Voltage detection circuit, charging / discharge control device, and battery device

Abstract

To provide a voltage detection circuit that does not degrade detection accuracy even when a cell imbalance occurs.SOLUTION: A voltage detection circuit 101 that detects each battery voltage to collectively control battery cells 21 to 24 has: a bleeder resistor circuit BR that divides the battery voltage into voltage divider voltages VD1, VD2; a comparator C2 to which the voltage divider voltage VD1 and a reference voltage VREF are input and that outputs a signal indicating normal or overcharge; and a switch M3 that is connected in parallel with a resistor part R2 and is turned on in overcharge; a comparator C1 to which the voltage divider voltage VD2 and the reference voltage VREF are input and that outputs a signal indicating normal or low voltage; a switch M1 that is connected between a high voltage side of the resistor R3 and an input part of the comparator C1, and is turned on at low voltage; a switch M2 that is connected between a low voltage side of the resistor R3 and the input part of the comparator C1, and is turned off at low voltage. The voltage detection circuit 101 is connected in parallel to each of the battery cells 21 to 24, and the respective bleeder resistor circuits BR are connected in series.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a voltage detection circuit, a charge / discharge control device, and a battery device.

Background Art

[0002] Secondary batteries such as lithium-ion batteries are used in a wide range of fields as power sources for, for example, portable devices, power tools, and transportation devices. For devices that require a high voltage, a battery pack in which a plurality of battery cells are connected in series is used. In order to use this battery pack safely, a protection circuit that monitors overcharging and overdischarging (low voltage) of each battery cell is connected to the battery pack to prevent the balance from being lost due to individual differences in the battery cells.

[0003] In many cases, this protection circuit has a comparator that compares the divided battery voltage with a reference voltage in order to detect overcharging and the like. In such a case, "hysteresis" in which the detection voltage is different from the release voltage is provided so that the operation does not become unstable when the battery voltage fluctuates. For example, an overcharge and overdischarge prevention circuit has been proposed that has hysteresis for overcharge detection and overdischarge detection and can balance individual battery cells during overcharge and overdischarge (see Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] One aspect of the present invention is to provide a voltage detection circuit that can be miniaturized, has hysteresis for overcharge detection and overdischarge detection, and does not reduce overcharge detection accuracy even if cell balance is lost.

Means for Solving the Problem

[0006] A voltage detection circuit according to an embodiment of the present invention is a voltage detection circuit that detects each battery voltage in order to collectively charge and discharge a plurality of battery cells, a bleeder resistor circuit in which a plurality of resistor parts are connected in series and divide the battery voltage into a first divided voltage and a second divided voltage; an overcharge detection comparator to which the first divided voltage and a reference voltage of the bleeder resistor circuit are input and which outputs an output signal indicating a normal state or an overcharged state; an overcharge switch connected in parallel with a first resistor part in the bleeder resistor circuit, which is turned off in the normal state and turned on in the overcharged state; a low voltage detection comparator to which the second divided voltage and the reference voltage of the bleeder resistor circuit are input and which outputs an output signal indicating the normal state or a low voltage state; a first low voltage switch connected between the high voltage side of a second resistor part in the bleeder resistor circuit and an input part to which the first divided voltage of the low voltage detection comparator is input, which is turned off in the normal state and turned on in the low voltage state; a second low voltage switch connected between the low voltage side of the second resistor part in the bleeder resistor circuit and the input part of the low voltage detection comparator, which is turned on in the normal state and turned off in the low voltage state; and each is connected in parallel to each battery cell, and a plurality of the bleeder resistor circuits are connected in series.

Advantages of the Invention

[0007] According to one aspect of the present invention, it is possible to provide a voltage detection circuit that can be miniaturized, has hysteresis in overcharge detection and overdischarge detection, and whose overcharge detection accuracy does not decrease even if cell balance is lost.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Embodiment for Carrying Out the Invention

[0009] In the circuit configuration described in Patent Document 1, when trying to give hysteresis to the detection of overcharge and overdischarge, two switches are required for each, so the number of transistors as those switches increases, the wiring becomes complicated, and miniaturization is difficult. This invention is based on such a finding. For miniaturization, if a simple circuit configuration with one switch each for the detection of overcharge and overdischarge as shown in FIG. 3 is adopted, although details will be described later, the overcharge detection voltage may change due to cell balance breakdown, resulting in a decrease in the detection accuracy of overcharge. Particularly in a protection circuit called "second protect", it is important to cut off the circuit in case of overcharge. Even if the detection accuracy is improved in other parts, it is not perfect that the overcharge detection accuracy decreases due to cell balance breakdown. Therefore, in one embodiment of the present invention, a circuit configuration is adopted that can achieve miniaturization, can have hysteresis in overcharge detection and overdischarge detection, and the overcharge detection accuracy does not decrease even if cell balance breakdown occurs.

[0010] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the drawings, the same reference numerals are given to the same components, and redundant explanations may be omitted.

[0011] Figure 1 is a circuit diagram showing a battery device and a charge / discharge control circuit according to an embodiment of the present invention. The battery device 1 is a power supply device that can safely use battery cells of lithium-ion batteries connected in series, and has a protection IC (Integrated Circuit) called "Second Protect" that can stop charging. This battery device 1 includes a charge and discharge control device 10 as its protection IC, a battery pack 20 in which four battery cells 21 to 24 are connected in series, an SCP (Self Control Protecter) 30, a charge control FET (Field Effect Transistor) 40, an external terminal EB+, and an external terminal EB-. When the battery device 1 is being charged, a charging device is connected between the external terminal EB+ and the external terminal EB-. Also, when the battery device 1 is in use, a load device is connected between the external terminal EB+ and the external terminal EB-.

[0012] The charge and discharge control device 10 detects the battery voltages of the battery cells 21 to 24 of the battery pack 20 respectively, and controls charging and discharging according to the detected battery voltages. When any one of the battery cells 21 to 24 is in an overcharged state, the charge and discharge control device 10 performs control to stop charging the battery pack 20. Also, when any one of the battery cells 21 to 24 is in a low voltage state (over-discharged state), the charge and discharge control device 10 stops other functions and performs control to stop discharging from the battery pack 20.

[0013] Here, the "overcharged state" means a state in which the battery voltage of any one of the battery cells 21 to 24 exceeds a predetermined overcharge detection voltage, and the time during which the battery voltage exceeds the overcharge detection voltage exceeds a predetermined time. Also, when the battery voltage drops below the overcharge release voltage within a predetermined time, it returns to the "normal state". Furthermore, in order not to make the operation unstable when the battery voltage fluctuates near the overcharge detection voltage, an "overcharge hysteresis voltage" (= overcharge detection voltage - overcharge release voltage) is provided by setting the overcharge release voltage lower than the overcharge detection voltage to stabilize the operation. Specifically, the overcharge detection voltage is set to 4.6V and the overcharge release voltage is set to 4.3V, etc.

[0014] The "low voltage state" refers to a state in which the battery voltage of any one of the battery cells 21 to 24 drops below a predetermined low voltage detection voltage, and the time during which the battery voltage is lower than the low voltage detection voltage continues for a predetermined time or longer. Also, when the battery voltage rises above the return voltage within a predetermined time, it returns to the "normal state". Furthermore, in order to prevent the operation from becoming unstable when the battery voltage fluctuates near the low voltage detection voltage, the return voltage is set higher than the low voltage detection voltage to provide a "low voltage hysteresis voltage" (= return voltage - low voltage detection voltage) to stabilize the operation. Specifically, the low voltage detection voltage is set to 2.5V and the return voltage is set to 2.7V, etc.

[0015] The "normal state" refers to a state in which the battery voltages of all of the battery cells 21 to 24 are less than or equal to the overcharge detection voltage and greater than or equal to the low voltage detection voltage.

[0016] Also, as other functions, without particular limitation, they can be appropriately selected according to the purpose. For example, when the load device is a notebook computer and the charge and discharge control device 10 includes a constant voltage circuit that supplies a constant voltage to the real-time clock circuit of its external circuit, the constant voltage circuit corresponds to other functions.

[0017] The charge and discharge control device 10 includes a voltage detection unit 100, a control unit 110, a power supply terminal VDD, a ground terminal VSS, input units VC1 to VC4, and an output terminal CO. The battery cells 21 to 24 of the battery pack 20 are connected to the power supply terminal VDD, the input units VC1 to VC4, and the ground terminal VSS of the charge and discharge control device 10 so that the battery voltages of the battery cells 21 to 24 can be detected respectively.

[0018] The battery pack 20 has its positive side connected to the external terminal EB+ and its negative side connected to the external terminal EB-. A fuse circuit SCP30 for blocking the charging path during charging is connected between the external terminal EB+ and the positive side of the battery pack 20. The terminal T1 of SCP30 is connected to the battery pack 20, and the terminal T2 of SCP30 is connected to the external terminal EB+.

[0019] In SCP30, fuses 31 and 32 are connected in series between terminal T1 and terminal T2, and a resistance element 33 is connected between the connection part of fuses 31 and 32 and terminal T3. When in an overcurrent state, fuses 31 and 32 blow to cut off the circuit. Also, when in an overcharge state, charge control FET 40 turns on, the resistance element 33 that serves as a heater is energized, and fuses 31 and 32 are blown by the heat generated from the resistance element 33 to cut off the circuit. Note that the resistance element 33 is single in this embodiment, but it may be plural.

[0020] The gate terminal of charge control FET 40 is connected to the output terminal CO of charge and discharge control device 10. Charge control FET 40 is turned on and off based on a control signal from charge and discharge control device 10, is off in the normal state, and is on in the overcharge state.

[0021] Therefore, as the operation of charge and discharge control device 10, charge control FET 40 is kept off in the normal state, when it becomes in an overcharge state during charging, charge control FET 40 is turned on to energize resistance element 33, and fuses 31 and 32 are blown by the heat to stop charging. Also, when charge and discharge control device 10 is in a low voltage state, it stops other functions and stops discharging from battery pack 20.

[0022] Next, the voltage detection unit 100 and control unit 110 of this charge and discharge control device 10 will be described in detail.

[0023] Voltage detection unit 100 includes voltage detection circuits 101 to 104 connected to the positive and negative sides of each battery cell to detect the battery voltages of battery cells 21 to 24 respectively. Since voltage detection circuits 101 to 104 are all formed in the same way, hereinafter, voltage detection circuit 101 will be described, and the description of voltage detection circuits 102 to 104 will be omitted.

[0024] Figure 2 is a circuit diagram showing a voltage detection circuit in an embodiment of the present invention. As shown in FIG. 2, the voltage detection circuit 101 includes a bleeder resistor circuit BR, a reference voltage source VR, a low voltage detection comparator C1, low voltage switches M1 and M2, an overcharge detection comparator C2, and an overcharge switch M3.

[0025] The bleeder resistor circuit BR is a voltage dividing circuit, and is connected in series from the positive electrode to the negative electrode of the battery cell 21 in the order of a plurality of resistor portions R1, R2, R3, R4, and R5. Each resistor portion may be formed of a single resistor element or a plurality of resistor elements. Further, each resistor portion may include a fuse element so that the resistance value can be adjusted, and in many cases, the detection accuracy is improved by trimming with this fuse element.

[0026] This bleeder resistor circuit BR outputs a divided voltage VD1 (first divided voltage), a divided voltage VD2 (second divided voltage), and a divided voltage VD3 of the battery voltage of the battery cell 21. The divided voltage VD1 is output from the connection portion of the resistor portions R2 and R3, and is input to the first input portion of the low voltage detection comparator C1 via the low voltage switch M1. The divided voltage VD2 is output from the connection portion of the resistor portions R3 and R4, and is input to the first input portion of the low voltage detection comparator C1 via the low voltage switch M2. The divided voltage VD3 is output from the connection portion of the resistor portions R4 and R5, and is input to the first input portion of the overcharge detection comparator C2.

[0027] The reference voltage source VR outputs the generated reference voltage VREF to the second input portion of the low voltage detection comparator C1 and the second input portion of the overcharge detection comparator C2, respectively.

[0028] The comparator C1 for low voltage detection has the divided voltage VD1 or the divided voltage VD2 input to the first input part, and the reference voltage VREF input to the second input part. Then, the comparator C1 for low voltage detection compares either of the divided voltages with the reference voltage VREF, and outputs an output signal of H level or L level according to the result to the control unit 110, thereby detecting the low voltage of the battery cell 21.

[0029] The low voltage switches M1 and M2 can generate a "low voltage hysteresis voltage" and prevent the overcharge detection accuracy from decreasing even if any of the battery cells is in a low voltage state. First, an explanation will be given about the low voltage switches M1 and M2 generating a "low voltage hysteresis voltage".

[0030] The low voltage switch M1 is a transistor and is connected between the high voltage side of the resistor part R3, which is the second resistor part in the bleeder resistor circuit BR, and the first input part of the comparator C1 for low voltage detection. The gate terminal of the low voltage switch M1 is connected to the control unit 110, and according to the control signal from the control unit 110, it turns off in the normal state and turns on in the low voltage state.

[0031] The low voltage switch M2 is a transistor and is connected between the low voltage side of the resistor part R3 in the bleeder resistor circuit BR and the first input part of the comparator C1 for low voltage detection. The gate terminal of the low voltage switch M2 is connected to the control unit 110, and according to the control signal from the control unit 110, it turns on in the normal state and turns off in the low voltage state.

[0032] Therefore, due to the switching of the low voltage switches M1 and M2, the divided voltage VD1 is input to the first input part of the comparator C1 for low voltage detection in the low voltage state, and the divided voltage VD2 is input in the normal state, thereby generating a "low voltage hysteresis voltage".

[0033] The comparator C2 for overcharge detection compares the divided voltage VD3 input to the first input section with the reference voltage VREF input to the second input section, and outputs an H-level or L-level output signal according to the result to the control unit 110, thereby detecting overcharge of the battery cell 21.

[0034] The overcharge switch M3 is a transistor and is connected in parallel with the resistor section R2 as the first resistor section. The overcharge switch M3 has its gate terminal connected to the control unit 110, and turns off in the normal state and turns on in the overcharge state according to the control signal from the control unit 110. Therefore, by the switching of the overcharge switch M3, a voltage drop occurs across the resistor section R2 in the normal state, and no voltage drop occurs in the overcharge state, thereby generating an "overcharge hysteresis voltage". As a result, an "overcharge hysteresis voltage" can be generated by one switch, and miniaturization can be achieved without routing complex wiring.

[0035] Also, in the entire voltage detection unit 100, as shown in FIG. 1, the bleeder resistor circuits BR are respectively provided for the battery cells 21 to 24, and the four bleeder resistor circuits are connected in series.

[0036] Based on the output signal of any one of the voltage detection circuits 101 to 104, the control unit 110 outputs a control signal for collectively turning on and off the charge control FETs 40, the low-voltage switches M1 and M2, and the overcharge switch M3 in all of the voltage detection circuits 101 to 104.

[0037] Specifically, if all of the battery cells 21 to 24 are in the normal state, the control unit 110 turns off the low-voltage switch M1, turns on the low-voltage switch M2, and turns off the overcharge switch M3 for all of the voltage detection circuits 101 to 104. Also, if any of the battery cells 21 to 24 is in the low-voltage state, the control unit 110 turns on the low-voltage switch M1, turns off the low-voltage switch M2, and turns off the overcharge switch M3 for all of the voltage detection circuits 101 to 104. Furthermore, if any of the battery cells 21 to 24 is in an overcharged state, the control unit 110 turns on the low-voltage switch M1, turns off the low-voltage switch M2, and turns on the overcharge switch M3 for all of the voltage detection circuits 101 to 104.

[0038] Then, for example, consider a case where battery cells 21 to 23 are in a normal state and only battery cell 24 changes from the normal state to a low-voltage state, resulting in "cell balance breakdown". In this case, the control unit 110 turns on the low-voltage switch M1, turns off the low-voltage switch M2, and keeps the overcharge switch M3 off for all of the voltage detection circuits 101 to 104 in order to control the low-voltage state. Even when such cell balance breakdown occurs, the voltage detection circuits 101 to 104 have the overcharge detection voltage VCU at the same value in the normal state and the low-voltage state. Specifically, assuming that the resistance values of the resistance parts R1, R2, R3, R4, and R5 are r1, r2, r3, r4, and r5 in order, it becomes as follows.

[0039] In the normal state, since the low-voltage switch M1 is off, the low-voltage switch M2 is on, and the overcharge switch M3 is off, the overcharge detection voltage VCU is given by the following formula (1). VCU = ((r1 + r2 + r3 + r4 + r5) / r5) × VREF ···(1)

[0040] In the low-voltage state, since the low-voltage switch M1 is on, the low-voltage switch M2 is off, and the overcharge switch M3 is off, the overcharge detection voltage VCU is given by the following formula (2). VCU = ((r1 + r2 + r3 + r4 + r5) / r5) × VREF ···(2)

[0041] Therefore, as shown in the above formulas (1) and (2), even when cell balance collapse occurs, the overcharge detection voltage VCU in one embodiment of the present invention does not change between the normal state and the low voltage state, so the overcharge detection accuracy does not decrease. That is, although it is rare for one battery cell to be in a low voltage state and another battery cell to be in an overcharged state, the battery device 1 can accurately cut off the circuit in the overcharged state by the charge and discharge control device 10 equipped with the voltage detection circuits 101 to 104.

[0042] Hereinafter, for comparison with one embodiment of the present invention, a conventional voltage detection circuit will be described.

[0043] FIG. 3 is a circuit diagram showing a conventional voltage detection circuit. As shown in FIG. 3, the conventional voltage detection circuit is the same as the voltage detection circuit 101 except that a low voltage switch M4 is connected in parallel with the resistance section R3 instead of the low voltage switches M1 and M2 in the voltage detection circuit 101 which is one embodiment of the present invention. This low voltage switch M4 is a transistor, which is off in the normal state and on in the low voltage state, thereby generating a "low voltage hysteresis voltage".

[0044] Such a conventional voltage detection circuit causes the overcharge detection voltage VCU to change between the normal state and the low voltage state due to cell balance collapse. Specifically, it is as follows.

[0045] In the normal state, since the overcharge switch M3 is off and the low voltage switch M4 is on, the overcharge detection voltage VCU is given by the following formula (3). VCU = ((r1 + r2 + r4 + r5) / r5) × VREF ···(3)

[0046] In the low voltage state, since the overcharge switch M3 is off and the low voltage switch M4 is off, the overcharge detection voltage VCU is given by the following formula (4). VCU = ((r1 + r2 + r3 + r4 + r5) / r5) × VREF ···(4)

[0047] Therefore, as shown in the above formulas (3) and (4), when cell balance collapse occurs, the overcharge detection voltage VCU in the conventional voltage detection circuit changes between the normal state and the low voltage state, and the overcharge detection accuracy decreases. In particular, in a charge and discharge control device called "second protect", it is important to cut off the circuit in the overcharged state. Even if the detection accuracy is improved by trimming with a fuse element as described above, it is not perfect that the overcharge detection accuracy decreases due to cell balance collapse.

[0048] Therefore, in one embodiment of the present invention, as shown in FIG. 2, by arranging low voltage switches M1 and M2 instead of the low voltage switch M4, the overcharge detection voltage is prevented from changing between the normal state and the low voltage state, and the overcharge detection accuracy does not decrease even when cell balance collapse occurs.

[0049] As described above, the voltage detection circuit in one embodiment of the present invention is a circuit that detects each battery voltage in order to collectively charge and discharge a plurality of battery cells. This voltage detection circuit has a bleeder resistance circuit in which a plurality of resistance parts are connected in series and divide the battery voltage into a first divided voltage and a second divided voltage. Further, this voltage detection circuit includes an overcharge detection comparator to which the first divided voltage and a reference voltage are input and which outputs a signal indicating a normal state or an overcharged state, and an overcharge switch that is connected in parallel with the first resistance part, is turned off in the normal state, and is turned on in the overcharged state. Furthermore, this voltage detection circuit further includes a low voltage detection comparator to which the second divided voltage and a reference voltage are input and which outputs a signal indicating a normal state or a low voltage state. Still further, this voltage detection circuit includes a first low voltage switch that is connected between the high voltage side of the second resistance part and the input part of the low voltage detection comparator, is turned off in the normal state, and is turned on in the low voltage state, and a second low voltage switch that is connected between the low voltage side of the second resistance part and the input part of the low voltage detection comparator, is turned on in the normal state, and is turned off in the low voltage state. And this voltage detection circuit is connected in parallel to each of a plurality of battery cells, and a plurality of bleeder resistance circuits are connected in series. As a result, this voltage detection circuit can be miniaturized, and can have hysteresis in overcharge detection and overdischarge detection, and the overcharge detection accuracy does not decrease even if cell balance is lost.

[0050] In this embodiment, the second protection of the lithium-ion battery has been described as an example, but the present invention is not limited to this, and it can be applied to any device using a circuit for detecting voltage. Note that the control unit may include various circuits such as a delay circuit and an oscillation circuit for controlling the charge and discharge of the battery pack based on the output signals from the comparators. Also, in the above embodiment, the number of resistance parts of the bleeder resistance circuit is five, but the present invention is not limited to this, and at least the first resistance part and the second resistance part are sufficient. Furthermore, in the above embodiment, the number of the plurality of battery cells is four, but the present invention is not limited to this, and any plurality is sufficient. Moreover, although the various switches are described in the drawings as NMOS (N type Metal Oxide Semiconductor) transistors, the present invention is not limited to this, and any element having a switching function may be used. For example, the various switches may be PMOS (P type Metal Oxide Semiconductor) transistors or junction type FETs in consideration of the conductivity type of the semiconductor substrate, the substrate bias effect, etc.

Explanation of Reference Numerals

[0051] 1 Battery device 10 Charge and discharge control device 20 Battery pack 30 SCP 40 Charge control FET 100 Voltage detection unit 101~104 Voltage detection circuit 110 Control unit BR Bleeder resistance circuit C1 Comparator for low voltage detection C2 Comparator for overcharge detection EB+ External terminal EB - External Terminal R2 Resistance Section (First Resistance Section) R3 Resistance Section (Second Resistance Section) M1 Low - Voltage Switch (First Low - Voltage Switch) M2 Low - Voltage Switch (Second Low - Voltage Switch) M3 Overcharge Switch

Claims

1. A voltage detection circuit that detects each battery voltage in order to collectively control charging and discharging of a plurality of battery cells, a bleeder resistor circuit in which a plurality of resistor portions are connected in series and divide the battery voltage into a first divided voltage and a second divided voltage, an overcharge detection comparator to which the first divided voltage and a reference voltage of the bleeder resistor circuit are input and which outputs an output signal indicating a normal state or an overcharged state, an overcharge switch connected in parallel with a first resistor portion in the bleeder resistor circuit, which is turned off in the normal state and turned on in the overcharged state, a low voltage detection comparator to which the second divided voltage and the reference voltage of the bleeder resistor circuit are input and which outputs an output signal indicating the normal state or a low voltage state, a first low voltage switch connected between a high voltage side of a second resistor portion in the bleeder resistor circuit and an input portion to which the first divided voltage of the low voltage detection comparator is input, which is turned off in the normal state and turned on in the low voltage state, a second low voltage switch connected between a low voltage side of the second resistor portion in the bleeder resistor circuit and the input portion of the low voltage detection comparator, which is turned on in the normal state and turned off in the low voltage state, and having a voltage detection circuit characterized in that a plurality of the bleeder resistor circuits are connected in series, each connected in parallel to each of the battery cells.

2. The voltage detection circuit according to claim 1, each connected in parallel to the plurality of battery cells, and a control unit that collectively controls the plurality of voltage detection circuits, and having a charge and discharge control circuit, wherein the control unit controls the overcharge switch, the first low voltage switch, and the second low voltage switch respectively based on the output signal of the overcharge detection comparator and the output signal of the low voltage detection comparator.

3. The charge and discharge control circuit according to claim 2, a battery pack equipped with the plurality of battery cells connected in series, and a charge control FET connected in parallel with the battery pack between external terminal pairs, a battery device characterized by having.

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