JP4894865B2 - Bidirectional switch current detection circuit - Google Patents

Description

The present invention relates to a current detection circuit for a bidirectional switch that monitors a charging / discharging current of a DC power source such as a rechargeable battery and cuts off an excessive charging / discharging current to protect the DC power source. The present invention relates to a current detection circuit for a bidirectional switch that detects low-loss and high accuracy without inserting a sense resistor directly into the current path of the bidirectional switch.
In the following drawings, the same reference numerals denote the same or corresponding parts.

FIG. 3 shows an example of the configuration of the simplest conventional current measuring circuit of this type. In the figure, a current is detected by inserting a high-precision sense resistor Rs in series with a MOSFET Q1 as a main switch for opening and closing a load current Im to be measured, and observing a voltage generated at both ends thereof. However, in this circuit, when the current Im to be measured is particularly large, the power loss in the sense resistor Rs becomes large, which causes a problem.
On the other hand, there is an apparatus shown in FIG. 4 as an apparatus for detecting the load current Im with low loss. In FIG. 4, a current mirror circuit is provided by providing an Nch.MOSFET Q3 as a mirror switch having the same gate potential as that of the Nch MOSFET Q1 as a main switch through which the load current Im flows and having a W length of the transistor 1 / n that of the FET Q1. And the output terminal of the operational amplifier Op1 by a negative feedback circuit using a high input impedance element (the operational amplifier Op1 in this example) so that the voltage generated across the main FET Q1 and the voltage generated across the mirror FET Q3 are equal. The mirror current Is of exactly Im × (1 / n) is made to flow through the sense resistor Rs connected between the drains of the mirror FET Q3.
Since the mirror current Is is sufficiently smaller than the load current Im, the loss when detecting the mirror current Is is very small. The mirror current Is can be accurately detected by the element having a low output impedance (the operational amplifier Op1 in this example) and the sense resistor Rs, and the current Im of the main FET Q1 can be accurately detected.

However, in the current detection method shown in FIGS. 3 and 4, the direction of the current Im flowing through the main switch is constant, and thus the bidirectional current flowing through the bidirectional switch as the charge / discharge protection switch cannot be detected.
FIG. 5 shows a typical configuration example of a main switch portion of a charge / discharge protection switch (bidirectional switch) that cuts off an excessive charge / discharge current of the battery E. In the figure, main switches M1 and M2 each consisting of two Nch MOSFETs originally incorporating parasitic diodes are connected in series with opposite polarities so that the source terminals of the main switches M1 and M2 are at both ends in this example. And inserted into the charge / discharge path on the cathode side of the battery E.
Here, for example, if the main switch M1 is turned on and M2 is turned off, the charging current of the battery E can be cut off, and if the main switch M1 is turned off and M2 is driven on, the discharging current of the battery E can be cut off. .
As an example of a method for detecting a bidirectional current flowing through such a bidirectional switch, there is a disclosure of Patent Document 1. In this method, an operational amplifier that detects the voltage across the sense resistor while reducing the power loss of the sense resistor as a circuit configuration in which the value of the sense resistor inserted in the current path is switched depending on the magnitude of the charge / discharge current, An accurate current detection voltage with little influence of the offset is obtained.

  Further, Patent Document 2 enables detection of charging current and discharging current under the same operating conditions, and the characteristic factor of the amplifier circuit similarly affects the detection of charging current and discharging current, enabling accurate comparison. Thus, a charge / discharge current detection circuit that is easy to adjust offset is disclosed.

JP 11-69635 A JP 2003-215172 A

However, the method of Patent Document 1 has a problem that it is necessary to select a sense resistance value according to the current output specification of the device in order to suppress the loss of the sense resistor and increase the sense voltage. However, the loss of the sense resistor portion inserted in the charge / discharge current path still remains as a problem.
It is an object of the present invention to provide a current detection circuit for a bidirectional switch that solves these problems and can accurately detect a charge / discharge current with a simple circuit with low loss.

In order to solve the above problems, the current detection circuit of the bidirectional switch according to claim 1 includes two main transistors (M1, M2) connected in series with opposite polarities to constitute the bidirectional switch, Control electrodes (gates G1, G2, etc.) are coupled and connected in series with opposite polarities so as to correspond to the series connection, and the potentials between the main electrodes (source, drain, etc.) corresponding to the corresponding main transistor respectively. Two mirror transistors (M3, M4) formed to flow a current having a predetermined mirror ratio smaller than the current flowing through the corresponding main transistor under equal conditions
One end (one source or the like) corresponding to the series connection of the main transistor and the series connection of the mirror transistor are both connected to one electrode (cathode or the like) of a main DC power source (battery E or the like). Is inserted in the current path of the main DC power supply,
Further, the potentials of the other ends (such as the other source) of the two series connections are input with a high input impedance, and a sense resistor (Rs) is connected to the other end of the series connection of the mirror transistors so that the two potentials are equal. Feedback amplifying means for inputting and outputting the current of the mirror ratio through
The current of the bidirectional switch is detected from the voltage across the sense resistor (via the voltage difference unit 11 or the like).

The bidirectional switch current detection circuit according to claim 2 is the bidirectional switch current detection circuit according to claim 1, wherein the feedback amplification means has its positive and negative input terminals in addition to the two series connections. Two power sources (VDD, −VDD) having their own output terminals connected to the non-mirror transistor side of the sense resistor and having positive and negative potentials relative to the potential of the one electrode of the main DC power source, respectively. ) Is supplied from an operational amplifier (Op1).
A bidirectional switch current detection circuit according to a third aspect of the present invention is the bidirectional switch current detection circuit according to the first or second aspect, wherein the four transistors are formed of the same channel type MOSFET.
The bidirectional switch current detection circuit according to claim 4 is the bidirectional switch current detection circuit according to claim 3, wherein the four transistors are N-channel MOSFETs, and the two serially connected one ends are connected to each other. Are connected to the cathode of the main DC power source.
The bidirectional switch current detection circuit according to claim 5 is the bidirectional switch current detection circuit according to claim 3, wherein the four transistors are P-channel MOSFETs, and the two serially connected one ends are connected to each other. Are connected to the anode of the main DC power source.

The bidirectional switch current detection circuit according to claim 6 is the bidirectional switch current detection circuit according to any one of claims 1 to 5, wherein at least the four transistors and the feedback amplification means are formed of a semiconductor integrated circuit. Like that.
The operation of the present invention is as follows.
A bidirectional mirror switch comprising a combination of mirror transistors each configured to cause a small current (mirror current) having a predetermined mirror ratio to flow in the main transistor current to each of the two main transistors constituting the bidirectional main switch; An operational amplifier that forms a feedback amplifier circuit by inputting and outputting mirror current to and from the bidirectional mirror switch through a sense resistor so as to equalize the voltage across the bidirectional main switch and bidirectional mirror switch by being fed from two positive and negative power sources. The mirror current from the voltage across the sense resistor, and thus the bidirectional main switch current, is measured with a simple circuit with low loss and high accuracy .

According to the present invention, a bidirectional main switch having a role of cutting off an excessive charging / discharging current of a DC power source such as a battery has a small current (mirror) having a predetermined mirror ratio with respect to the current of the bidirectional main switch. In addition to the combination of bidirectional mirror switches configured to flow current), the bidirectional mirror switch has a sense resistor so that the voltage at both ends of the bidirectional main switch and bidirectional mirror switch is equalized by being fed from two positive and negative power sources. An operational amplifier that passes the mirror current through is provided, and the mirror current and therefore the bidirectional main switch current are measured from the voltage across the sense resistor.
While simplifying the current detection circuit, the bidirectional main switch current (battery charge / discharge current) can be detected with high accuracy and low loss .

It is a circuit diagram which shows the structure as one Example of this invention. It is explanatory drawing of the parasitic diode energization mode in FIG. It is a figure which shows the example of the conventional current detection circuit. It is a figure which shows the other example of the conventional current detection circuit. It is a circuit diagram which shows the structural example of a bidirectional switch.

FIG. 1 is a circuit diagram showing a configuration as one embodiment of the present invention. In FIG. 1, the main switches M1 and M2 composed of Nch MOSFETs constituting the bidirectional main switch similar to FIG. 5 are connected in series with the opposite polarity in parallel to each other, and the main switches M1 and M2 are gated respectively. There are provided mirror switches M3 and M4 composed of Nch.MOSFETs which form the mirror circuit of the main switches M1 and M2 with the same voltage. Here, the series connection of the mirror switches M3 and M4 is also referred to as a bidirectional mirror switch.
Strictly speaking, the main switches M1 and M2 are composed of Nch MOSFETs Q1 and Q2 and their parasitic diodes D1 and D2, respectively. Similarly, the mirror switches M3 and M4 are composed of Nch MOSFETs Q3 and Q4 and their parasitic diodes D3 and D4, respectively.
In this example, the source terminals (point c) of the main switch M1 and the mirror switch M3 are connected to the cathode of the battery E and maintained at the ground potential, and the source terminal (point a) of the main switch M2 is the negative external terminal VB. Connected to-. The anode terminal of the battery E is connected to the positive external terminal VB +, and a battery charger (not shown) is connected between the external terminals VB + and VB− when the battery E is charged (the positive electrode side is the external terminal VB +). When the battery E is discharged, a load (not shown) is connected.

On the other hand, for the source terminal (point a) of the main switch M2 and the source terminal (point b) of the mirror switch M4, the voltages of the source terminals (points a and b) are respectively input to the (+) input terminal and the (-) input. A high input impedance is input to the terminal, and a current is applied to the series connection of the mirror switches M3 and M4 via the sense resistor Rs so as to amplify the deviation between the two source terminal voltages and to make this deviation zero. An operational amplifier Op1 is provided.
In the circuit of FIG. 1, when the battery E is charged, a charging current flows through the path of the external terminal (VB +) → the battery E → the main switch M1 → the same M2 → the external terminal (VB−), and the point a with respect to the ground potential (point c). When the battery E is discharged, a discharge current flows through the path of the external terminal (VB−) → the main switch M2 → the same M1 → the battery E → (VB +). The potential becomes positive.
The operational amplifier Op1 that performs the feedback amplification operation needs to operate so that the potentials between the points a and b are equal during charging and discharging. Therefore, the operational amplifier Op1 is mirror-switched via the sense resistor Rs when the battery E is charged. It is necessary to sink current from M3 and M4, and to flow current to the mirror switches M3 and M4 via the sense resistor Rs when the battery E is discharged. That is, the output voltage of the operational amplifier Op1 needs to be a negative voltage with respect to the ground potential (point c) when the battery E is charged, and also needs to be a positive voltage when the battery E is discharged.

Therefore, in the present invention, the power supply voltage supplied to the operational amplifier Op1 is not a zero potential (0V) and a positive voltage (VDD) during normal use, but a positive and negative power supply such as a negative voltage (−VDD) and a positive voltage (VDD). By using it, the output voltage range of the operational amplifier Op1 extends over both positive and negative voltage ranges sandwiching 0V. In this case, when only a positive voltage is supplied from the outside, another battery is used inside to obtain a negative voltage, or a power supply circuit for generating a negative voltage from the positive voltage is incorporated.
In FIG. 1, the voltage difference unit 11 obtains the difference between the voltages at both ends of the sense resistor Rs, thereby measuring the current of the sense resistor Rs, and hence the current passing through the main switches M1 and M2. At this time, the current direction of the main switch can also be detected by looking at the polarity of the differential voltage, and it can be determined whether the charging state or the discharging state.
Then, the measured current (the output of the voltage differentiator 11, which is output separately as the current measurement value 11a in this example) is compared with a predetermined value by the comparator 12, and the measured current is changed to a predetermined current value. When exceeding, the gates G1 and G2 of the bidirectional main switch (and mirror switch) can be controlled by the output of the comparator 12 via the gate control circuit 13, and the bidirectional switch can be turned off.

For example, when an excessive discharge current flows due to a short circuit of an external circuit or the like, the gate G1 (and therefore the switches M1 and M3) is driven to the off side while the gate G2 (and therefore the switches M2 and M4) are driven to the on side, or When the battery E is destroyed and short-circuited, the gate G1 (and therefore the switches M1 and M3) is driven on while the gate G2 (and therefore the switches M2 and M4) are driven off and an excessive current is generated. Can be prevented.
In the present invention, an overcurrent flowing through a parasitic diode of the main switch, which has not been considered in the past, can also be detected and cut off. FIGS. 2A and 2B are explanatory diagrams of such energization mode. In FIG. 2, only the bidirectional main switches M1 and M2 in FIG. 1 are shown for simplicity.
That is, FIG. 2A shows an overdischarge prohibited and chargeable mode, that is, the gate G1 is driven to the off side and the gate G2 is driven to the on side. The current can flow through the path of the parasitic diode D1 → the main switch M2 of the main switch M1.
The current flowing through the parasitic diode D1 can be detected by the current flowing through the sense resistor Rs along the path of the parasitic diode D3 on the side of the bidirectional mirror switch in parallel with the bidirectional main switch and the mirror switch M4 (see FIG. 1). The overcurrent of the parasitic diode D1 can be blocked by further driving the gate G2 off.

Similarly, FIG. 2B shows the overcharge prohibited and dischargeable mode, that is, the state where the gate G1 is driven to the on side and the gate G2 is driven to the off side, respectively, and the charging current is blocked but the discharging side. Current can flow through the path of the parasitic diode D2 → main switch M1 of the main switch M2.
The current flowing through the parasitic diode D2 can also be detected by the current flowing from the sense resistor Rs to the path of the parasitic diode D4 in parallel to the bidirectional main switch and the mirror switch M3 (see FIG. 1). The overcurrent of the parasitic diode D2 can be further blocked by driving the gate G1 off.
In this case, in order to make the current characteristics of the parasitic diode equal to the mirror ratio between the mirror path and the main path, the area of the source region and the drain region and the area of the substrate contact region corresponding to the mirror path and the main path are accurately determined. To mirror ratio.
That is, if the current characteristics of the transistors Q1 and Q3 (and Q2 and Q4) are proportional to each other, it is important to match the W length to the mirror ratio, but the currents of the parasitic diodes D1 and D3 (and D2 and D4) In order to make the characteristics proportional to each other, it is important to match the opposing area of the PN junction with the mirror ratio.

If diode characteristics that completely match the mirror ratio are obtained, the applied voltages applied to both terminals of the diodes D1 and D3 (or applied voltages applied to both terminals of the diodes D2 and D4) are the same. Since the diode current is also equal to the mirror ratio, the mirror path current is proportional to the main path current even if the transistor is off.
The bidirectional switch is not limited to a type in which the drain is shared as shown in FIG. 1 and the source is connected to the external terminal, but a type in which the source is connected in common and the drain is connected to the external terminal is also possible. In addition to the type in which the Nch bidirectional switch is inserted on the cathode side of the battery E, it is possible to insert the Pch bidirectional switch on the anode side of the battery E.
When one end of the bidirectional switch is connected to the anode side of the battery E, the power supply VDD and −VDD of the operational amplifier Op1 needs to have positive and negative potentials with respect to the potential of the anode of the battery E, respectively.

E Battery M1, M2 Main switch M3, M4 Mirror switch Q1, Q2 Main MOSFET
Q3, Q4 mirror MOSFET
D1, D2 Parasitic diode of main MOSFET D3, D4 Parasitic diode of mirror MOSFET Rs Sense resistor Op1 Operational amplifier VDD, power supply of operational amplifier 11 Voltage difference 11a Current measurement value 12 Comparator 13 Gate control circuit G1 Main switch M1 and mirror switch M3 Gate G2 Main switch M2 and mirror switch M4 gate VB +, VB- External terminal

Claims (6)

Two main transistors that are connected in series with opposite polarities to form a bidirectional switch, each main transistor and the control electrode are coupled to each other, and are connected in series with opposite polarities so as to correspond to the series connection. Two mirror transistors formed so as to flow a current having a predetermined mirror ratio smaller than the current flowing through the corresponding main transistor under the condition that the potentials of the main electrodes corresponding to the main transistor are equal;
One end corresponding to the series connection of the main transistor and the series connection of the mirror transistor are both connected to one electrode of the main DC power supply, the series connection of the main transistor is inserted into the current path of the main DC power supply,
Further, the potentials at the other ends of the two series connections are respectively input with a high input impedance, and the current at the mirror ratio is input to the other end of the series connection of the mirror transistors via a sense resistor so as to equalize the two potentials. Provide feedback amplification means to output,
A bidirectional switch current detection circuit that detects a current of the bidirectional switch from a voltage across the sense resistor. In the current detection circuit of the bidirectional switch according to claim 1,
The feedback amplification means has its own positive / negative input terminal connected to the other end of the two series connections, its own output terminal connected to the non-mirror transistor side of the sense resistor, and the main DC power supply described above. A bidirectional switch current detection circuit comprising an operational amplifier fed from two power sources each having a positive and negative potential with respect to the potential of one electrode.   3. The bidirectional switch current detection circuit according to claim 1, wherein the four transistors are MOSFETs of the same channel type. 4. In the current detection circuit of the bidirectional switch according to claim 3,
A bidirectional switch current detection circuit, wherein the four transistors are N-channel MOSFETs, and the corresponding one ends of the two series connections are connected to the cathode of the main DC power supply. In the current detection circuit of the bidirectional switch according to claim 3,
A bidirectional switch current detection circuit, wherein the four transistors are P-channel MOSFETs, and one end of the two serial connections corresponding to each other is connected to the anode of the main DC power supply. 6. The bidirectional switch current detection circuit according to claim 1, wherein at least the four transistors and the feedback amplifying means comprise a semiconductor integrated circuit.

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