Abstract:
A driving method for a discharge lamp having two cathodes includes providing a supply input voltage for providing an alternating voltage at the terminals of the cathodes, monitoring a condition of each of the cathodes and measuring a first direct voltage signal of the waveform of the voltage of the lamp that develops when the lamp approaches an ageing condition, deactivating the alternating voltage when a variation of the first direct voltage signal occurs, and supplying a second direct voltage signal proportional to the supply input voltage for deactivating the alternating voltage when a variation of the first direct voltage signal occurs in relation to the second direct voltage signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. national filing of PCT/IT2005/000258 filed May 4, 2005, which is incorporated herein in its entirety by this reference. 
     FIELD OF THE INVENTION 
     The present invention refers to a device for driving discharge lamps, in particular fluorescent lamps. 
     BACKGROUND OF THE INVENTION 
     Fluorescent lamps are typically composed of a glass tube which contains a small quantity of mercury, a low pressure, inert gas and phosphorous powders which coat the inside part of the tube. At the extremities two electrodes are present which, connected to a suitable driving circuit, create the arc that permits the discharge of the gas to be generated and maintained. 
     Among the possible driving circuits the so-called high frequency ballast circuits can be enumerated: these are circuits at whose output an alternating voltage signal is generated at a frequency and amplitude necessary to keep the lamp on; this waveform is produced by a circuit that comprises a couple of transistors that switch at a frequency of tens of KHz, a current limiting coil and a filtering capacitance. 
     When the ageing condition of the lamp approaches, the voltage at the extremities of the same will tend to increase because of the depletion of the emissive coating on the cathodes with the consequent increase in the drop in voltage at their ends. It is common that this phenomenon comes about asymmetrically as one cathode ages before the other; this phenomenon takes the name of “rectifying effect”. 
     The resulting increase of power dissipated in the lamp could lead to an excessive overheating with dangerous consequences such as the fusion of the glass that surrounds the lamp itself; for this reason the ballast circuits must detect this failure condition, when it exceeds a certain level, and undertake suitable preventive measures such as turning off the ballast. 
     Various attempts have been made to prevent the overheating of the lamp due to the ageing such as in the EP patent 0 681 414. In said patent a ballast circuit for a discharge lamp  10  is described having two cathodes ( FIG. 1 ) in which the ballast circuit comprises an inverter  1 , driven by a device  11 , that provides for an alternated voltage at its output terminals; the inverter  1  is fed through a voltage Val coming from a PFC or from a rectification stage  100  fed in turn by the mains voltage Vin. The ballast comprises a circuitry  2  to couple the discharge lamp  10  to said output terminals, another circuitry  3  that measures a direct voltage component Vdc 1  at the ends of the blocking capacitor C 1  placed between the lamp and ground GND and means  4  suitable for deactivating the inverter  1  when the lamp approaches the ageing conditions. In the means  4  the measured direct component Vdc 1  is compared with a signal Vdcm referred to ground GND and produced by means  5 ; when said component Vdc 1  is less or greater than the signal Vdcm by a given value, the means  4  turn off the inverter  1  by acting on the device  11  through a signal Dis. 
     The proposed solution does not consider a problem linked mainly to the variations of the input voltage of the inverter, whether it be the mains voltage rectified or the output of a stage of the power factor correction (PFC). These variations can be due to low values of the input capacity of the inverter, to short interruptions of the mains voltage that cause a voltage drop or to transitory phenomena that cause its variation. In addition an oscillation at a frequency equal to twice the mains voltage frequency is overlaid to the direct value of the input voltage of the inverter; the amplitude of this oscillation is inversely proportional to the value of the capacity (electrolytic) placed downstream of the rectifier stage (normally a diode bridge) or of the PFC. 
     The circuitry proposed in the abovementioned patent also intervenes in presence of one of the abovementioned variations of the inverter input voltage, even though the lamp does not present any type of ageing condition. 
     SUMMARY OF THE INVENTION 
     In view of the state of the technique described, the object of the present invention is to provide a device for driving discharge lamps that overcomes the abovementioned inconvenience. 
     In accordance with the present invention, this object is achieved by means of a driving device for a discharge lamp having two cathodes, comprising first means having a supply input voltage and suitable for providing an alternating voltage at the terminals of said cathodes, second means capable of monitoring a condition of each of said cathodes and suitable for measuring a first direct voltage signal of the voltage waveform of the lamp that is developed when said lamp approaches the ageing condition, third means coupled to said second means and suitable for deactivating said first means when a predetermined variation of said first direct voltage signal occurs, characterised in that it comprises fourth means suitable for supplying to said third means a second direct voltage signal proportional in value to said supply voltage, said third means being suitable for deactivating said first means when a predetermined variation of said first direct voltage signal in relation to said second direct voltage signal occurs. 
     Thanks to the present invention it is possible to produce a driving device for a discharge lamp that prevents the substitution of the discharge lamp in presence of variations of the supply voltage of the same driving device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The characteristics and the 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 circuit diagram of an apparatus for driving a discharge lamp according to the known art; 
         FIG. 2  is a circuit diagram of a device for driving a fluorescent lamp according to the present invention; 
         FIG. 3  is a more detailed circuit diagram of a part of the control device of  FIG. 2 ; 
         FIG. 4  is an even more detailed circuit diagram of a part of the device of  FIG. 2 ; 
         FIG. 5  is a time diagram of signals present in the apparatus of  FIG. 1  according to the known art; 
         FIG. 6  is a time diagram of signals in question in the driving device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIG. 2  a driving device for a discharge lamp, in particular for a fluorescent lamp, according to the present invention is described; the elements equal to those of the circuit shown in  FIG. 1  will be indicated with the same numerical references. The driving device of  FIG. 2 , preferably a ballast circuit, provides for driving a fluorescent lamp  10  having two cathodes. The ballast circuit comprises an inverter  1 , driven by a device  11 , that provides for an alternating voltage on its output terminals; the inverter  1  is fed by a voltage Val coming from a device for the power factor correction (PFC) or from a rectification stage  100  fed in turn by the mains voltage Vin. The ballast circuit comprises a circuitry  2  to couple the fluorescent lamp  10  to said output terminals, another circuitry  3  that measures a direct voltage component Vdc at the ends of the blocking capacitor C 1  placed between the lamp and ground GND and means  40  suitable for deactivating the inverter  1  when the lamp approaches the ageing (“end of life”) conditions, that is when a depletion of the emissive coating of one of the cathodes of the same lamp occurs, so as to prevent excessive heating of said cathode. 
     The driving circuit comprises means  50  suitable for supplying a direct voltage component Vdca depending on said supply voltage Val, more precisely aligned or proportional in value to said supply voltage Val. 
     In the means  40  the measured direct component Vdc is compared with the signal Vdca; when said component Vdc is lower or higher than the signal Vdca by a given value D the means  40  provide for turning off the inverter  1  by acting on the device  11  through a signal Dis. Said given value D is, for example, within a field of variation between 2 and 52 volts. 
     In  FIG. 3  the means  40  and  50  are shown more in detail. The means  50  comprise a device capable of supplying a voltage signal proportional to the voltage Val, preferably a resistive divider comprising two resistors R 51  and R 52  arranged in series between the supply voltage Val and ground GND. The common terminal of the two resistors R 51  and R 52  is the input terminal of a buffer  51  and on said terminal there is the signal Vr; the output terminal of the buffer  51  is the common terminal of two resistors R 1  and R 2  having the other terminals connected respectively to two current generators I 1  and I 2  in turn connected respectively to a supply voltage Vdd and to ground GND. The threshold voltages Vdct 1  and Vdct 2  are taken respectively on the common terminal of the resistor R 1  and of the current generator I 1  and on the common terminal of the resistor R 2  and of the current generator I 2 ; said voltages Vdct 1  and Vdct 2  are in input to the means  40 . The latter comprise a comparator having in input the voltages Vdct 1 , Vdct 2  and Vdc. The threshold voltages Vdct 1  and Vdct 2  represent the reference voltage Vdca of the means  40  that depends on the supply voltage Val; in fact the voltages Vdct 1  and Vdct 2  depend on the voltage Vr that varies in accordance with a variation of the voltage Val. 
     In  FIG. 4  an even more detailed circuit diagram of a part of the device of  FIGS. 2 and 3  is shown. The circuitry  3  comprises a series of two resistors R 31 , R 32  placed at the ends of the capacitor C 1 ; the signal detected on the common terminal of the two resistors R 31 , R 32  is the signal Vdc that is in input to the means  40 , more precisely in input to the comparator. The means  50  comprise a resistive divider comprising two resistors R 51  and R 52  arranged in series between the supply voltage Val and ground GND. The common terminal of the two resistors R 51  and R 52  is the inverting input terminal of an operational amplifier  52  and on said terminal there is the signal Vr; the output terminal of the operational amplifier  52  is the gate terminal of the transistor M 1  having the source terminal connected to ground GND and the drain terminal connected to a terminal of a resistor R 2 . The latter has its other terminal connected to the non-inverting terminal of the operational amplifier  52  and to the resistor R 1  having its other terminal connected to a current generator I 1 ; preferably said current generator I 1  is controlled by a circuit bandgap  53  to obtain a precise current reference. The threshold voltages Vdct 1  and Vdct 2  are taken respectively at the common terminals of the resistor R 1  and of the current generator I 1  and of the resistor R 2  and of the transistor M 1  and are in input to the comparator of the means  40 . The threshold voltages Vdct 1  and Vdct 2  represent the reference voltage Vdca of the means  40  that depends on the supply voltage Val; in fact the voltages Vdct 1  and Vdct 2  depend on the voltage Vr that varies in accordance with a variation of the voltage Val. 
     Preferably the resistors R 31 , R 32 , R 51  and R 52  are sized so that Vdc=Vr when the fluorescent lamp  10  is new. 
     Preferably the current generator I 1  is such that I 1 =f(b) and the resistors R 1  and R 2  are chosen so that R 1 =(L 1 /W 1 )*b and R 2 =(L 2 /W 2 )*b where with L 1 , L 2  the length of the resistive component R 1 , R 2  is indicated and with W 1 , W 2  the width of said resistive component. 
     In  FIG. 5  a time diagram of signals in question in the circuit of  FIG. 1  according to the known art is shown. We note that a variation of the supply voltage Val causes a similar variation of the voltage Vdc 1 . If we consider that the reference signal Vdcm is constituted by two fixed threshold voltages, a threshold voltage exceeding Vt 2  and a threshold voltage lower than Vt 1 , we note how the variation of the voltage Vdcm causes a lowering of the same voltage Vdc 1  below the threshold voltage Vt 1 . This causes the signal Dis to be sent to deactivate the device  11 . In this manner the ballast circuit according to the known art operates in an incorrect manner given that it turns off the fluorescent lamp  10  not in presence of an ageing condition of the same but in presence of a variation of the supply voltage Val. 
     This does not occur with the driving circuit in accordance with the present invention. In fact, as can be seen in  FIG. 6 , a variation of the supply voltage Val causes a similar variation of the voltage Vdc but not the sending of the signal Dis to the circuitry  11 . The variation of the voltage Val also causes a similar variation of the threshold voltages Vdct 1  and Vdct 2 , so that the voltage Vdc does not go lower than the threshold voltage Vdct 1 . The signal Dis remains therefore a nil signal. In this manner the driving circuit according to the invention operates correctly given that it does not turn off the fluorescent lamp  10  in presence of a variation of the supply voltage Val.