JPH0671296B2 - Overcurrent protection circuit - Google Patents

Description

[Detailed Description of the Invention] [Overview] Regarding an overcurrent protection circuit in a power supply circuit that supplies a call current to a subscriber line, an output voltage of an operational amplifier used in the power supply circuit is limited, and a B line ground fault occurs In order to limit the current to the power supply transistor in the case of
In a power supply circuit that includes an amplifier and supplies the call current to the subscriber line, the base of the transistor is grounded via a diode or a transistor, and the output voltage of the output stage circuit is limited to prevent overcurrent of the power supply circuit. To configure.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit in a power supply circuit that supplies a call current to a subscriber line.

In a conventional analog exchange, subscriber interface functions such as supply of a call current to a telephone, transmission of a read signal, and end-of-call monitoring are integrated in a common part such as a trunk and used commonly for all telephones. Also, in a digital exchange that uses an IC element or the like in the communication path that is the path of voice signals, a circuit for converting analog signals to digital signals is required for each telephone, so that individual subscriber interface functions can be provided. It is provided in a subscriber circuit provided for each subscriber.

The subscriber circuit is an all-electronic circuit composed of LSI, etc., and its functions include a current supply function that supplies the call current to the subscriber line, a current monitoring function that detects the on-hook and off-hook of the telephone, and a bell It has a calling signal sending function for ringing, an analog / digital conversion function, and the like.

Above all, operational amplifiers and transistors for power supply are used in the power supply circuit that constitutes the current supply function, so that even if a subscriber line fails, they will not be damaged or malfunction. It is necessary to design

2. Description of the Related Art FIG. 4 is a conventional power supply circuit diagram, and FIG. 5 is a circuit diagram showing a state where a B-line ground fault occurs.

Reference numeral 11 denotes an operational amplifier for power supply, to which + V _CC of +5 V is applied as a positive power supply voltage, and -V _{BB of} -48 V is applied as a negative power supply voltage. Transistor TR
Reference numeral 64 is a power supply transistor. The collector is connected to the ground (hereinafter referred to as GND), the base is connected to the operational amplifier 11, and the emitter is connected to the subscriber line B. A call current is supplied to the telephone set 62 via the B line of the subscriber line.

In such a power feeding circuit, when a B line ground fault of the subscriber line occurs, the state becomes as shown in FIG. Since the operational region of the operational amplifier 11 has a value slightly lower than the applied positive power supply voltage, a potential higher than GND is used for the positive power supply voltage. As a result, the output of the operational amplifier 11 is V _BEO (base
It becomes a voltage higher than GND by the voltage of (emitter voltage). Since a positive voltage is applied to the base and collector of the transistor TR64, an overcurrent flows from the output of the operational amplifier 211 to the GND connected to the collector of the transistor TR64.

Problems to be Solved by the Invention In the above-described conventional power supply circuit, when a B-line ground fault occurs, an overcurrent flows between the base and collector of the power supply transistor, possibly damaging the power supply transistor. There was a problem that there is. In order to prevent overcurrent, a method of connecting a high resistance between the output of the operational amplifier and the power supply transistor has been proposed, but it is very difficult to set the resistance value due to the balance of the driving ability of the transistor. However, there is a problem in that it is not suitable for application to LSI because the resistance value is considerably high even if it is set.

The present invention has been made in view of the above circumstances, and an object of the present invention is to limit the output voltage of an operational amplifier used in a power supply circuit to supply power when a B-line ground fault occurs. An object of the present invention is to provide an overcurrent protection circuit that limits current to a transistor.

Means for Solving the Problems FIG. 1 shows the principle of the present invention.

The output stage circuit 12 is provided with an operational amplifier 10 having a transistor TR30, and the base of the transistor TR30 is grounded via a diode D14 in a power supply circuit for supplying a call current to a subscriber line. Alternatively, instead of the diode D14, the base is grounded via a transistor TR16 whose collector is connected to the collector.

With this configuration, the output voltage of the output stage circuit 12 is limited to prevent overcurrent in the power feeding circuit.

Operation In a power supply circuit composed of R56, R58, R60, TR54 and operational amplifier 10, the operational amplifier
Transistor TR that constitutes output stage circuit 12 of amplifier 10
The voltage level applied to the base side of 30 is diode D1
It is less than the voltage across 4. Operational amplifier 1
The output voltage of 0 drops further than the voltage across the diode D14 due to the base-emitter voltage of the transistor TR30. This limits the output voltage of the operational amplifier 10 and prevents an overcurrent from flowing from the base of the power-supplying transistor TR54 to the collector when the earth-side B line connected to the telephone 62 is grounded.

Even when the transistor TR16 is connected to the output stage circuit 12,
The operation is similar to that when the diode D14 is connected.

Embodiment Hereinafter, an overcurrent protection circuit according to the present invention will be described in detail based on an embodiment shown in the drawings.

FIG. 2 shows a feeder circuit diagram to which the overcurrent protection circuit of the present invention is applied.

Reference numeral 10 is an operational amplifier for power supply, and the inside will be described below. The differential amplifier circuit of the differential input stage is composed of transistors TR18 and TR20, and its output is converted into a single output by a current mirror circuit composed of transistors TR22 and TR24. Transistor TR26, TR
28 constitutes a high gain amplification stage, and amplifies the output from the current mirror circuit. Reference numeral 12 denotes an output stage circuit, which is composed of transistors TR30 and TR32, and transistors TR34 and TR36 form a bias voltage generation circuit for stabilizing the bias of the output stage circuit 12. Transistor
A diode D14, one end of which is connected to GND, is connected to the base side of TR30 to limit the voltage at point a.

Further, 46 is a current source, and transistors TR38, TR40, TR4
2. A current mirror circuit is formed by TR44, and a constant current flows to the emitters of the transistors TR18 and TR20 of the differential amplifier circuit. Positive power supply voltage of + V _CC is resistor R
It is applied to the transistor TR38 via the resistor 48, to the transistor TR40 via the resistor R50, and to the transistor TR42 via the resistor R52. For example, the positive power supply voltage of + V _CC is + 5V and the negative power supply voltage of -V _BB is -48V.

Transistor TR54 is a power supply transistor. The collector is GND and the base is operational amplifier 1.
0, the emitter is connected to the B line of the subscriber line via a resistor R56. The telephone line 62 is connected to the subscriber line B line, and the subscriber line A line is connected to the other end of the telephone line 62.

The power supply circuit of this embodiment is an operational amplifier 10,
The transistor TR54 and the resistors R56, R58, and R60 form a constant resistance power supply circuit, and the call current I flowing to the B line is given by the call current I = (Va, Vb ) / R56.

The voltage V _{OO at the} point a to which the diode D14 is connected is equal to or lower than the voltage V _FOO across the diode D14, and V _FOO is about 0.6V to 0.7V in the case of a silicon diode. If the base-emitter voltage of the transistor TR30 is V _BEOO ,
The output voltage V of the operational amplifier 10 is V ≦ V _OO −V _BEOO . The voltage across diode D14, V _FOO, and transistor T
If the transistor TR30 and the diode D14 are set so that the base-emitter voltage V _BEOO of R30 is the same,
The output voltage of the operational amplifier 10 can be limited to 0V or less.

As a result, even if a B-line ground fault occurs, the operational
Since the output voltage of the amplifier 10 does not rise above GND, a positive voltage is not applied in the base-collector direction of the power-supplying transistor TR54, and an overcurrent can be prevented from flowing.

FIG. 3 is a circuit diagram in which a part of a power supply circuit to which an overcurrent protection circuit using transistors is applied is omitted.
Instead of the diode D14 of the illustrated embodiment, a transistor T
Since R16 is used and other components and circuits are the same as those in FIG. 2, the description thereof will be omitted.

The transistor TR16 uses an npn transistor to connect the emitter to GND and the base to the collector to utilize the base-emitter voltage. This voltage is about 0.6 to 0.7 V in the case of silicon type. If the base-emitter voltage of the transistor TR16 and the transistor TR30 are set to be the same, the output voltage from the operational amplifier 10 can be limited to 0V or less.

Since the overcurrent protection circuit of the present invention is configured as described above in detail, it becomes easy to limit the output voltage of the operational amplifier, and when a B-line ground fault occurs, a power supply transistor or the like due to overcurrent is generated. Can prevent damage to the LSI
The effect is that it can be applied to an operational amplifier formed inside.

[Brief description of drawings]

FIG. 1 is a principle diagram of the present invention, FIG. 2 is a power supply circuit diagram to which the overcurrent protection circuit of the present invention is applied, and FIG. 3 is a circuit diagram in which a transistor is used instead of the diode used in the embodiment of FIG. FIG. 4 is a power supply circuit diagram to which a current protection circuit is applied, FIG. 4 is a conventional power supply circuit diagram, and FIG. 5 is a circuit diagram showing a state in which a B line ground fault occurs. 10, 11 …… Operational amplifier, 12 …… Output stage circuit, D14 …… Diode, TR16, TR30, TR54 …… Transistor, 46 …… Current source, R56, R58, R60 …… Resistor, 62 …… Telephone.

Claims (1)

[Claims] 1. A transistor (TR30) in the output stage circuit (12).
In an electric power supply circuit that includes an operational amplifier (10) having a transistor (TR), the base of the transistor (TR30) is connected to a diode (D1
4) An overcurrent protection circuit that prevents overcurrent in the power supply circuit by limiting the output voltage of the output stage circuit (12) by grounding it via a transistor (TR16).