JP2001022456A - Current limiting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively and easily provide a stable current limiting function without depending on an operation environment temperature. SOLUTION: A current limiting circuit 30 limiting current so that current flowing in a current detection resistor 36 becomes equal to voltage between the base and the emitter of a transistor 37. The emitter electrode and the collector electrode of a different transistor 38 are connected to both ends of the current detection resistor 36 and the temperature characteristic of on-voltage between the base and the emitter of the current limiting transistor 37 is compensated by using the temperature characteristic of the current amplification rate of the transistor 38.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a current limiting circuit, and more particularly to a current limiting circuit for preventing an overcurrent in a voltage stabilizing circuit (regulator) or the like.

[0002]

2. Description of the Related Art Current limiting circuits have various uses.
A common application is to limit the current in a power supply (or power) line to a constant value to prevent damage to electronic devices / circuit elements due to overcurrent or overload current. It has a function of opening the circuit when such a current value reaches a predetermined limit level. It is preferable that such a limited current value is a constant value regardless of the operating environment temperature.

In order to meet such a demand, for example, Japanese Patent Laid-Open No.
Japanese Patent Publication No. 186909 proposes a circuit for controlling a base current of a transistor that opens and closes a line using a differential amplifier. This circuit is shown in FIG. This circuit is
Transistor Tr1 connected in series between input voltage source Vi and output voltage source Vo, parallel transistor Tr2 connected between input voltage source Vi and ground, and both transistors Tr
1, a control transistor Tr connected to the base of Tr2
Contains three. The output voltage Vo is controlled by the main control circuit 1 including the voltage dividing resistors R1 and R2, the reference voltage source 5 and the differential amplifier 3 connected in series between the voltage Vo and the ground so that the output voltage Vo becomes constant. Controls the base of Tr3. Thereby, both transistors Tr1, Tr
2 base.

The transistors Tr1 and Tr2 have their emitters and bases connected in parallel.
When the emitter current of the transistor Tr1 is Io by the parameter setting of 2, the transistor Tr2 has Io
/ N flows. A resistor R3 is connected between the collector of the transistor Tr2 and the ground, and a voltage drop (R3
× Io / N) and the reference voltage Vref of the reference voltage source 5 are provided to another differential amplifier 6, and the output of the current limiting circuit 2 is provided together with the output of the differential amplifier 3 and the output of the current source 4. Input to the base of the control transistor Tr3. With this configuration, the main control circuit 1 controls the output voltage Vo to a predetermined value and controls the base of the control transistor Tr3 at the time of overload, for example, an overcurrent such as a short circuit. This is kept within the limit value.

Japanese Patent Application Laid-Open No. 2-202208 discloses a current limiting circuit as shown in FIG.
That is, the input is applied to the collectors of the Darlington-connected transistors Q2 and Q3, and the emitter of the transistor Q3 is connected to the resistor R.
6. Connect the output via 6. The current source I1 is connected to the base of the transistor Q2 and the collector of another transistor Q1, and the emitter of the transistor Q1 is connected to the output. The base of the transistor Q1 is connected to a current source I2 via a resistor R4 and further to an output via a resistor R5. The current source I2 is connected to the diode D and the main current path of the transistor Q4, and the base of the transistor Q4 is connected to a common connection point between the emitter of the transistor Q3 and the resistor R6.

In this current limiting circuit, when the output current reaches a predetermined limit value by a current detecting resistor R6 connected in series between the input and the output, the transistors Q2 and Q3 are controlled in a non-conducting direction, whereby the output current is reduced. Restrict.

Further, Japanese Patent Application Laid-Open No. 7-239723 discloses a constant voltage power supply having a load current detecting resistor as shown in FIGS. 11 (A) and 11 (B). FIG. 11A shows a basic circuit, and FIG. 11B shows an improved circuit thereof.
First, referring to FIG. 11A, an input voltage source 10 of a voltage V1 is a power supply circuit for supplying an output voltage Vo and a load current Io to a load resistor 17, and a current detection resistor 13 and a P-channel Field effect transistor (FET) 1
6 are connected. The connection point voltage of the series resistors 14 and 15 connected between the source and the ground is applied to the gate of the FET 16. Further, the emitter and base of the control transistor 12 and the resistor 18 are connected to both ends of the current detecting resistor 13.
And the collector current Ic of the transistor 12
Is input to the gate of the FET 16 which is the connection point between the resistors 14 and 15 described above. With this configuration, when the load current Io is equal to or less than the limit value, that is, when the control transistor 12 is turned off, the gate voltage of the FET 16 is controlled by the input voltage V1, and the source-drain voltage corresponding to the input voltage V1 is adjusted. The output voltage Vo is changed to a substantially constant value. However, when the load current Io reaches the limit value, the control current Ic of the transistor 12 increases the voltage drop of the FET 16 or sets the FET 16 to the off state to limit the load current.

The circuit of FIG. 11B is intended to prevent or improve the fluctuation of the current limit point (drop point) due to the environmental temperature. Therefore, the emitter-base of the transistor 12 and the resistor 21
And a series circuit of a diode-connected transistor 20. Further, the base and collector of the transistor 20 are grounded via the resistors 22 and 23 and the Zener diode DZ. With such a configuration, the base of the transistor 12
The temperature dependence of the emitter junction is determined by the same transistor 2
By compensating for the temperature dependence of the base-emitter junction of 0, the above-mentioned fluctuation of the droop point is suppressed to a substantially constant value.

[0009]

However, in the above-mentioned prior art, a differential amplifier, a constant current source, and the like are required.
The circuit becomes complicated and expensive, and the power consumption increases. Furthermore, there is a problem that the number of required circuit elements is relatively large.

An object of the present invention is to provide a small and inexpensive current limiting circuit which requires a relatively small number of additional devices and has a substantially constant limiting current value regardless of the operating environment temperature. To provide.

[0011]

To solve the above-mentioned problems, the current limiting circuit according to the present invention employs the following characteristic configuration.

(1) A main current path of a current detecting resistor FET is connected in series between an input terminal and an output terminal, and an input side voltage of the FET is connected to a control electrode of the FET via a voltage dividing resistor. In a current limiting circuit for connecting a collector of a first transistor having an emitter and a base connected to both ends of a detection resistor to a control electrode of the FET, a second transistor for passing a base current proportional to an input voltage of the input terminal is provided. The collector current of the two transistors is
Current limiting circuit to supply to the input side of ET.

(2) The emitter of the second transistor
The current limiting circuit according to the above (1), wherein a series circuit of a base junction and a base resistor is connected in parallel with an input voltage of the input terminal.

(3) The current limiting circuit according to (1), wherein a switching transistor is connected to one end of the voltage dividing resistor, and the FET is turned on / off.

(4) The current limiting circuit according to (3), wherein a constant voltage element is connected in series to the emitter of the switching transistor.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a power supply limiting circuit according to the present invention will be described below in detail with reference to FIGS.

FIG. 1 is a circuit diagram of a first embodiment of a current limiting circuit according to the present invention. This current limiting circuit 30
Is an input terminal 3 connected to an input DC power supply (not shown).
1. It includes an output terminal 32 connected to a load (not shown), a control FET 33 and a current detection resistor 36 connected in series between the input terminal 31 and the output terminal 32. Further, resistors 34 and 35 are connected between the source of the FET 33 and the ground.
Are connected in series, and the gate of the FET 33
It is connected to connection points 4 and 35. The emitter of the first transistor 37 is connected to both ends of the current detection resistor 36.
The base is connected, and the collector is connected to the gate of FET33. Further, the input terminal 31 is connected to an emitter of the second transistor 38 and a series circuit of resistors 39 and 40. The base of the transistor 38 is a resistor 3
9 and 40, and the collector is connected to FET3.
3 to complete the circuit.

The current limiting circuit 30 shown in FIG.
When the voltage divided by the resistors 34 and 35 exceeds the gate-source ON voltage of the FET 33, the FET 33 is turned on, and the output voltage is supplied to the output terminal 32. The load current of the load connected to the output terminal 32 is the sum of I1 flowing through the current detection resistor 36 and the collector current I2 of the transistor 38.

Next, the current limiting operation of the current limiting circuit 30 of FIG. 1 will be described. Current I1 flowing through current detection resistor 36
When the load is normal, the voltage drop (I1 × R36) caused by the load is equal to or less than the ON voltage of the transistor 37, so that the transistor 37 is in a non-operating state (OFF). However, when the above-mentioned voltage drop reaches an overload state in which the transistor 37 is turned on, when the transistor 37 is turned on, this collector current flows into the resistor 35, and accordingly, the current flowing through the resistor 34 decreases, and the FE
It acts to reduce the gate-source voltage (bias voltage) of T33. On the other hand, considering the collector current of the transistor 38, the input voltage input to the input terminal 31 is equal to the base-emitter voltage (VB
E) Assuming that the input voltage is sufficiently larger than
And the current amplification factor β of the transistor 38.
Is determined by the product of

As described above, the current limiting function of the current control circuit 30 is determined by the base-emitter ON voltage of the current detection resistor 36 and the transistor 37.
As is well known, the base-emitter ON voltage 7 has a negative characteristic depending on the operating environment temperature. FIG. 2 shows the temperature characteristics of the base-emitter ON voltage of the transistor 37. FIG. 3 shows the current limiting characteristics in this case. this is,
This indicates that the current limit value decreases as the temperature increases. On the other hand, the collector current I2 of the transistor 38 is determined by the current amplification factor β of the transistor 38 as described above,
It has a positive characteristic with respect to the operating environment temperature. That is, the current amplification factor β of the transistor 38 has a temperature characteristic as shown in FIG.
2 is a value as shown in FIG. When the collector current I2 of the transistor 38 is supplied to the source of the FET 33,
The current I2 flows through the source and the drain of the FET 33 together with the current I1 flowing through the current detection resistor 36 and becomes a load current connected to the output terminal 32. Therefore,
When the two currents I1 and I2 having the temperature characteristics of the negative characteristics as shown in FIG. 3 and the positive characteristics as shown in FIG. 5 are added, the currents become substantially constant as shown in FIG. 6, and the temperature characteristics can be canceled or compensated.

In other words, the current I1 flowing through the current detection resistor 36 and the collector current I2 of the transistor 38 are added at an appropriate ratio, and the current I1 flows through the FET 33.
It can be understood that the current limiting characteristic of the current limiting circuit 30 can eliminate the temperature dependency.

[0022]

The first (or preferred) embodiment of the current limiting circuit according to the present invention has been described above with reference to FIGS. However, the present invention is not limited to only the specific embodiment, and various modifications and changes are possible. For example, FIG. 7 shows a current limiting circuit 30 'according to a second embodiment of the present invention. Note that most of the components are the same as those of the current limiting circuit 30 of FIG. 1, and therefore, the same reference numerals are given for convenience, and different points will be mainly described below.

The current limiting circuit 30 'shown in FIG. 7 is characterized in that a transistor 41 for turning on / off the FET 33 is added instead of grounding the lower ends of the resistors 35 and 40. That is, the lower ends of the resistors 35 and 40 are connected to the collector of the transistor 41, and the emitter is grounded. A control signal input terminal 42 is connected to the base of the transistor 41.
Is provided. When a sufficiently positive voltage control signal is input to the control signal input terminal 42, the transistor 41 is turned on,
Since the lower ends of the resistors 35 and 40 are substantially grounded, the circuit operates similarly to the current limiting circuit 30 of FIG. However, when such a control input signal is removed, the FET 33 is turned off, so that the output terminal 32 is electrically isolated from the input terminal 31. Thereby, on / off function (switching function)
Is obtained.

8 shows a current limiting circuit 30 "according to a third embodiment of the present invention. This current limiting circuit 30"
Is similar to the current limiting circuit 30 'of FIG. 7, but differs in that a Zener diode 43 is connected in series to the emitter of the on / off control transistor 41. Thereby, the control input voltage level input to the control signal input terminal 42 can be freely set, and the voltage at the lower ends of the resistors 35 and 40 when the transistor 41 is on is determined by the constant voltage of the zener diode 43. It can be set to a value.

Although not shown, the additional transistor 41 has an N
Instead of a PN transistor, a PNP transistor may be used. Further, the FET 33 may be an N-channel FET instead of a P-channel FET.

The preferred embodiments of the current limiting circuit according to the present invention have been described above. However, it will be easily understood by those skilled in the art that various modifications can be made without departing from the gist of the present invention.

[0027]

As is apparent from the above description, according to the current limiting circuit of the present invention, the additional transistor is provided in parallel with the transistor connected in parallel to the current detecting resistor, and the collector current depends on the current amplification of the transistor. With this additional transistor, a current limiting circuit having a stable current limiting characteristic can be obtained with a very simple circuit configuration for compensating for the temperature characteristic of the base-emitter ON voltage of the transistor.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment of a current limiting circuit according to the present invention.

FIG. 2 is a temperature characteristic diagram of a base-emitter on-voltage (VBE) of a transistor.

FIG. 3 is a temperature characteristic diagram of a current limit value according to the characteristic of FIG. 2;

FIG. 4 is a temperature characteristic diagram of a current amplification factor (β) of a transistor.

FIG. 5 is a temperature characteristic diagram of the collector current of the transistor having the characteristic of FIG.

FIG. 6 is a temperature characteristic diagram of a current control value of the current control circuit of FIG. 1;

FIG. 7 is a circuit diagram of a second embodiment of the current limiting circuit according to the present invention.

FIG. 8 is a circuit diagram of a third embodiment of the current limiting circuit according to the present invention.

FIG. 9 is a circuit diagram of a first example of a conventional current limiting circuit.

FIG. 10 is a circuit diagram of a second example of a conventional current limiting circuit.

FIG. 11 is a circuit diagram of a third example of a conventional current limiting circuit.

[Explanation of symbols]

30, 30 ', 30 "current limiting circuit 31 input terminal 32 output terminal 33 FET (field effect transistor) 36 current detecting resistor 34, 35 voltage dividing resistor 37 first transistor 38 second transistor 40 base resistor 41 switching transistor 43 zener Dao Iod (constant voltage element)

Claims (4)

[Claims] A current detection resistor and an F are connected between an input terminal and an output terminal.
The main current path of the ET is connected in series, the input side voltage of the FET is connected to the control electrode of the FET via a voltage dividing resistor, and the collector of a first transistor having an emitter and a base connected to both ends of the current detecting resistor. In a current limiting circuit connected to a control electrode of the FET, a second transistor for flowing a base current proportional to an input voltage of the input terminal is provided, and a collector current of the second transistor is set to the FET.
Current limiting circuit for supplying the current to the input side of the current limiter. 2. The current limiting circuit according to claim 1, wherein a series circuit of an emitter-base junction of said second transistor and a base resistor is connected in parallel with an input voltage of said input terminal. 3. The current limiting circuit according to claim 1, wherein a switching transistor is connected to one end of the voltage dividing resistor to control on / off of the FET. 4. The current limiting circuit according to claim 3, wherein a constant voltage element is connected in series to an emitter of said switching transistor.