Abstract:
A power supply circuit includes a direct current (DC) voltage source, a protecting circuit having a first switching element, a pulse width modulation (PWM) circuit having a first terminal, a switching circuit, and a transformer. The DC voltage source is configured to provide a first DC voltage. The first terminal is configured to receive the first DC voltage via the first switching element to enable the PWM circuit. The PWM circuit is configured to switch on or switch off the switching circuit. The transformer is configured to convert the first DC voltage to an alternating current (AC) voltage in cooperation with the switching circuit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a power supply circuit, and more particularly to a power supply circuit with a protecting circuit. 
       GENERAL BACKGROUND 
       [0002]    Power supply circuits have the advantages of low weight, small size, low power consumption; and have been widely used in various electronic devices, such as liquid crystal display (LCD) monitors and televisions. 
         [0003]    A typical power supply circuit includes a bridge rectifier circuit for converting an external alternating current (AC) voltage to a high level direct current (DC) voltage, a filter circuit for filtering the high level DC voltage to a stable DC voltage, and an inverter circuit for converting the stable DC voltage to a desired low level AC voltage. In order to maintain the stable DC voltage, the filter circuit generally needs a filter capacitor with large capacity. 
         [0004]    When an electronic device using the power supply circuit is powered off, the power supply circuit is turned off. Due to its large capacity, the filter capacitor may store a large quantity of electric energy. This may cause the inverter circuit to work in an abnormal state after the power supply circuit is turned off. As a result, the inverter circuit may be damaged or completely broken. Thus the reliability of the power supply circuit is somewhat low. 
         [0005]    What is needed is to provide a power supply circuit that can overcome the above-described deficiencies. 
       SUMMARY 
       [0006]    An aspect of the disclosure relates to a power supply circuit including a DC voltage source, a protecting circuit having a first switching element, a PWM circuit having a first terminal, a switching circuit, and a transformer. The DC voltage source is configured to provide a first DC voltage. The first terminal is configured to receive the first DC voltage via the first switching element to enable the PWM circuit. The PWM circuit is configured to switch on or switch off the switching circuit. The transformer is configured to convert the first DC voltage to an AC voltage in cooperation with the switching circuit. 
         [0007]    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 
         [0008]    The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of at least one embodiment. In the drawings, like reference numerals designate corresponding parts throughout the various views. 
           [0009]      FIG. 1  is a diagram of a power supply circuit according to a first embodiment of the present invention. 
           [0010]      FIG. 2  is a diagram of a power supply circuit according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0011]    Reference will now be made to the drawings to describe exemplary embodiments of the present invention in detail. 
         [0012]      FIG. 1  is a diagram of a power supply circuit  200  according to a first embodiment of the present invention. The power supply circuit  200  can be employed to provide power for an electronic device, such as an LCD. The power supply circuit  200  includes a full bridge rectifier circuit  211 , a filter capacitor  212 , a transformer  220 , a rectifier and filter circuit  230 , a PWM circuit  240 , a rectifying diode  252 , a protecting circuit  280 , and a switching circuit  290 . 
         [0013]    The protecting circuit  280  includes a first transistor  261 , a second transistor  271 , a diode  262 , a first current-limiting resistor  253 , a first bias resistor  264 , a second bias resistor  263 , and a third bias resistor  272 . The switching circuit  290  includes a third transistor  251  and a second current-limiting resistor  254 . Each of the first transistor  261  and the second transistor  271  is a negative-positive-negative (NPN) type bipolar junction transistor (BJT). The third transistor  251  is a n-channel metal oxide semiconductor field effect transistor (MOSFET). 
         [0014]    The full bridge rectifier circuit  211  is configured for converting an external AC voltage to a primary DC voltage, and the filter capacitor  212  is configured for converting the primary DC voltage to a stable DC voltage. The full bridge rectifier circuit  211  includes two input terminals  213  and  214 , a positive output terminal  215 , and a negative output terminal  216 . The two input terminals  213  and  214  are used to receive the external AC voltage. The positive output terminal  215  is connected to one end of the filter capacitor  212  via an anode and a cathode of the diode  262  in series. The negative output terminal  216  and the other end of the filter capacitor  212  are grounded. The filter capacitor  212  can be an electrolytic capacitor. 
         [0015]    The transformer  220  includes a first winding  221 , a second winding  222 , and a third winding  223 . One end of the first winding  221  is connected to the cathode of the first diode  262  for receiving the stable DC voltage, and the other end of the first winding  221  is grounded via a drain electrode and a source electrode of the third transistor  251  and the second current-limiting resistor  254  in series. The second winding  222  is connected to the rectifier and filter circuit  230 . One end of the third winding  223  is grounded, and the other end of the third winding  223  is connected to the PWM circuit  240  via an anode and a cathode of the rectifying diode  252  and the second transistor  271  in series. 
         [0016]    The PWM circuit  240  includes a first terminal  241  receiving the stable DC voltage for enabling the PWM circuit  240 , a second terminal  242  receiving a power voltage signal for working, and a controlling terminal  243  outputting a pulse signal to switch on or switch off the switching circuit  290 . The first terminal  241  is connected to the anode of the diode  262  via an emitter electrode and a collector electrode of the first transistor  261  and the first current-limiting resistor  253  in series. The second terminal  242  is connected to the cathode of the rectifying diode  252  via the emitter electrode and the collector electrode of the second transistor  271 . The controlling terminal  243  is connected to a gate electrode of the third transistor  251 . The anode of the first diode  262  is grounded via the first bias resistor  264  and the second bias resistor  263  in series. A base electrode of the first transistor  261  is connected to a node between the first bias resistor  264  and the second bias resistor  263 . A base electrode of the second transistor  271  is connected to the node between the first bias resistor  264  and the second bias resistor  263  via the third bias resistor  272 . 
         [0017]    Typical operation of the power supply circuit  200  is as follows. An external AC voltage is provided to the bridge rectifier circuit  211  and is converted into a primary DC voltage. The primary DC voltage is then provided to the filter capacitor  212  via the diode  262  and is converted to a stable DC voltage. The primary DC voltage is also provided to the base electrode of the first transistor  261  via the second bias resistor  263 , and to the base electrode of the second transistor  271  via the second bias resistor  263  and the third bias resistor  272  in series. Thereby, the first transistor  261  and the second transistor  271  are both switched on. The stable DC voltage is further provided to the first winding  221  of the transformer  220 ; and is also provided to the first terminal  241  of the PWM circuit  240  via the first current-limiting resistor  253 , the emitter electrode and the collector electrode of the first transistor  261 , in series. Thereby, the PWM circuit  240  is enabled and outputs a pulse signal via the controlling terminal  243  so as to switch on or switch off the third transistor  251  of the switching circuit  290 . 
         [0018]    When the third transistor  251  is switched on, the first winding  221  is grounded via the second current-limiting resistor  254 . A current I is generated and flows through the first winding  221 . When the third transistor  251  is switched off, the current I decreases. The first winding  221  generates a variable magnetic field during the transition from the switching on to the switching off of the third transistor  251 , and during the transition from the switching off to the switching on of the third transistor  251 . 
         [0019]    Due to the variable magnetic field, the second winding  222  and the third winding  223  respectively generate a first AC voltage signal and a second AC voltage signal. The first AC voltage signal is converted to a desired DC output voltage via the rectifier and filter circuit  230 , and is applied to a load circuit. The second AC voltage signal is rectified by the rectifying diode  252 , and is converted to an internal DC power voltage signal. The internal DC power voltage signal is then supplied to the PWM circuit  240  via the second transistor  271 . 
         [0020]    When the power supply circuit  200  is turned off, the electric energy stored in the filter capacitor  212  cannot be provide to the base electrodes of the first transistor  261  and the second transistor  271  because of the unilateral conduction characteristic of the first diode  262 . Thus, the first transistor  261  and the second transistor  271  are switched off immediately, and the PWM circuit  240  is turned off correspondingly. The electric energy stored in the filter capacitor  212  is prevented from being provided to the first terminal  241  and the second terminal  242  of the PWM circuit  240 , and is not released until the power supply circuit  200  is started up next time. 
         [0021]    In summary, when the power supply circuit  200  is turned off, the first transistor  261  and the second transistor  271  of the protecting circuit  280  are switched off immediately, so that the PWM circuit  240  is turned off immediately. Thereby the reliability of the PWM circuit  240  and the power supply circuit  200  is improved. Moreover, because the electric energy stored in the filter capacitor  212  is released when the power supply circuit  200  is started up next time, the turn-on time of the PWM circuit  240  and the power supply circuit  200  is also reduced. 
         [0022]      FIG. 2  is a diagram of a power supply circuit  300  according to a second embodiment of the present invention. The power supply circuit  300  is similar to the above-described power supply circuit  200 , differing only in that a first transistor  361  and a second transistor  371  are n-channel MOSFETs. 
         [0023]    It is to be understood, however, that even though numerous characteristics and advantages of the present 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, especially in matters of arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.