Battery cell balancing circuit using LC serial resonance

The present invention relates to a technology capable of improving the use time of a battery cell by generally or individually controlling charge or discharge with respect to a multi-battery cell by using LC serial resonance in a module with a multi-battery cell structure. To this end, the present invention includes a battery module including a plurality of serially connected battery cells, a serial resonant circuit including serially connected inductor and capacitor and performing a serial resonant function, and first to third switch units that set an electric energy collection path and an electric energy supply path between the battery module and the serial resonant circuit.

TECHNICAL FIELD

The present disclosure relates to a balancing technology of a multi-battery cell, and particularly, to a battery cell balancing circuit using LC serial resonance, by which it is possible to improve the use time of a battery cell by generally or individually controlling charge or discharge with respect to a multi-battery cell by using LC serial resonance in a module with a multi-battery cell structure.

BACKGROUND ART

In general, when a voltage of both terminals of an electric cell (a battery cell) exceeds a predetermined value, there is a risk of explosion, and when the voltage drops below the predetermined value, eternal damage occurs in the battery cell. Since a hybrid electric vehicle, a notebook computer and the like require relatively large electric power, when electric power is supplied using the battery cell, a battery module (a battery pack) obtained by serially connecting battery cells is used. However, when such a battery module is used, voltage unbalance may occur by performance deviation of the battery cells.

At the time of charge of the battery module, when one battery cell firstly reaches an upper limit voltage as compared with other battery cells in the battery module, since it is not possible to charge the battery module any more, it is necessary to end the charge in the state in which the other battery cells have not been sufficiently charged. In such a case, the charge capacity of the battery module does not reach rated charge capacity.

At the time of discharge of the battery module, when one battery cell firstly reaches a lower limit voltage as compared with other battery cells in the battery module, since it is not possible to use the battery module any more, the use time of the battery module is shortened.

As described above, at the time of charge or discharge of the battery module, electric energy of a battery cell having higher electric energy is supplied to a battery cell having lower electric energy, so that it is possible to improve the use time of the battery module, wherein such an operation is called battery cell balancing.

FIG. 1is a diagram illustrating a battery cell balancing circuit using a parallel resistor according to the conventional art. As illustrated inFIG. 1, the battery cell balancing circuit includes a battery module11having battery cells CELL1to CELL4serially connected to one another, resistors R11to R14serially connected to one another, and switches SW11to SW15that selectively connect both end terminals of the battery module11and respective connection terminals among the battery cells CELL1to CELL4to respective corresponding terminals of the resistors R11to R14.

Referring toFIG. 1, at the time of charge of the battery module11, when a charged voltage of an arbitrary battery cell of the battery cells CELL1to CELL4in the battery module11firstly reaches an upper limit voltage as compared with charged voltages of the other battery cells, a corresponding switch of the switches SW11to SW15is turned on, so that the charged voltage is discharged through a corresponding resistor of the resistors R11to R14.

For example, when a charged voltage of the second battery cell CELL2firstly reaches an upper limit voltage as compared with charged voltages of the other battery cells CELL1, CELL3, and CELL4, the switch SW12is turned on. Accordingly, the charged voltage of the second battery cell CELL2is discharged through the resistor R12, so that battery cell balancing is achieved.

However, in the case of using such a battery cell balancing circuit, since electric power is consumed through a resistor, efficiency is reduced. Furthermore, since it is not possible to supply an upper limit voltage to a battery cell having a low voltage during the use of a battery module, efficiency is reduced.

FIG. 2is a diagram illustrating a battery cell balancing circuit using a capacitor according to the conventional art. As illustrated inFIG. 2, the battery cell balancing circuit includes a battery module21having battery cells CELL1to CELL4serially connected to one another, capacitors C21to C23serially connected to one another, and switches SW21to SW24that selectively connect one side terminal of the capacitor C21, a connection terminal between the capacitors C21and C22, a connection terminal between the capacitors C22and C23, and the other side terminal of the capacitor C23to one of both terminals of each of the battery cells CELL1to CELL4.

Referring toFIG. 2, the battery cell balancing circuit using a capacitor has two connection states. In the first connection state, the one side terminal of the capacitor C21, the connection terminal between the capacitors C21and C22, the connection terminal between the capacitors C22and C23, and the other side terminal of the capacitor C23are respectively connected to one side terminal (a positive terminal) of each of the battery cells CELL1to CELL4as illustrated inFIG. 2. In the second connection state, the one side terminal of the capacitor C21, the connection terminal between the capacitors C21and C22, the connection terminal between the capacitors C22and C23, and the other side terminal of the capacitor C23are respectively connected to the other side terminal (a negative terminal) of each of the battery cells CELL1to CELL4.

However, such a battery cell balancing circuit has a problem that efficiency is low because a hard switching operation is generated between the capacitors and the battery cells. It is preferable that capacities of the battery cells in the battery module are equal to one another, but the capacities of the battery cells become different from one another due to various factors. In such a case, even though a charged voltage of a certain battery cell is lower than a charged voltage of another battery cell, it may have a larger capacity. In such a case, it is necessary to transfer a voltage of a battery cell having a high voltage to a battery cell having a low voltage. However, in such a conventional battery cell balancing circuit, it is not possible to perform such a voltage transfer function.

FIG. 3is a diagram illustrating a battery cell balancing circuit using a fly-back structure according to the conventional art. As illustrated inFIG. 3, the battery cell balancing circuit includes a battery module31having battery cells CELL1to CELL4serially connected to one another, a fly-back converter32, switches SW31to SW34that selectively connect a plurality of secondary coils of the fly-back converter32to both terminals of each of the battery cells CELL1to CELL4, and a switch SW35that selectively connects both ends of a primary coil of the fly-back converter32to both ends of the battery module31.

The battery cell balancing circuit ofFIG. 3is a battery cell balancing circuit using a fly-back structure belonging to SMPS (Switch Mode Power Supply) and has a structure in which it is possible to transfer electric energy to the battery cells CELL1to CELL4serially connected to one another in the battery module31by using the switches SW31to SW34, and it is possible to transfer electric energy between both end terminals of the battery module31.

Since such a battery cell balancing circuit has a SMPS type, it has superior efficiency. However, as the number of battery cells included in the battery module increases, the size of a magnetic core used in the fly-back converter becomes large. Therefore, the cost of the battery cell balancing circuit becomes expensive.

DISCLOSURE

Technical Problem

Various embodiments are directed to minimize loss due to hard switching by enabling exchange of electric energy among battery cells by using an LC resonant circuit and to transfer energy from a battery cell having high energy to a battery cell having low energy.

Technical Solution

In an embodiment, a battery cell balancing circuit using LC serial resonance includes: a battery module including a plurality of serially connected battery cells; a serial resonant circuit including serially connected inductor and capacitor and performing a serial resonant function; a first switch unit including a plurality of switches each connected between each terminal of the plurality of battery cells and a first common node in order to provide a path for collecting electric energy charged in one or more battery cells of the plurality of battery cells and storing the electric energy in the capacitor, or for supplying the electric energy collected and stored in the capacitor to the one or more battery cells; a second switch unit including a plurality of switches each connected between each terminal of the plurality of battery cells and a second common node in order to provide the path for supplying the electric energy collected and stored in the capacitor to the one or more battery cells, or for collecting the electric energy charged in the one or more battery cells; and a third switch unit including a plurality of switches connected between the first common node and both terminals of the serial resonant circuit and switches connected between the second common node and both terminals of the serial resonant circuit in order to provide the path for collecting the electric energy charged in the one or more battery cells of the plurality of battery cells, or for supplying the electric energy collected and stored in the capacitor to the one or more battery cells.

In an embodiment, a battery cell balancing circuit using LC serial resonance includes: a battery module including a plurality of serially connected battery cells; a serial resonant circuit including serially connected inductor and capacitor and performing a serial resonant function; a first switch unit including a plurality of MOS transistors and reverse current blocking diodes connected between each terminal of the plurality of battery cells and a first common node in order to provide the path for collecting electric energy charged in one or more battery cells of the plurality of battery cells and storing the electric energy in the capacitor, or for supplying the electric energy collected and stored in the capacitor to the one or more battery cells, each MOS transistor and each diode being connected in parallel to each other; a second switch unit including a plurality of MOS transistors and reverse current blocking diodes connected between each terminal of the plurality of battery cells and a second common node in order to provide a path for supplying the electric energy collected and stored in the capacitor to the one or more battery cells, or for collecting the electric energy charged in the one or more battery cells, each MOS transistor and each diode being connected in parallel to each other; and a third switch unit including MOS transistors and reverse current blocking diodes connected between the first common node and both terminals of the serial resonant circuit and MOS transistors and reverse current blocking diodes connected between the second common node and both terminals of the serial resonant circuit in order to provide the path for collecting the electric energy charged in the one or more battery cells of the plurality of battery cells, or for supplying the electric energy collected and stored in the capacitor to the one or more battery cells, each MOS transistor and each diode being connected in parallel to each other.

In an embodiment, a battery cell balancing circuit using LC serial resonance includes: a battery module including a plurality of serially connected battery cells; a serial resonant circuit including serially connected inductor and capacitor and performing a serial resonant function; a first switch unit including a plurality of switch paths connected between some of terminals of the plurality of battery cells and a first common node in order to provide a path for collecting electric energy charged in an arbitrary cell of the plurality of battery cells, or for supplying the electric energy collected and stored in the capacitor to the arbitrary cell battery cell, the plurality of switch paths each including two serially connected MOS transistors each connected in parallel to diodes; a second switch unit including a plurality of switch paths connected between remaining terminals, other than some of terminals of the plurality of battery cells, and a second common node in order to provide the path for supplying the electric energy collected and stored in the capacitor to the arbitrary cell, or for collecting the electric energy charged in the arbitrary cell, the plurality of switch paths each including two serially connected MOS transistors each connected in parallel to diodes; and a third switch unit including switch paths connected between the first common node and both terminals of the serial resonant circuit and switch paths connected between the second common node and both terminals of the serial resonant circuit in order to provide the path for collecting the electric energy charged in the arbitrary cell of the plurality of battery cells, or for supplying the electric energy collected and stored in the capacitor to the arbitrary cell, the plurality of switch paths each including two serially connected MOS transistors each connected in parallel to diodes.

In an embodiment, a battery cell balancing circuit using LC serial resonance includes: a battery module including a plurality of serially connected battery cells; a serial resonant circuit including an inductor and a capacitor serially connected between a first common node and a second common node and performing a serial resonant function; a first switch unit including a plurality of switches each connected between each terminal of the plurality of battery cells and a first common node in order to provide a path for collecting electric energy charged in one or more battery cells of the plurality of battery cells and storing the electric energy in the capacitor, or for supplying the electric energy collected and stored in the capacitor to the one or more battery cells; and a second switch unit including a plurality of switches each connected between each terminal of the plurality of battery cells and a second common node in order to provide the path for supplying the electric energy collected and stored in the capacitor to the one or more battery cells, or for collecting the electric energy charged in the one or more battery cells.

Advantageous Effects

According to the present invention, in a circuit that performs a battery cell balancing function by using a plurality of battery cells provided therein, exchange of electric energy becomes possible among battery cells by using an LC resonant circuit, so that loss due to hard switching is minimized, and energy can be transferred from a battery cell having high energy to a battery cell having low energy, so that battery performance is improved.

MODE FOR INVENTION

FIG. 4is a circuit diagram illustrating a battery cell balancing circuit using LC serial resonance according to a first embodiment of the present invention, and includes a battery module41, a serial resonant circuit42, and first to third switch units43to45.

The battery module41includes first to fourth battery cells CELL1to CELL0serially connected to one another.

The serial resonant circuit42includes an inductor Ls and a capacitor Cs serially connected to each other.

The first switch unit43is for forming collection and supply paths of electric energy, and includes first to fifth switches SW41to SW45each having one side terminal connected to respective terminals of the first to fourth battery cells CELL1to CELL4and the other side terminal commonly connected to a first common node N1.

The second switch unit44is for forming collection and supply paths of electric energy, and includes sixth to tenth switches SW46to SW50each having one side terminal connected to the respective terminals of the first to fourth battery cells CELL1to CELL4and the other side terminal commonly connected to a second common node N2.

The third switch unit45includes an eleventh switch SW51that connects one end terminal of the serial resonant circuit42to the first common node N1and a twelfth switch SW52that connects the other end terminal of the serial resonant circuit42to the second common node N2in an electric energy collection mode, and a thirteenth switch SW53that connects the other end terminal of the serial resonant circuit42to the first common node N1and a fourteenth switch SW54that connects the one end terminal of the serial resonant circuit42to the second common node N2in an electric energy supply mode.

The respective terminals of the first to fourth battery cells CELL1to CELL4indicate one side terminal of the first battery cell CELL1, a common connection terminal of the other side terminal of the first battery cell CELL1and one side terminal of the second battery cell CELL2, a common connection terminal of the other side terminal of the second battery cell CELL2and one side terminal of the third battery cell CELL3, a common connection terminal of the other side terminal of the third battery cell CELL3and one side terminal of the fourth battery cell CELL4, and the other side terminal of the fourth battery cell CELL4.

Electric energy charged in an arbitrary battery cell of the first to fourth battery cells CELL1to CELL4of the battery module41is temporarily charged in the capacitor Cs of the serial resonant circuit42through the first switch unit43and the third switch unit45, and the electric energy charged in the serial resonant circuit42is charged in an arbitrary battery cell of the first to fourth battery cells CELL1to CELL4through the third switch unit45and the second switch unit44.

InFIG. 4, the SPST (Single Pole Single Throw) switch has been described as an example of the switches provided in the first to third switch units43to45. However, the present invention is not limited thereto, and it may be implemented with another switch element, for example, a power switch of MOSFET (Metal Oxide Field Effect Transistor), BJT (Bipolar Junction Transistor), IGBT (Insulated Gate Bipolar Transistor) and the like.

When the number of battery cells of the battery module41is added, it is possible to add the number of switches of the first and second switch units43and44in correspondence to the added number.

FIG. 5Aillustrates an example in which electric energy relatively highly charged in the third battery cell CELL3(an arbitrary battery cell of the first to fourth battery cells CELL1to CELL4of the battery module41), as compared with other battery cells, is collected and is temporarily charged in the capacitor Cs of the serial resonant circuit42.

Referring toFIG. 5A, among the first to fifth switches SW41to SW45of the first switch unit43, the third switch SW43is turned on and the other switches are maintained in a turned-off state. At this time, among the sixth to tenth switches SW46to SW50of the second switch unit44, the ninth switch SW49is turned on and the other switches are maintained in a turned-off state. Furthermore, among the eleventh to fourteenth switches SW51to SW54of the third switch unit45, the eleventh and twelfth switches SW51and SW52are turned on and the other switches are maintained in a turned-off state. Accordingly, one side terminal of the third battery cell CELL3of the battery module41is connected to a third common node N3serving as one side terminal of the serial resonant circuit42through the third switch SW43and the eleventh switch SW51, and a fourth common node N4serving as the other side terminal of the serial resonant circuit42is connected to the second common node N2through the twelfth switch SW52.

Consequently, the charged energy of the third battery cell CELL3is collected through the third and eleventh switches SW43and SW51, and is charged in the capacitor Cs of the serial resonant circuit42. Since the capacity of the third battery cell CELL3is very larger than that of the capacitor Cs, when the serial resonant circuit42resonates, the charged voltage of the third battery cell CELL3finely drops. At this time, the charged voltage of the capacitor Cs rises in the form of a sine function as illustrated inFIG. 6A. As described above, since the charged voltage of the capacitor Cs slowly rises, hard switching loss does not almost occur. InFIG. 6B, the current graph ILindicates a change in the current amount of the third battery cell CELL3.

FIG. 5Billustrates an example in which the electric energy temporarily charged in the capacitor Cs of the serial resonant circuit42through the aforementioned process is supplied to the first battery cell CELL1as an arbitrary battery cell of the first to fourth battery cells CELL1to CELL4of the battery module41.

Referring toFIG. 5B, among the first to fifth switches SW41to SW45of the first switch unit43, the second switch SW42is turned on and the other switches are maintained in a turned-off state. At this time, among the sixth to tenth switches SW46to SW50of the second switch unit44, the sixth switch SW46is turned on and the other switches are maintained in a turned-off state. Furthermore, among the eleventh to fourteenth switches SW51to SW54of the third switch unit45, the thirteenth and fourteenth switches SW53and SW54are turned on and the other switches are maintained in a turned-off state. Accordingly, the third common node N3serving as the one side terminal of the serial resonant circuit42is connected to one side terminal of the first battery cell CELL1of the battery module41through the fourteenth switch SW54and the sixth switch SW46, and the fourth common node N4serving as the other side terminal of the serial resonant circuit42is connected to the first common node N1through the thirteenth switch SW53.

Consequently, the electric energy temporarily charged in the capacitor Cs of the serial resonant circuit42is supplied to the first battery cell CELL1of the battery module41through the fourteenth and sixth switches SW54and SW46. At this time, the charged voltage of the capacitor Cs falls in the form of a sine function as illustrated inFIG. 6B. InFIG. 6B, the current graph ILindicates a change in the current amount of the first battery cell CELL1.

As illustrated inFIG. 5AandFIG. 5B, an electric power amount when electric energy charged in an arbitrary battery cell is collected and is temporarily charged in the capacitor Cs of the serial resonant circuit42or the charged electric energy is supplied to an arbitrary battery cell is decided by values of the capacitor Cs and the inductor Ls of the serial resonant circuit42. For example, since a transferred electric power amount is large as the value of the capacitor Cs is large and the value of the inductor Ls is small, balancing is quickly achieved. However, as the amount of a resonance current becomes large, since a loss amount also becomes large, it is preferable to appropriately set the values of the capacitor Cs and the inductor Ls.

For example, when the value of the inductor Ls is set to 500 μH and the value of the capacitor Cs is set to 120 μF, an electric power amount transferred through resonance has been checked to about 0.5 W through an experiment.

The operations ofFIG. 5AandFIG. 5Bare repeatedly performed as required until balancing is achieved between the third battery cell CELL3and the first battery cell CELL1, so that the charged energy of the third battery cell CELL3is collected and is supplied to the first battery cell CELL1.

To this end, among the first to fourth battery cells CELL1to CELL4of the battery module41, a battery cell with the highest charged electric energy and a battery cell with the lowest charged electric energy can be selected, and a balancing algorithm for performing the aforementioned balancing function can be used.

So far, the case, in which the balancing function is performed for one battery cell with the highest charged electric energy and one battery cell with the lowest charged electric energy among the first to fourth battery cells CELL1to CELL4of the battery module41, has been described. However, the present invention is not limited thereto, and it is possible to perform the balancing function for n battery cells through the aforementioned process.

For example, when the tenth switch SW50and the first switch SW41are allowed to be turned on and the eleventh and twelfth switches SW51and SW52are allowed to be turned on, electric energy charged in the first to fourth battery cells CELL1to CELL4of the battery module41can be collected to the capacitor Cs of the serial resonant circuit42. Furthermore, when the sixth switch SW46and the fifth switch SW45are allowed to be turned on and the thirteenth and fourteenth switches SW53and SW54are allowed to be turned on, the electric energy temporarily charged in the capacitor Cs of the serial resonant circuit42can be supplied to the first to fourth battery cells CELL1to CELL4of the battery module41.

FIG. 7illustrates a second embodiment for a battery cell balancing circuit of the present invention, and is different from the battery cell balancing circuit of the first embodiment ofFIG. 4in that switches of first to third switch units43A,44A, and45A are implemented with first to fifth MOS transistors M41to M45, sixth to tenth MOS transistors M46to M50, and eleventh to fourteenth MOS transistors M51to M54.FIG. 7illustrates an example in which first to fourteenth reverse current blocking diodes D41_1to D54_1are serially connected between corresponding MOS transistors and corresponding nodes in order to prevent the first to fourth battery cells CELL1to CELL4of the battery module41from being damaged by a reverse current. An arrangement position of the MOS transistor and the diode serially connected to each other may be changed according to necessity, and the MOS transistor may include a P channel MOS transistor or an N channel MOS transistor. In addition, the first to fourteenth reverse current blocking diodes D41_1to D54_1are also connected between terminals of one side and terminals of the other side of first to fifth MOS transistors M41to M45, the sixth to tenth MOS transistors M46to M50, and the eleventh to fourteenth MOS transistors M51to M54.

FIG. 8illustrates a third embodiment for a battery cell balancing circuit of the present invention, and is different from that ofFIG. 4in that a current can be applied in two directions of first and second switch units43B and44B to reduce the number of switch paths to ½ and MOS transistors are used instead of the SPST switches.

That is, in the case ofFIG. 4, since five switch paths respectively exist between the first to fourth battery cells CELL1to CELL4of the battery module41and the first and second common nodes N1and N2, the total10switch paths exist. Meanwhile, in the case ofFIG. 8, since three switch paths exist in the first switch unit43B connected between the first to fourth battery cells CELL1to CELL4of the battery module41and the first common node N1and two switch paths exist in the second switch unit44B connected between the first to fourth battery cells CELL1to CELL4of the battery module41and the second common node N2, the total five switch paths exist.

In such a case, in order to allow a current to be applied in two directions in each of the five switch paths, a pair of a MOS transistor and a diode connected in parallel to each other are serially connected to another pair of a MOS transistor and a diode connected in parallel to each other, and these two pairs are provided on each of the five switch paths. For example, in the first switch unit43B, first and second MOS transistors M81and M82each connected in parallel to diodes are serially connected between one side terminal of the first battery cell CELL1and the first common node N1, third and fourth MOS transistors M83and M84each connected in parallel to diodes are serially connected between one side terminal of the third battery cell CELL3and the first common node N1, and fifth and sixth MOS transistors M85and M86each connected in parallel to diodes are serially connected between one side terminal of the fourth battery cell CELL4and the first common node N1. Similarly, in the second switch unit44B, seventh and eighth MOS transistors M87and M88each connected in parallel to diodes are serially connected between one side terminal of the second battery cell CELL2and the second common node N2, and ninth and tenth MOS transistors M89and M90each connected in parallel to diodes are serially connected between one side terminal of the fourth battery cell CELL4and the second common node N2.

In addition, also in a third switch unit45B, MOS transistors are connected as described above. That is, eleventh and twelfth MOS transistors M91and M92each connected in parallel to diodes are serially connected between the first common node N1and the third common node N3serving as one side terminal of the serial resonant circuit42, thirteenth and fourteenth MOS transistors M93and M94each connected in parallel to diodes are serially connected between the second common node N2and the fourth common node N4serving as the other side terminal of the serial resonant circuit42, fifteenth and sixteenth MOS transistors M95and M96each connected in parallel to diodes are serially connected between the first common node N1and the fourth common node N4, and seventeenth and eighteenth MOS transistors M97and M98each connected in parallel to diodes are serially connected between the second common node N2and the third common node N3.

For example, an electric energy collection path of the third battery cell CELL3is formed by the one side terminal of the third battery cell CELL3, a third diode D83, the third MOS transistor M84, the first common node N1, an eleventh diode D91, the twelfth MOS transistor M92, the serial resonant circuit42, a thirteenth diode D93, the fourteenth MOS transistor M94, the second common node N2, a ninth diode D89, the tenth MOS transistor M90, and the other side terminal of the third battery cell CELL3.

In another example, a path for supplying the electric energy collected and stored in the capacitor Cs to the fourth battery cell CELL4is formed by the other side terminal of the fourth battery cell CELL4, a fifth diode D85, the sixth MOS transistor M86, the first common node N1, a fifteenth diode D95, the sixteenth MOS transistor and M96, the serial resonant circuit42, a seventeenth diode D97, the eighteenth MOS transistor M98, the second common node N2, the ninth diode D89, the tenth MOS transistor M90, and the one side terminal of the fourth battery cell CELL4.

FIG. 9illustrates a fourth embodiment for a battery cell balancing circuit of the present invention, and is different from the battery cell balancing circuit of the first embodiment ofFIG. 4in that the third switch unit is omitted.

Referring toFIG. 9, the battery cell balancing circuit includes a battery module91, a serial resonant circuit92, a first switch unit93, and a second first switch unit94. Differently fromFIG. 4, the serial resonant circuit92is not selectively connected between the first and second common nodes N1and N2through a switch element, and one side terminal of an inductor Ls of the serial resonant circuit92is fixedly connected to the first common node N1and the other side terminal of a capacitor Cs of the serial resonant circuit92is fixedly connected to the second common node N2.

FIG. 10Ais an exemplary diagram of charge and illustrates an example in which electric energy charged in the third battery cell CELL3inFIG. 9is collected and is temporarily charged in the capacitor Cs of the serial resonant circuit92.

In such a case, a third switch SW93and a ninth switch SW99are turned on and the other switches are maintained in a turned-off state. Accordingly, a collection path connected to one side terminal of the third battery cell CELL3, the third switch SW93, the first common node N1, the serially connected inductor Ls and capacitor Cs of the serial resonant circuit92, the second common node N2, the ninth switch SW99, and the other side terminal of the third battery cell CELL3is formed. Consequently, the electric energy of the third battery cell CELL3is temporarily charged in the capacitor Cs through the third switch SW93, the first common node N1, and the inductor Ls.

FIG. 10Bis an exemplary diagram of supply of charged electric energy and illustrates an example in which the electric energy temporarily charged in the capacitor Cs is charged in the first battery cell CELL1.

In such a case, a first switch SW91and a seventh switch SW97are turned on and the other switches are maintained in a turned-off state. Accordingly, a supply path connected to one side terminal of the first battery cell CELL1, the first switch SW91, the first common node N1, the serially connected inductor Ls and capacitor Cs of the serial resonant circuit92, the second common node N2, the seventh switch SW97, and the other side terminal of the first battery cell CELL1is formed. Consequently, the electric energy charged in the capacitor Cs is supplied to and charged in the first battery cell CELL1through the inductor Ls, the first common node N1, and the first switch SW91.

FIG. 11is a diagram illustrating an example in which the SPST switches of the first and second switch units93and in the battery cell balancing circuit ofFIG. 9are implemented with MOS transistors.

Referring toFIG. 11, in a first switch unit93A, first and second MOS transistors M101and M102each connected in parallel to diodes are serially connected between one side terminal of the first battery cell CELL1and the first common node N1, third and fourth MOS transistors M103and M104each connected in parallel to diodes are serially connected between one side terminal of the second battery cell CELL2and the first common node N1, fifth and sixth MOS transistors M105and M106each connected in parallel to diodes are serially connected between one side terminal of the third battery cell CELL3and the first common node N1, seventh and eighth MOS transistors M107and M108each connected in parallel to diodes are serially connected between one side terminal of the fourth battery cell CELL4and the first common node N1, and ninth and tenth MOS transistors M109and M110each connected in parallel to diodes are serially connected between the other side terminal of the fourth battery cell CELL4and the first common node N1.

In a second switch unit94A, eleventh and twelfth MOS transistors M111and M112each connected in parallel to diodes are serially connected between the other side terminal of the first battery cell CELL1and the second common node N2, thirteenth and fourteenth MOS transistors M113and M114each connected in parallel to diodes are serially connected between the other side terminal of the second battery cell CELL2and the second common node N2, fifteenth and sixteenth MOS transistors M115and M116each connected in parallel to diodes are serially connected between the other side terminal of the third battery cell CELL3and the second common node N2, seventeenth and eighteenth MOS transistors M117and M118each connected in parallel to diodes are serially connected between the other side terminal of the fourth battery cell CELL4and the second common node N2, and nineteenth and twentieth MOS transistors M119and M120each connected in parallel to diodes are serially connected between the other side terminal of the fourth battery cell CELL4and the second common node N2.

FIG. 12illustrates another embodiment of the first and second switch units ofFIG. 9, and is different from that ofFIG. 9in that a current can be applied in two directions of first and second switch units93B and94B to reduce the number of switch paths to ½ and MOS transistors are used instead of the SPST switches. The configurations of the first and second switch units93B and94B are equal to those of the first and second switch units43B and44B inFIG. 8. However,FIG. 12is different fromFIG. 8in that the third switch unit is omitted and both end terminals of the serial resonant circuit92are directly connected between the first and second common nodes N1and N2.

By such a structure, there is a limitation in a current flow among the first to fourth battery cells CELL1to CELL4. That is, electric energy can be transferred only between the first and third cells CELL1and CELL3which are battery cells in an odd sequence, or can be transferred only between the second and fourth cells CELL2and CELL4which are battery cells in an even sequence.

FIG. 13illustrates an embodiment of a circuit for protecting the serial resonant circuit applied to the battery cell balancing circuit of the present invention.

Referring toFIG. 13, the protection circuit for the serial resonant circuit according to the present invention includes first and second diodes D131and D132connected in parallel to each other between both end terminals of a serial resonant circuit131including serially connected inductor Ls and capacitor Cs and an upper terminal TOP_STACK, third and fourth diodes D133and D134connected in parallel to each other between both end terminals of the serial resonant circuit131and a lower terminal BOTTOM_STACK, and a switch SW131connected in parallel to the capacitor Cs.

The upper terminal TOP_STACK is connected to a positive terminal of the uppermost first battery cell CELL1in the battery module91ofFIG. 12, and the lower terminal BOTTOM_STACK is connected to a negative terminal of the lowermost fourth battery cell CELL4in the battery module91.

When a current path is cut in the state in which a current is flowing through the inductor Ls, a strong voltage spike phenomenon occurs at both ends of the inductor Ls. For example, in the state in which a current is flowing from the third common node N3to the fourth common node N4, at the moment at which the current is blocked, a very high voltage with a positive polarity (+) is applied to the fourth common node N4and a very low voltage with a negative polarity (−) is applied to the third common node N3. Therefore, there is a risk that the switch elements connected to the serial resonant circuit131may be broken.

However, the switch elements are protected by the diodes D131to D134connected as illustrated inFIG. 13. For example, a current flows toward a battery stack through a path of the diode D134, the inductor Ls, the capacitor Cs, and the diode D131, so that only a voltage corresponding to the battery stack is applied to both ends of the fourth common node N4and the third common node N3. By such a principle, the elements are protected.

The protection circuit protects the switching elements and the serial resonant circuit131from voltage spike which may occur due to a rapid change in the inductor current when a current remaining in the inductor Ls is exhausted after a resonance operation of the serial resonant circuit131. Furthermore, the switch SW131resets charge remaining in the capacitor Cs after the resonance operation of the serial resonant circuit131, thereby allowing next resonance to be efficiently performed.