JPH05227748A - Power supply circuit - Google Patents

Abstract

PURPOSE:To provide a power supply circuit having a voltage stabilizing circuit outputting a predetermined stabilized voltage by rectifying and smoothing an AC input and then stepping down thus rectified and smoothed voltage wherein unstable state of output voltage is eliminated at the time of power ON. CONSTITUTION:Upon turn ON of power, voltage across a capacitor 3 increases gradually from 0V. So long as the voltage is lower than the Zener voltage of a Zener diode 5, an NPN transistor 8 is not turned ON nor a PEP transistor 4 is turned ON and thereby a three terminal regulator 9 is not fed with a rectified and smoothed voltage. When the voltage across the capacitor 3 exceeds the Zener voltage of the Zener diode 5, i.e., the minimum stabilizable voltage of the three terminal regulator 9, the transistors 8, 4 are turned ON and the three terminal regulator 9 can outputs a stabilized voltage from the begining.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit, and more particularly to a power supply circuit having a voltage stabilizing circuit that rectifies an AC input to generate a voltage drop in a smoothed output to obtain a DC output of a predetermined voltage.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram of a conventional power supply circuit.
For the AC input section, only the output winding (secondary winding) of the transformer is shown. The AC voltage input to the transformer and output from the output winding 1 is rectified and smoothed by the diode 2 and the capacitor 3, and is used as the input voltage of the three-terminal regulator 9.
The three-terminal regulator 9 is a voltage stabilizing circuit that generates a voltage drop according to the input voltage and outputs a predetermined stable voltage.

FIG. 5 shows the waveforms of the output voltage of the three-terminal regulator 9 and the voltage across the capacitor 3. Since the AC voltage output from the output winding 1 is rectified and smoothed by the diode 2 and the capacitor 3, the voltage Vc across the capacitor 3 is a three-terminal regulator when the capacitor 3 is not charged as when the power is turned on. 9 requires a certain period of time t to reach the minimum voltage value Vl at which regulation is possible. The minimum voltage value V at which the voltage Vc across the capacitor 3 can regulate the 3-terminal regulator 9
Until the voltage reaches 1, the three-terminal regulator 9 outputs the voltage Vd obtained by subtracting the minimum input-output potential difference of the three-terminal regulator 9 from the voltage Vc across the capacitor 3.
When the voltage across the capacitor 3 is charged to the minimum regulated voltage value of the three-terminal regulator 9 or more, a stable voltage Vo is output.

[0004]

In this conventional power supply circuit, an AC input is rectified and smoothed by a diode and a capacitor,
This rectified and smoothed voltage is used as the input voltage of a voltage stabilizing circuit such as a three-terminal regulator that stabilizes the output voltage by a voltage drop. Therefore, the voltage V across the smoothing capacitor
A certain time t is required until c reaches the minimum voltage Vl that can be stabilized by the voltage stabilizing circuit. When a voltage equal to or lower than Vl is input to the voltage stabilizing circuit, the voltage Vd obtained by subtracting the minimum input / output potential difference of the voltage stabilizing circuit from the input voltage.
Is output. That is, the voltage Vc across the smoothing capacitor
Until the voltage reaches Vl (time t), the output voltage of the voltage stabilization circuit has the same rise time as the rise time of the voltage across the smoothing capacitor, and the voltage value becomes unstable during this time.

Therefore, when the present power supply circuit is connected to the load device, the load device may malfunction because the voltage value is unstable at the rise of the output voltage of the three-terminal regulator.

[0006]

SUMMARY OF THE INVENTION A power supply circuit of the present invention is a rectifying / smoothing circuit for rectifying and smoothing an AC input, and a voltage stabilization for generating a DC output of a predetermined voltage by causing a voltage drop in the output of the rectifying / smoothing circuit. A power supply circuit having a circuit, a rectifying / smoothing circuit that outputs a detection signal when the output voltage value of the rectifying / smoothing circuit is equal to or greater than a preset reference value, and the rectification and smoothing circuit according to the presence or absence of the detection signal Switch means for controlling the supply of the output of the circuit to the voltage stabilizing circuit.

Another power supply circuit of the present invention is a rectifying / smoothing circuit for rectifying and smoothing an AC input, and a voltage stabilizing circuit for producing a DC output of a predetermined voltage by causing a voltage drop in the output of the rectifying / smoothing circuit. In a power supply circuit having: a voltage detection circuit that outputs a detection signal when the output voltage value of the rectifying and smoothing circuit is equal to or greater than a preset reference value; and the voltage stabilizing circuit according to the presence or absence of the detection signal. Switch means for short-circuiting or opening the output of.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a first embodiment of the present invention, in which an AC input section shows only an output winding of a transformer. As the AC input, the switching output of the transformer in the switching regulator circuit can be used in addition to the commercial power supply. The diode 2 and the capacitor (electrolytic capacitor) 3 form a half-wave rectifying and smoothing circuit, and rectify and smooth the output voltage of the output winding 1 of this transformer. By connecting the Zener diode 5 and the resistor 6 in series and connecting them to both ends of the capacitor 3, a voltage detecting means for outputting a detection signal when the output voltage value of the rectifying / smoothing circuit exceeds a reference value is configured. .. Zener diode 5 anode and resistor 6
The connection point with is the part that outputs this detection signal. The PNP transistor 4, the resistor 7, and the NPN transistor 8 constitute switch means for controlling the supply of the output of the rectifying and smoothing circuit to the voltage stabilizing circuit (three-terminal regulator 9) depending on the presence or absence of this detection signal. .. The base terminal of the NPN transistor 8 is connected to the connection point between the anode of the Zener diode 5 and the resistor 6, and the base terminal of the PNP transistor 4 is connected to the collector of the NPN transistor 8 via the resistor 7. The collector and emitter of the PNP transistor 4 are connected to the input terminal of the 3-terminal regulator 9 and the cathode of the diode 2, respectively, and the emitter of the NPN transistor 8 is connected to the GND terminal of the 3-terminal regulator 9.

Next, the operation will be described with reference to FIG. 4 showing the output voltage waveform of the present invention.

First, an AC voltage is output from the output winding 1 of the transformer to which AC is input, and is half-wave rectified and smoothed by the diode 2 and the capacitor 3. Since the capacitor 3 is not charged at all, the voltage Vc across the capacitor 3 is shown in FIG.
, It gradually rises from 0V. At this time, no current flows through the Zener diode 5 and the resistor 6 connected to both ends of the capacitor 3 until the Zener voltage of the Zener diode 5 is reached. Therefore, the NPN transistor 8 is not biased and remains off.
The P-transistor 4 also remains off. However, since the voltage Vc across the capacitor 3 gradually rises as shown in FIG. 4, when the voltage across the capacitor 3 reaches the Zener voltage Vc of the Zener diode 5 (time T), a current flows through the Zener diode 5 and the resistor 6. Begins to flow. When this value, the voltage across the resistor 6 reaches V _{BE (sat)} of the NPN transistor 8,
The NPN type transistor 8 is turned on and the PNP type transistor 4 is also turned on. When the PNP transistor 4 is turned on, a voltage is applied to the input terminal of the 3-terminal regulator 9, and the 3-terminal regulator 9 outputs a stable voltage Vo. That is, the three-terminal regulator 5 is selected by selecting the Zener diode 5 having the Zener voltage Vz, which is the sum of the regulated minimum voltage value Vl of the three-terminal regulator 9 and the collector-emitter saturation voltage (V _{CE (sat)} ) value of the PNP transistor 4. No. 9 does not apply a voltage below the regulated minimum voltage value Vl to its input terminal, and suddenly outputs a stable output voltage Vo.

FIG. 2 is a circuit diagram of the second embodiment of the present invention. Similar to the first embodiment of FIG. 1, the diode 2 and the capacitor 3 constitute a half-wave rectifying / smoothing circuit to rectify and smooth the AC voltage from the output winding 1. By connecting the Zener diode 5 and the resistor 6 in series and connecting them to both ends of the capacitor 3, a voltage detecting means for outputting a detection signal when the output voltage value of the rectifying / smoothing circuit becomes equal to or higher than a reference value is configured. There is.

This embodiment differs from the embodiment of FIG. 1 in that a resistor 7, an NPN transistor 8 and an NPN transistor 1 are provided.
4 and 4 constitutes a switch means for short-circuiting or opening the output of the three-terminal regulator 9 which is a voltage stabilizing circuit according to the presence or absence of the detection signal. The base terminal of the NPN transistor 8 is connected to the connection point between the anode of the Zener diode 5 and the resistor 6, and the base terminal of the NPN transistor 14 is connected to the collector of the NPN transistor 8. The resistor 7 is connected between the input terminal of the 3-terminal regulator 9 and the collector of the NPN transistor 8. The collector and the emitter of the NPN transistor 14 are connected to the output terminal and the GND terminal of the three-terminal regulator 9, respectively, and the emitter of the NPN transistor 8 is also connected to the GND terminal of the three-terminal regulator 9.

Next, the operation will be described. FIG. 4 also shows the output voltage waveform in this embodiment.

First, the AC voltage input to the transformer and output from the output winding 1 is half-wave rectified and smoothed by the diode 2 and the capacitor 3. Since the capacitor 3 is not charged at all, the voltage Vc across the capacitor 3 gradually rises from 0V. At this time, no current flows through the Zener diode 5 and the resistor 6 connected to both ends of the capacitor 3 until the Zener voltage Vz of the Zener diode 5 is reached. Therefore, the NPN transistor 8 is not biased and remains off. At this time, NPN transistor 1
4 is biased through the resistor 7 and turned on, and the output voltage of the three-terminal regulator 9 is short-circuited. However, since the voltage Vc across the capacitor 3 gradually rises as shown in FIG. 4, when the voltage Vc across the capacitor 3 reaches the Zener voltage Vz of the Zener diode 5 (time T), the Zener diode 5 and the resistor 6 are connected. The electric current begins to flow. At this time, when the voltage across the resistor 6 reaches V _{BE (sat)} of the NPN transistor 8, the NPN transistor 8 is turned on, and the NPN that has been flowing through the resistor 7 until now.
All the base current of the transistor 14 is drawn into the collector of the NPN transistor 8. As a result, the NPN transistor 14 is turned off, and the output of the three-terminal regulator 9 is switched from the short-circuited state to the open state, and the three-terminal regulator 9 supplies a stable output voltage Vo to the load side. That is, by selecting the Zener diode 5 having the Zener voltage Vz of the lowest regulated voltage value Vl of the three-terminal regulator 9, instability when a voltage not more than the lowest regulated voltage value Vl of the three-terminal regulator 9 is applied to the input terminal. Voltage is NPN transistor 1
When the reproduction voltage Vl which can be regulated is applied to the three-terminal regulator 9, the NPN transistor 14 is turned off and a stable voltage is supplied to the load side.

[0015]

As described above, the present invention provides a power supply circuit having a voltage stabilizing circuit such as a three-terminal regulator for rectifying an AC input and generating a voltage drop in a smoothed output to obtain a DC output of a predetermined voltage. (1) Do not apply a voltage lower than the minimum voltage value (reference value) that can be stabilized by the voltage stabilizing circuit to the input of the voltage stabilizing circuit.

(2) Alternatively, when a voltage equal to or lower than the reference value is applied to the input of the voltage stabilizing circuit, the output of the voltage stabilizing circuit is short-circuited and is not supplied to the load side.

As a result, the voltage output from the voltage stabilizing circuit when the power is turned on is not a voltage that gradually increases from 0V, but an output voltage that rises quickly and is generated when the load device rises. This has the effect of preventing malfunction. Especially when supplying power to the logic system,
This circuit also functions as a reset circuit, which has the effect of reducing costs.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a second embodiment of the present invention.

FIG. 3 is a circuit diagram of an example of a conventional power supply circuit.

FIG. 4 is an output voltage waveform diagram of the power supply circuit according to the present invention.

FIG. 5 is an output voltage waveform diagram of a conventional power supply circuit.

[Explanation of symbols]

 1 Output Winding 2 Diode 3 Capacitor 4 PNP Transistor 5 Zener Diode 6,7 Resistor 8,14 NPN Transistor 9 3 Terminal Regulator

Claims (2)

[Claims] 1. A rectifying / smoothing circuit comprising a rectifying / smoothing circuit for rectifying and smoothing an AC input, and a voltage stabilizing circuit for generating a voltage drop at an output of the rectifying / smoothing circuit to obtain a DC output of a predetermined voltage. Voltage detection means for outputting a detection signal when the output voltage value of the circuit becomes equal to or greater than a preset reference value; and a voltage detection circuit for outputting the output of the rectifying / smoothing circuit to the voltage stabilizing circuit according to the presence or absence of the detection signal. A power supply circuit comprising: switch means for controlling supply. 2. A rectifying / smoothing circuit having a rectifying / smoothing circuit for rectifying and smoothing an AC input, and a voltage stabilizing circuit for generating a voltage drop at an output of the rectifying / smoothing circuit to obtain a DC output of a predetermined voltage. Voltage detection means for outputting a detection signal when the output voltage value of the circuit becomes equal to or greater than a preset reference value, and switch means for short-circuiting or opening the output of the voltage stabilizing circuit according to the presence or absence of the detection signal. And a power supply circuit.