JPS61265608A - Current limiting circuit - Google Patents

Abstract

PURPOSE:To reduce the power loss, and also to improve the temperature characteristic by forming a circuit by a main transistor, a detecting resistor, a comparing circuit, a reference voltage generating resistor, etc., of a Darlington connection. CONSTITUTION:One end of a detecting resistor RS is connected to an emitter of a main transistor TRPTR, and between the other end and a base of the TRPTR, a constant-current output circuit 1 and a comparing circuit 2 are connected in parallel, respectively. Also, an output terminal of the output circuit 1 is connected to the other end of the resistor RS through a reference voltage generating resistor R3. In this state, a reference voltage V0 generated in the resistor R3 and a detected voltage VS generated in the resistor RS are compared by the comparing circuit 2, and an emitter current is limited so as to be constant by controlling a base current of the TRPTR. In this way, a power loss in the detecting resistor RS can be reduced, and also the temperature characteristic can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field to Which the Invention Pertains] The present invention relates to the configuration of a current limiting circuit that limits the output current of an output transistor to a constant value.

[Prior art and its problems]

As a conventional current limiting circuit, the one shown in Fig. g5 is known. That is, a detection resistor FL is connected to the emitter of the main transistor PTr, and a plurality of diodes D1eD2*03 are connected in series between the other end of the detection resistor Ps and the base of the main transistor PTr. , by keeping the base voltage of the main transistor constant, the main transistor P? It was configured to limit the emitter current of r to a constant value. B in the figure.

C and E indicate the terminals of this circuit, terminal B is connected to the base load and the positive terminal of the power supply, terminal C is connected to the load and the positive terminal of the power supply, and terminal E is connected to the negative terminal of the power supply. be done.

In the circuit shown in FIG. 5, the voltage generated across the detection resistor island is V, = 0.6 V, so the power loss here is large. -Male face direction voltage■ Since F is used as the voltage generation source, if the transistor is made of silicon, the temperature characteristic will be approximately 2K, so the temperature characteristic as a current limit of the main transistor PTr will be 2fnV0.6■. t [value. As described above, conventional circuits have disadvantages such as poor temperature characteristics and high power loss.

[Purpose of the invention]

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide a current limiting circuit in which the power loss in the detection resistor is small and the limiting current is not affected by temperature and a stable output current can be obtained.

[The highlight of invention]

In order to achieve the above object, in this invention, a comparison circuit and a constant current output circuit are respectively connected in parallel to a circuit in which a Darlington-connected main transistor and a detection resistor for detecting the emitter current of the main transistor are connected in series. A reference voltage generating resistor is connected between the positive input terminal of the comparator circuit and the terminal of the detection resistor on the side not connected to the emitter of the main transistor to configure a current limiting circuit, and the current flows through the reference voltage resistor. A comparison circuit compares a reference voltage generated by the current with a detection voltage generated when the emitter current of the main transistor flows through the detection resistor, and the base current of the main transistor is controlled by the output of the comparison circuit. This limits the emitter current.

[Embodiments of the invention]

FIG. 1 shows an electric circuit of a current limiting circuit according to an embodiment of the present invention, and the same components as in the conventional circuit (FIG. 5) are given the same reference numerals and their explanations will be omitted.

In this circuit, one end of the detection resistor RB is connected to the emitter side of the main transistor pta, and the other end and the main transistor p'
A constant 1jL output circuit 1 and a comparison circuit 2 are connected in parallel between the base of rn and the output terminal of the constant current output circuit is connected to a detection resistor R8 via a reference voltage generating resistor R3.
By connecting to the other end of the reference voltage generating resistor R3
The group that occurs in! The comparison circuit 2 compares the S voltage Vo and the detection voltage 8 generated in the detection resistor R, and the comparison circuit 2
The emitter current is kept constant by controlling the base current of the main transistor using the output of the main transistor.

With this configuration, the voltage generated across the detection resistor R8 is low (in principle), so that the power loss in the detection resistor R can be reduced.

Furthermore, the temperature characteristics of the current limit of the main transistor are also improved as described below. That is, the reference voltage v generated at the reference voltage generating resistor R3.

And the variation Δvo of vo per 1°C is determined by the following formula.

Here, T: Absolute temperature ■r: From the base-emitter voltage of the main transistor to j
This is the value obtained by subtracting the voltage between the base and emitter of the transistor Q1 of the gl, that is, the voltage corresponding to one diode junction. Approximately 2 fF! ■q: Electron charge amount: Boltzmann constant I (t: Emitter current of the first transistor Icz: i
E2 transistor emitter current 5th according to prior art
In the circuit shown in the figure, temperature fluctuations in vr directly result in fluctuations in the limiting current, but V, 7) Temperature fluctuations. ge
(VP+V・)R・C2 is extremely reduced due to the noise, and temperature fluctuations are improved. To explain the temperature characteristic diagram shown in FIG. 4, the horizontal axis is the temperature in degrees Celsius, and the vertical axis is the fluctuation rate of limiting current (%), where a is the temperature characteristic of the circuit according to the conventional technology, and b is the temperature characteristic according to an embodiment of the present invention. Represents temperature characteristics.

The temperature characteristics determined by Akira Kinei are determined by the above-mentioned equation (2), but here, in the constant current output circuit 1 shown in Fig. 2, the emitter current of the first diode Qt (rat) and the emitter current of the second diode Q2 Since it is IC2, when the absolute value of Vo is set to 200 to 5QQmV in the above-mentioned equation (1), Δvo shows a positive value, albeit slightly. That is, the temperature characteristic according to the present invention in FIG. 4 is the temperature characteristic a of the conventional circuit.
Although this is a significant improvement compared to , there is still some temperature fluctuation. In order to further improve this and obtain the ideal temperature characteristic shown in the failure measurement C, the correction was made more efficient by using a resistor as shown in FIG. 2, which is another embodiment of the present invention. It is a limiting circuit. That is, the first transistor Ql of the current output circuit l in the current limiting circuit shown in FIG.
The collector of the output transistor PTr is connected through the correction resistor R to the connection point between the emitter of the output transistor PTr and the detection resistor Rs. When the correction resistor R[ is added in this way, the above equation (2) becomes the following equation, that is, a negative correction is performed depending on the temperature characteristic of vF.

FIG. 3 is an electrical circuit diagram of a current limiting circuit which is still another embodiment of the present invention, in which the correction resistor R4 is incorporated into the comparator circuit 2. In FIG. That is, transistors Q4, Q
i and a fixed resistor Rc, and its output is passed through a current mirror circuit composed of transistors Qy and Qs to control output transistors Qe and QIG.
Transistor Q4 when compared by the temperature characteristics of transistor Q9 and correction resistors R and f
, Qs gives temperature characteristics to the current ratio.

This temperature characteristic can be expressed by the following formula: % By arbitrarily setting Rf and Rc in the second term of the equation (4), the second term can be made negative according to the temperature characteristic (2). This allows negative correction to be made to the temperature characteristics of the current limit of this circuit.

〔Effect of the invention〕

In this invention, a comparator circuit and a constant current output circuit are each connected in parallel to a circuit in which the emitter of the main transistor and a detection resistor are connected in series, and between the positive output terminal of the comparator circuit and the free terminal of the detection resistor. Since the reference voltage generating resistor is provided, the detection voltage can be set from about 100 mV compared to the conventional method, and the power consumption in the detection resistor can be extremely reduced. In addition, in the conventional technology, the temperature fluctuation between the base and emitter of the main transistor and the temperature fluctuation in VF caused an immediate fluctuation in the limiting current, but with this circuit configuration, the temperature fluctuation in vr becomes Joge VF ~, so it becomes an extremely small fluctuation and the temperature Characteristics have been improved. Further, by providing a correction resistor, the temperature characteristics can be further corrected.

[Brief explanation of drawings]

FIG. 1 is an electrical circuit diagram of a current limiting circuit which is an embodiment of the present invention, FIG. 2 is an electrical circuit diagram of a current limiting circuit which is another embodiment of the invention, and FIG. FIG. 4 is a temperature characteristic diagram of the current limiting circuit, and FIG. 5 is an electrical circuit diagram of the current limiting circuit according to the prior art. 1: Constant current output circuit, 2: Comparison circuit, R3: Reference voltage generation resistor, Rf: Correction resistor, R8: Detection resistor, PT
, : Main transistor.

Claims (1)

[Claims] 1) A comparator circuit and a constant current output circuit are each connected in parallel to a circuit in which a Darlington-connected main transistor and a detection resistor for detecting the emitter current of the main transistor are connected in series, and A circuit is constructed by connecting a reference voltage generating resistor between the positive input terminal of the comparator circuit and the terminal of the detection resistor that is not connected to the emitter of the main transistor, and the current flowing through the reference voltage generating resistor. A comparison circuit compares a reference voltage generated by the main transistor with a detection voltage generated when the emitter current of the main transistor flows through the detection resistor, and the output of the comparison circuit controls the base current of the main transistor. A current limiting circuit characterized by limiting emitter current. 2) In the current limiting circuit according to claim 1, a temperature compensation resistor is provided between the connection point between the emitter of the main transistor and the detection resistor, the negative input side terminal of the comparator circuit, and the constant current output circuit. A current limiting circuit characterized by having a body. 3) A current limiting circuit according to claim 2, characterized in that a temperature compensation resistor is incorporated into the comparator circuit. 4) In the current limiter according to any one of claims 1 to 3, the constant current output circuit
The base and collector of the first transistor are connected to the base of the second transistor, and the first resistor is connected to the sensing resistor at this connection point and the emitter of the main transistor. The second resistor is disposed between the emitter of the second transistor and the connection point between the emitter of the first transistor and the base of the main transistor. A current limiting circuit characterized by having a configuration.