Abstract:
A driving circuit for hot cathode fluorescent lamps aims to filter out signals with a voltage value lower than a duty voltage value of a switch unit through a commutation unit and a valley filled power correction circuit to prevent power from accumulating and the switch unit from being pierced.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a driving circuit and particularly to a driving circuit for hot cathode fluorescent lamps to filter out signals with a voltage value lower than a duty voltage of a switch unit through a commutation unit and a valley filled power factor correction circuit to prevent electricity from piercing the switch unit. 
       BACKGROUND OF THE INVENTION 
       [0002]    The general AC transmitted through the AC distribution system of power companies usually are connected to various types of loads that include resistance loads such as incandescent lamps, ovens and the like, and many other types of loads that mostly consist of resistance and induction. Hence on electronic circuits the current phase angle is lagged behind the voltage. In general the total current provided by the power company from generators through the power transmission and distribution lines includes effective current and ineffective current. Due to different rated voltage of different loads, the electrometer at user ends counts on the effective power (KW). However the ineffective power (KVAR) causes line voltage drops and loss. This is a loss to both the user ends and the power company. Hence to regulate power factor is a big issue for providers of user end systems. 
         [0003]    The techniques of power factor regulation can be divided into active type and passive type. They aim to reduce power loss, improve power quality, increase the life span of loads and save power expense. Valley filled power factor regulation circuit is one of the techniques adopted on electronic devices that can set a voltage value to regulate power. For instance R.O.C. patent No. M242969 discloses a driving circuit for electronic stabilizers of low resonant waves that mainly includes a power source input and a commutation circuit which consists of D 1 , D 2 , D 3  and D 4 , a transformation circuit (valley filled power factor correction circuit) consisting of C 2 , D 5 , D 6 , R 7  and C 6  to transform AC to DC at almost same voltage, a driving circuit (push-pull inverter circuit) consisting of two complimentary high voltage transistors (preferably MOSFET) to transform DC to a high frequency square wave, and a voltage division circuit to generate an one half power voltage that has an output end connecting to one end of a lamp. Its features include the driving circuit having a resistor switch R 3  to provide a small bias voltage to activate an inductor. The resistor switch is coupled with a capacitor C 3  to isolate DC from entering the inductor. The resistor switch further is coupled with DZ 1  to limit the bias voltage amplitude of the transistors within the range of 12V. The resistor switch also is connected to a LC resonant circuit consisting of C 4  and an inductor to transform oscillating square waves to sinusoidal waves. The resistor switch further is connected to a C 1  to improve wave filtering and enhance CF value. The resistor switch also is connected to one C to isolate DC portion so that the load is driven by AC. The resistor switch further is connected to C 5  and R 4  to improve the surge of the square waves. 
         [0004]    Other reference of the valley filled power factor regulation circuit can be found in R.O.C. patent Nos. M290932 and R.O.C. patent publication Nos. 519854, and U.S. Pat. Nos. 7,061,781, 6,909,622, 5,517,086, 6,667,586 and 6,297,613. 
         [0005]    The valley filled power factor regulation circuits in the aforesaid techniques mostly have a transistor switch at the rear end to split power signals. The transistor switch in practice has a duty voltage value which is the lowest voltage when the transistor switch is in an operating condition. When an electronic device equipped with a valley filled power factor regulation circuit is in an operating condition, if the signal transmitted to the transistor switch has a voltage value lower than the duty voltage of the transistor, the transistor cannot enter the operating condition due to not sufficient voltage. Then the power of signals transmitted to the transistor switch will accumulate on the input end of the transistor switch. When the accumulated power reaches the limit of the transistor switch, the transistor switch will be damaged or pierced. Hence how to prevent accumulation of low voltage power to damage or pierce the transistor switch is a big issue pending to be resolved in the industry. 
       SUMMARY OF THE INVENTION 
       [0006]    Therefore the primary object of the present invention is to provide a driving circuit for hot cathode fluorescent lamps to prevent a switch unit from being pierced by accumulating power. It filters out input signals that have a voltage value lower than the duty voltage of the switch unit through a cutoff voltage so that the switch unit can get signals that are operable. 
         [0007]    To achieve the foregoing object, the driving circuit for hot cathode fluorescent lamps according to the invention mainly includes a commutation unit to transform AC input cycle signals to DC cycle signals, a switch unit to divide driving power output to the hot cathode fluorescent lamps that has a duty voltage value and a valley filled power factor correction circuit which is electrically connected to the commutation unit and has a cutoff voltage value higher than the duty voltage value of the switch unit. Therefore when the voltage of the AC input cycle signals is lower than the cutoff voltage, the commutation unit cuts off conduction. As a result, the AC input cycle signals with the voltage lower than the cutoff voltage are filtered out before arriving the commutation unit. The switch unit at the rear end receives signals that have a voltage higher enough to allow the switch unit to enter an operating condition. Thus power accumulation that might otherwise occur to pierce the switch unit can be prevented. 
         [0008]    The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of an embodiment of the invention. 
           [0010]      FIG. 2  is a circuit diagram of an embodiment of the invention. 
           [0011]      FIG. 3A  is an equivalent circuit diagram of duty mode I of a valley filled power factor correction circuit according to an embodiment of the invention. 
           [0012]      FIG. 3B  is an equivalent circuit diagram of duty mode II of a valley filled power factor correction circuit according to an embodiment of the invention. 
           [0013]      FIG. 3C  is an equivalent circuit diagram of duty mode III of a valley filled power factor correction circuit according to an embodiment of the invention. 
           [0014]      FIG. 4  is a waveform chart of a valley filled power factor correction circuit in an operation mode according to an embodiment of the invention. 
           [0015]      FIG. 5  is a waveform chart of an AC input cycle signal Vin according to an embodiment of the invention. 
           [0016]      FIG. 6  is a waveform chart of a DC cycle signal S 1  according to an embodiment of the invention. 
           [0017]      FIG. 7  is a waveform chart of an AC input cycle signal S 2  in a filtered condition according to an embodiment of the invention. 
           [0018]      FIG. 8  is a waveform chart of a driving signal S 3  according to an embodiment of the invention. 
           [0019]      FIG. 9  is a waveform chart of an AC sinusoidal signal S 4  according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    Please refer to  FIGS. 1 and 2  for an embodiment of the invention. The driving circuit for a hot cathode fluorescent lamp (HCFL)  4  according to the invention mainly includes: 
         [0021]    a commutation unit  1  to transform an AC input cycle signal Vin from an AC power source to a DC cycle signal S 1  (also referring to  FIGS. 5 and 6 ). In this embodiment the commutation unit  1  is a full bridge rectifier consisting of D 1 -D 4 . It has a first output end B 1  and a second output end B 2 . 
         [0022]    Table 1 below shows circuit conditions of a valley filled filter in various duty modes according to an embodiment of the invention. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Duty 
                 Voltage 
                 Diode condition 
                 Current 
               
             
          
           
               
                 mode 
                 condition 
                 ON 
                 OFF 
                 condition 
               
               
                   
               
               
                 I 
                 |Vin| &lt; Vcp; 
                 D5, D6 
                 Other 
                 Iin = 0; 
               
               
                   
                 Vd = Vcp 
                   
                   
                 I Load  = I C1  + I c2   
               
               
                 II 
                 Vcp &lt; |Vin| &lt; 
                 D1, D4; Vin &gt; 0 
                 Other 
                 Iin = I Load ; 
               
               
                   
                 Vcs; Vd = |Vin| 
                 D2, D3; Vin &lt; 0 
                   
                 I C1  = I c2  = 0 
               
               
                 III 
                 |Vin| &gt; Vcs; 
                 D1, D4, D7; 
                 Other 
                 Iin = I Load  − I C1 ; 
               
               
                   
                 Vd = |Vin| 
                 Vin &gt; 0 
                   
                 I C1  = I c2   
               
               
                   
                   
                 D2, D3, D7; 
               
               
                   
                   
                 Vin &lt; 0 
               
               
                   
               
             
          
         
       
     
         [0023]    The invention further includes a valley filled power factor correction circuit  2  to receive the DC cycle signal from the commutation unit  1 , and has a cutoff voltage value V 1 . When the voltage value of the AC input cycle signal Vin is lower than the cutoff voltage V 1  the commutation unit  1  cuts off conduction. When the voltage value of the AC input cycle signal Vin is higher than or equal to the cutoff voltage V 1  conduction is established to form a filter wave condition S 2  (referring to  FIG. 7 ). In this embodiment the valley filled power factor correction circuit  2  includes a first circuit (not shown in the drawings) consisting of a first capacitor C 1  and a first diode D 5 , and a second circuit (also not shown in the drawings) consisting of a second capacitor C 2  and a second diode D 7 . The second diode D 7  is located between the first circuit and the second circuit. The first circuit and the second circuit are located between the first output end B 1  and the second output end B 2 . The valley filled power factor correction circuit  2  has three duty modes I, II and III (referring to  FIGS. 3A ,  3 B,  3 C,  4  and table 1). It is based on a principle that the capacitors C 1  and C 2  have a charging path and a discharging path that are different. Hence the conduction time period of the commutation unit  1  increases. Therefore power factor can be boosted up to 0.95. Moreover, the cutoff voltage value V 1  is ½ peak value of the AC input cycle signal Vin received by the commutation unit  1 . 
         [0024]    The invention further includes a switch unit  3  which has a duty voltage and divides the driving power output to the HCFL  4  to output a driving signal S 3  (referring to  FIG. 8 ) so that the HCFL  4  receives the power of an AC sinusoidal signal S 4  (referring to  FIG. 9 ) to generate light. The duty voltage is a minimum voltage of the power required by the switch unit  3  to enter an operating condition. The ½ peak value voltage is greater than the duty voltage. Hence through the valley filled power factor correction circuit  2  the AC input cycle signal lower than the cutoff voltage value V 1  can be filtered out before reaching the commutation unit  1 . And the voltage received by the rear end switch unit  3  is sufficient to enable the switch unit  3  to enter the operating condition. Thereby power accumulation that might otherwise happen on the input end of the switch unit  3  can be prevented and piercing of the switch unit  3  can be avoided. In this embodiment the switch unit  3  is two transistors. 
         [0025]    In short, the invention can filter out a portion of AC input cycle signal Vin that has a voltage value lower than the duty voltage through the commutation unit  1  and the valley filled power factor correction circuit  2  to prevent the voltage of input signals of the switch unit  3  lower than the duty voltage. Thus accumulation of power on the input end of the switch unit  3  can be prevented and piercing of the switch unit  3  can be avoided. It provides a significant improvement over the conventional techniques. 
         [0026]    While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.