JPH0670487A - Power supply switching-over circuit - Google Patents

Abstract

PURPOSE:To provide a power supply switching-over circuit whereby a useless power consumption can be reduced. CONSTITUTION:An external power supply 1 and a load circuit 2 are connected by a MOS type FET Q1, and a battery 3 and the load circuit 2 are connected by a MOS type FET Q2. Further, by a differential switching circuit 41 using a FET Q5 and transistors Q6-Q9, the voltage difference between the external power supply 1 and the battery 3 is sensed. Hereupon, according to the sensed result, either the FET Q1 or the FET Q2, which is on the side of the higher voltage one of the voltage sources comprising the external power supply 1 and the battery 3, is brought into ON-state. Therefore, a current is applied to the load circuit 2 from either the external power supply 1 or the battery 3 via either the FET Q1 or the FET Q2 respectively. Thereby, a useless power consumption can be reduced, and the time in which the battery 3 can drive a portable apparatus, etc., can be increased.

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 automatically switching between a battery and an external power supply.

[0002]

2. Description of the Related Art Conventionally, in a portable device or the like, a method has been adopted in which consumption of the built-in battery is reduced by automatically switching between the built-in battery and an external power source for use. As the power source switching circuit for automatically switching the power source in this way, the one shown in FIG. 2 is used. Figure 2
, D1 and D2 are diodes, and diode D1
The anode is connected to the positive terminal of the external power supply 1 via a connector or the like (not shown), and the cathode is connected to the load 2.
Furthermore, the anode of the diode D2 is connected to the positive terminal of the battery 3, and the cathode is connected to the load 2. The negative terminals of the external power source 1 and the battery 3 are grounded together with the negative terminal of the load.

With the above configuration, if either the external power source 1 or the battery 3 is connected, the load 2 is energized to drive the load. When both the external power source 1 and the battery 3 are connected, the load is energized from the one with the higher voltage.

[0004]

However, in the above-mentioned conventional power supply switching circuit, since the power supply voltages of the external power supply 1 and the battery 3 are compared by the diodes D1 and D2, the forward voltage drop of the diodes D1 and D2 is caused. However, there is a problem that power consumption is large and power consumption is large.

That is, when the fast recovery diodes are used as the diodes D1 and D2, the forward voltage drop becomes 1.0 V, and the current path 2A to the load is as follows:
Its power consumption is 2W. In addition, the forward voltage drop is 0.5
Even if a Schottky barrier diode with a small V is used,
1 W of power was wasted.

In view of the above problems, an object of the present invention is to provide a power supply switching circuit capable of reducing wasteful power consumption.

[0007]

In order to achieve the above object, the present invention provides a power supply switching circuit for supplying a current from a DC power supply having a higher voltage of a first DC power supply and a second DC power supply to a load. A first field effect transistor which is interposed between the first direct current power supply and the load, and has a drain and a source connected to the first direct current power supply and the second direct current power supply and the load, and A second field-effect transistor having a drain and a source connected to each other, a first transistor switching circuit having a voltage of the first DC power supply as a control voltage, and a voltage of the second DC power supply having a control voltage as a control voltage. And a second transistor switching circuit, the differential switch inverting the ON / OFF state of the first and second transistor switching circuits in response to the voltage difference between the first and second DC power supplies. And quenching circuit, the first and second
Proposes a power supply switching circuit provided with a voltage control circuit for controlling the gate voltages of the first and second field effect transistors based on the ON / OFF state of the transistor switching circuit.

[0008]

According to the present invention, the first field effect transistor is connected between the first DC power supply and the load, and the second field effect transistor is connected between the second DC power supply and the load. To be done. Further, the voltage of the first DC power supply is applied as a control voltage to the first transistor switching circuit of the differential switching circuit, and the voltage of the second DC power supply is applied as a control voltage to the second transistor switching circuit. . As a result, the ON / OFF states of the first and second transistor switching circuits are set to the first and second
Inverted according to the voltage difference of the DC power source. For example, when the voltage of the first DC power supply is higher than the voltage of the second DC power supply, the first transistor switching circuit is turned off and the second transistor switching circuit is turned on. The reverse bias voltage between the gate and the source of the first field effect transistor that has been set is lowered via the voltage control circuit, and the first field effect transistor is turned on. At this time, the voltage control circuit causes the gate of the second field effect transistor
The reverse bias voltage between the sources is increased and the second field effect transistor is turned off. As a result, the load is energized from the first DC power supply via the first field effect transistor. Further, when the voltage of the second DC power supply is higher than the voltage of the first DC power supply, the first transistor switching circuit is turned on and the second transistor switching circuit is turned off. Is set. At this time, the gate-source reverse bias voltage of the second field effect transistor is lowered via the voltage control circuit, the second field effect transistor is turned on, and the first field effect transistor is turned on. The reverse bias voltage between the gate and the source of the transistor is increased, and the first field effect transistor is turned off. As a result, the load is energized from the second DC power supply through the second field effect transistor.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention.
In the figure, the same components as those in the conventional example described above are represented by the same reference numerals. That is, 1 is an external power source such as an AC adapter,
Reference numeral 2 is a load circuit, 3 is a battery, and 4 is a power supply switching circuit.

The positive electrodes of the external power source 1 and the battery 3 are connected to the input terminals IN1 and IN2 of the power source switching circuit 4, and the negative electrodes are connected to the ground terminal G. The power input terminal of the load circuit 2 is the output terminal O of the power switching circuit 4.
It is connected to the UT.

The power supply switching circuit 4 comprises a differential switching circuit 41, MOS type N-channel field effect transistors (hereinafter referred to as FETs) Q1, Q2, Q3, Q4, and resistors Ra, Rb.

The drain of the FET Q1 is the first input terminal I
It is connected to the positive electrode terminal of the external power supply 1 via N1, the source is connected to the positive electrode input terminal of the load circuit 2 via the output terminal OUT, and is connected to the drain of the FET Q3. The gate of the FET Q1 is connected to the gate of the FET Q3 and also to the source of the FET Q3 via the resistor Ra. The drain of the FET Q2 is connected to the positive terminal of the battery 3 via the second input terminal IN2, and the source is connected to the output terminal OUT and the drain of the FET Q4. The gate of FET Q2 is FE
It is connected to the gate of TQ4 and also to the source of FET Q4 via a resistor Rb.

The differential switching circuit 41 is constructed symmetrically in the figure. That is, a P-channel field effect transistor (hereinafter referred to as FET) Q5, PNP
Type transistors Q6 and Q7, NPN type transistors Q8 and Q9, and resistors R1 to R7, and resistors R1 and R2 and resistors R3 and R4, and resistors R5 and R.
6 has the same resistance value.

The drain of the FET Q5 is connected to the output terminal OUT, and the source of the FET Q5 is connected to its gate and the emitters of the transistors Q6 and Q7 via the resistor R7.

The base of transistor Q6 is resistor R1.
Is grounded through and is also connected to the first input terminal IN1 through a resistor R2. The collector of the transistor Q6 is connected to the base of the transistor Q8 and is also grounded via the resistor R5. The collector of the transistor Q8 is connected to the gate of the FET Q2, and the emitter is grounded.

The base of the transistor Q7 is a resistor R3.
Is grounded through and is also connected to the second input terminal IN2 through a resistor R4. The collector of the transistor Q7 is connected to the base of the transistor Q9 and is also grounded via the resistor R6. The collector of the transistor Q9 is connected to the gate of the FET Q1 and the emitter is grounded.

According to this embodiment having the above-mentioned structure, for example, the voltage Vb of the battery 3 is the voltage Va of the external power source 1.
Higher, or when the battery 3 is connected and the external power supply 1 is not connected, the voltage applied to the base of the transistor Q7 is higher than the voltage applied to the base of the transistor Q6. Also, in general, MOS type F
In ETQ1 and Q2, a parasitic diode is formed between the drain and the source. Furthermore, transistors Q6 and Q7
Since their respective emitter voltages are the same, the base-emitter voltage of the transistor Q7 is
6 is larger than the base-emitter voltage, and the battery 3 to the FET Q2 parasitic diode and FET Q5
A current flowing through the transistor Q6 flows to the resistor R5 and the base of the transistor Q8 via the emitter / collector of the transistor Q6, the base-emitter voltage of the transistor Q8 rises, and the transistor Q8 is turned on. As a result, the gate of the FET Q4 is grounded, and F
A current flows from the drain of the ETQ4 to the ground via the source of the FET Q4 and the resistor Rb, and a reverse bias voltage is applied between the gate and the source of the FET Q2 to cause the FET Q2.
Is turned on, and the load circuit 2 is energized from the battery 3 via the FET Q2.

On the other hand, since no current flows to the transistor Q7, the base-emitter voltage of the transistor Q9 becomes almost 0 V, and the transistor Q9 is turned off.
As a result, the reverse bias voltage is not applied between the gate and the source of the FET Q3, so that the gate voltage and the source voltage of the FET Q1 become equal. As a result, the depletion layer region of the FET Q1 expands, the FET Q1 is turned off, and the external power supply 1 (first input terminal IN1) and the output terminal OUT are disconnected.

When the voltage Va of the external power supply 1 is higher than the voltage Vb of the battery 3, or when the external power supply 1 is connected and the battery 3 is not connected, a state symmetrical to the above is obtained.

That is, the voltage applied to the base of the transistor Q6 is higher than the voltage applied to the base of the transistor Q7, and the emitter voltages of the transistors Q6 and Q7 are the same. The voltage between the base of transistor Q7
It becomes larger than the voltage between the emitters and FE from the external power supply 1
The current flowing through the parasitic diode of TQ1 and the FET Q5 flows to the bases of the resistor R6 and the transistor Q9 via the emitter / collector of the transistor Q7, the base-emitter voltage of the transistor Q9 rises, and the transistor Q9 is turned on. Become. As a result, FETQ
The gate of the FET 3 is grounded, a current flows from the drain of the FET Q3 to the ground through the source and the resistor Ra, a reverse bias voltage is applied between the gate and the source of the FET Q2, the FET Q2 is turned on, and the external power supply is turned on. The load circuit 2 is energized from 1 through the FET Q1.

Further, since no current flows to the transistor Q6, the base-emitter voltage of the transistor Q8 becomes almost 0 V, and the transistor Q8 is turned off.
As a result, the reverse bias voltage is not applied between the gate and the source of the FET Q4, so that the gate voltage and the source voltage of the FET Q2 become equal. As a result, the depletion layer region of the FET Q2 expands, the FET Q2 is turned off, and the battery 3 (second input terminal IN2) and the output terminal OUT are disconnected.

In the above-mentioned configuration, the power consumed by the power supply switching circuit 4 is almost equal to the resistors R1 to R7, Ra and Rb.
It is considered that only the power consumption by the resistors R1 to R7,
By setting the respective resistance values so that the currents flowing through Ra and Rb become the necessary minimum, it is possible to reduce unnecessary power consumption as compared with the conventional case. According to the actual measurement value by the experiment, the power consumption in the power supply switching circuit 4 described above was 0.36W.

Therefore, useless power consumption can be reduced as compared with the conventional example, the drivable time by the battery 3 in a portable device or the like can be increased, and the performance of the battery-driven portable device can be improved.

[0024]

As described above, according to the present invention, the differential switching circuit using transistors detects the voltage difference between the first and second DC power supplies, and the voltage control circuit is based on the detection result. The load and the first and second
Since the ON and OFF states of the first and second field effect transistors provided between the DC power supplies are controlled,
It is possible to reduce wasteful power consumption in the circuit as compared with the related art, it is possible to increase the drivable time by a battery in a portable device or the like, and it is possible to improve performance.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... External power supply, 2 ... Load circuit, 3 ... Battery, 4 ... Power supply switching circuit, 41 ... Differential switching circuit, Q1-Q4
... N-channel field effect transistor, Q5 ... P-channel field effect transistor, Q6, Q7 ... PNP type transistor, Q8, Q9 ... NPN type transistor, R
1 to R7, Ra, Rb ... Resistors, IN1, IN2 ... Input terminals, OUT ... Output terminals, G ... Ground terminals.

Claims (1)

[Claims] 1. A power supply switching circuit that causes a current to flow from a higher voltage DC power supply of the first and second DC power supplies to a load, the power supply switching circuit being interposed between the first DC power supply and the load, A first field-effect transistor having a drain and a source connected to them; and a second field-effect transistor interposed between the second DC power supply and the load and having a drain and a source connected to them. A first transistor switching circuit that uses the voltage of the first DC power supply as a control voltage and a second transistor switching circuit that uses the voltage of the second DC power supply as a control voltage; Differential switching circuit that inverts the on / off state of the first and second transistor switching circuits in response to the voltage difference of the DC power source, and the first and second transistor switches. Based on the on-off state of the etching circuit, the first and provided with a voltage control circuit for controlling the gate voltage of the second field effect transistor, the power supply switching circuit, characterized in that.