Abstract:
An exemplary power supply circuit ( 20 ) includes a first commutating and filter circuit ( 21 ), a transformer ( 24 ), a second commutating and filter circuit ( 25 ), a transistor ( 27 ), a pulse width modulation circuit ( 26 ) outputting a control signal to control operation state of the transistor, and a feedback circuit ( 29 ). An external alternating current voltage is converted into a direct current with a cooperation operating of the transistor, the first commutating and filter circuit, the transformer, and the second commutating and filter circuit. The feedback circuit feeds an operating state of the transformer back to the pulse width modulation circuit, and the pulse width modulation circuit outputs corresponding control signals to turn on or turn off the transistor.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a power supply circuit that can be used for liquid crystal display devices, the power supply circuit including at least one feedback circuit feeding an operating state of a transformer back to a pulse width modulation circuit. 
   BACKGROUND 
   In general, a liquid crystal display (LCD) device needs to have a power supply circuit installed therein, for converting an external alternating current (AC) voltage into a direct current (DC) voltage. A typical power supply circuit is shown in  FIG. 4 . The power supply circuit  10  includes two input terminals  111  and  112 , an output terminal  150 , a first commutating and filter circuit  11 , a transformer  14 , a second commutating and filter circuit  15 , a pulse width modulation circuit  16 , a transistor  17 , and a current limiting resistor  170 . 
   The first commutating and filter circuit  11  includes a full-bridge rectifier circuit  113  and a first filter capacitor  114 . The full-bridge rectifier circuit  113  includes two input terminals (not labeled), a positive output terminal (not labeled), and a negative output terminal (not labeled). The two input terminals of the full-bridge rectifier circuit  113  are connected to the two input terminals  111  and  112 , respectively. The positive output terminal of the full-bridge rectifier circuit  113  is connected to ground via the first filter capacitor  114 . The negative output terminal of the full-bridge rectifier circuit  113  is directly connected to ground. 
   The transformer  14  includes a primary winding  141  and a secondary winding  142 . The primary winding  141  includes two taps (not labeled). One of the taps of the primary winding  141  is connected to the positive output terminal of the full-bridge rectifier circuit  113 , and the other tap of the primary winding  141  is connected to a source electrode of the transistor  17 . The secondary winding  142  also includes two taps (not labeled). 
   The second commutating and filter circuit  15  includes a first resistor  151 , a first capacitor  152 , a first diode  153 , a second diode  154 , a second filter capacitor  155 . The first resistor  151  and the first capacitor  152  are connected in series between a first one of the taps of the secondary winding  142  and the output terminal  150 . An anode of the first diode  153  is connected to the first tap of the secondary winding  142 , and a cathode of the first diode  153  is connected to the output terminal  150 . An anode of the second diode  154  is also connected to the first tap of the secondary winding  142 , and a cathode of the second diode  154  is connected to the output terminal  150 . The second tap of the secondary winding  142  is connected to ground. The output terminal  150  is connected to ground via the second filter capacitor  155 . 
   The pulse width modulation circuit  16  includes a control port  161 . The control port  161  is used to output a high level signal or a low level signal to turn on or turn off the transistor  17 . 
   A gate electrode of the transistor  17  is connected to the control port  161  of the pulse width modulation circuit  16 . A drain electrode of the transistor  17  is connected to ground via the current limiting resistor  170 . 
   The external AC voltage is inputted to the two input terminals  111  and  112 . The AC voltage is converted into a direct current (DC) voltage via the first commutating and filter circuit  11 . 
   When the transistor  17  is turned on, the first filter capacitor  114 , the primary winding  141  of the transformer  14 , the transistor  17 , and the current limiting resistor  170  form a first circuit path (not labeled). The first filter capacitor  114  can be regarded as a power source, and the primary winding  141  can be regarded as an inductor. A current flowing through the primary winding  141  linearly increases until the current reaches a maximum value when a voltage of the first filter capacitor  114  is constant. Such voltage can be expressed by the following equation (1): 
                 V   =     L   ⁢       ⅆ   I       ⅆ   t                 (   1   )               
where V represents the voltage of the first filter capacitor  114 , L represents an inductance of the primary winding  141 , I represents the current flowing through the primary winding  141 , and t represents time.
 
   When the transistor  17  is turned off, electrical energy stored in the primary winding  141  is transmitted to the secondary winding  142 , and is then converted into a steady DC voltage via the second commutating and filter circuit  15 . Then the steady DC voltage is outputted to circuits in other parts of the LCD device via the output terminal  150 . 
   However, in practice, the external AC voltage may increase suddenly. When this happens, electrical energy stored in the primary winding  141  and the secondary winding  142  of the transformer  14  may increase significantly. The voltage outputted by the second commutating and filter circuit  15  correspondingly increases significantly. Thus, the output terminal  150  may apply a large voltage to the circuits in the other parts of the LCD device. The large voltage is liable to disrupt normal operation of the LCD device. 
   Furthermore, when the external AC voltage increases suddenly, the current flowing through the first circuit path and a current flowing through the second commutating and filter circuit  15  correspondingly increase significantly. If any electronic component of the power supply circuit  10  is thereby impaired or damaged, the whole power supply circuit  10  is liable to be burned out. 
   Accordingly, what is needed is a power supply circuit for LCD devices that can overcome the above-described deficiencies. 
   SUMMARY 
   In one aspect, a power supply circuit includes a first commutating and filter circuit, a transformer, a second commutating and filter circuit, a transistor, a pulse width modulation circuit, and at least one feedback circuit. The transformer includes a primary winding, a secondary winding, and at least one auxiliary winding. The pulse width modulation circuit includes at least one feedback port and a control port. The control port is configured for outputting one of a high level signal and a low level signal to effect a selective one of turning on or turning off the transistor. An external alternating current voltage is converted into a first direct current voltage via the first commutating and filter circuit. When the transistor is turned on, electrical energy stored in the primary winding linearly increases until the electrical energy reaches a maximum value. When the transistor is turned off, the electrical energy stored in the primary winding is transmitted to the secondary winding, then converted into a second direct current voltage via the second commutating and filter circuit. Electrical energy stored in the at least one auxiliary winding is fed back into the at least one feedback port of the pulse width modulation circuit. The pulse width modulation circuit compares a value of a feedback signal of the at least one feedback port and a reference value stored in the pulse width modulation circuit, and when the value of the feedback signal is larger than the reference value, the control port of the pulse width modulation circuit outputs said one of the high level signal and the low level signal to effect turning off the transistor. 
   In another aspect, a power supply circuit includes a first commutating and filter circuit, a transformer, a second commutating and filter circuit, a transistor, a pulse width modulation circuit outputting control signals to turn on and turn off the transistor, and at least one feedback circuit. An external alternating current voltage is converted into a direct current voltage by the transistor, the first commutating and filter circuit, the transformer, and the second commutating and filter circuit in cooperation. The at least one feedback circuit feeds an operating state of the transformer back to the pulse width modulation circuit, the pulse width modulation circuit outputs corresponding control signals to turn on or turn off the transistor. 
   Other novel features and advantages will become apparent from the following detailed description of preferred and exemplary embodiments when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a power supply circuit according to a first embodiment of the present invention. 
       FIG. 2  is a diagram of a power supply circuit according to a second embodiment of the present invention. 
       FIG. 3  is a diagram of a power supply circuit according to a third embodiment of the present invention. 
       FIG. 4  is a diagram of a conventional power supply circuit. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference will now be made to the drawings to describe preferred and exemplary embodiments in detail. 
     FIG. 1  is a diagram of a power supply circuit  20  according to a first embodiment of the present invention. The power supply circuit  20  includes two input terminals  211  and  212 , an output terminal  250 , a first commutating and filter circuit  21 , a protection circuit  23 , a transformer  24 , a second commutating and filter circuit  25 , a pulse width modulation circuit  26 , a transistor  27 , a first feedback circuit  28 , a second feedback circuit  29 , and a current limiting resistor  270 . The transistor  27  is an N-channel enhancement mode metal-oxide-semiconductor field-effect transistor. 
   The first commutating and filter circuit  21  includes a full-bridge rectifier circuit  213  and a first filter capacitor  214 . The full-bridge rectifier circuit  213  includes two input terminals (not labeled), a positive output terminal (not labeled), and a negative output terminal (not labeled). The two input terminals of the full-bridge rectifier circuit  213  are connected to the two input terminals  211  and  212 , respectively. The positive output terminal of the full-bridge rectifier circuit  213  is connected to ground via the first filter capacitor  214 . The negative output terminal of the full-bridge rectifier circuit  213  is directly connected to ground. 
   The protection circuit  23  includes a first capacitor  231 , a first resistor  232 , and a first diode  233 . The first capacitor  231  is connected between the positive output terminal of the full-bridge rectifier circuit  213  and a cathode of the first diode  233 . The first resistor  232  is also connected between the positive output terminal of the full-bridge rectifier circuit  213  and the cathode of the first diode  233 . 
   The transformer  24  includes a primary winding  241 , a secondary winding  242 , and an auxiliary winding  243 . The primary winding  241  includes two taps (not labeled). One of the taps of the primary winding  241  is connected to the positive output terminal of the full-bridge rectifier circuit  213 , and the other tap of the primary winding  241  is connected to an anode of the first diode  233 . The secondary winding  242  and the auxiliary winding  243  include two taps (not labeled), respectively. 
   The second commutating and filter circuit  25  includes a second resistor  251 , a second capacitor  252 , a second diode  253 , a third diode  254 , and a second filter capacitor  255 . The second resistor  251  and the second capacitor  252  are connected in series between a first one of the taps of the secondary winding  242  and the output terminal  250 . An anode of the second diode  253  is connected to the first tap of the secondary winding  242 , and a cathode of the second diode  253  is connected to the output terminal  250 . An anode of the third diode  254  is connected to the first tap of the secondary winding  242 , and a cathode of the third diode  254  is connected to the output terminal  250 . The second tap of the secondary winding  242  is connected to ground. The output terminal  250  is connected to ground via the second filter capacitor  255 . 
   The pulse width modulation circuit  26  includes a control port  261  and a first feedback port  262 . The control port  261  is used to output a high level signal or a low level signal to turn on or turn off the transistor  27 . The first feedback port  262  is a voltage sampling port. 
   A gate electrode of the transistor  27  is connected to the control port  261  of the pulse width modulation circuit  26 . A source electrode of the transistor  27  is connected to the anode of the first diode  233 . A drain electrode of the transistor  27  is connected to ground via the current limiting resistor  270 . 
   The first feedback circuit  28  includes a third resistor  281  and a third capacitor  282 . The third resistor  281  is connected between the first feedback port  262  of the pulse width modulation circuit  26  and the drain electrode of the transistor  27 . The third capacitor  282  is connected between the first feedback port  262  of the pulse width modulation circuit  26  and the drain electrode of the transistor  27 . 
   The second feedback circuit  29  includes a fourth resistor  291 , a fifth resistor  292 , and a fourth diode  293 . A cathode of the fourth diode  293  is connected to the first feedback port  262  of the pulse width modulation circuit  26  via the fourth resistor  291 . An anode of the fourth diode  293  is connected to one of the taps of the auxiliary winding  243 . The other tap of the auxiliary winding  243  is connected to ground. The first feedback port  262  of the pulse width modulation circuit  26  is connected to ground via the fifth resistor  292 . 
   An external AC voltage is applied to the two input terminals  211  and  212 , and is converted into a direct current DC voltage when passing through the first commutating and filter circuit  21 . 
   When the transistor  27  is turned on, the first filter capacitor  214 , the primary winding  241  of the transformer  24 , the transistor  27 , and the current limiting resistor  270  cooperatively form a first circuit path (not labeled). The first filter capacitor  214  can be regarded as a power source, and the primary winding  241  can be regarded as an inductor. A current flowing through the primary winding  241  linearly increases until the current reaches a maximum value when a voltage of the first filter capacitor  214  is constant. The voltage it can be expressed by the following equation (2): 
                 V   =     L   ⁢       ⅆ   I       ⅆ   t                 (   2   )               
wherein V represents the voltage of the first filter capacitor  214 , L represents an inductance of the primary winding  241 , I represents the current flowing through the primary winding  241 , and t represents time.
 
   The auxiliary winding  243  of the transformer  24 , the fourth diode  293 , the fourth resistor  291 , and the fifth resistor  292  cooperatively form a second circuit path (not labeled). The auxiliary winding  243  can be regarded as a power source. An induction electromotive force of the auxiliary winding  243  is constant while the current flowing through the primary winding  241  is linearly increasing. The induction electromotive force can be expressed by the following equation (3): 
                 ɛ   =     M   ⁢       ⅆ   I       ⅆ   t                 (   3   )               
where ε represents the induction electromotive force of the auxiliary winding  243 , M represents a mutual inductance of the primary winding  241  and the auxiliary winding  243 , I represents the current flowing through the primary winding  241 , and t represents time.
 
   A voltage of the current limiting resistor  270  is inputted to the first feedback port  262  of the pulse width modulation circuit  26  via the first feedback circuit  28 . When the induction electromotive force of the auxiliary winding  243  is constant, the voltage of the fifth resistor  292  is constant accordingly. The voltage of the fifth resistor  292  is inputted to the first feedback port  262  of the pulse width modulation circuit  26 . That is, the first feedback port  262  receives a sum of the voltage of the current limiting resistor  270  and the voltage of the fifth resistor  292 . The pulse width modulation circuit  26  compares a value of the sum voltage and a reference value stored in the pulse width modulation circuit  26 . When the value of the sum voltage is larger than the reference value, the control port  261  of the pulse width modulation circuit  26  outputs the low level signal to turn off the transistor  27 . 
   When the transistor  27  is turned off, electrical energy stored in the primary winding  241  is transmitted to the secondary winding  242 , and is then converted into a steady DC voltage via the second commutating and filter circuit  25 . An excitation current of the primary winding  241  is consumed by the protection circuit  23 . 
   One test result of an over current threshold point of the output terminal  250  of the power supply circuit  20  is as follows. When the external AC voltage is 100V (volts), the over current point of the output terminal  250  is 2.61 A (amperes); and when the external AC voltage is 240V, the over current point of the output terminal  250  is 2.62 A. That is, the over current point of the output terminal  250  of the power supply circuit  20  generally remains constant regardless of the external AC voltage. 
   In practice, the external AC voltage may increase suddenly. When this happens, electrical energy stored in the primary winding  241  of the transformer  24  may increase significantly, and the induction electromotive force of the auxiliary winding  243  correspondingly increases. The change of the electrical energy stored in the auxiliary winding  243  is fed back to the pulse width modulation circuit  26  via the second feedback circuit  29 . When the value of the feedback signal is larger than the reference value stored in the pulse width modulation circuit  26 , the control port  261  of the pulse width modulation circuit  26  outputs the low level signal to turn off the transistor  27 . The first circuit path is in an open circuit state accordingly. The electrical energy stored in the primary winding  241  does not increase, and does not exceed a maximum safe threshold value in respect of the primary winding  241 . The electrical energy stored in the secondary winding  242  correspondingly does not exceed a maximum safe threshold value in respect of the secondary winding  242 . Thus, the DC voltage outputted by the output terminal  250  remains constant. That is, the LCD device still operates normally when the external AC voltage increases suddenly. 
   Because the electrical energy stored in the primary winding  241  is not larger than the maximum safe threshold value thereof when the external AC voltage increases suddenly, and the electrical energy stored in the secondary winding  242  is not larger than the maximum safe threshold value thereof correspondingly, the current flowing through the first circuit path does not exceed a maximum safe threshold value in respect of the first circuit path, and a current flowing through the second commutating and filter circuit  15  does not exceed a maximum safe threshold value in respect of the second commutating and filter circuit  15 . Thus, a risk of the power supply circuit  20  being burned out is effectively reduced or even eliminated. 
   Furthermore, when the external AC voltage is supplied by various electrical power sources, the over current point of the output terminal  250  of the power supply circuit  20  generally remains constant regardless of the external AC voltage. 
   In the first embodiment of the present invention, the transistor  27  can also be a P-channel depletion mode metal-oxide-semiconductor field-effect transistor. Under this condition, the transistor  27  is turned off when the gate electrode receives a high level signal, and the transistor  27  is turned on when the gate electrode receives a low level signal. 
     FIG. 2  is a diagram of a power supply circuit  30  according to a second embodiment of the present invention. The power supply circuit  30  is similar to the power supply circuit  20 . However, unique characteristics of the power supply circuit  30  are as follows: 
   The power supply circuit  30  further includes a third feedback circuit  35 , an eighth resistor  371 , and a fifth capacitor  372 . The transformer  34  further includes a second auxiliary winding  344 . The second auxiliary winding  344  includes two taps (not labeled). The pulse width modulation circuit  36  includes a second feedback port  363 . The second feedback port  363  is an over voltage protection port. The feedback circuit  35  includes a zener diode  351 , a sixth resistor  352 , a seventh resistor  353 , a fifth diode  354 , and a fourth capacitor  355 . One of the taps of the second auxiliary winding  344  is connected to an anode of the fifth diode  354  via the seventh resistor  353 , and the other tap of the second auxiliary winding  344  is connected to ground. A cathode of the fifth diode  354  is connected to a cathode of the zener diode  351 , and the cathode of fifth diode  354  is also connected to ground via the fourth capacitor  355 . The cathode of the zener diode  351  is also connected to ground via the sixth resistor  352 , and an anode of the zener diode  351  is connected to the second feedback port  363  of the pulse width modulation circuit  36 . The eighth resistor  371  and the fifth capacitor  372  are connected in parallel between the second feedback port  363  and ground. 
   When the transistor  37  is turned off, the second auxiliary winding  344  of the transformer  34 , the seventh resistor  353 , the fifth diode  354 , and the sixth resistor  352  form a third circuit path (not labeled). The second auxiliary winding  344  can be regarded as a power source. A voltage of the sixth resistor  352  increases while an induction electromotive force of the second auxiliary winding  344  is increasing. The voltage can be expressed by the following equation (4): 
                   V   6     =         R   6     ×     ɛ   2           R   6     +     R   D     +     R   7                 (   4   )               
where V 6  represents the voltage of the sixth resistor  352 , R 6  represents a resistance of the sixth resistor  352 , R D  represents an equivalent resistance of the fifth diode  354 , R 7  represents a resistance of the seventh resistor  353 , and ε 2  represents the induction electromotive force of the second auxiliary winding  344 .
 
   A voltage of the zener diode  351  is constant because of its own steady voltage characteristic. The second feedback port  363  of the pulse width modulation circuit  36  receives a sum of the voltage of the zener diode  351  and the voltage of the sixth resistor  352 . 
   When the voltage outputted by the output terminal  350  is larger than a predetermined maximum threshold, the electrical energy stored in the secondary winding  342  is larger than a normal value thereof. The electrical energy stored in the second auxiliary winding  344  is larger than a normal value thereof accordingly. The voltage of the sixth resistor  352  increases, and the sum of the voltage of the zener diode  351  and the voltage of the sixth resistor  352  correspondingly increases. The pulse width modulation circuit  36  compares a value of the sum voltage and a reference value stored in the pulse width modulation circuit  36 . When the value of the sum voltage is larger than the reference value, the control port  361  of the pulse width modulation circuit  36  outputs a low level signal to turn off the transistor  37 . That is, the power supply circuit  30  has an over voltage protection function, as compared with the power supply circuit  20 . 
     FIG. 3  is a circuit diagram of a power supply circuit  40  according to a third embodiment of the present invention. The power supply circuit  40  includes two input terminals  411  and  412 , an output terminal  450 , a first commutating and filter circuit  41 , a transformer  44 , a second commutating and filter circuit  45 , a pulse width modulation circuit  46 , a transistor  47 , a feedback circuit  49 , and a current limiting resistor  470 . The first commutating and filter circuit  41  is substantially the same as the first commutating and filter circuit  21  of the power supply circuit  20 . The transformer  44  is substantially the same as the transformer  24  of the power supply circuit  20 . The second commutating and filter circuit  45  is substantially the same as the second commutating and filter circuit  25  of the power supply circuit  20 . The pulse width modulation circuit  46  is substantially the same as the pulse width modulation circuit  26  of the power supply circuit  20 . The transistor  47  is substantially the same as the transistor  27  of the power supply circuit  20 . The current limiting resistor  470  is substantially the same as the current limiting resistor  270  of the power supply circuit  20 . Connection relationships of the two input terminals  411  and  412 , the output terminal  450 , the first commutating and filter circuit  41 , the transformer  44 , the second commutating and filter circuit  45 , the pulse width modulation circuit  46 , the transistor  47 , and the current limiting resistor  470  are substantially the same as those of the two terminals  211  and  212 , the output terminal  250 , the first commutating and filter circuit  21 , the transformer  24 , the second commutating and filter circuit  25 , the pulse width modulation circuit  26 , the transistor  27 , and the current limiting resistor  270  of the power supply circuit  20 . 
   The feedback circuit  49  includes a ninth resistor  491 , a tenth resistor  492 , an eleventh resistor  493 , a twelfth resistor  494 , a bipolar transistor  495 , a zener diode  496 , a sixth capacitor  497 , a sixth diode  498 , and a seventh capacitor  499 . The pulse width modulation circuit  46  further includes a third feedback port  464 . The third feedback port  464  is an under voltage protection port. The ninth resistor  491  and the tenth resistor  492  are connected in series between the positive output terminal of the full-bridge rectifier circuit  413  and ground. The eleventh resistor  493  is connected between the third feedback port  464  and a collector electrode of the bipolar transistor  495 . An emitter electrode of the bipolar transistor  495  is connected to a cathode of the sixth diode  498 , and a base electrode of the bipolar transistor  495  is connected to a cathode of the zener diode  496 . The cathode of the sixth diode  498  is also connected to ground via the sixth capacitor  497 . An anode of the sixth diode  498  is connected to one of the taps of the auxiliary winding  443 . The other tap of the auxiliary winding  443  is connected to ground. An anode of the zener diode  496  is connected to ground via the twelfth resistor  494 . The third feedback port  464  is connected to ground via the seventh capacitor  499 . 
   A voltage of the tenth resistor  492  is inputted to the third feedback port  464  of the pulse width modulation circuit  46 . When the external AC voltage is less than a predetermined minimum threshold, a value of the voltage of the tenth resistor  492  is less than a reference value stored in the pulse width modulation circuit  46 . The pulse width modulation circuit  46  is in a clamping protection state accordingly. 
   When a voltage outputted by the output terminal  450  is larger than a predetermined maximum threshold, electrical energy stored in the secondary winding  442  is larger than a normal value thereof. Electrical energy stored in the auxiliary winding  443  is larger than a normal value thereof accordingly. A voltage of the emitter electrode of the bipolar transistor  495  is larger than a normal value thereof, but a voltage of the base electrode of the bipolar transistor  495  is constant because of the voltage stabilizing function of the zener diode  496 . Thus, the bipolar transistor  495  is turned on. The auxiliary winding  443  of the transformer  44 , the sixth diode  498 , the bipolar transistor  495 , the eleventh resistor  493 , and the tenth resistor  492  form a fourth circuit path (not labeled). The auxiliary winding  443  can be regarded as a power source. When the electrical energy stored in the auxiliary winding  443  increases, the voltage of the tenth resistor  492  correspondingly increases. The voltage of the tenth resistor  492  is inputted to the third feedback port  464  of the pulse width modulation circuit  46 . The pulse width modulation circuit  46  compares the value of the voltage and the reference value stored in the pulse width modulation circuit  46 . When the value of the voltage is larger than the reference value, the control port  461  of the pulse width modulation circuit  46  outputs a high level signal to turn off the transistor  47 . That is, the power supply circuit  40  has an over voltage protection function, 
   The power supply circuit  40  utilizes the third feedback port  464  of the pulse width modulation circuit  46  to realize the over voltage protection function, and the third feedback port  464  is also the under voltage protection port of the power supply circuit  40 . Thus, an application range of the under voltage protection port of the pulse width modulation circuit  46  is extended. 
   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.