG. 3  shows, by way of example, waveforms of the output voltage of a half-bridge in a lamp ballast ( FIG. 3   a ), the waveform which results from this of a voltage measurement signal that is derived from this voltage ( FIG. 3   b ), as well as waveforms of drive signals for the half-bridge circuit ( FIGS. 3   c  and  3   d ) for non-zero-voltage switching operation of a first type, in order to explain the detection method according to the invention. 
           [0029]      FIG. 4  shows, by way of example, waveforms of the output voltage of a half-bridge in a lamp ballast ( FIG. 4   a ), the waveform which results from this of a voltage measurement signal that is produced on the basis of this voltage ( FIG. 4   b ), as well as waveforms of drive signals for the half-bridge circuit ( FIGS. 4   c  and  4   d ) for non-zero-voltage switching operation of a second type, in order to explain the detection method according to the invention. 
           [0030]      FIG. 5  shows a first exemplary embodiment of a detector circuit for detection of non-zero-voltage switching operation, which has a resistive voltage divider for determination of a voltage measurement signal. 
           [0031]      FIG. 6  shows, by way of example, waveforms of the signals which occur in the detector circuit shown in  FIG. 5 . 
           [0032]      FIG. 7  shows a further exemplary embodiment of a detector circuit with a resistive voltage divider. 
           [0033]      FIG. 8  shows an exemplary embodiment of a detector circuit according to the invention with a capacitive voltage divider. 
           [0034]      FIG. 9  shows a further exemplary embodiment of a detector circuit according to the invention with a capacitive voltage divider for determination of the voltage measurement signal. 
           [0035]      FIG. 10  shows waveforms of selected signals which occur in the detector circuit as shown in  FIG. 9 , for zero-voltage switching operation of the lamp ballast. 
           [0036]      FIG. 11  shows waveforms of selected signals which occur in the detector circuit as shown in  FIG. 9 , for non-zero-voltage switching operation of a first type. 
           [0037]      FIG. 12  shows examples of waveforms of selected signals in the detector circuit as shown in  FIG. 9  for non-zero-voltage switching operation of a second type. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Unless stated to the contrary, identical reference symbols denote identical circuit components and signals with the same meaning in the figures. 
         [0039]    By way of example,  FIG. 3  shows waveforms of drive signals S 1 , S 2  for switching elements Q 1 , Q 2  in a half-bridge in a lamp ballast, for example a lamp ballast as shown in  FIG. 1 , which is designed for zero-voltage switching operation and has a snubber capacitor C 3  connected to one output terminal K 3  of the half-bridge Q 1 , Q 2 . A time period between times t 1  and t 4  will be considered in more detail in the following text. At a first time t 1 , the first switch Q 1  in the half-bridge is switched off, driven by the first drive signal S 1 , at a second time t 2 , the second switch Q 2  in the half-bridge is switched on, driven by the second drive signal S 2 , at a third time t 3 , the second switch Q 2  is switched off, and at a fourth time t 4  the first switch Q 1  is switched on. In order to prevent the two switches Q 1 , Q 2  being driven such that they are switched on at the same time, a dead time Toff is provided between the first and the second times, t 1 , t 2  and between the third and fourth times t 3 , t 4 , during which neither of the two switches Q 1 , Q 2  is intended to be switched on. With reference to the statements relating to  FIG. 2 , this dead time between the first switch Q 1  being switched off and the second switch Q 2  being switched on is used in order to draw the potential at the output terminal K 3  to zero or to the reference ground potential GND, and the dead time between the second switch Q 2  being switched off and the first switch Q 1  being switched on is used to draw the output K 3  to the supply potential Vb. The voltage across the switching elements Q 1 , Q 2  when they are switched on is then zero. 
         [0040]    It should be noted that unavoidable delay times between the flanks of the drive signals S 1 , S 2  and the switching-on times of the switches S 1 , S 2  are ignored in  FIGS. 3 and 4 , for clarity reasons. 
         [0041]      FIG. 3   a  shows the waveform of the output voltage V 3  from the half-bridge in a lamp ballast for non-zero-voltage switching operation of a first type. In this case, although the output voltage V 2  falls from the first time t 1  when the first switch is switched off, the dead time Toff, however, is not sufficient in order to draw the output voltage V 2  to zero or to the reference ground potential GND before the second switch T 2  is switched on, so that a voltage which is not equal to zero is present across the second switch Q 2  when it is switched on, and this leads to increased switching losses. In a corresponding manner, during this operating state, the dead time between the second switch Q 2  being switched off and the first switch Q 1  being switched on is not sufficient in order to draw the output voltage V 2  to the value of the operating voltage Vb, so that a voltage which is not equal to zero is present across this first switch S 1  at its switching-on time t 4 , and this leads to increased switching losses. 
         [0042]    In order to detect this non-zero-voltage switching operation, the method according to the invention provides for a voltage measurement signal Vs to be produced, which is dependent on the output voltage V 2  from the half-bridge. The waveform of the signal Vs such as this, which is dependent on the waveform of the output voltage in the example, is illustrated in  FIG. 3   b . Provision is also made for a reference value Vref to be produced and for the voltage measurement signal Vs to be compared with the reference value Vref before the switching-on times of the first and/or second switch Q 1 , Q 2 , in order to detect non-zero-voltage switching operation. In  FIG. 3 , a first comparison time, which is located between the first and the second times t 1 , t 2 , is annotated tm 1 , and a second comparison time, which is located between the third and the fourth times t 3 , t 4 , is annotated tm 2 . 
         [0043]    Non-zero-voltage switching operation is detected using the method according to the invention when the voltage measurement signal Vs has not yet fallen below the reference value Vref at the first comparison time tm 1  before the second switch Q 2  is switched on (at the time t 2 ), and/or when the voltage measurement signal Vs has not yet risen above the reference value Vref at the second comparison time tm 2  before the second switch Q 2  is switched on (at the time t 4 ). The time interval between the respective comparison times tm 1 , tm 2  and the switching-on times t 2 , t 4  as well as the threshold of the reference value Vref are chosen such that, during correct zero-voltage switching operation, the voltage measurement signal Vs has already fallen below the reference value Vref at the first comparison time tm 1 , and has already risen above the reference value Vref at the second comparison time tm 2 . 
         [0044]      FIG. 4  shows the waveform of the output voltage V 2  from the half-bridge Q 1 , Q 2 , as a function of the first and second drive signals S 1 , S 2  ( FIGS. 4   c  and  4   d ), for non-zero-voltage switching operation of a second type, which can occur, by way of example, in the event of the florescent lamp  10  being broken during operation, or in the event of the florescent lamp being removed. Once the first switch Q 1  has switched off, the output voltage from the half-bridge circuit in this operating state would rise owing to the current induced in the resonant inductance L 1 . The potential at the output K 3  is, however, kept approximately at the supply potential Vb until the second switch Q 2  is switched on at the time t 2 , by means of a free-wheeling diode which is integrated in the switch Q 1  (which, for example, is in the form of a n-channel MOSFET) or, possibly, by means of a freewheeling diode which is connected in parallel with the switch Q 1 . The charge which is stored in the freewheeling diode in the first switch Q 1  while in the conducting state must be dissipated when the second switch Q 2  is switched on, and this leads to considerable switching losses in the two switches Q 1 , Q 2 . Once the second switch Q 2  has been switched off, the potential at the output K 3  initially remains at the reference ground potential, until the first switch Q 1  is switched on at the time t 4 , as a result of the freewheeling diode that is integrated in the second switch Q 2  or, possibly, as a result of a freewheeling diode connected in parallel with this switch. The charge which is stored in the freewheeling diode in the second switch Q 2  must in this case first of all be dissipated when the first switch Q 1  is switched on, and this also leads to considerable switching losses in the two switches Q 1 , Q 2  during this switching process. This operating state must be detected on the basis of the increased switching losses, in order if required to switch off the lamp ballast and to protect it against damage. 
         [0045]    This non-zero-voltage switching operation of the second type can also be detected by means of the method according to the invention by comparing the voltage measurement signal Vs (which is derived from the output voltage V 2  and whose waveform is illustrated in  FIG. 4   b ) with the reference value Vref. Since, during the illustrated non-zero-voltage switching operation, the voltage measurement signal Vs does not start to fall until the second switch S 2  is switched on at the time t 2 , the voltage measurement signal Vs at the first comparison time tm 1  is undoubtedly above the reference value Vref, and, since the voltage measurement signal Vs does not start to rise until the fourth time t 4  when the first switch Q 1  is switched on, the voltage measurement signal Vs is undoubtedly below the reference value Vref at the second comparison time tm 2 . 
         [0046]    The reference value Vref is chosen in such a way that it is located between the maximum possible signal value and the minimum possible signal value of the voltage measurement signal Vs, with the reference value preferably being closer to the minimum value than to the maximum value. These values are, in particular, dependent on the manner in which the voltage measurement signal Vs is obtained from the output voltage V 2  from the half-bridge Q 1 , Q 2 . 
         [0047]      FIG. 5  shows a first exemplary embodiment of a detector circuit according to the invention for detection of non-zero-voltage switching operation. In order to assist understanding, 
         [0048]      FIG. 5  also shows further components of the lamp ballast, specifically the half-bridge Q 1 , Q 2 , the resonant tuned circuit L 1 , C 1  with the blocking capacitor C 2 , the snubber capacitor C 3 , and a florescent lamp  10  inserted into the ballast. 
         [0049]    The detector circuit in the example has a resistive voltage divider R 1 , R 2 , which is connected between output K 3  of the half-bridge Q 1 , Q 2  and the reference ground potential GND and at whose center tap the voltage measurement signal Vs is available, as the voltage measurement arrangement for provision of a voltage measurement signal Vs which is dependent on the output voltage V 2  from the half-bridge Q 1 , Q 2 . This voltage measurement signal Vs is supplied to an evaluation circuit  30  which produces a status signal S 30 , which assumes a first level during zero-voltage switching operation, and a second level during non-zero-voltage switching operation. 
         [0050]    The evaluation circuit  30  has a reference voltage source  35 , which provides the reference value Vref. The reference value Vref and the voltage measurement signal Vs are supplied to a comparator  31 , which produces a comparison signal S 31  that is dependent on the comparison of the voltage measurement signal Vs with the reference value Vref. This comparison signal S 31  is supplied to the data input D of a D-flipflop  32 , which carries out the function of a sampling and storage unit. The comparison signal S 31  is sampled on the basis of a clock signal S 33 , which is derived from the second drive signal S 2  by inversion by means of an inverter  33  and is supplied to a clock input CLK of the flipflop  32 . The flipflop  32  is level-controlled and in each case receives the instantaneous value of the comparison signal S 31  while the clock signal is at a high level, and retains the most recently stored value after a falling flank of the clock signal S 33 . The value which is stored in the flipflop  31  is available at its output. This output signal S 32  from the flipflop  32  is linked by means of an AND gate  34  to the second drive signal S 2 , in order to produce the status signal S 30 . 
         [0051]    The method of operation of the detector circuit illustrated in  FIG. 5  will become clear from the waveform of the signals in  FIG. 6 , as illustrated in the evaluation circuit shown in  FIG. 5 . By way of example,  FIG. 6  shows the waveforms of the first and second drive signals S 1 , S 2 , of the clock signal, S 33  which corresponds to the inverted second drive signal S 2 , of the comparison signal S 31 , and the waveforms which result therefrom of the flipflop output signal S 32  and of the status signal S 30 . 
         [0052]    The comparison signal S 31  is evaluated by the detector circuit at each of the switching-on switching times of the second switch Q 2 , in which case, with reference to the statements relating to  FIGS. 4 and 5 , non-zero-voltage switching operation is assumed when the voltage measurement signal Vs is greater than the reference value Vref at the evaluation time. The evaluation times in the case of the detector circuit shown in  FIG. 5  are in each case predetermined by falling flanks of the clock signal S 33 , that is to say rising flanks of the second drive signal S 2 . In this case, use is made of the fact that there is an unavoidable time delay between the rising flank of the second drive signal S 2  and the actual switching of the second switch Q 2 , in which case this delay time predetermines the time interval between the comparison time and the switching-on time of the second switch Q 2 . The delay time between the rising flank of the second drive signal S 2  and the second switch Q 2  being switched on is normally considerably greater than the processing times or gate response times in the evaluation circuit  30 . The delay time between the rising flank of the second drive signal S 2  and the second switch Q 2  being switched on is governed predominantly by driver circuits which are not described in any more detail and convert the logic drive signals S 1 , S 2  to levels which are suitable for driving the switches Q 1 , Q 2 . It is optionally possible to connect delay elements (not illustrated) upstream of the drive connections of the switches Q 1 , Q 2  in order in this way to achieve a further delay between the rising flank of the second drive signal S 2  and the second switch Q 2  being switched on, with this delay time also governing the time interval, as explained with reference to  FIGS. 4 and 5 , between the first comparison time tm 1  and the time t 2  at which the second switch Q 2  is switched on. 
         [0053]    With reference to the waveforms shown in  FIG. 6 , the lamp ballast is first of all operated with zero voltage switching, that is to say the voltage measurement signal Vs has already fallen below the reference value Vref at a time t 5  of a rising flank of the second drive signal S 2 , thus resulting in the comparison signal S 31  being at a low level. During zero-voltage switching operation, a low level is produced at the output of the flipflop  32  when the second switch S 2  is switched on, thus resulting in the status signal S 30  being at a low level. As progress is made through the timing diagram shown in  FIG. 6 , non-zero-voltage switching operation starts, as a result of which the comparison signal S 31  assumes a high level at a time t 6  of a rising flank of the second drive signal S 2 , and this is transferred to the flipflop  32 . This high level at the output of the flipflop leads, in conjunction with the high level of the second drive signal S 2 , to the status signal S 30  being at a high level, in order to indicate non-zero-voltage switching operation of the lamp ballast. 
         [0054]    Instead of the level-controlled flipflop  32 , a flank-controlled flipflop could also be used in the evaluation circuit  30 , which stores the value of the comparison signal S 31  on each positive flank of the second drive signal S 2  and thus on a falling flank of the clock signal S 33 , and makes this available as the output signal at its output. The output signal from this flip-flop could then be used directly as the status signal S 30 . In this case, there would be no need for the AND gate  34 . 
         [0055]    The detector circuit which has been explained with reference to  FIG. 5  may, of course, be integrated in a central drive circuit, corresponding to the drive circuit  20  in  FIG. 1 . The resistors R 1 , R 2  in the resistive voltage part may in this case be provided as external components to the drive circuit  20 , which is normally in the form of an integrated circuit. 
         [0056]      FIG. 7  shows an exemplary embodiment of the detector circuit which allows the resistance elements R 1 , R 2  of the voltage divider to be integrated in an integrated circuit. In this exemplary embodiment, the voltage measurement arrangement has a further resistor R 3  and a diode in addition to the resistance elements R 1 , R 2  of the voltage divider, with the further resistor R 1  being connected in series with the diode D 1  between a supply potential Vcc and the output K 3  of the half-bridge. The resistive voltage divider R 1 , R 2  is in this example located between the reference ground potential GND and a node which is common to the further resistor R 3  and the diode D 1 . 
         [0057]    The diode D 1  in this case prevents high voltage from reaching the resistors R 1 , R 2 , while the resistor R 3  ensures that a defined voltage value is applied to the anode of the diode D 1  when the diode is reverse-biased. When the second switch Q 2  is switched on, the voltage at the anode of the diode D 1  corresponds to the output voltage V 2  from the half-bridge Q 1 , Q 2  plus the voltage drop across the forward-biased diode. When the first switch Q 1  is switched on, the resistor R 3  and the resistors R 1  and R 2  form a voltage divider, which divides the voltage Vcc. This circuit arrangement is used to detect whether the output voltage V 2  is less than the supply voltage Vcc minus the voltage drop across the resistor R 3  and the threshold voltage of the diode D 1 . 
         [0058]    A capacitive voltage divider C 4 , C 5  can also be used, instead of a resistive voltage divider, to produce the voltage measurement signal Vs from the output voltage V 3  from the half-bridge.  FIG. 8  illustrates a lamp ballast with a capacitive voltage divider such as this. The capacitive voltage divider has two capacitors C 4 , C 5 , which are connected in series between the output K 3  of the half-bridge circuit and the reference ground potential GND and which have a center tap at which the voltage measurement signal Vs can be tapped off. This voltage measurement signal Vs is supplied to the evaluation circuit  30  which, for example, is designed in a corresponding manner to the evaluation circuit in  FIG. 5 . 
         [0059]    In comparison to resistive voltage dividers, a capacitive voltage divider has the advantage of having a shorter signal delay when high-speed switching processes take place, and of having a lower power consumption. Furthermore, the capacitors C 4 , C 5  which are required for the capacitive voltage divider may, for example, be thick-oxide capacitors with an oxide thickness of between 2 and 3 .mu.m, or may be in the form of gate-oxide capacitors with an oxide thickness in the order of magnitude between 20 nm and 50 nm, so that the capacitors C 4 , C 5  in the capacitive voltage divider can be produced together with the control circuit  20  (illustrated by dashed lines in  FIG. 8 ) and the evaluation circuit  30  in a common semiconductor chip, so that no additional external components are required for the voltage divider. 
         [0060]    One of the two capacitors in the voltage divider C 4 , C 5  may, in particular, be part of a circuit arrangement, the rest of which is not illustrated in any more detail but which detects the presence of a florescent lamp. In addition to the capacitor, in the example the capacitor C 5  which is connected to the reference ground potential GND, a lamp identification circuit such as this requires a resistor R 5 , which is connected between this capacitor C 5  and the connection which is common to the lamp electrode  12  and the resonant capacitor C 1 . The capacitor C 5  and the resistor R 5  form a low-pass filter, with a lamp identification circuit, which is not illustrated in any more detail but is connected to the node that is common to the capacitor C 5  and the resistor R 5 , being designed to apply a test current to the resistor R 5  and to the lamp filaments  12 , and to monitor the voltage drop across the resistor R 5  and the lamp filaments  12 . When no lamp is inserted or the filament is defective, the voltage across the capacitor C 5  rises as a result of the test current and the lack of any discharge path. During normal operation, the operating current of the lamp results in a high-amplitude AC voltage across the filament. The low-pass filter that is formed from the capacitor C 5  and the resistor R 5  is used to keep this AC voltage away from the other circuit parts, which are formed in an integrated circuit. 
         [0061]    When a lamp identification circuit such as this is present, only one additional capacitor C 4  is required to produce the capacitive half-bridge, and is connected between the capacitor C 5  in the lamp identification circuit and the output K 3  of the half-bridge. 
         [0062]      FIG. 9  shows a further exemplary embodiment of an evaluation circuit  40 , which is particularly suitable for evaluation of a voltage measurement signal Vs obtained by means of a capacitive voltage divider C 4 , C 5 . 
         [0063]    This evaluation circuit  40  has a comparator  41 , one of whose inputs is supplied with the voltage measurement signal Vs, and whose other input is supplied with a reference value Vref produced by a reference voltage source  45 . A comparison signal, S 41  is produced at the output of this comparator  41  and is supplied to a data input D of a first flipflop  42  and to the one inverting data input D of a second flipflop  43 . The two flipflops  42 ,  43  are flank-triggered flipflops which in each case receive and store the respective signal applied to the data input on a rising flank of a clock signal that is supplied to them. The second drive signal S 2  is supplied as a clock signal to the first flipflop  42 , and the first drive signal S 1  is supplied as a clock signal to the second flip flop  43 . A first status signal S 42  is produced at one output of the first flipflop  42 , with a second status signal S 43  being produced at one output of the second flipflop  43 , which are used to indicate non-zero-voltage switching operation. 
         [0064]    Once again, it is assumed that the flanks of the first and second drive signals S 1 , S 2  each occur before the actual switching times of the switches, owing to unavoidable switching delays in the switches Q 1 , Q 2 . The comparison signal S 41  is then evaluated via the first flip flop  42 , which is driven by the second drive signal S 2 , in each case shortly before the second switch Q 2  is switched on, and the comparison signal S 41  is then evaluated via the second flip flop  43 , which is driven by the first drive signal S 1 , in each case shortly before the first switch Q 1  is switched on. This results in a distinction being drawn between two different non-zero-voltage switching operating modes, as will be explained in more detail in the following text with reference to  FIGS. 11 and 12 . 
         [0065]    In the exemplary embodiment, a further capacitor C 6  is optionally connected between the center tap of the capacitive voltage divider C 4 , C 5  and carries out the function of a coupling capacitor, with the voltage measurement signal being produced at its connection that is remote from the center tap. However, there is no need for this coupling capacitor C 6  if the capacitor C 5  in the capacitive voltage divider is not part of a lamp identification circuit, that is to say if no non-reactive resistance is connected between the center tap of the voltage divider and the lamp filaments or the lamp electrode  12 . 
         [0066]    The evaluation circuit  40  also has a switch  45 , which is connected between the reference ground potential GND and that input of the comparator  41  to which the voltage measurement signal Vs is supplied. This switch  45  is driven by the second drive signal S 2  and is switched on when the second semiconductor switching element Q 2  is switched on. The voltage measurement signal Vs is set to a defined potential by means of this switch  45  during the time in which the second switch Q 2  is switched on, and this results, after the second switch Q 2  has been switched off and the first switch Q 1  has been switched on, that is to say when the output voltage V 2  of the half-bridge is rising, in the voltage measurement signal Vs following the output voltage V 2  with respect to the reference ground potential GND, corresponding to the division ratio of the capacitive voltage divider C 4 , C 5 . The example is based on the assumption that the switch  45  in the evaluation circuit is driven by the second switch Q 2  in the half-bridge at the same time. However, correct operation is dependent on the voltage measurement signal Vs being set to a defined potential during the time period in which the second switch Q 2  is switched on. The switch  45  may for this purpose also be closed only after the switch Q 2  and may also be opened again before the second switch Q 2 . 
         [0067]    In summary, the further switch  45  results in the information which is normally not transmitted by a capacitive voltage divider being recovered via the DC component of the voltage V 2 , so that the voltage measurement signal Vs is proportional to the output voltage V 2 , and is related to the same reference ground potential GND. 
         [0068]    The method of operation of the evaluation circuit  40  shown in  FIG. 9  will be explained in the following text with reference to  FIGS. 10 ,  11  and  12 , with  FIG. 10  showing waveforms of the signals which occur in the evaluation circuit for zero-voltage switching operation, and  FIGS. 11 and 12  showing waveforms of the signals for non-zero-voltage switching operation of a first and of a second type. 
         [0069]      FIGS. 10   a  and  10   b  show the waveforms of the first and second drive signals S 1 , S 2 , and  FIG. 10   c  shows the waveform of the voltage measurement signal Vs which results from these drive signals S 1 , S 2  and is proportional to the output voltage V 2 , for zero-voltage switching operation of the half-bridge circuit. As can be seen, the voltage measurement signal rises during the dead times between the second switch Q 2  being switched off and the first switch Q 1  being switched on to its maximum value during the dead times between the second switch Q 2  being switched off and the first switch Q 1  being switched on, and falls to its minimum value during the dead times between the first switch Q 1  being switched off and the second switch Q 2  being switched on. At the times of rising flanks of the second drive signal S 2 , the voltage measurement signal Vs has in this case always already fallen below the reference value Vref, so that the first status signal S 42  assumes a low level. At times of rising flanks of the first drive signal S 1 , the voltage measurement signal Vs has always already exceeded the reference value Vref, thus resulting in the comparison signal S 41  being at a high level at these times and, inverted, these lead to low levels of the second status signal S 43 . In this evaluation circuit  40 , zero-voltage switching operation is thus indicated by low levels of both status signals S 42 , S 43 . 
         [0070]      FIG. 11   c  shows the waveform of the voltage measurement signal Vs for non-zero-voltage switching operation (as explained with reference to  FIG. 3 ) of the first type as a function of the first and second drive signals ( FIGS. 11   b  and  11   a ). During this operating state, when a rising flank of the second drive signal S 2  occurs, the voltage measurement signal Vs has never yet fallen below the reference value Vref, so that the first flipflop  42  receives a high level with a rising flank of the second drive signal S 2 . The first status signal S 42  then assumes a high level, as is illustrated in  FIG. 11   d.    
         [0071]    With reference to  FIG. 11   e , the second status signal S 43  remains at a low level, since the voltage measurement signal Vs will always have already exceeded the reference value Vref when rising flanks occur. 
         [0072]      FIG. 12  shows the waveform of the voltage measurement signal Vs for non-zero-voltage switching operation (which has already been explained with reference to  FIG. 4 ) of the second type, during which the output voltage V 2  and thus the voltage measurement signal Vs in each case rise only with a rising flank of the first drive signal S 1 , and in each case fall only after a rising flank of the second drive signal S 2 . This results in the comparison signal S 41  being at a high level when a rising flank of the second drive signal S 2  occurs, and thus in the first status signal S 42  being at a high level. When a rising flank of the first drive signal occurs, the comparison signal S 41  assumes a low level, resulting in the second status signal S 43  being at a high level. 
         [0073]    In summary, the evaluation circuit  40  as shown in  FIG. 9  can distinguish between two different non-zero-voltage switching operations operating modes, with only the first status signal S 42  assuming a high level in a first non-zero-voltage switching operating mode, while both status signals S 42 , S 43  assume a high level in a second non-zero-voltage switching operating mode. 
         [0074]    In general, in the case of the method which has been explained with reference to  FIGS. 10 to 12 , a first and a second comparison time occur during each period of the drive to the half-bridge, with the first comparison time being chosen as a function of the timing of a predetermined flank—the rising flank in the example—of the first drive signal S 1 , and with the second comparison time being chosen as a function of the timing of a predetermined flank—the rising flank in the example—of the second drive signal S 2 . Non-zero-voltage operation of a first type is detected in the case of this method when the voltage measurement signal Vs is greater than the reference value Vref at the first comparison time and at the second comparison time (see  FIG. 11 ). Non-zero-voltage operation of the second type is in the case of this method detected when the voltage measurement signal Vs is greater than the reference value Vref at the first comparison time is less than the reference value Vref at the comparison time and is greater than the reference value Vref at the second comparison time (see  FIG. 12 ). 
         [0075]    In both cases, the reference value Vref is chosen for determination of the operating state such that it is located asymmetrically between a maximum level and a minimum level of the voltage measurement signal Vs, and in this case preferably closer to the minimum level. In this case, the voltage measurement signal Vs assumes the minimum level when the output voltage V 2  from the half-bridge is zero, and the voltage measurement signal assumes the maximum level when the output voltage V 2  assumes the value of the supply voltage Vb. 
         [0076]    The different non-zero-voltage switching operating modes which have been explained above lead to different power losses being produced in the half-bridge circuit, with the zero-voltage switching operation explained with reference to  FIGS. 4 and 12  leading to higher power losses than the non-zero-voltage switching operation which has been explained with reference to  FIGS. 3 and 11 . Non-zero-voltage switching operation of the second type is thus permissible only for a shorter time period than non-zero-voltage switching operation of the first type, in order to prevent damage to the ballast. The information obtained by the evaluation circuit  40  as shown in  FIG. 9  about which non-zero-voltage switching operating mode has occurred can thus be used in the control circuit (which is not illustrated in any more detail in  FIG. 9 ) for the half-bridge circuit Q 1 , Q 2  in order to allow the different non-zero-voltage switching operating modes for time periods of different duration, before the drive to the half-bridge circuit is interrupted and the lamp ballast is switched off, in order to prevent damage resulting from overheating.