JPH11127543A - Protective circuit device for secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a protective circuit device which protects a secondary battery from overcorrect, when the battery discharges by detecting the overcorrect without causing the overheating of an FET for controlling the discharge or changing a reference voltage for detecting the overcurrent. SOLUTION: A protective circuit device is provided with a voltage-adding circuit 20 which outputs a discharge current detecting voltage by adding an offset voltage to the voltage between the drain and source of an FET 12, which relies upon the discharge current of a secondary battery 4 when the battery 4 discharges to a load and protects the battery 4 from an overcurrent by comparing the discharge current detecting voltage with a reference voltage Vref 3 for detecting an overcurrent by means of a voltage comparator circuit 15 for detecting overcurrent, and when the discharge current detecting voltage becomes higher than the reference voltage Vref 3, turning off the FET 12 by means of an FET drive circuit 17.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a protection circuit device for a secondary battery, and more particularly to a protection circuit device having an overcurrent protection function.

[0002]

2. Description of the Related Art Generally, a battery pack using a secondary battery has a built-in protection circuit device for protecting the secondary battery from overcharge, overdischarge and overcurrent. Specifically, this protection circuit device includes a charge control FET and a discharge control FET connected in series between a charge / discharge path of a secondary battery, that is, an external connection terminal to which a charger and a load are connected. When the battery voltage reaches the charging prohibition voltage during charging, the charge control FET is turned off to stop charging, and when discharging, the battery voltage reaches the discharging prohibition voltage or when an overcurrent flows. The discharge control FET is turned off to stop the discharge.

In such a secondary battery protection circuit device, as a method of detecting an overcurrent at the time of discharging, a method of inserting a discharging current detecting resistor in a charging / discharging path can be considered first. However, in this method, the voltage that can be supplied from the secondary battery to the load decreases due to the voltage drop in the discharge current detection resistor, and the energy stored in the battery cannot be sufficiently extracted.

Therefore, by utilizing the voltage drop due to the on-resistance of the discharge control FET at the time of discharge, that is, the voltage drop between the drain and source changes depending on the discharge current, this voltage drop is used for detecting the overcurrent. For this reason, there is known a protection circuit device in which when a predetermined reference voltage set for this purpose is reached, it is determined that an overcurrent has flown, and the discharge control FET is turned off to stop discharging.

The reference voltage for detecting an overcurrent is generally set in a control IC for controlling a charge control FET and a discharge control FET in many cases.
When the reference voltage for overcurrent detection is a fixed value set in advance as described above, when a low on-resistance FET is used as a discharge control FET in order to more efficiently extract the discharge capacity of the secondary battery, If a large current does not flow through the control FET, the drain-source voltage does not reach the reference voltage and cannot be detected as an overcurrent. As a result, when an overcurrent flows, the overcurrent cannot be detected because the drain-source voltage drop of the discharge control FET does not reach the reference voltage for overcurrent detection, and the charge control FET and the discharge control F
The ET may overheat, possibly exceeding the guaranteed operating temperature of the FET, and eventually may cause the FET to be destroyed.

This problem is the same even when the overcurrent detection is performed by using the discharge current detection resistor or by using both the voltage drop of the discharge control FET and the voltage drop of the discharge current detection resistor. If a discharge current detecting resistor having a low resistance value is used in order to more efficiently extract the discharge capability of the device, an overcurrent cannot be detected unless a larger current flows. Accordingly, since the overcurrent cannot be detected with high sensitivity, the charge control FET and the discharge control FET may overheat, and in the worst case, the FET may be destroyed.

On the other hand, changing the reference voltage for overcurrent detection set in the control IC in accordance with the on-resistance of the discharge control FET requires a great deal of cost and development time due to the design change of the IC. Necessary and not preferred. Furthermore, adopting a method of changing the reference voltage for overcurrent detection by an external resistor also requires the development of a special control IC having a terminal for adjusting the reference voltage, and similarly a large amount of design change cost. There is a problem that requires a development period.

[0008]

As described above, the overcurrent is detected by utilizing the voltage drop due to the on-resistance of the discharge control FET and / or the voltage drop of the discharge current detection resistor. When the voltage reaches the reference voltage for overcurrent detection, the discharge control FET is turned off to stop discharging and perform overcurrent protection. Since it is set inside the control IC, if a discharge control FET with a low on-resistance or a discharge current detection resistor with a low resistance value is used to sufficiently draw out the discharge capability of the secondary battery, a larger current will flow. If it does not flow, it cannot be detected as overcurrent, and when overcurrent occurs, the charge control FET and discharge control FET
May overheat and exceed the operation guarantee temperature, or may destroy the FET.

In order to address this problem, a control IC
Changing the reference voltage set internally or changing the reference voltage with an external resistor
There is a problem that it is necessary to change the design of the IC and to develop a special control IC having a terminal for adjusting the reference voltage.

SUMMARY OF THE INVENTION The present invention has been made to solve such a conventional problem, and does not allow an excessive current to flow through a discharge control switch element, and detects an overcurrent for an overcurrent set in an IC. An object of the present invention is to provide a protection circuit device for a secondary battery that can reliably detect an overcurrent without changing a reference voltage and perform overcurrent protection.

[0011]

In order to solve the above-mentioned problems, a protection circuit device for a secondary battery according to the present invention comprises a switch element inserted between a secondary battery and a load; A voltage adding circuit for adding a predetermined offset voltage to a voltage depending on a discharge current to a load, a voltage comparing circuit for detecting an overcurrent for comparing an output voltage of the voltage adding circuit with a predetermined reference voltage, A switch driving circuit for turning off the switching element when the output voltage of the voltage adding circuit becomes equal to or higher than the reference voltage in accordance with the output of the use voltage comparison circuit.

According to a more preferred aspect, the protection circuit device for a secondary battery according to the present invention includes a switch element inserted between the secondary battery and the load, and a voltage at the time of discharging the secondary battery to the load. Over-discharge detection voltage comparison circuit that compares the voltage with the discharge prohibition voltage, a voltage addition circuit that adds a predetermined offset voltage to a voltage that depends on the discharge current to the load of the secondary battery, and an output voltage of the voltage addition circuit. According to the outputs of the overcurrent detection voltage comparison circuit for comparing with a predetermined reference voltage, the overdischarge detection voltage comparison circuit and the overcurrent detection voltage comparison circuit, the voltage at the time of discharging to the load of the secondary battery is a discharge inhibition voltage. A switch drive circuit that turns off the switch element when the output voltage of the voltage addition circuit becomes equal to or less than the reference voltage, and a voltage at the time of discharging the load of the secondary battery to the discharge inhibition voltage or less. Become A power save control circuit for switching the power supply voltage of the switch drive circuit from the voltage of the secondary battery to the ground potential after the switch drive circuit is turned off, and the voltage adding circuit divides the output voltage of the switch drive circuit. Pressure to generate an offset voltage.

Here, the voltage that depends on the discharge current to the load of the secondary battery is, for example, a voltage that depends on the voltage drop of the switch element due to the discharge current. More specifically, the switch element is an FET (field effect transistor). ), FE
The drain voltage depending on the voltage drop due to the ON resistance of T corresponds to this.

As described above, in the protection circuit device of the present invention, the offset voltage is added to the voltage drop of the discharge control switch element, the voltage drop of the discharge current detection resistor, or a voltage dependent on both, and the offset voltage is added. The voltage after the addition is used as a discharge current detection voltage, and the overcurrent is detected by comparing the voltage with a reference voltage for overcurrent detection by a voltage comparison circuit for overcurrent detection.

Therefore, in order to sufficiently draw out the discharging ability of the secondary battery to a load, when a low resistance element is used for the discharge control switch element or a low resistance is used for the discharge current detection resistance. In addition, the discharge current detection voltage can be set to an appropriate value even when an unnecessarily large current flows through the discharge control switch element at the time of discharge, and the reference voltage for overcurrent detection is preset in the control IC. Even if the change cannot be made, the overcurrent can be reliably detected and the overcurrent protection function can be realized.

When the secondary battery is over-discharged, the output of the over-discharge detection voltage comparison circuit operates the power save control circuit, and the power supply voltage of the switch drive circuit drops to the ground potential. By reducing the output voltage to the ground potential, power consumption is reduced not only in the switch driving circuit but also in a voltage adding circuit that divides the output voltage of the switch driving circuit to generate an offset voltage.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a protection circuit device for a secondary battery according to one embodiment of the present invention. FIG.
, The protection circuit device 1 has battery connection terminals 2a, 2b and external connection terminals 3a, 3b, and the battery connection terminals 2a, 2b.
b, a secondary battery 4 is connected between the external connection terminals 3a and 3b.
The external device 5 is connected therebetween. As the external device 5, a charger is connected when the secondary battery 4 is charged, and a load, that is, various electronic devices that use the secondary battery 4 as a power source, is connected when discharging.

The protection circuit device 1 is configured as follows. First, a charge control F is provided between the external connection terminal 3b, which is a part of the charge / discharge path of the secondary battery 4, and the battery connection terminal 2b.
The ET 11 and the discharge control FET 12 are connected in series. In this example, the charge control FET 11 and the discharge control F
An N-channel FET is used for ET12. D11, D
Reference numeral 12 denotes a charge control FET 11 and a discharge control FE, respectively.
This is a parasitic diode between the drain and source of T12.
Here, the charge control FET 11 and the discharge control FET
The parasitic diode D11 is connected such that the direction of the discharge current of the secondary battery 4 is forward, and the parasitic diode D12 is connected so that the direction of the charge current of the secondary battery 4 is forward.

The charge control FET 11 is driven by an FET drive circuit 16 according to the output of the overcharge detection voltage comparison circuit 13. The discharge control FET 12 is driven by the FET drive circuit 17 according to the output of the over-discharge detection voltage comparison circuit 14 and the output of the over-current detection voltage comparison circuit 15.

The overcharge detection voltage comparison circuit 13 compares a terminal voltage (hereinafter referred to as a battery voltage) VB of the secondary battery 4 with a first reference voltage Vref1 corresponding to a charging prohibition voltage, and the battery voltage VB is used as a reference. When the voltage becomes equal to or higher than the voltage Vref1, the output is inverted from a low level to a high level. When the output of the overcharge detection voltage comparison circuit 13 becomes high, the FET drive circuit 16 turns off the charge control FET 11.

On the other hand, the over-discharge detection voltage comparison circuit 14
The battery voltage VB is compared with the second reference voltage Vref2 corresponding to the discharge prohibition voltage, and when the battery voltage VB falls below the reference voltage Vref2, the output is inverted from a low level to a high level. The overcurrent detection voltage comparison circuit 15 includes a voltage addition circuit 20 described later.
The discharge current detection voltage which is the output voltage of the third reference voltage V
When ref or more, the output is inverted from low level to high level. Then, when the output of the over-discharge detection voltage comparison circuit 14 or the over-current detection voltage comparison circuit 15 becomes high level, the FET drive circuit 17 turns off the discharge control FET 12.

The detection of the overcurrent at the time of discharging the secondary battery 4 is performed by utilizing the voltage drop between the drain and the source due to the ON resistance of the discharge control FET 12. That is, a drain voltage that changes depending on the voltage drop between the drain and source of the discharge control FET 12 (in this case, it is equal to the voltage between the drain and source) is added to the offset voltage by the voltage adding circuit 20 to perform voltage conversion. The output voltage of the voltage addition circuit 20 is input to the inverting input terminal of the overcurrent detection voltage comparison circuit 15 as an overcurrent detection voltage.

The voltage adding circuit 20 includes a discharge control FET 1
2 is a circuit for outputting a voltage obtained by adding a predetermined offset voltage to the drain-source voltage of the FET 2 and a charge control FET
11 and a voltage dividing resistor 2 connected between the drain of the discharge control FET 12 and the output terminal of the FET drive circuit 17.
1 and 22 and a protection resistor 23 connected between a connection point (voltage division point) of the voltage dividing resistors 21 and 22 and an inverting input terminal of the overcurrent detection voltage comparison circuit 15. That is, in the voltage adding circuit 20, the voltage obtained by dividing the output voltage of the FET drive circuit 17 by the voltage dividing resistors 21 and 22 is added as an offset voltage to the drain-source voltage of the discharge control FET 12, and the voltage after this offset voltage addition is performed. The voltage is input to the inverting input terminal of the overcurrent detection voltage comparison circuit 15 via the protection resistor 23 as the overcurrent detection voltage.

In this embodiment, a power save control circuit 18 is provided. This power save control circuit 1
8 indicates that the secondary battery 11 is FE
The FET drive circuit 1 is disconnected from the power supply of the T drive circuit 17.
7 is set to the ground potential, the secondary battery 11
This is a circuit that performs control for saving power.

Further, in the protection circuit device 1 of FIG. 1, for example, the parts other than the FETs 11 and 12 and the voltage adding circuit 20 are as follows.
It is configured in the control IC. Therefore, the first to third reference voltages Vref1, Vref2, and Vref31 are set inside the control IC, and the voltage values cannot be basically changed.

Next, the operation of the protection circuit device according to this embodiment will be described. (When charging) When charging the secondary battery 11, the external connection terminal 3
A charger is connected as an external device 5 between a and 3b. In this case, the charge control FET 11 and the discharge control FET
Reference numeral 12 indicates a normal ON state, and indicates “+ side output terminal of charger →
A charging current flows through a path of terminal 3a → terminal 2a → secondary battery 4 → terminal 2b → discharge control FET 12 → charge control FET 11 → negative output terminal of the charger.

If the battery voltage VB reaches the first reference voltage Vref1, which is a charging prohibition voltage, during this charging, the output of the overcharge detection voltage comparison circuit 13 goes high,
The drive circuit 16 turns off the charge control FET 11 and stops charging. Thus, the overcharge protection of the secondary battery 11 is performed.

(During discharge) When the secondary battery 12 is discharged, a load is connected as the external device 5 between the external connection terminals 3a and 3b. In this case, the charge control FET 11 and the discharge control FET 12 are normally in the ON state, and “+ side electrode of the secondary battery 4 → terminal 2 a → terminal 3 a → load → terminal 3 b → charge control FET 11 → discharge control FET 12 → Rechargeable battery 4
The discharge current flows through the path of “− side electrode”.

When the battery voltage VB reaches the second reference voltage Vref2, which is a discharge prohibition voltage, during this discharge, the output of the overdischarge detection voltage comparison circuit 14 goes high,
The drive control circuit 17 turns off the discharge control FET 12, and the discharge is stopped. In this way, the overdischarge protection of the secondary battery 11 is performed.

(When Overcurrent Occurs) When an overcurrent flows through the secondary battery 11 due to any cause during the above-described discharge, for example, a short circuit between the external connection terminals 3a and 3b, the drain current generated by the ON resistance of the discharge control FET 12 The source-to-source voltage increases. An offset voltage is added to the voltage between the drain and the source of the FET 12 by the voltage adding circuit 20, and this is input to the overcurrent detection voltage comparison circuit 15 as a discharge current detection voltage.

FIG. 2 is a diagram for explaining the operation of the voltage adding circuit 20. As shown in the figure, the values of the resistors 21 and 22 in the voltage addition circuit 20 are R1 and R2, and the drain-source voltage of the discharge control FET 12 is Va and F
Assuming that the output voltage of the ET driving circuit 17 is Vb, the output voltage Vc of the voltage adding circuit 20 is given by Vc = Va + {R1 / (R1 + R2)} · Vb. That is, from the voltage adding circuit 20, the drain-source voltage Va of the discharge control FET 12 is set to ｛R
A voltage Vc obtained by adding an offset voltage of 1 / (R1 + R2)｝ · Vb is output as a discharge current detection voltage. Note that the relationship between the resistance values R1 and R2 of the resistors 21 and 22 is R1 <
<R2, specifically, for example, R1 is several tens kΩ, R
2 is chosen to be a few MΩ.

The overcurrent detection voltage comparison circuit 15 determines that the discharge current detection voltage Vc is the third overcurrent detection reference voltage.
When the reference voltage Vref3 reaches the reference voltage Vref3, it is determined that an overcurrent has flowed through the secondary battery 4, and the output is inverted to a high level.
By the output of the overcurrent detection voltage comparison circuit 15, F
The discharge control FET 12 is turned off via the ET drive circuit 17, and overcurrent protection of the secondary battery 4 is performed.

As described above, in the protection circuit device of the present embodiment, when the overcurrent detection is performed by detecting the drain-source voltage due to the voltage drop of the on-resistance of the discharge control FET 12, this drain-source voltage is The offset voltage is added by the voltage addition circuit 20, and the output voltage of the voltage addition circuit 20 is used as the discharge current detection voltage, and this is compared with the reference voltage Vref3 for overcurrent detection by the overcurrent detection voltage comparison circuit 14. .

Therefore, the discharge current detection voltage can be made sufficiently large without causing an unnecessarily large current to flow through the discharge control FET 12 at the time of discharge.
Even if it cannot be changed because it is set in advance internally, overcurrent can be detected reliably and easily.

That is, as described above, in order to sufficiently extract the discharging ability of the secondary battery 4 to the load, it is necessary to use an FET having a low on-resistance as the discharging control FET 12. Here, if the drain-source voltage due to the voltage drop of the on-resistance of the discharge control FET 12 is used as the discharge current detection voltage as in the related art, it cannot be detected as an overcurrent unless the current flowing through the FET 12 becomes larger.

On the other hand, in this embodiment, since the voltage obtained by adding the offset voltage to the drain voltage of the FET 12 is used as the discharge current detection voltage, the offset voltage is appropriately selected.
2, the discharge current detection voltage becomes sufficiently large without a large current flowing, and the overcurrent can be reliably detected.

FIG. 3 shows the relationship between the discharge current Id of the secondary battery 4, the discharge current detection voltage Vc, and the junction temperature of the FET for explaining the effect of the present embodiment. In the case of the prior art, that is, when the offset voltage is not added, the discharge current detection voltage Vc reaches the reference voltage Vref3 for overcurrent detection when the discharge current Id reaches 6 A as shown by the broken line, and at this time, the junction of the FETs In contrast to the case where the temperature has reached the breakdown limit of 150 ° C., when the offset voltage is added as in this embodiment, when the discharge current Id reaches 5.3 A as shown by the solid line, the discharge current is detected. It can be seen that the voltage Vc reaches the reference voltage Vref3 and overcurrent detection is performed when the junction temperature of the FET is lower than 150 ° C.

When the secondary battery 4 is overdischarged, the power save control circuit 18 operates by the output of the overdischarge detection voltage comparison circuit 14, and the power supply voltage of the FET drive circuit 17 drops from the battery voltage to the ground potential. , FET drive circuit 1
7 also drops to the ground potential,
The power consumption of the T drive circuit 17 is reduced, and the power of the secondary battery 4 is saved.

When the power save control circuit 18 operates as described above, the power consumption of the voltage addition circuit 20 that generates the offset voltage by using the output voltage of the FET drive circuit 17 as input is reduced. Thus, there is little problem of an increase in power consumption. In this case, the offset voltage generated by the voltage adding circuit 20 becomes zero, but there is no problem because it is not necessary to perform overcurrent detection during overdischarge.

In the above embodiment, the discharge control FET is used.
The overcurrent detection is performed using the voltage drop due to the on-resistance of the FET. However, the overcurrent detection may be performed using the voltage drop due to the discharge current detecting resistor. Also in the case where overcurrent detection is performed using the voltage drop due to both of the detection resistors, the same as in the above embodiment, by adding an offset voltage to a voltage dependent on these voltage drops to obtain an overcurrent detection voltage. The effect of can be obtained.

In the above embodiment, the protection circuit device having the functions of overcharge protection, overdischarge protection and overcurrent protection has been described. However, the protection circuit device having only the functions of overdischarge protection and overcurrent protection is also applicable to the present invention. The invention is valid.

[0042]

As described above, according to the present invention, the offset voltage is added to the voltage depending on the voltage drop of the discharge control switch element or the discharge current detecting resistor, and the voltage after the addition of the offset voltage is discharged. The overcurrent detection voltage comparator circuit compares the current as the overcurrent detection reference voltage with the overcurrent detection reference voltage to detect the overcurrent, thereby fully discharging the rechargeable battery. Even if a low-resistance element is used for the resistor or a low-resistance resistor is used for the discharge current detection resistor, the discharge current detection voltage can be sufficiently increased without flowing more current than necessary to the discharge control switch element during discharging. Even if the reference voltage for overcurrent detection is preset in the control IC and cannot be changed, the overcurrent can be reliably detected to implement the overcurrent protection function. It is possible.

Further, when the secondary battery is overdischarged, the power save control circuit is operated by the output of the overdischarge detection voltage comparison circuit to lower the power supply voltage (output voltage) of the switch drive circuit to the ground potential. The power consumption of the switch driving circuit and the voltage adding circuit can be reduced, and the battery life can be prolonged.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a protection circuit device for a secondary battery according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the voltage adding circuit in FIG. 1;

FIG. 3 is a characteristic diagram showing a relationship between a discharge current and an over-discharge current of the secondary battery in the same embodiment as compared with a conventional protection circuit device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Protection circuit device 2a, 2b ... Battery connection terminal 3a, 3b ... External connection terminal 4 ... Secondary battery 5 ... External device (charger or load) 11 ... Charge control FET 12 ... Discharge control FET 13 ... Overcharge Detection voltage comparison circuit 14 ... Over discharge detection voltage comparison circuit 15 ... Over current detection voltage comparison circuit 16 ... Charge control FET drive circuit 17 ... Discharge control FET drive circuit 18 ... Power save control circuit 20 ... Voltage Adder circuits 21, 22 ... voltage dividing resistors 23 ... protective resistors

Claims (4)

[Claims] A switching element inserted between a secondary battery and a load; a voltage adding circuit for adding a predetermined offset voltage to a voltage dependent on a discharge current of the secondary battery to the load; An overcurrent detection voltage comparison circuit that compares an output voltage of the voltage addition circuit with a predetermined reference voltage, and an output voltage of the voltage addition circuit is equal to or higher than the reference voltage according to an output of the overcurrent detection voltage comparison circuit. And a switch driving circuit for turning off the switching element. 2. A switch element inserted between a secondary battery and a load; an overdischarge detection voltage comparison circuit for comparing a voltage of the secondary battery to the load with a discharge inhibition voltage; A voltage adding circuit for adding a predetermined offset voltage to a voltage dependent on a discharge current of the secondary battery to the load, and a voltage comparing circuit for overcurrent detection for comparing an output voltage of the voltage adding circuit with a predetermined reference voltage According to the output of the over-discharge detection voltage comparison circuit and the over-current detection voltage comparison circuit, when the voltage at the time of discharging the secondary battery to the load becomes equal to or less than a discharge inhibition voltage, or the voltage addition. A switch drive circuit for turning off the switch element when an output voltage of the circuit is equal to or higher than the reference voltage; and a switch when the voltage of the secondary battery discharged to the load is equal to or lower than a discharge inhibition voltage. A power save control circuit for switching a power supply voltage of the switch drive circuit from a voltage of the secondary battery to a ground potential after the drive circuit is turned off, wherein the voltage adding circuit includes an output of the switch drive circuit. A protection circuit device for a secondary battery, wherein the offset voltage is generated by dividing a voltage. 3. The protection circuit for a secondary battery according to claim 1, wherein the voltage addition circuit adds the offset voltage to a voltage dependent on a voltage drop of the switch element due to the discharge current. apparatus. 4. The device according to claim 1, wherein said switch element is a field effect transistor, and said voltage adding circuit adds said offset voltage to a voltage dependent on a voltage drop due to an on-resistance of said field effect transistor. 4. The protection circuit device for a secondary battery according to claim 3.