Abstract:
A power circuit includes a transformer, a full-bridge rectifier and a feedback circuit. The transformer is used to input an external AC voltage. The transformer includes a plurality of output terminals. The output terminals include a plurality of output terminal combinations, and at least two different output terminal combinations output a first AC voltage and a second AC voltage. The full-bridge rectifier is used to convert the first AC voltage or the second AC voltage into DC voltages. The feedback circuit makes the transformer switch between different output ports combinations according to the value of the second AC voltage.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a power circuit and a liquid crystal display device using the same. 
         [0003]    2. Description of Related Art 
         [0004]    Liquid crystal display (LCD) devices are widely used in various devices, such as computers, personal digital assistant (PDAs), and TVs. An LCD device often includes a power circuit electrically connected to an external alternating current (AC) power source to supply the device with direct current (DC) voltage. Such voltages, however, are unstable as the source AC voltage is unstable. The unstable voltages can damage components of the device. 
         [0005]    As shown in  FIG. 2 , a commonly used LCD device  20  includes a power circuit  25  and a driving circuit  26 . The power circuit  25  supplies the driving circuit  26  with DC voltage. The power circuit  25  includes a transformer  21 , a full-bridge rectifier  22 , and a filter capacitor  24 . The transformer  21  includes a primary winding and a secondary winding. The transformer  21  receives an external AC voltage from an external AC power source through the primary winding. The secondary winding includes two output terminals  211 ,  212 . An AC voltage is output from the two output terminals  211 ,  212 . The AC voltage is converted to DC voltage by the full-bridge rectifier  22 . The full-bridge rectifier  22  includes two input terminals  221 ,  222  and two output terminals  223 ,  224 . The two input terminals  221 ,  222  are electrically connected to the two output terminals  211 ,  212  of the transformer  21 , respectively. The two output terminals  223 ,  224  are electrically connected to the driving circuit  26  and are electrically connected to each other through the filter capacitor  24 . The filter capacitor  24  stabilizes the DC voltage output from the full-bridge rectifier  22 . 
         [0006]    However, the DC voltage output from the power circuit  25  changes with the external AC voltage. As the external AC voltage is unstable, the DC voltage output from the power circuit  25  is unstable. The unstable DC voltage makes the driving circuit  26  work abnormally or even damage components of the driving circuit  26 . 
         [0007]    What is needed, is a power circuit and an LCD device that can over come the above-described deficiencies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present power circuit and the LCD device. In the drawings, like reference numerals designate corresponding parts throughout various views, and all the views are schematic. 
           [0009]      FIG. 1  is a schematic circuit diagram of an LCD device according to an embodiment of the present disclosure. 
           [0010]      FIG. 2  is a schematic circuit diagram of an LCD device. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Reference will now be made to the drawings to describe exemplary embodiments of the present disclosure in detail. 
         [0012]      FIG. 1  is a schematic circuit diagram of an LCD device  10  according to an embodiment of the present disclosure. The LCD device  10  includes a power circuit  15  and a driving circuit  16 . The power circuit  15  supplies the driving circuit  16  with DC voltage. 
         [0013]    The power circuit  15  includes a transformer  11 , a full-bridge rectifier  12 , a feedback circuit  13 , a filter capacitor  14 , two diodes  115 ,  116 , and a voltage divider which includes two resistors  135 ,  136  connected in series. The transformer  11  includes a primary winding and a secondary winding. The transformer  11  receives an external AC voltage from an external AC power source through the primary winding. The secondary winding includes a first output terminal  111 , a second output terminal  112 , a third output terminal  113 , and a fourth output terminal  114 . Every two output terminals of the four output terminals  111 ,  112 ,  113 ,  114  define an output terminal combination, so the four output terminal  111 ,  112 ,  113 ,  114  define a first output terminal combination including the first and the fourth output terminals  111 ,  114 , a second output terminal combination including the first and the third output terminals  111 ,  113 , and a third output terminal combination including the second and the fourth outpour terminals  112 ,  114  in total. The number of turns of the second output terminal combination is the same as the number of turns of the third output terminal combination, and less than the number of turns of the first output terminal combination. 
         [0014]    The full-bridge rectifier  12  converts AC voltage from the transformer  11  to DC voltage. The full-bridge rectifier  12  includes a first input terminal  121 , a second input terminal  122 , a positive output terminal  123 , a negative output terminal  124 , a first diode  125 , a second diode  126 , a third diode  127 , and a fourth diode  128 . A cathode of the first diode  125  and an anode of the second diode  126  are electrically connected to the first input terminal  121 . Anodes of the first and third diodes  125 ,  127  are electrically connected to the negative output terminal  124 . A cathode of the third diode  127  and an anode of the fourth diode  128  are electrically connected to the second input terminal  122 . Cathodes of the second and fourth diodes  126 ,  128  are electrically connected to the positive output terminal  123 . The first input terminal  121  is electrically connected to the output terminal  111  of the transformer  11 . The second input terminal  122  is electrically connected to the fourth output terminal  114  of the transformer  11 . The positive output terminal  123  is electrically connected to the second output terminal  112  through the voltage divider, an anode and a cathode of diode  115  in that order. An anode of the diode  116  is electrically connected to the anode of the diode  115 , and a cathode of the diode  116  is electrically connected to the third output terminal  113 . 
         [0015]    The feedback circuit  13  includes a comparator  130  and a switch  132 . The comparator  130  includes a bipolar transistor  131 , a diode  134 , a zener diode  133 , and a resistor  137 . The switch  132  is a field effect transistor, The field effect transistor is an NMOS transistor, and the bipolar transistor  131  is an npn bipolar transistor. A base of the bipolar transistor  131  is electrically connected to a node between the two resistors  135 ,  136  through an anode and a cathode of the zener diode  133 . A collector of the bipolar transistor  131  is electrically connected to the positive output terminal  123  of the full-bridge rectifier  12  through the resistor  137 . An emitter of the bipolar transistor  131  is electrically connected to ground. A source of the field effect transistor is electrically connected to the emitter of the bipolar transistor  131 . A gate of the field effect transistor is electrically connected to the collector of the bipolar transistor  131  through an anode and a cathode of the diode  134 . A drain of the field effect transistor is electrically connected to the negative output terminal  124 . 
         [0016]    An end of the filter capacitor  14  is electrically connected to the positive output terminal  123  of the full-bridge rectifier  12 , and another end of the filter capacitor  14  is electrically connected to the source of the field effect transistor. As the output AC voltage of the transformer  11  keeps steady, a steady DC voltage with predetermined voltage value is generated between the two ends of the filter capacitor  14 . The driving circuit is electrically connected to the two ends of the filter capacitor  14  and drives an LCD panel. 
         [0017]    Operation of the LCD device  10  follows: 
         [0018]    An external AC voltage is transformed by the transformer  11  to an AC voltage with value lower than that of the external AC voltage, such as an AC voltage of 20 V or an AC voltage of 22 V. The AC voltage is proportional to the external AC voltage and changes with the external AC voltage. When the AC voltage is rectified by the full-bridge rectifier  12  and the two diodes  115 ,  116 , a positive voltage is output from the positive output terminal  123  of the full-bridge rectifier  12  and a negative voltage is output from the negative output terminal  124  of the full-bridge rectifier  12 , the second output terminal  112 , and the third output terminal  113 . 
         [0019]    The voltage value between the two ends of the filter capacitor  14  is equal to that between the positive output terminal  123  of the full-bridge rectifier  12  and ground. The value of positive voltage output from the positive output terminal  123  is proportional to the value of the positive voltage output from the first output terminal  111  or the fourth output terminal  114 . The value of the positive voltage output from the first output terminal  111  or the fourth output terminal  114  changes as different output terminal combination is used, and is proportional to the external AC voltage. The first output terminal combination outputs a first AC voltage. The second output terminal combination or the third output terminal combination outputs a second AC voltage. As the value of the output AC voltage is proportional to the number of turns of the output terminal combination, the value of the first AC voltage is greater than that of the second AC voltage. 
         [0020]    The zener diode  133  defines a predetermined voltage value, such as a value of 19 V. As the second voltage changes with the external AC voltage, when the value of second AC voltage is greater than the predetermined voltage value, a higher voltage is applied to the zener diode  133 , which is then turned on. The higher voltage is applied to the base of the bipolar transistor  131  through the zener diode  133  and the bipolar transistor  131  is turned on. The gate of the field effect transistor is electrically connected to ground. A lower voltage is applied to the gate of the field effect transistor, the field effect transistor is turned off. When a positive voltage is output from the first output terminal  111 , the voltage generated by the secondary winding outputs from the second output terminal combination to the full-bridge rectifier  12 . When a positive voltage is output from the fourth output terminal  114 , the voltage generated by the secondary winding outputs from the third output terminal combination to the full-bridge rectifier  12 . The transformer  11  thus outputs the second AC voltage to the full-bridge rectifier  12 . Thus, when the value of the second AC voltage is greater than the predetermined voltage value, the power circuit  15  outputs a DC voltage with correspondingly lower value. 
         [0021]    When the value of second AC voltage is lower than the predetermined voltage value, a lower voltage is applied to the zener diode  133  so as to turn off the zener diode  133 . The bipolar transistor  131  is turned off as the zener diode  133  is turned off. A higher voltage is applied to the gate of the field effect transistor through the resistor  137  and the diode  134 . The field effect transistor is turned on. The voltage generated by the secondary winding outputs from the first output terminal combination to the full bridge rectifier  12 . The transformer  11  outputs the first AC voltage. Thus, when the value of the second AC voltage is lower than the predetermined voltage value, the power circuit  15  outputs a DC voltage with correspondingly higher value. 
         [0022]    In summary, the power circuit  15  includes a transformer  11  with a plurality of output terminals and a feedback circuit  13 . The transformer  11  is controlled by the feedback circuit  13  according to the second AC voltage. The transformer  11  outputs a first AC voltage to the full-bridge rectifier  12  through the first output terminal combination when the value of the second AC voltage is lower than a predetermined voltage value. The transformer  11  outputs a second AC voltage to the full-bridge rectifier  12  through the second output terminal combination, or the third output terminal combination when the value of the second AC voltage is greater than a predetermined voltage value. Thus, the output DC voltage of the power circuit  15  is relatively steady. The power circuit  15  is applied to the LCD device  10 , and provided the driving circuit  16  with steady DC voltage. The LCD device  10  with the power circuit  15  works steadily as well. 
         [0023]    In alternative embodiments, the switch  132  can be an npn bipolar transistor and the bipolar transistor  131  can be replaced by an NMOS transistor. 
         [0024]    In addition, the gate of the field effect transistor can be generalized as a control terminal, the source of the field effect transistor can be generalized as a first terminal, and the drain of the field effect transistor can be generalized as a second terminal. 
         [0025]    It is to be further understood that even though numerous characteristics and advantages of preferred and exemplary embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail within the principles of the present disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.