JPH09247854A - Direct-current power supply circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a direct-current power supply circuit for which a small- sized, low-rating overvoltage absorbing element is good enough, and allows a load to be kept operating even at overvoltage. SOLUTION: The title direct-current power supply circuit is provided with an impedance switching circuit 14 that makes a power line section 12, connecting a direct-current power supply section 11 and a load L, switch between high impedance state and low impedance state. It is also provided with a voltage monitoring circuit 15 so that the power line section will be made to switch to the high impedance state only when a predetermined level of overvoltage is detected through the voltage monitoring circuit 15. In a protective circuit, a switching transistor is parallel-connected with a current limiting resistor so that the switching transistor is brought into the off state by a control circuit only when the level of the output voltage of the direct-current power supply section 11 exceeds a predetermined value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC power supply circuit capable of protecting a load from overvoltage.

[0002]

2. Description of the Related Art In a DC power supply circuit for supplying a DC voltage of a prescribed level to various loads, the power supply line is usually placed in a low impedance state to minimize power loss therein. Is desirable. Further, in this type of circuit, a configuration is known in which a surge absorbing circuit using a surge absorbing element such as various diode elements is added in order to protect a load from an overvoltage such as a surge in a power supply line when the surge occurs in the power line. Is.

[0003]

By the way, from the viewpoint of effectively connecting such a surge absorbing element to the power supply line, it is desirable that the power supply line has rather high impedance. This is because it is possible to use a low-rated, small-sized, lightweight surge absorbing element by limiting the current. However, it is difficult to design a circuit that satisfies these conflicting conditions, and it is currently difficult to reduce the cost of the surge absorption circuit. In order to solve this, a configuration has been proposed in which, when it is detected that a surge voltage has occurred in the power supply line, the power supply line is disconnected from the load and the power supply line is placed in a high impedance state. However, in this configuration, the power supply to the load is temporarily cut off, which causes another problem that the operation of the load becomes unstable.

Therefore, an object of the present invention is to provide a surge absorption element of a small size and a low rating by switching the energization path to the load when an overvoltage due to a surge or the like occurs in the DC power supply circuit, and Another object of the present invention is to provide a DC power supply circuit capable of maintaining the operation of the load.

[0005]

The features of the invention of claim 1 for solving the above-mentioned problems are as follows: a DC power supply unit for supplying DC power; and an output of the DC power supply unit for connecting to a load. In a DC power supply circuit comprising a power supply line section and a surge absorbing element provided on the load side of the power supply line section so as to be connected in parallel with the load, the power supply line section has a high impedance state or a low impedance state. And a voltage monitoring circuit for monitoring whether or not the output level of the DC power supply unit has reached a predetermined overvoltage level state, and the voltage monitoring circuit provides a predetermined overvoltage level state. The point is that the power supply line section is switched to the high impedance state only when is detected.

According to this structure, in a normal state in which the level of the power supply line portion is at a predetermined prescribed level, the power supply line portion is in a low impedance state, the voltage drop hardly occurs in the power supply line portion, and the DC power supply The voltage from the section is applied to the load as it is. On the other hand, if a surge voltage is generated in the power supply circuit for some reason and an abnormal state occurs in which a predetermined overvoltage level state occurs, the voltage monitoring circuit detects this and the power supply line section is switched to the high impedance state. . Therefore, in the operation in which a current flows through the surge absorbing element due to the overvoltage and the load is protected from the overvoltage, the level of the current flowing through the surge absorbing element at this time is suppressed to a low level due to the high impedance state of the power line section, and the surge absorbing element is It is small and lightweight with a small rating, and does not need to interrupt the power supply to the load.

According to a second aspect of the present invention, there is provided a DC power supply unit for supplying DC power, and an output voltage from the DC power supply unit is supplied to the load through a protection circuit for protecting the load from overvoltage. In the DC power supply circuit, the protection circuit, a surge absorbing element connected in parallel with the load, a current limiting resistance element inserted and connected in series between the output of the DC power supply unit and the load, A switching element connected in parallel with the current limiting resistance element; and a control circuit which responds to the output voltage level of the DC power supply unit and turns off the switching element only when the voltage level exceeds a predetermined value. The point is to be prepared.

When the voltage level of the output of the DC power supply unit is a predetermined normal value, the semiconductor switching element is turned on by the control circuit, and the connection state between the DC power supply unit and the load becomes a low impedance connection state. The required DC power is sent from the DC power supply unit to the load with almost no loss.
On the other hand, if the voltage level of the output of the DC power supply becomes higher than a predetermined value due to the occurrence of surge voltage or for some other reason, the switching element is turned off by the control circuit, and the DC power supply and the load are A high impedance connection state is established in which the connection is made via the limiting resistance element. Therefore, when a surge voltage is generated, the level of the current flowing through the surge absorbing element is limited by the current limiting resistance element due to this surge voltage, so it can be suppressed to a low level. In addition, it is not necessary to cut off the power supply to the load.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing an example of an embodiment of a DC power supply circuit according to the invention of claim 1. The DC power supply circuit 10 includes a DC power supply unit 11 for supplying DC power,
A power supply line unit 12 for connecting the output of the DC power supply unit 11 to the load L for supplying an output voltage E from the DC power supply unit 11 to the load L, and a load L connected in parallel with the load L on the load side of the power supply line unit 12. And a constant voltage diode 13 which is a surge absorbing element provided as described above.

The power supply line section 12 includes:
Impedance switching circuit 14 for switching the high impedance state or the low impedance state
Are provided in series on the line 12A on the positive side. In the present embodiment, the impedance switching circuit 14 is configured so that the current limiting resistor 14B can be connected in series to the line 12A by the switching switch 14A that operates in response to an electric signal from the outside. .

Reference numeral 15 indicates a power supply line section 1
2 is a voltage monitoring circuit for monitoring whether or not the voltage of 2 is in a predetermined overvoltage level state. The voltage monitoring circuit 15
When the voltage of the power supply line section 12 is in a normal state where the voltage is at a predetermined level, the output line 15A is set to a low level state, and the changeover switch 14A is switched to the state shown by the solid line in response to this, and the power supply line section 12 receives a current. Limiting resistor 14
B is in a low impedance state in which it is not connected in series. Therefore, the output voltage E from the DC power supply unit 11 is applied to the load L as it is, and the power can be supplied from the DC power supply unit 11 to the load L without power loss in the power supply line unit 12. The voltage monitoring circuit 15 may have a known configuration.

On the other hand, the output line 15A of the voltage monitoring circuit 15 is in a high level state when the voltage of the power supply line section 12 is in a predetermined overvoltage level state due to a surge voltage or the like, whereby the changeover switch 14A is indicated by a dotted line. Can be switched to the state that As a result, the current limiting resistor 14B is connected in series to the line 12A so that the power line unit 12
Becomes a high impedance state, and the overvoltage generated in the power supply line section 12 makes the constant voltage diode 13 conductive.
At this time, the magnitude of the current flowing from the power supply line section 12 to the constant voltage diode 13 is limited by the current limiting resistor 14B. Therefore, the constant voltage diode 13 need only have a low rating, and an inexpensive, small, and lightweight element can be used as the surge absorbing element. Furthermore, in this case, since the load L is continuously supplied with electric power via the current limiting resistor 14B, the load L is supplied with a current that does not destroy the load L even at the time of overvoltage. The required operation of L is performed without interruption.

As described above, according to the configuration shown in FIG. 1, since the impedance of the power supply line section 12 is in a low impedance state when it is normal, the DC power supply section 11 loses power to the load L. It is possible to supply electric power in a state where is substantially zero. The voltage monitoring circuit 15 is provided only when an overvoltage level condition occurs in the power line section 12.
The changeover switch 14A is changed over as shown by the dotted line, whereby the current limiting resistor 14B is connected in series to the power supply line section 12 and the overvoltage generated in the power supply line section 12 makes the low-voltage diode 13 conductive. It Therefore, the current flowing through the low-voltage diode 13 can be suppressed to a relatively small value while maintaining the operation of the load L, and the rating of the low-voltage diode 13 can be reduced. The rating of the low voltage diode 13 can be appropriately set by adjusting the size of the current limiting resistor 14B.

Another advantage of suppressing the current flowing through the constant voltage diode 13 to a small value by the current limiting resistor 14B is that the pattern width of the printed board can be narrowed, so that a small printed board can be used. It is possible and economical. If the current limiting resistor 14B is not used to limit the current, the wiring pattern on the printed board will be burned by the current due to the surge voltage unless the wiring pattern on the printed board is thickened, which increases the cost of the printed board.

FIG. 2 is a circuit diagram showing an example of an embodiment of a DC power supply circuit according to the invention of claim 2. The DC power supply circuit 20 includes a DC power supply unit 2 for supplying DC power.
1, and the output voltage E from the DC power supply unit 21 is supplied to the load L via the protection circuit 30.

The protection circuit 30 absorbs the overvoltage when the voltage of the output of the DC power supply 21 exceeds the predetermined value due to a surge voltage or the like, and absorbs the overvoltage.
Is a circuit for preventing a load L from being damaged by a voltage higher than the rated voltage applied to the load L, and is connected in parallel with the load L to act as a surge absorbing element.
2 and a current limiting resistor 33 that is inserted and connected in series between the output of the DC power supply unit 21 and the load L. A collector-emitter circuit of a switching transistor 34 that functions as a switching element is connected in parallel to the current limiting resistor 33, and the switching transistor 34 operates in response to the output voltage level of the DC power supply unit 21. ON / OFF is controlled by. In the present embodiment, the constant voltage diodes 31 and 32 are connected in series to function as a surge absorbing element, but it is also possible to use only one constant voltage diode, or three constant voltage diodes. A configuration in which the above is connected in series is also possible.

The control circuit 40 determines whether or not the level of the output voltage E of the DC power supply unit 21 exceeds a predetermined specified value, and only when the level of the output voltage E exceeds the predetermined specified value, the switching transistor 34. Is a circuit for making a high impedance state by the current limiting resistor 33 between the output of the DC power supply unit 21 and the load L by turning off the switch. In the control circuit 40, resistors 41 and 42 are connected in series between the output of the DC power supply unit 21 and the ground to form a voltage dividing circuit, and the resistors 41 and 42 output the DC power supply unit 21. The voltage is divided. The divided voltage thus obtained is applied between the base and emitter of the transistor 44 via the constant voltage diode 43. The collector of the transistor 44 is connected to the base of another transistor 45 whose emitter is grounded, and is also connected to the output of the DC power supply unit 21 via the resistor 46. The collector of the transistor 45 is connected to the base of the switching transistor 34 via the resistor 47.

The constant voltage diode 43 is provided with resistors 41, 42 when the output voltage E is in a normal state where the output voltage E is in a predetermined specified value.
It is not conducted by the divided voltage obtained by, but is obtained by the resistors 41, 42 when the output voltage E becomes an overvoltage level larger than a predetermined specified value due to generation of surge voltage or the like. An element having a Zener voltage value that conducts by a divided voltage is selected.

Therefore, in a normal state in which the value of the output voltage E does not exceed the predetermined specified value, the transistor 44 is in the off state and the transistor 45 is kept in the on state, so that the switching transistor 34 becomes conductive. It is in a state. Therefore, the load L from the DC power supply unit 21
Most of the current to the load L is supplied through the switching transistor 34 in the low impedance state, and power is supplied from the DC power supply unit 21 to the load L with a very small power loss in the protection circuit 30. .

On the other hand, in an abnormal state in which the value of the output voltage E exceeds a predetermined specified value, the transistor 44 is turned on, so that both the transistor 45 and the switching transistor 34 are turned off, and the DC power supply unit is turned on. A high impedance state is established between 21 and the load L. For this reason,
Most of the current from the DC power supply unit 21 to the load L is supplied through the current limiting resistor 33. In this case, the constant voltage diodes 31 and 32 are prevented from being turned on due to the overvoltage state of the output voltage E and an excessive level of voltage is applied to the load L, but the constant voltage diodes 31 and 32 are turned on. The current limiting resistor 33 limits the level of the current flowing therethrough. Therefore, the constant voltage diodes 31 and 32 need only have a low rating, and an inexpensive, small and lightweight element can be used as a surge absorbing element. Further, even when the overvoltage is generated, the load L is supplied with a current that does not destroy the load L, and thus the load L is continuously supplied with power via the current limiting resistor 33.
The required operation of the load L takes place without interruption.

As described above, according to the configuration shown in FIG. 2, since the switching transistor 34 is in the ON state between the DC power supply unit 21 and the load L in a normal state, the DC power supply unit 21 has a low impedance state. Electric power is supplied from the portion 21 to the load L with a power loss of substantially zero. Then, when the output of the DC power supply unit 21 becomes an overvoltage state, the switching transistor 34 is turned off and the current limiting resistor 33 is provided between the DC power supply unit 21 and the load L.
The high impedance state due to
The constant voltage diode 3 due to the overvoltage generated at the output of 1
1, 32 are made conductive. Therefore, the current due to the overvoltage flows into the constant voltage diodes 31 and 32 via the current limiting resistor 33, so that the current flowing into the constant voltage diodes 31 and 32 can be suppressed to a relatively small value, and the constant voltage diode 13 has a small rated small size. It can be made lightweight and inexpensive, which is advantageous for mounting. In addition, even if the switching transistor 34 is turned off, the operating current that does not destroy the load L is supplied to the load L via the current limiting resistor 33, so that the required operation of the load L is overvoltage. It will not be interrupted due to occurrence.

Another advantage of keeping the current flowing through the constant voltage diodes 31, 32 small by the current limiting resistor 33 is that the pattern width of the printed circuit board can be made narrower, and a smaller printed circuit board is sufficient. It is possible and economical. If the current is not limited by the current limiting resistor 33, the wiring pattern of the printed board must be thickened to prevent the burnout, which increases the cost of the printed board.

[0024]

According to the present invention, since a small and lightweight low-rated surge absorbing element can be used, it is economical and advantageous in mounting, and it is not necessary to cut off the power supply to the load. Therefore, the operation of the load can be maintained even in the overvoltage state.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an example of an embodiment of a DC power supply circuit according to the invention of claim 1.

FIG. 2 is a circuit diagram showing an example of an embodiment of a DC power supply circuit according to the invention of claim 2.

[Explanation of symbols]

 10, 20 DC power supply circuit 11, 21 DC power supply section 12 Power supply line section 13, 31, 32 Constant voltage diode 14 Impedance switching circuit 15 Voltage monitoring circuit 30 Protection circuit 33 Current limiting resistor 34 Switching transistor 40 Control circuit E Output voltage L load

Claims (2)

[Claims] 1. A DC power supply unit for supplying DC power, a power supply line unit for connecting an output of the DC power supply unit to a load, and a load side of the power supply line unit connected in parallel with the load. In a DC power supply circuit comprising a surge absorbing element provided as described above, the power supply line section is configured to be switchable to either a high impedance state or a low impedance state,
Provided is a voltage monitoring circuit for monitoring whether the output level of the DC power supply unit is in a predetermined overvoltage level state,
A DC power supply circuit, wherein the power supply line section is switched to a high impedance state only when a predetermined overvoltage level state is detected by the voltage monitoring circuit. 2. A direct-current power supply circuit having a direct-current power supply unit for supplying direct-current power, wherein an output voltage from the direct-current power supply unit is supplied to a load through a protection circuit for protecting the load from overvoltage. The protection circuit includes a surge absorption element connected in parallel with the load, a current limiting resistance element inserted and connected in series between the output of the DC power supply unit and the load, and the current limiting resistance element in parallel. And a control circuit which responds to the output voltage level of the DC power supply unit and turns off the switching element only when the voltage level exceeds a predetermined value. DC power supply circuit.