Abstract:
A switching power supply equipment has a function to protect a power supply from abnormalities such as an output short-circuit. An output signal from an error amplification circuit to amplify a difference voltage between an output voltage and a predetermined voltage is input into a feedback terminal of an integrated circuit power supply control circuit, and a power transistor is controlled by a pulse-width control circuit base on the output signal. n addition, a comparator having a reference voltage and a series circuit of a capacitor and a resistor making operation of an overload protection circuit delay are connected to a connecting point of the error amplification circuit and the pulse-width control circuit. The overload protection circuit starts operation by an output of the comparator. The number of required terminals of the integrated circuit is decreased and also a package size of the integrated circuit is decreased in comparison to convention switching power supply equipment incorporating similar protection circuitry.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application PA 2006-179238, filed Jun. 29, 2006, the contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a switching power supply such as a DC-DC converter, and more specifically to switching power supply equipment including a protection function to protect a power supply from an over load. 
     In a switching power supply equipment such as a DC-DC converter, a protection function is provided to prevent heat generation or destruction in the case of an over load such as a connected load short-circuit. One measure that can be taken, for example, is to stop the operation of the power supply when the over load occurs. When such a protection circuit is used, in a case in which the protection function is attempted to be operated in at the moment the over load is caused, the protection function might malfunction though the switching power supply equipment operates normally since it is impossible to discriminate whether the over current flows transiently in starting or the over current flows transiently during a sudden change in the load condition. 
     To avoid such a malfunction, it is disclosed in, for example, Japanese Patent Laid-Open No. 2004-40858 that a delay time is provided so that the protection function works after the over load is caused, and that the over load state is discriminated as an abnormality when the over load state continues for a constant time and then the protection function starts operation. Further, Japanese Patent Laid-Open No. 2004-40858 discloses a method by which a delay time is provided by using a charge time of a capacitor and a method by which a delay time is provided by combining a reference clock and a counter. 
       FIG. 8 , for example, is a schematic diagram showing a circuit configuration of conventional switching power supply equipment including such a protection function. The switching power supply equipment includes a power transistor PT 1  connected in series with a primary winding of an output transformer T 1 , and an input DC voltage Vin is switched ON and OFF by the power transistor PT 1  and a pulsating flow generated to a secondary winding of an output transformer T 1  is rectified and smoothed by a diode D 1  and a capacitor C 1  to output a rectified and smoothed voltage. The switching power supply equipment further includes a power supply control circuit  100 , being an integrated circuit, and an error amplifier which amplifies a difference voltage between an output voltage (a voltage across the capacitor C 1 ) and a predetermined voltage (when resistance values of resistors R 201  and R 202  are respectively assumed R 201  and R 202 , Vr×(R 201 +R 202 )/R 202  becomes the predetermined voltage) based on inputting a divided voltage which the output voltage is divided by resistors R 201  and R 202  and the output voltage Vr of a reference voltage source Vr 1 . 
     In addition, the power supply control circuit  100  includes an internal power supply  101  which an internal power supply voltage regulated from a source voltage Vcc is generated and supplies the internal power supply voltage to internal circuits of the power supply control circuit  1 , a pulse-width control circuit  102  which controls a drive pulse width of the power transistor PT 1  based on an output of an error amplification circuit  200 , and an overload protection circuit  13  which functions in the over load. The power supply control circuit  100  further includes comparators IC 101  and IC 102 , a resistor R 101 , a current source Is 101  and an electric switch SW 101 , and the power supply control circuit  100  is an integrated circuit including these circuits and components. The comparator IC 102  compares an output of the error amplification circuit  200  with a reference voltage Vref 2 , and the electric switch SW 101  is opened or closed by the output of the comparator IC 102 . A capacitor C 10  being a delay (an integral) element is connected to the power supply control circuit  100 , and a comparator IC 101  compares an integral voltage of the capacitor C 10  integrating an electric current from the current source Is 101  with a reference voltage Vref 1 . 
     The error amplification circuit  200  includes resistors R 201  and R 202  dividing the output voltage, and an amplifier IC  201  which amplified a difference voltage between a divided voltage of the output voltage and a reference voltage Vr 1 , and an output voltage of the amplifier IC  201  is input to the power supply control circuit  100  through a photocoupler  201  as a feedback voltage FB. 
     In the switching power supply equipment of the above configuration, when an over load is caused and the output voltage falls to less than the predetermined voltage in the switching power supply equipment, the output of the amplifier IC  201  rises, and a signal being in response to the output of the amplifier IC 201  through the photocoupler  201  is transmitted to a feedback terminal of the power supply control circuit  100 , so that the feedback voltage FB of the feedback terminal rises. When the feedback voltage FB becomes more than the reference voltage Vref 2 , the electric switch SW 101  is closed due to an output of comparator IC 102  being inverted, and the capacitor C 10  is charged (integrated) by the current source Is 101  due to the current source Is 101  being connected to the capacitor C 10 , so that the voltage of the capacitor C 10  rises slowly. And when the voltage of the capacitor C 10  becomes more than the reference voltage Vref 1 , the output of the comparator IC 101  is inverted and the overload protection circuit  103  starts operation, so that the power transistor PT 1  becomes OFF, and the power supply stops. 
     However, in the conventional switching power supply equipment described above, a delay time till the overload protection circuit starts operation after the over load is caused is different for every used circuit. Thus, when the control circuit having such a protection function is integrated and used, it is necessary to provide a special terminal, for example, to connect externally a capacitor (which corresponds to C 10  of  FIG. 8 ) for charging (integrating) so that an arbitrary delay time can be set. Accordingly, there are problems that the number of the terminals of the integrated circuit increases and a package size increases. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems discussed above, and aims to provide a switching power supply equipment in which the number of terminals of an integrated circuit is not increased and also a package size of the integrated circuit is not increased to set an operating delay time of a protection function. 
     According to a first aspect of the invention, a switching power supply equipment which a pulsating flow generated by a direct current being switched ON and OFF by a switching element is smoothed, or rectified and smoothed, and outputs an output of the switching power supply equipment, including an error amplification circuit amplifying a difference voltage between an output voltage of the switching power supply equipment and a predetermined voltage, the error amplification circuit outputting an amplified output, a switching control circuit which controls ON and OFF of the switching element by the output of the error amplification circuit, a comparator which compares a reference voltage with a voltage of an input terminal of the switching control circuit which the output of the error amplification circuit is input, a protection circuit which operation is started based on output of the comparator, the protection circuit which functions at the time of abnormality of the output voltage of the switching power supply equipment, and a delay circuit for delaying operation of the protection circuit, the delay circuit connected to the input terminal. 
     In such equipment, because the delay circuit to delay operation of the protection circuit is connected to the input terminal of the switching control circuit which the output of the error amplification circuit is input, the number of terminals of an integrated circuit is not increased and a package size of the integrated circuit is not increased to set the operating delay time of the protection function. 
     Because a delay circuit to make operation of the protection circuit delay is connected to an input terminal of the switching control circuit which an output of an error amplification circuit is input, the switching power supply equipment according to the present invention has advantages in that the number of terminals of an integrated circuit is not increased and also a package size of the integrated circuit is not increased to set the operating delay time of the protection function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to certain preferred embodiments thereof and the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram showing a circuit configuration of a switching power supply equipment according to a first embodiment of the present invention; 
         FIGS. 2(   a ) to  2 ( c ) are operation waveforms showing respectively an output voltage, a feedback terminal voltage and a gate drive pulse of the switching power supply equipment according to the first embodiment; 
         FIG. 3  is a schematic diagram showing a circuit configuration of a switching power supply equipment according to a second embodiment of the present invention; 
         FIG. 4  is a schematic diagram showing a circuit configuration of a switching power supply equipment according to a third embodiment of the present invention; 
         FIGS. 5(   a ) to  5 ( c ) are operation waveforms showing respectively an output voltage, a feedback terminal voltage and a gate drive pulse of the switching power supply equipment according to the third embodiment; 
         FIG. 6  is a schematic diagram showing a circuit configuration of a switching power supply equipment according to a fourth embodiment of the present invention; 
         FIGS. 7(   a ) to  7 ( c ) are operation waveforms showing respectively an output voltage, a feedback terminal voltage and a gate drive pulse of the switching power supply equipment according to the fourth embodiment; 
         FIG. 8  is a schematic diagram showing a circuit configuration of a conventional switching power supply equipment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments according to the present invention will now be explained with reference to the attached drawings.  FIG. 1  is a schematic diagram showing a circuit configuration of a switching power supply equipment according to a first embodiment of the present invention. The switching power supply equipment is configured as a DC-DC converter of a flyback type, and includes a power transistor PT 1  of an N channel being a switching element connected in series with a primary winding of an output transformer T 1 . An input voltage Vin from a DC power source Vs 1  is switched ON and OFF by the power transistor PT 1 . A pulsating flow generated hereby to a secondary winding of an output transformer T 1  is rectified and smoothed by a diode D 1  and a capacitor C 1 , and becomes an output voltage. The output voltage is output to a load (not illustrated). 
     A power supply control circuit  1  for controlling a drive of the power transistor PT 1  is assembled in a package as an integrated circuit, and includes an input terminal Tv which a source voltage Vcc from a DC power source Vs 2  is input, an output terminal to outputting a gate drive pulse OUT of the power transistor PT 1 , an input feedback terminal Tfb (or, merely, an input terminal Tfb) which a feedback voltage FB from an error amplification circuit  2  is input and a ground (GND) terminal Tg connected to a ground (GND) potential. 
     In addition, the power supply control circuit  1  includes an internal power supply  11  which an internal power supply voltage regulated from source voltage Vcc is generated and supplies the internal power supply voltage to internal circuits of the power supply control circuit  1 , a pulse-width control circuit  12  being a switching control circuit for controlling ON and OFF of the power transistor PT 1  based on an output of the error amplification circuit  2 , a comparator IC  11  which compares a reference voltage Vref of a voltage source Vs 11  with a voltage of the feedback terminal Tfb which the feedback voltage FB being output from the error amplification circuit  2  is input, and an overload protection circuit  13  which operation is started based on an output of the comparator IC  11  and functions in abnormality of an output voltage (a voltage across the capacitor C 1 ). A charging resistor R 11  supplied the internal power supply voltage from the internal power supply  11  is connected to a connection point of the pulse-width control circuit  12  and a non-inverting input terminal of the comparator IC  11 . The error amplification circuit  2  for amplifying and outputting a difference voltage between the output voltage (the voltage across the capacitor C 1 ) and a predetermined voltage includes resistors R 21  and R 22  that divide the output voltage and an amplifier IC 21 , and amplifies a difference voltage between a divided voltage and a reference voltage Vr of a voltage source Vs 21  by the amplifier IC 21  and outputs as a detecting voltage. Vr×(R 21 +R 22 )/R 22  corresponds to the predetermined voltage to the output voltage, and is the same as the conventional switching power supply equipment shown in  FIG. 8  (resistance values of resistors R 21  and R 22  are respectively assumed R 21  and R 22 ). The error amplification circuit  2  further includes a diode D 21 , and the diode D 21  is connected between the amplifier IC  21  and the pulse-width control circuit  12 . 
     In addition, a series circuit of a resistor R 1  and a capacitor C 2  is connected as a delay circuit to make operation of the overload protection circuit  13  delay to a connecting point (input terminal Tfb) of the error amplification circuit  2  and the pulse-width control circuit  12 . In the case that, here, there is a source capability (which is able to supply a current of plus to outside) in an output of the amplifier IC  21 , a timer by the series circuit of the resistor R 1  and the capacitor C 2  does not function as the timer due to the capacitor C 2  being charged by the output of amplifier IC  21  instantly when the timer starts operation. Therefore the diode D 21  is connected to the output side of the amplifier IC  21 . 
       FIGS. 2(   a ) to  2 ( c ) are operation waveforms showing respectively an output voltage, a feedback terminal voltage and a gate drive pulse of the switching power supply equipment according to the first embodiment. In a usual (operation) state, a voltage of the feedback terminal Tfb of the power supply control circuit  1  is adjusted by a function of the error amplification circuit  2  so that the output voltage becomes the predetermined voltage. Since an output of amplifier IC  21  becomes over a maximum range when the output voltage decreases due to being caused an over load for a switching power supply output, the capacitor C 2  integrates a current supplied through a charging resistor R 11  in the power supply control circuit  1  and then a voltage of the feedback terminal Tfb rises. And the output of the comparator IC  11  is inverted and the overload protection circuit  13  starts operation when the voltage of the feedback terminal Tfb reaches the reference voltage Vref of the comparator IC  11 . 
     In other words the delay time which it is necessary to charge the capacitor C 2  is produced till the overload protection circuit  13  starts operation after the over load state is caused. For this reason, when a over current flows transiently in starting or the over current flows transiently in the load sudden change, malfunctions of the overload protection circuit  13  can be avoided without the overload protection circuit  13  malfunctioning in the case that an operation of the switching power supply is normal. In other words even if the output of the amplifier IC  21  becomes over a maximum range instantly and an integral of the capacitor C 2  starts, in the case that the output voltage returns to be normal, the output of the amplifier IC  21  falls down and a charge of capacitor C 2  is discharged through the diode D 21  (the diode D 21  is connected to a forward direction for discharge), so that the voltage of the feedback terminal Tfb can be returned to a normal value. 
     According to the first embodiment, in the power supply control circuit  1  having the overload protection circuit  13  with the delay time, other terminal (an existing input terminal) can be commonly used as a terminal to connect the external capacitor C 2  to determine the delay time, so that the number of the terminal of the integrated circuit and the number of components can be decreased, and miniaturization of a overall size can be realized. 
     In other words, because the delay circuit to make the operation of the overload protection circuit  13  delay is connected to the connecting point of the error amplification circuit  2  and the pulse-width control circuit  12  to set the operating delay time of the overload protection circuit  13 , the number of the terminals of the integrated circuit is not increased, and also a package size of the integrated circuit is not increased. 
     In addition, the resistor R 1  is connected in series with the capacitor C 2 . The reason is that it is possible to instantly respond when the load is caused the sudden change during the normal operation. By way of example only, when only the capacitor C 2  is connected to the feedback terminal Tfb without providing the resistor R 1 , even if the voltage of the feedback terminal Tfb is tried to be changed instantly, a voltage change of the feedback terminal Tfb becomes too late since it is necessary for the capacitor C 2  to have a charging time or a discharging time. 
     On the other hand, in the case that the resistor R 1  is connected, when the load is caused the sudden change, a current flows to the resistor R 1  from the error amplification circuit  2  or the charging resistor R 11  in the power supply control circuit  1  (a current of the same value as the charging resistor R 11  flows also to the resistor R 1 ) and a voltage corresponding to the current through the resistors R 11  and R 1  is generated across the resistor R 1 . The voltage of the feedback terminal Tfb can be changed instantly by the generated voltage across the resistor R 1 . Since the scope which the voltage of the feedback terminal Tfb can be changed instantly in response to the change of the load is used as an effective scope in which can be used for controlling the output voltage, a delay for a transient response such as a load sudden change is not caused. 
       FIG. 3  is a schematic diagram showing a circuit configuration of a switching power supply equipment according to a second embodiment of the present invention. The output of amplifier IC  21  is transmitted to the feedback terminal Tfb through a photocoupler  21  according to the second embodiment. A configuration according to the second embodiment is the same as  FIG. 1  except use of the photocoupler  21 . 
     The second embodiment is applied to the switching power supply equipment that an input and an output are insulated. Even if such a connecting method is applied, the output of the photocoupler  21  can be considered to be the output of the error amplification circuit  2  and the series circuit of the resistor R 1  and the capacitor C 2  can be connected to the input terminal Tfb. 
       FIG. 4  is a schematic diagram showing a circuit configuration of a switching power supply equipment according to a third embodiment of the present invention. A diode D 11  is connected between the feedback terminal (the input terminal) Tfb and an input port of the pulse-width control circuit  12 , and a current source Is 11  is connected to the input terminal Tfb in the power supply control circuit  1  according to the third embodiment. The configuration is the same as  FIG. 3  except the diode D 11  and the current source Is 11 .  FIGS. 5(   a ) to  5 ( c ) are operation waveforms showing respectively an output voltage, a feedback terminal voltage and a gate drive pulse of the switching power supply equipment according to the third embodiment. 
     In the third embodiment, the diode D 11  is additionally connected between the feedback terminal Tfb and a connecting point of the input port of the pulse-width control circuit  12  and the charging resistor R 11 , and the current source Is 11  is additionally connected to the feedback terminal Tfb. In addition, the current source Is 11  can be exchanged with a resistor. 
     In the first embodiment and the second embodiment shown respectively in  FIG. 1  and  FIG. 3 , a charging voltage of the capacitor C 2  rises only as high as the internal power supply voltage when the delay time is generated. In the configuration of the third embodiment, the charging voltage of the capacitor C 2  can be raised to as high as the source voltage Vcc. Therefore to obtain a desired delay time, a capacitor of a comparatively small capacitance can be used. In other words, the source voltage Vcc is used for a source voltage of comparator IC  11 , and then the capacitor C 2  of the small capacitance can be used because the reference voltage Vref of the voltage source Vs 11  can be raised. 
       FIG. 6  is a schematic diagram showing a circuit configuration of a switching power supply equipment according to a fourth embodiment of the present invention. In the fourth embodiment, a comparator IC  12  having a hysteresis characteristic is used to the comparator which compares the reference voltage Vref with a voltage of a connecting point of the error amplification circuit  2  and the pulse-width control circuit  12 , and current sources Is 12  and Is 13  are respectively connected in series with electric switches SW 11  and SW 12  so that the capacitor C 2  is repeatedly charged and discharged by an output of the comparator IC  12 , and an inverter IC  13  is provided. In addition, a counter  14  counting the number of times that the output of the comparator IC  12  is inverted is provided, and the overload protection circuit  13  starts operation when the counter  14  counts the predetermined number of times. The configuration is the same as  FIG. 4  except the above description. 
     The comparator IC 12  having the hysteresis characteristics, the current sources Is 12  and Is 13  to repeatedly charge and discharge the external capacitor C 2  being connected to the feedback terminal Tfb, and the counter  14  are provided in the power supply control circuit  1  according to the fourth embodiment. In addition, the current sources Is 12  and Is 13  can be exchanged with resistors. In addition, the internal power supply voltage can be connected to the current source Is 12  which can be exchanged with the source voltage Vcc (when the current source Is 12  is exchanged with a resistor, the internal power supply voltage can be connected to the resistor being exchanged with the source voltage Vcc). 
     In the switching power supply equipment configured as above, when the over load state is caused, the external capacitor C 2  connected to the feedback terminal Tfb is repeatedly charged and discharged respectively through the current sources Is 12  and Is 13  due to the electric switches SW 11  and SW 12  complementarily switching ON and OFF (when one is in ON, the other is in OFF) by the output of the comparator IC  12 . An oscillation is performed between two threshold values of the comparator IC  12  having the hysteresis characteristics. The counter  14  counts the number of times of the discharge and charge and the overload protection circuit  13  starts operation after counting the predetermined number of times of the discharge and charge. 
     Thus the delay time can be determined by repetition of the discharge and charge of the capacitor C 2  by providing the counter  14 , and then the capacitance of the capacitor C 2  can be made smaller. 
       FIGS. 7(   a ) to  7 ( c ) are operation waveforms showing respectively an output voltage, a feedback terminal voltage and a gate drive pulse of the switching power supply equipment according to the fourth embodiment. Whenever the electric switches SW 11  and SW 12  are switched to ON and OFF, the voltage of the feedback terminal Tfb shifts instantly due to a voltage determined by a product of the resistance value of the resistor R 1  and the constant current value supplied by the source Is 12  or the current source Is 13  (the voltage of the feedback terminal Tfb=the constant current value×the resistance value of resistor R 1 +the voltage across capacitor C 2 , and the voltage across the capacitor C 2  does not change instantly). The voltage of the feedback terminal Tfb changes with a linear tilt afterwards since the capacitor C 2  is charged or discharged by the constant current. 
     In addition, the switching power supply equipment of the DC-DC converter of the flyback type is explained as an example in the above respective embodiment. The present invention is not restricted to the example, and the present invention can also be applied to a normal DC-DC converter such as a voltage step down converter which uses an inductor rather than the transformer. In addition, it does not need to say that there is a DC-DC converter that rectification of the output is unnecessary in the DC-DC converter to use the inductor. 
     In addition, in the above respective embodiment, regarding connection of the series circuit of the resistor R 1  and the capacitor C 2  as the delay circuit, it is shown in the embodiments that one end of the capacitor C 2  is connected to the input terminal Tfb and one end of the resistor R 1  is connected to the ground (GND) potential. The resistor R 1  and the capacitor C 2  can be exchanged each other in the connection of the series circuit. In other words, the one end of the resistor R 1  can be connected to the input terminal Tfb and the one end of the capacitor C 2  can be connected to the ground (GND) potential. In this case, operation of the series circuit is the same as the above explanation.