Patent Publication Number: US-7218060-B1

Title: Flyback ballast for fluorescent lamp

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention is related in general to a ballast, and more particularly, to a ballast of fluorescent lamps. 
   2. Description of Related Art 
   Fluorescent lamps are the most popular light source on the market today. By improving the efficiency of the fluorescent lamps and/or providing the dimming control will offer significant savings in energy. Therefore, in recent developments, the capability for improving the efficiency and power savings for fluorescent lamps is a major concern. In addition, prolonging the fluorescent lamp&#39;s lifespan is also important for reducing environmental pollution. The proper starting and operating of the lamp is able to provide a longer lamp lifespan. Prior to ignition, the cathode filaments of the lamp electrodes should be preheated to an appropriate emission temperature. During normal operation, the cathode filaments should be maintained at an emission temperature. Furthermore, the glow discharge should be prevented because it will cause the filaments to wear out. The glow discharge is occurred when the filaments are being preheated and during an instance of higher voltage across the lamp. Therefore, the lamp voltage should be limited to within controlled ranges during the preheating interval. In order to completely eliminate the glow discharge, an additional filament heating circuit may be needed. However, such an approach has led to higher costs. 
     FIG. 1  shows a conventional electronic ballast with a series resonant inverter. The half-bridge inverter includes two switches  10  and  11 . The two switches  10  and  11  are interchangeably switched on and off under a 50% duty cycle at the desired switching frequency, which can be controlled to fulfill the requirements during both starting and normal operations. The resonant circuit is formed by an inductor  13 , a capacitor  14  and a fluorescent lamp  15 . The fluorescent lamp  15  is coupled in parallel with a capacitor  16 . The capacitor  14  is operated as the starting circuit. During the preheating stage, the lamp voltage can be maintained low by deliberately operating the ballast at the resonance frequency of the starting circuit. After the cathode filaments have been preheated to an appropriate emission temperature, the ballast frequency is adjusted to generate the required high ignition voltage. Once the lamp is running, the frequency is controlled to produce the required lamp voltage. The drawback of the aforementioned circuit is the having of a high glow current. During the preheating interval, the lamp voltage is determined by the switching frequency of the switches  10 ,  11  and the resonant frequency of the starting circuit. Once the ballast has been switched on, a resonant current is flowed through the capacitor  16  and the filaments for preheating. At this time, the lamp voltage is simultaneously produced on the lamp, which causes an inevitable glow discharge. Another disadvantage of the aformentioned circuit is the reduced control of efficiency during normal operation. The resonant frequency is varied according with a change of the parasitic devices of the fluorescent lamp. The parasitic devices of the fluorescent lamp, such as equivalent capacitance, for example, are changed in response to a change in temperature and the age of the lamp. Furthermore, the input voltage, the resonant frequency, and the characteristics of the fluorescent lamp affect the power consumption of the lamp. 
   The objective of the present invention is to provide a flyback ballast with improved efficiency. Another objective of the present invention is to eliminate the glow current, and thus prolonging the lamp lifespan. 
   SUMMARY OF THE INVENTION 
   The present invention provides a flyback ballast for fluorescent lamps. A transformer is utilized to control the power delivered to a lamp. A first switch is coupled to a first winding of the transformer. A first filter and a second filter are coupled to a second winding and a third winding of the transformer, respectively. The first filter is coupled to the second filter through a plurality of first terminals of a lamp. The second switch is coupled to the third switch through a plurality of second terminals of the lamp. A switching signal is provided to the first switch to control the power of the transformer delivered to the first filter and the second filter. A second switching signal and a third switching signal are provided to the second switch and the third switch respectively to control the power delivered to the lamp. A first range of power is delivered to the first filter and the second filter during the preheating interval. Meanwhile, the second switch is turned on and the third switch is to perform on/off switching for preheating the lamp. No glow discharge is occurred during the preheating interval, which results in the extension of lamp life. A second range of power is delivered to the first filter and the second filter under normal operation, in which the second range of the power is higher than the first range. The flyback power mode operation is to provide dimming control with higher efficiency for the lamps. 

   
     BRIEF DESCRIPTION OF ACCOMPANIED DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the present invention. In the drawings, 
       FIG. 1  shows a conventional electronic ballast with a series resonant inverter. 
       FIG. 2  is a circuit schematic of a flyback ballast according to an embodiment of the present invention. 
       FIG. 3  shows the flyback ballast according to an embodiment of the present invention during a preheating interval. 
       FIG. 4  shows a first phase of the flyback ballast during a normal operation. 
       FIG. 5  shows a second phase of the flyback ballast during the normal operation. 
       FIG. 6  shows a plurality of waveforms of the flyback ballast according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE EMBODIMENTS 
     FIG. 2  shows the circuit schematic of a flyback ballast  100  for a fluorescent lamp  15  according to an embodiment of the present invention. The flyback ballast includes a transformer  50  having a first winding N P , a second winding N S1 , and a third winding N S2 . A switch  20  is coupled to the first winding N P  of the transformer  50  to form a flyback converter. A plurality of rectifiers  61  and  62  are coupled to the second winding N S1  and the third winding N S2 , respectively. A capacitor  85  is coupled to the rectifier  61  to form a first filter  70 . A capacitor  86  is coupled to the rectifier  62  to form a second filter  90 . A switch  30  is coupled to the first filter  70 . A switch  35  is coupled to the second filter  90 . The first filter  70  is coupled to the second filter  90  through a first terminal pair  15   a  of a lamp  15 . The switch  30  is coupled to the switch  35  through a second terminal pair  15   b  of the lamp  15 . A switching signal S P  is provided to the switch  20  to control the power of the transformer  50  delivered to the first filter  70  and the second filter  90 . When the input voltage V IN  is applied to the first winding N P , the energy ε P  will be stored inside the transformer  50 . It is given by the following equations:
 ε P =½ ×L   P   ×I   P   2   (1) 
   
     
       
         
           
             
               
                 
                   I 
                   P 
                 
                 = 
                 
                   
                     
                       V 
                       IN 
                     
                     
                       L 
                       P 
                     
                   
                   × 
                   
                     T 
                     ON 
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   In accordance with the equations (1) and (2), the power P transferred through the transformer  50  can be expressed as the following: 
   
     
       
         
           
             
               
                 P 
                 = 
                 
                   
                     
                       V 
                       IN 
                       2 
                     
                     × 
                     
                       T 
                       ON 
                       2 
                     
                   
                   
                     2 
                     × 
                     
                       L 
                       P 
                     
                     × 
                     T 
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   in which L P  is the inductance of the first winding N P , I P  is the switching current of the first winding N P  when the switch  20  is turned on, T ON  is the on time of the switch  20 , and T is the switching period. 
   The energy is stored into the transformer  50  when the switch  20  is turned on. The energy is discharged to the second winding N S1  and the third winding N S2  once the switch  20  is turned off. By controlling the on time T ON  of the switching signal S P , the power transferred to the first filter  70  and the second filter  90  is regulated. A switching signal S H  and a switching signal S L  are provided to the switch  30  and the switch  35  respectively for controlling the power delivered to the lamp  15 . 
     FIG. 3  shows the operation of the flyback ballast according to an embodiment of the present invention during a preheating interval. The flyback converter delivers a first range of the power to the first filter  70  and the second filter  90  during the preheating interval. Meanwhile, while the switch  30  is turned on, the switch  35  is used to perform on/off switching to preheat the lamp  15 . No glow discharge is occurred during the preheating interval, which extends the lamp life. 
   Under normal operation, the flyback converter delivers a second range of power to the first filter  70  and the second filter  90 . The second range of the power is higher than the first range.  FIG. 4  and  FIG. 5  show the first phase and the second phase of the operation of the flyback ballast, in which the switch  30  and the switch  35  are interchangeably switched on to transfer the power to the lamp  15 . The level of the second range of the power determines the brightness of the lamp  15 .  FIG. 6  shows a plurality of waveforms of the flyback ballast  100  according to an embodiment of the present invention. During a preheating interval T 0 , the first range of the power generates a voltage V P  at the first filter  70  and the second filter  90 . After the preheating, the switch  35  is turned off. A second range of the power is applied to the first filter  70  and the second filter  90 . A higher lamp voltage will be generated to ignite the lamp  15 . After the ignition, a voltage V B  will be produced at the first filter  70  and the second filter  90 , and the switch  30  and the switch  35  are interchangeably switched on (T 1  and T 2 ) to transfer the power to the lamp  15 . Since the flyback converter can accurately control the output power, high efficiency dimming control for the lamps is realized. 
   While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.