JPH07248853A - Power switching circuit - Google Patents

Abstract

PURPOSE:To reduce the power consumption and to prevent the occurrence of a momentary break to improve the reliability by connecting a PNP switching transistor TR and first and second diodes between an external power source and a battery and selectively supplying the output voltage to a load. CONSTITUTION:First and second diodes 11 and 12 for prevention of the reverse current are connected in parallel to the output end of an AC adapter for external power source, and the first diode 11 is connected to a load 13 and the collector of a transistor TR 14. A secondary battery 15 is connected to the emitter of the TR 14, and the earth is connected to the base through the second diode 12 and a resistance 16. That is, when output voltages of the battery 15 and the adapter 10 are prescribed values, the current doesn't flow between the emitter and the collector of the TR 14, and the output voltage of the AC adapter 10 is supplied to the load 13. If the output voltage of the AC adapter 10 is reduced, the current flowing between the emitter and the collector is increased, and the output voltage of the battery 15 is generated on the collector side, and the output voltage of the battery 15 is supplied to the load, 13.

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 for selectively connecting two power supplies to a load.

[0002]

2. Description of the Related Art At present, portable equipment is required for electronic equipment such as personal computers and word processors and communication equipment such as cordless telephones. It is ready for use.

Normally, the battery power supply is used when the external power supply is not connected or when the voltage value of the external power supply is below a predetermined voltage value. Therefore, the predetermined voltage is supplied from the external power supply while the battery power supply is in use. Then, the power source is immediately switched to the external power source (see, for example, Japanese Utility Model Laid-Open No. 1-127017).

FIG. 3 shows the simplest conventional configuration of this type of circuit, in which the voltage supplied from an external power source such as a commercial power source is AC.
The adapter (1) converts the voltage into a predetermined driving voltage and supplies the voltage to the main circuit via the first diode (2) and the voltage supplied from the built-in battery (3) to the second power feeding path. A general configuration is one in which a second power supply path that supplies the main circuit via the diode (4) is connected in parallel.

[0005]

However, in the above-mentioned conventional structure, the second diode (4) for preventing backflow of the power source supplied from the external power source to the battery (3) is not supplied from the external power source. Without it, it would simply be a load on the battery. Therefore, it has been a cause of causing unnecessary power consumption.

[0006]

The present invention has been made in view of the above-mentioned problems, and features thereof include a first power supply means and a second power supply means.
A power supply switching circuit for selectively supplying a voltage output from the power supply means to a load, and a first diode for backflow prevention connected between the first power supply means and the load, A PNP having a collector connected to the cathode of the first diode and the second power supply means connected to the emitter.
Type switching transistor, a second diode connected between the base of the transistor and the anode of the first diode, and a resistor connected between the base of the transistor and ground.

Another feature is that the first diode is a Schottky type.

[0008]

According to this structure, there is no component that causes power consumption between the second power supply means and the load. Further, by making the first diode a Schottky type, the forward voltage Vf of the first diode can be lowered.

[0009]

FIG. 1 is a circuit diagram showing an embodiment of the present invention,
The AC adapter (10) converts a commercial power supply into a DC voltage of 7.5 V and outputs it. A Schottky type backflow prevention first diode (1
1) and the anode of the second diode (12) for cutoff are connected in parallel. The first diode (1
The cathode of 1) is connected to the load (13) and the collector of a PNP type switching transistor (14). A secondary battery (15) made of nickel-cadmium, nickel-hydrogen, lithium, or the like and having a maximum output voltage of about 7.5 V is connected to the emitter of the transistor (14),
Further, the base is connected to the ground through the cathode of the second diode (12) and the voltage dividing resistor (16).

The transistor (14) is, for example, 2SA1562 manufactured by Sanyo Electric Co., Ltd., and the reverse withstand voltage value between its base and emitter is 15V. Further, the forward voltage Vf of the Schottky type first diode (11) is 0.3 V, and V of the second diode (12) is Vf.
f is about 0.6V.

Next, to explain the operation of the apparatus of this embodiment,
Now, assuming that the output voltage values of the secondary battery (15) and the AC adapter (10) are both 7.5 V, the secondary battery (1
The output voltage difference between 5) and the AC adapter (10) is about 0.6 V even if the step-down amount of the second diode (12) is taken into consideration, and no current flows between the emitter and collector of the transistor (14). As a result, the above load (13) has A
The voltage output from the C adapter (10) is supplied.

Thereafter, when the output voltage of the AC adapter (10) gradually decreases, the emitter-base voltage VEB of the transistor (14) gradually increases, and the current flowing between the emitter and collector also increases accordingly. .

The output voltage value of the AC adapter (10) is 7.
When the voltage becomes 0 V or less and the VEB value of the transistor (14) becomes substantially 1 V or more, almost complete conduction is established between the emitter and collector. As a result, the transistor (14)
The output voltage of the secondary battery (15) (7.5
V) occurs. On the other hand, the output of the AC adapter (10) is stepped down by the first diode (11) and supplied as a voltage of about 6.7V. As a result, the voltage output from the secondary battery (15) is supplied to the load (13).

FIG. 2 shows the load (13) in the above operation.
Supply voltage value (shown by the solid line in the figure) to the AC adapter (1
0) shows the correlation with the output voltage value (indicated by a broken line in the figure).

As is clear from FIG. 2, in this embodiment, as the output voltage value of the AC adapter (10) decreases, the power is supplied to the load (13) by the AC adapter (10).
To the secondary battery (15) gradually.

In this embodiment, when the output voltage value of the AC adapter (10) becomes 7.0 V, the power supply to the load (13) is completely switched to the secondary battery (15) side. As described above, such a switching point is determined based on the characteristics of the first and second diodes (11) (12) and the transistor (14).

A Schottky type diode is adopted as the first diode (11), which is provided as shown in FIG. 2 by reducing the forward voltage value of the first diode (11) as much as possible. This is to shorten the period during which the voltage value drops.

[0018]

According to the present invention, it is possible to obtain a highly reliable power supply circuit without a momentary interruption or the like with a simple circuit. Also,
Since only the transistor is interposed between the secondary battery and the load, power loss can be suppressed.

[Brief description of drawings]

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

FIG. 2 is a schematic diagram for explaining the operation of the present embodiment.

FIG. 3 is a circuit diagram showing a conventional example.

[Explanation of symbols]

 10 AC Adapter 11 First Diode 12 Second Diode 13 Load 14 Transistor 15 Secondary Battery 16 Resistance

Claims (2)

[Claims] 1. A power supply switching circuit for selectively supplying a voltage output from each of the first power supply means and the second power supply means to a load, wherein the power supply switching circuit is provided between the first power supply means and the load. A first diode connected to prevent backflow, a PNP switching transistor having a collector connected to the cathode of the first diode and the second power supply means connected to the emitter, a base of the transistor, and the first transistor. A power supply switching circuit comprising a second diode connected between the anode of the diode and a resistor connected between the base of the transistor and ground. 2. The power supply switching circuit according to claim 1, wherein the first diode is a Schottky type.