Patent Publication Number: US-2022216717-A1

Title: Charging system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Priority is claimed on Japanese Patent Application No. 2021-001662, filed Jan. 7, 2021, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a charging system. 
     Description of Related Art 
     Conventionally, for example, a power supply device that supplies power to a plurality of battery modules connected in series is known (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2011-67021). This power supply device includes a rectifier circuit connected to each of the plurality of battery modules, an AC electric circuit that sequentially connects the plurality of rectifier circuits, and an AC generation circuit that applies an AC voltage to the AC electric circuit. 
     SUMMARY OF THE INVENTION 
     In the conventional power supply device described above, a rectifier circuit formed of a combination of a plurality of diodes is included, and thus a leakage current in a reverse direction may increase when a diode having a characteristic of a small voltage in a forward direction is used for loss reduction, and the like. When the leakage current of a rectifier circuit increases, there is a problem that a battery module is discharged and charge amounts of each of a plurality of battery modules become non-uniform. 
     For such a problem, for example, when a relay having a mechanical contact is added, there are restrictions on a mechanical operation guarantee, and there is a problem that the contact portion is liable to deteriorate due to an inrush current at the time of being turned on or an arc at the time of being turned off. When a photo coupler, an isolated DC-DC converter, or the like is added in response to potentials of each of the plurality of battery modules connected in series being different from each other, there is a problem that a cost required for a device configuration increases. 
     Aspects of the present invention have been made in consideration of such circumstances, and an object of the present invention is to provide a charging system that can cut off a leakage current while suppressing an increase in cost required for the configuration. 
     The present invention has adopted the following aspects to solve the problems described above. 
     (1) A charging system according to one aspect of the present invention is a charging system that charges a plurality of power storage modules that form a power storage device and includes an AC power source, a plurality of rectifiers that are connected between the AC power source and each of the plurality of power storage modules, and supply DC power obtained by rectifying AC power supplied from the AC power source to the plurality of power storage modules, and a plurality of current cutoff units that are connected between each of the plurality of power storage modules and each of the plurality of rectifiers, and automatically switch between conduction and cutoff of a current between the plurality of power storage modules and the plurality of rectifiers according to the AC power input to each of the plurality of rectifiers. 
     (2) In the aspect of (1) described above, each of the plurality of current cutoff units may include a switch unit that is connected between the power storage module and the rectifier and includes at least one switching element, and a drive unit that is connected between an AC input end of the rectifier and a control terminal of the switch unit and generates a control signal for switching between closing and opening of the switch unit according to the AC power input to the AC input end. 
     (3) In the aspect of (2) described above, the drive unit may include a capacitor connected to the AC input end, a rectifier circuit connected to the capacitor, and a discharge resistor connected between the rectifier circuit and the switch unit. 
     According to the aspect of (1) described above, it is possible to suppress an increase in leakage current of a power storage module via a rectifier while suppressing an increase in cost required for a configuration by including a plurality of current cutoff units that automatically switch between conduction and cutoff of a current between a plurality of power storage modules and a plurality of rectifiers according to AC power input to each rectifier. 
     In a case of the aspect of (2) described above, it is possible to suppress an increase in leakage current of a power storage module via a rectifier while suppressing an increase in cost required for a configuration by including a drive unit that automatically switches between on (conduction) and off (cut off) of a switching element. 
     In a case of the aspect of (3) described above, it is possible to automatically switch between conduction and cutoff of a current according to the presence or absence of application of AC power. When the AC power is not applied, a switching element is cut off by a discharge of a discharge resistor, and when the AC power is applied, a potential of a control terminal of a switch unit is increased and a switching element is conducted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram which shows a configuration of a charging system in an embodiment of the present invention. 
         FIG. 2  is a diagram which shows a configuration of an AC power source of the charging system in the embodiment of the present invention. 
         FIG. 3  is a diagram which shows a configuration of a rectifier of the charging system in the embodiment of the present invention. 
         FIG. 4  is a diagram which shows a configuration of a positive electrode-side current cutoff circuit of the charging system in the embodiment of the present invention. 
         FIG. 5  is a diagram which shows an example of a change in each of a charging current, a source-gate potential, and an AC voltage amplitude in the charging system of the embodiment of the present invention. 
         FIG. 6  is a diagram which shows a configuration of a charging system in a first modified example of the embodiment of the present invention. 
         FIG. 7  is a diagram which shows a configuration of a negative electrode side current cutoff circuit of the charging system in the first modified example of the embodiment of the present invention. 
         FIG. 8  is a diagram which shows a configuration of a charging system in a second modified example of the embodiment of the present invention. 
         FIG. 9  is a diagram which shows a configuration of a positive electrode side current cutoff circuit and a negative electrode side current cutoff circuit of the charging system in the second modified example of the embodiment of the present invention. 
         FIG. 10  is a diagram which shows a configuration of a positive electrode side current cutoff circuit of a charging system in a third modified example of the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a charging system  10  according to the embodiment of the present invention will be described with reference to the accompanying drawings. 
       FIG. 1  is a diagram which shows a configuration of the charging system  10  in the embodiment. 
     The charging system  10  according to the present embodiment is mounted in, for example, a vehicle such as an electric vehicle. The charging system  10  is connected to a power storage device mounted in the vehicle. The electric vehicle is an electric vehicle, a hybrid vehicle, a fuel cell vehicle, or the like. An electric vehicle is driven by a battery as a power source. A hybrid vehicle is driven by a battery and an internal combustion engine as power sources. A fuel cell vehicle is driven by a fuel cell as a power source. 
     As shown in  FIG. 1 , the power storage device connected to the charging system  10  is, for example, a high-voltage battery  1  which is a power source of the vehicle. The battery  1  includes, for example, a string  3  formed of a plurality of cells  2  connected in series, and positive electrode terminals and negative electrode terminals at both ends of the string  3 . The battery  1  includes a plurality of battery modules  4  formed by dividing the string  3  into a plurality of sub-strings in series. The plurality of battery modules  4  are, for example, a first battery module  4   a,  a second battery module  4   b,  a third battery module  4   c,  and a fourth battery module  4   d  formed by dividing the string  3  into four parts. For example, the first battery module  4   a,  the second battery module  4   b,  the third battery module  4   c,  and the fourth battery module  4   d  are sequentially connected in series. 
     The charging system  10  includes an AC power source  11 , a plurality of circuit modules  13 , and a control device  15 . 
       FIG. 2  is a diagram which shows a configuration of an AC power source  11  of the charging system  10  in the embodiment. 
     As shown in  FIG. 2 , the AC power source  11  includes a DC power supply  21 , a first power conversion unit  22 , and a second power conversion unit  23 . 
     The DC power supply  21  is, for example, a solar cell or the like. 
     The first power conversion unit  22  includes, for example, a DC-DC converter that performs two types of power conversion of step-up and step-down. The first power conversion unit  22  includes a first positive electrode terminal P 1 , a first negative electrode terminal N 1 , a second positive electrode terminal P 2 , and a second negative electrode terminal N 2 . 
     The first positive electrode terminal P 1  and the first negative electrode terminal N 1  of the first power conversion unit  22  are connected to a positive electrode terminal DP and a negative electrode terminal DN of the DC power supply  21 . The second positive electrode terminal P 2  and the second negative electrode terminal N 2  of the first power conversion unit  22  are connected to a positive electrode terminal PT and a negative electrode terminal NT of the second power conversion unit  23 . 
     The first power conversion unit  22  includes, for example, a switching element of a low-side arm and a high-side arm paired using two phases, and a reactor. The switching element is a transistor such as a metal oxide semiconductor field effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT), and is, for example, an N channel type MOSFET. The reactor is a choke coil L. 
     Each transistor may include a rectifying element. The rectifying element is a diode connected in parallel to each transistor. The rectifying element is, for example, a freewheel diode that is connected between a drain and a source of the MOSFET from the source to the drain in a forward direction. 
     The first power conversion unit  22  includes high-side arm and low-side arm first-phase transistors S 1 H and S 1 L paired using a first phase, and high-side arm and low-side arm second-phase transistors S 2 H and S 2 L paired using a second phase. 
     A drain of the high-side arm first phase transistor S 1 H is connected to the first positive electrode terminal P 1 . A drain of the high-side arm second phase transistor S 2 H is connected to the second positive electrode terminal P 2 . A source of the low-side arm first phase transistor S 1 L is connected to the first negative electrode terminal N 1 . A source of the low-side arm second phase transistor S 2 L is connected to the second negative electrode terminal N 2 . A source of the high-side arm first phase transistor S 1 H and a drain of the low-side arm first phase transistor S 1 L are connected to first ends at both ends of the choke coil L. A source of the high-side arm second phase transistor S 2 H and a drain of the low-side arm second phase transistor S 2 L are connected to second ends at both ends of the choke coil L. 
     The first power conversion unit  22  includes a first smoothing capacitor SC 1  connected between the first positive electrode terminal P 1  and the first negative electrode terminal N 1  and a second smoothing capacitor SC 2  connected between the second positive electrode terminal P 2  and the second negative electrode terminal N 2 . The first smoothing capacitor SC 1  and the second smoothing capacitor SC 2  smooth voltage fluctuations generated by an on or off switching operation of each of the transistors S 1 H, S 1 L, S 2 H, and S 2 L. 
     The first power conversion unit  22  switches between on (conduction) and off (cut off) of each of the transistors S 1 H, S 1 L, S 2 H, and S 2 L based on a gate signal which is a switching command input to a gate of each of the transistors S 1 H, S 1 L, S 2 H, and S 2 L. 
     The first power conversion unit  22  steps up power input from a DC power supply  21  to the first positive electrode terminal P 1  and the first negative electrode terminal N 1  at the time of stepping up a voltage, and outputs the stepped-up power from the second positive electrode terminal P 2  and the second negative electrode terminal N 2 . The first power conversion unit  22  maintains an on state (conduction) of the high-side arm first phase transistor S 1 H and an off state (cut off) of the low-side arm first phase transistor S 1 L at the time of stepping up a voltage. 
     The first power conversion unit  22  accumulates magnetic energy by direct current excitation of the reactor (choke coil L) when the high-side arm second phase transistor S 2 H is turned off (cut off) and the low-side arm second phase transistor S 2 L is turned on (conduction). The first power conversion unit  22  causes a voltage higher than those of the first positive electrode terminal P 1  and the first negative electrode terminal N 1  to be generated in the second positive electrode terminal P 2  and the second negative electrode terminal N 2  by superimposing an induced voltage generated by the magnetic energy of the reactor (choke coil L) when the high-side arm second phase transistor S 2 H is turned on (conduction) and the low-side arm second phase transistor S 2 L is turned off (cut off) and the voltage applied to the first positive electrode terminal P 1  and the first negative electrode terminal N 1 . 
     The first power conversion unit  22  steps down a voltage of the power input from the first positive electrode terminal P 1  and the first negative electrode terminal N 1  at the time of stepping down a voltage, and outputs the power whose voltage is stepped down from the second positive electrode terminal P 2  and the second negative electrode terminal N 2 . The first power conversion unit  22  maintains an on state (conduction) of the high-side arm second phase transistor S 2 H and an off state (cut off) of the low-side arm second phase transistor S 2 L at the time of stepping down a voltage. 
     The first power conversion unit  22  accumulates magnetic energy by direct current excitation of the reactor (choke coil L) when the high-side arm first phase transistor S 1 H is turned on (conduction) and the low-side arm first phase transistor S 1 L is turned off (cut off). The first power conversion unit  22  causes a voltage lower than that of the first positive electrode terminal P 1  and the first negative electrode terminal N 1  to be generated in the second positive electrode terminal P 2  and the second negative electrode terminal N 2  by stepping down an induced voltage generated by the magnetic energy of the reactor (choke coil L) when the high-side arm first phase transistor S 1 H is turned off (cut off) and the low-side arm first phase transistor S 1 L is turned on (conduction). 
     The second power conversion unit  23  includes, for example, an inverter that converts DC power input from the first power conversion unit  22  into AC power and outputs it to an AC electric circuit  12 . 
     The second power conversion unit  23  includes, for example, a bridge circuit formed of a plurality of switching elements that are bridge-connected by two phases, an A phase and a B phase. The switching element is a transistor such as a MOSFET or an IGBT, and is, for example, an N channel-type MOSFET. Each transistor may include a rectifying element. The rectifying element is a diode connected in parallel to each transistor. The rectifying element is, for example, a freewheel diode that is connected between the drain and the source of the MOSFET in a forward direction from a source to a drain. 
     The second power conversion unit  23  includes high-side arm and low-side arm A-phase transistors SaH and SaL that form a pair using the A phase, and high-side arm and low-side arm B-phase transistors SbH and SbL that form a pair using the B phase. 
     Each drain of the high-side arm A-phase transistor SaH and the high-side arm B-phase transistor SbH is connected to the positive electrode terminal PT. Each source of the low-side arm A-phase transistor SaL and the low-side arm B-phase transistor SbL is connected to the negative electrode terminal NT. The source of the high-side arm A-phase transistor SaH and the drain of the low-side arm A-phase transistor SaL are connected to an A-phase terminal AT. The source of the high-side arm B-phase transistor SbH and the drain of the low-side arm B-phase transistor SbL are connected to a B-phase terminal BT. 
     The second power conversion unit  23  switches between on (conduction) and off (cut off) of a transistor pair of each phase based on a gate signal which is a switching command input to a gate of each transistor SaH, SaL, SbH, or SbL. The second power conversion unit  23  converts DC power input from the positive electrode terminal PT and the negative electrode terminal NT into single-phase AC power and outputs it from the A-phase terminal AT and the B-phase terminal BT. The A-phase terminal AT of the second power conversion unit  23  is connected to an A-phase terminal  11 A of the AC power source  11 , and the B-phase terminal BT of the second power conversion unit  23  is connected to a B-phase terminal  11 B of the AC power source  11 . 
     The AC power source  11  supplies the same current (power) to each battery module  4  of the battery  1 , for example, when an alternating current having a frequency close to a resonance frequency of an AC electric circuit  31  (a resonance electric circuit) to be described below is generated. 
     As shown in  FIG. 1 , the plurality of circuit modules  13  are connected between the AC power source  11  and each of the plurality of battery modules  4 . The number of the plurality of circuit modules  13  is the same as the number of the plurality of battery modules  4 . The plurality of circuit modules  13  are, for example, a first circuit module  13   a,  a second circuit module  13   b,  a third circuit module  13   c,  and a fourth circuit module  13   d.    
     For example, the first battery module  4   a  and the first circuit module  13   a  are integrally connected, the second battery module  4   b  and the second circuit module  13   b  are integrally connected, the third battery module  4   c  and the third circuit module  13   c  are integrally connected, and the fourth battery module  4   d  and the fourth circuit module  13   d  are integrally connected. Each battery module  4  and each circuit module  13  are connected by, for example, a bus bar (not shown) having an insulating coating. The plurality of circuit modules  13  are sequentially connected from the AC power source  11  by wiring (not shown). For example, the second circuit module  13   b  and the first circuit module  13   a  are sequentially connected from the AC power source  11 , and the third circuit module  13   c  and the fourth circuit module  13   d  are sequentially connected from the AC power source  11 . 
     As shown in  FIG. 1 , each of the plurality of circuit modules  13  includes an AC electric circuit  31 , a rectifier circuit  33 , and a positive electrode side current cutoff circuit  35 . For example, the first circuit module  13   a  includes a first AC electric circuit  31   a,  a first rectifier circuit  33   a,  and a first positive electrode side current cutoff circuit  35   a.  The second circuit module  13   b  includes a second AC electric circuit  31   b,  a second rectifier circuit  33   b,  and a second positive electrode side current cutoff circuit  35   b.  The third circuit module  13   c  includes a third AC electric circuit  31   c,  a third rectifier circuit  33   c,  and a third positive electrode side current cutoff circuit  35   c.  The fourth circuit module  13   d  includes a fourth AC electric circuit  31   d,  a fourth rectifier circuit  33   d,  and a fourth positive electrode side current cutoff circuit  35   d.    
     Each AC electric circuit  31  includes an A-phase electric circuit  41  directly or indirectly connected to the A-phase terminal  11 A of the AC power source  11  and a B-phase electric circuit  43  directly or indirectly connected to the B-phase terminal  11 B of the AC power source  11 . Each of the A-phase electric circuit  41  and the B-phase electric circuit  43  includes an LC row  45  of a first capacitor C 1  and a first reactor L 1  connected in series on an input side of AC power, and a second reactor L 2 . 
     The second reactor L 2  is connected between a first connection point  47  provided from the input side of AC power via the LC row  45  in the AC electric circuit  31  and a second connection point  49  on the rectifier circuit  33  side. The second connection point  49  is connected to the second reactor L 2 , the rectifier circuit  33 , and the positive electrode side current cutoff circuit  35 . 
     A combination of a combined capacitance of capacitors and a combined inductance of inductors of the AC electric circuit  31  in each of the plurality of circuit modules  13  (for example, a product of the combined capacitance and the combined inductance) may be an appropriate combination. For example, when the product (LC product) of the combined capacitance and the combined inductance of resonance electric circuits of each stage corresponding to each battery module  4  of the battery  1  is the same, a current gain for each battery module  4  is the same, and the same current (power) is uniformly supplied to each battery module  4 . 
     For example, when a first LC product for the first battery module  4   a,  a second LC product for the second battery module  4   b,  a third LC product for the third battery module  4   c,  and a fourth LC product for the fourth battery module  4   d  are all the same, the same current (power) is uniformly supplied to each of the battery modules  4   a,    4   b,    4   c,  and  4   d.  For example, each LC product is a product of the combined capacitance and the combined inductance of capacitors and reactors other than the second reactor L 2  in each electric circuit from the AC power source  11  to each rectifier circuit  33 . 
     The first LC product is a product of the combined capacitance and the combined inductance of a first capacitor C 1  and a first reactor L 1  of the first circuit module  13   a  indirectly connected to the AC power source  11  via the second circuit module  13   b,  and a first capacitor C 1  and a first reactor L 1  of the second circuit module  13   b.    
     The second LC product is a product of the combined capacitance and the combined inductance of the first capacitor C 1  and the first reactor L 1  of the second circuit module  13   b  directly connected to the AC power source  11 . 
     The third LC product is a product of the combined capacitance and the combined inductance of a first capacitor C 1  and a first reactor L 1  of the third circuit module  13   c  directly connected to the AC power source  11 . 
     The fourth LC product is a product of the combined capacitance and the combined inductance of a first capacitor C 1  and a first reactor L 1  of a fourth circuit module  13   d  indirectly connected to the AC power source  11  via the third circuit module  13   c,  and the first capacitor C 1  and the first reactor L 1  of the third circuit module  13   c.    
     Each rectifier circuit  33  of the plurality of circuit modules  13  is connected between the AC electric circuit  31  and a corresponding battery module  4  in the battery  1 . In each of the plurality of circuit modules  13 , a second connection point  49  of the A-phase electric circuit  41  of the AC electric circuit  31  is connected to an A-phase terminal AS of the rectifier circuit  33 . A second connection point  49  of the B-phase electric circuit  43  of the AC electric circuit  31  is connected to a B-phase terminal BS of the rectifier circuit  33 . 
       FIG. 3  is a diagram which shows a configuration of the rectifier circuit  33  of the charging system  10  in the embodiment. 
     As shown in  FIG. 3 , the rectifier circuit  33  includes, for example, a bridge circuit formed of a plurality of diodes that are bridge-connected in two rows of a first row and a second row. 
     The rectifier circuit  33  is, for example, a full-wave rectifier circuit. The rectifier circuit  33  includes a first diode  51   a  and a second diode  51   b  connected in the forward direction in the first row, and a third diode  51   c  and a fourth diode  51   d  connected in the forward direction in the second row. 
     A connection point  33 A of an anode of the first diode  51   a  and a cathode of the second diode  51   b  is connected to the A-phase terminal AS. A connection point  33 B of an anode of the third diode  51   c  and a cathode of the fourth diode  51   d  is connected to the B-phase terminal BS. 
     Each cathode of the first diode  51   a  and the third diode  51   c  is connected to a positive electrode terminal PR. Each anode of the second diode  51   b  and the fourth diode  51   d  is connected to the negative electrode terminal NR. The positive electrode terminal PR and the negative electrode terminal NR of the rectifier circuit  42  are connected to a positive electrode end and a negative electrode end of a corresponding battery module  4  in the battery  1 . 
     The rectifier circuit  33  full-wave rectifies AC power input from the A-phase terminal AS and the B-phase terminal BS, and outputs the rectified DC power from the positive electrode terminal PR and negative electrode terminal NR. 
       FIG. 4  is a diagram which shows a configuration of the positive electrode side current cutoff circuit  35  of the charging system  10  in the embodiment. 
     As shown in  FIG. 4 , each positive electrode side current cutoff circuit  35  of the plurality of circuit modules  13  is connected between the rectifier circuit  33  and the positive electrode end of a corresponding battery module  4  in the battery  1 . The positive electrode side current cutoff circuit  35  includes a positive electrode side switch unit  61  and a positive electrode side switch drive unit  63 . 
     The positive electrode side switch unit  61  is connected between a positive electrode terminal PR of the rectifier circuit  33  and the positive electrode end of a corresponding battery module  4  in the battery  1 . The positive electrode side switch unit  61  is, for example, a bidirectional switch formed of two switching elements. The switching element is a transistor such as a MOSFET or an IGBT, and is, for example, an N channel type MOSFET. Each transistor may have a rectifying element. The rectifying element is a diode connected in parallel to each transistor. The rectifying element is, for example, a freewheel diode that is connected between the drain and source of the MOSFET in the forward direction from a source to a drain. 
     The positive electrode side switch unit  61  includes a positive electrode side first transistor  61   a  and a positive electrode side second transistor  61   b  connected in anti-series. 
     Gates G of the positive electrode side first transistor  61   a  and the positive electrode side second transistor  61   b  are connected to the positive electrode end of the positive electrode side switch drive unit  63  (for example, the connection point  65 P of a positive electrode side rectifier circuit  65  to be described below). Sources of the positive electrode side first transistor  61   a  and the positive electrode side second transistor  61   b  are connected to the negative electrode end of the positive electrode side switch drive unit  63  (for example, a connection point  65 N of the positive electrode side rectifier circuit  65  to be described below). A drain of the positive electrode side first transistor  61   a  is connected to the positive electrode end of a corresponding battery module  4  in the battery  1 . A drain of the positive electrode side second transistor  61   b  is connected to the positive electrode terminal PR of the rectifier circuit  33 . 
     The positive electrode side switch unit  61  switches between on (conduction) and off (cut off) of the positive electrode side first transistor  61   a  and the positive electrode side second transistor  61   b  based on a gate signal, which is a switching command based on a voltage applied from the positive electrode side switch drive unit  63  between the gate and the source of each of the positive electrode side first transistor  61   a  and the positive electrode side second transistor  61   b.  The positive electrode side switch unit  61  switches between conduction and cutoff of a current between each battery module  4  and the rectifier circuit  33  according to on (conduction) or off (cut off) of the positive electrode side first transistor  61   a  and the positive electrode side second transistor  61   b.    
     The positive electrode side switch drive unit  63  includes two positive electrode side capacitors CPs for DC insulation, a positive electrode side rectifier circuit  65 , and a positive electrode side resistor RP for discharge. 
     The two positive electrode side capacitors CPs are connected to the second connection point  49  of each of the A-phase electric circuit  41  and the B-phase electric circuit  43  of the AC electric circuit  31 . 
     The positive electrode side rectifier circuit  65  includes, for example, a bridge circuit formed of a plurality of diodes that are bridge-connected in two rows of the first row and the second row. 
     The positive electrode side rectifier circuit  65  is, for example, a full-wave rectifier circuit. The positive electrode side rectifier circuit  65  includes a positive electrode side first diode  65   a  and a positive electrode side second diode  65   b  connected in the forward direction in the first row, and a positive electrode side third diode  65   c  and a positive electrode side fourth diode  65   d  connected in the forward direction in the second row. 
     A connection point (an A-phase connection point)  65 A between an anode of the positive electrode side first diode  65   a  and a cathode of the positive electrode side second diode  65   b  is connected to a second connection point  49  of the A-phase electric circuit  41  via the positive electrode side capacitor CP. A connection point (a B-phase connection point)  65 B between an anode of the positive electrode side third diode  65   c  and a cathode of the positive electrode side fourth diode  65   d  is connected to a second connection point  49  of the B-phase electric circuit  43  via the positive electrode side capacitor CP. 
     A connection point  65 P between cathodes of the positive electrode side first diode  65   a  and the positive electrode side third diode  65   c  is connected to gates G of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61 . A connection point  65 N between anodes of the positive electrode side second diode  65   b  and the positive electrode side fourth diode  65   d  is connected to sources of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61 . 
     The positive electrode side rectifier circuit  65  full-wave rectifies AC power input from the A-phase electric circuit  41  and the B-phase electric circuit  43  to the A-phase connection point  65 A and the B phase connection point  65 B, and outputs the rectified DC power from the connection point  65 P and the connection point  65 N. 
     The positive electrode side resistor RP is connected between the connection point  65 P and the connection point  65 N of the positive electrode side rectifier circuit  65 . 
     The positive electrode side switch drive unit  63  automatically switches between conduction and cutoff of a current of the positive electrode side switch unit  61  according to AC power input from the A-phase electric circuit  41  and the B-phase electric circuit  43  to the rectifier circuit  33 . 
     First, in the positive electrode side switch drive unit  63 , when the AC power is not applied to the AC electric circuit  31 , the positive electrode side resistor RP for discharge discharges charges between the gates and the sources of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61 , and thereby each of the transistors  61   a  and  61   b  is put into a cut-off state, and a leakage current of the battery module  4  via the rectifier circuit  33  becomes extremely small. 
     When the AC power source  11  is activated and AC power is applied from the AC electric circuit  31  to the positive electrode side switch drive unit  63 , only an AC component is input to the positive electrode side rectifier circuit  65  via the positive electrode side capacitor CP. When capacitance between the gates and the sources of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61  is charged with the DC power rectified by the positive electrode side rectifier circuit  65 , and a gate-source potential becomes sufficiently high, each of the transistors  61   a  and  61   b  is conduced, and the DC power generated by the rectifier circuit  33  is used to charge the battery module  4 . 
     When the AC power source  11  is stopped and AC power of the AC electric circuit  31  is cut off, the transistors  61   a  and  61   b  of the positive electrode side switch unit  61  are put into the cut-off state again, and a leakage current of the battery module  4  through the rectifier circuit  33  becomes extremely small. 
       FIG. 5  is a diagram which shows an example of changes in each of a charging current, a source-gate potential, and an AC voltage amplitude in the charging system  10  of the embodiment. 
     As shown in  FIG. 5 , as an amplitude of an AC voltage increases, for example, after a time t 1 , a potential between the source and the gate of each of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61  changes in an increasing tendency. 
     Then, as shown after a time t 2 , after the amplitude of the AC voltage reaches a predetermined amplitude Vb and the potential between the source and the gate of each of the transistors  61   a  and  61   b  reaches a predetermined potential Va, each of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61  is turned into a conduction state, and a predetermined charging current Ia flows through the battery module  4 . 
     Then, as shown after a time t 3 , the amplitude of the AC voltage decreases from the predetermined amplitude Vb, such that the charging current of the battery module  4  decreases from the predetermined charging current Ia. Along with this, as shown after a time t 4 , the potential between the source and the gate of each of the transistors  61   a  and  61   b  drops from the predetermined potential Va. 
     Then, as shown at a time t 5 , the amplitude of the AC voltage and the charging current of the battery module  4  reach zero, the potential between the source and the gate of each of the transistors  61   a  and  61   b  falls, and each of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61  is put into the cut-off state. 
     As described above, by applying the AC power, each of the transistors  61   a  and  61   b  is automatically conduced, and each of the transistors  61   a  and  61   b  is automatically put into the cut-off state by stopping the AC power. 
     As shown in  FIG. 1 , the control device  15  controls an operation of the charging system  10 . For example, the control device  15  is a software functional unit that functions by a processor such as a central processing unit (CPU) executing a predetermined program. The software functional unit is an electronic control unit (ECU) that includes a processor such as a CPU, a read only memory (ROM) for storing a program, a random access memory (RAM) for temporarily storing data, and an electronic circuit such as a timer. At least a part of the control device  15  may also be an integrated circuit such as a large scale integration (LSI). 
     For example, the control device  15  sets a timing to drive each switching element of the AC power source  11  to be turned on (conduction) or off (cut oft), and generates a gate signal for actually driving each switching element to be turned on (conduction) or off (cut off). 
     As described above, the charging system  10  of the embodiment includes the positive electrode side current cutoff circuit  35  that automatically switches between the conduction and the cut off of a current between the battery module  4  and the rectifier circuit  33  according to the AC power input to the rectifier circuit  33 , and thereby it is possible to suppress an increase in leakage current of the battery module  4  via the rectifier circuit  33  while suppressing an increase in cost required for the configuration. 
     The positive electrode side current cutoff circuit  35  includes the positive electrode side switch drive unit  63  that automatically switches between on (conduction) and off (cut off) of each of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61 , and thereby, for example, it is possible to suppress an increase in cost required for the configuration as compared with a case of adding a photo coupler, an isolated DC-DC converter, or the like. 
     The positive electrode side switch drive unit  63  can automatically switch between the conduction and the cut off of the current of the positive electrode side switch unit  61  according to the presence or absence of application of AC power. When the AC power is not applied, each of the transistors  61   a  and  61   b  is cut off by discharge of the positive electrode side resistor RP, and, when the AC power is applied, the gate-source potential of each of the transistors  61   a  and  61   b  increases and each of the transistors  61   a  and  61   b  is conduced. 
     MODIFIED EXAMPLE 
     In the following description, modified examples of the embodiment will be described. The same parts as those in the embodiment described above will be denoted by the same reference numerals, and the description thereof will be omitted or simplified. 
     FIRST MODIFIED EXAMPLE 
     In the embodiment described above, each of the plurality of circuit modules  13  includes a positive electrode side current cutoff circuit  35  between the rectifier circuit  33  and the positive electrode end of the battery module  4 , but the present invention is not limited to this. 
       FIG. 6  is a diagram which shows a configuration of a charging system  10 A in a first modified example of the embodiment.  FIG. 7  is a diagram which shows a configuration of a negative electrode side current cutoff circuit  37  of the charging system  10 A in the first modified example of the embodiment. 
     As shown in  FIG. 6 , the charging system  10 A in the first modified example includes an AC power source  11 , a plurality of circuit modules  13 A, and a control device  15 . The plurality of circuit modules  13 A are connected between the AC power source  11  and each of the plurality of battery modules  4 . The number of the plurality of circuit modules  13 A is the same as the number of the plurality of battery modules  4 . The plurality of circuit modules  13 A are, for example, a first circuit module  13 Aa, a second circuit module  13 Ab, a third circuit module  13 Ac, and a fourth circuit module  13 Ad. 
     Each of the plurality of circuit modules  13 A includes an AC electric circuit  31 , a rectifier circuit  33 , and a negative electrode side current cutoff circuit  37 . For example, the first circuit module  13 Aa includes a first AC electric circuit  31   a,  a first rectifier circuit  33   a,  and a first negative electrode side current cutoff circuit  37   a.  The second circuit module  13 Ab includes a second AC electric circuit  31   b,  a second rectifier circuit  33   b,  and a second negative electrode side current cutoff circuit  37   b.  The third circuit module  13 Ac includes a third AC electric circuit  31   c,  a third rectifier circuit  33   c,  and a third negative electrode side current cutoff circuit  37   c.  The fourth circuit module  13 Ad includes a fourth AC electric circuit  31   d,  a fourth rectifier circuit  33   d,  and a fourth negative electrode side current cutoff circuit  37   d.    
     As shown in  FIG. 7 , each negative electrode side current cutoff circuit  37  of the plurality of circuit modules  13 A is connected between the rectifier circuit  33  and the negative electrode end of a corresponding battery module  4  in the battery  1 . The negative electrode side current cutoff circuit  37  includes a negative electrode side switch unit  71  and a negative electrode side switch drive unit  73 . 
     The negative electrode side switch unit  71  is connected between the negative electrode terminal NR of the rectifier circuit  33  and the negative electrode end of a corresponding battery module  4  in the battery  1 . The negative electrode side switch unit  71  is, for example, a bidirectional switch formed of two switching elements. The switching element is a transistor such as a MOSFET or an IGBT, and is, for example, an N channel-type MOSFET. Each transistor may include a rectifying element. The rectifying element is a diode connected in parallel to each transistor. The rectifying element is, for example, a freewheel diode that is connected between the drain and the source of the MOSFET in the forward direction from a source to a drain. 
     The negative electrode side switch unit  71  includes a negative electrode side first transistor  71   a  and a negative electrode side second transistor  71   b  connected in anti-series. 
     The gates G of the negative electrode side first transistor  71   a  and the negative electrode side second transistor  71   b  are connected to a positive electrode end of the negative electrode side switch drive unit  73  (for example, a connection point  75 P of a negative electrode side rectifier circuit  75  to be described below). The sources of the negative electrode side first transistor  71   a  and the negative electrode side second transistor  71   b  are connected to a negative electrode end of the negative electrode side switch drive unit  73  (for example, a connection point  75 N of the negative electrode side rectifier circuit  75  to be described below). A drain of the negative electrode side first transistor  71   a  is connected to the negative electrode terminal NR of the rectifier circuit  33 . A drain of the negative electrode side second transistor  71   b  is connected to the negative electrode end of a corresponding battery module  4  in the battery  1 . 
     The negative electrode side switch unit  71  switches between on (conduction) and off (cut off) of the negative electrode side first transistor  71   a  and negative electrode side second transistor  71   b  based on a gate signal, which is a switching command based on a voltage applied from the negative electrode side switch drive unit  73  between each gate and each source of the negative electrode side first transistor  71   a  and the negative electrode side second transistor  71   b.  The negative electrode side switch unit  71  switches between the conduction and the cut-off of a current between each battery module  4  and the rectifier circuit  33  according to on (conduction) or off (cut off) of the negative electrode side first transistor  71   a  and the negative electrode side second transistor  71   b.    
     The negative electrode side switch drive unit  73  includes two negative electrode side capacitors CNs for DC insulation, a negative electrode side rectifier circuit  75 , and a negative electrode side resistance RN for discharge. 
     The two negative electrode side capacitors CNs are connected to the second connection point  49  of each of the A-phase electric circuit  41  and the B-phase electric circuit  43  of the AC electric circuit  31 . 
     The negative electrode side rectifier circuit  75  includes, for example, a bridge circuit formed of a plurality of diodes that are bridge-connected in two rows of the first row and the second row. 
     The negative electrode side rectifier circuit  75  is, for example, a full-wave rectifier circuit. The negative electrode side rectifier circuit  75  includes a negative electrode side first diode  75   a  and a negative electrode side second diode  75   b  connected in the forward direction in the first row, and a negative electrode side third diode  75   c  and a negative electrode side fourth diode  75   d  connected in the forward direction in the second row. 
     A connection point (an A-phase connection point)  75 A between an anode of the negative electrode side first diode  75   a  and a cathode of the negative electrode side second diode  75   b  is connected to the second connection point  49  of the A-phase electric circuit  41  via the negative electrode side capacitor CN. A connection point (a B-phase connection point)  75 B between an anode of the negative electrode side third diode  75   c  and a cathode of the negative electrode side fourth diode  75   d  is connected to the second connection point  49  of the B-phase electric circuit  43  via the negative electrode side capacitor CN. 
     A connection point  75 P between cathodes of the negative electrode side first diode  75   a  and the negative electrode side third diode  75   c  is connected to the gate G of each of the transistors  71   a  and  71   b  of the negative electrode side switch unit  71 . A connection point  75 N between anodes of each of the negative electrode side second diode  75   b  and the negative electrode side fourth diode  75   d  is connected to the source of each of the transistors  71   a  and  71   b  of the negative electrode side switch unit  71 . 
     The negative electrode side rectifier circuit  75  full-wave rectifies AC power input from the A-phase electric circuit  41  and the B-phase electric circuit  43  to the A-phase connection point  75 A and the B-phase connection point  75 B, and outputs the rectified DC power from the connection point  75 P and the connection point  75 N. 
     The negative electrode side resistance RN is connected between the connection point  75 P and the connection point  75 N of the negative electrode side rectifier circuit  75 . 
     The negative electrode side switch drive unit  73  automatically switches between the conduction and cut-off of a current of the negative electrode side switch unit  71  according to AC power input from the A-phase electric circuit  41  and the B-phase electric circuit  43  to the rectifier circuit  33 . 
     SECOND MODIFIED EXAMPLE 
     In the embodiment or the first modified example described above, each of the plurality of circuit modules  13  includes a positive electrode side current cutoff circuit  35  or a negative electrode side current cutoff circuit  37  between the rectifier circuit  33  and the positive electrode end or the negative electrode end of the battery module  4 , but the present invention is not limited to this. 
       FIG. 8  is a diagram which shows a configuration of a charging system  10 B in a second modified example of the embodiment.  FIG. 9  is a diagram which shows a configurations of the positive electrode side current cutoff circuit  35  and the negative electrode side current cutoff circuit  37  of the charging system  10 B in the second modified example of the embodiment. 
     As shown in  FIGS. 8 and 9 , the charging system  10 B in the second modified example includes an AC power source  11 , a plurality of circuit modules  13 B, and a control device  15 . The plurality of circuit modules  13 B are connected between the AC power source  11  and each of the plurality of battery modules  4 . The number of the plurality of circuit modules  13 B is the same as the number of the plurality of battery modules  4 . The plurality of circuit modules  13 B are, for example, a first circuit module  13 Ba, a second circuit module  13 Bb, a third circuit module  13 Bc, and a fourth circuit module  13 Bd. Each of the plurality of circuit modules  13 B includes an AC electric circuit  31 , a rectifier circuit  33 , a positive electrode side current cutoff circuit  35 , and a negative electrode side current cutoff circuit  37 . For example, the first circuit module  13 Ba includes a first AC electric circuit  31   a,  a first rectifier circuit  33   a,  a first positive electrode side current cutoff circuit  35   a,  and a first negative electrode side current cutoff circuit  37   a.  The second circuit module  13 Bb includes a second AC electric circuit  31   b,  a second rectifier circuit  33   b,  a second positive electrode side current cutoff circuit  35   b,  and a second negative electrode side current cutoff circuit  37   b.  The third circuit module  13 Bc includes a third AC electric circuit  31   c,  a third rectifier circuit  33   c,  a third positive electrode side current cutoff circuit  35   c,  and a third negative electrode side current cutoff circuit  37   c.  The fourth circuit module  13 Bd includes a fourth AC electric circuit  31   d,  a fourth rectifier circuit  33   d,  a fourth positive electrode side current cutoff circuit  35   d,  and a fourth negative electrode side current cutoff circuit  37   d.    
     THIRD MODIFIED EXAMPLE 
     In the embodiment described above, each positive electrode side current cutoff circuit  35  of the plurality of circuit modules  13  includes a bridge circuit formed of a plurality of diodes, but the present invention is not limited to this, and may include other rectifier circuits. 
       FIG. 10  is a diagram which shows a configuration of a positive electrode side current cutoff circuit  35 A of a charging system  10 C in a third modified example of the embodiment. 
     As shown in  FIG. 10 , the positive electrode side current cutoff circuit  35 A of the charging system  10 C in the third modified example includes a positive electrode side switch unit  61  and a positive electrode side switch drive unit  63 A. The positive electrode side switch drive unit  63 A includes two negative electrode side capacitors CNs for DC insulation, a positive electrode side rectifier circuit  81 , and a positive electrode side resistor RP for discharge. 
     The positive electrode side rectifier circuit  81  includes, for example, a first diode  81   a,  a second diode  81   b,  and a third diode  81   c.    
     A cathode of the first diode  81   a  is connected to the positive electrode side capacitor CP on an A-phase side, and an anode of the first diode  81   a  is connected to the positive electrode side capacitor CP on a B-phase side. 
     A cathode of the second diode  81   b  is connected to the gate G of each of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61 , and an anode of the second diode  81   b  is connected to the cathode of the first diode  81   a.    
     A cathode of the third diode  81   c  is connected to the anode of the first diode  81   a,  and an anode of the third diode  81   c  is connected to the source of each of the transistors  61   a  and  61   b  of the positive electrode side switch unit  61 . 
     In the embodiment described above, it is described that the first power conversion unit  22  performs two types of power conversion of step-up and step-down, but the present invention is not limited to this, and may include a step-up circuit or a step-down circuit. 
     In the embodiment described above, the charging system  10  is mounted in a vehicle, but the present invention is not limited to this, and the charging system  10  may be mounted in other devices. 
     In the embodiment described above, the charging system  10  is connected to a power storage device, but the present invention is not limited to this, and the charging system  10  may be connected to another load to supply power. 
     The embodiment of the present invention is presented as an example, and is not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made within a range not departing from the gist of the invention. These embodiments and modified examples thereof are included in the scope and gist of the invention, as well as in the invention described in the scope of the claims and the equivalent scope thereof.