JPH09322399A - Power supply changeover circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the power loss generated in a changeover circuit, by using a transistor instead of a reverse current preventing diode. SOLUTION: When the positive voltage of a DC power supply 1 is higher than the positive voltage of a DC power supply 2, a P-type power transistor 13 is turned off, and the output voltage of a power-supply changeover circuit 4 becomes equal to the one obtained through the subtraction of the forward voltage drop of a diode 11 from the positive voltage of the DC power supply 1. On the other hand, when the positive voltage of the DC power supply 2 is higher than the positive voltage of the DC power supply 1, the P-type power transistor 13 is turned on to make the output voltage of a power-supply changeover circuit 4 equal to the one obtained through the subtraction of the voltage drop caused by the ON-resistance of the P type power transistor 13 from the positive voltage of the DC power supply 2. Since the voltage drop caused by the ON-resistance of the P-type power transistor 13 is smaller than the forward voltage drop of the diode 11, the power loss in the case of feeding a power from the DC power supply 2 to a load 3 can be reduced.

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,
In particular, it relates to a power supply switching circuit with less power loss.

[0002]

2. Description of the Related Art A conventional power supply switching circuit will be described in detail with reference to the drawings.

FIG. 3 is a circuit diagram showing a conventional example.
The power supply switching circuit shown in FIG. 3 is configured to include diodes 11 and 21 for preventing backflow. Although the DC power supply 1 and the DC power supply 2 are operated in parallel, it is necessary to prevent the current from directly flowing from the power supply with the higher voltage to the power supply with the lower voltage.

Now, assuming that one battery is used as a power source and one battery is considerably consumed, a supplementary battery is inserted into the apparatus, and power must be continuously supplied to the load 3. A circuit for satisfying the condition that does not occur corresponds to FIG. It is assumed that the DC power supply 2 is inserted when the DC power supply 1 first supplies power to the load 3 and the output voltage thereof gradually decreases to 6 V to 5 V and approaches the lower limit of the required voltage. Without the diodes 11 and 21, the output (6 volts) of the DC power supply 2 is mainly used for charging the DC power supply 1, and the load 3 cannot be supplied with a sufficient voltage. Therefore, when the DC power supply 1 and the DC power supply 2 are operated in parallel via the diodes 11 and 21, the diode 11 blocks the current flowing into the DC power supply 1, and the energy of the DC power supply 2 is directed toward the load 3. Only flows.

[0005]

The above-mentioned conventional power supply switching circuit consumes electric power determined by the product of the forward voltage drop of the diode and the current flowing through the diode, and therefore has the drawback of lowering the efficiency of the power supply system. there were.

[0006]

In the power supply switching circuit of the first invention, a transistor is used in place of the backflow prevention diode to reduce the power loss generated in the switching circuit. A power supply switching circuit characterized by the above.

According to a second aspect of the present invention, there is provided a power source switching circuit in which a diode connected in a forward direction from a positive voltage terminal of a first DC power source to a positive voltage terminal of a load and an emitter electrode of the second DC power source have a positive voltage terminal. And a collector electrode connected to the positive voltage terminal of the load and a base electrode connected to the voltage dividing point of the voltage dividing resistor of the second DC power supply.

The power supply switching circuit of the third invention detects a voltage appearing at the positive voltage terminals of a plurality of DC power supplies (1a to 1n) and outputs a plurality of terminals corresponding to the DC power supply showing the maximum voltage. Power supply voltage detection circuit (30) that selects from the terminals (31a to 31n) and sets its logic level to LOW
And each emitter electrode is a DC power supply (1a to 1n)
Connected to the positive voltage terminal of the load (3) and connected to the positive voltage terminal of the load (3) and the respective base electrodes of the output terminals (31a to 31) of the power supply voltage detection circuit (30).
n) power transistors (13a-13) connected to
n) and are included.

In the power supply switching circuit of the fourth invention, in the third invention, the switching time of the output terminal of the power supply voltage detection circuit overlaps.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing a first embodiment of the present invention. The power supply switching circuit shown in FIG.
From the positive voltage terminal of the load 3 to the positive voltage terminal of the load 3, the emitter electrode is connected to the positive voltage terminal of the DC power supply 2, the collector electrode is connected to the positive voltage terminal of the load 3, and the base electrode is DC voltage source 2 voltage dividing resistors 11 and 2
And a power transistor 13 connected to the voltage dividing point of 2.

When the positive voltage of the DC power supply 1 is higher than the positive voltage of the DC power supply 2, the P-type power transistor 13 is turned off, and the output voltage of the power supply switching circuit 4 is in the order from the positive voltage of the DC power supply 1 to the diode 11. It is the value obtained by subtracting the directional voltage drop.

When the positive voltage of the DC power supply 2 is higher than the positive voltage of the DC power supply 1, the P-type power transistor 13 is turned on, and the output voltage of the power supply switching circuit 4 changes from the positive voltage of the DC power supply 2 to the P-type power. It has a value obtained by subtracting the voltage drop due to the on-resistance of the transistor 13. Since the voltage drop due to the on-resistance of the power transistor 13 is smaller than the forward voltage drop of the diode 11, the loss can be reduced when power is supplied from the DC power supply 2 to the load 3. If the diode 11 is replaced with a power transistor, the loss can be reduced even when power is supplied from the DC power supply 1 to the load 3.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention. The power supply switching circuit shown in FIG.
The terminal corresponding to the DC power supply that has detected the voltage appearing at the positive voltage terminals a to 1n and shows the maximum voltage is selected from the plurality of output terminals 31a to 31n, and its logic level is set to LOW.
Power supply voltage detection circuit 30, each emitter electrode is connected to the positive voltage terminals of the DC power supplies 1a to 1n, each collector electrode is connected to the positive voltage terminal of the load 3, and each base electrode is the power supply voltage detection circuit 30. Output terminal 3
Power transistors 13a-connected to 1a-31n
13n and.

Now, the output terminal 3 of the power supply voltage detection circuit 30
If 1a is LOW, the DC power supply 1a supplying current to the load 3 is consumed, and the voltage of the DC power supply 1b becomes the highest, the power supply voltage detection circuit 30
Output terminal 31b becomes LOW and other output terminals 31b
31n are turned HIGH, the power transistor 13b is turned ON, and the DC power supply 1b supplies current to the load 3.

If the switching times of the output terminals of the power supply voltage detection circuit are set to overlap,
No power interruption to the load 3 occurs.

[0017]

The power supply switching circuit of the present invention has the effect that the power loss generated in the switching circuit can be reduced by using a transistor instead of the diode for preventing backflow.

[Brief description of drawings]

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

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

FIG. 3 is a circuit diagram showing an example of the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2 DC power supply 3 Load 4 Power supply switching circuit 11 Diode 12 Resistance 13 Power transistor 14 Diode 22 Resistance

Claims (4)

[Claims] 1. A power supply switching circuit characterized by using a transistor instead of a diode for preventing backflow to reduce power loss generated in the switching circuit. 2. A diode connected in a forward direction from a positive voltage terminal of a first DC power supply to a positive voltage terminal of a load, an emitter electrode connected to a positive voltage terminal of a second DC power supply, and a collector electrode of the load. And a power transistor having a base electrode connected to the positive voltage terminal of the second DC power supply connected to the voltage dividing point of the voltage dividing resistor of the second DC power supply. 3. A plurality of output terminals (31a to 31) corresponding to the DC power supply that has detected the voltage appearing at the positive voltage terminals of the plurality of DC power supplies (1a to 1n) and exhibits the maximum voltage.
n) and a power supply voltage detection circuit (30) for setting its logic level to LOW, each emitter electrode is connected to the positive voltage terminal of the DC power supply (1a to 1n), and each collector electrode is connected to the load (3). Power supply switching circuit, which includes a power transistor (13a to 13n) connected to the output terminal (31a to 31n) of the power supply voltage detection circuit (30), the base electrode of which is connected to the positive voltage terminal of FIG. . 4. The power supply switching circuit according to claim 3, wherein switching times of output terminals of the power supply voltage detection circuit overlap.