Switch circuit, semiconductor device, and battery device

There is provided a semiconductor device capable of preventing the passage of current that is unexpected in a circuit operation even in the case of reverse connection, thus ensuring higher safety. The semiconductor device has a switch circuit which includes: a first transistor; a second transistor having a drain thereof connected to a drain of the first transistor, a source and a back gate thereof connected to a back gate of the first transistor, and a gate thereof connected to a source of the first transistor; and a third transistor having a drain thereof connected to the source of the first transistor, a source and a back gate thereof connected to the back gate of the first transistor, and a gate thereof connected to the drain of the first transistor.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-031365 filed on Feb. 20, 2013, the entire content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switch circuit and a semiconductor device.

2. Background Art

FIG. 4is a circuit diagram illustrating a conventional voltage controller. The conventional voltage controller is constituted of a voltage detector circuit503, a positive power supply connection terminal509providing a positive power supply of the voltage detector circuit503, a negative power supply connection terminal510providing a negative power supply of the voltage detector circuit503, a control circuit508connected between the positive power supply connection terminal509and the negative power supply connection terminal510, an N-ch transistor501having a gate thereof connected to an output of the control circuit508, a drain thereof connected to an output terminal511, and a source and a back gate thereof connected to the negative power supply connection terminal510, and a resistor512.

The control circuit508is constituted of a reference voltage generating circuit506, resistors504and505, which divide a voltage between the positive power supply connection terminal509and the negative power supply connection terminal510(the voltage between the positive power supply connection terminal and the negative power supply connection terminal will be hereinafter referred to as “the supply voltage”), and a comparator507, which compares the voltage divided by the resistors504and505with a voltage of the reference voltage generating circuit506. A parasitic diode502exists between the drain and the back gate of the N-ch transistor501. The N-ch transistor501will act as a switch circuit of the voltage detector circuit503.

Connecting the positive power supply connection terminal509and the output terminal511of the voltage detector circuit503and connecting one terminal of the resistor512thereto, and connecting the other terminal of the resistor to the positive power supply of a voltage generator513constitute the voltage controller, which controls the supply voltage of the voltage detector circuit503to a constant level.

A description will now be given of the operation. The supply voltage of the voltage detector circuit503is controlled to a voltage obtained by dividing the voltage of the voltage generator513by the resistance value of the resistor512connected to the positive power supply connection terminal509and an ON resistance value of the N-ch transistor501. If the supply voltage of the voltage detector circuit503changes to a higher level, then the voltage divided by the resistors504and505increases to the voltage of the reference voltage generating circuit506or more, and the output of the control circuit508increases the gate voltage of the N-ch transistor501. As the gate voltage increases, the ON resistance value of the N-ch transistor501decreases, thus decreasing the voltage between the output terminal511and the negative power supply connection terminal510. The output terminal511is connected to the positive power supply connection terminal509, so that the supply voltage of the voltage detector circuit503is also controlled to decrease. If the supply voltage of the voltage detector circuit503changes to a lower level, then the ON resistance value of the N-ch transistor501increases and the supply voltage of the voltage detector circuit503is controlled to increase, which is opposite from the foregoing case. Repeating the foregoing operation causes the voltage controller to operate to continue outputting a constant voltage (refer to, for example,FIG. 1in Patent Document 1).

However, according to the conventional voltage controller, if the positive power supply connection terminal509and the negative power supply connection terminal510of the voltage detector circuit503are inversely connected to a positive power supply terminal and a negative power supply terminal of the voltage generator513(hereinafter referred to as “reverse connection”), then the parasitic diode502of the N-ch transistor501of the switch circuit will be in the forward direction, because the positive power supply connection terminal509and the output terminal511are connected. As a result, current that is unexpected in the circuit operation flows through the voltage detector circuit503. There has been a problem in that the passage of much current that is unexpected in the circuit operation tends to lead to heat generation or ignition, causing deteriorated safety of the voltage controller.

SUMMARY OF THE INVENTION

The present invention has been made with a view toward solving the problem described above and an object of the invention is to achieve a voltage controller capable of preventing the passage of current that is unexpected in a circuit operation even in the case of reverse connection, thus ensuring higher safety.

To this end, a switch circuit according to the present invention is configured as follows.

The switch circuit includes: a first transistor; a second transistor having a drain thereof connected to a drain of the first transistor, a source and a back gate thereof connected to a back gate of the first transistor, and a gate thereof connected to a source of the first transistor; and a third transistor having a drain thereof connected to the source of the first transistor, a source and a back gate thereof connected to the back gate of the first transistor, and a gate thereof connected to the drain of the first transistor.

According to the present invention, the voltage of the back gate of an N-ch transistor of the switch circuit connected to an output terminal is changed according to a connection state, namely, normal connection or reverse connection. This makes it possible to short-circuit the anode and the cathode of a parasitic diode, which is in a forward direction regardless of the connection state, thereby turning the diode off The present invention makes it possible to advantageously prevent, even in the case of reverse connection, the passage of current that is unexpected in a circuit operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe embodiments of the present invention with reference to the accompanying drawings.

First Embodiment

FIG. 1is a circuit diagram of a voltage detector provided with a switch circuit according to a first embodiment. The voltage detector provided with the switch circuit according to the present embodiment is constituted of a voltage detector circuit106, a positive power supply connection terminal112, a negative power supply connection terminal113, an output terminal114and a control circuit111of the voltage detector circuit106, a switch circuit comprised of N-ch transistors101,104and105, and a pull-up resistor312. Resistors107and108, a reference voltage generating circuit109, and a comparator110constitute the control circuit111.

A non-inverting input terminal of the comparator110is connected to a connection point of the resistors107and108, an inverting input terminal thereof is connected to the reference voltage generating circuit109, and an output terminal thereof is connected to the gate of the N-ch transistor101. The other terminal of the resistor108is connected to the negative power supply connection terminal113, while the other terminal of the resistor107is connected to the positive power supply connection terminal112. The other terminal of the reference voltage generating circuit109is connected to the negative power supply connection terminal113. One terminal of the pull-up resistor312is connected to the output terminal114, while the other terminal thereof is connected to the positive power supply connection terminal112. The N-ch transistor101of the switch circuit has a source thereof connected to the negative power supply connection terminal113and a drain thereof is connected to the positive power supply connection terminal112. The N-ch transistor104has a gate thereof connected to the source of the N-ch transistor101, a drain thereof connected to the drain of the N-ch transistor101, and a source and a back gate thereof connected to a back gate of the N-ch transistor101. The N-ch transistor105has a gate thereof connected to the drain of the N-ch transistor101, a drain thereof connected to the source of the N-ch transistor101, and a source and a back gate thereof connected to the back gate of the N-ch transistor101. A parasitic diode102exists between the back gate and the drain of the N-ch transistor101, and a cathode of the parasitic diode102is connected to the drain of the N-ch transistor101, while an anode thereof is connected to the back gate of the N-ch transistor101. A parasitic diode103exists between the back gate and the source of the N-ch transistor101, and a cathode of the parasitic diode103is connected to the source of the N-ch transistor101, while an anode thereof is connected to the back gate of the N-ch transistor101.

A description will now be given of the operation of the voltage detector provided with the switch circuit of the present embodiment.

If a voltage generator311is in normal connection with the positive power supply connection terminal112and the negative power supply connection terminal113, which function also to monitor voltages and if the voltage of the voltage generator311is not more than a predetermined voltage set by the voltage detector circuit106, then the N-ch transistor101is turned off by the control circuit111, and the voltage of the output terminal114is set to a positive voltage of the voltage generator311by the pull-up resistor312. If the voltage of the voltage generator311reaches a predetermined voltage, which is set by the voltage detector circuit106, or more, then the N-ch transistor101is turned on by the control circuit111and the voltage of the output terminal114becomes a negative voltage of the voltage generator311. Thus, the voltage detector operates to be capable of detecting that the voltage generator311has reached a predetermined voltage.

At this time, the direction of the parasitic diode103of the N-ch transistor101of the switch circuit of the voltage detector circuit106is the forward direction. However, the N-ch transistor105turns on, so that the parasitic diode103is short-circuited, thus preventing the passage of current that is unexpected in the circuit operation. If the positive power supply connection terminal112and the negative power supply connection terminal113of the voltage detector circuit106are inversely connected to the positive power supply terminal and the negative supply terminal of the voltage generator311, then the direction of the parasitic diode102of the N-ch transistor101of the switch circuit becomes the forward direction. However, the N-ch transistor104turns on, so that the parasitic diode102is short-circuited, thus preventing the passage of current that is unexpected in the circuit operation.

Therefore, the N-ch transistor104or105of the switch circuit turns on according to the connection state, causing the parasitic diode in the forward direction to be short-circuited. This makes it possible to prevent the passage of current that is unexpected in the circuit operation even in the case of the reverse connection.

In the present embodiment, the N-ch transistors101,104and105have been used in describing the switch circuit. Alternatively, however, P-ch transistors may be used, and the transistors used are not limited to those in the present embodiment. Further, the control circuit111is not limited to the one used in the present embodiment.

As described above, the voltage detector provided with the switch circuit according to the present embodiment is capable of preventing the passage of current that is unexpected in a circuit operation regardless whether a voltage generator, such as a secondary battery, is in normal connection or reverse connection, thus ensuring higher safety.

Second Embodiment

FIG. 2is a circuit diagram of a voltage controller provided with a switch circuit according to a second embodiment. A voltage detector circuit106becomes a voltage control circuit that monitors the voltage between the drain and the source of an N-ch transistor101to control the N-ch transistor101, when a positive power supply connection terminal112and an output terminal114are connected. Further, one terminal of a resistor313is connected to the positive power supply connection terminal112and the output terminal114, while the other terminal of the resistor313is connected to the positive power supply of a voltage generator311. The voltage controller operates with the configuration described above.

A description will now be given of the operation of the voltage controller provided with the switch circuit according to the present embodiment.

The supply voltage of the voltage control circuit115is controlled to a voltage obtained by dividing the voltage of the voltage generator311by the resistance value of the resistor312and the ON resistance value of the N-ch transistor101. If the supply voltage of the voltage control circuit115changes to a higher level, then the voltage divided by resistors107and108reaches the voltage of a reference voltage generating circuit109or higher, and the output of the control circuit111increases the gate voltage of the N-ch transistor101. As the gate voltage increases, the ON resistance value of the N-ch transistor101decreases, thus decreasing the voltage between the output terminal114and a negative power supply connection terminal113. The output terminal114is connected to the positive power supply connection terminal112, so that the supply voltage of the voltage control circuit115is also controlled to decrease. If the supply voltage of the voltage control circuit115changes to a lower level, then the ON resistance value of the N-ch transistor101increases and the supply voltage of the voltage control circuit115is controlled to increase, which is opposite from the foregoing case. Repeating the foregoing operation causes the voltage controller to operate to continue outputting a constant voltage. At this time, an N-ch transistor104is off, while an N-ch transistor105is on, and a parasitic diode103in the forward direction is short-circuited, thus preventing the passage of current that is unexpected in the circuit operation. Thus, the passage of current that is unexpected in the circuit operation is prevented by the N-ch transistor105of the switch circuit in the case of the normal connection.

If a voltage generator311is inversely connected between the positive power supply connection terminal112and the negative power supply connection terminal113, then the N-ch transistor104of the switch circuit turns on, while the N-ch transistor105thereof turns off. This causes a parasitic diode102in the forward direction to be short-circuited, thus making it possible to prevent the passage of current that is unexpected in the circuit operation.

Hence, the N-ch transistor104or105of the switch circuit turns on according to the connection state, causing the parasitic diode in the forward direction to be short-circuited. Thus, the passage of current that is unexpected in the circuit operation can be prevented even in the case of reverse connection.

In the present embodiment, the N-ch transistors101,104and105have been used in describing the switch circuit. Alternatively, however, P-ch transistors may be used, and the transistors used are not limited to those in the present embodiment. Further, the control circuit111is not limited to the one used in the present embodiment.

As described above, the voltage controller provided with the switch circuit according to the present embodiment is capable of preventing the passage of current that is unexpected in a circuit operation regardless of normal connection or reverse connection, thereby ensuring higher safety.

Third Embodiment

FIG. 3is a circuit diagram of a battery device provided with a switch circuit according to the present embodiment.

The battery device includes a plurality of secondary batteries204_1to204_n connected in series, a plurality of voltage control circuits115_1to115_n connected in series, a charge/discharge control circuit201, a discharge control N-ch FET transistor203, and a charge control N-ch FET transistor202. All the voltage control circuits115_1to115_n have the same configuration as that of the voltage control circuit115.

The secondary battery204_1and the voltage control circuit115_1are connected in parallel and connected to input terminals of the charge/discharge control circuit201. In the case of normal connection, a positive power supply terminal of the secondary battery204_1is connected to a positive power supply connection terminal112of the voltage control circuit115_1, while a negative power supply terminal thereof is connected to a negative power supply connection terminal113of the voltage control circuit115_1. The same connection applies to the secondary batteries204_2to204_n and the voltage control circuits115_2to115_n. The discharge control N-ch FET transistor203and the charge control N-ch FET transistor202are connected to the charge/discharge control circuit201so as to be controlled to turn on/off.

A description will now be given of the operation of the battery device provided with the switch circuit according to the present embodiment.

If the charge/discharge control circuit201detects that the voltages of the secondary batteries204_1to204_n charged by a voltage generator, such as a charger, connected between an external terminal205and an external terminal206reach a predetermined voltage, which is set by the charge/discharge control circuit201, or higher, then the charge/discharge control circuit201turns off the charge control N-ch FET transistor202to interrupt charging current. Further, if the charge/discharge control circuit201detects that the voltages of the secondary batteries204_1to204_n discharged by a load, such as a resistor, connected between the external terminal205and the external terminal206have reduced to another predetermined voltage, which is set by the charge/discharge control circuit201, or lower, then the charge/discharge control circuit201turns off the discharge control N-ch FET transistor203to interrupt discharging current.

If the voltage control circuits115_1to115_n detect that the voltages of the secondary batteries204_1to204_n charged by a voltage generator, such as a charger, connected between the external terminal205and the external terminal206reaches a predetermined voltage, which is set in the voltage control circuits115_1to115_n, or higher, then the voltage control circuits115_1to115_n turns on an N-ch transistor101so as to redirect the charging current, which is flowing to the secondary batteries204_1to204_n, to the voltage control circuits115_1to115_n. For example, if the voltage of the secondary battery204_1reaches the predetermined voltage, which is set in the voltage control circuit115_1, or higher, then the N-ch transistor101of the voltage control circuit115_1connected in parallel turns on. When the N-ch transistor101turns on, the charging current flows to the N-ch transistor101but does not flow to the secondary battery204_1. When the N-ch transistor101turns on and the ON resistance value of the N-ch transistor101decreases, the voltage of a positive power supply connection terminal112of the voltage control circuit115_1decreases and if the voltage of the positive power supply connection terminal112reduces to the predetermined voltage, which is set in the voltage control circuit115_1, or lower, then the N-ch transistor101turns off. When the N-ch transistor101turns off and the ON resistance value of the N-ch transistor101increases, the voltage of the positive power supply connection terminal112of the voltage control circuit115_1is increased and reaches the voltage, which is set in the voltage control circuit115_1, or higher. This causes the N-ch transistor101to turn on, redirecting the charging current again. The series of the foregoing operations is repeated to maintain the voltage of the secondary battery204_1to the predetermined voltage set in the voltage control circuit115_1. If the secondary batteries204_2to204_n are charged while the secondary battery204_1is holding the constant voltage, then the same operations as those of the secondary battery204_1will be performed. Setting the predetermined voltages to be set in the voltage control circuits115_1to115_n to the same voltage holds all the voltages of the plurality of the secondary batteries204_2to204_n connected in series at a constant voltage, thus maintaining a voltage balance. This arrangement enables the voltage control circuits115_1to115_n to operate in the battery device as the voltage control circuits for accomplishing the voltage balance in the plurality of the secondary batteries connected in series.

When the secondary batteries204_1to204_n are in normal connection, the direction of a parasitic diode103of the N-ch transistor101of the switch circuit becomes the forward direction. However, an N-ch transistor105turns on, enabling the voltage control circuits115_1to115_n to prevent the passage of current that is unexpected in the circuit operation. If the voltage control circuits115_1to115_n are inversely connected to a positive power supply terminal and a negative power supply terminal of the secondary batteries204_1to204_n, then the direction of a parasitic diode102of the N-ch transistor101of the switch circuit becomes the forward direction. However, an N-ch transistor104turns on, thus making it possible to prevent the passage of current that is unexpected in the circuit operation. Hence, the N-ch transistor104or105of the switch circuit turns on according to the connection state, causing the parasitic diode in the forward direction to be short-circuited. This makes it possible to prevent the passage of current that is unexpected in the circuit operation even in the case of reverse connection.

The N-ch transistors101,104and105have been used in describing the switch circuit. Alternatively, however, P-ch transistors may be used, and the transistors used are not limited to those in the present embodiment.

As described above, the battery device provided with the switch circuit according to the present embodiment is capable of controlling the voltage balance among a plurality of secondary batteries connected in series. Further, it is possible to provide a battery device capable of preventing the passage of current that is unexpected in a circuit operation regardless of whether secondary batteries are in normal connection or reverse connection, thus ensuring higher safety.