GB2120474A

GB2120474A - Standby power supply system

Info

Publication number: GB2120474A
Application number: GB08312685A
Authority: GB
Inventors: John Edward Crowe
Original assignee: Harmer & Simmons Ltd
Current assignee: Harmer & Simmons Ltd
Priority date: 1982-05-11
Filing date: 1983-05-09
Publication date: 1983-11-30
Also published as: GB8312685D0; GB2120474B

Abstract

The system includes two inverters 4a, 7 driven simultaneously in synchronism by a pulse width modulated controller 6a. Each inverter has an output winding A, B which forms a primary winding of a transformer system 7, having a plurality of d.c. outputs. Each inverter also includes isolating diodes D1 to D4 responsive, via the transformer core T2, to the voltage applied by the other inverter. In normal operation, the main power supply output reverse biasses the diodes D3, D4 of the second inverter 7 which is thereby held ineffective, but should this output fall below a predetermined value the reverse bias on the battery stand-by circuit is automatically reduced so that the stand-by system provides the required output and, again via the transformer core T2, reverse biasses the diodes D1, D2 of the first inverter 4a to hold the first inverter ineffective. <IMAGE>

Description

SPECIFICATION An electrical power supply system This invention relates to an electrical power supply system and particularly to an electrical system having a main power supply source and a standby power supply source which provides the required output should the main supply source fail.

In certain installations, such as telephone exchange equipment, or computorised equipment, it is a requirement that should the main source of power supply, usually the a.c. mains fail, then a standby system, usually batteries, should immediately take over to provide the required output so that the apparatus continues to function.

Figure 1 of the accompanying drawings illustrates one known arrangement used for this purpose. In this system, the normal a.c. mains of 240 volts is applied to an a.c. to d.c. converter 1 which converts the 240 volts a.c. input to a 48 volt d.c. output. This d.c. output is used to charge a standby battery 2 and is also supplied to a d.c. to d.c. converter 3 from which a multiplicity of d.c.

outputs of the desired voltage may be obtained.

In this system, under normal operation when the apparatus is powered from the a.c. mains, the total output power is processed in two power handling stages, once in the a.c. to d.c. converter 1 and subsequently by the d.c. to d.c. converter 3.

This known system, therefore has a number of disadvantages since it tends to be relatively inefficient, unreliable and costly.

The present invention seeks to provide a power supply system having a standby facility in which only one power handling stage is active whether the power supply is derived from the main source or the standby source.

According to the present invention there is provided a pulse width modulated electrical power supply system including a first power source, a second standby power source, a first inverter arranged to derive power from the first source, a second inverter arranged to derive power from the second source, a transformer system having a plurality of outputs, each inverter having its own output winding which comprises a primary winding of said transformer system, and a pulse width modulated controller adapted to synchronously control oscillation of the two inverters to thereby drive the inverters simultaneously, each inverter including isolating means responsive to the voltage applied to the other inverter, the output from the first inverter when above a first predetermined level driving the transformer system and impressing a voltage on the output winding of the second inverter, the said voltage acting on the isolating means of the second inverter to maintain the second inverter non-effective, and when the output from the first inverter derived from the first power source is below said predetermined level, the output of the second inverter drives the transformer system and impresses a voltage on the output winding of the first inverter, the voltage impressed on the winding of the first inverter acting on the isolating means thereof to maintain the first inverter non effective.

By this means, in which both inverters are at all times driven in synchronism by the pulse width modulated controller, should the main supply, the first power source, fail, the standby system is automatically brought into operation without any break in the power supply.

Preferably, the first inverter derives its power from a.c. mains and the standby system from a rechargable battery system. The isolating means are preferably diodes.

A preferred embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows the known system already described, Figure 2 shows a known pulse width modulated circuit, Figure 3 shows a modified form of the pulse width modulated power supply circuit of Figure 2 with the addition of a standby system in accordance with the present invention and Figure 4 shows a modification of the circuit of Figure 3.

Referring now to Figure 2, there is shown an a.c. to d.c. multi-output power supply system in which power is derived from the a.c. mains, rectified and then transferred through an inverter 4 to a primary winding A of an output transformer system 5. The power in the primary winding A is controlled by means of a pulse width modulated controller 6 which controls the switching cycle of two transistors TR1 andTR2 in the inverter through appropriate windings on a core TRi.

The output of the transformer system 5 is derived from multiple secondary windings to provide the required plurality of the outputs. In this example, outputs of 5 volts, +12 volts and -12 volts d.c. are provided.

Figure 3 shows the addition of a standby facility to the circuit which is shown in Figure 2. The standby circuit includes a battery 10 giving an output of 48 volts d.c. which is connected to an inverter 7 similar to the inverter 4a supplied from the mains. This inverter 7 is connected to a primary winding B wound on the same core of the transformer system 5 as the primary winding A driven by the main power supply inverter 4a. Both the primary winding A and the primary winding B are capable of independently driving the secondary windings of the transformer system 5 from which the d.c. outputs are derived.The second inverter 7 is driven synchronously with the first inverter 4a by the pulse width modulated controller 6a which simultaneously drives the two inverters through respective parailel mounted output windings 8, 9 and respective T1, T3.Thus, both inverters are driven by the controller 6a at all times.

The main power supply inverter 4a includes isolating means in the form of diodes D1 and D2 in the collector paths of the two transistors TR 1 and TR2. Similarly, two isolating diodes D3 and D4 are provided in the collector paths of corresponding transistors TR3 and TR4 of the standby inverter 7.

The operation of this circuit arrangement is as follows: When the input mains voltage applied to the first inverter is within the normal range of 240 volts + 6%, and for a battery voltage within the normal range of 42 to 56 volts d.c., the number of turns in the primary windings A and B are chosen so that winding A assumes the function of driving all of the secondary windings in the transformer system 5 and, additionally imposes a voltage wave form on winding B which is large in amplitude compared with the battery voltage.In these circumstances, the diodes D3 and D4 of the second inverter are reversed biased by the impressed wave form on B so that although TR3 and the transistor TR4 being driven by the pulse width modulated drive circuit, the reverse biasing on the diodes D3, D4 prevents the two transistors taking part in determining the wave forms in the core T2 of the transformer system. The transistor TR3 and TR4 are effectively "idling" as they are isolated from the load by the reverse bias on the diodes D3 and D4.

If the mains voltage faiis below the lower limit of the normal range, the wave form voltage impressed on the primary winding B through primary winding A and core T2 by the first inverter becomes too small to keep the diodes D3 and D4 reverse biased. Winding B now assumes the function formerly provided by the winding A in as much as the transistors TR3 and TR4 impress their wave form via the winding B and the core T2 onto the secondary winding of the transformer system 5. The wave form impressed on the primary winding A by the winding B is now largely compared with the voltage therein derived from the diminished rectified mains voltage. In these circumstances, the diodes D1 and D2 in the first inverter are reversed biassed to isolate the transistors TR1 and TR2 from the output transformer system.In a similar manner to that described previously for the standby inverter, transistors TR1 and TR2 are now "idling" even though they are still being driven by the controller 6a.

In one practical embodiment of the invention designed to operate at a frequency of 25 KHz, using a ferrite core type DIN E 42 in the core T2, the winding A has 31 turns and the winding B has two opposed windings having 11 turns on each.

The mains voltage is nominally 240 volts and the standby battery voltage is nominally 48 volts.

Winding A will dominate and deliver power for voltages greater than 21 6 volts (i.e. 240 volts less 10%) but if the battery voltage is at its maximum float-charged level of 56 volts, then winding B will take over as the mains voltage fails below 21 6 volts. Whilst the mains voltage remains below 216 volts the power supply will thus be provided by the standby system. If the battery voltage is lower than 56 volts then the cross over point will be at a correspondingly lower mains voltage.

It will be understood that the apparatus of the invention is readily adaptable for different mains voltages and standby voltages by the selection of suitable components. Figure 4 shows a modification of the circuit shown in Figure 3 which enables the circuit to be used with a mains voltage of either 240 volts or 1 15 volts AC.

The modified circuit includes two capacitors C1 and C2 in series across the output terminals of a full wave bridge rectifier, the 240 volts AC supply being connected to the input terminals of the rectifier. The 11 5 volt AC connections are made between, respectively, one of the input terminals of the rectifier and the junction between the series-connected capacitors C1 and C2.

With this configuration, an AC input applied to the 240 volt AC terminals will "see" a bridge rectifier which is arranged to produce approximately 240 volts DC across the capacitor reservoir formed by the two series connected capacitors C1 and C2.

An Ac input applied across the 11 5 volts AC terminals will, however, "see" a voltage doubler circuit and thus an input of 1 15 volts AC will also produce an output at the capacitor reservoir of the same value, 240 volts DC. Thus, input voltages of either 11 5 volts AC or 240 volts AC may be utilized, selectively, to provide the same DC output. Since the reservoir acts as the power source for the mains inverter, no other circuit details need not be modified to accommodate the alternative input voltages.

It will be seen that the driving of the secondary windings of the transformer system, and hence the provision of their associated d.c. outputs, is assumed by either one or other of the inverters, depending on whether the mains voltage is within its normal limits. Because both inverters are at all times driven in synchronism by the pulse width modulated controller, the transition from main supply to standby battery operation and vice versa will be smooth and automatic without any of the problems caused by breaks and interactive effects which would attend any attempt to change from one mode of operation to the other by means of switching transistors or other switching devices.

Claims

1. A pulse width modulated electrical power supply system including a first power source, a second standby power source, a first inverter arranged to derive power from the first source, a second inverter arranged to derive power from the second source, a transformer system having a plurality of outputs, each inverter having its own output winding which comprises a primary winding of said transformer system, and a pulse width modulated controller adapted to synchronously control oscillation of the two inverters to thereby drive the inverters simultaneously, each inverter including isolating means responsive to the voltage applied to the other inverter, the output from the first inverter when above a first predetermined level driving the transformer system and impressing a voltage on the output winding of the second inverter, the said voltage acting on the isolating means of the second inverter to maintain the second inverter non-effective, and when the output from the first inverter derived from the first power source is below said predetermined level, the output of the second inverter drives the transformer system and impresses a voltage on the output winding of the first inverter, the voltage impressed on the winding of the first inverter acting on the isolating means thereof to maintain the first inverter noneffective.

2. The system according to Claim 1 wherein the first power source is an a.c. mains supply.

3. The system according to Claim 1 or 2 wherein the second power source is derived from a rechargeable battery.

4. The system according to Claim 1 or 2 or 3 wherein the isolating means of the inverters are diodes.

5. The system according to any one of Claims 1 to 4 wherein the transformer system provides a plurality of outputs.

6. A pulse width modulated electrical power supply system substantially as described herein with reference to and as illustrated in Figures 2 and 3 or these Figures as modified by Figure 4 of the accompanying drawings.