JPH0447510B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Background of the Invention) [Technical Field of the Invention] The present invention relates to a transformer and a control circuit element used in a power supply, specifically a communication system, to provide a pulsed input current to the transformer. Regarding the power supply.

Description of the Prior Art In some applications, it is desirable to limit the output voltage of a power supply when a high impedance, e.g. an open circuit load, is connected to the power supply voltage at the output of the power supply. This is true of power supplies that provide power to subscriber lines in communication systems.
It is desirable to provide approximately constant output power to the subscriber line to accommodate variations in line length and impedance, and to avoid higher than normal output voltage levels. Many power supply designs use a pulse-based transformer with a switching circuit that controls the input current to the transformer's primary winding. In one particular case, a feedback winding coupled to the transformer's primary winding is used to control the current flowing through the primary winding. However, such conventional power supplies have the disadvantage that under open circuit load conditions, the output voltage increases as a result of imperfections in the circuit elements. Furthermore, the output voltage of such conventional currents tends to vary with input voltage variations. Another problem in the design of subscriber line power supplies is the ability to accurately detect hook-on and hook-off of connected subscriber equipment.

(Summary of the Invention) In the present invention, a power supply circuit consisting of a transformer having a primary winding and a secondary winding and a circuit element that controls an input current flowing to the primary winding is a transformer connected to the control circuit element. It has a sensing winding that detects the magnitude of the output voltage of the transformer and controls a control circuit element that reduces the input current to the primary winding when it exceeds a certain voltage level.

Further, the power supply according to the present invention provides a constant output power when the output impedance becomes less than a certain critical value, and provides a constant output voltage when the output impedance becomes greater than the critical value.

In one embodiment of the invention, a transistor switch is connected in series with the primary coil to control the input current flowing through the coil and to periodically switch the switch.
It is controlled by a clock circuit that turns it on, causing current to flow through the primary winding. A current sensing element connected in series with the transistor switch, which may be a conventional resistor, provides a voltage signal proportional to the current flowing in the primary winding. In this example, the sensing winding is connected to a voltage conversion circuit. This voltage conversion circuit provides an output signal that matches a certain reference voltage level when the voltage on the sensing winding is less than the reference value, and a certain reduced voltage level when the voltage on the sensing winding exceeds the reference value. provides an output signal. The output of the conversion circuit is used as a reference signal for a comparator circuit in which the output of the current detection element is used. When the voltage provided by the current sensing element exceeds the reference signal, the comparator provides an output signal that causes the transistor to be cut off. Therefore, when the voltage level of the sensing winding representing the output voltage of the transformer rises above a certain reference value, the reference signal to the comparator is reduced and the primary winding current is cut off at a lower value. Furthermore, when the output voltage of the transformer is lower than a certain value, this corresponds to a critical load impedance, but if the input current to the primary winding is constant and the impedance presented to the secondary increases above the critical value. decreases when This method avoids extremely high output voltages under high impedance or open circuit load conditions. Additionally, a fixed reference voltage source is a circuit that is independent of input voltage variations, and the circuit output is completely independent of input voltage variations.

Examples of the invention include a monitoring circuit that provides an output signal indicating the hook-on/hooks-off status of any subscriber equipment connected to the power supply circuit. In another example,
The monitoring circuit is generated by a signal derived from the detection winding output signal. Additionally, these circuits generate supervisory signals without the need for extra transformer windings, and do not rely on external signals that may be induced in the loop circuit.

DESCRIPTION OF THE EMBODIMENTS FIG. 1 shows an example of the invention having a transformer 101, a primary winding 103, a secondary winding 105 and a detection winding 107. This circuit was designed for a telephone switch system where a standard voltage source provides -48 volts direct current. Therefore, current in primary winding 103 flows from ground to power supply Vss in accordance with standard current flow laws. The direction of mutual inductance between the transformer windings is indicated by the black dots. The input current to the primary winding 103 is controlled by a transistor switch 110, which is a well-known field effect transistor that operates when a gate voltage is applied and allows current to flow through a path that includes the primary winding. . The gate voltage is provided to the base of transistor 110 by drive circuit 112, which is a standard amplifier. Also, a standard S/R flip-flop 114 is alternately set and reset to provide a high or low voltage signal to drive circuit 112. The drive circuit is flip-flop
In response to a high voltage input signal from the flop, sufficient gate voltage is provided to the transistor to cause it to conduct. flip-flop 1
14 is set periodically by clock pulses and reset in response to a signal from comparator circuit 122 when the primary winding current reaches a predetermined level. The series circuit including the primary winding 103 and the transistor 110 further includes a resistor 120. It is used as a current sensor to measure the current flowing in a series circuit due to the voltage across the resistor. The resistor connected between the transistor and the -48 volt supply may be a standard resistor, for example 10 ohms. The terminal of resistor 120 connected to the transistor is also connected to a comparator circuit 122 which provides a signal to the reset side of flip-flop 114 when the voltage across the resistor reaches a reference level. The flip-flop is now in a reset state, and drive circuit 112 turns off transistor 110, thereby causing primary winding 1
The input current flowing to 03 is cut off. The set terminal of flip-flop 114 is connected to a clock pulse that periodically sets the flip-flop. It activates the drive circuit 112,
Transistor 110 operates for the next cycle of primary winding current. When current is flowing in the primary side, the diode prevents current from flowing in the secondary side. When the primary current is cut off, the secondary current flows through the diode 111 to the capacitor 1.
13 and the load 109.

2 and 3 show the primary and secondary current waveforms corresponding to the secondary and sense winding voltage waveforms for the set and reset pulses applied to flip-flop 114. Figure 2 shows these waveforms under a load condition where the loop impedance is less than the critical impedance Z-crit, and the third
The figure shows the waveform when the loop impedance is greater than the critical impedance. At the rising edge of the clock pulse as shown in FIG. 2, the flip-flop 114 is set and the primary winding current Ip
stands up. The current reaches a certain level and resistor 12
A reset pulse is applied to flip-flop 114 when the voltage across zero exceeds the reference signal level applied to comparator 12α. As a result, the primary current is cut off, and the secondary current Is begins to flow. Due to the polarity of the winding, when the primary current is flowing, the secondary voltage at the black dot end is
Vs and the voltage V-sense applied to the detection winding are negative, and positive when the secondary current is flowing. The secondary winding 105 has a diode 111 and a capacitor 1
load 10 through a network consisting of 13
Connected to 9. In this example, the capacitor is 5μF.

One terminal of the detection winding 107 is connected to the power supply Vss, and the other terminal is connected to the peak detection circuit 124 through a resistor 150. A resistor 150 and a resistor 152 connected between peak detection circuit 124 and power supply Vss are used to attenuate the signal. The value of resistors 150 and 152 is 43KΩ respectively.
It is 4.5KΩ. Peak detection circuits are known circuits that provide an output voltage equal to the peak value of the input voltage. The output of the peak detection circuit 124 is connected through a 1.21KΩ resistor 130 to the input of a conventional operational amplifier 126. A 100KΩ resistor 132 connects a diode 134 from the output of the operational amplifier and a 0.02μF capacitor 136 to maintain feedback stability.
connected with an inverting input in parallel with The output of the operational amplifier is connected to the reference input of comparator circuit 122 through a 4.64KΩ resistor 154. A resistor 154 and a 1K resistor 156 connected from the reference input to the reference voltage source 128 are used to attenuate the operational amplifier output signal. The operational amplifier has a reference input terminal connected to a reference voltage source 128 of approximately 3 volts. The reference voltage source may be a known bandgap reference circuit that provides a reference voltage that is independent of variations in the power supply Vss.

The operation when the impedance of the load 109 is low and no excessive voltage is generated in the secondary winding will be described with reference to FIGS. 1 and 2.

When the load impedance is not high, the secondary side voltage Vs (Vsec in the figure) and the detection voltage Vsense
does not show a relatively high value. In this state, if the peak detection voltage of the peak detector 124 is set to be lower than the reference voltage Vr of the reference voltage source 128, the output voltage of the operational amplifier 126 will be equal to the reference voltage Vr, and therefore the comparator 12
A constant voltage Vr is applied to the reference input terminal No. 2.

In this state, the clock CLK rises (hereinafter referred to as
(see FIG. 2), when the flip-flop 114 is set and the Q output becomes high level, the drive circuit 1
12 causes transistor 110 to conduct. When the transistor 110 becomes conductive, the primary side current Ip input to the primary winding gradually increases, and the current detection voltage generated by the resistor 120 also increases in proportion to it. Since this current detection voltage is applied to the other input terminal of the comparator 122, the comparator 122 compares the constant voltage Vr and the current detection voltage.

While the current detection voltage is lower than the constant voltage Vr, the output of the comparator 122 is low level, but when the primary current Ip increases and the current detection voltage exceeds the constant voltage Vr, the comparator 122 outputs a high level reset pulse. Flip-flop 114
output to the reset terminal. This reset pulse causes the Q output of the flip-flop 113 to go low, and the drive circuit 112 turns off the transistor 110. Therefore, the primary current Ip flowing through the primary winding 103 suddenly decreases to zero when the reset pulse is generated, and conversely, the secondary current Is starts flowing through the secondary winding 105. The above operation is repeated at the cycle of clock CLK.

In this way, when the impedance of the load 109 is low and the secondary side voltage Vs (Vsec in the figure) and detection voltage Vsense are relatively low, the output voltage of the operational amplifier 126 is equal to the reference voltage Vr, and therefore the comparator The voltage Vr is applied to the reference input terminal 122. Therefore, comparator 12
The reset pulse from 2 is always generated when the primary current Ip reaches a certain level, and the time and waveform during which Ip flows are constant as shown in FIG. That is, the energy (power) given to the primary winding 103 is basically the same in each cycle, and the input power and output power of the ideal transformer are the same, so the output power obtained by the secondary winding 105 is virtually constant. Referring to FIG. 4, a graph of VoutIout=constant (power) is a "constant power" curve.

Next, a case where the impedance of the load 109 increases will be explained with reference to FIGS. 1 and 3.

As the load impedance increases, the secondary current
Is decreases, and the secondary current Vs (Vsec in the figure) increases. This development is also reflected on the detection winding 107,
This appears as an increase in the detection voltage Vsense, and therefore the peak detection voltage that is the output of the peak detection circuit 124 increases.

The peak detection voltage increases and the reference voltage source 128
When the reference voltage Vr exceeds the reference voltage Vr, the output voltage of the operational amplifier 126 decreases from the reference voltage Vr accordingly. Therefore, the voltage applied to the reference input terminal of comparator 122 also decreases. This means that the reset pulse from comparator 122 occurs at an earlier point in time.

That is, as mentioned above, when the clock CLK rises, the primary current Ip rises, and the current detection voltage rises in proportion to it.
Since the voltage at the reference input terminal of 22 (the output voltage of the operational amplifier 126) has decreased to a voltage lower than the constant voltage Vr, the current detection voltage reaches the reference input terminal voltage at an earlier point in time and is reset by the comparator 122. A pulse is output. Therefore, when the load impedance increases and the detection voltage Vsense by the detection winding 107 becomes higher, the flip-flop 114 is reset at an earlier point in time and the primary current Ip is increased.
The time (period) in which it flows becomes shorter. This reduces the power supplied to the secondary winding 105, allowing the voltage across the high impedance load to remain constant.

FIG. 4 is a diagram showing the output voltage applied to the load 109 by the power supply circuit as a function of the output current. The figure shows that the voltage is approximately constant at current values lower than the current at the critical load impedance Z-crit. In the experimental circuit, the output voltage was approximately 60 volts. The output current is prevented from going to zero by a Zener diode 115 connected to the load. The Zener diode conducts at 70 volts when the load current approaches zero. When the output current is greater than the current at the critical load impedance, the output power remains constant and the voltage therefore decreases as the current increases. This means that the output current is approximately equal to the value at critical impedance.
Valid when between the values at the minimum impedance in the telephone subscriber line of 200 Ω. When the load current is between the Zener current and the value at the critical load impedance, the output voltage is completely constant. Critical load impedance can be calculated using the following formula.

Z-crit = 2x (R ₁ + R ₂ ) ² xRs ² /N ² xR ² / ₂ xFxLp (1) Here, R ₁ = Resistor 150 = 43KΩ R ₂ = Resistor 152 = 4.5KΩ R _S = Resistor 120 = 10KΩ N =Turns ratio between primary winding and detection winding=
0.5 F = Clock frequency = 256KHz L _p = Primary winding inductance 200μH The circuit in Figure 1 was designed for the telephone line of a communication switch system.
A monitoring circuit 135 is provided whose output is sampled in time by the system controller to determine the hook-off condition. The monitoring circuit consists of a comparator 140, one terminal of which is connected to the current sensing resistor 120 through a peak detector, and the other terminal of which is connected to a reference voltage source 144. This may be a known bandgap reference circuit of about 1.5 volts that does not depend on voltage fluctuations of the power supply Vss. The voltage across the current sensing resistor 120 varies according to the pulsed input current in the primary winding;
To provide a signal to comparator 140, peak detector 142 is designed to provide an output signal in response to the peak value of the resistor voltage.
When the switch is on, the impedance of the subscriber line becomes high and becomes greater than the critical impedance shown in FIG. As a result, there is a relatively low peak voltage at the inputs corresponding to resistor 120 and comparator 140. When the hook is off, the loop impedance is low and therefore the voltage across sense resistor 120 is relatively high. The reference voltage source 144 value is selected to be higher than the voltage signal when the hook is on and lower than the voltage signal when the hook is off. Therefore, comparator 14
A zero output provides a different voltage signal when the hook is on than when the hook is off. In this method, a monitoring output indicative of subscriber status is provided by comparator 140.

Another example of hook-on/hook-off detector 145 shown in FIG.
26 output. When hooked off, the load impedance is relatively low and the amplifier therefore provides an output voltage level that is relatively higher than the level of reference voltage source 128. When the hook is on, the loop impedance is the maximum hook-on loop impedance Z-max shown in Figure 4.
becomes higher. When the equal voltage lines in Figure 4 approach the horizontal axis, they have a slight slope from about 60 volts to 65 volts. This voltage difference is Z at the output of amplifier 126.
It is sufficient to distinguish between −max and normal.
The amplifier output level is compared to a reference voltage source 147 by comparator 149 to provide an appropriate monitoring output signal.

Obviously, changes or modifications may be made in these techniques without departing from the spirit or scope of the invention as herein described. Accordingly, these changes and modifications are included within the scope of the claims of this patent.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a power supply circuit showing an embodiment of the present invention; FIGS. 2 and 3 are diagrams showing current and voltage waveforms at different load impedances in the circuit of FIG. 1;
FIG. 4 is a diagram depicting the output voltage of the power supply as a function of the output current. 101...Transformer, 103...Primary winding, 10
5...Secondary winding, 107...Detection winding, 109...
...Load, 110...Transistor switch, 11
1... Diode, 112... Drive circuit, 113
... Capacitor, 114 ... Flip-flop, 115 ... Zener diode, 120 ...
Resistor, 122... Comparator circuit, 124...
...Peak detection circuit, 126...Operation amplifier, 12
8... Reference voltage source, 130, 132... Resistor, 1
34...Diode, 135...Monitoring circuit, 13
6... Capacitor, 140... Comparator,
142...Peak detector, 144...Reference voltage source, 145...Detector, 147...Reference voltage source,
149...Comparator, 150, 152, 1
54,156...Resistance.

Claims (1)

[Claims] 1. Switch means 110 for switching and controlling the current flowing through the primary winding 103 of the transformer circuit 101
in the current supply circuit which supplies constant power to the communication terminal load 109 connected to the secondary winding 105 by keeping the periodic conduction period of the switching means constant. , a detection winding 107 for detecting the magnitude of the output voltage of the secondary winding; peak voltage detection means 124 for detecting the peak voltage of the detection winding; The periodicity of said switching means is such that when the peak voltage exceeds a certain predetermined reference voltage (VR), the input current to said primary winding is reduced to make the output voltage of said secondary winding substantially constant. Control means 126, 122, 11 for shortening the conduction period
4,112; A current supply circuit for a communication terminal, characterized in that it has the following.