JP2009225087A - Overcurrent protection circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an overcurrent protection circuit by which precision for shutting off a circuit does not fall off by the change of environmental temperature. <P>SOLUTION: First and second transistors T1, T2 constituting a current mirror circuit 9 which detects overcurrent which flows in a detection resistor R3 have the same temperature characteristics. There is a relation of R1=R2-(I3/I1)R3 among first and second limiting resistors R1, R2, the detection resistor R3, and current I1, I2, the resistors are set so that the current becomes I1>I2 before the overcurrent flows and becomes I2>I1 after the moment that the overcurrent flows. Since no parameter, such as voltage between base and emitter, which fluctuates by the temperature characteristics is present in the formula, even when the environmental temperature changes, timing that the second transistor T2 turns a fourth transistor T4 on does is not varied. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an overcurrent protection circuit.

  FIG. 5 is a circuit diagram showing a conventional overcurrent protection circuit. The conventional overcurrent protection circuit 7 includes a detection resistor R3 for detecting an overcurrent supplied from the current supply source side P1 to the current supply target side P2, and a current supply when an overcurrent flows from the current supply side P1. An NPN bipolar transistor T3 for cutting off the electric circuit 5 to the target side P2, a current cut circuit 8 for turning off the transistor T3 when an overcurrent flows, and a PNP for monitoring the current flowing through the detection resistor R3 Type bipolar transistor T5, limiting resistor R7 that limits the operating current of transistor T5, NPN bipolar transistor T6 that turns on when transistor T5 is turned on, and limiting resistor R6 that limits the operating current of transistor T6 Prepare.

  During normal times when no overcurrent occurs, the base-emitter voltage Vf of the transistor T5 is low and the transistor T5 does not turn on, so that the transistor T6 remains off. For this reason, the current cut circuit 8 outputs a high level signal, the transistor T3 is kept on, and a current is supplied to the current supply target side P2. When an overcurrent flows through the detection resistor R3, the transistor T5 is turned on, so that the transistor T6 is turned on, the current cut circuit 8 outputs a low level signal, the transistor T3 is turned off, and the electric circuit 5 is cut off. The current supply target side P2 is protected from overcurrent.

JP-A-7-170295 (9th to 11th paragraphs, FIG. 1).

However, since the transistor has temperature characteristics, if the ambient temperature in which the overcurrent protection circuit 7 is placed changes, the base-emitter voltage Vf of the transistor T5 varies, and the overcurrent detection accuracy decreases. . For example, when the ambient temperature rises above room temperature (25 ° C.), when an overcurrent smaller than the overcurrent assumed at the time of designing under room temperature conditions flows, the transistor T5 is turned on and the electric circuit 5 is cut off. There is a fear. Conversely, when the environmental temperature drops below room temperature, even when an overcurrent larger than the assumed overcurrent flows, the transistor T5 is not turned on, the electric circuit 5 is not cut off, and the current supply target is destroyed. There is a fear.
In other words, the conventional overcurrent protection circuit has a problem that the accuracy of interrupting the electric circuit is lowered due to a change in environmental temperature.

  Therefore, the present invention has been made to solve the above-described problem, and an object of the present invention is to realize an overcurrent protection circuit in which the accuracy of interrupting an electric circuit is not lowered by a change in environmental temperature.

  In order to achieve the above object, according to the present invention, the overcurrent connected to the electric circuit (5) for supplying current from the current supply source (P1) to the current supply target (P2) is reduced. In the overcurrent protection circuit (1), which has a detection resistor (R3) for detecting, and cuts off the electric circuit when the overcurrent is detected, an emitter is connected to the current supply target side of the detection resistor A first transistor (T1) having a base and a collector coupled to each other, a first limiting resistor (R1) connected to an emitter of the first transistor, and a collector of the first transistor A first constant current source (10) connected to the second transistor (T) and an emitter connected to the current supply source side of the detection resistor, and a second transistor (T) having the same temperature characteristics as the first transistor ), A second limiting resistor (R2) connected to the emitter of the second transistor, and a second constant current source (11) connected to the collector of the second transistor. And a current mirror circuit (9) in which the bases of the first and second transistors are connected to each other and a collector of the second transistor, and a collector current flowing through the collector is a predetermined value. A current cutoff circuit (6) that operates in the case of the above to cut off the electric circuit, and when the collector current of the second transistor is not overcurrent flowing through the detection resistor, the predetermined current A technical means is used in which the current mirror circuit is configured so as to be equal to or greater than the predetermined value when an overcurrent flows through the detection resistor while maintaining less than the value.

  According to a second aspect of the present invention, in the overcurrent protection circuit (1) according to the first aspect, the current value of the current flowing through the first limiting resistor is I1, and the current value of the current flowing through the second limiting resistor is When the value is I2, when the overcurrent does not flow through the detection resistor, I1> I2, and when the overcurrent flows through the detection resistor, the current mirror circuit so that I2> I1 The technical means that (9) is configured is used.

  According to a third aspect of the present invention, in the overcurrent protection circuit (1) according to the second aspect, the resistance value of the first limiting resistor is R1, the resistance value of the second limiting resistor is R2, and the detection When the resistance value of the resistor is R3, the current value of the current flowing through the detection resistor is I3, and I1 = I2 at the moment when the overcurrent flows through the detection resistor, R1 = R2- (I3 / I1) The technical means that the current mirror circuit (9) is configured so that the relationship of R3 is established is used.

  According to a fourth aspect of the present invention, in the overcurrent protection circuit (1) according to any one of the first to third aspects, the current interrupt circuit (6) can interrupt the electric circuit (5). The third transistor (T3) connected to the electric circuit and the base are connected to the collector of the second transistor (T2), and the collector current of the second transistor has exceeded the predetermined value. A technical means is used that includes a fourth transistor (T4) that is turned on at times and a transistor drive circuit (2) that turns off the third transistor when the fourth transistor is turned on.

  According to a fifth aspect of the present invention, in the overcurrent protection circuit (1) according to any one of the first to fourth aspects, the first and second transistors (T1, T2) are each bipolar. The technical means of being a transistor is used.

  According to a sixth aspect of the present invention, in the overcurrent protection circuit (1) according to any one of the first to fourth aspects, the first and second transistors (T1, T2) are respectively MOS transistors. The technical means of being a transistor is used.

  According to a seventh aspect of the present invention, in the overcurrent protection circuit (1) according to any one of the fourth to sixth aspects, the transistor drive circuit (2) includes the fourth transistor (T4). A technical means is used that is a comparison circuit (4) that compares the operation voltage and the reference voltage and applies a voltage according to the comparison result to the base of the third transistor (T3).

  In addition, the code | symbol in each said parenthesis shows the correspondence with the specific example shown in embodiment of invention mentioned later.

(Operation of the Invention of Claim 1)
A change in the current flowing through the detection resistor connected to the electric circuit that supplies current from the current supply source to the current supply target is detected by current mirror circuits connected to both ends of the detection resistor. The collector current that flows through the collector of the second transistor is maintained below a predetermined value that allows the current interrupt circuit to operate when no overcurrent flows through the detection resistor, and the overcurrent flows through the detection resistor. When it falls, it becomes a predetermined value or more.
Here, the temperature characteristics of the first and second transistors constituting the current mirror circuit are the same.
Accordingly, the collector current of the second transistor is maintained below a predetermined value at which the current cutoff circuit can be operated when no overcurrent flows through the detection resistor regardless of the environmental temperature, and the overcurrent is supplied to the detection resistor. Will flow above the predetermined value regardless of the environmental temperature.

(Effect of the invention according to claim 1)
It is possible to realize an overcurrent protection circuit in which the accuracy of interrupting the electric circuit is not lowered by a change in environmental temperature.

(Effect of the invention according to claim 3)
Since the current mirror circuit is configured so that the relationship of R1 = R2- (I3 / I1) R3 is established, parameters such as base-emitter voltages of the first and second transistors that change with temperature characteristics Irrespective of the resistance values of the detection resistor and the first and second limiting resistors, the current mirror circuit can be configured.
Therefore, it is possible to realize an overcurrent protection circuit in which the accuracy of interrupting the electric circuit is not lowered by a change in the environmental temperature.

(Operation of the Invention of Claim 4)
When an overcurrent flows through the detection resistor, a collector current that is not absorbed by the second constant current source among the collector current of the second transistor flows to the base of the fourth transistor, and the base current exceeds a predetermined value. Then, the fourth transistor is turned on, the third transistor connected to the electric circuit is turned off by the transistor driving circuit, and the electric circuit is cut off.

(Effect of the invention according to claim 4)
Since the fourth transistor is turned on for the first time when an overcurrent flows through the detection resistor and the electric circuit is cut off, there is no possibility that the electric circuit is cut off when no overcurrent flows through the detection resistor.
In addition, since the temperature characteristics of the first and second transistors constituting the current mirror circuit are the same, the timing at which the base current of the fourth transistor begins to flow does not fluctuate due to the influence of the environmental temperature, so the circuit is interrupted. The timing accuracy can be increased.

(Effect of the invention according to claim 5)
Since each of the first and second transistors is a bipolar transistor, the breakdown voltage of the current mirror circuit can be increased.

(Effect of the invention according to claim 6)
Since each of the first and second transistors is a MOS transistor, the response speed of the current mirror circuit can be increased.

(Effect of the invention according to claim 7)
Since the third transistor can be turned on or off by the output of the comparison circuit, the electric circuit can be instantaneously interrupted with good response. Further, when the overcurrent protection circuit is manufactured by IC or LSI, the comparison circuit can be formed on the same semiconductor substrate, so that the manufacturing efficiency can be improved.

<First Embodiment>
An embodiment according to the present invention will be described with reference to FIG. FIG. 1 is a circuit diagram showing a schematic configuration of an overcurrent protection circuit according to this embodiment.

(Outline configuration)
The overcurrent protection circuit 1 is connected in series to a circuit 5 that supplies current from the current supply source side P1 to the current supply target side P2, and detects a detection resistor R3 for detecting an overcurrent, and a voltage across the detection resistor R3. And a current interrupt circuit 6 that interrupts the electric circuit 5 by the operation of the current mirror circuit 9. The overcurrent protection circuit 1 is used, for example, to protect the power source of an ECU (electric control device) provided in the vehicle from overcurrent, the current supply source side P1 is connected to the vehicle battery, and the current supply target side P2 is connected to the power source of the ECU.

  The current mirror circuit 9 includes a first limiting resistor R1 having one end connected to the current supply target side of the detection resistor R3, an emitter connected to the other end of the first limiting resistor R2, and a base and a collector coupled. The first bipolar transistor T1, the first constant current source 10 having one end connected to the collector of the first bipolar transistor T1 and the other end grounded, and one end on the current supply source side of the detection resistor R3. The connected second limiting resistor R2, the second bipolar transistor T2 having an emitter connected to the other end of the second limiting resistor R2, one end connected to the collector of the second bipolar transistor, the other And a second constant current source 11 whose end is grounded. The bases of the first and second bipolar transistors T1, T2 are connected to each other.

  The overcurrent protection circuit 1 is a monolithic IC or LSI formed on the same semiconductor substrate, and the first and second bipolar transistors T1, T2 are monolithic dual type. For this reason, the first and second bipolar transistors T1, T2 have the same temperature characteristics.

  The current cutoff circuit 6 includes a third bipolar transistor T3 having a collector connected to the current supply source side of the detection resistor R3 and an emitter connected to the current supply target side in the electric circuit 5, and a base of the third bipolar transistor T3. Are connected to the collector of the second bipolar transistor T2, the base is connected to the collector of the second bipolar transistor T2, the emitter is grounded, and the base of the fourth bipolar transistor T4 is connected to the base of the fourth bipolar transistor T4. Are connected, and the other end is grounded, and a limiting resistor R4 connected in series to the collector of the fourth bipolar transistor T4.

  In this embodiment, the first and second bipolar transistors T1 and T2 are each a PNP type, and the third and fourth bipolar transistors T3 and T4 are each an NPN type.

  FIG. 2 is an example of a circuit configuration of the current cut circuit 2 shown in FIG. In this embodiment, the current cut circuit 2 uses a comparison circuit (comparator). The non-inverting input of the comparison circuit 4 is connected to the collector of the fourth bipolar transistor T4, and the reference voltage Vref is applied to the inverting input. The output of the comparison circuit 4 is connected to the base of the third bipolar transistor T3. Further, when the overcurrent protection circuit 1 is manufactured by an IC or LSI, the comparison circuit 4 can be formed on the same semiconductor substrate, so that the manufacturing efficiency can be improved.

(Determination of resistance value and current value)
The resistance values of the first limiting resistor R1, the second limiting resistor R2, and the detection resistor R3 are R1, R2, and R3, respectively, and the current values that flow through the resistors are I1, I2, and I3, respectively.
Also, when no overcurrent flows through the detection resistor R3, each resistance value is set so that I1> I2, when the overcurrent flows, I1 = I2, and after the overcurrent flows, I2> I1. decide.
I1 and I2 can be obtained by the following well-known expressions (1) and (2), respectively.

  I1 = Is · exp (q · VBE1 / kT) (1)

  I2 = Is · exp (q · VBE2 / kT) (2)

Here, Is is a saturation current, q is a charge amount, VBE1 is a voltage between the base and emitter of the first bipolar transistor T1, VBE2 is a voltage between the base and emitter of the second bipolar transistor T2, k is a Boltzmann constant, and T is an absolute temperature. It is.
Further, from FIG. 1, the following expression (3) is established.

  R3 · I3 + R1 · I1 + VBE1 = R2 · I2 + VBE2 (3)

  Here, when overcurrent is detected, since I1 = I2 is set, the following equation (4) is established from equations (1) and (2).

  VBE1 = VBE2 (4)

  Substituting equation (4) into equation (3) and substituting I1 into I2, the following equation (5) is established.

  R3 · I3 + R1 · I1 = R2 · I1 (5)

  When R1 is obtained from this equation (5), the following equation (6) is established.

  R1 = R2- (I3 / I1) R3 (6)

  That is, the resistance values R1 and R2 of the first and second limiting resistors R1 and R2 constituting the current mirror circuit 9 may be determined so as to satisfy the above formula (6). In addition, since the equation (6) does not include the base-emitter voltage VBE, the resistance values R1 and R2 can be determined without being affected by the temperature characteristics of the transistor.

(Main operations)
When no overcurrent flows through the detection resistor R3, I1 is larger than I2, so that the current I2 is absorbed by the constant current source 11 through the bipolar transistor T2 and the base current (I2-I) of the fourth bipolar transistor T4. ) Does not flow (I2−I = 0), the fourth bipolar transistor T4 maintains the OFF state.

  For this reason, the fourth bipolar transistor T4 maintains the off state, and the voltage applied to the non-inverting input of the comparison circuit 4 is lower than the reference voltage Vref. Therefore, the comparison circuit 4 sends the high level signal to the third bipolar transistor. Output to the base of T3. As a result, the third bipolar transistor T3 is kept on, and the current I3 is supplied to the current supply target side P2.

  Further, even when the environmental temperature at which the overcurrent protection circuit 1 is placed changes, the base current of the fourth bipolar transistor T4 does not flow, so that when the overcurrent does not flow, There is no possibility that the electric circuit 5 will be interrupted.

  Next, at the moment when the overcurrent flows to the detection resistor R3, I1 = I2, but since the base current (I2-I) of the fourth transistor T4 does not flow, the fourth transistor T4 is not turned on. The electric circuit 5 is not interrupted.

  After the moment when the overcurrent flows through the detection resistor R3, I2 becomes larger than I1, and the collector current of the second transistor T2 is large enough to turn on the fourth transistor T4 (claim of the present application). (Corresponding to the predetermined value described in item 1). For this reason, the base current (I2-I) of the fourth bipolar transistor T4 increases, and the fourth bipolar transistor T4 is turned on.

  For this reason, the voltage applied to the non-inverting input of the comparison circuit 4 becomes higher than the reference voltage Vref, and the comparison circuit 4 outputs a low level signal to the base of the third bipolar transistor T3. As a result, the third bipolar transistor T3 is turned off, the electric circuit 5 is interrupted, and no overcurrent is supplied to the current supply target side P2. Further, since the output of the comparison circuit 4 is binary (digital), the electric circuit 5 can be instantaneously interrupted with good responsiveness.

  Even if the environmental temperature at which the overcurrent protection circuit 1 is placed changes, the change in the base current of the fourth bipolar transistor T4 is extremely small. There is no possibility that the electric circuit 5 will not be interrupted by the change.

Even if the collector current of the second bipolar transistor T2 leaks to the base of the fourth bipolar transistor T4 when no overcurrent is flowing through the detection resistor R3, the leakage current of the fourth bipolar transistor T4 Absorbed by the pull-down resistor R5 connected to the base. Therefore, there is no possibility that the fourth bipolar transistor T4 malfunctions when the overcurrent does not flow through the detection resistor R3 and the electric circuit 5 is interrupted.
Further, the pull-down resistor R5 can be omitted by setting a base current value necessary for turning on the fourth bipolar transistor T4 to a value sufficiently larger than the current value of the leakage current.

(Effect of embodiment)
(1) As described above, according to the overcurrent protection circuit 1 of the above embodiment, the accuracy of overcurrent detection can be increased regardless of changes in the environmental temperature, so that the current supply target is reliably protected from overcurrent. can do.

(2) Moreover, since the temperature characteristics of the first and second transistors T1 and T2 constituting the current mirror circuit 9 are the same, the timing at which the base current of the fourth transistor T4 starts to flow is affected by the environmental temperature. Therefore, it is possible to improve the accuracy of the timing at which the electric circuit 5 is cut off.

(3) Since the first and second transistors T1 and T2 are bipolar transistors, the breakdown voltage of the current mirror circuit 9 can be increased.

Second Embodiment
Next, a second embodiment of the present invention will be described with reference to FIG.
FIG. 3 is a circuit diagram showing an overcurrent protection circuit according to the second embodiment. The overcurrent protection circuit 3 shown in FIG. 3 is characterized in that a MOS transistor (Metal Oxide Semiconductor FET) is used instead of the bipolar transistor. Since the overcurrent protection circuit 3 has the same configuration except that each bipolar transistor of the overcurrent protection circuit 1 of the first embodiment is replaced with a MOS transistor, the description of the same configuration and operation is omitted.

  The source of the first MOS transistor T1 constituting the current mirror circuit 9 is connected to the first limiting resistor R1, the drain is connected to the first constant current source 10, and the gate and the drain are combined. Yes. The source of the second MOS transistor T2 is connected to the second limiting resistor R2, and the drain is connected to the second constant current source 11. The gates of the first and second MOS transistors T1 and T2 are connected to each other.

  The source of the third MOS transistor T3 is connected to the detection resistor R3, the drain is connected to the current supply target side P2, and the gate is connected to the output of the current cut circuit 2. The base of the fourth MOS transistor T4 is connected to the drain of the second MOS transistor T2, and the source is connected to the current cut circuit 2. The first and second MOS transistors T1, T2 have the same temperature characteristics.

  As described above, the overcurrent protection circuit 3 has the same configuration as the overcurrent protection circuit 1 of the first embodiment except that each transistor is a MOS transistor, and operates in the same manner. The same effects as 1) and (2) can be achieved. Further, since each transistor is a MOS transistor, the response from the detection of overcurrent to the interruption of the electric circuit 5 can be improved as compared with the case where it is constituted by a bipolar transistor.

<Third Embodiment>
Next, a third embodiment of the present invention will be described with reference to FIG.
FIG. 4 is a circuit diagram showing an overcurrent protection circuit according to the third embodiment. The overcurrent protection circuit 1 shown in FIG. 4 is characterized in that an NPN type bipolar transistor is used instead of each PNP type bipolar transistor constituting the overcurrent protection circuit 1 of the first embodiment. The overcurrent protection circuit 1 has the same configuration except that each PNP type bipolar transistor of the overcurrent protection circuit 1 of the first embodiment is replaced with an NPN type bipolar transistor. Description is omitted.

  The current mirror circuit 9 includes a first limiting resistor R1 having one end connected to the current supply target side of the detection resistor R3, an emitter connected to the other end of the first limiting resistor R2, and a base and a collector coupled. The first bipolar transistor T1, the first constant current source 10 having one end connected to the collector of the first bipolar transistor T1 and the other end connected to the current supply source side, and the ground side of the detection resistor R3 A second limiting resistor R2 having one end connected to the second bipolar transistor T2, an emitter connected to the other end of the second limiting resistor R2, and a collector connected to the collector of the second bipolar transistor. And a second constant current source 11 having the other end connected to the current supply source side. The bases of the first and second bipolar transistors T1, T2 are connected to each other.

Each of the transistors T1 to T4 is an NPN type transistor. The transistors T1 and T2 have the same temperature characteristics.
As described above, the overcurrent protection circuit 1 has the same configuration as the overcurrent protection circuit 1 according to the first embodiment except that each transistor is an NPN-type bipolar transistor. The effects (1) to (3) can be obtained.

1 is a circuit diagram showing a schematic configuration of an overcurrent protection circuit according to an embodiment of the present invention. It is an example of the circuit structure of the current cut circuit 2 shown in FIG. It is a circuit diagram which shows the overcurrent protection circuit which concerns on 2nd Embodiment. It is a circuit diagram which shows the overcurrent protection circuit which concerns on 3rd Embodiment. It is a circuit diagram which shows the conventional overcurrent protection circuit.

Explanation of symbols

1 .... Overcurrent protection circuit, 2 .... Current cut circuit, 5 .... Electric circuit, 6 .... Current cut-off circuit,
9 .. Current mirror circuit, 10... First constant current source, 11.
P1 ... current supply source side, P2 ... current supply target side, R1 ... first limiting resistor,
R2 ... second limiting resistor, R3 ... detection resistor, T1 ... first transistor,
T2 ... second transistor, T3 ... third transistor,
T4 .. Fourth transistor.

Claims (7)

An overcurrent protection circuit that includes a detection resistor for detecting an overcurrent, connected to an electric circuit that supplies current from an electric current supply source to an electric current supply target, and that interrupts the electric circuit when the overcurrent is detected In
A first transistor having an emitter connected to the current supply target side of the detection resistor and having a base and a collector coupled; a first limiting resistor connected to the emitter of the first transistor; A first constant current source connected to the collector of the first transistor, a second emitter having an emitter connected to the current supply source side of the detection resistor, and having the same temperature characteristics as the first transistor. A second limiting resistor connected to the emitter of the second transistor, a second constant current source connected to the collector of the second transistor, and the second A current mirror circuit in which the bases of the first and second transistors are connected to each other;
A current cutoff circuit that is connected to the collector of the second transistor and operates when the collector current flowing through the collector exceeds a predetermined value, and shuts off the electrical circuit;
The collector current of the second transistor is maintained below the predetermined value when no overcurrent flows through the detection resistor, and is equal to or greater than the predetermined value when an overcurrent flows through the detection resistor. An overcurrent protection circuit comprising a current mirror circuit. When the current value of the current flowing through the first limiting resistor is I1, and the current value of the current flowing through the second limiting resistor is I2,
The current mirror circuit is configured such that I1> I2 when no overcurrent flows through the detection resistor, and I2> I1 when overcurrent flows through the detection resistor. The overcurrent protection circuit according to claim 1. The resistance value of the first limiting resistor is R1, the resistance value of the second limiting resistor is R2, the resistance value of the detection resistor is R3, and the current value of the current flowing through the detection resistor is I3. If I1 = I2 at the moment when the overcurrent flows,
R1 = R2- (I3 / I1) R3
The overcurrent protection circuit according to claim 2, wherein the current mirror circuit is configured so that the relationship is established. The current interrupt circuit is
A third transistor connected to the circuit so as to be capable of interrupting the circuit;
A base connected to the collector of the second transistor, and a fourth transistor that turns on when a collector current of the second transistor becomes equal to or greater than the predetermined value;
4. The overcurrent protection circuit according to claim 1, further comprising: a transistor drive circuit that turns off the third transistor when the fourth transistor is turned on. 5.   The overcurrent protection circuit according to claim 1, wherein each of the first and second transistors is a bipolar transistor.   5. The overcurrent protection circuit according to claim 1, wherein each of the first and second transistors is a MOS transistor.   The transistor drive circuit is a comparison circuit that compares an operation voltage of the fourth transistor with a reference voltage and applies a voltage corresponding to the comparison result to a base of the third transistor. The overcurrent protection circuit according to any one of claims 4 to 6.

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