JPS632389B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power supply filter inserted between a power supply and a load in order to remove noise from the power supply when the power supply needs to be in a high-impedance AC state when viewed from the load side. It is.

Power supply filters as described above are used, for example, in supplying bias current to telephone subscriber circuits. FIG. 1 is a conceptual diagram of a telephone subscriber circuit. In this figure, 1 is a high impedance filter in terms of AC, 2 is a load (telephone), 3 is a signal detection circuit, 4 is a power supply, R is a resistor for overcurrent prevention,
I _L is the load current, and V _L is the load voltage. As shown in the figure, a DC bias current is supplied to the subscriber circuit by a power supply 4, and the voice signal from the telephone set 2 is superimposed on this bias current. The audio signal superimposed on the bias current is extracted by the signal detection circuit 3. At this time, unless the alternating current high impedance filter 1 is inserted, the voltage across the signal detection circuit 3 will be constant and no audio signal can be extracted. It also prevents crosstalk from the power supply and
In order to remove noise when a DC-DC converter is used, a high impedance AC filter 1 is inserted between the power supply 4 and the telephone set.

Conventionally, a coil has been used for the above-mentioned AC high impedance filter 1. However, if the filter is constructed using a coil, an inductance in the unit of Henry is required, so the shape becomes large and it is difficult to downsize the filter. Furthermore, there is a drawback that the power supply efficiency deteriorates due to the power consumption of the coil itself.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a high-impedance AC power filter that is small in size and consumes little power.

In the present invention, as shown in FIG. 4, current control means includes a current control transistor T that controls the output current based on the output from the comparator 8, and reference voltage generation means includes a power supply and a variable resistor _VR . ,
a voltage detection means 5 for detecting the DC bias voltage between the input and output of the constant current source type filter; a difference detection means 6 for detecting the difference between the output from the reference voltage generation means and the output from the voltage detection means; A low-pass filter means 7 is provided for removing alternating current components from the output of the difference detection means.

By inputting the output of the difference detection means from which the alternating current component has been removed by the low-pass filtering means to the comparator of the current control means, a DC bias voltage between the reference voltage and the input/output of the constant current source type power filter is generated. The output of the current control means is controlled so that the current control means becomes equal.

The present invention will be explained in detail below using the figures.

The present invention focuses on the fact that a constant current source is high impedance in principle, and inserts a constant current source circuit as a filter between a power source and a load.

FIG. 2 is a diagram showing the output characteristics of a constant current source.
In this figure, the vertical axis shows the current I, and the horizontal axis shows the voltage V. In this way, in a constant current source, the current change is zero with respect to the voltage change, so the voltage change can be expressed as ΔV,
If the current change is ΔI and the impedance is Z, then
Z = ΔV / ΔI = ΔV / 0 = ∞, and since it is an AC high impedance, noise from the power supply will not be transmitted through the constant current source, and the audio signal from the telephone will not leak to the power supply side. .

Now consider the load characteristics in FIG. Third
The figure shows the load characteristics in the subscriber circuit of the telephone shown in FIG. In this figure, the vertical axis is the load voltage,
The horizontal axis shows the load current _IL . If the voltage of the power supply 4 is V, then V = V _L + I _L R, V _L = V - I _L R, so
The load characteristics will be as shown in Figure 3. In other words, the larger the load, the more the load voltage V _L increases, and the load current I _L
It has a characteristic that the smaller the load, the smaller the load voltage V _L and the more the load current I _L increases.

Therefore, in the constant current source type filter according to the present invention, the constant current source is provided with a self-bias function in order to have high impedance in terms of alternating current and satisfy the characteristics shown in FIG. I'm trying to change the. for example,
The load impedance R ₁ at point A in Figure 3 is B
The above condition is satisfied by changing the set current of the constant current source from I ₁ to I ₂ using the self-bias function when the load impedance at the point changes to R ₂ .

An embodiment of the present invention will be shown and further explained below. FIG. 4 shows an embodiment of the present invention. In this figure, 5 is a buffer amplifier, 6 is an integrator, 7 is a low-pass filter, 8 is a comparator, 9 is a DC/DC converter, 10 is a constant current source filter, VR is a variable resistor, T is a current control transistor, R′ is resistance,
Vc is the reference voltage, Vref is the output voltage of the low-pass filter, E _R is the voltage across the resistor R', and V _B is the bias voltage of the constant current source filter. Note that the same numbers as in FIG. 1 indicate the same parts.

The transistor T is used for current control, and its bias voltage V _B is detected by the buffer amplifier 5.
The variable resistor VR sets the reference voltage V _C that determines the bias voltage V _B. The integrator 6 inputs the reference voltage V _C and the detection output of the buffer amplifier 5 and integrates the difference between them, so that the bias voltage of the filter becomes V _B =
Create a Vref that becomes V _C. The output of the integrator 6 is passed through a low-pass filter 7 to remove the alternating current component, and then sent to a comparator 8.
is input. The comparator 8 controls the current control transistor T so that the output voltage Vref of the low-pass filter and the voltage drop E _R across the resistor R' are equal. As described above, in the constant current source type filter according to the present invention, control is performed to keep the bias voltage V _B of the filter constant at the reference voltage V _C in a DC manner.

Here, load 2 is constant and DC-DC converter 9
Consider the case where noise is generated from load 2
When V is constant, V _B = V _C is maintained in direct current terms, but since the audio signal from the telephone is superimposed on the bias current, the bias voltage of the filter detected by the buffer amplifier 5 is not equal to the audio signal. If the voltage fluctuation caused by the voltage change is ΔV, it becomes V _B +ΔV. Therefore, the integrator 6 integrates the difference ±ΔV from the reference voltage V _C , but since this is an AC component, the low-pass filter 7
Therefore, as long as the magnitude of the load 2 does not change, the output voltage Vref of the low-pass filter remains constant regardless of whether the audio signal is superimposed on the bias current. On the other hand, the voltage E _R across the resistor R′
=I _L R' is pulsating due to noise from the DC/DC converter 9. However, comparator 8 is Vref=E _R
Since the current control transistor T is controlled so that =I _L R', there is a negative feedback effect on the current flowing into the current control transistor T, and the DC bias current supplied to the load 2 depends on the presence or absence of noise from the power supply. Regardless, the current is constant. That is, I _L R′＞Vref
When , I _L is decreased, and when I _L R′<Vref, I _L is increased to maintain a constant current. As above, DC/
Noise generated from DC converter 9 is removed.

Next, consider a case where the magnitude of load 2 changes.
For example, if load 2 increases and changes from R ₁ to R ₂ (R ₂ > R ₁ ), the constant current source causes a constant current to flow, so the bias voltage V _B of the filter increases and V _B > V _C
becomes. The bias voltage V _B is detected by a buffer amplifier 5, and the difference from the reference voltage V _C is integrated by an integrator 6 to create Vref such that V _B =V _C. That is, since Vref<I _L R', the comparator 8 controls the output current of the current control transistor T to decrease. Here, it is assumed that the reference voltage V _C is set by the output voltage of the constant current source filter so as to be the same as the conventional power supply characteristic. In this way, when the size of load 2 changes, the set current of the constant current source changes due to the self-bias function,
The load characteristics shown in FIG. 3 are obtained.

As described above, the constant current source filter according to the present invention can remove maze without changing the load characteristics before the filter is inserted.

The above explanation has been given using a telephone subscriber circuit as an example, but the application is not limited to telephone subscriber circuits, but can also be used in other applications that require a high impedance state on the power supply side when viewed from the load side. It is possible.

As described above, according to the present invention, it is possible to realize a small-sized, low-power consumption, high-impedance AC power supply filter, and the power supply efficiency is also improved.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of a telephone subscriber circuit, FIG. 2 is an output characteristic of a constant current source, FIG. 3 is a load characteristic of a telephone subscriber circuit, and FIG. 4 is an embodiment of the present invention. 2...Load (telephone), 3...Signal detection circuit,
5... Buffer amplifier, 6... Integrator, 7... Low pass filter, 8... Comparator, 9... DC/DC
Converter, 10...constant current source type filter,
R′...Resistor, VR...Variable resistor, T...Current control transistor, V _B ...Filter bias voltage, E _R ...Voltage across resistor R, V _C ...Reference voltage, Vref... ...Output voltage of low-pass filter, I _L
...Load current, V _L ...Load voltage.

Claims (1)

[Claims] 1. In a constant current source filter, current control means includes a current control transistor 8 that controls the output current based on the output from the comparator 8, and a reference voltage comprising a power source and a variable resistor _VR . generating means; voltage detecting means 5 for detecting a DC bias voltage between the input and output of the constant current source type filter; and difference detecting means for detecting the difference between the output from the reference voltage generating means and the output from the voltage detecting means. 6
and low-pass filtering means 7 for removing alternating current components from the output of the difference detecting means, and inputting the output of the difference detecting means from which the alternating current components have been removed by the low-pass filtering means to the comparator of the current control means. A constant current source type having a self-biasing function, wherein the output of the current control means is controlled so that the reference voltage and the DC bias voltage between the input and output of the constant current source type power filter are equal. filter.