JPH0247765B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a current supply circuit, and more particularly to a communication current supply circuit for a telephone subscriber line.

Conventionally, the circuit for supplying DC talking current from the line circuit of a telephone exchange to a telephone set was as shown in Fig. 1.
It consists of a DC voltage E and an internal resistance R, and the transformer T
A current I _L is supplied to the telephone TEL in the direction of the arrow through the primary windings W ₁ and W ₁ '. In the figure, R _L
is the equivalent resistance of the subscriber line, C is a capacitor for AC short-circuiting the internal resistance R, and W ₂ and W ₂ ' are the secondary windings of the transformer.

The internal resistance R of the above current supply circuit is the line resistance
This is to prevent the communication current from becoming excessive when R _L is small, and has a fixed value determined by the system specifications. Here, if the DC resistance of the primary windings W ₁ and W ₁ ' of the transformer and the telephone TEL are ignored, the circuit of FIG. 1 becomes equivalent to the circuit of FIG. 2 in terms of DC.

From the circuit diagram shown in Figure 2, the magnitude of the DC talking current I _L is expressed by the following equation (1), and the relationship between I _L and R _L is expressed as
It will look like the solid line in the figure.

I _L = E/R + R _L ...(1) In the existing system, R = 440Ω and E = -48V, so if the line resistance R _L is set to zero, I _L = 0.11 A, and the power consumption due to the internal resistance R is It becomes about 5.3W.

In other words, in the conventional DC current supply circuit described above, power is wasted in the internal resistance R, and this power consumption becomes several watts if the line resistance R _L is small.
The heat generated by this makes it difficult to downsize the exchange.

As a solution to the above problem, the communication current supply circuit consisting of the power source E and the resistor R is replaced with, for example, a constant current circuit, and the DC current I _L flows constantly regardless of the line resistance R _L as shown by the broken line in Fig. 3. There is a plan to do so. However, this proposal cannot be applied to current telephone systems for the following reasons.

In other words, the telephone is equipped with a call loss compensation circuit to equalize the voice call level, and this circuit operates according to the size of the resistance (R + R _L ).
When R _L is small, an attenuator for the audio signal is inserted, and when R _L is large, the attenuator is removed. In this case, the magnitude of the line resistance can be determined by the magnitude of the DC talking current I _L flowing through the line, so as mentioned above,
If I _L is set to a constant current, the magnitude of the resistance cannot be determined, and the problem arises that the call loss compensation function cannot be achieved.

The present invention has been made to address the above-mentioned reasons, and aims to provide a novel call current supply circuit that is compatible with the current telephone system, solves the problem of heat generation, and allows the switching equipment to be made smaller. .

The communication current supply circuit according to the present invention includes a voltage source that generates a predetermined DC voltage E, and a voltage source that is connected to the voltage source and converts the DC voltage E into a voltage according to a duty.
a first circuit that converts E into ₀ and applies it to the subscriber line;
It is characterized by comprising a second circuit that detects the DC talk current _IL flowing through the subscriber line and controls the duty of the first circuit so that E-E ₀ is proportional to the talk current _IL .

That is, from equation (1), R _L・I _L (=E ₀ )=E−R・I _L …(2) In other words, since E−E ₀ =R・I _L (E−
If E ₀ ) is controlled to be proportional to I _L , it is possible to supply a DC talking current I _L whose value depends on the subscriber line resistance R _L as in the conventional circuit shown in FIG. 1, and (E − E ₀ )
The desired purpose can be achieved by generating this with a switching regulator that consumes little power, such as a switching regulator. In this case, the proportionality constant R represents the equivalent internal resistance.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 4 shows a communication current supply circuit according to the present invention connected to a subscriber line shown in an equivalent circuit.
constitutes a switching regulator that stores energy in the inductor 12 during the conduction period T _ON of the transistor 10, and releases it to the load side, that is, the subscriber line side, during the cut-off period T _OFF of the transistor 10.

Here, if the output voltage of the first circuit, that is, the terminal voltage of the capacitor 13 is E ₀ and the DC power supply voltage is E, then the accumulated energy of the inductor 12 (E-
Since E ₀ )・I _L・T _ON is equal to the released energy E ₀・I _L・T _OFF , E ₀ /E=T _ON /T _ON +T _OFF =T _ON /T …(3) However, T=T A relationship of _ON + T _OFF is established, and the output voltage E ₀ is proportional to the duty of the conduction time of the transistor 10.

In the present invention, the first circuit is controlled as follows. When rewritten as equation (2), E ₀ /E=1−R・I _L /E (4) is obtained.

Therefore, from equations (3) and (4) above, the necessary conditions for operating the first circuit as a talking current supply circuit are T _ON /T=1-R・I _L /E (5) It is.

In FIG. 4, a second circuit B consisting of transistors 14 to 17, a resistive element 18, a constant current circuit 19, and a sawtooth voltage generating circuit 20 is used to control the duty of the first circuit A according to the above-mentioned conditions. belongs to.

Transistor 15 and transistor 14 connected in series to the subscriber line constitute a current mirror circuit, and collector current I _C of transistor 15 is proportional to DC talk current I _L .

Here, if the proportionality constant of both is k and the resistance value of the resistance element 18 is R ₁₈ , then the collector potential V _t of the transistor 15 is V _t =E−k・I _L・R ₁₈ (6), and the above resistance If the value R ₁₈ is chosen to be 1/k of the design target value of the internal resistance R in the conventional circuit, for example 440Ω, then V _t =E−R・I _L …(7) or V _t /E=1−R・I _L /E...(8) holds true and matches the right side of equation (5). Therefore,
Transistor 10 of the first circuit in the ratio of V _t and E
If the conduction period of is determined, then in the talking current generating circuit of FIG. 4 according to the present invention, the relationship between the current voltage E and the output voltage _E0 satisfies the equation (4), and E- _E0 = R・I _L It can be seen from the relationship that the internal resistance of this circuit is equal to the target value R.

The sawtooth wave voltage generating circuit 20 generates a sawtooth wave voltage V _r having an amplitude E and a period T, as shown in FIG. 5A. This voltage _Vr is inputted to the base of one transistor 17 constituting the comparator, and the above-mentioned detection voltage is applied to the base of the other transistor 16 of the comparator.
When V _t is input, the transistor 17
As shown in Figure B, it operates on at a duty ratio of V _t /E. Therefore, when the transistor 10 of the first circuit is driven by the transistor 17, the first circuit satisfies the condition of equation (5), and the current supply using the conventional R shown in equation (1) and FIG. It operates as a circuit equivalent to the circuit.

According to the above circuit configuration of the present invention, the on/off operation of the switching regulator (first circuit) achieves a function equivalent to that of the internal resistance element R of the conventional circuit, and therefore the power consumption in the first circuit can be extremely reduced. Can be made smaller. Also, in the second circuit,
Since the current I _C flowing through the resistive element 18 can be made sufficiently smaller than the communication current I _L of the subscriber circuit, it is possible to extremely reduce the power consumption of the resistive element 18, which corresponds to the internal resistance.

For example, if the emitter area of transistor 15 is designed to be smaller than that of transistor 14 so that I _C =1/100・I _L , that is, k=1/100, the resistance element 18 when E=-48V The power consumption is only about 50mV. Further, if the current amplification factor β of the transistor 10 is 100, the current flowing through the constant current source 19 is 1/100I _L , so the power consumption due to this current is only about 50 mW. A current I _L flows through the transistor 14, but the base
Since the voltage V _BE between the emitters is small, its power consumption is only a few tens of mW. Therefore, power consumption as a communication current supply circuit can be suppressed to several hundred mW, and heat generation within the exchange can be significantly reduced compared to a conventional circuit using a resistor R.

Incidentally, in the present invention, for example, a Hall element may be used as the communication current detection element 14 of the second circuit, or circuits A and B may be integrated into ICs in order to reduce the overall size.
The circuit configuration is suitable for
Various modifications other than the embodiments are possible, such as combining them into an IC device.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the configuration of a conventional current supply circuit connected to a subscriber line, Figure 2 is a DC equivalent circuit diagram of the circuit in Figure 1, and Figure 3 shows the current-load characteristics in the subscriber circuit. 4 is a circuit diagram showing one embodiment of the current supply circuit according to the present invention, and FIGS. 5A and 5B are signal waveform diagrams for explaining the operation of the second circuit B in the above embodiment. be. In Figure 4, E is a DC voltage source, A is a first circuit that outputs voltage _E0 , B is a second circuit that controls the duty of the first circuit, T is a transformer, and R _L is the equivalent resistance of the subscriber line. shows.

Claims (1)

[Scope of Claims] 1. A current supply circuit having a predetermined internal resistance R and supplying a DC talking current I _L to a subscriber line of a telephone set with a value depending on the resistance value of the subscriber line, the current supply circuit having a predetermined internal resistance R, A voltage source that generates voltage E, and a voltage source connected to the voltage source that converts the DC voltage E into a voltage E ₀ according to the duty.
The DC talking current I _L flowing through the subscriber line is detected, and the duty cycle of the first circuit is adjusted so that E-E ₀ is proportional to I _L. the first circuit includes a transistor, an inductor, and a capacitor connected in series with respect to the voltage source, and a conduction time proportional to the conduction time of the transistor is connected to both ends of the capacitor. The second circuit includes a switching regulator that outputs a voltage _E0 , and the second circuit detects the DC talking current I _L of the subscriber line.
and means for converting the DC talking current I _L from the DC voltage E.
and _an internal resistance R required for the current supply circuit _.
and third means for controlling the conduction period of the transistor according to the ratio of the current supply circuit to the current supply circuit. 2. The first means is configured to control the DC talking current I _L by 1/
k (k>1), the second means comprises a resistor connected between the voltage source and the transistor circuit, and the resistor is connected to the internal 2. The current supply circuit according to claim 1, wherein the current supply circuit has a resistance value k times as large as the resistance R, and the detection voltage V _t is output as a terminal voltage of the resistance element.