POWER SUPPLY FOR TELECOMMUNICATIONS DEVICES

Abstract: A power supply has a plurality of single line interface circuits (20, 22, 24 and 26) each connected by a respective line, to a respective telephone (4, 6, 8 and 10) or other telecommunications device. Each SLIC applies a sufficient voltage derived from a battery to the line to operate the respective telephone, and dissipates any excess power supplied by the battery. The apparatus includes common power regulation means (30), which incorporates a switch mode power supply unit (68), and which monitors the voltages being applied by the SLICs at any one time and reduces the voltage applied to the SLICs to substantially the maximum voltage required by any one of the SLICs, thereby reducing the amount of power dissipated by the SLICs.

Description: POWER SUPPLY FOR TELECOMMUNICATIONS DEVICES Field of the Invention This invention relates to power supply apparatus for supplying electrical power ro a number of remotely situated telecommunications devices, such as telephones, and to an assembly comprising such apparatus connected to a plurality of telephones. Background to the Invention A conventional telephone exchange can serve a large number of telephones connected to the exchange along corresponding lines which are of differing lengths as a result of differences in the distances of the telephones from the exchange. As well as relaying telecommunications signals, the exchange also feeds to each line sufficient power to operate the telephones. That power can vary from one telephone to another in view of the differing lengths of the lines, which give rise to differences in τ_he amount of resistive losses on the various lines. The variation in power requirements is accommodated by means of a number of subscriber line interface circuits (SLICs), each of which relays through a respective line a sufficient amount of power to operate its respective telephone. If linear SLICs are used, excess power supplied to each circuit from the source is dissipated as hear. In this case, the voltage applied to each SLIC from the power source has to be sufficient to operate the telephone connected by the longest line. If a large number of other telephones are connected on much shorter lines, a relatively large amount of power has to be dissipated by the SLICs associated wirh the shorter lines, which can be uneconomical and leads to cooling problems in the exchange. Some exchanges use SLICs which have individual internal switch mode power supply units which reduce the power dissipation. However, such SLICs are relatively expensive. Summary of the Invention According to the first aspect of the invention, there is provided power supply apparatus for connection tc a plurality of remotely situated telecommunications devices along corresponding lines, rhe apparatus compriεing a plurality of interface circuits, each having an input for connection to a common power source and an output for connection to a respective line, each interface circuit, when active, feeding sufficient power, derived from the source, to the line to provide rhe required amount of power to operate the respective telecommunications device, dissipating any excess power supplied to the interface circuit from the source, wherein the apparatus includes common power regulation means for monitoring a characteristic of the amount of power being fed by each active interface circuit to its respective line and so controlling the power supplied to each active interface circuit that said supplied power is substantially equal to the largest of the amounts of power required by the active interface circuitε. Thus the power regulation means ensures that the power supplied to all the interface circuirs is governed by the requirements of the active interface circuit which has to feed the most power to its respective line. The invention therefore helps to reduce the amount of power which needs to be dissipated by the active interface circuits, particularly when the interface circuits with the highest power requirements are not active, and thus enables the apparatuε to use relatively cheap linear SLICs, whilst avoiding, or at least ameliorating problemε ariεing from the power dissipation in the SLICs. Conveniently, the characteriεtic monitored by the regulating means is the respective voltage applied by each active interface circuit to its reεpective line, and the regulating means controlε the power supplied to the interface circuitε by controlling the voltage applied to the inputs of the interface circuitε. Conveniently, the apparatus is adapted to be connected to a source of dc power, for example, a battery. The power regulation means preferably includes a switch mode power supply unit operable to reduce the voltage supplied by the dc source to the interface circuits. Preferably, the interface circuits are mounted on a single card for mounting in a rack. Preferably, the apparatus has a plurality of εuch cardε, and the regulating meanε iε one of a plurality of εuch means each aεεociated with a reεpective card. The use of a plurality of cards and asεociated regulating means enableε the allocation of lineε to interface circuits to be such that the interface circuits on each card are connected to a number of lines of similar lengths, thereby reducing the range of powers to be fed by the interface circuits in each respective card. That reduction in range enables the amount of power disεipated by the interface circuits to be further reduced. According to the second aspect of the invention, there is provided a telephone exchange having power supply apparatuε in accordance with the firεt aεpect of the invention connected to a plurality of remotely situated telephoneε along correεponding lineε which are not neceεεarily of the εame resistance, and to a dc power source. Preferably, the apparatus has a plurality of cards, each having a reεpective εet of interface circuits and all the lineε connected to the circuits on each cards are of lengths which lie in a respective one of a plurality of different ranges. The invention also lieε in power regulation meanε for use in apparatus in accordance with the firεt aεpect of the invention, the power regulation meanε compriεing a power input for connection to a battery, a plurality of further inputs, each for connection to the output of a respective interface circuit, monitoring meanε connected to εaid further inputε for monitoring the voltage, or current fed, by each active interface unit to a reεpective line connecting that interface unit with a reεpective telecommunicationε device, the monitoring meanε being operable to determine, at any one time, the maximum of said voltages or currents; and control meanε for applying to all εaid interface circuitε an output voltage, derived from εaid batrery voltage, εubstantially equal to the largest of the voltages required by an active interface circuit. Preferably, the monitoring means is arranged to monitor the voltages applied by said active interface circuits and to determine the largest of said voltages. Brief Description of the Drawinσε Power supply apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schemaric block diagram of the apparatuε when connected to a number of telephones; Figure 2 is a block diagram of an interface circuit forming part of rhe apparatuε shown in Figure 1; Figure 3 is a block diagram of another part of the apparatus; Figures and 5 are diagrams illustrating the operation of the interface circuits; Figure 6 shows an alternative type of interface circuit for use in apparatus not in accordance with rhe invention; and Figure 7 shows an exchange naving a subrack containing a plurality of power supply apparatuεes in accordance with the invention. Detailed Description The power εupply apparatus shown in Figure 1 forms part of a telephone exchange 2 which is connected to a number of telephones, 4, 6, 8 and 10 along corresponding lines 12, 14, 16 and 18, each comprising a pair of conductors (A and B). For the sake of clarity, only 4 telephones are εhown in the drawing, although typically the apparatus will serve between 6 and 15 telephones (or other telecommunications devices such as fax machineε). The apparatus in this example operates under a constant current protocol, which applies in the UK, by supplying a predetermined current ro each telephone which is in uεe. However, apparatuε in accordance with rhe invention can operate under different protocolε, for example, the conεtant reεiεtance requirementε applicable in the USA. The telephoneε are located at variouε different diεtances (in the range 0-5 km) from the exchange 2, and aε a result, the connecting lines are not all of the same length. However, the lineε have the same reεiεtivity, and therefore do not house the εame reεiεtance. Each of the lineε 12, 14, 16 and 18 is connecred to a respective one of four identical linear subscriber line interface circuits (SLICs) referenced 20, 22, 24 and 26. The subscriber line interface circuitε all have inputs for the electrical power needed to operate the telephones 4, 6, 8 and 10, and those inputs are connected in parallel, as indicated by line 28 to regulating means 30 in the form of a compensating power supply. The single line interface circuit 20 is shown in greater detail in Figure 2, in which the resiεtor Rloop repreεentε the reεistance of the line 12 and telephone 4. The circuit 20 comprises an amplifier 50 controlled by a feedback loop which includes an anti-εaruration circuit 52 and an external programming resistor Rdc. The amplifier 50 applies a voltage, Vab, to rhe line 12 causing a current to flow therealong. In the case of a consrant current syεtem, the current εupplied by the amplifier iε fixed unleεε the anti-saturation circuit 52 iε activated. In the caεe of a conεtant reεiεtance system, the voltage at the output of the amplifier 50 is fed back, aε indicated by the dotted line 54, to εi ulate a conεtant resistance. Again, thiε may be modified by rhe anti-saturation circuit 52 (in the case of longer lineε). The anti-εaturation circuit 52 provideε a constant headroom voltage for the transmisεion of voice frequency (VF) signals, and will, if necessary, reduce the current (and hence Vab) to maintain voltage headroom for VF signals. The amplifier 50 is εupplied with a voltage Vreg by the regulating meanε 30, and the difference between Vreg and Vab is dissipated in the amplifier 50. With reference to Figure 3, the compensating power supply 30 has an input 32 for connection to a battery (not shown) or other source of dc power, and four monitor inputs 56, 58, 60 and 62, each of which iε connected to the output of a reεpective one of the interface circuitε 20, 22, 24 and 26. Each of the monitor inputε iε in turn, connected to monitor circuitry 64 which monitorε the voltages, V(AB) 1-4, being applied to each interface circuit to its respective line (when the telephone connected thereto is in use), determines the maximum cf those voltages and feeds a signal representative of that maximum to a volrage control unit 66. The unit 66 is in turn connected to a power supply εwitching regulator 68 which iε alεo connected to the power εupply input 32, and to an output 70 (connected ro the line 28). The regulator applieε to the line 28 a voltage which is less than or equal to the battery voltage, and which is controlled by the unit 66 εo aε to be related to the maximum voltage in a way described below. In this example, the constant current requirement is uεed to determine the voltage which haε to be εupplied by each SLIC to itε respective line, when in use; the voltage applied has to be sufficient to produce 30mA of current along the respective line. In view of the differences in reεiεtanceε of the lineε 12, 14, 16 and 18, rhat voltage will vary from one SLIC to another. Since each SLIC iε a linear device, any excesε power (i.e. power which iε not then transmitted along the respective one of the lines 12, 14, 16 and 18) is disεipated aε heat. In uεe, εwitching regulator 68 of the compenεating power supply 30 controls the voltage applied to the line 28 so that it is equivalenr to that maximum voltage plus a bias voltage needed in order for the SLIC to operate. As a result, the power dissipated by the active SLIC which is applying the largest voltage to its respective line is minimised, and the power dissipated in the other active SLICs is less than would be the case if the full battery voltage- were applied to all the SLICε. It will be appreciated that the voltage supplied along the line 28 can vary depending upon which of the SLICs is in use at any one time. As shown in Figure 7, the components of the power supply apparatus can be mounted on a single card 74 accommodated in a rack 76 of a plurality of such cards, 74, 78, 80, each of which εerveε a reεpective εet of telephones. The allocation of telephoneε to cardε is such that the distance of each member in a set from the exchange lies in a respective range associated with thar set, so that the range of different voltages which have to be applied by the SLICs on any one card can, as far as is practicable, be kept to a minimum. For example, all the telephones connected to the card 74 are situated more than 1km from the rack 76, the telephones connected to the card 78 are situated in the range 300m to 1km and the telephones connected to the card 80 in the range 0 to 300m. A number cf examples of the operation of the power supply apparatus on any one card, and of the resultant savingε in diεεipated power, are diεcuεsed below. Aε explained above, all of the monitor inputε V(AB) 1-4 are examined and the output of the supply 30 is then adjusted to supply the voltage which will maintain the required subscriber line conditions on the worst active line (i.e. the line with the highest resiεtance). Some example cases are given below in which the battery voltage iε 48 voltε, V(ab) is the voltage applied ro the line 28, (and is equal to the voltage detecred at the respective one of the inputs 50, 58, 60, 62) and which assume a requirement of 30mA constant current, telephone resistance of 400 ohms, subscriber line resistance of 200 ohms/km, and 5V biaε required for line driverε. With reference to Figure 4, Rlcop iε the sum of the resistanceε of the telephone and the line, Rline and Rphone. Vab iε the voltage which the SLIC haε to apply to itε line and Vreg iε the voltage supplied to the SLIC by the supply 30. Case (a) All subscribers active on very short lines (negligible line resistance) . Vab (max) ' = 0.03 x Rloop = 0.03 x 400 = 12V. Vreg = (0.03 x 400) -r 5 = 17V. In this case V(reg) will be set to 17V, and the dissipation in each SLIC will be 0.15W (total of 0.6W for all 4 channels). Without compensating power supply, the power dissipated would be 0.03 x (48 - (0.03 x 400) = 1.08W for each channel, would have to be dissipated. Power saving is equivalent to 0.93W per channel. Caεe (b 3 subεcribers active on very εhort lines (12, 14, 16 - negligible line resiεtance), one εubεcriber active on 300M line (18). V(ab) {12, 14 and 16}= (0.03 X 400) = 12V. V(ab) {18} = (0.03 x (400 + (0.3 X 200)) = 13.8V. Vreg = 13 . 8 + 5 = 18 . 8V . In this case V(reg) will be set to 18.8V, and the dissipation in SLICs 20, 22, 24 will be 0.204W and in SLIC 26 will be 0.15W (total of 0.762W for all 4 channels). Without compensating power supply, the power dissipated would be 0.03 x (48 - (0.03 x 400)) = 1.08W for SLICS 20, 22 and 24 and 0.03 x (48 - (0.03 x (400 + (0.3 x 200)))) = 1.026W for SLIC 26. Power saving is equivalent to 0.876W per channel. Case ( c ) 3 εubscribers active on very εhort lineε (12, 14, 16) (negligible line resistance), one subεcriber active on 3km line, (18) V(ab) {12, 14 and 16} = (0.03 x 400) = 12V V(ab) {18} = (0.03 x (400 i- ( 3 x 200)) = 30V V(reg) = 30 + 5 = 35V. In this case V(reg) will be set to 35V, and the dissipation in SLIC 20, 22 and 24 will be 0.69W and in SLIC 26 0.15W (total of 2.22W for all 4 channels). Without compensating power supply the power dissipated would be 0.03 x (48 - (0.03 x 400)) = 1.08W for SLICs 20, 22 and 24 0.03 x (48 - (0.03 x (400 ■ •- ( 3 x 200)))) = 0.54W for SLIC 26. Power saving is equivalent to C.39W per channel. Case (d) 1 subscriber active on very short line 12 (negligible line resistance), three subscriberε active on 3km lineε (14, 16, 18). V(ab) {12} = (0.03 x 400) = 12V. V(ab) {14, 16, 18} = (0.03 x (400 + (3 x 200)) = 30V V(reg) = 30 ■ >- 5 = 35V. In this caεe V(reg) will be set to 35V, and the dissipation in SLIC 20 will be 0.69W and SLIC 22, 24, 26 is 0.15W (total of 1.14W for all 4 channels). Without compensating power supply the power dissipated would be 0.03 x (48 - (0.03 x 400)) = 1.08W for SLIC 20 and 0.03 x (48 - (0.03 X (400 + (3 X 200)))) = 0.54W for SLICS {22, 24, 26 . Power saving is equivalent to 0.39W per channel. When the apparatus is used in a constant resistance system, the model illustrated in Figure 5 applies. The following calculations asεume a requirement of 2 x 200 ohm conεtant resistance feed (Rfeed) from 50V (Vfeed), a telephone resistance (Rphone) cf 400 ohms, a line resiεtance (Rline) of 200 ohmε/Km (multiplied by the line length), and a 5V biaε requiremenr for the line driverε. Again, Vab iε the voltage applied by a SLIC to its line and Vreg is the voltage applied to the SLICS by the supply 30. Hoop is the current flowing along the line. Case (e) All subscribers active on very short lines (negligible line resiεtance) . Vab = 50 x 400/(400 + 400) = 25V Hoop = Vab/Rloop = 25/400 = 62.5mA Vreg = 25 + 5 = 30V In this case Vreg will be set to 30V, and the dissipation in each SLIC will be 0.3125W (total of 1.25W for all 4 channels). Without compensating power supply the power dissipated would be 0.0625 x (48 - 25) = 1.44W for each channel. Power saving is equivalent to 1.125W per channel. Case (f ) 3 subscriberε active on very εhort lineε (negligible line resistance), one subεcriber active on 3km line. Vab {1} = 50 x (400 ÷ 600)/(400 + 600 + 400) = 35.7V Hoop = Vab/Rloop = 35.7/(400 + 600) = 35.7mA Vab {2, 3, 4} = 50 x 400/(400 + 400) = 25V Hoop = Vab/Rloop = 25/400 = 62.5mA Vreg = 35.7 + 5 = 40.7V In this case Vreg will be set to 40.7V, and the dissipation in each SLIC{1} will be 0.179W and in SLICs {2, 3, 4} will be 0.981W per channel (total power = 3.122W). Without compensating power εupply the power dissipated would be 0.0357 X (48 - 35.7) = 0.439W for channel 1 and channels 2, 3, 4 power = 0.0625 x (48 - 25) = 1.44W per channel (total power = 4.76W) . Power saving is equivalent to 0.41W per channel. An alternative control loop method would employ the SLIC output εignal (uεually available on commercial componentε) which gives an indication of the loop current. In constant resistance feed applications the minimum loop current will directly show the longest line and hence highest Vab. For constant current feed, where all lines are in the constant current region, the loop currents will be equal, in thiε case the control voltage would be reduced until one of the SLICs anti-saturation circuits (the one with the longest line) began to reduce the loop current. Consequently the corresponding monitor input would begin to fall below the constant current value and at this point the regulated supply would maintain its voltage keeping the SLIC jusr on the edge of the constant current and anti-εaturation regionε of operation. The monitor block in thiε caεe outputε the minimum voltage (Vmin) . A control voltage (Vreg) iε produced from the maximum/minimum loop monitor voltage to enable adjustmenr of the switching regulator to supply the voltage which will maintain the required subεcriber line conditionε on thiε worst case line. This block also ensures the regulated voltage is within SLIC battery input operating range. The SLIC shown in Figure 6 is of a type having itε own builr-in regulator which operareε, in a similar fashion to regulator 68, to reduce the voltage applied from a battery, without diεsipating excess power. An exchange incorporating such SLICs, however, is relatively costly. Although the invention has been described in relation to telephone exchangeε, ir can alεo be uεed in various types of primary iπulriplexing equipment, such as primary multiplexers supplying subscriber line access and situared remotely from the telephone exchange.

Claims: CLAIMS 1. Power supply apparatus for connection to a plurality of remotely situated telecommunicarions devices along corresponding lines, the apparatus comprising a plurality of interface circuits, each having an input for connection to a common power source and an output for connection to a respective line, each interface circuit, when active, feeding sufficient power, derived from the source, to the line to provide the required amount of power to operate the respective telecommunications device, and dissipating any excess power εupplied from the εource, wherein the apparatus includes common power regulation means for monitoring a characteristic of the amount of power being fed by each active interface circuit to its respective line and so controlling the power εupplied to each active interface circuir that εaid εupplied power is substantially equal to the largest of the amounts of power required by the active interface circuits. 2. Apparatus according to claim 1 in which the regulating means is εo arranged, in uεe, that it monitors the respective voltage applied by each active interface circuit to its respective line, controls rhe power supplied to the interface circuits by controlling the voltage applied to the inputs of the interface circuitε. 3. Apparatus according to claim 1 or claim 2 in which the apparatus is adapted to be connected to a source of dc power. 4. Apparatuε according to any of the preceding claimε, in which the power regulation means includes a switch mode power εupply unir operable to reduce rhe voltage εupplied by the εource to the interface circuitε. 5. Apparatus according to any of the preceding claimε, in which the interface circuits are mounted on a single card for mounting in a rack. 6. Apparatuε according to claim 5 , in which the card iε one of a plurality of such cardε, and the regulating meanε iε one of a plurality of εuch means each associated with a respective one or more cards. 7. A telephone exchange having power supply apparatus in accordance with any of the preceding claimε connected to a plurality of remotely εituated telephoneε along correεponding lineε, and to a dc power εource. 8. An exchange according ro claim 7 in which the power εupply apparatuε has a plurality of cards, each having a respective set of interface circuits and all the lines connected to the circuits on each cards being of lengths which lie in a reεpective one of a plurality of different rangeε. 9. Apparatuε substantially as described herein with reference to, and as illustrated in, Figures 1 to 5 of the accompanying drawings. 10. An exchange substantially as described herein with reference to, and as illustrated in, Figure 7 of the accompanying drawingε.

Inventors: PHILLIPS SIMON CHRISTOPHER DI, PHILLIPS, SIMON, CHRISTOPHER +DI
Applicants: DAVIES ALISON JANE & LF, NOKIA TELECOMMUNICATIONS OY, DAVIES, ALISON, JANE +LF
Application year: 1996
Main classification: H04M19
IPC codes: H04M19