Uninterruptible Power Supply Circuit

This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. .sctn.119 from an application for Power Supply Voltage Supplying Circuit earlier filed in the Korean Industrial Property Office on the 18th day of Oct. 1995 and there duly assigned Serial No. 35967/1995.

1. Field of the Invention

The present invention relates to uninterruptible power supply circuits, and more particularly to uninterruptible power supply circuits including a main power source and a back-up power source, the back-up power source being charged by the main power source.

2. Description of the Related Art

Generally, an uninterruptible power supply circuit is constructed with a main power source unit for supplying power during normal times and a back-up power source for supplying back-up power when the main power source is cut off A battery which is charged by the main power source is used for the back-up power source.

In an earlier designs of circuits for an uninterruptible power supply, a first power supply voltage V.sub.1 is supplied to an output node via a first diode. The first power supply voltage at the output node appears at an output terminal V.sub.0 and is applied to simultaneously charge a back-up battery via a resistor. A second diode is connected between the back-up battery and the output terminal V.sub.0 and is used as a reverse current protection diode for cutting off the path to the battery. When the first power supply voltage V.sub.1 is lower than the voltage of the back-up battery, the back-up battery voltage is supplied to the output terminal V.sub.0 via the second diode. The first diode is used as a reverse current protection for preventing the back-up power from being supplied to a first power supply circuit from which the first power supply voltage is supplied.

The back-up battery is charged by a voltage lower than the first power supply voltage and the back-up battery is not fully charged and is discharged via the second diode. Thus, if the back-up battery voltage which is lower than the turn on voltage of the second diode is discharged, the second diode is turned off and this causes a shorter discharge time and the incomplete discharging of the back-up battery.

Exemplars of recent attempts to provide uninterruptible power supplies include: U.S. Pat. No. 5,440,179 to Severinsky entitled UPS with Bi-Directional Power Flow, U.S. Pat. No. 5,418,445 to Alpert et al. entitled Switching Circuit For Simultaneous Rapid Battery Charge And System Operation, U.S. Pat. No. 5,300,874 to Shimamoto et al. entitled Intelligent Power Supply System For A Portable Computer, U.S. Pat. No. 5,278,798 to Miyawaki entitled Semiconductor Memory Device, U.S. Pat. No. 5,150,032 to Ho entitled Combined Charging And Supply Circuit, U.S. Pat. No. 5,073,837 to Baek entitled Low Voltage Protection Circuit, U.S. Pat. No. 5,033,882 to Vanderpool et al. entitled Circuit For Conserving Power Of A Backup Battery, U.S. Pat. No. 4,730,121 to Lee et al. entitled Power Controller For Circuits With Battery Backup. I have found that these circuits lack an ability to both obtain a full charge across a battery by controlling the charging voltage and to prolong the useful life of a back-up battery after power has been interrupted and the back-up battery has been placed into service.

An object of the present invention is, therefore, to provide an improved circuit for a power supply.

It is another object to provide an uninterruptible power supply circuit able to fully charge a battery by raising the charging voltage.

It is still another object to provide an uninterruptible power supply circuit which can lengthen the discharging time of a back-up battery after the power is cut off and the back-up battery has been placed into service.

To achieve these and other objects of the present invention, an uninterruptible power supply circuit is constructed with a first power supply unit for supplying a first power supply voltage; a second power supply unit for supplying a second power supply voltage that is lower in amplitude than the first power supply voltage; a back-up power supply unit for supplying a back-up power supply voltage; a charge current control unit for receiving the first power supply voltage and for controlling the charge current to be applied to the back-up power supply unit according to the capacity of the back-up power supply unit; a reverse current protection unit connected between the second power supply unit and an output node connected to an output terminal. An overcharge protection unit may be connected between the back-up power supply unit and the output node, for protecting the back-up power supply unit from being overcharged, and a relay unit driven by the first power supply voltage, may be connected to form a path for applying the back-up power supply voltage to the output node when the first power supply voltage is cut off.

A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:

FIG. 1 is a circuit diagram showing an exemplar of an earlier uninterruptible power supply circuit; and

FIG. 2 is a circuit diagram showing an uninterruptible power supply circuit constructed according to the principles of the present invention.

Turning now to the drawings, FIG. 1 shows an earlier uninterruptible power supply circuit. If a first power supply voltage V.sub.1 is at a normal level, it is supplied to a node N via a diode 12. The power supply voltage at the node N is output to an output terminal V.sub.o and is also used to simultaneously charge a battery 18 via a resistor 14. A diode 16 is used as a reverse current protection diode for cutting off the path to battery 18. If the first power supply voltage V.sub.1 is at an abnormal level, i.e., if the first power supply voltage V.sub.1 is lower than a back-up voltage V.sub.2 of battery 18, the back-up power V.sub.2 is supplied to the output terminal V.sub.o via the diode 16. Diode 12 is used as a reverse current protection diode for preventing the back-up voltage V.sub.2 from being supplied to a first power supply circuit (not shown) from which the first power supply voltage V.sub.1 is supplied. Battery 18 is charged by a voltage lower than the first power supply voltage V.sub.1. Battery 18 is not fully charged and is discharged via the diode 16. Thus, if the back-up voltage V.sub.2 of the battery 18 which is lower than the turn-on voltage of the diode 16 is discharged, then the diode 16 is turned off. This causes a shorter discharging time and an incomplete discharging of the battery 18.

As shown in FIG. 2, a first power supply voltage V.sub.1 is supplied to a charge current control unit 221. The charge current control unit 221 is connected to a battery 222 via a node N.sub.2 and controls the amount of the charge current according to the capacity of the battery 222. The battery 222 is charged by the first power supply voltage V.sub.1 via the charge current control unit 221. A second power supply voltage V.sub.2 is supplied to an output node N via a diode 33 and then is output to an output terminal V.sub.o. An overcharge protection unit 224 connected between the node N.sub.2 and the output node N.sub.3 is composed of diodes 31 and 32 and functions to protect the battery 222 from being overcharged. A relay unit 223 which inputs the first power supply voltage V.sub.1 as a driving power has a relay driver coil 29 and a relay contract switch 30. If the first power supply voltage V.sub.1 is supplied to the coil 29, then the switch 30 is opened, while if the first power supply voltage V.sub.1 is cut off, then the switch 30 is closed.

Firstly, a discussion follows for the case in which first and second power supply voltages of normal levels are supplied to the charge current control unit 221 and the diode 33, respectively. The first power supply voltage V.sub.1 is supplied to resistors 21 and 23 and a diode 26. A dividing voltage V.sub.N1 obtained by voltage-dividing the first power supply voltage V.sub.1 by the resistors 21 and 22 is applied to the base of a PNP transistor 24 (assuming that the base current of the transistor 24 is small in comparison with the current through the resistors 21 and 23). The resistance values of the resistors 21 and 22 are set such that the PNP transistor 24 can be turned on by the dividing voltage V.sub.N1. The first power supply voltage V.sub.1 is supplied to the emitter of the turned-on PNP transistor 24 via the resistor 23. The battery 222 is then charged by the first power supply voltage V.sub.1. The first power supply voltage V.sub.1 passes through the diodes 26, 27 and 28 and drives the coil 29, thereby opening the switch 30. The battery 222 is charged by the power supply voltage which is higher than the voltage at the output node N.sub.3 by the turn-on voltage level of the diodes 31 and 32. The reason is that is the battery 222 is discharged when a power supply voltage higher than the voltage at the output node N.sub.3 plus the turn-on voltage of the diodes 31 and 32 is supplied to the battery 222. The power supply voltage obtained by subtracting the turn-on voltage of the diode 33 from the second power supply voltage V.sub.2 is supplied to the output terminal V.sub.o. The subtracted power supply voltage is the power supply voltage of the output node N.sub.3.

Secondly, a discussion follows for the case in which first and second power supply voltages of abnormal levels are supplied to the charge current control unit 221 and the diode 33. If the first power supply voltage V.sub.1 is cut off, no circuit flows in the coil 29 and thus the switch 30 is closed. The back-up power supply voltage V.sub.3 of the battery 222 is supplied to the output node N.sub.3 via the node N.sub.2 and the closed switch 30. The back-up power supply voltage V.sub.3 supplied to the output node N.sub.3 is not discharged to a second power supply unit (not shown) by the diode 33. In this case, the diode 33 is used as a reverse current protection diode. The discharging operation continues until the battery 222 becomes completely discharged. Thereafter, if the first power supply voltage V.sub.1 is supplied to a completely discharged battery 222, the maximum charge current is limited by the resistors 21, 22 and 23, thereby protecting the battery 222.

As described above, it is possible to fully charge the battery by raising the level of the back-up power supply voltage and it is also possible to increase the discharging time of the battery, thereby enabling the complete discharge of the battery, by using the closing of the switch 30. In addition, the expected life span of the battery can be extended by protecting the battery from being overcharged

While there has been illustrated and described what is considered to be a preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the present invention. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the present invention, but that the present invention includes all embodiments falling within the scope of the appended claims.