Abstract:
Herein described is a driver circuit of a fluorescent lamp having a first and a second electrode and igniting when the voltage between the first and second Electrode exceeds a given threshold voltage. The driver circuit comprises an inductance coupled to a supply voltage and to a terminal of the first electrode a first condenser coupled to the other terminal of the first electrode and to a terminal of the second electrode, a control device comprising a first and a second system of switches capable of guaranteeing oscillations of a voltage signal on the inductance and on the first condenser up to the ignition of the lamp. The driver circuit comprises a device associated to the control device and capable of acting on the first system of switches so as to regulate the frequency of the oscillations from a frequency greater than the resonance frequency of the inductance and of the first condenser to the same resonance frequency so as to guarantee a preheating of the first and second electrodes. The device is sensitive to the depletion of gas of the lamp and is capable of sending a turn-off signal to the control device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention refers to a fluorescent lamp driver circuit. 
     Fluorescent lamps are generally composed of a glass tube  1  which contains fluorescent gas  2  and at the ends of which there are two electrodes F 1  and F 2 , as shown in FIG.  1 . Under normal conditions the lamp is assimilable to an open circuit and presents an infinite impedance between its electrodes, as shown in diagram I(V) of FIG.  2 . If the voltage between its electrodes exceeds a threshold voltage Vth the lamp ignites as there is an ionization of the gas  2  by means of emission of electrons by the two electrodes. The value of the voltage Vth depends on the temperature of the two electrodes given that, at equal voltage applied, as the temperature increases the quantity of electrons emitted increases and therefore at a higher temperature of the electrodes corresponds a voltage Vth of lower value. Once the threshold voltage Vth is exceeded the gas  2  changes state and the tube  1  becomes assimilable to a resistive load; so that said condition remains it is necessary to supply a small current Imin. 
     Fluorescent lamps must be driven by circuits that are capable of permitting their turning on and off, like the circuit shown in FIG. 3. A first block  31  converts an alternating voltage, generally a mains voltage, into a direct voltage between Vdd and ground Gnd. To a terminal of the voltage Vdd is connected a resistor R 2  connected in turn to an inductance L 1  connected with a terminal P 1  of the electrode F 1  of the fluorescent lamp. The other terminal P 2  of the electrode F 1  is connected to a terminal of a capacitance C 3  connected in turn to a terminal P 4  of the electrode F 2 ; the other terminal P 3  of the electrode F 2  is connected to a condenser C 1  connected to the voltage Vdd and to a condenser C 2  connected to ground Gnd. The inductance L 1  and the condenser C 3  form a circuit L-C series. Two secondaries Ls 1  and Ls 2  are wound on the inductance L 1  that transfer the state of the circuit L-C to the control logic formed by two blocks  32  and  33 . The block  32  comprises a diode DIAC  34  connected to a resistor R 1  connected to the voltage Vdd and connected to a condenser C 4  having the other terminal grounded Gnd; the DIAC  34  is capable of giving a first impulse to a system of switches  35  then disabling itself. The system of switches  34  acts so that the circuit L-C series begins to oscillate at a resonance frequency given by          f                 r        1     2                 Σ                 L1C3         ,                          
     which generally has a value comprised between 60 Khz and 70 Khz, and there will be a square wave signal of frequency fr and amplitude Vdd in a node PC on the terminals in common of the inductance L 1  and of the resistor R 2 . The resonant circuit L-C shall determine overvoltages on the condenser C 3  such that after a few cycles of oscillation the value of the threshold voltage Vth is exceeded causing the ignition of the fluorescent lamp. Between the nodes PC and P 3  there will no longer be the resonant circuit L-C but a circuit R-L 1  where R is the resistor of the fluorescent lamp and the control logic inside the blocks  32  and  33  will determine the working frequency, generally between 30 Khz and 50 Khz. The block  33  is similar to the block  32  but does not comprise the diode Diac  34  and instead comprises a system of switches  101  similar to the system of switches  35  of the block  32 . 
     In the place of the two blocks  32  and  33  and of the respective inductances Ls 1  and Ls 2  an integrated circuit  41 , as shown in FIG. 4, that controls the operations described above, can be inserted in the circuit of FIG.  3 . 
     THE SUMMARY OF THE INVENTION 
     To increase the life of the fluorescent lamp a function of preheating of the electrodes F 1  and F 2  is required, in the phase prior to the ignition of the lamp; said preheating of the electrodes F 1  and F 2  enables them to be more emissive and to obtain a threshold voltage Vth of lower value. A circuit  51  that implements the preheating function is shown in FIG. 5; said circuit  51  comprises a condenser C 6  inserted between the terminal P 4  and a terminal of the condenser C 3  and placed in parallel with a block PTC that comprises a resistor variable with the temperature. The preheating function is carried out by passing a current through the electrodes F 1  and F 2  that is the same as the current that passes through the oscillating circuit L-C where at the beginning, seeing the PTC is a low impedance, the C corresponds to C 3  which is chosen sufficiently big so as not to generate high voltages near the threshold voltage Vth. The heating of the PTC causes an increase of its resistance and after a certain time it presents an infinite impedance at the limit. In this case the capacitor C of the resonant circuit L-C is given by the series of condensers C 3  and C 6  and the value of C 6  must be chosen much lower than C 3 . The capacitive impedance is high and such as to generate at its ends a higher voltage than the threshold voltage Vth and therefore ignites the lamp. 
     In particular applications where the replacement of the worn fluorescent lamp is envisaged, a protection function of the lamp driver circuit called “End of life” is required. In fact if the gas in the lamp is depleted, the lamp will never ignite and the driver circuit will remain in perpetual free oscillation with high overvoltages and overcurrents that lead to the destruction of the driver circuit of the lamp. 
     In FIG. 6 a driver circuit is shown in which the End of Life function is made by means of a block  61  comprising a series of elements connected in series between the terminal P 1  and ground Gnd: a condenser C 7 , a diode Zener Dz 1 , a resistor R 3 , a condenser C 8 . A condenser Cp is placed in parallel with the elements Dz 1 , R 3  and C 8 , while a resistor R 4  is placed in parallel with only the condenser C 8 . A diode Zener Dz 2  is connected to the terminal in common with the elements R 3  and C 8 , connected to a turn-off block of the type SCR  62  connected to the block  32 , to the terminal in common of the elements R 1  and C 4  and a ground. With the lamp ignited the voltage value on the terminal P 1  is low and therefore the voltage value on the element Cp does not exceed the breakdown voltage BVDz 1  of the diode Dz 1 . In the phase prior to the ignition of the lamp the terminal P 1  reaches high voltage values, generally between 0,8 KV and 1 KV, and therefore the voltage on the element Cp manages to exceed the voltage BVDz 1 ; in that case current flows through the condenser C 8  whose value is limited to the resistor R 3 . The time constant of R 3  and C 8  is a very few seconds, generally 2 or 3 seconds, therefore in conditions of normal ignition with or without preheating the voltage at the ends of the element C 8  does not exceed the breakdown voltage BVDz 2  of the diode Dz 2 . In condition of gas depleted in the lamp the driver circuit remains in perpetual free oscillation and that implies that on the condenser C 8  current continues to flow; the voltage at the ends of C 8  exceeds the voltage BVDz 2  and that enables the activation of the turn-off block  62 . This block causes the turn-off of the block  32  causing the stop of the oscillations and impedes the voltage on the element C 4  from reaching an ignition value of the block  32 . 
     In view of the state of the technique described, object of the present invention is to present a driver circuit for fluorescent lamps which is simpler than known circuits and carries out the functions of preheating and end of life. 
     In accordance with the present invention, said object is reached by means of a driver circuit of a fluorescent lamp having a first and a second electrode and igniting when the voltage between said first and second electrode exceeds a given threshold voltage, said driver circuit comprising a inductance coupled to a supply voltage and to a terminal of said first electrode, a first condenser coupled to the other terminal of said first electrode and to a terminal of said second electrode, a control device comprising a first and a second system of switches capable of guaranteeing oscillations of a voltage signal on said inductance and on said first condenser up to the ignition of said lamp, characterized in that it comprises a device associated to said control device and capable of acting on said first system of switches so as to regulate the frequency of said oscillations from a frequency greater than the resonance frequency of said inductance and of said first condenser to the same said resonance frequency so as to guarantee a preheating of said first and second electrode, said device being sensitive to the depletion of gas of said lamp and being capable of sending a turn-off signal to said control device. 
     Thanks to the present invention a driver circuit for fluorescent lamps can be made which is simpler than the known circuits and comprises less expensive elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the present invention will appear evident from the following detailed description of an embodiment thereof, illustrated as non-limiting example in the enclosed drawings, in which: 
     FIG. 1 is a schematic view of a fluorescent lamp; 
     FIG. 2 is a graph of the current of the lamp of FIG. 1 in function of the voltage on the electrodes of the lamp; 
     FIG. 3 is a first circuit diagram of a driver circuit for the lamp of FIG. 1 according to the known art; 
     FIG. 4 is a second circuit diagram of a driver circuit for the lamp of FIG. 1 according to the known art; 
     FIG. 5 is a circuit diagram of the driver circuit of FIG. 4 with a device for implementing the preheating function; 
     FIG. 6 is a circuit diagram of the driver circuit of FIG. 5 with a device for implementing the end of life function; 
     FIG. 7 is a circuit diagram of a driver circuit for the lamp of FIG. 1 according to a first embodiment of the invention; 
     FIG. 8 is a circuit diagram of the preheating device and the end of life device of the circuit of FIG. 7; 
     FIG. 8 a  is an example of circuit implementation of the block  32  of the circuit of FIG. 7; 
     FIG. 8 b  is an example of circuit implementation of the block  33  of the circuit of FIG. 7; 
     FIG. 9 is a more detailed circuit diagram of the device of FIG. 8; 
     FIG. 10 is a circuit diagram of a driver circuit for the lamp of FIG. 1 according to a variant to the first embodiment of the invention; 
     FIG. 11 is a detailed circuit diagram of only the preheating device of FIG.  10 . 
    
    
     DETAILED DESCRIPTION 
     A circuit diagram of a driver circuit for the lamp of FIG. 1 according to a first embodiment of the invention is shown with reference to FIG.  7 . said driver circuit is similar to the circuit of FIG. 3 except for the presence of a device  71  capable of carrying out the functions of preheating and end of life; the same circuit elements will be indicated by the same numeric references. The device  71  is connected to the inductance Ls 1  of the secondary and to the block  32  comprising a system of switches  35  whose closing and opening frequency fint is controlled by the device  71 . Said device  71  is capable of sending a Power off signal to turn off the block  32  and to impede Diac  34  from sending pulses for new oscillations of the system. 
     The device  71  of FIG. 7 is shown in FIG. 8 which comprises a block  81  which constitutes the only preheating circuit while the entire device  71  also carries out the end of life function. 
     The block  81 , which is drawn with a dotted line in FIG. 8, comprises: a comparator Comp 1  capable of comparing a voltage signal Sec coming from the secondary with the inductance Ls 1  and a reference signal Vref 1 . The signal in output from the comparator Comp 1  controls a switch  86  capable of connecting a resistor R 10 , connected to the secondary Ls 1 , to an electrolytic capacitance Cel of a high value connected to ground. The capacitor Cel is connected to a block  83 , capable of controlling the frequency fint opening and closing the system of switches  35  that guarantees a square wave signal of frequency f on the node PC. The block  83  therefore controls said frequency f in dependency on a voltage Vc at the ends of the capacitor Cel; more precisely the block  83  starting from a frequency f greater than that of oscillation fr of the circuit L-C series formed by the inductance L 1  and by the capacitor C 3 , tends to lower said frequency f in function of the voltage Vc up to arrive at said frequency of oscillation fr. In this manner given that at frequencies f greater than the frequency of oscillation fr the voltage on the condenser C 3  is lower than the voltage Vth of ignition of the fluorescent lamp, it is possible to activate a preheating of the filaments F 1  and F 2  of the lamp by means of the same current that flows in the circuit L 1 -C 3 . Said heating has a duration that is chosen on the basis of the dimensions of the capacitor Cel, for example 0,5 s, 1 s or 2 s. The block  83  has a characteristic which is inversely proportional to the voltage Vc, that is f inversely proportional to Vc, and tends to lead to f being equal to fr; with f=fr we have, after several cycles of oscillation, the ignition of the lamp. Once the ignition of the lamp has come about, the voltage Sec on the secondary diminishes to the same degree the current that circulates on the inductance L 1  diminishes; the capacitor Cel can discharge through a resistor Rs 4  placed in parallel with it and of high value when the circuit is no longer supplied by the mains voltage. 
     The device  71  of FIG. 8 comprises in addition to the block  81  also a comparator Comp 2  capable of comparing the voltage Vc with a reference voltage Vref 2  where Vref 2 &gt;Vref 1 , and a turn-off block  84  which is supplied through the supply circuit R 1 -C 4  of the Diac  34  of the circuit of FIG. 7, schematized in the figure by a block  340 , and, once activated, sees to turning-off the block  32  by means of the signal Power off. Starting from the condition in which the frequency f is equal to the frequency fr, if the lamp is not ignited (that is in conditions of depleted gas in the lamp), the voltage Sec on the secondary continues to increase and consequently also the voltage Vc increases. When the voltage Vc exceeds the voltage Vref 2  the comparator Comp 2  activates the turn-off block  84  that confirms the load on Cel by means of the retroaction R, turns off the block  32  with the signal Power off and, given that it is supplied through the same supply circuit as the Diac, impedes the Diac to send impulses for new oscillations of the system. 
     An example of circuit implementation of the block  32  of the circuit of FIG. 7 is shown in FIG. 8 a . Said block  32  comprises the diode Diac  34  connected to the secondary Sec and to the mains block  340  and to the anode of a diode D 60  having the cathode connected to the node PC. The block  32  comprises a block of diodes formed by diodes D 10 -D 30  connected in series with the cathode of the diode D 10  connected to ground Gnd and the anode of the diode D 30  coupled by means of the resistor Ri 1  to the secondary Sec, and two diodes D 40 -D 50  connected in series with the anode of the diode D 50  connected to ground and the cathode of the diode D 40  coupled by means of the resistor Ri 1  to the secondary Sec; a resistor Ri 2  is placed in parallel to the diode D 40 . The system of switches  35  comprises a diode D 70  whose anode is connected to the anode of the diode D 40  and whose cathode is connected to the base terminal of a transistor npn Qoff having the emitter terminal grounded Gnd and the collector terminal coupled to the secondary Sec by means of a resistor Rg, a transistor NMOS MPWR having the source terminal connected to ground, the gate terminal connected to the collector terminal of the transistor Qoff, the drain terminal connected to the emitter terminal of a transistor npn QPWR having the base terminal coupled to the secondary Sec by means of a resistor Rb and the collector terminal connected to the node PC, a diode DPWR having the anode connected to the source terminal of the transistor MPWR and the cathode connected to the node PC. 
     An example of circuit implementation of the block  33  of the circuit of FIG. 7 is shown in FIG. 8 b . As can be seen in FIG. 8 b  the block  33  shows the same circuit elements as the block  32  except for the block  34 . 
     A more detailed circuit diagram of the device of FIG. 8 is shown in FIG. 9. A first resistor Rs 1  connected in series to a second resistor Rs 2  connected to ground is connected to the secondary with inductance Ls 1  at the terminal Sec. A diode Zener Dsz 1  is connected in parallel to the resistor Rs 2  and a second diode Zener Dsz 2  is connected to the terminal in common of the resistors Rs 1  and Rs 2  connected in turn to an armature of the capacitor Cel. Another armature of the capacitor Cel is connected to condenser Cf 1  connected to ground and connected to a current generator Ig 1  connected to the block  32 . A third diode Zener Dsz 3  in series to a resistor Rs 7  and a resistor Rs 4  in parallel with the capacitor Cel are connected at the ends of the capacitor Cel. The block  84  is formed by a first bipolar transistor npn Q 1  having the emitter terminal connected to the terminal in common of the capacitors Cf 1  and Cel, the base terminal connected to the terminal in common of the elements Dsz 3  and Rs 7 , and the collector terminal connected both to the base terminal of a second bipolar transistor pnp Q 2  and to a terminal of the resistor Rs 3 . The emitter terminal of the transistor Q 2  is connected to the other terminal of the resistor Rs 3  and to the supply mains R 1 -C 4  of the Diac  34 , schematized in the figure by a block  340 , while the collector terminal is connected to the terminal in common of the elements Dsz 2 , Dsz 3  and Cel. 
     The condenser Cf 1  receives the current Ig 1  added to a current Ir coming from the secondary Sec and passes through the elements R 1 , Dz 2  and the only electrolytic capacitor Cel (it is with a very high value capacitance so much so that at high frequencies its impedance is approximately null) given that the resistor Rs 4  has a high value. The capacitor Cf 1  receiving the current Ig 1 +Ir loads rapidly and as said capacity controls the frequency fint of the system of switches  35  we have on the circuit L 1 -C 3  a square wave with a frequency f greater than the resonance frequency fr of the circuit L-C; that is the inductive part of the current-voltage characteristic of the circuit is worked on and this entails low voltage values on the condenser C 3  such that the lamp is not ignited but a current capable of heating the electrodes F 1  and F 2  is circulated. With the passing of time the passage of the current Ir on the capacitor Cel loads this capacitor which cannot discharge as Rs 4  is a very large resistor and does not permit the discharge of the capacitor Cel in a short time (the capacitor Cel cannot discharge through Rs 2  because the diode Dz 2  does not permit the passage of inverse current). The voltage at the ends of Cel increases and consequently diminishes the value of the current Ir given that the voltage at the ends of the capacitors Cel and Cf 1  becomes similar to the voltage SecRs 2 /(Rs 1 +Rs 2 ); this entails the diminishing of the frequency f by Cf 1  which tends to the resonance frequency fr of the circuit L-C series. When f=fr and after the ignition of the lamp  1 , the voltage on the secondary Ls 1  diminishes as the voltage on the primary L 1  is low; this entails a low partition voltage SecR 2 /(R 1 +R 2 ) and less than the voltage at the ends of the condenser Cel and therefore the current Ir tends to zero. The voltage on the terminal K in common with the capacitor Cel and of the diode Zener Dsz 2  can be considered as the voltage Vref 1  of FIG.  8  and is a variable voltage. The comparator Comp 1 , the resistor R 10  and the switch  86  are implemented by the complex of the elements R 1 , R 2 , Dsz 2  and Cel; in fact according to the value of the voltage SecR 2 /(R 1 +R 2 ) and of the voltage on the node K we have the passage of the current Ir with a determined value. The capacitor Cf 1  implements the block  83  of FIG. 9; in this case the capacitor Cel is not connected to ground but to the capacitor Cf 1  to make use of the current Ir that circulated therein as current that enables the variation of the frequency f (nevertheless instead of the capacitor Cf 1  it is possible to have a device which is sensitive to Vc and capable of varying the frequency f). 
     In the part of the circuit of FIG. 9 that implements the end of life function, the element Comp 2  is given by the diode Zener Dsz 3 ; the block  84  turns on when the voltage at the ends of the capacitor Cel exceed the voltage value Vbe 1 +BVDsz 3 . The connection R of the collector terminal of the transistor Q 2  with positive armature of the capacitor Cel implements a function of loading confirmation. 
     A circuit diagram of a driver circuit for the lamp of FIG. 1 is shown in the FIG. 10 according to a variant to the first embodiment of the invention. Said circuit differs from the circuit of FIG. 7 for the presence of a second preheating circuit  100  connected to the block  33  which enables there to be a form of square wave with a duty-cycle of 50% on the node PC which is obtained making the two capacitors Cf 1  and Cf 2  work alternately. 
     The circuit diagram of the preheating device  100  is shown in FIG. 11 where it can be seen that it consists of block  81  of the circuit of FIG. 9 without the presence of the block  84  and without the third diode Zener Dsz 3  (the terminal of the secondary with inductance Ls 2  is indicated with the same reference used for the secondary with inductance Ls 1 , that is, Sec). In place of the current generator Ig 1  and of the condenser Cf 1  there is the current generator Ig 2  and the condenser Cf 2 . The device  100  is connected to the secondary with inductance Ls 2  and sends a signal in output capable of modulating the frequency of a system of switches  101  of the block  33 . The functioning of the preheating circuit  100  is similar to the functioning of the part of the circuit of FIG. 9 that implements the preheating function.