Abstract:
The present invention relates to a circuit for controlling a fluorescent lamp, including circuitry that provides a low frequency alternating current to the fluorescent lamp, this circuitry being controlled by a controllable switched-mode current source operating at high frequency.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of fluorescent lamps supplied from the high voltage a.c. mains power system (for example, 220 volts/50 Hz or 115 volts/60 Hz). The present invention more specifically relates to the lamp control, essentially, in current limitation in nominal operation and in turn-on triggering. 
     2. Discussion of the Related Art 
     In nominal operation, it is necessary to provide in the lamp supply circuit a current limiting element due to the structure of fluorescent lamps. Indeed, this type of lamp behaves, in nominal operation, as a voltage limiting component, that is, the voltage across the lamp is independent from the supply voltage, and is determined by the power of the lamp itself. Accordingly, to supply a fluorescent lamp on the mains voltage, it is necessary to provide a current limiting component, generally called a “ballast”. 
     At turn-on, it is necessary to provide a triggering or starting component, generally called a “starter”, meant to, first, heat up the filaments of the lamp, then start the lamp with an overvoltage. 
     FIG. 1 shows an example of a diagram of a conventional fluorescent lamp power supply circuit. A lamp  1  is generally formed of a tubular piece T filled with gas and at both ends of which are provided two excitation filaments f, f′. Each filament is meant to be electrically connected by both its ends and is thus associated with two supply terminals  1 ,  2 , respectively,  1 ′,  2 ′. The two filaments f, f′ are meant to be supplied by an a.c. voltage Vac, for example, the mains voltage applied between two supply terminals  3 ,  4 , of the lamp circuit. 
     In the example shown in FIG. 1, the current limiting component is formed of a high value iron inductance L interposed between a first a.c. supply terminal  3  and a first terminal  1  of one of the filaments f of lamp T. The second terminal of filament f is connected, via a starting component  5 , to a terminal  2 ′ of the second filament f′, the second terminal  1 ′ of which is connected to the second mains supply terminal  4 . A capacitor C interconnects terminals  3  and  4 . 
     Triggering or starting element  5  is most often a thermal switch meant to heat up filaments f and f′, of lamp T by short-circuiting terminals  2  and  2 ′ as long as the filaments are cold. The thermal switch opens as soon as it has reached a given temperature, which causes an overvoltage which triggers the lamp by means of the power storage formed by inductance L. 
     The function of inductance L is, in nominal operation, to limit the current in lamp T so that its voltage does not exceed the value for which it is designed. The function of capacitor C is to compensate the dephasing associated with the inductive assembly in order to improve the power factor and to make it acceptable for a connection to the network. 
     A disadvantage of a conventional supply system such as shown in FIG. 1 is that the use of a high inductance (generally on the order of 1 Henry) results in a bulky and heavy system. Further, the inductive nature of the assembly which requires a compensation of the dephasing by capacitor C requires a capacitor of high value (generally of more than 10 μF), which thus has to be an electrolytic capacitor. 
     Another disadvantage of such a system is that there exists no light dimmer for use with this system. 
     FIG. 2 shows an example of a conventional diagram of a so-called “electronic” limiting circuit, that is, a circuit using active components to limit the current of the fluorescent lamp in nominal operation. 
     Such a circuit is formed by a diode bridge  10 , having two terminals that receive an a.c. voltage connected to two terminals  3 ,  4 , that receive mains voltage Vac. A first rectifying output terminal  11  of bridge  10  forms a ground terminal of the circuit. A second rectifying output terminal  12  of bridge  10  provides, by means of a high value electrolytic capacitor C′, a d.c. supply to a switched-mode converter  13  used to supply fluorescent lamp T. Switched-mode converter  13  generally is formed by a control circuit  14  associated with two MOS power transistors M 1 , M 2  (or two bipolar transistors) connected in series between terminal  12  of bridge  10  and the ground, capacitor C′ being connected in parallel to this series association. A terminal  15  of the switched-mode converter is connected to a first terminal of a high frequency inductance L′ mounted, as in the case of FIG. 1, in series with one of the filaments f of lamp T. A capacitor C″ of low value interconnects filaments f and f′ and contributes to the lamp triggering. The second terminal  1 ′ of filament f′ is grounded via a capacitor  16 . Another capacitor  17  connects terminal  1 ′ to an input terminal  18  of switched-mode capacitor  13 . Capacitors  16  and  17  are used to filter the d.c. component in lamp T. Terminal  18  receives the d.c. voltage provided by capacitor C′. Transistor M 1  is connected between terminal  18  and terminal  15  and transistor M 2  is connected between terminal  15  and the ground. Transistors M 1  and M 2  are controlled by circuit  14  which also includes a feedback input connected to terminal  15  and which is supplied from terminal  18  via a resistor R. A capacitor  19  interconnects terminals  15  and  18  and contributes to the generation of an auxiliary power supply necessary for the control of transistor M 1 . 
     Circuit  14  may include other configuration and parametering terminals, not shown. The operation of an electronic limiting circuit such as shown in FIG. 2 is perfectly well known. Bridge  10  and capacitor C′ provide, for a 220 -volt a.c. voltage, a power supply on the order of 300 d.c. volts to the switched-mode converter which is of “symmetrical half-bridge” type. This converter provides an alternating current at a frequency which is generally approximately 30 kHz to fluorescent lamp T via the high frequency (series) inductance L′, which may be of low value (on the order of one mH). 
     A system such as shown in FIG. 2 eliminates the use of a high inductance (L, FIG.  1 ). 
     However, a disadvantage of a circuit such as shown in FIG. 2 is that it still requires an electrolytic capacitor C′ of high value (generally higher than 10 μF) to filter the voltage rectified by bridge  10 . The use of electrolytic capacitors may result in a reduced circuit lifetime. 
     Another disadvantage of the system shown in FIG. 2 is that it requires two high voltage MOS power transistors which operate at high frequency. 
     Another disadvantage of such a system is that it is required to add to bridge  10  a power factor correction circuit  20 . Without circuit  20 , the harmonics of the supply current strongly adversely affect the power factor. 
     SUMMARY OF THE INVENTION 
     The present invention aims at overcoming the disadvantages of known fluorescent lamp supply systems. 
     The present invention aims, in particular, at providing a novel system for controlling a fluorescent lamp which limits the nominal current while being of low bulk and at least partially integrable. 
     The present invention also aims at providing a solution which allows addition of a light dimming function to the control system. 
     The present invention also aims at improving the reliability of the control system by avoiding the use of electrolytic capacitors. 
     A characteristic of the present invention is to provide a supply of the fluorescent lamp by a sine current at a low frequency corresponding to the mains frequency (for example, 50 hertz) while avoiding the use of a high value inductance by means of an active circuit operating at a higher frequency (for example, on the order of 100 kHz). 
     Thus, according to the present invention, an active device is used to supply the fluorescent lamp while controlling its current. This device provides a low frequency a.c. current to the fluorescent lamp of a shape similar to that of a ferromagnetic-type limiting circuit, that is, using a single inductance of high value. 
     More specifically, the present invention provides a circuit for controlling a fluorescent lamp, including means for providing a low frequency alternating current to the fluorescent lamp, these means being controlled by a controllable switched-mode current source operating at high frequency. 
     According to an embodiment of the present invention, the switched-mode current source is formed by a switch connected in series with a measurement resistor between two rectifying output terminals of a bridge for rectifying a low frequency a.c. supply voltage, the switch being controlled by a circuit based on a measurement of the current in the resistor. 
     According to an embodiment of the present invention, the switched-mode current source is supplied between the two rectifying output terminals of the bridge. 
     According to an embodiment of the present invention, the lamp is connected between a first terminal of the low frequency a.c. power supply and a first a.c. input terminal of the bridge, the other a.c. input terminal of which is connected to a second terminal of the a.c. power supply. 
     According to an embodiment of the present invention, a capacitor having a low value is connected in parallel with a series connection of an inductance having a low value and the lamp. 
     According to an embodiment of the present invention, an inductive element is connected in series with the switch between the rectifying output terminals of the bridge. 
     According to an embodiment of the present invention, the bridge comprises, between its first a.c. input terminal and each of the rectifying output terminals, an inductive element. 
     According to an embodiment of the present invention, the bridge comprises a first inductive element in series with a first diode of the bridge between the second terminal of the a.c. power supply and a first rectifying output terminal, and a second inductive element in series with a second diode between the first a.c. input terminal and the first rectifying output terminal. 
     According to an embodiment of the present invention, the control circuit includes means for recovering the power of the inductive element(s) when the switch is open. 
     According to an embodiment of the present invention, said means include an inverter controlled at the frequency of the a.c. supply voltage and likely to operate an inductive element in free wheel mode during the opening periods of the switch. 
     According to an embodiment of the present invention, the first inductive elements form respective primary windings of two transformers for isolating the lamp with respect to the a.c. supply voltage. 
     According to an embodiment of the present invention, each transformer includes a first secondary winding for heating a filament of the lamp. 
     According to an embodiment of the present invention, each transformer further includes a second secondary winding for starting the lamp by forming, with a capacitor connected between first terminals of the filaments, a charge pump circuit. 
     According to an embodiment of the present invention, each transformer includes a third secondary winding for supplying the lamp once started. 
     According to an embodiment of the present invention, the control circuit includes means for varying the light intensity of the fluorescent lamp. 
    
    
     The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2, previously described, are meant to show the state of the art and the problem to solve; 
     FIG. 3 shows a first embodiment of a fluorescent lamp control circuit according to the present invention; 
     FIG. 4 shows a second embodiment of a fluorescent lamp control circuit according to the present invention; 
     FIGS. 5A to  5 D illustrate, in the form of timing diagrams, the operation of a control circuit according to the present invention; 
     FIG. 6 shows an embodiment of an inverter of a control circuit such as shown in FIGS. 3 and 4; and 
     FIG. 7 shows a third embodiment of a fluorescent lamp control circuit according to the present invention, including a starting function. 
    
    
     DETAILED DESCRIPTION 
     The same elements have been designated with the same references in the different drawings. For clarity, only those elements of the supply circuit which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. 
     FIG. 3 shows a first embodiment of a circuit that supplies a fluorescent lamp T according to the present invention. 
     Such a circuit is formed of a bridge  10  of diodes D 1 , D 2 , D 3 , D 4 , providing on two output terminals  12 ,  11 , a rectified a.c. voltage. A first a.c. supply terminal of the bridge is connected to a first terminal  3  that receives an a.c. supply voltage, for example, mains voltage Vac. According to the embodiment of FIG. 3, a second a.c. supply terminal  30  of bridge  10  is connected to a second terminal  4  of the mains power supply, at least via lamp T. 
     Thus, a characteristic of the present invention according to this embodiment, which fundamentally distinguishes it from a conventional control circuit such as shown in FIG. 2, is that lamp T is connected to a.c. voltage terminals of the bridge and not to rectified voltage terminals. 
     According to the present invention, the current limitation in the lamp is performed by a switched-mode current source associated with a current measurement resistor Rs. This current source is essentially formed of a switch  31  (for example, a MOS transistor, a bipolar transistor, etc.) connected in series with resistor Rs between rectifying output terminals  12  and  11  of bridge  10 . Switch  31  is controlled by an electronic circuit  32  based on voltage Vr measured across resistor Rs. Circuit  32  is supplied between terminals  12  and  11 . In the embodiment shown in FIG. 3, an inductive element  33 , here, the primary winding of a transformer  39 , is interposed in series between terminal  12  and switch  31 . 
     An inductance L′ is interposed in series between terminal  30  and the first filament f of lamp T. According to the present invention, this inductance L′ is of low value (for example, on the order of 1 to 10 mH) since its function is not, conversely to a circuit such as shown in FIG. 1, to limit the current in lamp T, but to curb the current peak associated with the presence of a capacitor C 1  between terminals  30  and  4 . 
     During a positive halfwave of supply voltage Vac and while switch  31  is closed, a current flows through diode D 1 , winding  33  of transformer  39 , resistor Rs, and diode D 3 , and thus charges capacitor C 1 . When this current reaches a reference value, known by circuit  32  and measured by proportion of voltage Vr, switch  31  opens and, in a simplified embodiment (not shown), capacitor C 1  discharges. 
     In the embodiment shown in FIG. 3, winding  33  of the transformer is used to store power and to slow down the current slope in resistor Rs at each closing of switch  31 . Upon opening of switch  31 , one of secondary windings  34 ,  35 , is used to recover the reactive power to limit the power dissipation. In this case, upon opening of switch  31 , the energy stored in winding  33  is transferred to capacitor C 1  by one of secondary windings  34 ,  35 , of the transformer, according to the considered halfwave. 
     A first secondary winding  34  is connected in series with a diode D 6  between terminal  30  and a first terminal  36  of an inverter  37  having its second terminal  38  connected in series with a second secondary winding  35  and a diode D 7  to the same terminal  30 . The common node of inverter  37  is connected to terminal  4  and diodes D 6  and D 7  are connected reversely with respect to each other. Inverter  37  is controlled at the frequency of a.c. voltage Vac and alternately switches from a position where it connects terminal  30  to terminal  4  via diode D 7  and winding  35  and a position where it connects terminal  4  to terminal  30  via winding  34  and diode D 6 . 
     Assuming that inverter  37  is closed on contact  36  during a positive halfwave, the power stored in winding  33  before opening of switch  31  is transferred to capacitor C 1  by winding  34  through diode D 6 . 
     Similarly, if inverter  37  is closed on contact  38  during the negative halfwave, a current flows in diode D 2 , resistor Rs, switch  31 , winding  33 , and diode D 4  during closing periods of switch  31  and the power stored in winding  33  is transferred, upon opening of switch  31 , by winding  35  through diode D 7 . 
     The value to which the current in lamp T is limited by means of a system according to the present invention is set by the opening threshold of switch  31  by means of control circuit  32 . It should be noted that circuit  32  is designed so that the frequency of the closing cycles of switch  31  is much higher than the frequency of the a.c. power supply. For example, circuit  32  is designed so that switch  31  is closed at a 100 -kHz frequency, its opening being triggered at each switched-mode period, by measurement of the current flowing through resistor Rs. 
     Thus, capacitor C 1  can be of low value (for example, on the order of one hundred nanofarads). In the embodiment described hereabove, fluorescent lamp T is associated with a conventional starter  5 . 
     The system described hereabove generates a low frequency a.c. current which is adapted to supplying a fluorescent lamp. Inductance L′ smooths the residual high frequency component of the current supplied to the lamp. The latter can thus be of low size/value. It should be noted that inductance L′ also intervenes upon the operation of starter  5 . 
     It should be noted that the two secondary windings  34  and  35  of transformer  39  must be of the same size and must be adapted to the nominal voltage of the fluorescent lamp. However, it is not required that the size of secondary windings  34  and  35  corresponds to half the size of primary winding  33 . For example, considering a primary winding of 220 spirals and a lamp designed to have a nominal operation at 90 volts, secondary windings  34  and  35  include on the order of 90 spirals each. 
     FIG. 4 shows a second embodiment of a circuit of control of a florescent lamp T according to the present invention. 
     This embodiment differs from that shown in FIG. 3 in that the function of recovery of the reactive power upon opening of the switch (here, symbolized by a MOS transistor M) is no longer performed by a transformer but by inductances  40 ,  41 . 
     In the embodiment shown in FIG. 4, two inductances  40  and  41  are respectively connected in series with a diode D 4 , D 3  of rectifying diode bridge  10 ′. The respective midpoints of the series associations of the diodes, respectively, D 3 , D 4 , and of the inductances, respectively,  41 ,  40 , are connected to the respective terminals  36 ,  38  of inverter  37  via the diodes, respectively D 6 , D 7 . 
     As in the first embodiment, a circuit  32  of control of switch M measures voltage Vr across a resistor Rs connected in series with switch M between terminals  12  and  11  of bridge  10 ′. The rest of the circuit is not modified with respect to the assembly described in relation with FIG.  3 . 
     During a positive halfwave, the current flows, when switch M is closed, through diode D 1 , through transistor M, through resistor Rs, through diode D 3 , and through inductance  41  to charge capacitor C 1 . Upon opening of the switch, that is, as transistor M is turned off, the current then flows in free wheel mode through inductance  41  and diode D 6 , inverter  37  being closed on contact  36  during positive halfwaves. The same line of argument applies during negative halfwaves, through inductance  40 , diode D 4 , transistor M, resistor Rs, and diode D 2  during periods of switch closing and free-wheeling through inductance  40  and diode D 7  when the switch is open, inverter  37  being closed on contact  38  during negative halfwaves. 
     The choice of the first or second embodiment depends, for example, on the number of inductive elements desired in the circuit and/or on the availability of a high frequency transformer (FIG.  3 ). 
     The operation of a control system such as shown in FIGS. 3 and 4 is illustrated by FIGS. 5A to  5 D. FIGS. 5A and 5B respectively show the shapes of voltage V T  and of current I T  of the fluorescent lamp. It is assumed that the lamp is in nominal operation, that is, that starter  5  is open. FIGS. 5C and 5D respectively illustrate the closing periods of switch  31  and current Is through measurement resistor Rs. In FIG. 5C, the closing periods of switch  31  have been symbolized by states  1  and the opening periods by states  0 . 
     As illustrated in FIGS. 5C and 5D, switch  31  is controlled at a high frequency, for example, on the order of 100 kHz, and current Is has the shape of a saw-tooth train, the amplitude of which is set by the predetermined threshold, known by circuit  32 . Here, the current damping function in resistor Rs, performed by the inductive element ( 33 , FIG.  3 — 40  or  41 , FIG. 4) in series with the switch, which slows down the current slope upon closing of switch  31  or M, can be seen. 
     As concerns fluorescent lamp T, voltage V T  (FIG. 5A) is limited to a value VO depending on the power of the fluorescent lamp. For example, for a fluorescent lamp of a 20 -watt power, which results in a nominal operating voltage on the order of 50 volts RMS, control circuit  32  is then sized to provide a mean current on the order of 400 milliamperes RMS over one period of supply voltage Vac. 
     As illustrated in FIG. 5B, current I T  in lamp T is at the frequency of a.c. supply Vac by being smoothed by inductance L′. The delay Δt present at each halfwave beginning corresponds to the time taken by voltage Vac to reach the lamp starting voltage (for example, its limiting voltage VO). 
     An advantage of the present invention, in particular with respect to a conventional circuit such as shown in FIG. 2, is that the control system takes current substantially over the entire halfwave of the a.c. voltage. Accordingly, the present invention considerably reduces the current harmonics due to the current peaks occurring upon each halfwave in a conventional system. In an optimized embodiment, circuit  32  is designed to modulate the current taken from the mains, for example, according to a sine wave in phase with the mains voltage. Such a modulation especially improves the power factor and/or obtains a dimming of the light intensity, as will be seen hereafter. 
     It should be noted that other means of control of the switched-mode current source so implemented and, in particular, other means of current detection than resistor Rs, may be provided. 
     FIG. 6 shows an embodiment of an inverter  37  controlled at the frequency of the mains a.c. voltage. It should however be noted that other adapted inverter structures may be provided. 
     In the example shown in FIG. 6, an embodiment of the control system conforming to that of FIG. 3 has been considered. However, the respective positions of diodes D 6  and D 7  and of windings  34  and  35  with respect to terminals  30  and  4  have been inverted. This has no effect upon the operation since these elements are, anyways, connected in series. 
     The inverter here is formed by two thyristors Th 1 , Th 2 , respectively with a cathode gate and with an anode gate, connected in series with secondary windings  34  and  35 . Thus, the cathode of thyristor Th 1  is connected to terminal  30  of the bridge ( 10 , FIG. 3) and its anode is connected to a first end of secondary winding  34 , a second end of which is connected to terminal  4  via diode D 6 . The anode of thyristor Th 2  is connected to terminal  30  and its cathode is connected to terminal  4  via the series connection of winding  35  and of diode D 7 . The respective cathode and anode gates of thyristors Th 1  and Th 2  are connected, each via a resistor R, to a.c. supply terminal  3 . Resistors R are of high value (for example, on the order of several hundred kΩ) to limit the current in the gates of thyristors Th 1  and Th 2 . 
     In positive halfwaves, thyristor Th 1  turns on as soon as its gate current is sufficient to trigger it, while thyristor Th 2 , which is an anode-gate thyristor, cannot be turned on due to the direction of its gate current. Conversely, in negative halfwaves, thyristor Th 1  remains off while thyristor Th 2  is turned on. 
     An inverter directly controlled by the mains power supply has thus been implemented. 
     Of course, inverter  37  at the frequency of the a.c. supply voltage can be implemented by other means than those illustrated as an example in FIG.  6 . 
     An advantage of the present invention is that it replaces a heavy and bulky ferromagnetic limiting circuit (FIG. 1) with an active electronic circuit of low bulk in which all components can be integrated. Indeed, the respective values of the inductive and capacitive elements are perfectly adapted to such an integration. In particular, the inductive elements have values on the order of one mH. 
     Another advantage of the present invention with respect to the electronic limiter illustrated in FIG. 2 is that it does not require a high value electrolytic capacitor. Indeed, capacitor C 1  is, according to the present invention, a capacitor of a value on the order of one hundred nanofarads. The absence of use of an electrolytic capacitor considerably improves the lifetime of the control system. 
     Another advantage of the present invention is that it requires only a single power transistor operating at high frequency. 
     To implement a light intensity dimming function by means of a control circuit of the present invention, it is enough to modify the duration of the closing periods of switch  31  between a minimum duration resulting in a minimum power and a maximum duration provided, with a sufficient security margin, to be adapted to the maximum nominal voltage that the fluorescent lamp can withstand. 
     The practical implementation of a light dimming system is within the abilities of those skilled in the art according to the functional indications given hereabove. For example, a variable resistor (potentiometer) may be introduced in control circuit  32  to modify the measurement voltage used by this circuit and thus modify the duration of the switch closing periods. 
     FIG. 7 shows a third embodiment of a fluorescent lamp control circuit according to the present invention. The embodiment illustrated in FIG. 7 is particularly well adapted to a control circuit in which a power dimming is performed and is meant to replace, not only the conventional current limiter (ballast), but also the starter. 
     This circuit also provides a controllable switched-mode current source  31 ,  32 , operating at high frequency. This current source is, as in the previous embodiments, for example formed of a switch  31  connected in series with a resistor Rs of measurement of the current between rectified output terminals  12  and  11  of a diode bridge  10 ″. Switch  31  is, as previously, controlled by an electronic circuit  32  based on voltage Vr measured across resistor Rs. 
     In the embodiment shown in FIG. 7, rectifying bridge  10 ″ includes, in two of its branches, a primary winding of a transformer associated with the halfwaves concerned by the branch. Thus, the anode of diode D 1  of bridge  10 ″ is connected to terminal  3  that receives a.c. voltage Vac, while the cathode of diode D 1  is connected to a first terminal of a winding  33   p  of a first transformer  50 , the second terminal of which forms positive output terminal  12  of the rectified power supply. Similarly, the anode of diode D 4  is connected to the second a.c. supply terminal  30  of bridge  10 ″ (connected to the second terminal  4  of the mains power supply), the cathode of diode D 4  being connected to a first terminal of a primary winding  33   n  of a second transformer  51  associated with the negative halfwaves and the second terminal of which is connected to terminal  12 . The rest of the bridge (diodes D 2 , D 3 ) is similar to a conventional diode bridge such as shown in FIG.  3 . 
     Thus, on the mains side, the assembly of FIG. 7 essentially differs from the assembly of FIG. 3 by the use of two transformers  50 ,  51  respectively associated with the positive and negative halfwaves of the mains voltage. 
     According to the embodiment of FIG. 7, the circuit portion which will be described hereafter and which is associated with the secondaries of transformers  50  and  51  is isolated from the mains. Each secondary of transformers  50  and  51  includes three windings, respectively  52   p,    53   p,    54   p,  and  52   n,    53   n,    54   n.  Windings  53   p  and  54   p  (respectively  53   n  and  54   n ) have a common node  55   p  ( 55   n ). Winding  52   p  ( 52   n ) has no common node with the other secondary windings. Finally, windings  54   p  and  54   n  have a common node  56 . 
     The structure of the circuits, on the secondary side of the transformers, is similar for each transformer respectively associated with the positive or negative halfwaves. The components involved in the positive halfwaves are identified by letter p. The identical components involved in the negative halfwaves have the same reference associated with letter n. 
     The circuit shown in FIG. 7 enables starting of lamp T by a charge pump operation using windings  53  and  54 . This circuit corresponds, for each transformer  50  and  51 , to a so-called “flyback” converter. 
     The midpoint  55   p  ( 55   n ) of windings  53   p  and  54   p  ( 53   n  and  54   n ) is connected, via two series diodes D 8   p,  D 9   p  (D 8   n,  D 9   n ), to a first terminal  1  ( 1 ′) of a filament f (f′) of lamp T. Terminal  1  ( 1 ′) is connected, via winding  52   p  ( 52   n ) mounted in series with a diode D 10   p  (D 10   n ), to the second terminal  2  ( 2 ′) of the filament involved f (f′). A storage capacitor C 2  is connected between terminals  1  and  1 ′. It should be noted that this capacitor is, according to the present invention, of a value on the order of some ten nanofarads and thus does not need to be electrolytic. The terminal of winding  53   p  ( 53   n ), opposite to node  55   p  ( 55   n ), is connected to the midpoint of the series connection of diodes D 8   p  and D 9   p  (D 8   n  and D 9   n ) via a capacitor C 3   p  (C 3   n ) of low value (on the order of some hundred picofarads). 
     For each halfwave, the circuit associated with the secondaries of one of the transformers is closed via a thyristor, respectively Thp or Thn, connected between the terminal of capacitor C 2  opposite to the terminal of filament  1  ( 1 ′) involved in the halfwave, and midpoint  56  between windings  54   p  and  54   n.  Thus, the anode of thyristor Thn is connected to terminal  1 . The anode of thyristor Thp is connected to terminal Th 1 ′. The respective cathodes of thyristors Thn and Thp are connected to node  56 . To control thyristors Thp and Thn, their respective gates are connected to the terminal of capacitor C 2  opposite to that connecting their anode, via a control resistor, respectively R 1   n,  R 1   p.  The gate of each thyristor is also connected to node  56  via a resistor R 2   p,  R 2   n  forming, with the resistor R 1   p,  R 1   n  involved, a resistive dividing bridge between node  56  and terminal  1 , respectively  1 ′, of lamp T. 
     The operation of the circuit illustrated in FIG. 7 will be described in relation with a positive halfwave. The operation during negative halfwaves can be easily induced from the following description. 
     It is assumed that lamp T is cold, that is, that it behaves as an open circuit. In this case, upon each closing of switch  31 , a current increases linearly in primary winding  33   p  of transformer  50 . The secondary and primary windings are connected so that diode D 8   p  is blocked (non-conducting) when switch  31  is conductive. During this closing period of switch  31 , no current flows in the secondaries of transformer  50 . The opening of switch  31  causes the inversion of the voltage direction across the secondary windings of transformer  50 . Diodes D 8   p  and D 9   p  then become conductive and the power stored in transformer  50  is transmitted to capacitors C 3   p  and C 2 . The current, in the secondary windings, decreases linearly until it cancels. Switch  31  is closed back and this cycle starts again. During opening periods of switch  31  and during the positive halfwaves, thyristor Thp is on, while thyristor Thn is off. It should be noted that charge pump capacitor C 2  stores power and sees the voltage thereacross increase during positive halfwaves as well as during negative halfwaves. 
     It should be noted that, as previously, as long as the voltage across lamp T has not reached a sufficient value and as long as filaments f and f′ are not hot enough, the lamp behaves as an open circuit. During positive halfwaves and during periods of opening of switch  31 , a current flows through winding  52   p  in diode D 10   p  to heat up filament f. When the voltage across capacitor C 2  becomes sufficient, lamp T starts. Once the lamp is started, the voltage across winding  52   p  decreases. Accordingly, the heating current of filament f decreases without, however, stopping. 
     As long as the lamp has not been started, the voltages of the secondary windings of the transformers depend on the respective number of spirals of the windings. Attention will be paid to having a higher voltage for winding  53   p  than for winding  54   p,  and to having the lowest voltage for winding  52   p.  Further, winding  54   p  will be provided to be made with a wire of sufficient cross-section, since this winding has to be designed to carry the nominal current of lamp T once started. 
     It should be noted that, during the preheating of lamp T, the voltages across the secondary windings are higher than in the nominal state. This is not disturbing since, as long as it has not been started, the lamp takes no current. As a specific example of implementation, winding  52   p  of preheating of filament f has a voltage from 8 to 10 volts as long as the lamp has not been started. This voltage drops to 4 volts once the lamp is started. Winding  54   p  has a voltage of some hundred volts. Winding  53   p  has the highest voltage among the secondary windings, for example, on the order of 500 to 1000 volts, as long as the lamp has not been started. 
     An advantage of the embodiment illustrated in relation with FIG. 7 is that it is particularly well adapted to a light dimming function. Indeed, due to the presence of windings  52   p,    52   n  of pre-heating of filaments f and f′, any flickering of lamp T by its cooling down in case of a decrease of the light intensity, and thus of a decrease in the power transmitted to the secondaries of the transformers, is avoided. With a conventional starting circuit which operates according to a thermal principle, the starting circuit causes untimely and random failures of the lamp. 
     It should be noted that transformers  50  and  51  which have been shown in FIG. 7 with a common node  12  of primary windings  33   p  and  33   n  are particularly well adapted to the implementation of the circuit according to the present invention by means of an integrable transformer. It could however be provided, as an alternative, to associate each primary of transformers  50  and  51  to the anode of diodes D 1  and D 4 , respectively. Node  12  then becomes the anodes of diodes D 1  and D 4  again, as in the preceding drawings. The circuit operation is not modified for all this with respect to that described in relation with FIG.  7 . 
     It should be noted that the embodiment illustrated by FIG. 7 has the same advantages as the preceding embodiments as concerns the absence of use of a high inductance and the absence of use of an electrolytic capacitor. Further, the circuit of FIG. 7 corresponds to a complete electronic “ballast” (starting, preheating, dimming possibility). Further, this circuit applies particularly well to the case where lamp T is desired to be isolated from the mains. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the respective sizings of the different components used will be adapted according to the power of the fluorescent lamp. Further, the assemblies described hereabove as an example can be modified provided that they respect the functional characteristic of creating a high frequency switched-mode current source while providing a low frequency a.c. current to the fluorescent lamp. 
     Further, although, in the foregoing description of FIGS. 3 and 4, the connection, in series with inductance L′, of a fluorescent lamp T in parallel with starting circuit  5  has been considered, the control circuit of the present invention can also be used to connect, in series with inductance L′, a fluorescent lamp already associated with a conventional electronic control circuit (FIG.  2 ). In other words, the embodiments of FIGS. 3 and 4 can be implemented as the complement of a conventional electronic “ballast”, in particular, if the latter uses bipolar transistors and an element of control transformer type. The present invention then offers the advantage of providing a light dimming function by means of a simple circuit of low bulk. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.