GB2023949A

GB2023949A - Power supply circuits

Info

Publication number: GB2023949A
Application number: GB7919390A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1978-06-21
Filing date: 1979-06-04
Publication date: 1980-01-03
Also published as: GB2023949B

Abstract

In a power supply system where a supply battery 12 is connected between a positive line 10 and earth 11 a negative supply V- is provided by a circuit comprising an operational amplifier oscillator (13, R1 to R4, C1) a buffer amplifier (14, R5, R6) and a diode pump (C2, C3, D1, D2). The supply terminals of the operational amplifier (13) are connected to the positive line (10) and the output of the diode pump respectively, so that when the circuit is first connected to the supply the operational amplifier is initially connected between the lines (10, 11) through the intermediary of the diodes of the diode pump. The operational amplifier then starts to oscillate and switches the buffer amplifier, in the form of transistor 14 connected between the positive line and earth, on and off alternately to cause a charge to be pumped into capacitor C3 connected between earth and the required negative supply in a direction so that the negative supply voltage is built up. <IMAGE>

Description

SPECIFICATION Power supply circuits This invention relates to power supply circits for battery operated electrical equipment, for use, for example, in automotive vehicle electrical systems.

Many electrical circuits operate most satisfactorily when provided with a dual rail power supply providing both positive and negative supply voltages relative to the local earth. In the case of a vehicle electrical system however, one battery terminal, usually the negative one, is customarily connected to the vehicle earth.

It has already been proposed to derive a negative supply voltage utilizing an astable multivibrator, a buffer amplifier and a diode pump. Where operational amplifiers are being employed in the electrical equipment it is convenient to use such an amplifier as the active element of the astable multivibrator, but frequently it is most convenient to utilize multiple operational amplifers, of the generally known kind in which two or more operational amplifiers are included in a common package, having common supply terminals. With such an arrangement the conventional ci rcuit for deriving a negative supply cannot be used.

It is accordingly an object of the present invention to provide a circuit which can be used in these circumstances.

In accordance with the invention there is provided a power supply circuit comprising first and second power supply rails for connection to opposite poles of a power supply, an oscillator including an operational amplififer, a buffer amplifier connected across said first and second rails and connected to be driven by the output of said operational amplifier and a diode pump circuit connected between the output of said buffer amplifier and said second rail, characterised in that the supply terminals of the operational amplifier are connected to said first rail and the output terminal of the pump circuit respectively.

With such an arrangement, when the circuit is energised the supply terminals of the operational amplifier are initially connected to the first rail, and through the diodes of the diode pump circuit, to the second rail so that the oscillator starts to run. As the diode pump circuit produces an increasing output voltage, the working voltage of the operational amplifier increases.

It will be noted that any operational amplifiers in the same integrated circuit package as the operational amplifier of the astable multivibrator receive a correctdual voltage supply once the diode pump is producing its full output.

Preferably, the operational amplifier has a bias circuit comprising a pair of resistors in series between the first and second rails with their common point connected to the non-inverting terminal of the operational amplifier, a further resistor connecting the output terminal of the operational ampli fiertothe non-inverting inputterminal thereof and a timing circuit comprising a resistor and capacitor in series betwen the output terminal of the operational amplifier and the second rail, the junction of said resistor and capacitor being connected to the inverting input terminal of the operational amplififer.

The accompanying drawing shows the circuit diagram of one example of a power supply circuit in accordance with the invention.

The power supply circuit shown includes first and second supply rails 10, 11 connected respectively to the positive and negative terminals of a vehicle battery 12, the negative terminal of the battery being grounded to the vehicle frame.

An operational amplifier 13, which is in fact one of four operational amplifiers in a quad operational amplifiers integrated circuit package (for example type LM324), has its non-inverting input terminal connected to the junction of two resistors R1 and R2 connected in series between the rails 10,11. A positive feedback resistor R3 connected between the output terminal of the amplifier 13 and its noninverting input terminal gives the operational amplifier the transfer function of an inverting voltage comparatorwith hysteresis. Afurther resistor R4 is connected in series with a capacitor C1 between the output terminal of the amplifier 13 and the rail 11, the junction of the resistor R4 and the capacitor Cn being connected to the inverting input terminal of the amplifier 13.

The output terminal of the amplifier 13 is connected by a resistor R5 to the base of an npn transistor 14, which has its emitter connected to the rail 11 and its collector connected by a load resistor Re to the rail 10. The transistor 14 acts as an inverting buffer amplifier.

The collector of the transistor 14 is connected to one side of a capacitor C2, the other side of which is connected to the anode of a diode D1 and to the cathode of a diode D2. The cathode of the diode D1 is connected to the rail 11 and the anode of the diode D2 is connected to one side of a capacitor C23, the other side of which is connected to the rail 11.

The anode of the diode D2 is also connected to an output terminal V- and it will be noted that the supply terminals of the operational amplifier 13 are connected respectively to the rail 10 and the terminal V-.

When the circuit shown is not connected to the battery 12, all three capacitors C1, C2 and C3 discharge. On connection to the battery, the operational amplifier 13 will receive its power supply from the battery 12, via the forwardly biassed diodes D1, D2 in series. Since the capacitor Ci is discharged the inverting input of the amplifier 13 will be at ground potential, but the resistor network R1, R2, Rub will put some higher voltage on the non-inverting input terminal. As a result the output of the amplifier 13 will rise to a voltage close to that on rail 10, thereby turning the transistor 14 hard on and maintaining the capacitor C2 is its discharged state.The voltage on the non-inverting input of the amplififer 13 now becomes fixed at approximately > 2V+ (assuming that R1 = R2 = R3) and the capacitor C1 starts to charge via the resistor R4. When the voltage on capacitor C1 reaches 3V+, the output of the amplifier 13 goes low, i.e. to a voltage close to that on rail 11, and transistor 14 will be turned off. In this state the capacitor C1 starts to discharge through the resistor R4 and the capacitor C2 starts to charge through the resistor R5.

The time constant R6 C2 is arranged to be much less than the time constant R4 C1 so that before the voltage on capacitor C1 has fallen to 3V+, the capacitor C2 has become substantially fully charged to a voltage equal to V+ less the forward voltage drop of diode D1. When the output of amplifier 13 goes high again and transistor 14 turns hard on the left hand side of capacitor C2 assumes almost ground voltage so that its right hand side takes up a negative voltage. This causes diode D2 to be forwardly biased so that the charge on capacitor C2 is shared between capacitors C2 and C3, leaving a negative voltage on capacitor C3.

From this point in the operation the operational amplifier 3 will pass current into capacitor C3 instead ofthrough the diodes D1, D2. When the output of amplifier 13 next goes low it will fall to a voltage below that of the rail 11, thereby decreasing the voltage at the non-inverting input terminal below 3V+ and correspondingly increasing the rate of discharge of capacitor C1.

During each cycle of operation of the astable multivibratorthe voltage at terminal V-falls until it stablises at a level almost as far below earth as the voltage on rail V+ is above earth, the difference being accounted for by the forward voltages of the diodes D1 and D2 and the saturation voltage of the transistor 14.

It will be appreciated that the other operational amplifiers in the integrated circuit package are powered from the rail 10 and the terminal V-. Other operational amplifiers may also be connected to the power supply circuit described above.

Claims

1. A power supply circuit comprising first and second power supply rails for connection to opposite poles of a power supply, an oscillator including an operational amplifier, a buffer amplifer connected across said first and second rails and connected to be driven bythe output of said operational amplifier and a diode pump circuit connected between the output of said buffer amplifier and said second rail, characterised in that the supply terminals of the operational amplifier are connected to said first rail and the output terminal of the pump circuit rspectively.

2. A power supply circuit as claimed in claim 1 in which the operational amplifier has a bias circuit comprising a pair of resistors in series between the first and second rails with their common point connected to the non-inverting terminal of the operational amplifier, a further resistor connecting the output terminal of the operational amplifier to the non-inverting input terminal thereof and a timing circuit comprising a resistor and capacitor in series between the output terminal of the operational amplifier and the second rail, the junction of said resistor and capacitor being connected to the inverting input terminal of the operational amplifier.

3. A power supply circuit as claimed in claim 1 or claim 2 in which said operational amplifer is one of a plurality of operational amplifiers in an integrated circuit, said plurality of operational amplifiers having common supply terminals.

4. A power supply circuit as claimed in any preceding claim in which the buffer amplifer comprises a transistor having its base connected by a resistor to the output of the operational amplifier, its emitter connected to one supply rail and its collector connected by a resistor to the other supply rails.

5. A power supply circuit as claimed in claim 4 in which said diode pump comprises a first capacitor connected at one side to the collector of the transistor, a first diode having its anode connected to the other side of said first capacitor and its cathode connected to said one supply rail, a second diode having its cathode connected to said other side of said first capacitor and its anode connected to the diode pump output terminal and a second capacitor connected between the diode pump output terminal and said one supply rail.

6. A power supply circuit substantially as hereinbefore described with reference to the accompanying drawings.