Abstract:
A ballast for discharge lamp. The ballast for discharge lamp for utilizing high frequency voltage is generated by alternatively switching two switching elements to light the discharge lamp. The ballast includes a primary winding of a switching transformer to which the high frequency voltage is applied; a secondary winding of a switching transformer for alternatively switching the two switching elements according to reverse electromotive voltage generated by application of high frequency voltage to the primary winding; and a stabilizer for stabilizing reverse electromotive voltage generating at the secondary winding.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ballast for a discharge lamp, and more particularly to a ballast for a discharge lamp adapted to prevent a flicker effect and acoustic resonant phenomenon generated in the lighting of high voltage discharge lamp, such as a metal lamp, sodium lamp and the like. 
     2. Description of the Prior Art 
     When a discharge lamp is lighted and after the lamp is lighted in a conventional ballast for discharge lamp, there is a problem in that the voltage and a current difference are generated due to discharge and resonant frequency variation, resulting in acoustic resonant phenomenon and flicker effect. 
     Particularly, there is another problem in that a voltage or a current characteristic differs a little bit even in discharge lamps of identical design that one manufactured by the same company, and such difference frequently changes resonant frequency of the discharge lamps, and there is great difficulty in solving the acoustic resonant phenomenon and flicker effect. 
     SUMMARY OF THE INVENTION 
     The present invention solves the aforementioned problems of the prior art and it is an object of the present invention to provide a ballast for a discharge lamp adapted to prevent resonant frequency of the discharge lamp from fluctuating due to voltage and current difference of the discharge lamp when the discharge lamp is lighted and after the discharge lamp is lighted, thereby avoiding the acoustic resonant phenomenon and flicker effect, and prolonging the life of the discharge lamp as well. 
     There is another object of the present invention to provide a ballast for a discharge lamp adapted to constantly maintain the resonant frequency of the discharge lamp even though intrinsic voltage and current characteristics of the discharge lamp are different, thereby preventing acoustic resonant phenomenon and flicker effect and prolonging life of the discharge lamp as well. 
     In accordance with the objects of the present invention, there is provided a ballast for discharge lamp, the ballast for discharge lamp fur utilizing high frequency voltage generated by alternatively switching two switching elements to light the discharge lamp, the ballast comprising: 
     a primary winding of a switching transformer to which the high frequency voltage is applied; 
     a secondary winding of a switching transformer for alternatively switching the two switching elements according to reverse electromotive voltage generated by application of high frequency voltage to the primary winding; and 
     stabilizing means for stabilizing reverse electromotive voltage generating at the secondary winding, wherein the stabilizing means comprises: 
     a control winding where reverse electromotive voltage is generated according to high frequency voltage applied to the primary winding; 
     a diode for half-rectifying the reverse electromotive voltage generated at the winding for control; 
     charging/discharging circuit for receiving the reverse electromotive voltage half-rectified by the diode to charge and discharge according to charging/discharging time constant; and 
     a transistor for switching according to a voltage value of the charge/discharge circuit to control reverse electromotive voltage generated at the secondary winding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a circuit diagram of a ballast for discharge lamp according to the present invention; and 
     FIG. 2 is a schematic structural drawing of a switching transformer according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a circuit diagram of a ballast for discharge lamp according to the present invention, where the ballast for the discharge lamp includes AC/DC converter  10 , a starter  20 , a first switching controller  30 , a second switching controller  40 , discharge lamp driver  50  and a stabilizer  60 . 
     The AC/DC converter  10  for serving to convert an alternating current AC source input from outside to a direct current DC source includes a bridge diode (BD) and a smoothing condenser (C 2 ). The starter  20  outputs a starting signal when a DC voltage is applied from the AC/DC converter  10 , and includes resistors (R 1  and R 2 ), a condenser (C 3 ) and a diode AC switch (DIAC). 
     The first switching controller  30  and the second switching controller  40  alternatively switch a first switching element (Q 1 ) and a second switching element (Q 2 ) when a starting signal is output from the starter  20  to supply high frequency voltage to the discharge lamp driver  50 , and includes secondary windings of switching transformer (T 1 - a  and T 1 - b ), resistors (R 3  and R 4 ) diodes (D 1  and D 2 ) and zenor diodes (Z 1  and Z 2 ). 
     The discharge lamp driver  50  serves to receive the high frequency voltage supplied by the switching operation of the first switching element (Q 1 ) and the second switching element (Q 2 ) to light the discharge lamp  51  and includes a first winding (T 1 - c ) of switching transformer, inductors (T 2  and T 3 ), condensers (C 4 ˜C 9 ) and diodes (D 6 , D 8  and D 9 ). 
     The stabilizer  60  serves to stabilize the reverse electromotive voltage induced to the secondary windings (T 1 - a , T 1 - b ) of the first switching controller  30  and the second switching controller  40  according to high frequency voltage supplied to the discharge lamp driver  50  by switching operation of the first switching element Q 1  and the second switching element Q 2  and includes a control winding (T 1 - d ) of switching transformer, a diode (D 7 ), a resistor (R 7 ), a condenser (C 10 ) and a transistor (Q 3 ). 
     Furthermore, the switching transformer according to the present invention is constructed such that the control winding (T 1 - d ) thereof is overlapped on the first winding (T 1 - c ) and the second windings (T 1 - a , T 1 - b ) to be wound on a magnetic core  70  at inner sides of the first winding (T 1 - c ) and the second windings (T 1 - a , T a-b ), as illustrated in FIG.  2 . 
     Now, operational effect of the ballast for discharge lamp thus constructed according to the present invention will be described in detail with reference to the accompanying drawings. 
     When a user turns on a discharge lamp switch (SW), an alternating current (AC) source supplied from outside is input to a bridge diode (BD) of the AC/DC converter  10  through a rush preventing resistor (NTC), a fuse (F), a noise removing transformer (LFT) and a condenser (C 1 ). 
     The bridge diode (BD) full-wave rectifies the AC to output same to the smoothing condenser (C 2 ), where the smoothing condenser (C 2 ) smooths the full-wave rectified DC source to supply same via a power factor improving inductor  21 . 
     At this time, DC voltage input through the power factor improving inductor  21  and the resistor R 1  is charged in the condenser C 3  of the start  20 , where the DIAC (DA) is conducted the moment the charged voltage surpasses the driving voltage of the DIAC (DA) and a starting signal, that is, a high-leveled voltage signal, is applied to a gate terminal of the second switching element (Q 2 ) to turn on the second switching element (Q 2 ). 
     When the second switching element (Q 2 ) is thus turned on, current supplied from the power factor improving inductor  21  flows through condensers (C 5 , C 6 , C 4 ) of the discharge lamp driver  50 , inductor (T 2 ), primary winding (T 1 - c ) of the switching transformer and the second switching element (Q 2 ). 
     Furthermore, when charge of the condensers (C 5 , C 6 ) is completed, current flow of the primary winding at the switching transformer is blocked to cause reverse electromotive voltage to form at the secondary winding (T 1 - a ) of the switching transformer and the first switching element (Q 1 ) is turned on by the reverse electromotive voltage. 
     When the first switching element (Q 1 )is turned as described in the above, current supplied through the power factor improving inductor  21  flows through diode (D 6 ), first switching element (Q 1 ), primary winding (T 1 - c ) of the switching transformer, inductor (T 2 ) and condensers (C 4 , C 5  and C 6 ). 
     When the condenser (C 6 ) is completed in charge thereof, current flow at the primary winding (T 1 - c ) of the switching transformer is stopped, preventing the reverse electromotive voltage from being generated at the secondary winding (T 1 - b ) of the switching transformer, resulting in the second switching element (Q 2 ) to be turned on according to the reverse electromotive voltage. 
     In other words, reverse electromotive voltage is alternatively generated at the secondary windings (T 1 - a , T 1 - b ) of the switching transformer by discharge of the condenser (C 6 ) to cause the first and second switching elements (Q 1  and Q 2 ) to repeatedly perform the switching operation such that the high frequency voltage is supplied to the discharge lamp driver  50 . 
     At this time, over current and over voltage are prevented from being applied to gate terminals of the first and second switching elements (Q 1  and Q 2 ) by the resistors (R 3  and R 4 ) and zenor diodes (Z 1  and Z 2 ) connected to gate terminals of the first and second switching elements (Q 1  and Q 2 ), such that the first and second switching elements (Q 1  and Q 2 ) are avoided from being damaged. 
     Furthermore, source terminals of the first and second switching elements (Q 1  and Q 2 ) are connected with diodes (D 3  and D 4 ) and current attenuating resistors (R 5  and R 6 ), such that, when the first and second switching elements (Q 1  and Q 2 ) are turned on, current is supplied through the diodes (D 3  and D 4 ), and after the first and second switching elements (Q 1  and Q 2 ) are turned on, the current supplied from the discharge lamp driver  50  through the current attenuating resistors (R 5  and R 6 ) is restricted to thereby prevent the flicker effect. 
     Meanwhile, when frequency of the high frequency voltage applied to the discharge lamp driver  50  by the switching operation of the first and second switching elements (Q 1  and Q 2 ) becomes equal to resonant frequency of a resonant circuit having inductor (T 2 ) and condenser (C 4 ) of the discharge lamp driver  50 , a high voltage is generated at both ends of the condenser (C 4 ) to light the discharge lamp  51 . 
     After the discharge lamp  51  is lighted, the resonant frequency starts to be changed due to different voltage and current characteristics of the discharge lamp  51 , where the stabilizer  60  maintains the resonant frequency at a constant level to prevent the acoustic resonant phenomenon and the flicker effect from happening. 
     In other words, reverse electromotive voltage is inducted on the control winding (T 1 - d ) of the stabilizer  60  according to the resonant frequency and the electromotive voltage is half-wave rectified by the diode (D 7 ) to be charged at the condenser (C 1 O) through resistor (R 7 ), such that at every half cycle of the resonant frequency, the condenser (C 1 O) is charged. 
     When the condenser (C 10 ) is charged, the transistor (Q 3 ) is turned on to reduce the reverse electromotive voltage inducted on the secondary windings (T 1 - a , T a-b ) of the switching transformer, thereby charging flow of current flowing through the first and second switching elements (Q 1  and Q 2 ). 
     Accordingly, when charge/discharge time constant of the charge/discharge circuit having the resistor (R 7 ) and the condenser (C 10 ) is adjusted to fixedly place the resonant frequency at a bandwidth where no acoustic resonance occurs, the acoustic resonant phenomenon and the flicker effect are not generated even though voltage and current characteristics of the discharge lamp  51  or input voltage are changed. 
     As apparent from the foregoing, there is an advantage in the ballast for discharge lamp thus described according to the present invention in that resonant frequency of a discharge lamp is prevented from fluctuating due to difference in voltage and current characteristics when the discharge lamp is lighted and after the discharge lamp is lighted, thereby avoiding acoustic resonant phenomenon and flicker effect and prolonging life of the discharge lamp as well. 
     There is another advantage in that resonant frequency of the discharge lamp is constantly maintained regardless of intrinsic voltage and current characteristics of the discharge lamp, thereby preventing acoustic resonant phenomenon and flicker effect and prolonging life of the discharge lamp as well.