JP2001100851A - Surge clamp circuit for power source circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surge clamp circuit for power source circuit, with which dark currents are reduced as much as possible. SOLUTION: Concerning this circuit, a power source circuit 2 is composed of a serially connected surge clamp circuit 26 and a conventional CMOS-type constant voltage power source circuit 28, which reduces dark current to an extremely low level, and the dark current is reduced to an extremely low level by modifying the configuration of the surge clamp circuit 26 and the dark current is reduced to an extremely low level also as a power source circuit of a microcomputer 3 being a load. Namely, this surge clamp circuit 26 is composed of a drive transistor 20, a first Zener diode 21, a second Zener diode 22, a first resistor 23, a second resistor 24 and a third resistor 25, and further an NPN type control transistor 27 is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surge clamp circuit for a power supply circuit.

[0002]

2. Description of the Related Art As this kind, for example, there is a circuit as shown in FIG. 2. A circuit 1 has a function as a surge clamp circuit and a function as a constant voltage circuit. The 12 V DC output supplied via the backflow prevention diode 11 is stabilized at 5 V and supplied to the microcomputer (load circuit) 3. The circuit 1 includes a first transistor 4, a second transistor 5, a Zener diode 6, a diode 7, a first resistor 8, a second resistor 9, and a third resistor 10, and usually includes the first resistor 8, the Zener diode 6, A voltage regulated to 5 V by the Zener diode 6 is generated at the connection point A of the second transistor A, and the second transistor 5 is turned on by this voltage,
The connection point B between the emitter terminal of the second transistor 5 and the third resistor 10 is maintained at 4.5V. As a result, a stabilized voltage of 5 V obtained by adding the forward voltage drop of the diode 7 to the voltage at the point B to the collector terminal of the first transistor 4 is supplied to the microcomputer 3 serving as a load.

On the other hand, even if a surge current flows into the input terminal of the circuit 1 and a voltage of, for example, 30 V is applied to the emitter terminal of the first transistor 4, the second transistor 5
Since the bias voltage of the first transistor 4
Is maintained at 5 V as described above. That is, the first transistor 4 does not transmit the surge voltage from the emitter terminal to the collector terminal.

[0004]

However, in the configuration described above, the entire circuit 1 always operates while the DC power supply 2 is connected.
Since a dark current always flows as indicated by arrows I1 and I2, there is a problem that the power of the DC power supply is wasted.

The present invention has been made in view of such circumstances, and has as its object to provide a surge clamp circuit for a power supply circuit in which dark current is reduced as much as possible.

[0006]

According to a first aspect of the present invention, there is provided a power supply circuit for inputting an output from a DC power supply via a control transistor and stabilizing and outputting a voltage, wherein the control transistor is provided from the DC power supply side. In a surge clamp circuit of a power supply circuit that turns off the control transistor when a surge current is supplied, the control transistor is an NPN
And the power supply circuit is a CMOS type, and the surge clamp circuit is turned on when an input limit voltage of the power supply circuit exceeds an upper limit, and a voltage lower than the input limit voltage is supplied. A surge clamp circuit of a power supply circuit which is sometimes maintained in an off state.

According to a second aspect of the present invention, in the control transistor according to the first aspect, a terminal is connected to an input terminal of the power supply circuit, a terminal is connected to an output side of the DC power supply, and a base terminal is connected to an output terminal of the surge clamp circuit. It is a surge clamp circuit of a power supply circuit characterized by being connected.

According to a third aspect, in the surge clamp circuit according to the first aspect, a first Zener diode for detecting a surge current applied to an input of the control transistor, and the first Zener diode detects a surge current. And a drive transistor that activates a second Zener diode that controls the output voltage of the control transistor to limit the output voltage to an appropriate input voltage of the power supply circuit.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments will be described below. Embodiment 1 FIG. In FIG. 1, the same components as those of the conventional configuration already described in FIG. 2 or equivalent components are denoted by the same reference numerals, detailed description thereof will be omitted, and only different portions will be described below. This circuit comprises a power supply circuit comprising a surge clamp circuit 26 connected in series and a commercially available CMOS constant voltage power supply circuit 28 having a very low dark current. The current is extremely reduced, and the dark current can be extremely reduced also as a power supply circuit of the microcomputer 3 serving as a load.

That is, the surge clamp circuit 26
Is the drive transistor 20, the first Zener diode 2
1, a second Zener diode 22, a first resistor 23, a second resistor 24, and a third resistor 25, and an NPN-type control transistor 27 is used.

The driving transistor 20 has an emitter terminal grounded and a collector terminal connected to the second Zener diode 22.
And a control transistor 27 via a third resistor 25 in series.
, And controls the control transistor 27. The base terminal of the driving transistor 20 is the first terminal.
It is connected to a connection point A between the Zener diode 21 and the second resistor 24. The Zener voltage of the first Zener diode 21 is selected so as to match the output voltage of the DC power supply 2, for example, 22V higher than 12V, in other words, the maximum voltage of the input limiting voltage of the CMOS type constant voltage power supply circuit 28. I have.

The Zener voltage of the second Zener diode 22 is higher than the output voltage of the DC power supply 2 but lower than the Zener voltage of the first Zener diode 21 by 15 V, in other words, the input of the CMOS constant voltage power supply circuit 28. It is selected to match the minimum voltage of the limiting voltage.

Next, the operation of the above configuration will be described. Normally, the output voltage of the DC power supply 2 is
, The drive transistor 20 remains off, and the surge clamp circuit 26 does not operate. Therefore, the control transistor 27 has only a function as a mere diode function, and the output voltage of the DC power supply 2 is directly supplied to the CMOS regulator 28 so that no dark current flows.

Next, when a surge occurs in the signal line between the output of the DC power supply 2 and the control transistor 27 and the voltage caused by the surge is larger than the Zener voltage of the first Zener diode 21, the second resistor 24 A current flows, a bias voltage of the drive transistor 20 is generated at the point A, and the drive transistor 20 is turned on. As a result, the surge current flows through the resistors 9 and 25 and the second Zener diode 22 in series, and the voltage of the base terminal of the control transistor 27 becomes the second current.
The voltage almost coincides with the Zener voltage of the Zener diode 22, and is maintained at a constant voltage at the emitter terminal (output terminal) of the control transistor 27.

[0015]

According to the present invention, the effect that the dark current can be extremely reduced is exhibited.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram showing a first embodiment according to the present invention.

FIG. 2 is an explanatory diagram of a conventional circuit block.

[Explanation of symbols]

 2 DC power supply 3 Microcomputer (load circuit) 4,5,20,27 Transistor 6,21,22 Zener diode 7 Diode 8,9,10,23,24,25 Resistance 11 Backflow prevention diode

Claims (3)

[Claims] 1. A power supply circuit which receives an output from a DC power supply via a control transistor, stabilizes and outputs a voltage, and wherein the control transistor receives a surge current from the DC power supply side. In a surge clamp circuit of a power supply circuit for turning off a control transistor, the control transistor is an NPN transistor and the power supply circuit is a CMOS type, and the surge clamp circuit exceeds an upper limit of an input limit voltage of the power supply circuit. A surge clamp circuit for a power supply circuit, wherein the surge clamp circuit is turned on when the voltage is lower than the input limit voltage and is normally kept off. 2. The control transistor has a terminal connected to an input terminal of the power supply circuit, a terminal connected to an output side of the DC power supply,
2. The surge clamp circuit for a power supply circuit according to claim 1, wherein a base terminal is connected to each of output terminals of said surge clamp circuit. 3. The surge clamp circuit includes a first Zener diode for detecting a surge current applied to an input of the control transistor, and an output voltage of the control transistor when the first Zener diode detects a surge current. 2. A surge clamp circuit for a power supply circuit according to claim 1, further comprising: a driving transistor for controlling a second Zener diode for controlling the input voltage of said power supply circuit so as to limit the input voltage.