GB2242082A - "An uninterruptible power supply" - Google Patents

Abstract

An uninterruptible electrical power supply of the on-line type comprises an inverter 5 having an input for receiving rectified AC power from a primary commercial mains supply 1 via a rectifier 2 in normal operation of the power supply. A back-up battery 4 is connected to the output 6 of the inverter 5 via a battery charger 3 which maintains the battery 4 charged during normal operation. The battery 4 is connectible to the input of the inverter 5 to power the inventor 5 in the event of an interruption in the primary power source. The recharging of the battery 4 from the output of the inventor in normal operation enables the battery charger 3 to be simplified. <IMAGE>

Description

Description of Invention "Uninterruptible electrical power supply" THIS INVENTION relates to an uninterruptible electrical power supply.

An uninterruptible power supply is an apparatus for providing a back-up source of electrical power in the event of an interruption in a primary electrical power source. Usually, the primary electrical power source is the commercial mains electrieity supply and the back-up source is a battery, the output of which is electrically processed to replicate the characteristics of the primary power source.

The present invention relates more particularly to an uninterruptible electrical power supply of the on-line type. In an uninterruptible power supply of this type, the output electrical power is always derived from an inverter of the power supply regardless of whether the source of energy is the primary power source or the backup power source. Such on-line uninterruptible power supplies are thus different from uninterruptible power supplies of the off-line, or standby, type in which the output power is derived from an inverter only when the primary power source is interrupted and the source of energy is the back-up power source.

Known on-line uninterruptible power supplies comprise an inverter for receiving a DC input and generating a required AC output, a battery constituting the back-up source of energy in the event of an interruption in the primary power source, and a battery charger for receiving energy from the primary power source during normal operation to replenish the energy stored in the battery. Where the primary power source is an AC source, an AC to DC converter is provided to convert the AC of the primary source to a DC input for the inverter.

A first known uninterruptibie power supply of the on-line type is shown in Figure 1 of the accompanying drawings, in which it is assumed that the primary power source is the commercial AC mains electricity supply and the output of the uninterruptible power supply replicates the ideal characteristics of the mains supply.

As shown in Figure 1, this first known on-line uninterruptible power supply comprises a rectifier 2, a battery charger 3, a back-up battery 4 and an inverter 5 connected to form a series path between a power input terminal 1 and a power output terminal 6 of the power supply. The mains input is applied to the input terminal 1 and is converted from AC to DC by the rectifier 2 before passing to the charger 3 which regulates the received DC to maintain the battery 4 fully charged. The inverter 5 converts the DC battery voltage to the required AC output voltage and, in the event of uninterruption of the mains input at terminal 1, the battery 4 continues to supply the inverter 5 with power until either the battery is discharged or until the mains input is restored.

In normal operation of this first known on-line uninterruptible power supply, the input power applied to terminal 1 passes through both the battery charger 3 and the inverter 5. This results in low electrical efficiency, since there are therefore two converters in the power supply path between terminals 1 and 6, each of which is responsible for a power loss. Moreover, the charger 3 must have sufficient capacity to supply all of the output power, plus the power loss in the inverter 5 and additional power to recharge the battery 4.

A second known form of on-line uninterruptible power supply is shown in Figure 2. In this second known configuration, the input of the inverter 5 is connected directly to the output of the rectifier 2, so as to operate directly from the rectified mains supply delivered by the rectifier. The battery charger 3 also draws energy from the mains supply, either from the output of the rectifier 2 or via its own rectifier (not shown). The output of the battery charger 3 is connected to the battery 4 which is connected to the input of inverter 5 through a battery voltage converter 7. The battery charger 3, battery 4 and battery voltage converter 7 are thus connected in a parallel loop between the output of the rectifier 2 and the input of the inverter 5.In this configuration, if the mains input is interrupted, the battery 4, which is maintained fully charged during normal operation, provides power to the inverter through the battery voltage converter 7 which acts to raise the battery voltage to a level comparable to the rectified mains voltage normally delivered at the output of rectifier 2. The battery voltage converter 7 is normally used for reasons of cost on low powered uninterrupted power supplies. For higher power levels, high voltage batteries may be used, rendering the battery voltage converter 7 unnecessary.

This second known on-line uninterruptible power supply overcomes the low efficiency of the serial circuit architecture shown in Figure 1, since only the inverter 5 lies in the main power path during normal operation. The requirement for a high capacity battery charger 3 is also eliminated, since the battery charger is now only needed to recharge the battery 4 during normal operation.

The present invention aims to provide a further improved uninterruptible power supply of the on-line type and, to this end, the invention provides an uninterruptible power supply comprising an inverter having an input for receiving a DC voltage from a primary power source and an output providing a desired AC voltage, a back-up battery connectible to the input of the inverter in the event of an interruption in the primary power source and a battery charger connected between the back-up battery and the inverter to receive a stabilized voltage from the inverter for recharging the battery during normal operation of the power supply.

In one embodiment of the invention, a rectifier is connected to the input of the inverter for converting the output of an AC primary source to DC.

If required the battery may be connected to the input of the inverter via a battery voltage converter for converting the DC voltage of the battery to a desired DC input voltage for the inverter.

In order that the invention may be more readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a first known uninterruptible power supply of the on-line type; Figure 2 is a block diagram of a second known uninterruptible power supply of the on-line type; Figure 3 is a block diagram of an uninterruptible power supply embodying the present invention; Figure 4 is a more detailed block diagram of the power supply embodying the invention; Figure 5 schematically illustrates the construction of a rectifier of the power supply embodying the invention; Figure 6 schematically illustrates the construction of an inverter of the power supply emb ying the invention;; Figure 7 schematically illustrates the construction of a reference signal generator of the power supply embodying the invention; Figure 8 schematically illustrates the construction of a battery charger of the power supply embodying the invention;and Figure 9 schematically illustrates the construction of a battery voltage converter of the power supply embodying the invention.

Referring firstly to Figure 3, an on-line uninterruptible power supply embodying the present invention comprises a rectifier 2 having its input 1 connected to primary AC power source, such as the commercial mains supply, and its DC output connected directly to the input of an inverter 5. The AC output of the inverter 5 provides power to the load served by the uninterruptible power supply but is also connected to a battery charger 3 to charge a back-up battery 4. The output of the battery 4 is connectible via a battery voltage converter 7 to the input of inverter 5 in the event of an interruption in the primary AC source connected to rectifier input 1. The power supply embodying the invention is thus similar t6 the known uninterruptible power supply of Figure 2, except that the battery charger 3 is connected between the back-up battery 4 and the output of the inverter 5. This circuit architecture enables simplification of the battery charger 3 since the output of the inverter 5 is precise and predictable and the charger 3 therefore no longer has to compensate for variations and fluctuations of the primary power source, manifested as variations and fluctuations in the DC supplied to the inverter via the rectifier 2 in the present embodiment.

The power supply embodying the present invention retains, however, the advantages of the known parallel circuit architecture of Figure 2.

The battery voltage converter 7 is only required if it is necessary to raise the voltage of the battery 4 to a higher level for the inverter 5.

Figure 4 is a block diagram showing the power source embodying the present invention in more detail.

In addition to the components shown in the basic block diagram of Figure 3. Figure 4 shows the power supply as comprising a primary source or mains supervisor 10, a system supervisor 11 and a reference signal generator 12.

The mains supervisor 10 detects any failure in the primary power source and signals the system supervisor 11 which reacts by connecting the back-up battery 4 to the input of the inverter 5 and disconnecting the battery charger 3 from the inverter output. The mains supervisor 10 also provides any low voltage power supplies required for the control electronics of the uninterruptible power supply.

The system supervisor 11 controls and supervises the operation of the power supply, carrying out signalling and sequencing functions as determined by the particular performance requirements of the power supply.

The reference signal generator 12 provides the inverter 5 with a reference signal in the form of a scaled replica of the waveform required at the output of the inverter 5. In the present embodiment, the reference signal generator t2 delivers a sine wave reference signal at 50/60 Hz.

The compo3ition of the various components of the Figure 3. Power supply is schematically illustrated in Figures 5 to 9.

Referring firstly to Figure 5, the mains rectifier 2 comprises a rectifying diode bridge 15 connected across a pair of series connected re3ervoir capacitors 16 and 17 to provide, in normal operation of the power supply, a smoothed higher rectified output voltage of +340 volts on output lines 18 and 19 connected across the pair of capacitors 16 and 17 and a lower voltage of +t70 volts on output lines 180 and 19 across the capacitor 17 alone.

In the event of a failure in the primary source connected to the input of rectifier 2, an output voltage of about 300 volts delivered by the battery voltage converter 7 on lines 181 and 191 is connected across the 340 volt output of the rectifier.

As illustrated in Figure 6, the inverter 5 comprises a half bridge FET transistor switching power stage 20 driven by a pulse width modulator 21 which varies the timing of drive signals fed to the two power switches 22 and 23 on lines 210 by comparing feedback signals from feedback circuitry 24 with the reference signal from the reference signal generator 12 so as to cause the inverter output waveform on lines 28 and 29 to follow the reference waveform. The output of the power stage 20 consists of a series of pulses 25 with a magnitude of either zero volts or a magnitude equal to the rectified mains voltage of nominally 340 volts DC.

The pulses 25 pass through a low pass filter 26, comprising inductor 260 and capacitor 261, which removes the high frequency switching components leaving only those lower frequency components required to form the output sine waveform. The output from the filter 26 is a replica of the reference waveform provided by reference signal generator 12 and may be raised to a higher voltage by a transformer, if desired, within output stage 27.

The inverter output is delivered by the output stage 27 on lines 28 and 29 via a transformer 270, the output stage also comprising sensing circuitry to sense the DC and AC voltage and current content of the inverter output delivered on lines 28 and 29 and provide respective measurement signals. The measurement signals are delivered to the feedback circuitry 24 on lines 240, 241 and 242. Corresponding feedback signals are generated in the feedback circuitry and fed to the pulse width modulator on lines 211 for comparison with the reference signal in the modulator 21. The principles of operation of control loops of this type in switch-mode power regulators are well known.

Figure 7 shows the reference signal generator 12.

The output waveform required from the uninterruptible power supply is assumed to be a 50Hz sine wave, although of course other frequencies or wave shapes could be specified. The reference signal to the inverter 5 is derived from a quartz crystal controlled clock generator 30 in which the oscillator frequency of 32768 MHz determined by crystal 35 is divided down to 800 Hz by a digital divider and the resulting 800 Hz clock signal on output line 300 of the clock generator is applied to digital-to-analogue converter 32 to clock a pattern through a shift register of the digital-to-analogue converter 32 which, as a result, delivers a triangular waveform output 36 at 50Hz. The triangular waveform is converted to a sine wave by a low pass filter 34.A master reset signal is provided to reference signal generator 12 by the system supervisor 11 on line 37 and is fed to various points in the reference signal generator to establish the correct initial conditions.

The battery charger 3 is illustrated in Figure 8 and comprises a simple series pass linear voltage and current regulator comprising a diode bridge rectifier 40, a series pass transistor 41, reverse blocking circuitry 42 and a controller 43. The rectifier 40 is connected to receive the AC output of the inverter 5 and delivers rectified DC to the transistor 41 which is controlled by voltage and current control circuits 44 and 45 in the controller 43 to deliver a required charging current to battery 4 via the reverse blocking circuitry 42. Current feedback is provided on-line 46 to current control circuit 45. A voltage feedback lead 47 from circuitry 42 is connected to voltage control circuit 44 which compares the feedback voltage with a reference voltage supplied to the voltage control circuit 44 by a reference voltage circuit 48 in the controller 43.To prevent reverse discharge of the battery 4 into the charger 3, respective series blocking diodes 49, 50 are included in power output line 51 and voltage feedback line 47. The reference voltage supplied by circuit 48 may, if desired, have two levels to provide for two different high and low charging states.

The battery voltage converter 7 is schematically indicated in Figure 9 and is a standard switch mode voltage converter comprising a push-pull FET transistor power stage 60 having its input connected to battery 4, a transformer 61 connected to the output of power stage 60, rectifying and filtering circuitry 62 connected to the output of the transformer 61 and a controller 63 connected to receive a current feedback signal from the transformer 61 on feedback line 64 and having two control output lines 65 and 66 connected to drive respective stages 67 and 68 of the power stage 60.In the illustrated example, the battery 4 delivers an input voltage of +48 volts to the power stage which delivers two trains of pulses in antiphase to the primary winding 69 of centre-tapped transformer 61, the secondary winding 70 of which delivers an output which is rectified and filtered by the rectifying and filtering circuitry 62 to provide the required +300 volts DC to the input of the inverter 5.

Whilst in the above-described embodiment of the invention the charger 3 is connected to receive the output delivered by output stage 27 on lines 28 and 29 via the transformer 270, it is envisaged that the charger 3 could be connected to any convenient point along the inverter output path where the voltage is stabilized by the regulating action of the inverter 5. For example, the charger 3 could be designed to receive the 50/60Hz low voltage delivered by the filter 26 or a voltage delivered by a separate winding on the transformer 270 in the output stage 27.

Claims (5)

CLAIMS:

1. An uninterruptible power supply of the on-line type comprising an inverter having an input for receiving a DC voltage from a primary power source and an output providing a desired AC voltage, a back-up battery connectible to the input of the inverter in the event of an interruption in the primary power source and a battery charger connected between the back-up battery and the inverter to receive a stabilized voltage from the inverter for recharging the battery during normal operation of the power supply.

2. A power supply according to claim 1, wherein a rectifier is connected to the input of the inverter for converting the output of an AC primary source to DC.

3. A power supply according to claim 1 or 2, wherein the battery is connected to the input of the inverter via a battery voltage converter for converting the DC voltage of the battery to a desired DC input voltage for the inverter.

4. An uninterruptible power supply substantially as hereinbefore described with reference to Figures 3 to 9 of the accompanying drawings.

5. Any novel feature or combination of features described herein.