Abstract:
A switching power supply includes an energy-storing device, a power switch, a driving circuit and a thermal sensing device. The energy-storing device is coupled to an input power source and controlled by the power switch to increase or decrease the power therein. The power switch has a control terminal connected to the driving circuit for switching. The thermal sensing device is connected to the control terminal of the power switch and powered by the driving circuit. When sensing an operation temperature exceeding a predetermined range, the thermal sensing device disables the driving circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an over-temperature protection method and apparatus thereof for a switching power supply. 
     2. Description of the Prior Art 
     At present, most consumer electronic devices adopt switching power supplies as power supplies. The switching power supply controls the energy storage and release of an inductor via switching a power switch to provide a power supply fulfilling specification requirements. In order to prevent damage to equipments and risk to public security, most switching power supplies are equipped with various protection mechanisms, e.g., over-voltage protection (OVP), over-current protection (OCP), over-load protection (OLP), over-temperature protection (OTP), etc. to prevent the occurrence of the aforementioned damage or risk. 
     An OTP mechanism usually adopts a thermistor whose resistance varies with temperature to observe the temperature variation of a monitored object. When the temperature of the monitored object exceeds a permitted range, at least part of the operation of the switching power supply will be stopped. 
       FIG. 1  is a diagram of a switching power supply  60  adopting conventional OTP. Switching power supply  60  is a flyback power converter which converts energy inputted by the AC (alternating current) power source V AC  into an output power source V OUT  which meets a requirement of a specification. Bridge rectifier  62  substantially rectifies the AC power source V AC . Power switch  72  controls a current in primary coil L P  in transformer  64 . When power switch  64  is turned on, the energy stored in transformer  64  is increased; when power switch  64  is turned off, the energy stored in transformer  64  is released via second coil L S . The released energy is stored in output capacitor  69  through rectifier  66  and therefore generates the output power source V OUT . Feedback circuit  68  monitors a magnitude (e.g., a current, a voltage, or a power) of the output power source V OUT  and provides a feedback signal to controller  74 . 
     An OTP mechanism is provided by resistor  78  and thermistor  76 , which are connected between an input power source V in  and an electrical ground GND. For example, assuming the resistance of thermistor  76  inversely proportional to the temperature, when thermistor  76  has a higher resistance at a lower temperature, controller  74  receives a logic “0” signal from enable pin “enb” and is thereby enabled. Switching power supply  60  normally provides the output power source V OUT . Once the temperature of thermistor  76  exceeds a certain extent and the resistance thereof becomes smaller, controller  74  receives a logic “1” signal from enable pin “enb”, thereby stopping switching of the power switch  72 . 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, a switching power supply is disclosed. The switching power supply includes an energy-storing device, a power switch, a driving circuit and a thermal sensing device. The energy-storing device is coupled to an input power source. The power switch controls the energy-storing device to increase or decrease an electric power within the energy-storing device, and has a control terminal. The driving circuit, coupled to the control terminal of the power switch, is implemented for switching the power switch. The thermal sensing device, coupled to the control terminal of the power switch, is powered by the driving circuit. When sensing that an ambient temperature exceeds a predetermined range, the thermal sensing device disables the driving circuit. 
     In another embodiment of the present invention, over-temperature protection method is provided. The over-temperature protection method includes: providing an integrating circuit chip which comprises a driving circuit for driving one terminal of a power switch, wherein the integrated circuit chip further comprises a pin and a detecting circuit; coupling a thermal sensing device to the control terminal and the pin; detecting a characteristic of the pin when the power switch is turned on; and disabling the driving circuit when the characteristic of the pin exceeds a predetermined range. 
     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 
         FIG. 1  is a diagram of a switching power supply adopting conventional over-temperature protection. 
         FIG. 2  is a diagram of a switching power supply according to an embodiment of the present invention. 
         FIG. 3  is a zoom-in diagram of partial circuits in  FIG. 2 . 
         FIG. 4  is a diagram of a switching power supply according to another embodiment of the present invention. 
         FIG. 5  is a zoom-in diagram of partial circuits in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     To facilitate a further comprehension of objectives, characteristics and advantages of the present invention, the following paragraphs bring out preferred embodiments in conjunction with accompanying drawings for detailed explanation. 
     For ease of explanation, same or similar functions will be represented by the same element symbol. Therefore, the same symbols in different embodiments do not necessarily mean that two elements are completely the same. The scope of the present invention is dependent on the limitations recited in the claims. 
       FIG. 2  is a diagram of a switching power supply  80  according to an embodiment of the present invention. Switching power supply  80  is a flyback power converter converting energy inputted by the AC power source V AC  into an output power source V OUT . All the same or similar elements represented by the same symbol in  FIG. 1  and  FIG. 2  are explained in the prior art, and therefore further description will be omitted here for brevity. Unlike the conventional configuration shown in  FIG. 1 , thermistor  86  and resistor  88  in this embodiment are connected in series between the control terminal of power switch  72  and the electrical ground GND; the connecting point between thermistor  86  and resistor  88  is connected to pin “enb” of controller  74   a . When controller  74   a  turns off power switch  72  with a low voltage, thermistor  86  is not powered; when controller  74   a  turns on power switch  72  with a high voltage, thermistor  86  is powered and thereby a divided voltage is generated at pin “enb”. 
     Thermistor  86  could be an NTC (negative temperature coefficient) resistor whose resistance falls when an ambient temperature rises. Controller  74   a  could be an integrated circuit chip. 
       FIG. 3  is a zoom-in diagram of partial circuits shown in  FIG. 2 . In  FIG. 3 , controller  74   a  includes a driving circuit  96   a , an oscillator  92   a  and a detecting circuit  94   a . Driving circuit  96   a  is connected to thermistor  86  via a pin “gate”. Oscillator  92   a  is connected to resistor  88  and thermistor  86  via pin “enb”. Detecting circuit  94   a  detects a current flowing through pin “enb”. 
     When the ambient temperature is within a predetermined permitted range, the resistance of thermistor  86  is so large that it could be viewed as open-circuited. Therefore, the driving signal, no matter whether a high voltage or a low voltage, provided by driving circuit  96   a  to power switch  72  can be viewed as non-influential to resistor  88 . Resistor  88  determines a charging/discharging current of oscillator  92   a  so as to determine the oscillating frequency for providing a clock signal to driving circuit  96   a . At this time, detecting circuit  94   a  determines that the current flowing through pin “enb” is a proper value and thus enables driving circuit  96   a  to periodically control power switch  72 . 
     When the ambient temperature is higher than a predetermined permitted range, the resistance of thermistor  86  becomes relatively small. When driving circuit  96   a  provides a high voltage to turn on power switch  72 , the voltage at pin “enb” becomes higher, leading to a relatively smaller current flowing through pin “enb”. When the current flowing through pin “enb” becomes smaller than a predetermined value, detecting circuit  94   a  determines that an over-temperature event occurs, thus disabling the driving circuit  96   a  to stop driving circuit  96   a  from switching power switch  72 . Detecting circuit  94   a  can be designed to acquire a latching function. Once an over-temperature event occurs, the output will be latched and will not be released even after driving circuit  96   a  turning off the power switch  72 . 
     Detecting circuit  94   a  could also be designed to detect a voltage at pin “enb”. When the voltage of pin “enb” is higher than a predetermined value, an occurrence of the over-temperature event is detected. 
     In the embodiment of  FIG. 3 , pin “enb” is a multi-function pin, which not only has a function of over-temperature protection, but also has a function of setting the charging/discharging current in oscillator  92   a.    
     Thermistor  76  within the conventional switching power supply  60  in  FIG. 1  is powered by an input power source V in . Input power source V in  may offer hundreds of volts continuously. Thus, a conducting path constructed by thermistor  76  and resistor  78  could consume a considerable amount of electric power. 
     Thermistor  86  within switching power supply  80  shown in  FIG. 2  and  FIG. 3  is powered by driving circuit  96   a . On one hand, the high driving voltage provided by driving circuit  96   a  may be only tens of volts, and the amount of power consumed by the path formed by thermistor  86  and resistor  88  is relatively small; on the other hand, the high driving voltage provided by driving circuit  96   a  only exists when power switch  72  is turned on. When power switch  72  is turned off, thermistor  86  and resistor  88  almost consume no power at all. Therefore, compared with the prior art in  FIG. 1 , switching power supply  80  in  FIG. 2  can save a great deal of electric power. 
       FIG. 4  is a diagram of a switching power supply  90  according to an embodiment of the present invention. Switching power supply  90  is a flyback power converter which converts energy inputted by AC power source V AC  into output power source V OUT  which meets specification requirements. Same or similar elements represented by the same symbol in  FIG. 2  and  FIG. 4  are explained above, and therefore further description will be omitted here for brevity. Resistor  88  in  FIG. 2  is replaced by a capacitor  93  in  FIG. 4 . When controller  74   b  turns off the power switch  72  with a low voltage, thermistor  86  is not powered; when controller  74   b  turns on power switch  72  with a high voltage, thermistor  86  is powered to change a voltage of pin “enb”. Controller  74   b  could be an integrated circuit chip. 
       FIG. 5  is a zoom-in diagram of partial circuits shown in  FIG. 4 . In  FIG. 5 , controller  74   b  includes a driving circuit  96   b , an oscillator  92   b  and a detecting circuit  94   b . Driving circuit  96   b  is connected to thermistor  86  via pin “gate”. Oscillator  92   b  is connected to capacitor  93  and thermistor  86  via pin “enb”. Detecting circuit  94   b  detects a current flowing through pin “enb”. 
     When the ambient temperature is within a predetermined permitted range, the resistance of thermistor  86  is so large that it could be viewed as open-circuited. Therefore, the driving signal, no matter whether a high voltage or a low voltage, provided by driving circuit  96   b  to power switch  72  could be viewed as non-influential to capacitor  93 . Capacitor  93  is charged/discharged by a charging/discharging current of oscillator  92   b  so as to determine the oscillating frequency. In this way, a triangular wave is generated at one terminal of capacitor  93  and provided to driving circuit  96   b . At this time, detecting circuit  94   b  determines that the voltage at pin “enb” is within a proper range and thus enables driving circuit  96   b  to periodically control power switch  72 . 
     When the ambient temperature is higher than a predetermined permitted range, the resistance of thermistor  86  becomes relatively small. When driving circuit  96   b  provides a high voltage to turn on power switch  72 , the voltage at pin “enb” becomes high. At this moment, detecting circuit  94   b  determines that an over-temperature event occurs according to the voltage at pin “enb”, and thereby disabling and stopping driving circuit  96   b  from switching power switch  72 . Detecting circuit  94   b  can be designed to acquire a latching function. Once an over-temperature event occurs, the output will be latched and will not be released even the driving circuit  96   b  turning off power switch  72 . 
     Similarly, thermistor  86  within switching power supply  90  in  FIG. 4  and  FIG. 5  is powered by driving circuit  96   b . On one hand, the high driving voltage provided by driving circuit  96   b  is relatively lower; on the other hand, the high driving voltage from driving circuit  96   b  is not continuously provided. Therefore, compared with the prior art design in  FIG. 1 , switching power supply  90  in  FIG. 4  can save a great deal of electric power. 
     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.