Abstract:
A switching regulator includes an input power supply connected to an input of the switching regulator; at least one input/output disconnection switch disposed between the input power supply and a device connected to an output of the switching regulator, for interrupting the supply of an energy to the device from the input power supply; and a device for limiting a rush current that flows into an output capacitor from the input power supply when the input/output disconnection switch turns on to start step-up operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a switching regulator that is capable of reducing a rush current that flows at the time of starting. 
     2. Description of the Related Art 
     In general, a chopper type step-up switching regulator is mainly made up of, as shown in a circuit diagram of FIG. 4, an inductor  403 , a rectifying device  404 , a smoothing capacitor  405 , a switching device  407  and a control circuit  406 . In the switching regulator thus structured, even if the switching device  407  is turned off to stop the step-up operation, an energy is unavoidably supplied to a load device  402  from an input power supply  401  through the inductor  403  and the rectifying device  404 . 
     Under the above circumstances, there has been conventionally known, as shown in FIG. 3, a switching regulator in which a switch  310  is disposed between an input power supply  301  and an inductor  303 , a switch  311  is disposed between the inductor  303  and a switching device  307 , a switch  312  is disposed between the switching device  307  and a rectifying device  304 , a switch  313  is disposed between the rectifying device  304  and a smoothing capacitor  305 , and a switch  314  is disposed between a smoothing capacitor  305  and a device  302 , wherein those respective switches are turned off in order to interrupt the supply of an energy to the device  302  from the input power supply  301  when step-up operation stops. 
     However, in the conventional switching regulator, when the switch  310 , the switch  311 , the switch  312 , the switch  313  or the switch  314  are turned on to start the step-up operation, a large rush current unavoidably flows into the smoothing capacitor  305  and a capacitor  309  added to the device from the input power supply  301  with an adverse effect that an input supply voltage drops or the rectifying device, the smoothing capacitor and so on are damaged by the rush current. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the present invention is to solve the above problems with the conventional switching regulator. 
     Another object of the present invention is to provide a switching regulator which is capable of suppressing a rush current in which a switch for interrupting the supply of an energy to a device from an input power supply is not suddenly changed over from an inconductive state to a conductive state to permit a current to flow infinitely as soon as the switch turns on as in the conventional switching regulator, but a switch that limits a current or a switch that limits a current which flows for a given period of time, is employed. 
     In order to achieve the above objects, according to the present invention, there is provided a switching regulator, comprising: 
     an input power supply connected to an input of the switching regulator; 
     at least one input/output disconnection switch disposed between the input power supply and a device connected to an output of the switching regulator, for interrupting the supply of an energy to the device from the input power supply; and 
     means for limiting a rush current that flows into an output capacitor from the input power supply when the input/output disconnection switch turns on to start step-up operation. 
     Also, according to the present invention, in the switching regulator, the rush current limiting means of the input/output disconnection switch has a current limit function added to the output disconnection switch to set a limit value of a current that is allowed to flow. 
     Further, according to the present invention, in the switching regulator, the rush current limiting means of the input/output disconnection switch has a transistor as the output disconnection switch, the transistor operating in the same manner as that of an integral amplifying circuit to limit a current that flows for a given period of time. 
     Still further, according to the present invention, in the switching regulator, the rush current limiting means of the input/output disconnection switch repeats the on/off operation of the output disconnection switch to control an on-time and off-time, thereby limiting a current that is allowed to flow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a diagram showing the entire circuit of a chopper type step-up switching regulator with an input/output disconnection switch in accordance with a first embodiment of the present invention; 
     FIG. 2 is a diagram showing a specific circuit example 1 of an input/output disconnection switch portion in accordance with the first embodiment of the present invention; 
     FIG. 3 is a diagram showing the entire circuit of a conventional chopper type step-up switching regulator with an input/output disconnection switch; 
     FIG. 4 is a diagram showing the entire circuit of a conventional chopper type step-up switching regulator; 
     FIG. 5 is a diagram showing a specific circuit example 2 of an input/output disconnection switch portion in accordance with the first embodiment of the present invention; 
     FIG. 6 is a diagram showing a specific circuit example 1 of a chopper type step-up switching regulator with an input/output disconnection switch in accordance with a second embodiment of the present invention; 
     FIG. 7 is a diagram showing a specific circuit example 2 of a chopper type step-up switching regulator with an input/output disconnection switch in accordance with the second embodiment of the present invention; and 
     FIG. 8 is a diagram showing a specific circuit example of a chopper type step-up switching regulator with an input/output disconnection switch in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, a description will be given in more detail of preferred embodiments of the present invention with reference to the accompanying drawings. 
     First Embodiment 
     FIG. 1 is a diagram showing a chopper type step-up switching regulator in accordance with a first embodiment of the present invention. In the circuit, an inductor  103 , a rectifying device  104 , a smoothing capacitor  105 , a switching device  107  and a control circuit  106  are identical with those in the conventional switching regulator. A switch that limits a current is employed as one of a switch  110 , a switch  111 , a switch  112 , a switch  113  and a switch  114  which are disposed between an input power supply  101  and a device  102 . This structure can limit a rush current flowing from the input power supply  101  to the smoothing capacitor  105  and a capacitor  109  added to the device  102  since the stop of step-up operation when the switch disposed between the input power supply  101  and the device  102  turns off until the start of the step-up operation when the switch turns on. 
     FIG. 2 shows a specific example of the switch that limits a current. The switch is made up of a current limit resistor  204 , a switch  202  and a switch  203 . At the time of starting step-up operation, the switch  202  first turns on, the smoothing capacitor  105  and the capacitor  109  added to the device are charged by the input power supply  101  through the current limit resistor  204 , and the switch  203  is turned on after the charging operation is completed. In this situation, assuming that an input supply voltage is VIN and a current limit resistance is R, the maximum charge current Icharge which flows in the capacitor  105  and the capacitor  109  is represented by the following expression: 
     
       
         Icharge=VIN/R  (1) 
       
     
     Thus, the current limit is realized by setting the current limit resistor R to such a current limit resistance as to produce a charge current without any problems. 
     Similarly, in an example of a switch shown in FIG. 5, the current limit can be realized in such a manner that a switch  503  first turns on and a switch  502  turns on after the charging operation. 
     It is apparent that the current limit resistor can be replaced by another element that limits a current and the same effect can be obtained if the switch is formed of a transistor that is an electronic switch device. Also, it is apparent that the same effect is obtained without using the current limit resistor if a switch that limits a current is employed. 
     Second Embodiment 
     FIG. 6 is a circuit diagram showing a chopper type step-up switching regulator in accordance with a second embodiment of the present invention. A difference of the second embodiment from the first embodiment resides in that a switch can limit a current flowing for a given period of time. As in the first embodiment, this structure shown in FIG. 6 can limit a rush current flowing from an input power supply  601  to a smoothing capacitor  605  and a capacitor  609  added to a device  602  since the stop of step-up operation when a switch disposed between an input power supply  601  and the device  602  turns off until the start of the step-up operation when the switch turns on. 
     FIG. 6 shows a specific example of a switch  610  that can limit a current that flows for a given period of time. The switch  610  is made up of a transistor  611 , a capacitor  612 , a resistor  613 , and a switch  614 . In the start of the step-up operation when the switch  614  turns on, the same operation as that of an integral amplifying circuit is obtained by addition of the capacitor  612  and the resistor  613  to the transistor  611  so that the switch  614  can be slowly turned on. 
     A resistor  615  and a switch  616  shown in FIG. 6 are used as an example for turning off the transistor  611  when the step-up operation stops. 
     When the switch  614  turns on, charges charged in the capacitor  612  are discharged to GND through the resistor  613 , and when a gate voltage at which the transistor  611  turns on is set to a threshold value VTH, a gate voltage  617  of the transistor  611  drops to the threshold value to turn on the transistor  611 . When the transistor  611  turns on, a drain voltage  618  of the transistor  611  which is an output of the chopper type step-up switching regulator goes up, the capacitor  612  is charged so that the gate voltage  617  of the transistor  611  goes up, and the on-operation of the transistor  611  is suppressed, with the result that an output voltage  618  of the chopper type step-up switching regulator slowly goes up. Assuming that the capacitance of the capacitor  612  is Cc and the discharge current value limited by the resistor  613  is ic, a period of time tc during which the output voltage  618  goes up to the input supply voltage VIN from 0 V is represented by the following expression: 
     
       
           tc=Cc×VIN/ic   (2) 
       
     
     Thus, the current limit is realized by setting an output voltage going-up period tc so that the rush current flowing in the capacitor  605  and the capacitor  609  becomes a value producing no problems. 
     The example in which the transistor  611  is formed of a MOS field effect transistor was described. However, it is apparent that the same is applied if a bipolar transistor is used, and the resistor  613  may be replaced by another element that limits a current. Also, it is apparent that the same effect is obtained if the switch  614  is replaced by a transistor which is an electronic switching device, and the same effect is obtained if the positions of the resistor  613  and of the switch  614  are changed. 
     Further, it is apparent that the same effect is obtained if a parasitic capacitance existing between a gate  617  and a source  618  of the transistor  611  is employed as the capacitor  612 . 
     Also, the same effect can be realized even in a structural example shown in FIG. 7, which is the same as FIG. 6 but has the smoothing capacitor  705  located at the input stage of the switch  710 . 
     FIG. 8 is a circuit diagram showing a chopper type step-up switching regulator in accordance with a third embodiment of the present invention. A switch that limits a current is used as in the first embodiment, but a difference of this embodiment from the first embodiment resides in a current limiting method. More specifically, the method is that an inductor  803  disposed between an input power supply  801  and a device  802  is used, and a switching device  811  and a diode  813  are used to control an on/off period of time to limit a rush current flowing into a smoothing capacitor  805  and a capacitor  809  added to the device  802  from the input power supply  801 . 
     Assuming that a period of time after the switch  811  turns on is ton, an inductance of the inductor  803  is L, a voltage of an input power supply  814  is VIN, and a voltage of an output  815  is Vout, a current i flowing into capacitors  805  and  809  from the input power supply  801  which increases with a time t after the switch  811  turns on is represented by the following expression: 
     
       
           i =( VIN−V out)/ L×t on  (3) 
       
     
     As is apparent from the above expression (3), the current i can be adjusted by the on-time of the switch  811 . 
     Also, assuming that a period of time after the switch  811  turns off is toff, and a forward voltage drop of the diode  813  is VF, a current−i flowing into the capacitors  805  and  809  from the diode  813  which decreases with a time t after the switch  811  turns off is represented by the following expression: 
     
       
         − i =( V out− VF )/ L×t off  (4) 
       
     
     As is apparent from the above expression (4), the current −i can be adjusted by the off-time of the switch  811  similarly. 
     Accordingly, the current limit can be realized by adjusting the on/off time of the switch  811  by the current control circuit  812  so that the rush current that flows into the capacitor  805  and the capacitor  809  becomes a value producing no problems. 
     The example in which the transistor  811  is formed of a MOS field effect transistor was described. However, it is apparent that the same is applied if the transistor  811  is replaced by another switching device, and the same effect is obtained if the diode  813  is replaced by another rectifying device. 
     As was described above, the switching regulator according to the present invention has an advantage that a rush current that flows into a smoothing capacitor and a capacitor added to a device is suppressed when the step-up operation starts from a state in which the step-up operation stops, to thereby prevent an adverse effect that the input supply voltage drops and the rectifying device, the smoothing capacitor and so on are damaged. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.