JP3092126B2 - Power supply switching circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply switching circuit, and more particularly, to a circuit for performing power supply backup by switching from one power supply to another power supply among two supply power supplies.

[Conventional technology]

For example, two systems of power supplies are used for a CPU of a microcomputer, and when one power supply voltage drops to a certain level, the other power supply is switched to a memory backup or the like.

FIG. 4 shows a circuit diagram of a conventional example. In the figure,
First and transistor Tr1 is turned on by the output of amplifier A _1, and supplies the voltages V ₁ from the power source Vcc1 to an output terminal Vout. Here, when the voltage of the power supply voltage Vcc1 drops as shown in FIG. 3A, the comparator Comp comparing the terminal voltage VD of the Zener diode _D with the input voltage Vin.
There switched amplifier A ₁ operates off, turn on the amplifier A _2,
Turn off transistor Tr1, turn on transistor Tr2,
Thus, supplying a voltage V ₂ from the power source Vcc2 to the output terminal Vout. By switching from the power supply Vcc1 to the power supply Vcc2, any power backup is performed.

[Problems to be solved by the invention]

Conventional circuit shown in Fig. 4, switches the amplifier A _1, A ₂ of two amplifiers on from the power supply Vcc1 to supply Vcc2, since carried off switching operation, as shown in FIG. 3 (B) In this case, both the amplifiers A ₁ and A ₂ may be momentarily turned off (that is, both the transistors Tr1 and Tr2 may be turned off). As a result, the power supply voltage does not appear at the output terminal Vout and the CPU is reset. There was a problem. On the other hand, as shown in FIG. 3 (C), both A ₁ and A ₂ may be turned on (that is, both transistors Tr 1 and Tr ₂ may be turned on). Vcc1 and power supply Vcc2 are short-circuited, and power supply Vcc1 to Vcc2 or power supply
Vcc2 is charged from Vcc2, which has a bad effect on the power supply itself. Further, the amplifier A ₁ or A ₂ is malfunctioning, the output terminal
There has been a problem that a correct voltage does not appear at Vout and the CPU malfunctions.

The present invention provides a method for switching transistors Tr1 and Tr2 during power supply switching.
It is an object of the present invention to provide a power supply switching circuit capable of reliably performing power supply switching without making both ON and OFF states.

[Means for solving the problem]

According to the present invention, a first switching element (Tr1) connected between a first power supply (Vcc1) and an output terminal (Vout), and a second switching element (Trc2) connected between the second power supply (Vcc2) and the output terminal (Vout) A second switching element (Tr2) connected to the power supply, detects a voltage drop of one of the power supplies and switches on and off the first and second switching elements (Tr1, Tr2) to switch the other power supply Is supplied to the output terminal (Vout), the voltages (V1, V2) of the first and second power supplies (Vcc1, Vcc2) are detected, and the first and second power supplies are detected in accordance with the detection result. And a differential circuit (Tr3, Tr4) for differentially switching the second switching element (Tr1, Tr2), and a reference voltage circuit (D and a resistor connected in series to D) for generating a reference voltage (VD) An output voltage detection circuit (a resistor connected in series to the output terminal) for detecting a voltage (Vout) of the output terminal; The difference between a reference voltage (VD) from the reference voltage circuit (D and a resistor connected in series to D) and a detection voltage (Vout) from the output voltage detection circuit (a resistor connected in series to the output terminal). And a differential amplifier circuit (A3) for controlling the operation current of the differential circuits (Tr3, Tr4) according to the above.

[Operation] According to the present invention, since the first and second switching elements are driven by the differential circuit, the two switching elements are both off or both on when switching on and off. The moment is extremely short. Therefore, when switching,
The power supply can be reliably switched without causing the inconvenience that the power supply voltage does not appear at the output terminal or that the two power supplies are short-circuited.

Further, by detecting the reference voltage and the output voltage of the output terminal by the reference voltage circuit, the output voltage detection circuit, and the differential amplifier circuit, and controlling the operation current of the differential circuit according to the detection result, The second switching element is connected to the first
In addition, since the output voltage of the second switching element can be controlled to be a constant voltage regardless of the load fluctuation of the output terminal, the differential circuit can operate stably.

〔Example〕

FIG. 1 shows a circuit diagram of an embodiment of the present invention.
The same components as those in FIG. 4 are denoted by the same reference numerals. In FIG. 1, a differential circuit composed of transistors Tr3 and Tr4 is connected to the bases of the transistors Tr1 and Tr2, respectively. The base of the transistor Tr3 is a voltage dividing terminal Vin1 on the power supply Vcc1 side, and the base of the transistor Tr4 is a power supply Vcc2. It is connected to the side of the voltage dividing terminal Vin2, these common emitter connected to the output terminal of the amplifier a _3. Two input terminals of the amplifier A ₃ is applied to the terminal voltage V _D and the voltage V ₀ which Zener diode D.

Now, if the transistor Tr3 constituting the differential circuit is on and the transistor Tr1 is also on, the power supply V
The voltage of cc1 is supplied to the output terminal Vout via the transistor Tr1. In this case, the output of the amplifier A ₃ is output terminal Vo
The so-called constant voltage control operation is performed by controlling the base current of the transistor Tr1 so as to supply a constant voltage irrespective of the load fluctuation of ut.

Here, the voltage V ₁ of the power source Vcc1 is descends transistor Tr3 is turned off, the transistor Tr4 is turned on,
The transistor Tr1 turns off and the transistor Tr2 turns on. By turning on the transistor Tr2, the voltage V ₂ of the power source Vcc2
Is supplied to the output terminal Vout via the transistor Tr2. In this case, the output of the amplifier A ₃ controls the base current of the transistor Tr2, and performs the constant voltage control operation.

By the way, since the transistors Tr3 and Tr4 form a differential circuit, the transistors Tr3 and Tr4 are switched on and off.
The moment when both Tr4 are off or both are very short, and the switching is performed as shown in FIG. 3 (D). As a result, the switching of the transistors Tr1 and Tr2 is performed differentially, and the power supply voltage does not appear at the output terminal Vout and the CPU resets or goes out of control, or the power supplies Vcc1 and Vcc2 are short-circuited as in the conventional example. Thus, there is no inconvenience that the CPU malfunctions.

Note that the differential circuit is not limited to a configuration including only two transistors Tr3 and Tr4, and as shown in FIG. 2, transistors Tr3 and Tr3 'and a transistor T3
The configuration may be such that the r4 and Tr4 'are connected by Darlington.

〔The invention's effect〕

According to the present invention, since the first and second switching elements are driven by the differential circuit, at the time of switching, the power supply voltage does not appear at the output terminal or the two power supplies are short-circuited. Does not occur, and the power supply can be reliably switched.

According to the invention, the reference voltage circuit, the output voltage detection circuit, and the differential amplifier circuit detect the reference voltage and the output voltage of the output terminal, and control the operation current of the differential circuit according to the detection result. Accordingly, the first and second switching elements can be controlled so that the output voltages of the first and second switching elements become constant regardless of the load fluctuation of the output terminal. It can be operated stably.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention, FIG. 2 is a circuit diagram of another embodiment of the present invention, FIG. 3 is an operation timing chart of the present invention and the conventional example, and FIG. FIG. Vcc1, Vcc2: Power supply, Tr1 to Tr4: Transistor, Vout
…… Output terminal, A ₃ …… Amplifier, D …… Zener diode.

Claims (1)

(57) [Claims] A first switching element connected between a first power supply and an output terminal; and a second switching element connected between a second power supply and the output terminal. A power supply switching circuit configured to detect a voltage drop of the power supply and to switch on and off the first and second switching elements and to supply the other power supply to the output terminal; And a differential circuit for differentially switching the first and second switching elements according to the detection result; a reference voltage circuit for generating a reference voltage; and detecting a voltage at the output terminal. An output voltage detection circuit, and a differential amplifier circuit that controls an operation current of the differential circuit according to a difference between a reference voltage from the reference voltage circuit and a detection voltage from the voltage detection circuit. Power supply switching circuit.