Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
1999-04-06
2001-01-16
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C307S066000
Reexamination Certificate
active
06175510
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved uninterruptible power supply (UPS) and relates particularly, though not exclusively, to a UPS for supplying uninterrupted power to a computer.
BACKGROUND TO THE INVENTION
Most companies and organisations rely on the public power supply grid for the supply of electrical power to machinery, lighting, laboratory and office equipment. In many modem buildings there is generally a back-up power supply, typically a diesel engine powered generator, to supply internal power for the building in the event of power failure. However sensitive electrical equipment such as computer systems and telecommunications equipment are frequently provided with a dedicated uninterruptible power supply (UPS) to ensure that there is no interruption to the supply of electrical power that may result in the loss of valuable information held in volatile memory. A UPS is also intended to regulate the supply of electrical power to an appliance so as to minimise fluctuations in the voltage supplied to the appliance.
A typical prior art UPS is connected to the AC power grid and converts the incoming AC voltage to a bipolar DC voltage across two storage capacitors. This bipolar DC voltage is then reconverted to a regulated AC output voltage. The back-up battery voltage is converted to a high bipolar DC voltage and feeds into the storage capacitors in the event that the input AC voltage falls below an acceptable level. There are a number of disadvantages with such conventional UPS technology. Firstly, there are inefficiencies and power losses associated with the need to convert the voltage twice, from AC to DC and vice versa. Secondly, the two storage capacitors are quite bulky because they need to filter out the 50/60 Hz voltage ripple. They are also costly because they need to have high ripple current capability, high capacitance and high voltage characteristics. Further, this type of UPS circuit does not readily lend itself to a high level of integration.
SUMMARY OF THE INVENTION
The present invention was developed with a view to providing an improved UPS which is less susceptible to at least some of the above-noted disadvantages of the prior art.
According to one aspect of the present invention there is provided an uninterruptible power supply device, the device comprising:
an input stage connected to an AC power supply and an output stage for supplying AC power to an appliance;
wherein said input stage has a first switching means connected in series with the output stage and a second switching means connected in parallel with the output stage, wherein each of said switching means is capable of switching current in two directions; and,
a controller operatively connected to said first and second switching means for controlling the direction of current through each of the respective switching means in response to detected changes in power conditions whereby, in use, regulated AC power can be supplied from the output stage.
Preferably the output stage has a third switching means connected in series with the input stage and a fourth switching means connected in parallel with the input stage, and said controller is also operatively connected to said third and fourth switching means.
Preferably said input stage and output stage share a common neutral line and said first switching means and third switching means are connected in series with an inductor therebetween.
Preferably the device further comprises:
a battery-powered switching section for supplying back-up power in the event of interruption of power to the input stage.
Typically said input stage and output stage each have a capacitor connected in parallel with the input and the output respectively.
Advantageously the device further comprises a single storage capacitor connected to an output of said battery-powered switching section for filtering an output voltage from said switching section. Preferably fifth and sixth switching means are provided for connecting said battery-powered switching section to said input stage of the device, said fifth and sixth switching means also being operatively connected to said controller.
In an alternative embodiment said first switching means is one of a pair of first switching means respectively connected to the respective ends of a first winding of a high frequency transformer, and said second switching means is one of a pair of second switching means respectively connected to the respective ends of a second winding of the high frequency transformer.
Preferably, the input stage also has a third switching means connected in parallel with the output stage. Preferably said third switching means is one of a pair of third switching means respectively connected to the respective ends of the second winding of the transformer.
Preferably said high frequency transformer has three separate windings, and said battery-powered switching section is coupled to the input stage via a third winding of the high frequency transformer.
According to another aspect of the present invention there is provided a method of controlling an uninterruptible power supply device, the method comprising the steps of:
monitoring AC power supplied to an input stage of the device;
monitoring AC power supplied from an output stage of the device;
providing first and second switching means in the input stage connected in series and in parallel respectively with the output stage, and wherein each of said switching means is capable of switching current in two directions; and,
controlling the direction of current through each of the first and second switching means in response to detected changes in power conditions whereby, in use, regulated power can be supplied from the output stage.
Preferably the method further comprises:
providing third and fourth switching means in the output stage connected in series and in parallel respectively with the input stage; and,
controlling the direction of current through said third and fourth switching means in response to detected changes in power conditions.
Preferably the method further comprises:
providing a battery-powered switching section for supplying back-up power in the event of interruption of power supplied to the input stage, and wherein a single storage capacitor is connected to an output of said battery-powered switching section for filtering an output voltage from said switching section.
Preferably the method further comprises:
monitoring the voltage across said storage capacitor and comparing it with the voltage of the AC power supplied from the output stage.
Preferably the method further comprises comparing the voltage of the AC power supplied from the output stage with a reference voltage representing the desired output voltage of the device.
REFERENCES:
patent: 5631814 (1997-05-01), Zak
patent: 5959369 (1999-09-01), Descombes et al.
Han Jessica
Michael & Best & Friedrich LLP
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