FIELD OF THE INVENTION 
       [0001]    The present invention relates to power supply circuits used in liquid crystal display (LCD) devices; and particularly to a power supply circuit having two direct current/direct current (DC/DC) converters, one of which specially supplies an operating voltage to a main chip of an inverter of an LCD device. 
       BACKGROUND 
       [0002]    LCD devices are commonly used as displays for compact electronic apparatuses. This is because they not only provide good quality images with little power consumption, but also they are very thin. A typical LCD device includes a power supply circuit, which supplies operating voltages for various kinds of working units in the LCD device. 
         [0003]    Referring to  FIG. 3 , a conventional power supply circuit  1  for an LCD device (not labeled) includes a power board  11  and an interface board  10 . The power board  11  is used to supply power voltages to various kinds of circuit elements of the LCD device. The interface board  10  is used to supply signals to a driving circuit  18  of the LCD device. The signals include various kinds of control signals. The power board  11  includes an alternating current/direct current (AC/DC) converter  17  and an inverter  13 . The interface board  10  includes a DC/DC converter  12 , a plurality of ports (not shown), and other circuits (not shown) such as a scaling circuit, a main control unit, and the like. The ports are used to transmit control signals or other signals to the driving circuit  18 . 
         [0004]    The AC/DC converter  17  converts an AC voltage supplied by an external source to a DC voltage of 12V, and supplies the 12V DC voltage to the inverter  13  and the DC/DC converter  12 . The DC/DC converter  12  converts the 12V DC voltage to a DC voltage of 5V, and supplies the 5V DC voltage as an operating voltage to other circuits of the interface board  10  and a main chip (not shown) of the inverter  13 . The inverter  13  converts the 12V DC voltage to an AC voltage, and supplies the AC voltage to a light source  19  of the LCD device for driving the light source  19  to emit light beams. 
         [0005]    The power board  11  and the interface board  10  are different kinds of circuit boards. Most of circuits of the interface board  10  are digital circuits, which have very strict specifications such as those relating to stability of operating voltage, noise interference, magnitude of signal ripples, and the like. In order to satisfy the strict requirements of the digital circuits of the interface board  10 , the DC/DC converter  12  is disposed on the interface board  10 . Thereby, transmission paths of the operating voltage of 5V between the DC/DC converter  12  and the digital circuits of the interface board  10  are shortened. However, the operating voltage of 5V required by the main chip of the inverter  13  is supplied by the same DC/DC converter  12 . Due to the DC/DC converter  12  being disposed on the interface board  10 , a transmission path of the 5V operating voltage along a connecting wire between the DC/DC converter  12  and the main chip of the inverter  13  is long. The voltage of the long transmission path is liable to induce noise in neighboring circuits. In addition, a resistance of the long connecting wire may be substantial, and is liable to cause a large voltage drop from the original operating voltage of 5V. In such case, the power requirements of the main chip of the inverter  13  may not be met. 
         [0006]    To overcome the above-described deficiencies, a multiplex AC/DC converter is used. The multiplex AC/DC converter can convert an AC voltage to several different DC voltages. 
         [0007]    Referring to  FIG. 4 , this shows another conventional power supply circuit  2  for an LCD device (not labeled). The power supply circuit  2  is similar to the power supply circuit  1 . However, the power supply circuit  2  includes an AC/DC converter  27 , which is a multiplex AC/DC converter. The AC/DC converter  27  supplies a DC voltage of 12V to a DC/DC converter  22  of an interface board  20  and an inverter  23  of a power board  21 , and a DC voltage of 5V to a main chip (not shown) of the inverter  23 . 
         [0008]    The DC voltage of 5V required by the main chip of the inverter  23  is supplied directly by the AC/DC converter  27 . Thereby, a transmission path of the 5V DC voltage between the AC/DC converter  27  and the main chip of the inverter  23  is shortened, and noise induced in neighboring circuits is correspondingly reduced. However, the multiplex AC/DC converter  27  has a complicated circuit construction consisting of many internal electronic components. Thus the multiplex AC/DC converter  27  is expensive, and the cost of the power supply circuit  2  is correspondingly high. 
         [0009]    What is needed, therefore, is a power supply circuit that can overcome the above-described deficiencies. 
       SUMMARY 
       [0010]    A power supply circuit used for a liquid crystal display device is provided. In one aspect, the power supply circuit includes an interface board having a first DC/DC converter, and a power board having an AC/DC converter, a second DC/DC converter, and an inverter. The AC/DC converter is configured for supplying a DC voltage to the first and second DC/DC converters. The first DC/DC converter is configured for supplying operating voltages to other circuits of the interface board, and the second DC/DC converter is configured for supplying an operating voltage to a main chip of the inverter. 
         [0011]    In another aspect, the power supply circuit includes an interface board having a first DC/DC converter, and a power board having a second DC/DC converter and an inverter. The first DC/DC converter is configured for receiving a DC voltage from the power board and converting the DC voltage to operating voltages needed by other circuits of the interface board, and the second DC/DC converter is configured for converting the DC voltage to an operating voltage needed by a main chip of the inverter. 
         [0012]    Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The emphasis in the drawings is placed upon clearly illustrating the principles of various embodiments of the present invention. Like reference numerals designate corresponding parts throughout various drawings. 
           [0014]      FIG. 1  is a block diagram of a power supply circuit according to a first embodiment of the present invention. 
           [0015]      FIG. 2  is a block diagram of a power supply circuit according to a second embodiment of the present invention. 
           [0016]      FIG. 3  is a block diagram of a conventional power supply circuit for an LCD device. 
           [0017]      FIG. 4  is a block diagram of another conventional power supply circuit for an LCD device. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    Reference will now be made to the drawings to describe preferred embodiments of the present invention in detail. 
         [0019]    Referring to  FIG. 1 , a power supply circuit  3  for an LCD device according to a first embodiment of the present invention is shown. The power supply circuit  3  includes an interface board  30  and a power board  31 . The interface board  30  includes a first DC/DC converter  32 , a plurality of ports (not shown), and other circuits (not shown). The ports are used to transmit control signals or other signals to a driving circuit  38  of the LCD device (not labeled). The power board  31  includes an AC/DC converter  37 , a second DC/DC converter  35 , and an inverter  33 . 
         [0020]    The second DC/DC converter  35  includes an input terminal  350 , a first resistor  351 , a first diode  352 , a second diode  353 , a Zener diode  354 , a second resistor  355 , and an output terminal  356 . The input terminal  350  is connected to the output terminal  356  via the first resistor  351  and the first diode  352 , and an anode (not labeled) of the first diode  352  is connected to the first resistor  351 , and a cathode (not labeled) of the first diode  352  is connected to the output terminal  356 . The input terminal  350  is grounded via the first resistor  351 , the second diode  353 , and the Zener diode  354 . An anode (not labeled) of the second diode  353  is connected to the first resistor  351 , a cathode (not labeled) of the second diode  353  is connected to a cathode (not labeled) of the Zener diode  354 , and an anode (not labeled) of the Zener diode  354  is grounded. The output terminal  356  is grounded via the second resistor  355 . 
         [0021]    The first and second diodes  352 ,  353  are used to provide a cutoff function in order to protect the Zener diode  354  when an unwanted negative voltage is applied to the second DC/DC converter  35  via the input terminal  350 . The first resistor  351  is used as a current-limiting resistor. A resistance R 1  of the first resistor  351  is governed by the following formula: R 1 &lt;(V i −V o −V d1 )/I R2 , where V i  is an input voltage of the input terminal  350  received from the AC/DC converter  37 , V o  is an output voltage of the output terminal  356  supplied to a main chip (not shown) of the inverter  33  as an operating voltage of the main chip, V d1  is a forward working voltage of the first diode  352 , and I R2  is a load current passing through the second resistor  355 . A forward working voltage V d2  of the second diode  353  is equal to the forward working voltage V d1  of the first diode  352 , and a Zener voltage V Z  is determined according to the output voltage V o . For example, if the operating voltage of the main chip of the inverter  13  is 5V, the Zener diode  354  can be chosen a Zener diode having a Zener voltage V Z  of 5V, and then the output voltage V o  is 5V. When the input voltage V i  is 12V, both forward working voltages V d1 , V d2  are 0.6V, and the resistance R 1  is in the range: R 1 &lt;(V i −V o −V d1 )/I R2 ; i.e. R 1 &lt;(12V−5V−0.6V)/I R2 ; which reduces to R 1 &lt;6.4V/I R2 . 
         [0022]    When an external source (not shown) supplies an AC voltage to the AC/DC converter  37 , the AC/DC converter  37  converts the AC voltage to a first DC voltage, and supplies the first DC voltage to the inverter  33 , the first DC/DC converter  32 , and the second DC/DC converter  35 . The first DC/DC converter  32  converts the first DC voltage to a second DC voltage, and supplies the second DC voltage as an operating voltage to other circuits of the interface board  30 . The second DC/DC converter  35  receives the first DC/DC voltage via the input terminal  350 , converts the first DC voltage to a third DC voltage, and supplies the third DC voltage as an operating voltage to the main chip of the inverter  33  via the output terminal  356 . The inverter  33  converts the first DC/DC voltage to an AC voltage by using the main chip, and supplies the AC voltage to a light source  39  of the LCD device for driving the light source  39  to emit light beams. 
         [0023]    In summary, the second DC/DC converter  35  is disposed in the power board  31 , and supplies the operating voltage directly to the main chip of the inverter  33 . Therefore a transmission path between the second DC/DC converter  35  and the main chip of the inverter  33  is shortened, and noise induced in neighboring circuits is correspondingly reduced. In addition, the second DC/DC converter  35  is comprised of relatively few standard electronic components, such as resistors, diodes, and the like, and is formed as a shunt regulating circuit to convert the first DC voltage to the third DC voltage. That is, the structure of the second DC/DC converter  35  is simple and inexpensive, so that the cost of the power supply circuit  3  is correspondingly inexpensive. Thus, the power supply circuit  3  not only provides good, efficient performance, but also has a low cost. 
         [0024]    Referring to  FIG. 2 , a power supply circuit  4  for an LCD device according to a second embodiment of the present invention is shown. The power supply circuit  4  is similar to the power supply circuit  3 . However, a second DC/DC converter  45  of a power board  41  includes an input terminal  450 , a first resistor  451 , a Zener diode  452 , a second resistor  453 , a transistor  454 , and an output terminal  460 . The input terminal  450  is used to receive a DC voltage supplied by an AC/DC converter  47  of the power board  41 . The output terminal  460  is used to supply an operating voltage to a main chip (not shown) of an inverter  43 . The input terminal  450  is connected to the output terminal  460  via the second resistor  453  and a collector (not labeled) and an emitter (not labeled) of the transistor  454 . The input terminal  450  is grounded via the first resistor  451  and the Zener diode  452 . A cathode (not labeled) of the Zener diode  452  is connected to the first resistor  451 , and an anode (not labeled) of the Zener diode  452  is grounded. A base (not labeled) of the transistor  454  is connected to the cathode of the Zener diode  452 . 
         [0025]    The transistor  454  may be a 2N3904 type transistor. When the second DC/DC converter  45  is working, the transistor  454  works in an amplifying mode; and an emitter junction of the transistor  454  is forward biased, and a collector junction of the transistor  454  is reverse biased. A voltage V CE  between the collector and the emitter may be larger than 1V, and a voltage V BE  between the base and the emitter may be 0.7V. The types of the Zener diode  452  and the transistor  454  are determined according to an output voltage V o  of the output terminal  460 . A value of the output voltage V o  is governed by the following formula: V o =V Z −V BE , where the voltage V Z  is a working voltage of the Zener diode  452 . For example, if the operating voltage of the main chip of the inverter  33  is 5V, the Zener diode  452  may be chosen to be a 1N4734 type Zener diode. Accordingly, a working voltage V Z  of the Zener diode  452  is 5.7V, and a test current I ZT  of the Zener diode  452  is 45 mA. Thus, by means of voltage stabilizing provided by the Zener diode  452 , a voltage V B  of the base is 5.7V, and a voltage V E  of the emitter is equal to V B −V BE =5.7V−0.7V=5V. That is, the output voltage V o  of the output terminal  460  is 5V. 
         [0026]    When the second DC/DC converter  45  works for a long time, it is necessary to avoid thermal deterioration of the Zener diode  452  caused by the existence of an inverse current. Thus, a resistance R 1  of the first resistor  451  is governed by the following formula: R 1 &gt;(V i −V Z )/0.7I ZT , where the voltage V i  is an input voltage of the input terminal  450  from the AC/DC converter  47 . To ensure that the transistor  454  works with an amplification characteristic, a resistance R 2  of the second resistor  453  is governed by the following formula: R 2 &lt;(V i −V o −V CE )/I 2 , where the current I 2  is an emitter load current or a working current of the main chip of the inverter  43 . For example, if the input voltage V i  is 12V and the output voltage V o  is 5V, the resistance R 1  of the first resistor  451  is in the range: R 1 &gt;(V i −V Z )/0.7I ZT ; i.e. R 1 &gt;(12V−5.7V)/(0.7×45 mA); which reduces to R 1 &gt;200Ω (ohms). Furthermore, if the working current I 2  of the main chip of the inverter  43  is 2.5 mA, the resistance R 2  of the second resistor  453  is in the range: R 2 &lt;(V i −V o −V CE )/I 2 ; i.e. R 2 &lt;(12V−5V−1V)/2.5 mA; which reduces to R 2 &lt;2400Ω. 
         [0027]    In summary, the second DC/DC converter  45  is disposed in the power board  41 , and supplies the operating voltage directly to the main chip of the inverter  43 . Therefore a transmission path between the second DC/DC converter  45  and the main chip of the inverter  43  is shortened, and noise induced in neighboring circuits is correspondingly reduced. In addition, the second DC/DC converter  45  is comprised of relatively few standard electronic components, such as resistors, diodes, and the like. That is, the structure of the second DC/DC converter  45  is simple and inexpensive, so that the cost of the power supply circuit  4  is correspondingly inexpensive. Thus, the power supply circuit  4  not only provides good, efficient performance, but also has a low cost. 
         [0028]    It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.
An exemplary power supply circuit ( 3) for a liquid crystal display device includes an interface board ( 30) having a first DC/DC converter ( 32), and a power board ( 31) having an AC/DC converter ( 37), a second DC/DC converter ( 35), and an inverter ( 33). The AC/DC converter is configured for supplying a DC voltage to the first and second DC/DC converters. The first DC/DC converter is configured for converting the DC voltage to operating voltages desired by other circuits of the interface board, and the second DC/DC converter is configured for converting the DC voltage to an operating voltage desired by a main chip of the inverter.