JPH0721740B2 - Constant voltage power supply circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant voltage power supply circuit used in various electronic devices.

2. Description of the Related Art FIG. 2 shows a conventional constant voltage power supply circuit.

In the right part of Fig. 2, D ₁ is the anode of the power supply voltage Vin to guarantee the operating voltage Vcc for the load R at the time of power interruption.
A diode whose anode is connected to the load R side, Co, whose one end is connected to the anode of the diode D ₁ ,
The other end is a capacitor that is grounded.

In Q ₁ , the emitter is connected to the anode of the diode D ₁ , the collector is connected to one end of the load R, and the base is PN.
The PNP type transistor R ₆ connected to the emitter of the P type transistor Q ₂ is a resistor whose _one end is connected to the anode of the diode D _{1 and} whose other end is connected to the base of the transistor Q ₁ .

In the left side of Fig. 2, C is the operating voltage Vcc at the momentary power failure.
A capacitor 1 connected across a load R to ensure that a reference voltage generator generates a reference voltage and outputs it to the emitter of the transistor Q ₁ and the + terminal of the differential amplifier 2,
R ₁ and R ₂ are resistors that divide the voltage Vcc for the load R and output the divided voltage to one terminal of the differential amplifier.

R ₃ is a resistor connected between the base of the output terminal and the transistor Q ₃ of the differential amplifier 2, Q ₃ has a collector connected to the base of the transistor Q _2, the emitter connected to the collector of the transistor Q ₂ NPN type transistor, R ₄ is a resistor connected between the emitter and base of transistor Q ₂ ,
R ₅ is a resistor whose one end is connected to the collector of the transistor Q _{2 and} the emitter of the transistor Q _{3 and} whose other end is grounded.

In the above configuration, in the steady state, the voltage Vc with respect to the load R is
c is constant with respect to the power supply voltage Vin, and therefore an electronic device equipped with this constant voltage power supply circuit operates normally.

On the other hand, when the power supply voltage Vin is momentarily short-circuited to the ground side, a current flows from the collector to the emitter due to the reverse transistor effect of the transistor Q ₁ , so the voltage Vcc is calculated by the formula Vcc = Vin · e ^{-t / R1} ( However, a R1 = R // r, r decreases from the collector of the transistor Q ₁ base, by the impedance) with respect to the emitter.

Therefore, in the conventional constant voltage power supply circuit, the capacitance of the capacitor C is determined in consideration of the operation guarantee voltage and the instantaneous guarantee time for the electronic device.

Problems to be Solved by the Invention However, in the above-mentioned conventional constant voltage power supply circuit, since the diode D ₁ is connected in series to the power supply voltage Vin, the diode D ₁ causes a voltage drop (about 0.6 V). In order to obtain the required voltage Vcc for the load R, there is a problem that the power supply voltage Vin must be set higher by this voltage drop.

Although the above problem can be solved by removing the diode D ₁ , there is a problem that a capacitor C having a large capacitance must be selected in order to obtain a stable voltage Vcc with respect to the load R.

In view of the above problems, the present invention can reduce the capacity of a capacitor connected to both ends of a load in order to guarantee the operating voltage at the time of power interruption, and to guarantee the operating voltage at the time of power interruption. An object is to provide a constant voltage power supply circuit capable of removing a diode.

Means for Solving the Problems In order to solve the above problems, the present invention includes a switching means, which is turned off when a resistor and a power supply voltage are short-circuited to the ground side, connected in series between an emitter and a base of a PNP transistor. Connected to.

Action The present invention, by virtue of the configuration described above, can eliminate the diode, so that it is not necessary to set the power supply voltage higher by the amount of voltage drop of the diode, and the reverse transistor phenomenon of the PNP type transistor at the time of power interruption due to the switching means is eliminated. Therefore, the capacitance of the capacitors connected to both ends of the load can be reduced.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a constant voltage power supply circuit according to the present invention, and the same components as those shown in FIG. 2 are designated by the same reference numerals.

In the right side of FIG. 1, Co is a capacitor whose one end is connected to the input terminal of the power supply voltage Vin and the emitter of the transistor Q ₁ , and the other end is grounded. Q ₁ has a collector connected to one end of the load R, A PNP transistor whose base is connected to one end of a resistor R ₆₀ and the emitter of a PNP transistor Q ₂ , R
₆₀ is a resistor having the other end connected to the collector of the PNP type transistor Q ₄ .

Q ₄ is a transistor whose emitter is connected to the input terminal of the power supply voltage Vin and whose base is connected to one end of the resistor R ₇ , and whose Q ₅ is whose collector is connected to the other end of the resistor R _{7 and} whose emitter is grounded. The power supply voltage Vi is applied to the base by resistors R ₈ and R _9.
It is an NPN type transistor to which a divided voltage is applied.

In the left part of Fig. 1, C is the operating voltage Vcc at the momentary power failure.
A capacitor, 1 connected across the load R to ensure that a reference voltage generator for generating a reference voltage by the power supply voltage Vin and outputting the reference voltage to the emitter of the transistor Q ₁ and the + terminal of the differential amplifier 2, R ₁ and R ₂ are the voltage Vcc for the load R
Is a resistor that divides and outputs to one terminal of the differential amplifier.

R ₃ is a resistor connected between the base of the output terminal and the transistor Q ₃ of the differential amplifier 2, Q ₃ has a collector connected to the base of the transistor Q _2, the emitter connected to the collector of the transistor Q ₂ NPN type transistor, R ₄ is a resistor connected between the emitter and base of transistor Q ₂ ,
R ₅ is a resistor whose one end is connected to the collector of the transistor Q _{2 and} the emitter of the transistor Q _{3 and} whose other end is grounded.

Next, the operation of the embodiment having the above configuration will be described.

In Fig. 1, when the power supply voltage Vin is input, the resistors R ₈ and R
₉ the power supply voltage Vin divided voltage to the base application of the transistor Q _5, the transistor Q ₅ is turned on, therefore, the transistor Q ₄ is turned on.

Here, in the initial state, the voltage Vcc with respect to the load R on the collector side of the transistor Q ₁ is substantially the same as the level on the ground side, and the reference voltage generator 1 generates the reference voltage by the power supply voltage Vin. , The output of the differential amplifier 2 is at a high level, and since the transistor Q ₄ is turned on as described above, the resistors R ₆₀ and R ₄ are turned on and the transistor Q ₄ is turned on.
Q ₂ and Q ₃ turn on.

When the transistors Q ₂ and Q ₃ are turned on, the base current of the transistor Q ₁ flows and the voltage Vc to the load R on the collector side
c increases, and the resistors R ₁ and R ₂ divide this voltage Vcc and apply it to one terminal of the differential amplifier 2.

Therefore, when the level of the reference voltage is equal to the divided voltage level, the voltage Vcc with respect to the load R becomes stable and enters a steady state.

Next, the operation when the power supply voltage Vin is instantly short-circuited to the ground side from this steady state will be described.

In this case, the reference voltage generated by the reference voltage generator 1 becomes low level due to the voltage drop of the power supply voltage Vin, and the output of the differential amplifier 2 also becomes low level.
The transistors Q ₂ and Q ₃ are turned off.

On the other hand, the transistors Q ₄ and Q ₅ are also turned off due to the voltage drop of the power supply voltage Vin, and thus the transistor Q ₁
Since there is almost no current flowing from the base to the ground side, the level of the voltage Vcc with respect to the load R on the collector side is maintained.

Here, since the base voltage of the transistor Q ₁ is higher than the collector-side voltage Vcc, the reverse transistor phenomenon in which the transistor Q ₁ flows backward from the collector side to the base side does not occur.

Therefore, the level of the voltage Vcc with respect to the load R on the collector side of the transistor Q ₁ only decreases according to the time constant determined by the load R and the capacitor C connected across the load R, and the capacitor having a small capacitance is used. C can be selected.

As described above, according to the present invention, between the emitter and the base of the PNP transistor, the resistor and the switching means that is turned off when the power supply voltage is short-circuited to the ground side are connected in series. Therefore, it is not necessary to set the power supply voltage higher by the amount of the voltage drop of the diode, and the reverse transistor phenomenon of the PNP transistor at the time of momentary power failure can be prevented. The capacitance of the capacitors connected at both ends can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a constant voltage power supply circuit according to the present invention, and FIG. 2 is a circuit diagram showing a conventional constant voltage power supply circuit. Q ₁ , Q ₄ ... PNP type transistor, Q ₅ ... NPN type transistor, R ₆₀ ... resistance.

Claims (1)

[Claims] 1. A PNP-type first transistor having an emitter connected to a power input terminal and a collector connected to a load, and the first transistor having an emitter connected to the power input terminal and a collector via a resistor. A second transistor connected to the base of the second transistor, and a third transistor having a base to which the divided power supply of the power supply input is applied, a collector connected to the base of the second transistor, and an emitter grounded. A constant voltage power supply circuit, characterized in that when the power supply input terminal is short-circuited to the ground side, a connection between the emitter and the base of the first transistor is turned off.