JPH099523A - Power supply switching circuit - Google Patents

Abstract

PURPOSE: To avoid the limitless charging to an internal power supply by a method wherein two semiconductor switching circuits which are connected in series to each other so as to have both the anodes of respective parasitic diodes connected to each other are used and, when a power is supplied to a load, the semiconductor switching circuit on a supply line side is turned off. CONSTITUTION: When a power is supplied to a load 9 through a feeder line 3 from a DC power supply 1, a P-type MOS-FET 4 is turned OFF by a DC power supply voltage monitor circuit 7 and a P-type MOS-FET 5 is turned ON by a battery voltage monitor circuit 8. In this state, as the ideal MOS-FET 4a of the P-type MOS-FET 4 is in an OFF-state and the voltage of a battery 6 is lower than the output voltage of the DC power supply 1, the direction of a diode 4b in the P-type MOS-FET 4 is reverse, so that the charging to the battery 6 from the feeder line 3 can be avoided. Further, the overdischarging and the momentary interruption of the feeder line 3 can be avoided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply switching circuit, and more particularly to a technique effective when applied to a constant voltage power supply for supplying a stable DC voltage to small electronic equipment.

[0002]

2. Description of the Related Art Generally, a power supply for electronic equipment is constituted by a combination of an external DC power supply, for example, an AC adapter and a battery which is an internal DC power supply.

In a power supply composed of a combination of an AC adapter and a battery, the output voltage of the AC adapter is usually set higher than the battery voltage, so that uncontrolled charging of the battery from the power supply line is prevented. Alternatively, it is necessary to prevent over-discharge when the battery capacity is exhausted, and thus a switch circuit for disconnecting the battery from the power supply line is required.

As the switch circuit, a P-channel MOS type field effect transistor (hereinafter referred to as P-channel MOS-FET), which is usually a semiconductor switching element, is used as the switch circuit.
A semiconductor switch circuit is used.

Also, by monitoring the output voltage of the AC adapter,
A circuit for turning on / off the P-channel MOS-FET, and further for preventing instantaneous interruption of the power supply line due to a delay in turning on the P-channel MOS-FET after detecting the output interruption of the AC adapter. It is common to use capacitors and diodes.

FIG. 3 is a diagram showing an example of a conventional power supply switching circuit, and FIG. 3 is a power supply switching circuit using a capacitor for preventing instantaneous interruption.

In FIG. 3, 1 is an external DC power supply (hereinafter,
(Referred to as DC power supply), 2 is a connector for connecting the DC power supply 1 to electronic equipment, 3 is a power supply line, 4 and 5 are P-channel MOS-FETs which are semiconductor switching elements, 6 is a battery which is an internal DC power supply, and 7 is DC power supply voltage monitoring circuit, 8 is a battery voltage monitoring circuit, 9 is a load such as a logic element, 14
Is a capacitor for preventing instantaneous interruption.

The DC power supply 1 is a DC power supply obtained by rectifying and smoothing a commercial AC power supply outside the electronic equipment, or an AC adapter.

The DC power supply voltage monitoring circuit 7 is a circuit for monitoring the output voltage of the DC power supply 1 to turn on / off the P-channel MOS-FETs (4, 5) which are semiconductor switching elements, and are resistors 7a and 7b. Series circuit with resistor 7c
And a constant voltage diode 7d in series, and an operational amplifier 7
e, a pnp type transistor 7f.

The battery voltage monitoring circuit 8 is a circuit for monitoring the output voltage of the battery and turning on / off the P-channel MOS-FETs (4, 5) which are semiconductor switching elements. The resistor 8a and the resistor 8b are connected in series. Circuit, a series circuit of a resistor 8c and a constant voltage diode 8d, and an operational amplifier 8
e, a pnp type transistor 8f.

Further, the P-channel MOS-FET 4 and the P-channel MOS-FET 5 form a semiconductor switch circuit, and the P-channel MOS-FET (4, 5) is shown in FIG.
As shown in, the parasitic diode (4b, 5b) is formed therein.

FIG. 4 shows a P-channel MOS-FET (4,
Parasitic diodes (4b, 5b) formed inside 5)
It is a figure for explaining.

In FIG. 4, 20 is an n region, 21 is a drain region, 22 is a gate electrode, 23 is a gate oxide film, 2
Reference numeral 4 is a source region and 25 is a contact region. In the P-channel MOS-FET, a parasitic diode 4b is formed between the drain region 21 and the contact region 25.

Therefore, as shown in FIG. 4, when the source region 24 and the contact region 25 are made to have the same potential, as a result, the parasitic diode 4b is formed between the drain region 21 and the source region 24.

Therefore, the P-channel MOS-FET 4
The source and the source of the P-channel MOS-FET 5 are connected to each other, and the parasitic diode 4b and the parasitic diode 5b prevent a current from flowing to the battery 6.

The gate and source of the P-channel MOS-FET 4 are connected to the series circuit of the pnp-type transistor 7f and the pnp-type transistor 8f via the resistor 11 and the resistor 10, respectively, and further the P-channel MO-FET 4 is connected.
The gate and source of the S-FET 5 have a resistance of 1
2 and resistor 10 to connect pnp type transistors 7f and p
It is connected to a series circuit with the np-type transistor 8f.

Next, the operation of the power supply switching circuit shown in FIG. 3 will be described.

The inverting input terminal of the operational amplifier 7e of the DC power supply voltage monitoring circuit 7 shown in FIG.
Of the power supply line 3 divided by the resistors 7a and 7b at the non-inverting input terminal of the operational amplifier 7e.
Is applied.

When power is supplied from the DC power supply 1 to the load 9 via the power supply line 3, the voltage of the supply line 3 divided by the resistors 7a and 7b is the output voltage of the constant voltage diode 7d. The output of the operational amplifier 7e becomes "H level", and the output of the operational amplifier 7e is applied to the cathode pn.
The p-type transistor 7f is turned off.

Further, when it becomes impossible to supply electric power from the DC power source 1 to the load 9 through the power feeding line 3 and the voltage of the power feeding line 3 becomes equal to or lower than the output voltage of the battery 6, the resistor 7a is provided. Since the voltage of the power supply line 3 divided by the resistor 7b and the resistor 7b is set to be lower than the output voltage of the constant voltage diode 7d, the operational amplifier 7e
Becomes "L level", and the pnp-type transistor 7f to which the output of the operational amplifier 7e is applied to the cathode is turned on.

At the non-inverting input terminal of the operational amplifier 8e of the battery voltage monitoring circuit 8 shown in FIG.
The output voltage of d and the output voltage of the battery 6 divided by the resistors 8a and 8b are applied to the inverting input terminal of the operational amplifier 8e.

When power is supplied from the DC power supply 1 to the load 9 via the power supply line 3, the output voltage of the battery 6 divided by the resistors 8a and 8b is the output voltage of the constant voltage diode 8d. Since it is set to be higher than the above, the output of the operational amplifier 8e becomes "L level", and the output of the operational amplifier 8e is applied to the cathode pn.
The p-type transistor 8f is turned on.

Further, power is supplied from the battery 6 to the load 9 through the power supply line 3, the capacity of the battery 6 is lost, and the output voltage of the battery 6 divided by the resistors 8a and 8b is a constant voltage diode. When it becomes lower than the output voltage of 8d, the output of the operational amplifier 8e becomes "H level".
Then, the pnp-type transistor 8f to which the output of the operational amplifier 8e is applied to the cathode is turned off.

In addition, the pnp type transistors 7f and pnp
When the type transistor 8f is turned on, the P-channel MOS-FET 4 and the P-channel MOS-FET 5 are turned on.

Therefore, from the DC power supply 1 to the power supply line 3
When power is supplied to the load 9 via the DC power supply voltage monitoring circuit 7, the P-channel MOS-FET (4,
5) is turned off and the capacitor 14 is charged.

When it becomes impossible to supply electric power from the DC power supply 1 to the load 9 via the power supply line 3 due to a power failure of the AC power supply, a failure of the AC adapter, a disconnection of the connector 2, or the like, the load 9 is loaded. Power is supplied to the capacitor 14 by discharging the capacitor 14.

In the meantime, the P-channel MOS-FETs (4, 5) are turned on by the DC power supply voltage monitoring circuit 7, and before the capacitor 14 is completely discharged, from the battery 6 to the load 9 via the power supply line 3. Electric power is supplied to prevent the power supply line 3 from being instantaneously cut off.

After that, when the discharge of the battery 6 progresses and the capacity of the battery 6 is exhausted, the battery voltage monitoring circuit 8 turns off the P-channel MOS-FETs (4, 5) to prevent over-discharge. I am trying.

FIG. 5 is a diagram showing another example of a conventional power supply switching circuit, which is a power supply switching circuit using a diode for preventing instantaneous interruption.

In FIG. 5, 1 is a DC power supply, 2 is a connector for connecting the DC power supply 1 and electronic equipment, 3 is a power supply line,
4, 5 are P-channel MO which are semiconductor switching elements
S-FET, 6 is a battery which is an internal DC power supply, 8 is a battery voltage monitoring circuit, 9 is a load such as a logic element, and 15 is a diode for preventing instantaneous power failure.

In FIG. 5, components designated by the same reference numerals as those in FIG. 3 are the same components as those of the power supply switching circuit shown in FIG.

In the power supply switching circuit shown in FIG. 5, parts different from the power supply switching circuit shown in FIG. 3 will be described.

The P-channel MOS-FET 4 and the P-channel MOS-FET 5 form a semiconductor switch circuit, and the gate and the source of the P-channel MOS-FET 4 are connected to the pnp type transistor 8f via the resistor 11 and the resistor 10, respectively. Connected to a P-channel MOS
-The gate and the source of the FET 5 are each a resistor 12
Is connected to the pnp type transistor 8f via the resistor 10.

The diode 15 is a P channel MO.
It is connected between the S-FET 4 and the power supply line 3.

Next, the operation of the power supply switching circuit shown in FIG. 5 will be described.

The non-inverting input terminal of the operational amplifier 8e of the battery voltage monitoring circuit 8 shown in FIG.
The output voltage of d and the output voltage of the battery 6 divided by the resistors 8a and 8b are applied to the inverting input terminal of the operational amplifier 8e.

Here, when power is supplied from the DC power supply 1 to the load 9 via the power supply line 3, the output voltage of the battery 6 divided by the resistors 8a and 8b is the output voltage of the constant voltage diode 8d. Since it is set to be higher than the above, the output of the operational amplifier 8e becomes "L level", and the output of the operational amplifier 8e is applied to the cathode pn.
The p-type transistor 8f is turned on.

Further, power is supplied from the battery 6 to the load 9 through the power supply line 3, the capacity of the battery 6 is exhausted, and the output voltage of the battery 6 divided by the resistors 8a and 8b is a constant voltage diode 8d. When the output voltage of the operational amplifier 7e becomes lower than the output voltage of the operational amplifier 7e, the output of the operational amplifier 7e becomes "H level", and the output of the operational amplifier 8e is applied to the cathode p.
The np type transistor 8f is turned off.

When the pnp transistor 8f is turned on, the P channel MOS-FET 4 and the P channel MOS-FET 5 are turned on.

Therefore, from the DC power supply 1 to the power supply line 3
When power is supplied to the load 9 via the battery voltage monitoring circuit 8, the P-channel MOS-FET (4,
Since 5) is in the ON state, the diode 15 prevents uncontrolled charging of the battery 6.

When it becomes impossible to supply electric power from the DC power supply 1 to the load 9 through the power supply line 3 due to a power failure of the AC power supply, a failure of the AC adapter, a disconnection of the connector 2, or the like, a P channel MOS- Since the FETs (4, 5) are in the ON state, when the output voltage of the battery 6 becomes higher than the output voltage of the DC power supply 1, the diode 15 conducts to supply the power from the battery 6 to the load 9 via the power supply line 3. However, the power line 3 is prevented from being instantaneously disconnected.

When the battery 6 runs out of capacity,
The battery voltage monitoring circuit 8 allows the P-channel MOS-F
ET (4,5) is turned off to prevent over-discharge.

[0043]

FIG. 3 or,
In the conventional power supply switching circuit shown in FIG. 5, the capacitor 14 or the diode 15 is used to prevent the power supply line 3 from being instantaneously disconnected.

However, when the load 9 becomes large, in the power supply switching circuit shown in FIG.
Is required to be increased, and in the power supply switching circuit shown in FIG. 5, heat generation due to the loss of the diode 15 is increased and a heat sink or the like is required.

Therefore, the conventional power supply switching circuit shown in FIG. 3 or FIG. 5 has a problem that the miniaturization required for a small electronic device is hindered.

The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to reduce the size, increase the reliability, and reduce the cost of a power supply switching circuit. To provide a technology that enables

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0048]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows.

(1) In the power supply switching circuit for switching the power supply connected to the power supply line to the internal DC power supply when it becomes impossible to supply power from the external DC power supply connected to the power supply line to the load, It is connected between the power supply line and the internal DC power supply and has one or more semiconductor switching elements, and is connected in series so that the anodes of the parasitic diodes formed in each semiconductor switching element are connected to each other. Two semiconductor switch circuits and an external DC power supply voltage for switching the semiconductor switch circuits on the power supply line side from the OFF state to the ON state when the output voltage of the external DC power supply becomes equal to or lower than the output voltage of the internal DC power supply. When the output voltage of the monitoring circuit and the internal DC power supply falls below a predetermined voltage, the internal DC power supply Characterized in that it comprises an internal DC power supply voltage monitoring circuit for switching the semiconductor switching circuit side from the ON state to the OFF state.

[0050]

According to the above means, when it becomes impossible to supply power to the load from the external DC power supply connected to the power supply line, the power supply switching to switch the power supply connected to the power supply line to the internal DC power supply. In the circuit, one or more semiconductor switching elements are provided between the power supply line and the internal DC power supply, and the anodes of the parasitic diodes formed in the respective semiconductor switching elements are connected in series. When connecting two connected semiconductor switch circuits and supplying power from the external DC power supply to the load via the power supply line, turn off the semiconductor switch circuit on the power supply line side and disconnect the internal DC power supply from the power supply line. Prevents uncontrolled charging of the internal DC power supply.

When it becomes impossible to supply power from the external DC power supply to the load, the semiconductor switch circuit on the power supply line side is turned on to supply power from the internal DC power supply to the load, and at the same time the power supply line side is supplied. While the semiconductor switch circuit is turned on, electric power is supplied to the load from the internal DC power supply via the parasitic diode of the semiconductor switching element forming the semiconductor switch circuit on the power supply line side to prevent instantaneous interruption of the power supply line.

After that, when the capacity of the internal DC power supply is exhausted, the semiconductor switch circuit on the internal DC power supply side is turned off to prevent the internal DC power supply from being over-discharged.

[0053]

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

In all the drawings for explaining the embodiments, parts having the same function are designated by the same reference numerals and their repeated description will be omitted.

FIG. 1 is a diagram showing a power supply switching circuit according to an embodiment of the present invention.

In FIG. 1, 1 is an external DC power supply (hereinafter,
(Referred to as DC power supply), 2 is a connector for connecting the DC power supply 1 to electronic equipment, 3 is a power supply line, 4 and 5 are P-channel MOS-FETs which are semiconductor switching elements, 6 is a battery which is an internal DC power supply, and 7 is A DC power supply voltage monitoring circuit, 8 is a battery voltage monitoring circuit, and 9 is a load such as a logic element.

The DC power supply 1 is a DC power supply obtained by rectifying and smoothing a commercial AC power supply outside the electronic equipment, or an AC adapter.

The DC power supply voltage monitoring circuit 7 is a circuit for monitoring the output voltage of the DC power supply 1 and turning on / off the P-channel MOS-FET 4 which is a semiconductor switching element.
A series circuit of a resistor 7a and a resistor 7b, a series circuit of a resistor 7c and a constant voltage diode 7d, operational amplifiers 7e and pn
It is composed of a p-type transistor 7f.

The battery voltage monitoring circuit 8 is a circuit for monitoring the output voltage of the battery and turning on / off the P-channel MOS-FET 5 which is a semiconductor switching element. The series circuit of the resistors 8a and 8b and the resistor 8c. And a constant voltage diode 8d in series circuit, operational amplifiers 8e, pnp
Type transistor 8f.

The P-channel MOS-FET 4 and the P-channel MOS-FET 5 form a semiconductor switch circuit. The P-channel MOS-FET 4 and the P-channel MOS-
FET5 has a drain and a drain connected to each other,
By the parasitic diode (4b, 5b), the battery 6
The current is prevented from flowing in.

The source of the P-channel MOS-FET 4 is connected to the gate via the resistor 10, and further, P
The gate of the channel MOS-FET 4 is connected to the pnp type transistor 7f via the resistor 11.

The source of the P-channel MOS-FET 5 is connected to the gate via the resistor 13, and further, P
The gate of the channel MOS-FET 5 is connected to the pnp type transistor 8f via the resistor 12.

Next, the operation of the power supply switching circuit of this embodiment will be described.

The inverting input terminal of the operational amplifier 7e of the DC power supply voltage monitoring circuit 7 of this embodiment has a constant voltage diode 7d.
Of the power supply line 3 divided by the resistors 7a and 7b at the non-inverting input terminal of the operational amplifier 7e.
Is applied.

Here, when power is supplied from the DC power supply 1 to the load 9 via the power supply line 3, the voltage of the power supply line 3 divided by the resistors 7a and 7b is the output voltage of the constant voltage diode 7d. The output of the operational amplifier 7e becomes "H level", and the output of the operational amplifier 7e is applied to the cathode pn.
The p-type transistor 7f is turned off.

When it becomes impossible to supply electric power from the DC power supply 1 to the load 9 through the power supply line 3, and the voltage of the power supply line 3 becomes equal to or lower than the output voltage of the battery 6, the resistor 7a Since the voltage of the power supply line 3 divided by the resistor 7b and the resistor 7b is set to be lower than the output voltage of the constant voltage diode 7d, the operational amplifier 7e
Becomes "L level", and the pnp-type transistor 7f to which the output of the operational amplifier 7e is applied to the cathode is turned on.

When the pnp type transistor 7f is turned on, the P channel MOS-FET 4 is turned on.

The non-inverting input terminal of the operational amplifier 8e of the battery voltage monitoring circuit 8 of this embodiment has a constant voltage diode 8
The output voltage of d and the output voltage of the battery 6 divided by the resistors 8a and 8b are applied to the inverting input terminal of the operational amplifier 8e.

When power is supplied from the DC power supply 1 to the load 9 via the power supply line 3, the output voltage of the battery 6 divided by the resistors 8a and 8b is the output voltage of the constant voltage diode 8d. Since it is set to be higher than the above, the output of the operational amplifier 8e becomes "L level", and the output of the operational amplifier 8e is applied to the cathode pn.
The p-type transistor 8f is turned on.

When the pnp type transistor 8f is turned on, the P channel MOS-FET 5 is turned on.

In addition, power is supplied from the battery 6 to the load 9 through the power supply line 3, the capacity of the battery 6 is lost, and the output voltage of the battery 6 divided by the resistors 8a and 8b is the constant voltage diode 8d. When the output voltage of the operational amplifier 8e becomes lower than the output voltage of the operational amplifier 8e, the output of the operational amplifier 8e becomes "H level", and the output of the operational amplifier 8e is applied to the cathode.
The p-type transistor 8f is turned off.

Therefore, from the DC power source 1 to the power feeding line 3
When power is supplied to the load 9 via the DC power supply voltage monitoring circuit 7, the P-channel MOS-FET 4 is turned off by the DC power supply voltage monitoring circuit 7, and the P-channel M-channel is monitored by the battery voltage monitoring circuit 8.
The OS-FET 5 is on.

In this case, the P-channel MOS-FET 4
Of the ideal MOS-FET 4a in the off state,
Since the voltage of the battery 6 is lower than the output voltage of the DC power supply 1, the parasitic diode 4b inside the P-channel MOS-FET 4 is in the opposite direction, preventing uncontrolled charging of the battery 6 from the power supply line 3. There is.

DC power supply 1 to power supply line 3 due to AC power supply failure, DC power supply failure, disconnection of connector 2, etc.
When it becomes impossible to supply electric power to the load 9 through the P-channel MOS-FET 5, the P-channel MOS-FET 5 is in the ON state, and therefore the ideal MOS-FET of the P-channel MOS-FET 5 is formed.
From the battery 6 to the load 9 via the FET 5a, the parasitic diode 4b of the P-channel MOS-FET 4, and the power supply line 3.
Power is supplied to the power supply line 3 to prevent instantaneous interruption.

In the meantime, the voltage of the battery 6 changes to the DC power source 1
The output voltage of the P-channel MOS-FET 4 becomes higher than that of the P-channel MOS-FET 4 by the DC power supply voltage monitoring circuit 7.
Since T4a is turned on, it is possible to suppress heat generation of the parasitic diode 4b.

Thereafter, when the capacity of the battery 6 is exhausted while the battery 6 is being discharged, the battery voltage monitoring circuit 8 causes the ideal M of the P-channel MOS-FET 5 to change.
Since the OS-FET 5a is turned off and the parasitic diode 5b of the P-channel MOS-FET 5 is in the opposite direction, it is possible to prevent the battery 6 from being over-discharged.

Therefore, according to the power supply switching circuit of the present embodiment, even if the load increases, the P-channel MOS-FET (4, which is a semiconductor switching element corresponding to the increase of the load).
By selecting 5), it becomes possible to prevent uncontrolled charging, over-discharging, and instantaneous interruption of the power supply line 3.

Further, in the power supply switching circuit shown in FIG. 1, the case where the semiconductor switch circuit is composed of one P-channel MOS-FET (4, 5) has been described, but as shown in FIG. , A semiconductor switch circuit, two or more P-channel MOS-FET (40, 41,
..) and two or more P-channel MOS-FETs (50,
51, ···).

In this case, when the load increases, the number of P-channel MOS-FETs (40, 41, ...
By using (.), It is possible to prevent uncontrolled charging, over-discharging, and instantaneous interruption of the power supply line 3.

Although the present invention has been specifically described based on the embodiments, the present invention is not limited to the above embodiments, and it goes without saying that various modifications can be made without departing from the scope of the invention.

[0081]

The effects obtained by the typical ones of the inventions disclosed in the present application will be briefly described as follows.

(1) According to the present invention, in the power supply switching circuit for switching the power supply connected to the power supply line to the internal DC power supply when the power cannot be supplied from the external DC power supply to the load, Even if the discharge current of the internal DC power supply increases according to the load, by using a semiconductor switch circuit that corresponds to it, uncontrolled charging and
It is possible to prevent over-discharge and instantaneous interruption of the power supply line.

As a result, it is possible to reduce the number of parts, reduce the size, increase the reliability, and reduce the cost, unlike the conventional case, which does not require a part for preventing instantaneous interruption, such as a capacitor or a diode. Becomes

[Brief description of drawings]

FIG. 1 is a diagram showing a power supply switching circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing another example of a semiconductor switch circuit in the power supply switching circuit shown in FIG.

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

FIG. 4 is a diagram for explaining a parasitic diode formed inside a P-channel MOS-FET.

FIG. 5 is a diagram showing another example of a conventional power supply switching circuit.

[Explanation of symbols]

1 ... External DC power supply, 2 ... Connection connector, 3 ... Power supply line, 4, 5, 40, 41, 50, 51 ... P channel MO
S-type field effect transistor (P-channel MOS-FE
T), 4a, 5a ... Ideal MOS-FET, 4b, 5b
... Parasitic diode, 6 ... Battery, 7 ... DC power supply voltage monitoring circuit, 8 ... Battery voltage monitoring circuit, 7a, 7b, 7
c, 8a, 8b, 8c, 10, 11, 12, 13, ... Resistance, 7d, 8d ... Constant voltage diode, 7e, 8e ... Operational amplifier, 7f, 8f ... Pnp type transistor, 9 ... Load, 14 ... Capacitor, 15 ... Diode, 20 ... N region, 21 ... Drain region, 22 ... Gate electrode, 23 ... Gate oxide film, 24 ... Source region, 25 ... Contact region.

Claims (1)

[Claims] 1. When it becomes impossible to supply electric power to a load from an external DC power supply connected to a power supply line,
A power supply switching circuit for switching a power supply connected to a power supply line to an internal DC power supply, which is connected between the power supply line and the internal DC power supply, has one or more semiconductor switching elements, and is formed in each semiconductor switching element. Two semiconductor switch circuits connected in series so that the anodes of the parasitic diodes are connected to each other, and the power supply when the output voltage of the external DC power supply becomes equal to or lower than the output voltage of the internal DC power supply. An external DC power supply voltage monitoring circuit that switches the semiconductor switch circuit on the line side from an OFF state to an ON state, and a semiconductor switch circuit on the internal DC power supply side when the output voltage of the internal DC power supply falls below a predetermined voltage. And an internal DC power supply voltage monitoring circuit for switching from an on state to an off state. Power switching circuit that.