Abstract:
A power supply circuit comprising: a load; a resistor coupled the load; a first secondary battery; a second secondary battery; a switch configured to switch between a first state in which the first secondary battery and the second secondary battery are charged and a second state in which load current is supplied from the first secondary battery and the second secondary battery to the load based on current flowing through the resistor; and a shunt regulator configured to control the switch.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-62892, filed on Mar. 19, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a power supply circuit and a base station. 
     BACKGROUND 
     In a typical power supply circuit, a regulated power supply circuit is provided at an upstream of a load and a secondary battery is provided at an upstream of the regulated power supply circuit. The secondary battery is used to supply load current in such a power supply circuit. 
     For example, a related technology is disclosed in Japanese Laid-open Patent Publication No. 2002-369407. 
     SUMMARY 
     According to one aspect of the embodiments, a power supply circuit includes: a load; a resistor coupled the load; a first secondary battery; a second secondary battery; a switch configured to switch between a first state in which the first secondary battery and the second secondary battery are charged and a second state in which load current is supplied from the first secondary battery and the second secondary battery to the load based on current flowing through the resistor; and a shunt regulator configured to control the switch. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an exemplary power supply circuit; 
         FIG. 2  illustrates an exemplary base station; 
         FIG. 3  illustrates an exemplary power supply circuit; 
         FIG. 4  illustrates an exemplary power supply circuit; 
         FIG. 5  illustrates an exemplary power supply circuit; 
         FIG. 6  illustrates an exemplary relationship between time and load current; 
         FIG. 7  illustrates an exemplary power supply circuit; and 
         FIG. 8  illustrates an exemplary power supply circuit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates an exemplary power supply circuit. 
     In the power supply circuit illustrated in  FIG. 1 , a regulated power supply circuit is provided at an upstream of a load and a secondary battery BT is provided at an upstream of the regulated power supply circuit. For example, an output side OUT of the regulated power supply circuit is coupled to one end of the load. The other end of the load, which is different from the one end of the load coupled to the regulated power supply circuit, is grounded. An input side IN of the regulated power supply circuit is coupled to one end of a resistor R 0  and the plus side of the secondary battery BT. The other end of the resistor R 0 , which is different from the one end of the resistor R 0  coupled to the regulated power supply circuit, is an input terminal to which a voltage V 0  is applied. The minus side of the secondary battery BT is grounded. 
     A power supply applies the voltage V 0  to the input terminal. The voltage V 0  is to be applied to the load. If the application of the voltage V 0  is stopped upon occurrence of, for example, power outage, current Ia flows from the secondary battery BT to the load. For example, the power supply is backed up by the secondary battery BT. A voltage V 1  of the secondary battery BT is set to a value lower than the voltage V 0  of the power supply. 
     The current may be directly supplied from the secondary battery BT to the load to compensate for the current in the power supply circuit. When the current is directly supplied from the secondary battery BT to the load, a voltage Vz to be applied to the load is varied. 
     In order to reduce the variation in the voltage Vz, the current is supplied from the secondary battery BT to the input side IN of the regulated power supply circuit in the power supply circuit illustrated in  FIG. 1 . Loss PI occurring in the regulated power supply circuit is represented by Equation (1):
 
 PI =( I 0+ Ia ) V 1− Iz×Vz   (1)
 
     where I 0  denotes the current flowing through the resistor R 0 , Ia denotes the current supplied from the secondary battery BT to the load, V 1  denotes the voltage at the input side IN of the regulated power supply circuit, Iz denotes the current flowing through the load, and Vz denotes the voltage at the output side OUT of the regulated power supply circuit. 
     According to Equation (1), the current Ia is supplied from the secondary battery BT to the input side IN of the regulated power supply circuit, thereby causing a loss corresponding to the efficiency of the regulated power supply circuit. 
     In a compact apparatus, such as a remote radio head (RRH), installed outside, an unmetered electric service may be used, instead of a metered electric service. The RRH may be referred to as remote radio equipment (RRE). 
     The power consumption may be controlled so as to be lower than or equal to a predetermined power in the unmetered electric service. However, a peak power higher than the predetermined power in the unmetered electric service may occur in the power supply circuit. 
     In the power supply circuit, if the current of the load exceeds a predetermined value of the current supplied from the power supply, the supply voltage is decreased. The decrease in the supply voltage decreases the current to decrease the power consumed by the load. Although the decrease in the power consumed by the load degrades the performance or the function of the load, the supply voltage and the current increase in response to the degradation of the performance or the function of the load. 
     The same reference numerals are used in the following diagrams to identify the components having substantially the same function or similar functions. A repeated description of such components may be omitted or reduced. 
       FIG. 2  illustrates an exemplary base station. Referring to  FIG. 2 , a base station  100  may be a remote radio head. 
     The base station  100  includes a radio equipment (RE)  102  and a radio equipment control (REC)  120 . The radio equipment  102  may be coupled to the radio equipment control  120  via, for example, an optical fiber. The radio equipment  102  may be separated from the radio equipment control  120  because of the connection between the radio equipment  102  and the radio equipment control  120  via, for example, the optical fiber. Since the radio equipment  102  is provided near an antenna  104 , the cable loss may be reduced and the power consumption in the base station  100  may be reduced. 
     The radio equipment  102  includes the antenna  104 , an antenna duplexer  106 , a power amplifier (PA)  108 , a transmission circuit  110 , a low noise amplifier (LNA)  112 , a reception circuit  114 , an interface  116 , and a central processing unit (CPU)  118 . 
     A radio signal from the antenna  104  is input into the low noise amplifier  112  through the antenna duplexer  106 . The antenna duplexer  106  is coupled to the antenna  104 . The antenna duplexer  106  supplies the radio signal from the antenna  104  to the low noise amplifier  112 . The low noise amplifier  112  is coupled to the antenna duplexer  106 . The low noise amplifier  112  amplifies the radio signal from the antenna duplexer  106 . The low noise amplifier  112  supplies the amplified radio signal from the antenna duplexer  106  to the reception circuit  114 . 
     The reception circuit  114  is coupled to the low noise amplifier  112 . The reception circuit  114  performs reception processing on the radio signal from the low noise amplifier  112 . The reception circuit  114  supplies the signal that is subjected to the reception processing to the radio equipment control  120  via the interface  116 . The interface  116  is coupled to the reception circuit  114 . The interface  116  may be an interface between the reception circuit  114  and the radio equipment control  120 . The interface  116  may be, for example, a common public radio interface (CPRI). The CPRI may include a field programmable gate array (FPGA). The FPGA may be controlled by the CPU  118 . 
     The radio equipment control  120  is coupled to the interface  116 . The radio equipment control  120  processes the signal supplied from the reception circuit  114  through the interface  116 . A data signal from the radio equipment control  120  is supplied to the transmission circuit  110  through the interface  116 . 
     The transmission circuit  110  is coupled to the interface  116 . The transmission circuit  110  performs transmission processing on the data signal supplied from the radio equipment control  120  through the interface  116 . The transmission circuit  110  supplies the data signal subjected to the transmission processing to the power amplifier  108 . 
     The power amplifier  108  is coupled to the transmission circuit  110 . The power amplifier  108  amplifies the data signal subjected to the transmission processing in the transmission circuit  110 . The data signal amplified by the power amplifier  108  is transmitted from the antenna  104 . 
     The power amplifier  108  includes a power supply circuit  200 . 
       FIG. 3  illustrates an exemplary power supply circuit. An equivalent circuit of part of the power supply circuit  200  may be illustrated in  FIG. 3 . For example, an equivalent circuit when a secondary battery included in the power supply circuit  200  is charged with current may be illustrated in  FIG. 3 . 
     Multiple secondary batteries are provided in the power supply circuit  200 . Referring to  FIG. 3 , two secondary batteries, a secondary battery (BT 1 )  230  and a secondary battery (BT 2 )  236 , are provided. Three or more secondary batteries may be provided. 
     A resistor (R 0 )  204 , a resistor (R 7 )  228 , and a resistor (R 8 )  242  are provided in the power supply circuit  200 . 
     The plus side of the secondary battery (BT 1 )  230 , the plus side of the secondary battery (BT 2 )  236 , one end of the resistor (R 0 )  204 , and one end of a load  202  are coupled to each other. 
     A voltage V 0  is applied from a power supply (not illustrated) to the other end of the resistor (R 0 )  204 , which is different from the one end of the resistor (R 0 )  204  coupled to the plus side of the secondary battery (BT 1 )  230 , the plus side of the secondary battery (BT 2 )  236 , and the one end of the load  202 . 
     The minus side of the secondary battery (BT 1 )  230  is grounded via the resistor (R 7 )  228 . The minus side of the secondary battery (BT 2 )  236  is grounded via the resistor (R 8 )  242 . 
     The other end of the load  202 , which is different from the one end of the load  202  coupled to the one end of the resistor (R 0 )  204 , the plus side of the secondary battery (BT 1 )  230 , and the plus side of the secondary battery (BT 2 )  236 , is grounded. 
     A voltage resulting from addition of the voltage of the secondary battery (BT 2 )  236  to the voltage of the secondary battery (BT 1 )  230  may be set to a voltage lower than the voltage V 0  of the power supply. The capacitance of the secondary battery (BT 1 )  230  may be different from that of the secondary battery (BT 2 )  236 . The capacitance of the secondary battery (BT 1 )  230  may be substantially equal to that of the secondary battery (BT 2 )  236 . 
     Referring to  FIG. 3 , the current flowing through the resistor (R 0 )  204  is denoted by “current I 0 ” and the load current flowing through the load  202  is denoted by “load current Iz.” When the load current Iz is smaller than the current I 0 , the current I 0  flowing through the resistor (R 0 )  204  flows through the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236 . The current flowing through the secondary battery (BT 1 )  230  is denoted by “current Ib 1 ” and the current flowing through the secondary battery (BT 2 )  236  is denoted by “current Ib 2 .” For example, I 0 =Ib 1 +Ib 2 +Iz. The secondary battery (BT 1 )  230  is charged with the flowing current Ib 1 . The secondary battery (BT 2 )  236  is charged with the flowing current Ib 2 . 
       FIG. 4  illustrates an exemplary power supply circuit. An equivalent circuit of part of the power supply circuit  200  may be illustrated in  FIG. 4 . For example, an equivalent circuit when a secondary battery included in the power supply circuit  200  discharges may be illustrated in  FIG. 4 . 
     The resistor (R 7 )  228  and the resistor (R 8 )  242  illustrated in  FIG. 3  may be omitted from the exemplary power supply circuit in  FIG. 4 . 
     In the power supply circuit  200  in  FIG. 4 , multiple secondary batteries are coupled in series to each other. Referring to  FIG. 4 , the two secondary batteries, the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236 , are coupled in series to each other. Three or more secondary batteries may be coupled in series to each other. The capacitance of the secondary battery (BT 1 )  230  may be different from that of the secondary battery (BT 2 )  236 . The capacitance of the secondary battery (BT 1 )  230  may be substantially equal to that of the secondary battery (BT 2 )  236 . The substantial equality between the capacitance of the secondary battery (BT 1 )  230  and the capacitance of the secondary battery (BT 2 )  236  may reduce non-flowing of the current based on completion of the discharge of either of the secondary batteries. 
     The plus side of the secondary battery (BT 2 )  236 , one end of the resistor (R 0 )  204 , and one end of the load  202  are coupled to each other. The voltage V 0  is applied from a power supply to the other end of the resistor (R 0 )  204 , which is different from the one end of the resistor (R 0 )  204  coupled to the plus side of the secondary battery (BT 2 )  236  and the one end of the load  202 . 
     The minus side of the secondary battery (BT 2 )  236  is coupled to the plus side of the secondary battery (BT 1 )  230 . The minus side of the secondary battery (BT 1 )  230  is grounded. 
     The other end of the load  202 , which is different from the one end of the load  202  coupled to the one end of the resistor (R 0 )  204  and the plus side of the secondary battery (BT 2 )  236 , is grounded. 
     When the load current Iz is larger than the current I 0 , the current flows from the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  to the load  202 . The current flowing through the battery including the series connection of the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  is denoted by “Ia.” For example, I 0 +Ia=Iz. The current Ia supplied from the battery including the series connection of the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  and the current I 0  supplied from the power supply are supplied to the load  202 . 
     Multiple secondary batteries are provided in the power supply circuit  200 . When the load current Iz is smaller than the current I 0 , the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  are charged with the current (the current Ib 1 +the current Ib 2 ) resulting from subtraction of the load current Iz from the current I 0 . When the load current Iz is larger than the current J 0 , the current Ia resulting from subtraction of the current I 0  from the load current Iz is supplied from the battery including the series connection of the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236 . For example, when the load current Iz has a value near the peak, the parallel connection between the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  is switched to the series connection from the series connection and the current Ia is supplied from the battery including the series connection. Since the current Ia is supplied from the battery including the series connection of the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236 , the current may be stably supplied to the load  202 . 
       FIG. 5  illustrates an exemplary power supply circuit. The power supply circuit  200  may be included in the power amplifier  108 . 
     Referring to  FIG. 5 , the power supply circuit  200  includes the resistor (R 0 )  204 , a resistor (R 1 )  206 , a resistor (R 2 )  208 , a resistor (R 3 )  220 , and a resistor (R 4 )  238 . The power supply circuit  200  includes a resistor (R 5 )  222 , a resistor (R 6 )  224 , the resistor (R 7 )  228 , the resistor (R 8 )  242 , a resistor (R 9 )  212 , and a resistor (R 10 )  213 . 
     The power supply circuit  200  includes a shunt regulator  210 , a capacitor (C 0 )  218 , the secondary battery (BT 1 )  230 , and the secondary battery (BT 2 )  236 . 
     The power supply circuit  200  includes a transistor (Tr 1 )  232 , a transistor (Tr 2 )  240 , and a transistor (Tr 3 )  214 . 
     The power supply circuit  200  includes a diode (D 1 )  226 , a diode (D 2 )  234 , and a diode (D 3 )  216 . 
     One end of the resistor (R 0 )  204  is an input terminal. The other end of the resistor (R 0 )  204  is coupled to the resistor (R 1 )  206 , the resistor (R 9 )  212 , the emitter of the transistor (Tr 3 )  214 , the cathode of the diode (D 3 )  216 , the capacitor (C 0 )  218 , and the load  202 . 
     The other end of the load  202 , which is different from one end of the load  202  coupled to the resistor (R 0 )  204 , is grounded. 
     The other end of the resistor (R 1 )  206 , which is different from one end of the resistor (R 1 )  206  coupled to the resistor (R 0 )  204 , is coupled to the resistor (R 2 )  208  and a Reference terminal of the shunt regulator  210 . 
     The other end of the resistor (R 2 )  208 , which is different from one end of the resistor (R 2 )  208  coupled to the resistor (R 1 )  206 , is grounded. 
     The other end of the resistor (R 9 )  212 , which is different from one end of the resistor (R 9 )  212  coupled to the resistor (R 0 )  204 , is coupled to a Cathode terminal of the shunt regulator  210  and the base of the transistor (Tr 3 )  214 . The other end of the resistor (R 9 )  212 , which is different from the one end of the resistor (R 9 )  212  coupled to the resistor (R 0 )  204 , is coupled to the Cathode terminal of the shunt regulator  210  via the resistor (R 10 )  213 . 
     The other end of the capacitor (C 0 )  218 , which is different from one end of the capacitor (C 0 )  218  coupled to the resistor (R 0 )  204 , is grounded. 
     The collector of the transistor (Tr 3 )  214  is coupled to the resistor (R 3 )  220 , the resistor (R 5 )  222 , the anode of the diode (D 1 )  226 , and the anode of the diode (D 2 )  234 . 
     The other end of the resistor (R 3 )  220 , which is different from one end of the resistor (R 3 )  220  coupled to the collector of the transistor (Tr 3 )  214 , is coupled to the resistor (R 4 )  238  and the base of the transistor (Tr 2 )  240 . 
     The other end of the resistor (R 5 )  222 , which is different from one end of the resistor (R 5 )  222  coupled to the collector of the transistor (Tr 3 )  214 , is coupled to the resistor (R 6 )  224  and the base of the transistor (Tr 1 )  232 . 
     The other end of the resistor (R 6 )  224 , which is different from one end of the resistor (R 6 )  224  coupled to the resistor (R 5 )  222 , is grounded. 
     The cathode of the diode (D 1 )  226  is coupled to the resistor (R 7 )  228 . The other end of the resistor (R 7 )  228 , which is different from one end of the resistor (R 7 )  228  coupled to the cathode of the diode (D 1 )  226 , is coupled to the emitter of the transistor (Tr 1 )  232  and the plus side of the secondary battery (BT 1 )  230 . The minus side of the secondary battery (BT 1 )  230  is grounded. 
     The collector of the transistor (Tr 1 )  232  is coupled to the minus side of the secondary battery (BT 2 )  236  and the collector of the transistor (Tr 2 )  240 . 
     The cathode of the diode (D 2 )  234  is coupled to the anode of the diode (D 3 )  216  and the plus side of the secondary battery (BT 2 )  236 . 
     The other end of the resistor (R 4 )  238 , which is different from one end of the resistor (R 4 )  238  coupled to the resistor (R 3 )  220 , is grounded. 
     The emitter of the transistor (Tr 2 )  240  is coupled to the resistor (R 8 )  242 . The other end of the resistor (R 8 )  242 , which is different from one end of the resistor (R 8 )  242  coupled to the emitter of the transistor (Tr 2 )  240 , is grounded. 
     Referring to  FIG. 5 , the current flowing through the resistor (R 0 )  204  is denoted by the “current I 0 ” and the load current flowing through the load  202  is denoted by the “load current Iz.” The current flowing through the diode (D 1 )  226  is denoted by the “current Ib 1 ” and the current flowing through the diode (D 2 )  234  is denoted by the “current Ib 2 .” The current flowing through the diode (D 3 )  216  is denoted by the “current Ia.” The current I 0  may be the current supplied by applying the voltage V 0  to the power supply circuit  200 . The current I 0  flowing through the resistor (R 0 )  204  may be referred to as a limited current value. 
     The resistor (R 0 )  204  limits the load current Iz of the load  202 . The shunt regulator  210  controls the load voltage Vz to be applied to the load  202  so as to be kept the load voltage Vz at a certain value. For example, the shunt regulator  210  may function as a switch that causes current to flow through a bypass circuit in order to keep the load voltage Vz of the load at the certain value. 
     In the power supply circuit  200 , when the load current Iz is smaller than the limited current value, the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  are charged with the current Ib 1 +the current Ib 2 . When the load current Iz is larger than the limited current value, the charge is stopped and the shortfall is compensated for by the discharge from the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236 . The current compensating for the load current Iz may be the current Ia. For example, the current I 0  and the current Ia flow through the load  202 . 
     When the voltage of the secondary battery (BT 1 )  230  is denoted by the “voltage Vb” and the voltage of the secondary battery (BT 2 )  236  is dented by the “voltage Vb”, “the voltage Vb&lt;the load voltage Vz&lt;2×the voltage Vb” may be established. For example, the load voltage Vz may be higher than the voltage Vb. The load voltage Vz may be set to a value that is lower than twice of the voltage Vb. When the secondary battery (BT 1 )  230  is coupled in parallel to the secondary battery (BT 2 )  236 , no current may flow through the load  202  because the voltage Vb&lt;the load voltage Vz. For example, the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  are charged. When the secondary battery (BT 1 )  230  is coupled in series to the secondary battery (BT 2 )  236 , the current flows through the load  202  because the load voltage Vz&lt;2×the voltage Vb. For example, the current is supplied from the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  to the load  202 . 
     The shunt regulator  210  includes an Anode (A) terminal, a Cathode (K) terminal, and a Reference (R) terminal. The shunt regulator  210  includes a shunt transistor  2102 , an error amplifier  2104 , a diode  2106 , and a diode  2108 . The collector of the shunt transistor  2102  functions as the Cathode terminal and the emitter of the shunt transistor  2102  functions as the Anode terminal. A non-inverting input of the error amplifier  2104  functions as the Reference terminal. 
     The shunt regulator  210  controls the load voltage Vz to be applied to the load  202  so as to be made the load voltage Vz a certain setting value. The setting value of the load voltage Vz is set by the resistor (R 1 )  206  and the resistor (R 2 )  208 . When the load voltage Vz is increased to a voltage higher than the setting value, short-circuit occurs between the Cathode terminal and the Anode terminal. When the load voltage Vz is decreased to a value lower than the setting value, the circuit is opened between the Cathode terminal and the Anode terminal. 
     Since the load voltage Vz makes the Cathode terminal and the Anode terminal short-circuit or open, the current flowing through the resistor (R 0 )  204  is controlled. For example, the shunt regulator  210  performs the control so as to make the voltage drop based on the resistor (R 0 )  204  constant to stably supply the voltage to the load  202 . 
     The capacitor (C 0 )  218  may be a capacitor for removing ripple components. The capacitor (C 0 )  218  removes the ripple components that occur based on the switching function of the shunt regulator  210 . 
     Difference current corresponding to the difference between the limited current value I 0  and the load current Iz flowing through the load  202  may be referred to as the current Ib 1 +the current Ib 2 . The secondary battery (BT 1 )  230  is charged with the current Ib 1  in the current Ib 1 +the current Ib 2  and the secondary battery (BT 2 )  236  is charged with the current Ib 2  in the current Ib 1 +the current Ib 2 . 
     When the load current Iz flowing through the load  202  becomes smaller than the current I 0  flowing through the resistor (R 0 )  204 , the voltage drop based on the resistor (R 0 )  204  is decreased. The decrease in the voltage drop based on the resistor (R 0 )  204  increases the load voltage Vz to be applied to the load  202 . If the load voltage Vz increases to a value higher than or equal to a certain value, the shunt regulator  210  makes the Cathode terminal and the Anode terminal short-circuit. The occurrence of the short-circuit between the Cathode terminal and the Anode terminal of the shunt regulator  210  decreases the base voltage of the transistor (Tr 3 )  214 . The decrease in the base voltage of the transistor (Tr 3 )  214  turns on the transistor (Tr 3 )  214 . The transistor (Tr 1 )  232  is turned off in response to an increase in the base voltage. The transistor (Tr 2 )  240  is turned on in response to an increase in the base voltage. 
     Accordingly, the secondary battery (BT 1 )  230  is coupled in parallel to the secondary battery (BT 2 )  236 . For example, the shunt regulator  210  switches the transistor (Tr 1 )  232 , the transistor (Tr 2 )  240 , and the transistor (Tr 3 )  214  so that the secondary battery (BT 1 )  230  is coupled in parallel to the secondary battery (BT 2 )  236 . For example, each of the transistor (Tr 1 )  232 , the transistor (Tr 2 )  240 , and the transistor (Tr 3 )  214  may function as a switch. 
     The secondary battery (BT 1 )  230  is charged with the current Ib 1  via the diode (D 1 )  226  and the resistor (R 7 )  228 . The secondary battery (BT 2 )  236  is charged with the current Ib 2  via the diode (D 2 )  234  and the resistor (R 8 )  242 . For example, one secondary battery is divided into the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  by the transistor (Tr 1 )  232  and the transistor (Tr 2 )  240 , and the secondary battery (BT 1 )  230  is coupled in parallel to the secondary battery (BT 2 )  236 . 
     When the load  202  needs a current value exceeding the limited current value J 0 , contrary to the charge, the discharge from the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  causes the difference current Ia between the load current Iz which the load  202  needs and the current I 0  to be supplied to the load  202 . 
     When the load current Iz flowing through the load  202  becomes larger than the current I 0  flowing through the resistor (R 0 )  204 , the voltage drop is increased due to the resistor (R 0 )  204 . The increase in the voltage drop due to the resistor (R 0 )  204  decreases the load voltage Vz to be applied to the load  202 . When the load voltage Vz decreases to a value lower than a setting value, the shunt regulator  210  operates so as to open the circuit between the Cathode terminal and the Anode terminal. Since the shunt regulator  210  operates so as to open the circuit between the Cathode terminal and the Anode terminal, the base voltage of the transistor (Tr 2 )  240  and the base voltage of the transistor (Tr 3 )  214  increase. The transistor (Tr 2 )  240  and the transistor (Tr 3 )  214  are turned off in response to the increase in the base voltages. The base voltage of the transistor (Tr 1 )  232  is decreased. The decrease in the base voltage turns on the transistor (Tr 1 )  232 . 
     The secondary battery (BT 1 )  230  is coupled in series to the secondary battery (BT 2 )  236  via the diode (D 3 )  216  and the transistor (Tr 1 )  232 . For example, the shunt regulator  210  switches the transistor (Tr 1 )  232 , the transistor (Tr 2 )  240 , and the transistor (Tr 3 )  214  so that the secondary battery (BT 1 )  230  is coupled in series to the secondary battery (BT 2 )  236 . For example, each of the transistor (Tr 1 )  232 , the transistor (Tr 2 )  240 , and the transistor (Tr 3 )  214  may function as a switch. 
     The secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  that are coupled in series to each other cause the current Ia to be supplied to the load  202 . 
       FIG. 6  illustrates an exemplary relationship between time and load current. The load current may be the load current Iz in the power supply circuit  200 . 
     Referring to  FIG. 6 , the horizontal axis represents time T [sec] and the vertical axis represents the load current Iz [A]. The limited current value is also illustrated in  FIG. 6 . 
     When the load current Iz is larger than the limited current value I 0 , the discharge from the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  occurs. Since the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  discharges, the current Ia is supplied to the load  202  by the battery including the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  couples in series. Referring to  FIG. 6 , the supply of the current Ia is performed during a time period from zero seconds to about 1.25 seconds (the time T), a time period from about 4.5 seconds to about 6.5 seconds (the time T), and a time period from about 9.8 seconds to about 10.5 seconds (the time T). 
     When the load current Iz is smaller than the limited current value I 0 , the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  are charged with current. When the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236  are charged with current, the secondary battery (BT 1 )  230  is coupled in parallel to the secondary battery (BT 2 )  236 . The secondary battery (BT 1 )  230  is charged with the current Ib 1  and the secondary battery (BT 2 )  236  is charged with the current Ib 2 . Referring to  FIG. 6 , the charge is performed during a time period from about 1.25 seconds to about 4.5 seconds (the time T), a time period from about 6.5 seconds to about 9.8 seconds (the time T), and a time period from about 10.5 seconds to about 12 seconds (the time T). 
       FIG. 7  illustrates an exemplary power supply circuit. 
     In the power supply circuit  200  illustrated in  FIG. 7 , a capacitor (C 1 )  244  and a capacitor (C 2 )  246  may be used, instead of the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236 , respectively, illustrated in  FIG. 5 . 
     In the power supply circuit  200  illustrated in  FIG. 7 , when the load current Iz is smaller than the limited current value I 0 , the capacitor (C 1 )  244  and the capacitor (C 2 )  246  are charged with the current Ib 1 +the current Ib 2 . When the load current Iz is larger than the limited current value J 0 , the charge is stopped and the shortfall of the load current Iz is compensated for from the capacitor (C 1 )  244  and the capacitor (C 2 )  246 . For example, when the shortfall of the load current Iz is equal to the current Ia, the current I 0  and the current Ia flow through the load  202 . Since the increase in capacitance of the capacitor (C 1 )  244  and the capacitor (C 2 )  246  increases the accumulated amount of electricity, the discharge time may be increased. 
     The capacitance of the capacitor (C 1 )  244  may be different from that of the capacitor (C 2 )  246 . The capacitance of the capacitor (C 1 )  244  may be substantially equal to that of the capacitor (C 2 )  246 . The substantial equality between the capacitance of the capacitor (C 1 )  244  and the capacitance of the capacitor (C 2 )  246  may reduce non-flowing of the current based on completion of the discharge of either of the capacitors. 
     When the voltage of the capacitor (C 1 )  244  is denoted by the “voltage Vb” and the voltage of the capacitor (C 2 )  246  is denoted by the “voltage Vb”, an inequality the voltage Vb&lt;the load voltage Vz&lt;2×the voltage Vb may be established. For example, the load voltage Vz may be higher than the voltage Vb. The load voltage Vz may be set to a value that is lower than twice of the voltage Vb. When the capacitor (C 1 )  244  is coupled in parallel to the capacitor (C 2 )  246 , no current flows through the load  202  because the voltage Vb&lt;the load voltage Vz. For example, the capacitor (C 1 )  244  and the capacitor (C 2 )  246  are charged with current. When the capacitor (C 1 )  244  is coupled in series to the capacitor (C 2 )  246 , the current flows through the load  202  because the load voltage Vz&lt;2×the voltage Vb. For example, the current is supplied from the capacitor (C 1 )  244  and the capacitor (C 2 )  246  to the load  202 . 
     In the power supply circuit  200  illustrated in  FIG. 7 , either of the capacitor (C 1 )  244  and the capacitor (C 2 )  246  may be a secondary battery. 
     When the voltage of the secondary battery is substantially equal to the “voltage Vb”, an inequality the voltage Vb&lt;the load voltage Vz may be established. For example, the load voltage Vz may be higher than the voltage Vb. When the capacitor is coupled in parallel to the secondary battery, no current flows through the load  202  because the voltage Vb&lt;the load voltage Vz. For example, the capacitor and the secondary battery are charged with current. 
       FIG. 8  illustrates an exemplary power supply circuit. 
     In the power supply circuit  200  illustrated in  FIG. 8 , a fan (FT 1 )  248  and a fan (FT 2 )  250  are used, instead of the secondary battery (BT 1 )  230  and the secondary battery (BT 2 )  236 , respectively, illustrated in  FIG. 5 . The fan (FT 1 )  248  and the fan (FT 2 )  250  may each have a function to generate power based on the rotation of the fan. Each of the fan (FT 1 )  248  and the fan (FT 2 )  250  may be a power generator. 
     In the power supply circuit  200  illustrated in  FIG. 8 , when the load current Iz is smaller than the limited current value I 0 , the fan (FT 1 )  248  and the fan (FT 2 )  250  rotate by the current Ib 1 +the current Ib 2 . When the load current Iz is larger than the limited current value I 0 , the charge is stopped and the shortfall of the load current Iz is compensated for from fan (FT 1 )  248  and the fan (FT 2 )  250 . For example, the electromotive force caused by the power generated based on the rotation of the fan (FT 1 )  248  and the fan (FT 2 )  250  causes the current to be supplied to the load  202 . The current Ia occurring due to the power generated based on the rotation of the fan (FT 1 )  248  and the fan (FT 2 )  250  is supplied to the load  202 . 
     In the power supply circuit  200  illustrated in  FIG. 8 , either of the fan (FT 1 )  248  and the fan (FT 2 )  250  may be a secondary battery or a capacitor. 
     Since the current is supplied also from the capacitor that has been charged in advance before the performance or the function of the load (circuit) is degraded, a decrease in the performance or the function of the load, which is caused by the excess of the current supplied from the power supply over the current which the load needs, may be reduced. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.