GB2061642A

GB2061642A - Back-up power supply

Info

Publication number: GB2061642A
Application number: GB8032623A
Authority: GB
Original assignee: Contronic Controls Ltd
Current assignee: Contronic Controls Ltd
Priority date: 1979-10-12
Filing date: 1980-10-09
Publication date: 1981-05-13
Also published as: DE3038496A1

Abstract

The invention relates to a back- up power supply for use in association with a primary supply and a rechargeable battery, 26. The back- up supply includes a current control device, 32, which serves to regulate the magnitude and direction of battery current in response to a first control signal, across its current transfer terminals. When the first control signal has a polarity indicating that the primary supply is operating, the current control device charges the battery from the primary supply. When the first control signal has the opposite polarity, indicating that the primary supply is not operating, the current control device permits the battery to discharge freely through the load otherwise powered by the primary supply. The current control device is also responsive to a second control signal, one of the load and battery voltages, to detect when the battery has discharged beyond a predetermined level and to prevent the battery from then discharging any further through the load. <IMAGE>

Description

SPECIFICATION Back-up power supply The invention relates to a back-up power supply for use in association with a primary power supply and a rechargable battery and, more particularly, to a back-up supply for use in supplying a fixed voltage to a load when a primary supply fails.

A back-up power supply is used to provide what is commonly described as uninterruptable service. A back-up supply might be used, for example, in association with burglar or fire alarm systems to ensure that such systems continue to operate even when a line source which typically powers such systems fails. In such circumstances the back-up supply would be adapted to respond to a failure of the line source to begin to supply power to the system for some predetermined period of time and to thereby ensure that alarm state detection would continue.

In the past back-up power supplies have commonly consisted of a storage battery that generates the same fixed voltage as the primary supply connected essentially in parallel to the primary supply. When the primary supply fails, the battery simply assumes the function of supplying power to the load otherwise powered by the primary supply. When the primary supply once again resumes operation, the battery, owing to its essentially parallel connection to the primary supply, tends to recharge from the primary supply. Once recharged the battery is again capable of providing auxiliary power in the event of a further failure of the primary supply.Although the connection of the storage battery in parallel to a primary supply is a simple means for providing uninterruptable service, such an arrangement has commonly suffered drawbacks relating to failure to adequately regulate the rate of charging the storage battery, to effective loss of a portion of the available battery voltage owing to efforts to restrict the battery charge rate, and to failure to adequately regulate the discharge of the battery.

The service life of a storage battery can be adversely affected if the battery is subjected to excessive charging currents. In the past excessive charging currents have been avoided by connecting a power resistor between one battery terminal and the primary supply. Although the resistor so connected does serve to limit the charging current otherwise experienced by the battery, the resistor can also have the effect of unduly extending the time required to fully charge the battery. This extension of the charging time owes principally to the tendency of the battery to charge in a decaying exponential fashion and to the fact that the time constant for the exponential charging tends to vary directly with the resistive value of the power resistor.It will be readily appreciated that use of a power resistor to restrict charging current can extend the period of time during which uninterruptable service is unavailable.

Use of the power resistor as described can also cause a reduction in the battery voltage which can be impressed across a load when the primary supply fails. A portion of the battery voltage is effectively lost because the battery must generally discharge through the power resistor when providing power to the load. Such a voltage loss has commonly been avoided by shunting the resistor with a diode which provides an alternative pathway for the discharge of the battery when the primary supply fails. However, even when the power resistor is shunted with the diode, the battery voltage available to the load will typically be reduced by several hundred millivolts, namely, the voltage required to fully turn on the diode.

The useful life of the storage battery is affected not only by excessive charge rates but also by excessive discharging. Furthermore, if the battery is excessively drained, an excessively long recharge period may be required and it may be difficult to predict when uninterrupted service is once again available.

In the past, excessive discharge of the battery was sometimes avoided by use of a relay responsive to the differential voltage across the load. The relay would open the connection between the battery and the load thereby preventing further discharge of the battery when the load voltage dropped below a predetermined level indicating that the battery had discharged a certain amount. However the use of relays is not altogether satisfactory as they tend to be bulky, costly, and to a certain extent unreliable.

The invention provides a back-up power supply for use with a primary power supply that impresses a fixed voltage across a load connected between first and second primary supply terminals. The back-up supply is operated from a rechargable battery having first and second battery terminals, the first battery terminal being electrically connected to the first primary supply terminal.

The back-up supply includes current control means for controlling the magnitude and direction of the battery current. The current control means has first and second current transfer terminals and a control terminal. The first current transfer terminal is adapted to be connected to the second battery terminal and the second current transfer terminal is adapted to be connected to the second primary supply terminal, so that the battery current is constrained to flow through the current control means between the first and second current transfer terminals.

The current control means is responsive to a first control signal, namely, the polarity of the differential voltage between the first and sec ond current transfer terminals. Owing to the relative connection of the primary supply, battery and current control means, as described above, this differential voltage has a first polarity when the primary supply is operating and an opposite polarity when the battery rather than the primary supply is providing current to the load. The current control means detect the differential voltage and cause the battery to tend to charge from the primary supply to substantially the fixed voltage when the differential voltage has the first polarity and to discharge through the load when the differential voltage has the opposite polarity.

The current control means is also responsive to a second control signal applied to the control terminal to regulate the magnitude of the battery current. The back-up supply includes second control signal supply means for generating the second control signal.

In preferred embodiments, the second control signal supply means are coupled to one of the load and battery to detect one of the load and battery voltages and the magnitude of the second control signal is substantially proportional to the one of the load and battery voltages. In these preferred embodiments, the current control means have a conduction characteristic such that the magnitude of the current conducted between the first and second current transfer terminals increases and decreases respectively as the magnitude of the second control signal increases and decreases and drops to substantially zero when the magnitude of the second control signal drops to substantially zero.These preferred embodiments further comprise means for reducing the magnitude of the second control signal to substantially zero when the primary supply is not operating and the one of the load and battery voltages falls below a predetermined level. As the magnitude of the load and battery voltages are indicative of the extent of which the battery is charged or discharged, the battery is thereby prevented from discharging beyond a predetermined level.

The invention will be better understood with reference to drawings in which Figs. 1-3 diagrammatically illustrate various embodiments of the invention.

Reference is made to Fig. 1 which illustrates a back-up power supply 10 connected to a primary power supply 12 and a load 14.

The primary supply 12 includes a conventional voltage regulator 1 6 which receives power at power input terminals 18, 20 from an unregulated power supply (not shown).

When operating the regulator 16 generates a 1 2 volt potential difference between positive and negative primary supply terminals 22, 24.

The back-up supply 10 operates from a storage battery 26 having positive and negative battery terminals 28, 30. When fully charged the battery 26 develops a 12 volt potential difference between the battery terminals 28, 30. The positive battery terminal 28 is connected to- the positive primary supply terminal 22 and the negative battery terminal 30 is electrically coupled to the negative primary supply terminal 24, so that the polarity of the battery voltage when applied to the load 14 is the same as that of the primary supply 12.

The back-up supply 10 includes an npn transistor 32 which serves to control the magnitude and direction of current flowing from the battery 26. The transistor 32 is preferrably connected as shown with emitter electrically connected to the negative battery terminal 24. Although the emitter and collector connections can be interchanged, such an arrangement does not produce a preferred mode of operation for the transistor 32, as will be discussed below.

When the primary supply 1 2 is not operating, the transistor 32 is biased by the battery 26 to operate in the normal mode. Since the emitter is connected to the negative battery terminal 30 and the collector is connected through the load 14 to the positive battery terminal 28, positive transistor current tends to flow from collector to emitter in this mode of operation. Sufficient base current is provided by a transistor 34 to saturate the transistor 32 thereby permitting the battery 26 to discharge freely through the load 14. Since the transistor 32 is saturated, the battery voltage is fully impressed across the load 14 except for the saturation collector-emitter voltage of the transistor 32 which is in the order of 100 millivolts.

When the primary supply 1 2 is operating, the transistor 32 is biased to operate in the inverse mode. In this mode of operation, with emitter coupled through the battery 26 to the positive primary supply terminal 22 and collector connected to the negative primary supply terminal 24, the normal functions of the emitter and collector are interchanged, with positive transistor current flowing from the emitter to the collector. Any base current now provided by the transistor 34 to the transistor 32 causes a current flow in the transistor 32 which tends to charge the battery 26.

The battery 26 charges at an essentially constant rate, the charge rate being determined by the magnitude of the base current received by the transistor 32, which is varied principally by the resistive value of a resistor 36 and the magnitude of the primary supply voltage, and by the current gain P of the transistor 32. Since the base current provided is relatively constant regardless of the mode -of operation of the transistor 32, and since the ss of the transistor 32 will be lower in the inverse mode than in the normal mode, the charge rate will be substantially less than the discharge rate, and substantially less than it might otherwise be if the emitter and collector connections of the transistor 32 where interchanged.However, it is expected that in most applications it will be preferable to keep the potential discharge rate sufficiently high that the load effectively limits the rate of discharge of the battery 26 and to have a slower charge rate.

In practice, the battery 26 tends to charge to within millivolts (typically about 10 mv.) of the primary supply voltage. As the transistor 32 is saturated by base current provided by the transistor 34 during the charging of the battery 26, the battery voltage tends to approach the primary supply voltage less the saturation collector-emitter voltage of the transistor 32. Because the saturation collectoremitter voltage of the transistor is generally lower in the inverted rather than the normal mode, the emitter and collector connections shown permit the battery 26 to charge more closely to the primary supply voltage than would be otherwise possible if the emitter and collector connections were interchanged.

The extent to which the battery 26 can discharge through the load 14 is limited by the resistors 38, 39 which in association with the second transistor 34 couple the base of the transistor 32 to the load 1 4. The resistors 38, 39 serve to detect the load voltage, and the base-emitter junction of the transistor 34 serves to compare the detected load voltage to a reference voltage, namely, the 1.4 volt potential difference normally existing across the base-emitter junction of the transistor 34 when fully turned on (the transistor 34 being a Darlington pair). When the load voltage drops below a predetermined level, and the reference voltage can no longer be maintained across the base-emitter junction of the transistor 34, the transistor 34 begins to conduct less current with a consequent drop in base drive to the transistor 32.The conductivity of the transistor 32 is thereby reduced causing a further drop in the load voltage so that by virtue of positive feedback the transistor 32 is effectively constrained to shut off. It will be appreciated that this arrangement also provides short-circuit protection so that the backup supply 10 need not be fused. If the load 1 4 is short-circuited, the transistor 32 will shut off preventing further discharge of the battery 26.

If desired, the back-up supply 10 can be greatly simplified by eliminating its facility for detecting and controlling the extent to which the battery 26 discharges when the primary supply 1 2 is not operating. This can be done by eliminating the transistor 34 and the resistors 38, 39, and by then connecting the base of the transistor 32 directly to positive primary supply terminal 22 through the resistor 36.

The transistor 32 will then at any time receive a substantially constant base current from either the primary supply 1 2 or the battery 26, and will then charge or discharge the battery 26 in the manner described above, depending respectively on whether the primary supply 12 and battery 26 bias the transistor 32 to operate in the inverted or normal mode. This is not a preferred mode of operation for the back-up supply 10, however, as not only is the facility to control discharge of the battery 26 eliminated but the shortcircuit protection otherwise provided is incidentally lost.

Fig. 2 illustrates a back-up supply 40 which is an alternative embodiment of the invention.

The back-up supply 40 is shown connected to the primary supply 12, the load 14 and the battery 26, referred to above. The transistor 32 is common to the circuit configurations of both back-up supplies 10, 40 and operates in each in essentially the same manner. The principal difference between the back-up supply 10 and the back-up supply 40 is that the latter permits more accurate control of the discharge of the battery 26 and in particular permits more accurate specification of the drop in the load voltage which will cause the battery 26 to cease discharging any further.

The back-up supply 40 includes an operational amplifier 42 which is powered by the potential difference between the primary supply terminals 22, 24. A reference voltage is provided at the amplifier's non-inverting terminal 44 by a resistor 46 and zener diode 48 connected between the primary supply terminals 22, 24 as shown. A predetermined fraction of the load voltage is provided at the amplifier's inverting terminal 44 by a resistive voltage divider comprising the resistors 52, 54 connected as shown. It will be appreciated that the predetermined fraction of the load voltage and the reference voltage as described herein are referenced with respect to the voltage at the positive primary supply terminal 22.

When the magnitude of the predetermined fraction of the load voltage exceeds the magnitude of the reference voltage, the output signal of the amplifier 42 assumes a value which is substantially the voltage at the positive primary supply terminal 22. When this occurs, a pnp transistor 56 whose base is connected to the negative primary supply terminal 20 and whose emitter is coupled by a resistor 58 to the amplifier 42 is biased to provide base current through its collector to the transistor 32. This base current is obtained in part from the amplifier 42 through the resistor 58 but additional base current is obtained for the charging of battery 26 from the primary supply 1 2 through a resistor 60.

The magnitude of the base current provided will be relatively constant and dependent on the resistive values of the resistors 58, 60.

Provided with such a base current, the transistor 32 charges or discharges the battery 26 in the manner described above, depending on the polarity of the differential voltage between its collector and emitter.

If the primary supply 1 2 is not operating, and if the magnitude of the predetermined fraction of the load voltage-drops below the magnitude of the reference voltage, indicating that the battery 26 is discharged a predetermined amount, then the output of the amplifier 42 assumes a value which is substantially the voltage at the negative primary supply terminal 24. When this occurs, the transistor 56 ceases to provide base current to the tansistor 32 and the battery 26 thereby ceases to discharge any further. In this manner, excessive discharge of the battery is prevented.

A diode 62 is connected as shown to protect the voltage regulator 1 6 against reverse voltages which might occur when the unregulated supply (not shown) fails and the battery 26 begins to impress a positive voltage at the primary supply terminal 22. It will be appreciated that the voltage which can be impressed by the primary supply 1 2 across the load 1 4 and the voltage to which the battery 26 can be charged will be reduced from the voltage at terminal 22 by several hundred millivolts, namely, the voltage drop required to fully turn on the diode 62. The diode 62 does not otherwise significantly affect the operation of the back-up supply 40, and supply 1 2 may be adjusted to compensate for the drop in diode 62.

Reference is next made to Fig. 3 which illustrates a back-up supply 70 connected to the primary supply 12, the load 14 and thebattery 26, referred to above. The back-up supply 70 is essentially the back-up supply 40 modified to directly monitor the battery voltage to prevent excessive discharge. Except as described below, the back-up supplies 40, 70 operate in substantially the same manner and consequently circuit components common to the back-up supplies 40, 70 have been labelled with like reference numerals.

In the back-up supply 70, the amplifier 42 is powered by the battery 26. Excessive discharge pf the battery 26 is prevented essentially as follows. A reference voltage is provided at the amplifier's non-inverting terminal 44 by a zenor diode 72 and resistor 74 connected as shown between the battery terminals 28, 30. A resistive voltage divider comprising resistors 76, 78 is connected across the battery terminals 28, 30 to sense the battery voltage. A predetermined fraction of the battery voltage, the predetermined fraction being the voltage divider ratio of the resistors 76, 78, is provided at the amplifier's inverting terminal 50. The amplifier 42 thus effectively compares the battery voltage to tha reference voltage.

Assume that the primary supply 1 2 turns off. Then so Ipng as the magnitude of the predetermined fraction of the battery voltage exceeds the magnitude of the reference voltage, the output of the amplifier 42 assumes a value corresponding substantially to the voltage of the positive battery terminal 28. The transistor 56 is then biased to supply current to the first transistor 32 to permit the battery 26 to discharge in the manner described more fully above with reference to the back-up supply 40.

When the magnitude of the predetermined fraction bf the battery voltage becomes-less than the magnitude of the reference voltage, the output of the amplifier 42 assumes a value corresponding substantially to the voltage at the negative battery terminal 30. A diode 80 connected as shown prevents this output voltage from reverse biasing the transistor 56. The emitter of the transistor 56 does remain, however, at substantially the same potential as the base of the transistor 56 so that the transistor 56 ceases to provide base current to the transistor 32 which in turn prevents any further discharge of the battery 26. The battery 26 is thus prevented from discharging beyond a predetermined level.

When the primary supply 1 2 turns on again, the amplifier 42 continues to sense the low battery voltage at its terminals 44, 50 and does not at this time turn on again.

However the voltage from terminal 22 applied through resistor 60 to the emitter of transistor 56 turns on transistor 56, which supplies base current to transistor 32. Transistor 32 then turns on, causing batery 26 to charge.

When the battery voltage increases sufficiently, amplifier 42 turns on again, supplying additional base current to transistor 32 through transistor 56.

Although amplifier 42 and resistors 72, 78 and zener 72 and resistor 74 remain permanent load across battery 26, their current drain can be made very small (virtually equivalent to the battery's shelf discharge rate), since amplifier 42 need not supply all the base current for transistor 32 during battery charge. During battery discharge all the base current for transistor 32 is supplied by amplifier 42 but in its forward mode transistor 32 has a much higher current gain than in its inverted mode so less base current is needed for transistor 32 during discharge.

The back-up supply 40 which monitors the load voltage to detect excessive discharge of the battery 26 will shut down preventing discharge of the battery 26 when the load 14 is short-circuited. It will be appreciated that the back-up supply 70 which monitors the battery voltage still provides shortcircuit protection. This is because when the load voltage drops during a short circuit, the voltage across transistor 32 rises, decreasing the voltage across resistor 58, thereby reducing the base current to transistor 32 supplied by transistor 56. This continues until transistor 32 is completely turned off.

Particular polarities have been assigned both to the primary supply terminals 22, 24 and the battery terminals 28, 30 in the various embodiments of the invention illustrated above. Also, particular transistor devices, either npn or pnp devices, have been shown. It will be appreciated that the polarities of the primary supply 1 2 and the battery 26 can be reversed and that with appropriate circuit modifications complimentary semi-conductor devices can be substituted for the particular transistor devices illustrated.

Claims

1. A back-up power supply for use in association with the combination of a primary supply which impresses a fixed voltage across a load connected between first and second primary supply terminals and a rechargable battery having a first battery terminal electrically connected to the first primary supply terminal and a second battery terminal, and back-up supply comprising:: solid state current means for controlling the magnitude and direction of battery current, the current control means including first and second current transfer terminals and a control terminal, the first current transfer terminal being adapted to be connected to the second battery terminal and the second current transfer terminal being adapted to be connected to the second primary supply terminal so that the battery current is constrained to flow through the current control means between the first and second current transfer terminals, the current control means being respective to a first control signal, the first control signal comprising the polarity of the differential voltage between the first and second current transfer terminals, to tend to conduct current between the first and second current transfer terminals in a first direction to charge the battery from the primary supply when the differential voltage has a first polarity indicating that the primary supply is operating and to tend to conduct current between the first and second current transfer terminals in a second direction to discharge the battery through the load when the differential voltage has an opposite polarity indicating that the primary supply is not operating, the current control means being responsive to a second control signal applied to the control terminal to regulate the magnitude of the battery current; and, second control signal supply means coupled to the second control terminal for supplying the second control signal.

2. A back-up supply as claimed in claim 1 in which the current control means has a conduction characteristic such that when a pedetermined second control signal is applied to the control terminal and the differential voltage has a predetermined absolute value, the control means will conduct more current when the differential voltage has the first polarity than when the differential voltage has the opposite polarity.

3. A back-up supply as claimed in claim 1 or 2 in which the second control signal supply means are coupled to one of the load and battery to detect one of the load and battery voltages and the magnitude of the second control signal is substantially proportional to the one of the load and battery voltages and in which the current control means have a conduction characteristic such that the magnitude of the current conducted between the first and second current transfer terminals increases and decreases respectively as the magnitude of the second control signal increases and decreases and drops to substantially zero when the magnitude of the second control signal drops to substantially zero.

4. A back-up supply as claimed in claim 1, 2 or 3 in which the second control signal supply means comprise means for reducing the magnitude of the second control signal to substantially zero when the primary supply is not operating and the one of the load and battery voltages falls below a predetermined level, thereby causing the battery to cease to discharge any further.

5. A back-up supply as claimed in any of claims 1 to 4 in which the second control signal is substantially proportional to the load voltage, whereby, when the primary supply is not operating and the battery is supplying current to the load, the battery ceases to discharge when the load is short-circuited.

6. A back-up supply as claimed in any of claims 1 to 5 in which the current control means have a conduction characteristic such that, when the magnitude of the second control signal drops below a predetermined level, the magnitude of the differential voltage between the first and second current transfer terminals tends to increase as the magnitude of the second control signal decreases.

7. A back-up supply as claimed in any of claims 1 to 6 in which the second control signal is substantially proportional to the load voltage, whereby, when the primary supply is not operating and the battery is supplying current to the load, any drop in the load voltage causing a drop in the magnitude of the second control signal below the predetermined level tends to cause an increase in the magnitude of the differential voltage and consequently a further drop in the magnitude of the load voltage thereby causing the magnitudes of the load voltage and control signal to tend to drop to substantially zero.

8. A back-up supply as claimed in any of claims 1 to 7 in which the current control means comprise a transistor whose emitter and collector serve respectively as the first and second control terminals and whose base serves as the control terminal.

9. A back-up supply as claimed in claim 8 in which the primary supply and the battery are coupled to the transistor so that the trans istor is biased to operate in the inverted mode tending to charge the battery when the pri mary supply is operating and in the normal mode tending to discharge the battery through the load when the primary supply is not operating and the battery is supplying current to the load.

1 0. A back-up supply as claimed in claim 8 or 9 in which the second control signal is base current supplied by the second control signal supply means to the transistor.

11. A back-up supply as claimed in claim 8, 9 or 10 in which the second control signal supply means normally supply a fixed base current to the transistor when the primary supply is operating so that the battery tends to charge at a constant rate dependent on the ss of the transistor in the inverse mode.

1 2. A back-up supply as claimed in claim 11 in which the second control signal supply means is coupled to one of the load and battery to detect one of the the load and battery voltages and in which the second control signal supply means include means for reducing the base current supplied to the transistor to substantially zero when the primary supply is not operating and the one of the load and battery voltages has dropped below a predetermined level thereby causing the battery substantially to cease to discharge any further.

1 3. A back-up supply as claimed in claim 11 in which the magnitude of the base current supplied by the second control signal supply means is substantially proportional to the load voltage, whereby, when the primary supply is not operating and the battery is supplying current to the load, any drop in the load voltage below a predetermined level tends to cause an increase in the magnitude of the differential voltage between the emitter and the collector of the transistor and consequently a further drop in the magnitude of the load voltage thereby causing the magnitudes of the load voltage and transistor base current to tend to drop to substantially zero.

14. A back-up power supply constructed and adapted to operate substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 5. A back-up power supply constructed and adapted to operate substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

1 6. A back-up power supply constructed and adapted to operate substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.