Patent Publication Number: US-6657403-B2

Title: Circuit arrangement for operating a fluorescent lamp

Description:
TECHNICAL FIELD 
     The present invention relates to a circuit arrangement for operating a fluorescent lamp, comprising a half-bridge arrangement whose output signal can be fed to a load circuit, it being possible to connect a fluorescent lamp with filament electrodes to the load circuit, a preheating device for preheating the filament electrodes of the fluorescent lamp, and a power determining device for determining a preheating power value that represents a measure of the power that is converted in the filament electrodes during preheating. It also relates to a corresponding method for operating a fluorescent lamp. 
     BACKGROUND ART 
     The present invention addresses the problem that fluorescent lamps are produced which have different electrical data in the same dimensions or same versions. For example, one type is optimized for the converted electric power, and therefore for the luminous flux output by the lamp, while another type is directed toward high efficiency and in this case converts substantially less electric power. Since it is possible as regards their dimensions to interchange corresponding lamp types with one another in a given luminaire, operating devices have been developed that automatically recognize the type of lamp respectively being used and set the correct operating parameters. Such a circuit arrangement for automatic recognition of the type of lamp being used is known from the Energy Savings Company; the essential part of this is illustrated in FIG.  1 . In this case, a half-bridge arrangement that includes the two switches S 1  and S 2  is driven by an intermediate circuit voltage that is usually a DC voltage of the order of magnitude of 400 V. The midpoint of the half-bridge arrangement is connected to a lamp inductor L S  that serves for current limitation after ignition of the lamp LA. Two capacitors C K1 , C K2  serve as coupling capacitors. In addition to the already mentioned lamp inductor L S , a load circuit includes a resonance capacitor C R . They serve to generate the voltage required to ignite the lamp. 
     The lamp LA comprises two filament electrodes W 1 , W 2 , which are preheated as follows: together with an inductor L 1  of a preheating circuit VK, the lamp inductor LS forms a transformer. As long as the lamp LA has still not yet been ignited, the preheating circuit VK can be supplied with energy on this path via the lamp inductor. Also arranged in the preheating circuit VK is a switch S 3  that is closed for the purpose of switching on the preheating. With the switch S 3  closed and the lamp LA not ignited, the inductor L 1  drives a current through the preheating circuit VK. Said current flows through an inductor L 2  which, as primary inductor, forms a transformer with two further inductors L 21 , L 22 . As soon as current flows through the inductor L 2 , current also flows through the inductors L 21  and L 22 , thus producing through the filament electrodes W 1  and W 2  a flow of current that results in heating of the filament electrodes W 1 , W 2 , that is to say preheats them. The level of the currents I 21 , I 22  is a function of the impedance of the filament electrodes W 1 , W 2 . The currents I 21 , I 22  are at a fixed ratio to the current I 1  of the preheating circuit VK. It is possible by measuring the voltage drop U R1  across a measuring shunt R 1  to determine the current I 1 , and thus to establish which type of lamp is being used in the circuit arrangement. To control the half-bridge arrangement automatically, the voltage U R1  dropping across the resistor R 1  is fed to a processor that sets the operating parameters in accordance with the type of lamp determined. After the preheating of the lamp LA, the switches S 1 , S 2  of the half-bridge arrangement are activated at a suitable frequency to raise the voltage present across the lamp so high as a consequence of the resonance circuit formed by the lamp inductor L S  and the resonance capacitor C R  that the lamp LA is ignited. After the ignition of the lamp LA, switches S 1 , S 2  of the half-bridge arrangement are operated at a frequency that corresponds to the normal operation of the lamp. As is evident to the person skilled in the art, the circuit arrangement illustrated in FIG. 1 uses an actively switched preheating circuit VK. 
     DISCLOSURE OF THE INVENTION 
     The object of the present invention consists in developing a circuit arrangement of the type mentioned at the beginning so as to permit automatic recognition of the type of lamp being used in the circuit arrangement even in the case of circuit arrangements having a preheating circuit not actively switched. A corresponding method for operating a fluorescent lamp is also to be made available. 
     These objects are achieved by means of a generic circuit arrangement in the case of which a power determining device can be used to determine the power flowing into the load circuit during preheating. 
     The object in terms of method is achieved by means of a method in which the first step is to preheat a fluorescent lamp while feeding energy from a half-bridge arrangement into a load circuit, the fluorescent lamp, which has filament electrodes, being connected to a load circuit. In this case, a preheating power value is determined that represents a measure of the power flowing into the load circuit during preheating. This can be preferable the current flowing through the half-bridge arrangement. One of at least two sequences for operating the circuit arrangement is selected as a function of the determined preheating power value. 
     The solutions according to the invention are based on the finding that in the case of circuit arrangements having a preheating circuit that is not actively switched the power flowing into the load circuit is correlated with the type of lamp being used in the circuit arrangement, as long as the lamp is not ignited. In particular, it is possible to work out the portions converted in the load circuit, since the values of the components are known and constant. To this extent, the power flowing into the load circuit is essentially a function of the impedance of the filament electrodes, and so it is possible to deduce which type of lamp is being used from the power flowing into the load circuit. The suitable operating parameters can then be set as a function of the type of lamp established. In contrast with the known circuit arrangement illustrated in FIG. 1, instead of using preheating inductors it is also possible to preheat in the case of circuit arrangements according to the invention with the aid of preheating resistors or by constructing a resonance circuit. The range of preheating options is substantially widened to this extent. In the case of the circuit arrangement according to the invention, preheating, ignition and normal operation of the lamp are performed via one and the same circuit arrangement, solely by varying the frequency with which the switches of the half-bridge arrangement are opened and closed. The current flowing in one half-bridge arm is advantageously evaluated by the power determining device. It is thereby possible to determine the power flowing into the load circuit in a particularly simple way, for example by using a measuring shunt or an inductor. 
     The circuit arrangement preferably further comprises a sequence control system that selects one of at least two sequences for operating the circuit arrangement, doing so as a function of the preheating power value. It can be ensured by this measure that the circuit arrangement is operated automatically, that is to say without interaction of an operator, with the aid of operating parameters that correspond to the type of lamp established. 
     The at least two sequences for operating the circuit arrangement can comprise operating modes of the circuit arrangement for preheating and/or igniting and/or normal operations of the fluorescent lamp or measures in the event of a defective or unused fluorescent lamp. 
     This ensures, firstly, that in the case of a recognized type of fluorescent lamp the lamp is preheated, ignited and operated as normal in a suitable way, that is to say without the circuit arrangement or the fluorescent lamp being damaged. Furthermore, it can be detected thereby whether the lamp filaments are intact or whether a fluorescent lamp is indeed being used. In the prior art, by contrast, the filament detection is relatively time-consuming and cost-intensive, and so in the case of the circuit arrangement according to the invention, in which the active power in the load circuit is detected in any case, it is possible to make distinct saving on material and to have an electrically more favorable configuration of the load circuit. 
     A particularly preferred embodiment of the circuit arrangement according to the invention further comprises a controlling or regulating device, it being possible to use the power determining device to determine the power flowing into the load circuit even in the normal operation of the fluorescent lamp, and the controlling or regulating device being designed to change parameters for the normal operation of the circuit arrangement as a function of this determined power. Controlling or regulating the power flowing into the load circuit in the normal operation is necessary because of the fact that the fluorescent lamp is operated at different operating points as a function of temperature, that is to say shortly after switch on, in particular, the operating point has different current and voltage values than in the case of the operating temperature. To this extent, the lamp does not constitute a constant load. This existing power detection can also be used to achieve the object according to the invention, given a suitable design. The controlling or regulating device can be designed to compare the determined preheating power value against a desired value in order to change the parameters for the normal operation as a function of the result of this comparison. 
     It is particularly preferred for the sequence control system to comprise a bistable stage in order to select one of two operating modes of the circuit arrangement as a function of the determined preheating power value. This variant is particularly suitable for circuit arrangements in which it is possible to use only two types of lamp that correspond with regard to their dimensions. In this case, this simple solution suffices for automatically recognizing the type of lamp being used and for setting the associated operating parameters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are described in more detail below with reference to the attached drawings, in which: 
     FIG. 1 shows a part of a circuit arrangement, known from the prior art, with an actively switched preheating circuit; 
     FIG. 2 shows a first embodiment of a circuit arrangement according to the invention, and 
     FIG. 3 shows a schematic block diagram of an operating device for operating a circuit arrangement according to the invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     FIG. 2 shows a circuit arrangement according to the invention, elements and variables corresponding to those of FIG. 1 being denoted by the same reference numerals. The circuit arrangement illustrated in FIG. 2 likewise comprises a half-bridge arrangement with two switches S 1 , S 2 , that are fed by a voltage U Z . The midpoint of the half-bridge arrangement is connected to a lamp inductor L S  whose other end is connected to a first filament electrode W 1  of the lamp LA. Together with a resonance capacitor C R  that is connected between the filament electrode W 1  and a second filament electrode W 2 , the lamp inductor L S  forms a resonance circuit for igniting the lamp LA. Two capacitors C K1 , C K2  serve, in turn, as coupling capacitors. The power flowing into the load circuit LK is correlated with a current I RM  that flows through a measuring shunt R M  arranged in a half-bridge arm of the half-bridge arrangement. The voltage U RM  dropping across the measuring shunt R M  is preferably evaluated to determine the power flowing into the load circuit LK. 
     In the case of the circuit arrangement illustrated in FIG. 2, no actively switched preheating circuit is used to preheat the filament electrodes W 1 , W 2  of the fluorescent lamp LA. Rather, the operating states of the fluorescent lamp LA such as preheating, ignition and normal operation are effected by suitably setting the frequency at which the switches S 1 , S 2  of the half-bridge arrangement are opened and closed. 
     Shown in FIG. 3 in a block diagram is an operating device on which a preferred embodiment of a circuit arrangement according to the invention is shown. The operating device shown in FIG. 3 serves, in particular, to set suitable operating parameters of a first and a second type of lamp. During preheating, the first type of lamp is assigned a power P 1 , while the second type of lamp is assigned a power P 2  during preheating. The operating device illustrated in FIG. 3 has an input E for connecting a line voltage U N  to a component  10 . The component  10  comprises an HF filter, a rectifier and a circuit for limiting the line current harmonics. The output signal of the component  10  is typically a stabilized DC voltage of approximately 400 V, what is termed the intermediate circuit voltage, which feeds a half-bridge arrangement  12 . The intermediate circuit voltage U Z  is converted by the half-bridge arrangement  12  into a square-wave voltage signal U R  whose frequency can be determined by an oscillator  14 . The square-wave voltage U R  is conducted to a load circuit  16  that comprises a resonance circuit for igniting a lamp LA connected to it. The load circuit  16  serves, furthermore, to limit the lamp current in normal operation of the lamp LA. The oscillator  14  is driven by a sequence control system  18  that can be used to set the frequencies and/or pulse duty factors of the square-wave voltage U R  that are required for preheating, igniting and normal operation of the lamp LA. The sequence control system  18 , oscillator  14  and half-bridge arrangement  12  are not necessarily separate circuits, but can form a unit as in the case of a free-running inverter. The voltage U RM  dropping across a measuring shunt R M  (not illustrated) that is arranged in a half-bridge arm of the half-bridge arrangement is fed to a power determining device  20 . The power P act  flowing into the load circuit can be determined from the voltage U RM  and with knowledge of the components of the load circuit  16 . A threshold switch  22  compares the power P act  with a threshold value P thres  that is selected such that the power P act  is definitely smaller than P thres  during the entire preheating phase in the case of the connection of a first type of lamp, while the connection of a second type of lamp has the effect that the threshold value P thres  is exceeded at least temporarily during preheating. If the threshold value P thres  is not exceeded, the output of the threshold switch remains in the state L, while otherwise the output of the threshold switch  22  goes over at least temporarily into the state H. The sequence control system  18  can raise a signal only during the preheating phase, the electric switch  24  thereby being closed. As a result, the output signal of the threshold switch  22  is conducted to a bistable stage  26  and accepted. The bistable stage  26  provides at its output in the idle state a signal that corresponds to the state L of the threshold switch  22 . If this output signal is provided at the output of the bistable stage  26 , a switch  28  switches into the position illustrated in FIG. 3, so that a control amplifier  30  is fed a desired variable that corresponds to a power P 1 . The power P act  is fed to the control amplifier  30  as variable to be regulated. 
     For the case in which the threshold switch  22  provides at its output a signal that corresponds to the H state, the bistable stage  26  switches over the switch  28  and communicates a signal that is correlated with the power P 2  to the control amplifier  30 . Even a brief occurrence of the H state at the output of the threshold switch  22  suffices to bring the switch  28  into the P 2  position. The switch  28  remains in the state P 2  until the operating device is switched off. 
     In the case of the operating device illustrated in FIG. 3, the power fed to the lamp LA during preheating is regulated. In a simpler embodiment, the power fed to the lamp LA during preheating is controlled after presetting of a desired power. 
     With regard to the type of lamp established, the sequence control system  18  provides suitable signals for operating the lamp not only during the preheating phase, but also for ignition and in normal operation, in particular by varying the amplitude, frequency and pulse duty factor of the voltage U R . 
     As is evident to the person skilled in the art, it is also possible instead of a bistable stage to use other methods to ensure an appropriate drive of the lamp after the type of lamp being used is established. 
     Of course, as is also known from FIG. 1, the preheating of the filament electrodes can be implemented by means of preheating inductors in one embodiment (not illustrated).