Abstract:
A method for controlling an inverter under altering voltage aims to change the electric conductive interval of the electric conductive cycle of the inverter corresponding to alteration of an input voltage so that the dimming duty cycle, electric conductive cycle and transformer oscillation duty cycle of the inverter can be maintained at a selected level. Thereby when the input voltage is altered, the existing dimming range can be maintained and actuation electricity output is stabilized. The transformer can be protected and the life span of the load can be extended.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for controlling an inverter under altering voltage and particularly to a method to change the electric conductive interval of the electric conductive cycle of the inverter to maintain the existing dimming range and stabilize actuating electric output and protect the life span of transformers and loads.  
       BACKGROUND OF THE INVENTION  
       [0002]     Backlight module is a key element of the actuating light source of a display panel. Besides, providing a lighting source, the dimming function to alter the actual light projection effect in response to the environment illuminating condition is the basic function of the backlight module in practical applications.  
         [0003]     The actuating electric source for the backlight modules now on the market mostly adopts high voltage inverters. They can be classified in current feeding push-pull parallel resonant inverters and single stage inverters. The transformers used in the inverters include winding transformers and piezoelectric transformers. Their duty cycle waveforms are shown in  FIGS. 1A and 2A  ( FIG. 1A  is a burst mode dimming method,  FIG. 2A  is a standby mode dimming method, technical details can be found in U.S. Pat. No. 6,839,253). For discussion purpose, assuming input voltage is DC 10V, the dimming efficiency (dimming duty cycles are  1   a  and  1   c ) is 100%, then the electric conductive interval of the electric conductive cycles  2   a  and  2   c  is 50% ON and 50% OFF. The transformer oscillation duty cycles  3   a  and  3   c  are 100% sinuous waveform based on the amplitude of 10V. Assuming the load (cold cathode lamp) outputs a lamp feedback current of 6 mA, when the input voltage is altered to 20V, as shown in  FIGS. 1B and 2B , the existing dimming mechanism relatively increases the lamp feedback current (such as 12 mA) when the input voltage alters. In order for the backlight module to maintain the illumination at the existing dimming efficiency, the lamp current feeds back electricity to the dimming controller (or getting a voltage feedback electricity from the transformer output end or input end as the comparison value). By comparing the feedback electricity with a reference value built in the dimming controller, a second dimming duty cycle is determined. As shown in the drawings, when the input voltage is altered to DC of 20V, the dimming duty cycles  1   b  and  1   d  are transformed to 50% ON and 50% OFF. The electric conductive cycles  2   b  and  2   d  are changed to 50% ON and 50% OFF when the electric conductive interval is maintained 50% ON and 50% OFF. The transformer oscillation duty cycles  3   a  and  3   d  are 50% ON and 50% OFF at the amplitude of 20V. Referring to  FIGS. 3A and 4A , the input voltage is DC 10V, the dimming efficiency (dimming duty cycles  1   e  and  1   g ) is 50%.  FIGS. 3B and 4B  show that the input voltage is DC 20V, the dimming efficiency (dimming duty cycles  1   f  and  1   h ) is changed to 25%. The assumed conditions for the rest electric conductive cycles  2   e ,  2   g ,  2   f  and  2   h , and the oscillation duty cycles  3   e ,  3   g ,  3   f  and  3   h  are same as previously discussed. While such a dimming control mechanism can maintain the existing efficiency and illumination for the cold cathode lamp, as shown in the drawings, there are still drawbacks, notably:  
         [0004]     1. As the dimming duty cycle is squeezed, the actual applicable dimming range of the backlight module is affected. As shown in  FIG. 3B , the applicable dimming range of the existing backlight module mostly is between 20% and 100%. But when the input voltage has great alterations, the transformer that originally has 50% of dimming efficiency could result in a single sinuous waveform at 25%. As a result, the backlight module cannot continuously correct the dimming efficiency downwards, Hence the actual dimming efficiency range is limited between 50% and 100%, not the original setting of 20% to 100%.  
         [0005]     2. The oscillation amplitude of the transformer and actuator is changed (such as from 10V to 20V) when the input voltage is altered. This will shorten the life span of the transformer and actuator.  
         [0006]     3. When the input voltage fluctuates greatly and is not stable, the lamp current of the cold cathode lamp will generate a higher waveform factor. As a result, blacking phenomenon is easily occurred to the ignition end of the cold cathode lamp.  
       SUMMARY OF THE INVENTION  
       [0007]     Therefore the primary object of the present invention is to solve the aforesaid disadvantages occurred to the conventional techniques of altering dimming duty cycle under the varying input voltage. The present invention alters the electric conductive interval of the electric conductive cycle without changing the dimming duty cycle, electric conductive cycle and transformer oscillation duty cycle to maintain the existing dimming range and a constant voltage amplitude oscillation of the transformer, and generate symmetrical and even lamp current on the load (cold cathode lamp) so that the life span of the transformer and load can be maintained without suffering.  
         [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]      FIGS. 1A through 4B  are schematic views of duty cycles of conventional dimming mechanisms.  
         [0010]      FIGS. 5A through 8B  are schematic views of duty cycles of the present invention.  
         [0011]      FIG. 9  is a circuit block diagram of an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]     Please refer to  FIGS. 5A, 6A  and  9  for an embodiment of the method for controlling an inverter under altering voltage of the invention. To facilitate comparison with the conventional dimming mechanism previously discussed, the assumed conditions are same as those set forth above. However, they are not the limitation of the invention. The control method and circuit embodiment include:  
         [0013]     A. A first controller  1  to receive an external dimming signal  11  to determine dimming duty cycles  1   i ,  1   k ,  1   p , and  1   r . The first controller  1  is a pulse-width modulation (PWM) impulse signal generator. The external dimming signal  11  is set by users through an external dimming knob. As shown in  FIGS. 5A and 6A , the dimming duty cycles  1   i  and  1   k  are 100%. In  FIGS. 7A and 8A , the dimming duty cycles  1   p  and  1   r  are 50%. In  FIGS. 5A, 6A ,  7 A and  8 A, the input voltage  7  is 10V.  
         [0014]     B. A second controller  2  to receive the dimming duty cycles  1   l ,  1   k ,  1   p  and  1   r , and the input voltage  7  and respond to operating conditions of a transformer  4  of an inverter to determine output electric conductive cycles  2   i ,  2   k ,  2   p  and  2   r . The second controller  2  is a PWM frequency generator or a micro-controller integrated with the first controller  1 . The circuit of the embodiment adopts a piezoelectric transformer  4 . Depending on the size of the input voltage  7 , the second controller  2  may be coupled with a floating voltage level device  21 . In  FIGS. 5A and 6A , corresponding to the dimming duty cycles  1   i  and  1   k  of 100%, the electric conductive cycles  2   i  and  2   k  also are 100% ON. The electric conductive interval is 50% ON and 50% OFF. In  FIGS. 7A and 8A , corresponding to the dimming duty cycles  1   p  and  1   r  of 50% , the electric conductive cycles  2   p  and  2   r  also are 50% ON and 50% OFF. The electric conductive interval is 50% ON and 50% OFF. (In  FIGS. 6A and 8A , a maintenance voltage is still provided while the electric conductive interval is in the OFF condition. However, such a maintenance voltage does not affect the operation of the invention. To facilitate discussion, the variable of this maintenance voltage is omitted to avoid confusion).  
         [0015]     C. An actuator  3  to receive the electric conductive cycles  2   i ,  2   k ,  2   p  and  2   r  to determine oscillation duty cycles  3   i ,  3   k ,  3   p  and  3   r  to be output to the transformer  4  of the inverter. The actuator  3  may be a double-switch power transistor, and generates the sinuous oscillation duty cycles  3   i ,  3   k ,  3   p  and  3   r  through the charging effect of an inductor  41 . As shown in  FIGS. 5A and 6A , corresponding to the dimming duty cycles  1   i  and  1   k  of 100% , the oscillation duty cycles  3   i  and  3   k  also are 100% ON. In  FIGS. 7A and 8A , corresponding to the dimming duty cycles  1   p  and  1   r  of 50% , the oscillation duty cycles  3   p  and  3   r  also are 50% ON and 50% OFF.  
         [0016]     D. When the input voltage  7  alters, the invention provides a preset reference electricity value, and compares with the input voltage  7  and outputs a modulated signal to an electricity detector  6  of the second controller  2 , and according to the alteration of the input voltage  7 , changes the electric conductive interval of the electric conductive cycles  2   i ,  2   k ,  2   p  and  2   r  based on the oscillation duty cycles  3   i ,  3   k ,  3   p and  3   r  of the transformer  4  at step C. The electricity detector  6  may be a linear logic circuit containing a comparator or a comparison circuit of a micro-controller integrally built in the second controller  2 , or a micro-controller formed by integrating the first controller  1 , second controller  2  and electricity detector  6 . At step D, a feedback electricity  51  may be obtained and make the union comparison with the input voltage  7  to determine the electric conductive interval of electric conductive cycles  2   j ,  2   m ,  2   q  and  2   u . Meanwhile, the electricity detector  6  is a linear logic circuit of a window type comparator. The determination criteria of union are divided as follows:  
         [0017]     D1: When the feedback electricity  51  and the input voltage  7  are unchanged, the dimming duty cycles  1   i ,  1   k ,  1   p  and  1   r  and the electric conductive interval of the electric conductive cycles  2   l ,  2   k ,  2   p  and  2   r  remained unchanged.  
         [0018]     D2: When the feedback electricity  51  alters, but the input voltage  7  is unchanged, the dimming duty cycles  1   i ,  1   k ,  1   p  and  1   r  are changed according to the alteration of the feedback electricity  51 . But the electric conductive interval of the electric conductive cycles  2   l ,  2   k ,  2   p  and  2   r  remained unchanged. This situation mostly occurs to the lamp current of a cold cathode lamp  5  having an abrupt and a short abnormal condition or damage. In such an occasion, return to the normal condition usually takes place. If return to the normal condition fails, the cold cathode lamp  5  could be damaged and has to be replaced.  
         [0019]     D3: When the feedback electricity  51  remained unchanged, but the input voltage  7  alters, dimming duty cycles  1   j ,  1   m ,  1   q  and  1   u  remain unchanged. The electric conductive interval of the electric conductive cycles  2   j ,  2   m ,  2   q  and  2   u  are altered according to alteration of the input voltage  7 .  
         [0020]     D4: When the feedback electricity  51  and the input voltage  7  are changed, the dimming duty cycles  1   i ,  1   k ,  1   p  and  1   r  remain unchanged. The electric conductive interval of the electric conductive cycles  2   j ,  2   m ,  2   q  and  2   u  are altered according to alteration of the input voltage  7  if the allowing range of the actuator  3  is not exceeded. If the allowing range of the actuator  3  is exceeded, the dimming duty cycles  1   j ,  1   m ,  1   q  and  1   u  are changed according to alteration of the feedback electricity  51 , and the electric conductive interval of the electric conductive cycles  2   j ,  2   m ,  2   q  and  2   u  are altered according to alteration of the input voltage  7 .  
         [0021]     Based on the determination criteria of D3 and D4 previously discussed, also referring to  FIGS. 5B and 6B , when the input voltage  7  is changed to 20V, the dimming duty cycles  1   j ,  1   m ,  1   q  and  1   u , and electric conductive cycles  2   j ,  2   m ,  2   q  and  2   u  do not change because of alteration of the input voltage  7 . Hence the existing dimming range can be maintained. The electric conductive interval of the actuator  3  is altered from 50% ON and 50% OFF to 25% ON and 75% OFF. Alteration of the electric conductive interval shown in  FIGS. 7B and 8B  also adopts the same fashion.  
         [0022]     E: The electric conductive cycles  2   j ,  2   m ,  2   q  and  2   u  are generated after the electric conductive interval depicted at step D has been altered. Under the charge and discharge effect of the inductor  41 , the oscillation duty cycles  3   j ,  3   m ,  3   q  and  3   u  of the transformer  4  remain unchanged. The oscillation voltage amplitude also is maintained at 10V. Namely, the transformer  4  oscillates under the same voltage amplitude. Hence the life span of the transformer  4  can be maintained, and the lamp current of the cold cathode lamp  5  is maintained constant. Therefore blacking of one end can be reduced, and the service life of the cold cathode lamp  5  increases. While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments 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.