JPH0560708B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a current supply circuit, and more specifically, to electronic SLIC (Subscriber Line
This relates to a current supply circuit that has been improved to protect against ground faults and cross-contact, which are the biggest weaknesses of interface circuits.

[Conventional technology]

In recent years, subscriber interface circuits have been computerized, and the name SLIC is commonly used. Electronic interface circuits can be configured without using windings as in conventional circuits, which is highly effective in making it possible to downsize and increase packaging density.

On the other hand, since large amounts of power are handled, heat generation, overvoltage, etc.
It requires a circuit to prevent destruction due to overcurrent, so it is not necessarily an advantage.

FIG. 2 is a diagram showing an example of a conventional current supply circuit, in which 1 is a first current detection circuit, 2 is a second current detection circuit, 3 is a current comparison circuit, 4 and 5 are amplifiers, and 6
is a differential amplifier, 7 is a transfer function giving circuit with a transfer function of H, 8 is a telephone, 9 is a transfer function control circuit,
Control is performed by changing the value of the transfer function H or limiting the output of the bias power supply, which will be described later. 10 is a bias power supply, R ₁ , R ₂ , R ₃ , R ₄ , r ₁ , r ₂ are resistors, R ₅ , R ₆
is a line resistance, and Q ₁ and Q ₂ are first and second semiconductor elements, each of which is a transistor that supplies communication current to the telephone 8. Further, V ₀ is a power supply voltage, and is composed of, for example, earth ground (0V) and -48V.
Also, V ₀ ′ is 1/2 potential of the power supply voltage V ₀ , that is,
Indicates V ₀ /2=-24V.

Next, the operation of the conventional circuit will be explained.

First, the basic operation will be explained. Amplifiers 4, 5, differential amplifier 6, and transfer function providing circuit 7 constitute a feedback circuit via adders 12, 13, respectively, and an external bias power supply 10 generates a DC voltage (bias voltage) -V ₁ -24 is given to adders 12 and 13. By applying the DC voltage -V ₁ -24, a voltage V is generated between the terminals of the telephone 8, that is, between points A and B via the feedback circuit. In other words, earthly
GND → Transistor Q ₁ → Resistor r ₁ → Line resistance R ₅
→ Telephone 8 → Line resistance R ₆ → Resistor r ₂ → Transistor Q ₂ → Power supply circuit V ₀ Supplies communication current in the direction.
Differential amplifier 6 connects telephone 8 and line resistance R ₅ , R ₆
It operates with the voltage V generated across the terminal as input,
When R ₁ = R ₂ = R ₃ = R ₄ , -V is output.

When the resistance values of the line resistances R ₅ and R ₆ are small, the differential amplifier 6 makes the voltage V small, and the communication current is low even at the maximum.
A transfer function H is applied to the system by the transfer function applying circuit 7 so that the voltage becomes about 40 mA, and this -HV and the DC voltage -V ₁ of the bias power supply 10 are added by adders 12 and 13, and the output is sent to each amplifier. Return to 4 and 5. Further, when the line resistance value is large, the voltage V is made large and feedback is applied to the amplifiers 4 and 5 via the transfer function providing circuit 7 so as to secure a communication current of about 20 mA at the minimum. In this way, the communication current is controlled to be within a certain range.

This circuit operates based on -24V, so below:
The basic operation explanation is to express each potential using -24V as a reference potential (0V). DC voltage of bias power supply 10 with reference to earth GND −V ₁ −24 is −
Based on 24V standard, it is -V ₁ .

Here, if R ₁ =R ₂ =R ₃ =R ₄ , the output of the differential amplifier 6 becomes -V. The outputs of adder 12 and adder 13 become -(HV+V ₁ ). The emitter potential of transistor Q ₁ is (HV+V ₁ ), and the emitter potential of transistor Q ₂ is -(HV+V1).
becomes. Therefore, if r ₁ = r ₂ = R, then V = Z / 2R + Z・2 (HV + V ₁ ) ... (1) Therefore, V = 2Z / 2R + (1-2H) ZV ₁ ... (2) Therefore, I =V/Z=2/2R+(1-2H) _ZV1 ...(3) Here, Z is the total resistance of the telephone resistance and resistors _R5 and _R6 , and I is the communication current.

Also, the emitter potential of transistor Q ₁ is (HV + V ₁ ) = 2HZ/2R + (1-2H) ZV ₁ +V ₁ = 2R + Z/2R + (1-2H) ZV ₁ ...(4), and the emitter potential of transistor Q ₂ is The potential is −(HV+V ₁ )=−2R+Z/2R+(1−2H)ZV ₁ (5). Therefore, the potential difference V _EE between the emitter of transistor Q ₁ and the emitter of transistor Q ₂
becomes V _EE =2(2R+Z)/2R+(1-2H)ZV ₁ . Since Z is positive or O and R are positive, H
If is chosen to be positive, dV _EE /dZ>0, and V _EE is monotonically increasing with respect to Z. As explained above, by detecting V, adding a transfer function H to it, and applying feedback to the emitters of transistors Q ₁ and Q ₂ which are the supply sources of communication current,
The circuit operates in such a way that the potential difference between the emitters of transistors Q ₁ and Q ₂ increases as Z increases, that is, line resistances R ₅ and R ₆ increase. In other words, as the line resistances R ₅ and R ₆ increase, the communication current flowing through the telephone 8 decreases, but at the same time, the potential difference V _EE between the emitters of the transistors Q ₁ and Q ₂ increases. The circuit works to compensate for the decrease in passing current.

Next, to take actual values as an example, V ₁ = 9.8V,
If R = 244Ω and H = 0.326, when Z = 0Ω, I = 40.2mA, and when Z = 1400Ω, I = 20.1mA.

Furthermore, find the AC input impedance seen from points A and B.

Hv=-Hv-ir ₁ +v-ir ₂ (where v: AC signal) Therefore, the input impedance Zin is Zin=v/i=2r/1-2H [∵r=r ₁ = r ₂ ] It is possible to express the ideal input impedance.

The transfer function H is a parameter that determines the DC power source and input impedance.

The above is the basic operation, but next, the operation in case of ground fault or cross contact will be explained. Since SLIC is composed of electronic circuits, it will be destroyed if overcurrent flows, so protection against ground faults and cross-contact is required.

First, the operation at the time of a ground fault will be explained.

When a ground fault occurs, the potential at point A is GND (0V)
become. Then, an excessive current flows through resistor _r2 . This current value is detected by the second current detection circuit 2. On the other hand, the first current detection circuit 1 detects the current flowing through the resistor _r1 . The currents flowing through each resistor r ₁ and r ₂ are normally equal, but a difference occurs in the event of a ground fault or the like. This is monitored by the current comparison circuit 3. Even in the event of a ground fault, a small current must flow through transistor Q ₁ → resistor r ₁ → line resistance R ₅ → telephone 8 → line resistance R ₆ → resistor r ₂ → transistor Q ₂ . The reason for this is that when the transfer function control circuit 9 stops energization, the current flowing through each resistor r ₁ and r ₂ becomes 0, ground fault information is lost, and energization starts again, resulting in so-called racing. be. The transfer function control circuit 9 that obtains ground fault information changes the value of the transfer function H of the transfer function providing circuit 7 to suppress overcurrent.

Alternatively, V ₁ controls the direct current as seen from equation (3). This V ₁ is varied by the transfer function control circuit 9 to prevent overcurrent. For example, a switching element is provided in the transfer addend control circuit 9,
V ₁ can be controlled by turning it on and off.

In addition, even if there is common-mode noise induced in the line of the telephone 8, the difference in current flowing through each resistor r ₁ and r ₂ does not appear, so this differential detection method prevents false detection of hook information. .

However, for individual telephones such as PBXs and key telephones, the transmission distance is short and most of the cables are indoors, so there is no need to assume the excessive common-mode noise mentioned above. Therefore, for stand-alone telephones such as PBXs and key telephones, there is a demand for circuits that are simpler and have practically sufficient performance.

[Problem that the invention seeks to solve]

In this way, when a ground fault occurs, in order for the transfer function control circuit 9 to maintain its operation during the ground fault, it is necessary to apply a current to the telephone set 8 to an extent that other electronic circuits are not damaged. Therefore, there are problems such as the circuit scale becomes large and the structure is complicated, such as requiring a plurality of current detection circuits, current comparison circuits, and transfer function control circuits.

The present invention provides a current supply circuit with a circuit configuration that completely cuts off communication current in the event of a ground fault, thereby improving the conventional ground fault protection circuit including a plurality of current detection circuits, current comparison circuits, and transfer function control circuits. The purpose is to reduce the scale and reduce the price.

[Means for solving problems]

A current supply circuit according to the present invention includes a communication current supply source, a bias power supply, a first line connected to a telephone at one end, and a first resistor connected to the other end of the first line. , a first semiconductor element for supplying communication current to the telephone, which is provided between the first resistor and the communication current supply power source; a second line, one end of which is connected to the telephone; a second resistor connected to the other end; a second semiconductor element for supplying communication current to the telephone provided between the second resistor and the communication current supply power source; and the other end of the first line. The inverting input end of the third resistor is connected to the second line, and the non-inverting input end is connected to the other end of the second line via the fourth resistor, and between the inverting input end and the output end. a differential amplifier connected to a fifth resistor; a transfer function imparting circuit connected to the output end of the differential amplifier, imparting a transfer function imparting function according to the output thereof and outputting a corresponding transfer function imparting voltage; a first control means for controlling the first semiconductor element and the second semiconductor element respectively based on the summed output of the output voltage from the transfer function imparting circuit and the output voltage from the bias power supply; and a non-inverting input of the differential amplifier. A sixth resistor provided between the terminal and a predetermined potential point of the communication current supply power source compares the potential of the inverting input terminal of the differential amplifier with the reference potential, and a predetermined potential difference is generated. a comparator circuit that outputs ground fault information indicating that there is a ground fault condition;
and second control means for controlling the semiconductor element.

[Effect]

In this invention, when a ground fault occurs at the connection end of the telephone, the potential at the connection end of the telephone fluctuates with respect to the reference potential, so the comparator circuit detects this,
With this output, the third semiconductor element turns off the second semiconductor element, cutting off the current supply from the power supply circuit to the telephone.

〔Example〕

FIG. 1 is a circuit diagram showing one embodiment of the present invention. In this figure, _the same reference numerals as _those in _{FIG .}
It is a semiconductor device, and a transistor is a switching device.

Next, this operation will be explained using the case where the power supply voltage V ₀ =-48V as an example.

In this example, the differential amplifier 6 and the comparison circuit 1
For the power supply voltage of 1, the positive power supply is connected to GND, and the negative power supply is connected to V ₀ (-
48V). To obtain the best dynamic range of the differential amplifier 6, set the bias voltage to V ₀
It is desirable to reduce it to one-half of that. Therefore, the bias voltage of the differential amplifier 6 is set to -24V.

During normal operation, _{the current flowing through resistor r 1 and} the resistor
Since the currents i ₂ flowing through r ₂ are equal and it is determined that r ₁ =r ₂ , point A is always at a potential of -24V to -48V. Therefore, point C shows a voltage of -24V or less. Further, since the differential amplifier 6 is constituted by an operational amplifier, the points D are at the same potential. If point A has a ground fault, point C will be -
Since 24V is applied separately by resistors R ₃ and R ₄ , it shows a potential of -24V×R ₃ /(R ₄ +R ₃ ). This value is greater than -24V. When point B experiences a ground fault, two cases are possible. One is the total resistance value Z of the direct current resistance of telephone 8 and line resistance R ₅ and R ₆
is small and the potential at point C is greater than -24V. In this case, a ground fault is detected as described below. Another case is when Z is large and the potential at point C is -24V or less. In this case, no ground fault is detected and the ground fault current flows from point B → line resistance R ₅ → telephone 8 → line resistance R ₆ → resistor r ₂ → transistor Q ₂
However, since the above-mentioned total resistance value Z is large, the ground fault current is not large enough to destroy the transistor _Q2 , so there is no problem.

As is clear from the above, the potential at point C is -24 V or less and greater than -24 V during normal operation and during a ground fault, respectively. Here, in reality, the reference potential V ₂ is −24V
It is set to a slightly higher value. Further, since points C and D are at the same potential, a comparison circuit 11 with high input resistance monitors point D and outputs ground fault information. During normal operation, the comparator circuit 11 outputs -48V and turns on the transistor _Q3 , so that the transistor _Q2 becomes active.
That is, transistor Q ₁ → resistor r ₁ → line resistance
Current is supplied in the order of R ₅ → telephone 8 → line resistance R ₆ → resistor r ₂ → transistor Q ₂ . When point A is grounded, overcurrent will flow from point A to resistor r ₂ to transistor Q ₂ . On the other hand, points C and D are -24V
Since it becomes larger, the comparator circuit 11 outputs 0V,
Transistor Q ₃ is turned off so the transistor
Q ₂ is turned off. As a result, the route from point A to resistor r ₂ to transistor Q ₂ is cut off and the power supply is stopped.

Even if the power supply is stopped, if point A has a ground fault, point C
indicates a potential of −24V×R ₃ /(R ₄ +R ₃ ). That is, it is greater than -24V. Since this is the same potential as at the time of the ground fault before the power supply was stopped, the ground fault information is continuously outputted to the output of the comparator circuit 11. If the telephone 8 is on-hook after the power supply is stopped, the potential at point C becomes -24V since the transistor _Q2 is off when the ground fault is removed. Since the threshold value (reference potential V ₂ ) of the comparison circuit 11 is set to a value slightly higher than -24V, when the ground fault is removed, the output of the comparison circuit 11 becomes -48V and the transistor Q ₃ is turned on again. Start power supply.

As described above, power supply → ground fault → power supply stop → ground fault release → power supply is automatically performed. The same applies when there is contact.

Although the input terminal of the comparator circuit 11 is connected to the (-) input terminal (D) of the differential amplifier 6 in the embodiment, the same effect can be achieved even if the input terminal is configured to be taken out from the (+) input terminal (C). be able to.

〔Effect of the invention〕

As explained above, this invention completely cuts off the communication current supplied to the telephone in the event of a ground fault.
It has the advantage of reducing the scale of the ground fault protection circuit and lowering the cost, and is suitable for use in PBXs, key telephones, etc.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a ground fault protection circuit for a current supply circuit according to the present invention, and FIG. 2 is a circuit diagram showing an example of a conventional ground fault protection circuit for a current supply circuit. In the figure, 4 and 5 are amplifiers, 6 is a differential amplifier, 7 is a transfer function imparting circuit, 8 is a telephone, 11 is a comparison circuit, r ₁ , r ₂ , R ₁ to _{R 4} , R ₇ , R ₈ are resistors vessel, R ₅ , R ₆
is the line resistance, and Q ₁ , Q ₂ , and Q ₃ are transistors.

Claims (1)

[Claims] 1. A communication current supply source, a bias power supply, a first line connected to the telephone at one end, a first resistor connected to the other end of the first line, and the first resistor. a first semiconductor element for supplying a communication current to the telephone, which is provided between the communication current supply power source; a second line having one end connected to the telephone; and the other end of the second line. the second connected to the
a second semiconductor element for supplying a communication current to the telephone, which is provided between the second resistor and the communication current supply power source, and the first line;
An inverting input terminal is connected to the other end via a third resistor, and a non-inverting input terminal is connected to the other end of the second line via a fourth resistor, and the inverting input terminal and A differential amplifier with a fifth resistor connected between the output terminal and the differential amplifier, which is connected to the output terminal of the differential amplifier, is given a transfer function according to its output, and outputs a corresponding transfer function imparted voltage. a transfer function imparting circuit, and a first control for controlling the first semiconductor element and the second semiconductor element, respectively, based on the summed output of the output voltage from the transfer function imparting circuit and the output voltage from the bias power supply. means, a sixth resistor provided between the non-inverting input terminal of the differential amplifier and a predetermined potential point of the communication current supply power supply, and a potential of the inverting input terminal of the differential amplifier and a reference. a comparator circuit that outputs ground fault information indicating a ground fault state when a predetermined potential difference occurs by comparing the potentials; and second control means that turns off when the current supply circuit is turned off.