Abstract:
Power supplies together with related over voltage protection methods and apparatuses. A power supply has a transformer including a primary winding and an auxiliary winding. A power switch is coupled to the primary winding and a sensing resistor coupled between the power switch and a grounding line. A multi-function terminal of a controller is coupled to the sensing resistor. A diode and a first resistor is coupled between the auxiliary winding and the multi-function terminal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 14/538,813, filed on Nov. 12, 2014, which is a continuation of U.S. patent application Ser. No. 13/297,268, filed on Nov. 16, 2011, and all benefits of such earlier application are hereby claimed for this new continuation application 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    The present invention relates to a protection circuit of a power supply. 
       2. Description of the Prior Art 
       [0003]    Switching mode power supplies (SMPS), which control on and off states of a power switch to store and release energy in an inducting unit to provide required power to a load, make up the majority of power supplies due to conversion efficiency and product size thereof. 
         [0004]    For example,  FIG. 1  is a diagram showing a SMPS  10  with a flyback structure. A bridge rectifier  12  rectifies AC power to provide DC power VIN, which may be as high as 100 Volts to 300 Volts, at an IN end. A controller  18  detects a detection signal VCS across a current sensing resistor  16  via a CS end. The detection signal VCS is a voltage signal corresponding to an induction current through a primary winding  24  of a transformer  20  when a power switch  15  is turned on. The controller  18  increases or decreases the induction current by controlling on and off states of the power switch  15  via a GATE end. A secondary winding  22  provides output power VOUT to a load  30 . An auxiliary winding  23  provides operating power VCC to the controller  18 . 
         [0005]    Most switching mode power supplies need a protection mechanism to prevent abnormal conditions. A common protection mechanism is called over voltage protection, which turns off the power switch for a period of time when the output voltage VOUT is too high. 
         [0006]      FIG. 2  is a diagram showing an over voltage protection mechanism. In  FIG. 2 , an over voltage protection circuit is arranged in the controller  18 . If a comparator  32  determines a voltage of the operating power VCC exceeds a reference voltage VREF 1 , the over voltage protection is triggered. However, the voltage of the operating power VCC may not correctly correspond to the output power voltage VOUT due to inductor leakage. Therefore, the protection mechanism of  FIG. 2  is not proper. 
         [0007]      FIG. 3  is a diagram showing another over voltage protection mechanism. When the output power voltage VCC exceeds a predetermined voltage of a Zener diode  38 , a photo-coupler  36  pulls down a voltage at an input end of a comparator  34  to trigger an over voltage signal SOVP. However, the protection mechanism of  FIG. 3  needs the additional Zener diode  38  and photo-coupler  36 , which increases cost and product size. 
       SUMMARY OF THE INVENTION 
       [0008]    An embodiment of the present invention provides a power supply. The power supply includes a transformer, a power switch, a sensing resistor, a controller, and a diode and a first resistor. The transformer has a primary winding and an auxiliary winding. The power switch is coupled to the primary winding. The sensing resistor has a first terminal coupled to a grounding line and a second terminal coupled to the power switch. The controller has a multi-function terminal coupled to the sensing resistor. The diode and the first resistor are coupled between the auxiliary winding and the multi-function terminal. When the power switch is turned on, the controller through the multi-function terminal detects a current through the power switch, and when the power switch is turned off, the controller detects a voltage of the multi-function terminal, and determines to trigger a protection mechanism if the voltage of the multi-function terminal is over a reference voltage. 
         [0009]    Another embodiment of the present invention provides a controller for controlling a power switch of a power supply. The controller includes a multi-function terminal, a protection detecting circuit, and a gate controller. The multi-function terminal is coupled to the power switch and used for detecting a current through the power switch. The protection detecting circuit is used for detecting and comparing a voltage of the multi-function terminal with a reference voltage when the power switch is turned off, and triggering a protection signal if the voltage of the multi-function terminal is over a reference voltage. The gate controller is coupled to the power switch and used to turning on and turning off the power switch. If the protection mechanism is triggered, the power switch will be turned off. 
         [0010]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a diagram showing a switching mode power supply (SMPS) of the prior art. 
           [0012]      FIG. 2  and  FIG. 3  are diagrams showing over voltage protection mechanisms. 
           [0013]      FIG. 4  is a diagram showing a switching mode power supply (SMPS) of the present invention. 
           [0014]      FIG. 5  is a diagram showing part of a controller and components besides the controller. 
           [0015]      FIG. 6  is a diagram showing waveforms of signals of  FIG. 5 . 
           [0016]      FIG. 7  is another embodiment showing part of a controller and components besides the controller. 
           [0017]      FIG. 8  is a diagram showing another switching mode power supply (SMPS) of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 4  is a diagram showing a switching mode power supply  80  (SMPS  80 ) of the present invention. Different from the SMPS  10  of  FIG. 1 , the SMPS  80  comprises a Zener diode  83 , a diode  84 , resistors  86  and  88 , and a controller  82 . 
         [0019]    The controller  82  can be a single chip integrated circuit with a multi-function terminal CS/OVP. The Zener diode  83 , the diode  84 , and the resistor  86  are serially coupled between an auxiliary winding  23  and the multi-function terminal CS/OVP. The resistor  88  is coupled between the multi-function terminal CS/OVP and a current sensing resistor  16 . The multi-function terminal CS/OVP has at least two functions: (a) current detection, and (b) over voltage protection. 
         [0020]    When the controller  82  enables a gate signal V GATE  to turn on a power switch  15  via a GATE end, a voltage V CS  of the multi-function terminal CS/OVP corresponds to a current through the power switch  15 . And, the controller  82  determines when to disable the gate signal V GATE  to turn off the power switch  15  according to the voltage V CS  of the multi-function terminal CS/OVP. 
         [0021]    After the power switch  15  is turned off, a voltage V AUX  of the auxiliary winding  23  approximately corresponds to a voltage of a secondary winding as well as an output power voltage V OUT . If the voltage V AUX  is lower than a predetermined voltage of the Zener diode  83  and the diode  84 , the voltage V CS  of the multi-function terminal CS/OVP is around 0 Volts. If the voltage V AUX  is higher than the predetermined voltage, the voltage V CS  of the multi-function terminal CS/OVP is greater than 0 Volts. In order to prevent inaccuracies caused by inductor leakage, the controller  82  compares the voltage V CS  with a reference voltage after the power switch  15  is turned off for a period of delay time. If the voltage V CS  is higher than the reference voltage, an over voltage signal will be triggered, which indicates that the output power voltage V OUT  exceeds a corresponding value of the predetermined voltage of the Zener diode  83  and the diode  84 . 
         [0022]      FIG. 5  is a diagram showing part of the controller  82  and components other than the controller  82 .  FIG. 6  is a diagram showing waveforms of signals of  FIG. 5 . The controller  82  comprises a delay time generator  54 , an over voltage detection circuit  55 , and a gate controller  52 . The over voltage detection circuit  55  comprises a sampler  56  and a comparator  50 . 
         [0023]    When a signal V G  is logic “ 1 ”, the gate signal V GATE  is logic “ 1 ” as well to turn on the power switch  15 . The voltage V AUX  of the auxiliary winding  23  is negative. Since the voltage V AUX  is blocked by the diode  84 , the voltage V CS  of the multi-function terminal CS/OVP will not be affected. Therefore, a current through the power switch  15  is increased, such that the voltage V CS  increases as shown in  FIG. 6 . 
         [0024]    When the voltage V CS  reaches a certain level, the gate controller  52  switches the signal V G  to be logic “ 0 ” for turning off the power switch  15 . Once the power switch  15  is turned off, the voltage V AUX  of the auxiliary winding  23  will oscillate for a period of time and then settle to a positive value proportional to the output power voltage V OUT . The delay time generator  54  provides a delay time T DELAY  after the power switch  15  is turned off. The delay time T DELAY  is for preventing inaccuracies caused by the oscillation of the voltage V AUX . After the delay time T DELAY , the sampler  56  transmits a short pulse signal V P  for sampling the voltage of the multi-function terminal CS/OVP to generate a sampling signal V SAMP . When the short pulse signal V P  is logic “ 0 ”, the sampling signal V SAMP  is coupled to ground to be fixed at 0 Volts. When the short pulse signal V P  is logic “ 1 ”, the sampling signal V SAMP  is equal to the voltage V CS . As mentioned above, if the output power voltage V OUT  is high, the voltage V AUX  of the auxiliary winding  23  is high as well. If the voltage V AUX  of the auxiliary winding  23  is high enough to break down the Zener diode  83 , the sampling signal V SAMP  will be higher than a reference voltage V REF-OVP , such that the comparator  50  triggers an over voltage signal S OVP . For example, the triggered over voltage signal S OVP  can make the gate controller  52  keep the signal V G  at logic “ 0 ” through several on-off cycles. 
         [0025]      FIG. 7  is another embodiment showing part of a controller  82   a  and components other than the controller  82   a.    FIG. 6  can also be a diagram showing waveforms of signals of  FIG. 7 . In  FIG. 7 , the comparator  50  and a blocking unit  58  can be utilized as an over voltage detection circuit. As shown in  FIG. 7 , the comparator  50  compares the voltage of the multi-function terminal CS/OVP with the reference voltage V REF-OVP  in order to trigger a relay signal S MED . Most of the time, the relay signal S MED  is blocked by an AND gate of the blocking unit  58 . A logic level of the relay signal S MED  will be passed as a logic level of the over voltage signal S OVP  only when the short pulse signal V P  of the blocking unit  58  is logic “ 1 ”. Please refer to the illustration of  FIG. 5  for description of other operation principles of the embodiments of  FIG. 7 . Further illustrations are provided herein. 
         [0026]      FIG. 8  is a diagram showing another switching mode power supply (SMPS)  80   a  of the present invention. Different from the SMPS  80  of  FIG. 4 , the SMPS  80   a  does not comprise the Zener diode  83 . 
         [0027]    As shown in  FIG. 8 , after the power switch  15  is turned off, the voltage V CS  of the multi-function terminal CS/OVP approximately corresponds to the voltage V AUX  of the auxiliary winding  23  as well as the output power voltage V OUT . Therefore, the controller  82  compares the voltage V CS  with a reference voltage after the power switch  15  is turned off for a period of delay time. If the voltage V CS  is higher than the reference voltage, an over voltage signal will be triggered, which indicates that the output power voltage V OUT  exceeds a corresponding value of the reference voltage. Internal structure of the controller  82  of  FIG. 8  can be implemented according to the circuits of  FIG. 5  and  FIG. 7 , or other equivalent circuits. 
         [0028]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.