Abstract:
A protection circuit for a switching mode power supply circuit which outputs a constant voltage interrupts the ON-OFF switching operation of the switching transistor when an overvoltage state is detected. This protection circuit includes a switching transistor, a diode, a phototransistor, a thyristor, and a capacitor, in conjunction with a photodiode, a zener diode and a thyristor. If either an overvoltage is detected or an overcurrent is detected due to the load circuit being short circuited, the protection circuit simultaneously prevent the power supply input from the power input line from being unnecessarily lost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a switching mode power supply circuit (SMPS) using repetitive switching of the states which ON and OFF of a transistor to cause a direct current being applied to a primary coil of transformer to induce a current into a secondary coil, and more particularly, to a protection circuit for a switching mode power supply circuit using the ON and OFF operation of a transistor to allow an interruption when an over voltage is outputted. 
     2. Discussion of the Prior Art 
     As shown in FIG. 1, a protection circuit for a switching mode power supply circuit is constructed such that a power input terminal V IN  is connected to a collector of a transistor TR 1  switching ON and OFF a current flowing through a primary coil T 11  of a transformer T 1  ; an emitter of the transistor TR 1  is connected to an intermediate terminal of a secondary coil T 12  of transformer T 1  ; the end terminal of the secondary coil T 12  are connected to a base of the transistor TR 1  through a resistor R 14 , a capacitor C 1  and a feed back circuit 1; secondary coils T 13  T 14  are, respectively, connected to each input terminal of a rectifier 2 which is composed of a diode D 1 , capacitors C 2  C 3  and a coil L 1  and another rectifier 3 which is composed of a diode D 2 , capacitors C 4  C 5  and a regulator REG. On the other hand, an output terminal of the rectifier 2 is connected to a zenor diode ZD 1  and a resistor R 2  through a resistor R 1 . Resistor R 1  is also connected to an emitter of the transistor TR 2  through the resistor R 3  and to an inversion input terminal (-) of a comparator COMP through a resistor R 4 . The output terminal of rectifier 2 is also connected to a resistor R 6  and to a non-inversion input terminal (+) of the comparator COMP through a resistor R 5 . The output terminal of comparator COMP is connected to a base of the transistor TR 2  through a resistor R 8 . A collector of transistor TR 2  is connected to a base of a transistor TR 3  through a resistor R 10 . An emitter of the transistor TR 3  is connected to a gate of a thyristor SCR 1  and a capacitor C 6  through a resistor R 13 . An anode of thyristor SCR 1  is connected the diode D 1 , capacitor C 2 , and coil L 1 . 
     The operation of the conventional circuit will be explained below. 
     When a direct current is inputted to a power input terminal V IN , the direct current power is applied to a primary coil T 11  of transformer T 1 , thereby inducing a current in the secondary coils T 12  and T 13  in accordance with the ON and OFF switching of the transistor TR 1 . The power induced in a secondary coil T 12  is applied to a base of transistor TR 1  through the feed back circuit 1, thereby causing the ON and OFF switching of the transistor TR 1  to be repeated. 
     At this moment, the power is induced in another secondary coil T 13  and is rectified via the rectifier 2 which outputs a constant voltage V 1 . At the same time, the outputted constant voltage V 1  is applied to a zenor diode ZD 1  through a resistor R 1  to becomes another constant voltage. It is then applied to an inversion input terminal (-) of the comparator COMP through a resistor R 4 . Further, the power is induced in to another secondary coil T 14  is rectified via the rectifier 3 which outputs a constant voltage V 2  the output voltage V 2  is applied to a non-inversion terminal (+) of the comparator COMP through a resistor R 5 . 
     If the voltage which applied to the inversion input terminal (-) of the comparator COMP is lower than the voltage of non-inversion input terminal (+) of the comparator COMP, a high potential signal is outputted from the comparator COMP and the transistor TR 2  is turned OFF, accordingly the transistor TR 3  also turns OFF. In response to this OFF state, the gate of the thyristor SCR 1  is applied with a low potential, and the thyristor SCR 1  is turned OFF, thereby causing the rectifier to execute normal operations. 
     If the voltage applied to the inversion input terminal (-) of the comparator COMP is higher than the voltage applied to the non-inversion input terminal (+) of the comparator COMP, a low potential signal is outputted from the comparator COMP, and in contrary with above description, the transistor TR 2  turns ON causing the transistor TR 3  to also turn ON. A high potential signal is outputted to its emitter. This high potential signal is applied to the gate of the thyristor SCR 1  thereby causing the thyristor SCR 1  to become conductive and rendering the operation of the rectifier 2 to be terminated. 
     However, in such a conventional protection circuit for switching mode power circuit, if an overvoltage is outputted from the rectifier 2, the switching transistor TR 1   continuously repeats switching ON and OFF; therefore, the direct current power input to the power input terminal V IN  is unnecessarily lost such that the, switching mode power supply circuit cannot be protected. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a protection circuit for a switching mode power supply circuit which protects the switching mode power supply circuit from an overvoltage condition. 
     The object of the present invention, as described above, is realized in such a manner when an overvoltage is outputted from the switching mode power supply circuit, the first constant voltage is terminated by interrupting the ON.OFF switching operation of the switching transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a circuit diagram of a conventional protection circuit for a switching mode power supply circuit. 
     FIG. 2 is a circuit diagram of a protection circuit for a switching mode power supply circuit according to an embodiment of the present invention. 
     FIG. 3 is a circuit diagram of a protection circuit for a switching mode power supply circuit according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 2, a switching mode power supply circuit is constructed such that a power input terminal V IN  is connected to a collector of a switching transistor TR 21  through a primary coil T 21  of a transformer T 2 . An emitter of the transistor TR 21  is connected to an intermediate terminal of a secondary coil T 22  and one end terminal. The other end terminal of the secondary coil T 22  are connected to a base of the transistor TR 21  through a resistor R 21 , a capacitor C 21 , and a feed back circuit 221. The secondary coils T 23  and T 24  of the transformer T 2  are connected, respectively, to each of the input terminals of rectifiers 22 and 23 so that constant voltages V 21  and V 22  are outputted. 
     The output terminal of the rectifier 23 is connected in common to a photodiode PD 21 , a resistor R 23 , a capacitor C 26 , a gate of a thyristor SCR 21  through a resistor R 22  and a zenor diode ZD 21 . The anode of the thyristor SCR 21  is connected to the output terminal of the rectifier 22 through the resistor R 24 . The emitter of a phototransistor PTR 21  which senses the light of the photodiode PD 21  is connected in common to a gate of the thyristor SCR 22 , a resistor R 25  a capacitor C 27 , a connecting point between the resistors R 26  and R 27  which are connected in series to the power input terminal V IN . The connecting point between resistors R 26  and R 27  is connected to an anode of the thyristor SCR 22  and to a collector of the phototransistor PTR 21 . A base of the switching transistor TR 21  is connected to resistor R 27  and capacitor C 21 . 
     The operation of the present invention will be explained below. 
     When direct current power is inputted to the power input terminal V IN , the direct current power is applied to a primary coil T 21  of the transformer T 2  to induce a current in the secondary coils T 22 , T 23  and T 24  in accordance with the ON and OFF switching of the transistor TR 21 . The power induced in the secondary coil T 22  is applied to the base of the transistor TR 21  through the feedback circuit 21, thereby causing the ON and OFF switching of the transistor TR 21 . The power induced in the secondary coils T 23  and T 24  are rectified via the rectifiers 22 and 23 and to produce the constant voltages V 21  and V 22 . 
     When a rated voltage is outputted a zenor voltage of the zenor diode ZD 21  is established higher than the rated voltage to cause the zenor diode ZD 21  to turn OFF at this moment. Accordingly at this moment, the photodiode PD 21  is not lit, and the thyristor SCR 21  is OFF turned. Therefore, a rated voltage V 21  is normally outputted from the rectifier 22, and the phototransistor PTR 21  is turned OFF state in accordance with the lit off of the photodiode PD 21  ; therefore, the switching transistor TR 21  becomes to repeats the switching ON and OFF operations. If an overvoltage is outputted from the rectifier 23 and applied to the zenor diode ZD 21 , the zenor diode ZD 21   becomes conductive. Therefore, a rated voltage V 22  outputted from the rectifier 23 is applied to the photodiode PD 21  and to a gate of the thyristor SCR 21  through the resistor R 22  and zenor diode ZD 21 , thereby turning the photodiode PD 21  ON to produce light and causing the thyristor SCR 21  to become conductive. 
     Thus, when the thyristor SCR 21  becomes conductive, the constant voltage V 21  outputted from the rectifier 22 flows to earth through the resistor R 24  and the thyristor SCR 21 . 
     Further, since the phototransistor PTR 21  is ON in response to the lighting of the photodiode PD 21 , the power of the power input terminal V IN  is applied to a gate of the thyristor SCR 22  through the resistor R 26  and phototransistor PTR 21 . In accordance with this, the power applied to a base of the switching transistor TR 21  flows to the thyristor SCR 22  through a diode D 23  ; therefore, the switching transistor TR 21  is not operated in a switching operation and remains OFF. 
     FIG. 3 is a circuit diagram showing the another embodiment of a protection circuit for switching power circuit according to the present invention. The protections circuit is constructed such that a connecting point between the resistor R 46  and a collector of the transistor TR 34  which outputs a second constant voltage V 32  is connected to a base of a transistor TR 33  through a resistor R 47 . A collector of the transistor TR 33  is connected to the photodiode PD 32  through a resistor R 48 . An emitter of the phototransistor PTR 32   which senses the light of the photodiode PD 32  is connected in common to a gate of the thyristor SCR 31 , resistor R 36  and capacitor C 31 . A connecting point of the resistors R 32  and R 33  which are connected in series to the power input terminal V IN  is connected in common to an anode of the thyristor SCR 31  and to a collector of the phototransistor PTR 32 . A base of the switching transistor TR 31  is connected in common to an anode of the thyristor SCR 31  and to a collector of the phototransistor PTR 32  at through a diode D 32 . 
     The elements connected between a terminal end of secondary coil T 32  of the transformer T 3  and a base of the switching transistor TR 31  are formed with the usual feedback circuit. The resistors R 32 , R 35 , R 38  and R 41 , capacitor C 36 , transistor TR 35  and phototransistor PTR 31  are utilized to control the switching period of the switching transistor TR 31  by detecting the current. The resistor R 40 , zener diode ZD 31  and diode D 35  are utilized to control the primary side input voltage variation. The resistors R 39  and R 42 , capacitor C 32  and diode D 34  are utilized to protect the power supply source and secondary load from an overcurrent state. 
     The operation of the above embodiment according to the present invention will be explained in detail below. 
     When an over current is present such that a load which is supplied with second constant voltage V 32  is short circuited or the like, a voltage drop at the resistor R 45  becomes larger and a lower potential is applied to a base of the transistor TR 33 . Therefore, the transistor TR 33  becomes conductive, and accordingly, the photodiode PD 32  becomes lit. Since the phototransistor PTR 32  which senses the light of the photodiode PD 32  the power of the power input terminal V IN  is applied to a gate of the thyristor SCR 31  through the resistor R 32  and phototransistor PTR 32 . The thyristor SCR 31  is then turned ON. The power applied to a base of the switching transistor TR 31  flows through the diode D 32  and thyristor SCR 31  ; therefore the switching operator of the transistor TR 31  is not executed. 
     As explained in detail above the present invention presents the power from being unnecessarily lost because the switching operation of the switching transistor is interrupted by detecting when an overvoltage is outputted.