Abstract:
A method and apparatus for providing electrical current to a lamp, detecting a power supply voltage outage, detecting a return of the power supply voltage, determining how long the power supply voltage outage lasted, and preheating the lamp responsive to determining that the power supply voltage outage lasted greater than a threshold amount of time.

Description:
BACKGROUND 
       [0001]    Lamp ballasts usually include a converter having output terminals for connecting a fluorescent lamp and input terminals for applying an input voltage. The input voltage is a direct current (DC) voltage that is provided, for example, from a line voltage by a transformer stage. The converter generates from this DC voltage an AC voltage for operating the lamp, with the frequency of this alternating current (AC) voltage determining the operating state of the converter and thus of the lamp. 
         [0002]    It is known to preheat the lamp before it is turned on for the first time or before it is turned on after a long off time, say several minutes. To this end, an AC voltage having a frequency higher than a later operating frequency of the lamp is generated by the converter. Once a specified preheat time has been reached, the lamp can then be ignited by lowering the frequency of the AC voltage to an ignition frequency. After the lamp has been ignited, the AC voltage is provided at the operating frequency. This operating frequency lies in the range of the ignition frequency. 
         [0003]    If a fluorescent lamp turns off, for example because of an outage of the power supply, the lamp may be re-ignited immediately without a preheat phase once the power supply is restored, provided that the duration of the power outage is shorter than a maximum permissible waiting time, which is for example in the range of a second or a few seconds. Such brief outages of the power supply can occur for example in public buildings that have an emergency power supply and in which, upon an outage of a main power supply, an emergency supply is available—at least for selected circuits—within an interval of usually less than one second. 
       SUMMARY 
       [0004]    Various aspects as described herein are directed to a method and apparatus for providing electrical current to a lamp, detecting a power supply voltage outage, detecting a return of the power supply voltage, determining how long the power supply voltage outage lasted, and preheating the lamp responsive to determining that the power supply voltage outage lasted greater than a threshold amount of time. 
         [0005]    These and other aspects of the disclosure will be apparent upon consideration of the following detailed description of illustrative aspects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    A more complete understanding of the present disclosure may be acquired by referring to the following description in consideration of the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
           [0007]      FIG. 1  is a functional block diagram of an illustrative lamp ballast. 
           [0008]      FIG. 2  is an illustrative schematic circuit diagram of various individual functional blocks of the lamp ballast illustrated in  FIG. 1 . 
           [0009]      FIG. 3  is a series of graphs representing various illustrative waveforms of signals that may occur in a lamp ballast. 
           [0010]      FIG. 4  is an illustrative state diagram of various states of a drive circuit, consistent with the waveforms of  FIG. 3 . 
           [0011]      FIG. 5  is an illustrative variation of the state diagram of  FIG. 4 . 
           [0012]      FIG. 6  is a chart showing illustrative basic operations of a lamp ballast in the case of a brief power supply outage. 
           [0013]      FIG. 7  is a schematic circuit diagram of another illustrative lamp ballast. 
           [0014]      FIG. 8  is a series of graphs representing various illustrative signal waveforms, wherein after an outage of a power supply, an operating parameter of the lamp ballast is cyclically monitored on the basis of signal waveforms. 
           [0015]      FIG. 9  is an illustrative state diagram consistent with the waveforms of  FIG. 8 . 
           [0016]      FIG. 10  is another series of graphs of various illustrative signal waveforms, wherein after a power outage, an operating parameter of the lamp ballast is cyclically monitored on the basis of signal waveforms. 
           [0017]      FIG. 11  is an illustrative state diagram consistent with the waveforms of  FIG. 10 . 
           [0018]      FIG. 12  is an illustrative variation of the state diagram of  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The various aspects summarized previously may be embodied in various forms. 
         [0020]    The following description shows by way of illustration various examples in which the aspects may be practiced. It is understood that other examples may be utilized, and that structural and functional modifications may be made, without departing from the scope of the present disclosure. 
         [0021]    Except where explicitly stated otherwise, all references herein to two or more elements being “coupled,” “connected,” and “interconnected” to each other is intended to broadly include both (a) the elements being directly connected to each other, or otherwise in direct communication with each other, without any intervening elements, as well as (b) the elements being indirectly connected to each other, or otherwise in indirect communication with each other, with one or more intervening elements. 
         [0022]    Illustrative embodiments as described herein relate to a method for driving a lamp ballast, and the lamp ballast itself. The lamp ballast may include, for example output terminals for connecting a lamp thereto, and input terminals for receiving a power supply voltage and that is adapted for taking on at least one of a state of low power consumption and a lamp operating state. The method may provide, for example, for monitoring at least one operating parameter of the lamp ballast and converting the lamp ballast to the state of low power consumption if the operating parameter indicates an outage of the power supply voltage. After the lamp ballast has been converted to the state of low power consumption, the operating parameter is monitored cyclically and the lamp ballast is directly converted to the lamp operating state if the operating parameter indicates that a power supply voltage is present again and if an interval since the beginning of the state of low power consumption is less than a specified standby time. 
         [0023]    A drive circuit for a lamp ballast according to an illustrative embodiment is illustrated in  FIG. 1 . In the example illustrated, this drive circuit includes an evaluation and drive circuit  60  and a detector circuit  40  connected to evaluation and drive circuit  60 . For better understanding of the mode of functioning of this drive circuit,  FIG. 1  illustrates further components of the lamp ballast, which will be explained next. 
         [0024]    The lamp ballast as shown includes a converter  20  having input terminals  201 ,  202  for applying an input voltage Vi and having output terminals  203 ,  204  for connecting a fluorescent lamp  50  and for providing a power supply voltage Vb for the fluorescent lamp  50 . For better understanding, such a fluorescent lamp  50  is likewise illustrated in  FIG. 1 . In the illustrative embodiment shown, fluorescent lamp  50  includes two lamp coils  51 ,  52 , each having two terminals  501 ,  502  and  503 ,  504  respectively. A first terminal  501  of a first lamp coil  51  is connected to a first output terminal  201  of the inverter, and a first terminal  501  of a second lamp coil  52  is connected to a second output terminal  204  of converter  20 . 
         [0025]    In order to provide the input voltage Vi of converter  20 , there is a transformer stage  10 , which exhibits input terminals  101 ,  102  for applying a power supply voltage Vin and output terminals  103 ,  104  for providing the input voltage Vi of converter  20 . In what follows, this input voltage Vi of converter  20  is also referred to as intermediate circuit voltage. Transformer stage  10  may include, for example, a boost converter that is adapted to generate from the power supply voltage Vin an intermediate circuit voltage Vi that is larger in absolute value than the power supply voltage Vin. 
         [0026]    The power supply voltage Vin is available, for example, at the output of a bridge rectifier  91 , to which a line voltage Vn or a battery voltage Vbat is supplied as input voltage, with the input voltage being selected by changeover switches  92 ,  93 . In the case of a sinusoidal line voltage Vn as the input voltage of bridge rectifier  91 , the power supply voltage Vin of the lamp ballast is a voltage having the form of the absolute value of a sine wave. In the case of a battery voltage Vbat as the input voltage of bridge rectifier  91 , the power supply voltage Vin is a DC voltage. For the explanation that follows, suppose that a line voltage Vn as the input voltage of bridge rectifier  91  represents a normal operating case, while a battery voltage Vbat represents an emergency operating case in which, after an outage of the line voltage Vn, a battery  94  takes over the further supply. A battery-backed emergency power supply may be present, for example, in public buildings and may serve to ensure an emergency power supply in case of an outage of a main power supply. The emergency power supply may take effect within an extremely short time after an outage of the main power supply, for example within an interval shorter than one second. 
         [0027]    Transformer stage  10  is adapted to generate the intermediate circuit voltage Vi with a specified amplitude both from a power supply voltage Vin having the form of the absolute values of a sine wave and also from a DC voltage as the power supply voltage Vin. Transformer stage  10  is here controlled via evaluation and control circuit  60 , which is supplied with a signal dependent on the intermediate circuit voltage Vi via an input  604  and is adapted to control transformer stage  10  in such fashion that the intermediate circuit voltage Vi is controlled to a specified set point, for example 400 V, approximately independently of the current draw of converter  20  and approximately independently of the input voltage Vin. 
         [0028]    Evaluation and control circuit  60  is furthermore adapted to control converter  20  in such fashion that the converter provides a suitable power supply voltage Vb in dependence on the desired operating state of lamp  50 . There may be essentially four operating states for the operation of fluorescent lamp  50  and thus for converter  20  and the lamp ballast:
       1. An off state, in which the power supply voltage Vb is lower than a lamp operating voltage. Lamp  50  is off in this case.   2. A lamp operating state, in which the power supply voltage Vb is an AC voltage having an operating frequency suitable for lamp  50 , for example between 40 kHz and 60 kHz. The lamp is on (burns) during this operating state.   3. A preheat phase, in which the power supply voltage Vb is an AC voltage having a preheat frequency, for example between 80 kHz and 100 kHz, that is higher than the operating frequency. During this operating state the lamp is not yet on (burning).   4. An ignition phase, in which a frequency of the power supply voltage Vb is lowered from the preheat frequency to an ignition frequency. The ignition frequency here is in the range of the operating frequency of fluorescent lamp  50  or higher, for example between 45 kHz and 70 kHz. If the lamp ignites and if the ignition frequency is higher than the operating frequency, the frequency may subsequently be lowered to the operating frequency.       
 
         [0033]    Evaluation and control circuit  60  is adapted to set one of these operating states of converter  20  and, therefore, of the lamp ballast. When lamp  50  is turned on for the first time or when lamp  50  is turned on again after a prolonged waiting time, for example a waiting time of several seconds, the preheat phase may take place first for a specified preheat time. After the preheat time has elapsed, the ignition phase follows, and after successful ignition of the lamp comes the lamp operating phase. For the explanation that follows, it is assumed by way of example that the drive circuit may likewise take on at least four operating states that correspond to the lamp operating states. For instance, in an off state, the drive circuit controls converter  20  in such fashion that the lamp is turned off, in a preheat state so that the lamp is preheated, in an ignition state so that the lamp is ignited, and in an operating state so that the lamp burns. Depending on the particular embodiment, the drive circuit may take on further operating states, which will be explained by way of example. 
         [0034]    If a burning fluorescent lamp is turned off, the fluorescent lamp may be turned on again directly within a brief time window without a preheat phase. The time window may be, e.g., up to a few seconds. This time window, within which the fluorescent lamp may be directly re-ignited without a preheat phase, is referred to as the standby time in the following. Such brief intervals may play a role particularly in the power supply systems previously explained, in which a switchover to an emergency power supply takes place within a short time after an outage of the main power supply. In buildings having such power supply systems, it may be desired that fluorescent lamps that were burning before the outage of the power supply are turned on again as promptly as possible, without a preheat phase, once the emergency power supply takes effect, provided the emergency power supply is available within the standby time. 
         [0035]    In the following, a power supply outage (also referred to as an outage of the power supply) means a drop in the power supply voltage Vin to zero or to another voltage value at which a sufficient supply to the lamp ballast is no longer provided. In order to detect such an outage of the power supply to the lamp ballast, drive circuit  40 ,  60  is adapted to monitor an operating parameter of the lamp ballast and to convert converter  20  at least to a state of low power consumption, for example the off state, if the monitored operating parameter indicates an outage of the power supply. The monitored operating parameter may be, for example, the intermediate circuit voltage Vi. In the case of an outage of the power supply, the intermediate circuit voltage Vi decreases. Evaluation and control circuit  60  compares this intermediate circuit voltage Vi with a first threshold value and converts converter  20  to the state of low power consumption if the intermediate circuit voltage Vi falls below this threshold value. For the explanation that follows, it is assumed by way of example that the operating parameter can indicate two distinct supply states of the lamp ballast: a first supply state in which the power supply is out and a second supply state in which the power supply is in order. 
         [0036]    Because of an output capacitor  13  of transformer stage  10 , which may serve to smooth the intermediate circuit voltage Vi, the intermediate circuit voltage Vi decreases only very slowly after converter  20  is turned off, in comparison to a state with converter  20  turned on. A power supply of drive circuit  40 ,  60  can still be provided for some time after an outage of the power supply via the energy stored in output capacitor  13  of transformer stage  10 . Once converter  20  is turned off (that is, once converter  20  is converted to the off state), the power consumption of drive and evaluation circuit  60  may be already declining. A further reduction in the power consumption of evaluation and drive circuit  60  may be achieved by also turning off transformer stage  10  or otherwise converting it to a state of low power consumption upon the detection of an outage of the power supply. Upon the detection of an outage of the power supply, the drive circuit thus may also enter a state of low power consumption, which will be generically referred to herein as the off state, in accordance with the lamp operating state. The drive circuit is not, however, entirely turned off during this state but still possesses a power consumption covered via capacitor  13  of the transformer stage, which is required in order to maintain the basic functions of the drive circuit. 
         [0037]    One of these basic functions may be to detect the restoration of the power supply and suitably drive converter  20  and transformer stage  10  after such a detection. As used herein, power supply restoration (also referred to as restoration of the power supply) means the rise in the power supply voltage Vin to a voltage value sufficient to supply the lamp ballast or to a voltage value sufficient to provide the intermediate circuit voltage Vi. Detector circuit  40  serves to detect such a restoration of the power supply, this detector circuit having a first terminal  401  connected to one  101  of the input terminals of transformer stage  10  and providing a detector signal S 43 , which is supplied to evaluation and drive circuit  60 . This detector signal S 43  is dependent on a power supply voltage Vin present between input terminals  101 ,  102  of transformer stage  10  and is further dependent on the presence of a lamp  50  connected to output terminals  203 ,  204  of converter  20 . For generating the detector signal S 43 , detector circuit  40  in the example illustrated includes a resistance network  41 ,  42  and an evaluation circuit  43  connected to resistance network  41 ,  42 . The resistance network includes for example a first resistance  41 , which is connected between first input terminal  101  of transformer stage  10  and first output terminal  203  of converter  20 . If a lamp  50  is in place, first terminal  501  of first lamp coil  51  is connected to this first output terminal  203  of converter  20 . The resistance network further includes a second resistance  42 , which is connected between second terminal  502  of first lamp coil  51  and evaluation circuit  43  when a lamp is in place. Evaluation circuit  43  is adapted to evaluate a current I 1  flowing through this resistance network  41 ,  42 . A current I 1  greater than zero flows through resistance network  41 ,  42  only when a lamp  50  is in place, that is, when the break present between first and second resistances  41 ,  42  in resistance network  41 ,  42  is bridged by lamp coil  51  of lamp  50 , and when the power supply voltage Vin is greater than zero. Detector circuit  40  thus may have two functions: It may serve firstly to detect the presence of the lamp, and it may serve secondly to detect a power supply voltage Vin greater than zero. A zero power supply voltage Vin and an absent lamp  50  have the same effect on the current I 1  evaluated by evaluation circuit  43 . The detector signal S 43  may nevertheless be utilized to detect a restoration of the power supply after an outage of the power supply, as is explained in the following: 
         [0038]    Evaluation and control circuit  60  is adapted to control transformer stage  10  in order to provide the intermediate circuit voltage Vi, and to control converter  20  to ignite the lamp only when the detector signal S 43  indicates that a power supply voltage Vin is present and a lamp  50  is in place. If, after an outage of the power supply voltage Vin, the lamp is turned off in the manner that has been explained, the lamp may be directly turned on again without a prior preheat phase if the power supply voltage is again available within a short time of maximally several seconds. This time is so short that it is unlikely that a lamp replacement has taken place during this time. It may therefore be inferred that in the case of a previously burning lamp, the same already burning lamp is present and may be ignited without a preheat phase if the power supply is restored within the standby time after an outage of the power supply and thus after converter  20  has been turned off. 
         [0039]    If lamp  50  is removed during operation, so that detector signal S 43  indicates that no lamp  50  is in place, transformer stage  50  and converter  20  are likewise turned off via evaluation and control circuit  60 . Upon an outage of the power supply, detector signal S 43  takes on the same value as if lamp  50  were removed. A differentiation of cases between an outage of the power supply and a removal of lamp  50  during operation is possible because when lamp  50  is removed during operation, the intermediate circuit voltage Vi does not decrease immediately but only when transformer stage  10  is turned off by evaluation and control circuit  60 . In case of an outage of the power supply, the intermediate circuit voltage Vi decreases even before evaluation and control circuit  60  turns off transformer stage  10 . 
         [0040]    Evaluation and control circuit  60  as illustrated is adapted to control converter  20  for direct ignition of lamp  50  without a preheat phase if, after an outage of the power supply, which may be detected for example through the intermediate circuit voltage Vi, the detector signal S 43  indicates a restoration of the power supply within the standby time. The drive circuit then goes directly from the off state to the ignition state and, after successful ignition of the lamp, into the lamp operating state. In order to measure the interval between the outage of the power supply and the restoration of the power supply, evaluation and drive circuit  60  exhibits for example a clock generator  61 , which is schematically illustrated in  FIG. 1 . After an outage of the power supply and the transition of evaluation and control circuit  60  into the state of low power consumption, this clock generator  61  continues to be supplied with energy, for example directly or indirectly from output capacitor  13  of transformer stage  10 . Even after an outage of the power supply voltage Vin, a basic function of drive circuit  60 ,  40  for turning the lamp on again is thus provided at least for the standby time. 
         [0041]    Instead of by evaluating the intermediate circuit voltage Vi, an outage of the power supply may also be detected by evaluating a current I 50  in lamp  50 , which is available at the output of converter  20 . If this current I 50  falls below a specified current threshold for a specified interval that is longer than one period of the lamp voltage Vb, an outage of the power supply Vi is inferred. At least converter  20  and optionally also transformer stage  10  and evaluation and control circuit  60  are then converted to the state of low power consumption. 
         [0042]    For further explanation,  FIG. 2  depicts illustrative implementations of transformer stage  10 , converter  20  and evaluation circuit  43  of detector circuit  40 . In the example illustrated, converter  20  includes a half-bridge circuit having a first switch  21  and a second switch  22 , which are connected in series between input terminals  201 ,  202  of converter  20 . At one output of the half-bridge, which is formed by one of the common circuit nodes common to both switches  21 ,  22 , a series oscillatory circuit having an oscillatory circuit capacitance  25  and an oscillatory circuit inductance  24  is connected. A circuit node common to oscillatory circuit capacitance  25  and oscillatory circuit inductance  24  here forms the first output terminal  203  of converter  20  for the connection of lamp  50 . If a lamp is present, the lamp is connected in parallel with oscillatory circuit capacitance  25  in this arrangement. Between the output terminal of half-bridge  21 ,  22  and the series oscillatory circuit, the lamp ballast illustrated has a further capacitance  23 , which essentially serves to filter out a DC component of the output voltage Vhb of the half-bridge. A capacitance value of further capacitance  23  here is much greater than the capacitance value of oscillatory circuit capacitance C 25 , so that this further capacitance has no substantial effect on the resonant frequency of the oscillatory circuit  24 ,  25 . 
         [0043]    Both switches  21 ,  22  of the half-bridge are driven via first and second drive signals S 21 , S 22  of evaluation and control circuit  60 , which are available at outputs  605 ,  606  of evaluation and control circuit  60 . Switches  21 ,  22  are driven alternately, so that a rectangular or trapezoidal AC voltage, whose frequency corresponds to the drive frequency of switches  21 ,  22 , is available at the output of the half-bridge. The lamp voltage Vb when the lamp is ignited then corresponds to an approximately sinusoidal AC voltage having this frequency. The individual operating states of converter  20 , already explained, are set via control and evaluation circuit  60  through the frequency of the pulse-width-modulated drive signals S 21 , S 22  of half-bridge  21 ,  22 . 
         [0044]    In the example illustrated, transformer stage  10  is a boost converter and includes a series circuit of an inductive storage element  11  and a switch  14  between input terminals  101 ,  102 . Connected in parallel with switch  14  is a series circuit having a rectifier element  12  and output capacitor  13 . Connecting terminals of capacitor  13  here form output terminals  103 ,  103  of transformer stage  10 , at which the intermediate circuit voltage Vi is available. Switch  14  of transformer stage  10  is driven in pulse-width-modulated fashion via a third drive signal S 14 , which is available at one output  601  of evaluation and control circuit  60 . When switch  14  is closed, inductive storage element  11  absorbs energy via input terminals  101 ,  102 ; when switch  14  is subsequently opened, it delivers this energy via rectifier element  12  to output capacitor  13  and to converter  20 , which is connected downstream to the transformer stage  10 . monitor an operating parameter of the lamp ballast in order to detect an outage of the power supply. For the explanation that follows, suppose that this operating parameter is the intermediate circuit voltage Vi. Instead of the intermediate circuit voltage, however, an output current I 50  of converter  20  may also be evaluated. 
         [0045]    With reference to  FIG. 8 , suppose that the lamp ballast is initially in a lamp operating state, in which the lamp is burning, and that a power supply is present up to a time t 0 . Transformer stage  10  and converter  20  are activated by drive circuit  60 , converter  20  being activated in such fashion that it provides a lamp voltage Vb at a lamp operating frequency. If the power supply goes out at time t 0 , transformer stage  10  and converter  20  initially remain activated until the intermediate circuit voltage Vi has decreased to the first threshold value Vth 1 , as is the case at time t 1  in  FIG. 8 . At this time, drive circuit  60  detects an outage of the power supply and deactivates transformer stage  10  and converter  20  in such a way that these go into a state of low power consumption. As soon as transformer stage  10  and converter  20  are deactivated, drive circuit  60  also goes into a state of low power consumption, it being possible to deactivate further circuit components of drive circuit  60  that are not needed at the present time, in a manner not set forth in more detail, in order to reduce the power consumption of drive circuit  60  further. 
         [0046]    Drive circuit  60  is adapted to activate transformer stage  10  cyclically, each time for a specified interval Tb, after the power outage is detected, and to evaluate the behavior of the monitored operating parameter, the intermediate circuit voltage Vi in the example. If the intermediate circuit voltage Vi during such an evaluation time Tb exceeds the first threshold value Vth 1 , a restoration of the power supply is inferred, as illustrated at a time t 3  in  FIG. 8 . If the time interval Toff between the detection of the power outage and the detection of a restoration of the power supply voltage at time t 4  is shorter than the standby time, drive circuit  60  effects ignition of lamp  50  immediately, via converter  20 , without a prior preheat phase. 
         [0047]    It may be desirable to activate not only transformer stage  10  but also converter  20  during the evaluation times Tb, but at a frequency that can lie above the operating frequency and the ignition frequency of the lamp and can also lie above the preheat 
         [0048]    The pulse duty-cycle of the pulse-width-modulated third drive signal S 14 , set by evaluation and drive circuit  60 , determines the intermediate circuit voltage Vi in a basically known manner. In order to control the intermediate circuit voltage Vi to a nominal Value, evaluation and control circuit  60  is supplied via a first measurement input  602  with an intermediate circuit voltage signal S 30 , which is dependent on the intermediate circuit voltage Vi. This intermediate circuit voltage signal S 30  is provided, for example, by a voltage divider  30 , which includes voltage divider resistances  31 ,  32  and is connected between output terminals  103 ,  104  of transformer stage  10 . The duty-cycle of the third drive signal S 14  may be set in dependence on this intermediate circuit voltage signal S 30  with the objective of controlling the intermediate circuit voltage Vi to the specified nominal value, for example 400 V. 
         [0049]    Transformer stage  10  may in particular be a power factor controller (PFC), having a power factor correction capability. In some embodiments, in the case of such a power factor controller, the current draw is controlled in such a way that an average of an input current Iin is proportional to the applied input voltage Vin. This may be achieved for example by turning the switch on cyclically for an on time dependent on the intermediate circuit voltage Vi, with switch  14  being re-closed after switch  14  is opened as soon as, or otherwise after, inductive storage element  11  is partially or completely demagnetized. The control of the power consumption of transformer stage  10  for controlling the intermediate circuit voltage Vi is effected through the on time. In order to detect the times of demagnetization, evaluation and control circuit  60  is supplied, via a second input  603 , with a magnetization signal S 16 , which corresponds to the voltage across an auxiliary coil  16  that is inductively coupled with inductive storage element  11  and includes a terminal facing away from evaluation and control circuit  60  and connected to second input terminal  102  of transformer stage  10 . This second terminal  102  of transformer stage  10  is at a common reference potential GND, for example ground, with second output  104  of transformer stage  10 , second input  202  and second output  204  of converter  20 . 
         [0050]    Optionally, transformer stage  10  includes a current measuring resistance  15  connected in series with switch  14 , at which a current measurement signal S 15  can be picked up, which current measurement signal is supplied to evaluation and control circuit  60  via a third measurement input  604 . This current measuring resistance  15  may be present for safety reasons in order to detect an overcurrent when switch  14  is closed and thus to be able to turn off switch  14 . 
         [0051]    In the lamp ballast illustrated, there is a power supply circuit  70  for the power supply to drive circuit  60 ,  40 . This power supply circuit  70  in the example includes a starting resistance  71 , which is connected between output capacitor  13  of transformer stage  10  and a power supply input  608  of control and evaluation circuit  60 . When a power supply voltage Vin is applied, a charging current flows through inductive storage element  11  and rectifier element  12  of transformer stage  10  as well as starting resistance  71  to a power supply capacitor  72  in series circuit with starting resistance  71 , a power supply voltage Vcc being available for the drive circuit across the power supply capacitor. This current begins to flow as soon as a power supply voltage Vin is applied and does not necessarily require any drive of transformer stage  10 . This power supply voltage provided via starting resistance  71  makes it possible to turn on evaluation and control circuit  60  and thus to drive transformer stage  10  as well as converter  20 . On grounds of power loss, starting resistance  71  may be chosen such that the current flowing through starting resistance  71  is not sufficient to provide a supply to evaluation and control circuit  60  continuously, in particular not when evaluation and control circuit  60  is generating pulse-width-modulated control signals to drive transformer stage  10  and converter  20 . Power supply circuit  70  may therefore additionally include a charge pump  73 ,  74 ,  75 , which is connected between the output of half-bridge  21 ,  22  and power supply capacitor  72 . In the case of a half-bridge  21 ,  22  driven in clocked fashion, power supply capacitor  72  is supplied from the intermediate circuit voltage Vi principally via this charging pump  73 - 75  and first switch  21  of the half-bridge. 
         [0052]    In the example illustrated, evaluation circuit  43  of detector circuit  46  includes a current measurement arrangement  431  for acquiring a current I 1  flowing through second resistance  42 . Here, a terminal of second resistance  42  facing away from second terminal  502  of first lamp coil is connected to a terminal for a reference potential. This reference potential may correspond to the supply potential Vcc of evaluation and control circuit  60 , which lies for example in the range between 5 V and 20 V, or may correspond to the common reference potential GND of the circuit components of the lamp ballast. 
         [0053]    Current measurement arrangement  431  provides a current measurement signal V 431 , which is compared with a reference voltage Vth 2  provided by a reference voltage source  434  by a comparison element  432 , for example a comparator. Available at the output of comparison element  432  is the detector signal S 43 , which in the example illustrated takes on a high level when the current measurement signal V 431  is higher than the reference voltage and takes on a low level when the current measurement signal V 431  is lower than the reference voltage Vth 2 . Accordingly, S 43 =1 denotes a high level while S 43 =0 denotes a low level of the detector signal S 43 . 
         [0054]    Evaluation and control circuit  60  is adapted to detect an outage of the power supply, for example on the basis of the intermediate circuit voltage Vi, monitor the detector signal S 43  after an outage of the power supply and, via first and second control signals S 21 , S 22 , convert converter  20  directly to the ignition state without a preheat phase, and to the lamp operating state after the lamp has ignited, if the detector signal S 43  indicates a restoration of the power supply within the standby time. A restoration of the power supply may be inferred, for example, if the detector signal S 43  changes from a low level to a high level within the standby time. Evaluation and control circuit  60  and evaluation circuit  43  of detector circuit  40  are illustrated as separate circuit blocks for reasons of explanation. It should be pointed out, however, that evaluation and control circuit  60  and evaluation circuit  43  of detector circuit  40  may be jointly implemented such as in an integrated circuit arrangement. Resistances  41 ,  42  of detector circuit  40  in this case are implemented for example as external components of the integrated circuit. 
         [0055]    In the following, the mode of functioning of the drive circuit previously explained with evaluation and control circuit  60  as well as detector circuit  40  is explained on the basis of waveforms of the power supply voltage Vin, the intermediate circuit voltage Vi, a current draw  160  of evaluation and control circuit  60 , the output voltage Vhb of half-bridge  21 ,  22  of converter  20 , the drive signal S 14  of transformer stage  10 , and the current measurement signal V 431  of detector circuit  40 . The waveform of the current measurement signal corresponds to the waveform of the current through resistance network  41 ,  42 . For purposes of explanation, it is assumed by way of example that up to a time t 0  the lamp ballast is in a lamp operating state, as a state in which a lamp  50  is in place and in which lamp  50  is burning. For clarity,  FIG. 3  does not show the actual waveform of the input voltage Vin and rather shows whether a power supply voltage Vin is present, which is the case up to the time t 0  in the example illustrated. The solid line stands for a power supply voltage Vin resulting from the line voltage Vn in the example illustrated, while the dot-dash line stands for an input voltage Vin resulting from the battery voltage Vbat. 
         [0056]    When lamp  50  is on, half-bridge  21 ,  22  of converter  20  supplies a rectangular or trapezoidal AC voltage Vhb at a lamp operating frequency. In this operating state, transformer stage  10  is likewise in operation, which in  FIG. 3  is made clear by the pulse-width-modulated drive signal S 14  of switch  14  of transformer stage  10 . The intermediate circuit voltage Vi is thus at a nominal value higher than a first threshold value Vth 1 . The current I 1  through resistance  42  of resistance network  41 ,  42  possesses a sinusoidal waveform corresponding to the power supply voltage Vb of the lamp when lamp  50  is burning. 
         [0057]    In the example illustrated in  FIG. 3 , an outage of the power supply is in effect from the time t 0  on; the input voltage Vin begins to decline toward zero starting at this time. Transformer stage  10  and converter  20  initially continue to be driven, so that lamp  50  continues to burn. The energy used for this is taken from output capacitor  13  of transformer stage  10 , so that the intermediate circuit voltage Vi decreases. When at a time t 1  the intermediate circuit voltage Vi decreases to the first threshold value Vth 1 , the evaluation and control circuit turns converter  20  and transformer stage  10  off, for example by driving switches  14 ,  21  and  22  in blocking fashion. The output voltage Vhb of half-bridge  21 ,  22  then takes on a not exactly defined voltage value. In the waveform illustrated in  FIG. 3  and for purposes of further explanation, it is assumed that this output voltage Vhb, after the decay of the energy stored in series oscillatory circuit  24 ,  25 , settles to a voltage value that corresponds to roughly half the intermediate circuit voltage Vi. After time t 1  the intermediate circuit voltage Vi decreases further, but much more slowly than before this time t 1 . The further decrease of the intermediate circuit voltage Vi after deactivation of transformer stage  10  is principally due to a further current draw  160  of the drive circuit, but can also additionally result from parasitic effects. The current draw of the drive circuit is much reduced just because transformer stage  10  and converter  20  are deactivated, so that evaluation and control circuit  60  is not providing pulse-width-modulated drive signals for transformer stage  10  and converter  20 . In a manner not set forth in greater detail, circuit components inside evaluation and control circuit  60  can also be deactivated after transformer stage  10  and converter  20  have been deactivated, in order in this way to reduce further the current draw  160  of the evaluation and control circuit. A reduced current draw of the evaluation and control circuit after time t 1  is illustrated in  FIG. 3  by an abrupt drop in the input current  160  at time t 1 . 
         [0058]    Evaluation and control circuit  60  is adapted to monitor the detector signal S 43  after time t 1 , that is, after an outage of the power supply has been detected, in order to detect a restoration of the power supply on the basis of the signal level of this detector signal S 43 . Transient effects, however, may result in the current I 1  through the resistance network not yet being zero immediately after the outage of the power supply voltage Vin, but only decreasing slowly. The current measurement signal V 431  may therefore continue to lie above the reference value Vth 2  during a short interval after the outage of the power supply voltage Vin. In an illustrative embodiment, that evaluation and control circuit  60  therefore monitors the detector signal S 43 , looking for a restoration of the power supply voltage, only after the lapse of a delay time Td once an outage of the power supply voltage has been detected. A time starting at which such monitoring of the detector signal S 43  is in effect is denoted as t 2  in  FIG. 3 . 
         [0059]    For further explanation, suppose that at a later time t 3  the power supply is restored, for example by activation of a battery-backed emergency power supply. A current I 1  through the resistance network begins to rise at this time, the rise in current being limited by parasitic effects such as for example charging of capacitor  25 , which is in parallel circuit with lamp  50 . At a time t 4  the current measurement signal V 431  reaches the current reference value Vth 2 , so that the detector signal S 43  takes on a high level. Evaluation and control circuit  60  detects this level change in the detector signal S 43  and then activates transformer stage  10  and converter  20 . When transformer stage  10  is activated, the intermediate circuit voltage Vi begins to increase again toward the set point. The operating state in which converter  20  is placed by evaluation and control circuit  60  will now depend on the interval, referred to as the off time Toff in the following, between a detection of an outage of the power supply Vin at time t 1  and a detection of a restoration of the power supply Vin at time t 4 . If this off time Toff is shorter than the standby time Tstby, converter  20  is converted directly to the ignition state and subsequently to the lamp operating state; half-bridge  21 ,  22  is thus driven at the ignition frequency and subsequently the operating frequency of the lamp in order to ignite the lamp directly without waiting through a new preheat phase. If the off time Toff is longer than the standby time, the cycle executed is the same as in a cold start of the lamp; that is, evaluation and control circuit  60  converts converter  20  first to a preheat phase and subsequently, after an ignition phase, to the operating phase. 
         [0060]    For further elucidation of the method explained,  FIG. 4  depicts an illustrative state diagram in which individual operating states of the drive circuit and criteria for a state transition between the respective operating states are illustrated. For the explanation that follows, suppose that operating states of the drive circuit correspond to the respective operating states of the lamp ballast. The operating state of the drive circuit thus determines the operating state of the entire ballast. If for example the drive circuit is in the lamp operating state, then the lamp ballast is also in the lamp operating state. The individual operating states may differ, for example, in the frequency at which the half-bridge of converter  20  is driven or in the activation or deactivation of transformer stage  10 . 
         [0061]    In  FIG. 4  Z 1  denotes a lamp operating state in which the drive circuit drives transformer stage  10  to provide the intermediate circuit voltage Vi and drives converter  20  to provide a lamp voltage Vb at a lamp operating frequency. Z 21  denotes a first wait state, into which the drive circuit goes upon the detection of an outage of the power supply, for example when the intermediate circuit voltage Vi falls below the first threshold value Vth 1 . In the example illustrated in  FIG. 3 , the drive circuit takes on this first wait state at time t 1 . During this first wait state Z 21 , the drive circuit deactivates transformer stage  10  and converter  20 . There is not, however, any monitoring of the detector signal S 41  with a view to a restoration of the power supply, or a level of the detector signal is ignored during this interval. After the delay time Td has elapsed, the drive circuit goes into a second wait state Z 31 , in which transformer stage  10  and converter  20  still remain deactivated but the detector signal S 43  is monitored with a view to a restoration of the power supply. If during this second wait state Z 31  a restoration of the power supply voltage Vin is detected on the basis of the detector signal S 43 , for example (see  FIG. 3 ) because the detector signal S 43  takes on a high level, and if the off time Toff since the detection of the outage of the power supply is shorter than the standby time Tstby, then the drive circuit goes directly into an ignition state Z 6  and from the ignition state, after the lamp has ignited, into the lamp operating state Z 1  again. During the ignition state Z 6 , transformer stage  10  is activated to provide the intermediate circuit voltage Vi and converter  20  is activated in such fashion that it provides an AC voltage at an ignition frequency. The drive circuit may possess functionality for detecting ignition of the lamp, so that the drive circuit does not change over to the lamp operating state Z 1  until after the lamp has ignited. Such functionality is basically known for lamp ballasts, so that no further discussion of it is necessary. 
         [0062]    If the off time Toff since the detection of the outage of the power supply voltage exceeds the standby time Tstby, the drive circuit goes into a third wait state Z 41 , in which transformer stage  10  and the converter are deactivated and the detector signal S 43  is still monitored. If during this third wait state Z 41  a restoration of the power supply is detected on the basis of the detector signal S 43 , a turn-on cycle including lamp preheating and ignition is executed. Here the drive circuit goes first into a preheat state Z 6 , in which transformer stage is activated and converter  20  is activated in order to preheat lamp  50 . After a preheat time Th has elapsed, the drive circuit goes into the ignition state Z 6  and into the lamp operating state Z  1  after the lamp has ignited. An initial state of the drive circuit after a starting process, that is, after the power supply voltage Vcc is provided, is for example the third wait state Z 41 . This starting process is always executed if the power supply voltage Vcc of the drive circuit has fallen to zero, after the ballast has been turned off, or to voltage values not sufficient to supply the drive circuit. 
         [0063]    The drive circuit may change from the second wait state Z 31  to a shortened preheat state Z 51  (indicated by dashed lines) after a restoration of the power supply has been detected, and into the ignition state after a shortened first preheat time has elapsed. The first preheat time here is shorter than the “normal” preheat time Th executed during the turn-on cycle with a cold lamp. This normal preheat time is also referred to as second preheat time in the following. The first preheat time of the shortened preheat state Z 51  may be much shorter than the second preheat time and much shorter than the standby time. For example, the first preheat time is between 1% and 10% of the standby time, while the second preheat time Th can be in the range of this standby time or longer. In the following, the phrase “direct transition of the drive circuit into the lamp operating state,” and similar phrases, means a transition without a preheat state or a transition after a shortened preheat state. 
         [0064]      FIG. 5  depicts an illustrative modification of the method previously explained. In this method, the drive circuit goes into the first wait state Z 21  when an outage of the power supply voltage is detected and goes from this first wait state into the second wait state Z 31  at regular time intervals, each time for a monitoring time Tw′ during which the detector signal S 43  is monitored with a view to a restoration of the power supply voltage Vin. Starting from time t 1 , there is a transition from the first wait state Z 22  into the monitoring state Z 32 , for example every time t−T 1 =k·Tw, where k is a positive whole number and Tw denotes the duration of a period. The monitoring time Tw′ is smaller in each case than the period Tw. The period Tw here is longer than or equal to the wait time Td that elapses while waiting out transient processes after the power outage has been detected (see  FIG. 3 ). If, during the second wait state Z 31  during the monitoring time, a restoration of the power supply is detected and the off time Toff is shorter than the standby time, the drive circuit makes a direct transition to the ignition state Z 6 ; in other words, the lamp is immediately ignited without a prior preheat phase. If the standby time Tstby elapses during the first or the second wait state Z 21 , Z 31 , the drive circuit goes into the third wait state Z 41 . If a restoration of the power supply voltage is detected during the third wait state Z 42 , the cold-start cycle with preheat phase Z 5  and ignition phase Z 6  is executed. 
         [0065]    A basic cycle for a cold start of the lamp and a restoration of the lamp after an outage of the power supply voltage is illustrated in  FIG. 6 . Suppose that up to a time t 10  there is no power supply. The lamp is thus off until this time. If the power supply takes effect at a time t 10 , a preheat phase begins, in which control and evaluation circuit  60  drives the converter at a preheat frequency. After a preheat time has elapsed, an ignition phase follows, in which the frequency of converter  20  is reduced to an ignition frequency. The lamp burns after successful execution of the ignition cycle. If the power supply goes out at a time t 11 , the lamp may be immediately re-ignited without a preheat phase if the power supply is restored within an interval shorter than the standby time. In this case the ignition phase is executed directly, that is, without a preheat phase. 
         [0066]    In the following, a drive circuit for a lamp ballast and a method for driving a lamp ballast, without providing a detector signal that is dependent on a power supply voltage Vin and a lamp  50  in place, is explained with reference to  FIGS. 7 and 8 . The basic structure of the lamp ballast illustrated in  FIG. 7  corresponds to that of the lamp ballast illustrated in  FIG. 2 , with the difference that the drive circuit exhibits no detector circuit for providing a detector signal S 43  dependent on the power supply voltage Vin and the presence of a lamp. 
         [0067]    The mode of functioning of an illustrative embodiment of drive circuit  60  illustrated in  FIG. 7  will be visualized in terms of waveforms of the input voltage Vin, the intermediate circuit voltage Vi, a current draw  160  of drive circuit  60 , an output voltage Vhb of half-bridge  21 ,  22 , and the drive signal S 14  of switch  14  of transformer stage  10 , which are illustrated in  FIG. 8 . Drive circuit  60  is adapted to frequency of the lamp. The activation of converter  20  in this case is exclusively for the purpose of supplying power to drive circuit  60  via the charging pump  73 - 75  of power supply circuit  70 . By activating transformer stage  10 , the power consumption of drive circuit  60  can increase so much that its power demand cannot be covered solely via starting resistance  71 . With reference to  FIG. 8 , the higher power consumption of drive circuit  60  during the evaluation phases Tb, but above all the activation of converter  20 , leads to a decrease in the intermediate circuit voltage Vi during these evaluation phases Tb that is faster than during wait times Tw between the monitoring or activation times Tb. In  FIG. 8 , Tp denotes a period length after which transformer stage  10  is activated for an activation time Tb each time. 
         [0068]      FIG. 9  elucidates the method explained with reference to  FIG. 8 , using a state diagram. Here Z 1  denotes a lamp operating state, in which transformer stage  10  and converter are activated and in which lamp  50  burns. The drive circuit goes into a first wait state Z 23  if an outage of the power supply is detected, for example on the basis of the intermediate circuit voltage Vi. From this first wait state Z 23 , the drive circuit cyclically goes into an activation or monitoring state Z 33 , in which at least transformer stage  10  is activated for the specified activation time Tb. In  FIG. 9 , the expression t−t 1 =Tw+k·Tp, where k is a whole number greater than or equal to zero, denotes cyclically recurring times at which the drive circuit goes into the activation state Z 33 . If a restoration of the power supply is detected during this activation state Z 33 , for example because the intermediate circuit voltage Vi exceeds the first threshold value Vth 1 , and if a wait time since the detection of the power outage is shorter than the standby time Tstby, then the drive circuit goes directly into the ignition phase Z 6  without a prior preheat phase, and from the ignition phase Z 6  into the lamp operating phase Z 1 . The drive circuit may change to the ignition state Z 6  after a shortened preheat state Z 5   1 , in accordance with the example explained with reference to  FIG. 4 . 
         [0069]    If no restoration of the power supply is detected during the activation state Z 33 , that is, the intermediate circuit voltage Vi remains below the first threshold value Vth 1 , then the drive circuit returns to the first wait state Z 23  after the lapse of the activation time Tb. From both the wait state Z 23  and the activation state Z 33 , the drive circuit makes a transition to a second wait state Z 43  if the wait time is longer than the standby time Tstby. 
         [0070]    At the beginning of the second wait state Z 43 , converter  20  and transformer stage  10  are for example continuously activated. Because of the resulting power consumption, the intermediate circuit voltage Vi may continue to decrease until the supply to drive circuit  60  via power supply circuit  70  is no longer provided and drive circuit  60  deactivates itself on account of insufficient power supply voltage. If a power supply voltage Vin is again available after a deactivation of the drive circuit, intermediate circuit capacitor  13  of transformer stage  10  is charged, through inductance  11  and rectifier element  12 , to the peak value of the applied power supply voltage Vin, without transformer stage  10  being activated at first. The voltage value that comes into effect on intermediate circuit capacitor  13  is commonly lower than the intermediate circuit voltage that takes effect when transformer stage  10  is activated. At the same time, a current flows through starting resistance  71  to power supply capacitor  72 . The value of starting resistance  71  is chosen here such that restarting of drive circuit  60  is possible with the lower intermediate circuit voltage Vi and the resulting supply to drive circuit  60 . If the power supply to drive circuit  60  is restored during the third wait state Z 43 , the drive circuit activates transformer stage  10 . In this way, the intermediate circuit voltage Vi rises again. If the intermediate voltage Vi exceeds the first threshold value Vth 1 , the drive circuit goes into preheat state Z 5 , into the ignition state Z 6  after the preheat time has elapsed, and into the lamp operating state after the ignition IGN of the lamp. Depending on whether the intermediate circuit voltage Vi decreases so much during the third wait state that the supply to the drive circuit is interrupted, further operating states can come about during the third wait state, in the manner previously explained. 
         [0071]    In order to save electrical energy, which may be supplied exclusively by output capacitor  13  of transformer stage  10  when the power supply is interrupted, provision may be made in some embodiments to break off the activation state Z 33  prematurely, before the activation time Tb has elapsed, if on the basis of a further evaluation criterion it is determined that no increase in the intermediate circuit voltage Vi can be expected within the activation time Tb. In some embodiments, the magnetization signal S 16  provided by transformer stage  10  is evaluated. This magnetization signal S 16  is illustrated by way of example in  FIG. 10  during an activation phase of transformer stage  10 , in which the power supply is not yet present, and during an activation phase after the power supply is again present. If there is no power supply voltage, the magnetization signal S 13  does not exceed a third threshold value Vth 3 . Drive circuit  60  is adapted to monitor the magnetization signal S 16  during an activation time and to terminate the activation time prematurely if the magnetization signal S 16  does not exceed the third threshold value Vth 3  within a shortened activation time Tb′. In the example illustrated, the magnetization signal S 16  exceeds the third threshold value Vth 3  during an activation phase. The activation phase is then not terminated prematurely, but a check is performed throughout the activation time to determine whether the intermediate circuit voltage Vi exceeds the first threshold value Vth 1 . 
         [0072]      FIG. 11  depicts an illustrative state diagram relating to the method previously explained with reference to  FIG. 10 . This state diagram differs from the one illustrated in  FIG. 9  in that a transition from the activation state Z 33  to the wait state Z 23  takes place prematurely if the magnetization signal does not exceed the third threshold value Vth 3  before a shortened activation time Tb′ has elapsed. A transition into the wait state Z 23  furthermore takes place if, in the case of a non-shortened activation time, the intermediate circuit voltage Vi does not exceed the first threshold value Vth 1  during the maximum allowable activation time. 
         [0073]    In a further illustrative modification of the method explained previously, a change in the intermediate circuit voltage Vi during the activation time may be additionally examined and the activation time is terminated prematurely when, for example, the intermediate circuit voltage remains constant or even decreases.  FIG. 12  depicts an illustrative state diagram for this method. In this method, a transition from the activation state Z 33  to the first wait state Z 23  takes place prematurely if a constant or decreasing intermediate circuit voltage Vi is detected during the activation time. 
         [0074]    In some embodiments, transformer stage  10  and the converter are left activated after a detection of an outage of the power supply, but converter  20  is converted to an operating state in which the frequency of its output voltage Vb is higher than the operating frequency, so that the lamp does not burn, and converting the lamp again to ignition without a preheat phase if a restoration of the power supply is detected within the standby time.