Abstract:
The invention relates to a method for distributing electric power in telecommunication central office equipment intended to operate at voltages within a determined range, according to which method the voltage of the distributed power is allowed to drop temporarily below the lowest voltage of the range.

Description:
This invention relates to a method for distributing electric power in telecommunication central office equipment intended to operate at voltages within a determined range. 
     SUMMARY OF THE INVENTION 
     As the equipment operates continuously correctly only when the input voltage is situated within a limited range around a nominal voltage, the power is distributed in such way that the voltage is situated within a determined range. 
     For the power distribution of a telephone network, the reference voltage is for instance 48 V and said range from 38 V to 56 V. 
     Telecommunication standards, for example ETSI (European Telecommunications Standard Institute) and BELCORE standards, require that the equipment should keep operating normally when the input voltage, for example the DC voltage from a battery or a rectifier, drops temporarily below the lower limit of this range. 
     Such voltage drop can take place when the power is switched, for instance by means of relays, from one power supply to another. The duration of such input voltage drop can vary between 0 and 30 ms. 
     In order to avoid that the distribution is cut off during a short voltage drop, capacitors are used which keep the voltage constant, and thus above the lowest voltage of said voltage range, during the input voltage drop. 
     BACKGROUND OF THE INVENTION 
     These big capacitors are costly and voluminous. The distribution racks used for distributing are consequently relatively big and expensive. Extra sub-racks may be required for the big capacitors. 
     The aim of the invention is to obviate this drawback and to offer a method for power distribution, avoiding the use of big capacitors and permitting to use a relatively economic and compact equipment for the distribution. 
     In accordance with the invention, this object is accomplished in a method for distributing power according to which the voltage of the distributed power is allowed to drop below the lowest voltage of the range, but only temporarily. 
     This is completely against the general idea that the voltage may never drop below the lower limit of the prescribed range. 
     Unexpectedly, it was found that such voltage drop is permissible during a short time and for instance for the duration of the above mentioned input voltage drop which the prior art methods compensate with capacitors. 
     Preferably, according to the invention, the voltage of the distributed power is allowed to drop below this lowest voltage during the complete duration of said input voltage drop, this is thus in practice during 0 ms to 30 ms. 
     The invention also relates to a device for distributing electric power in telecommunication central office equipment intended to operate at voltages within a determined range, particularly suited for the application of the above described method of the invention. 
     The device is characterised in that it comprises means for allowing the voltage of the distributed power to drop temporarily below the lowest voltage of the range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example and with reference to the accompanying drawing. 
    
    
     In central office telecommunication equipment, power distribution racks distribute the power from one or more batteries  1  or a rectifier to further circuits. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Part of the power distribution system is constituted by equipment  2 , for instance a DC/DC converter, which normally operates continuously within a predetermined range, whereby the batteries  1  or rectifier have a nominal voltage situated in this range. 
     Therefore, the power from a battery  1  or a rectifier is provided to the equipment  2  in such way that the supplied voltage Vo normally remains within the above-mentioned range. 
     If the voltage of the distributed power rises above the upper limit of the range, it may be limited with for example a Zener-diode. 
     When the voltage of the battery, is continuously under the lower limit voltage, the equipment  2  does not have to work and, in fact, it is advisable that the equipment  2  should not operate as the used power is constant, but the voltage is lower, what means that the current is higher. 
     Such higher current influences the dimensioning of the distribution equipment  2  and the fuses. 
     Consequently, if the voltage input descends during a long time under the lower limit of the range, the power distribution is cut-off. 
     Therefore, the input voltage V i  is continuously compared with a comparison voltage V c  corresponding to said lower limit and the equipment is triggered in function of the result of this comparison. 
     As shown in the accompanying drawing, an electronic comparator  3  may be used for this comparison, the input voltage V i  being supplied to one entry of it, the other entry being connected to a comparison voltage input V c . 
     The output of the comparator  3  is a trigger signal sent to the equipment  2 . 
     While it should be avoided that the output voltage V o  of the power distribution is continuously beneath the lower limit of above-mentioned range, a temporary drop under this limit is however permitted. 
     This temporary drop may have a duration about equal of the duration of the input voltage drop caused by for instance relays that are switching from one power supply to another, for instance from one battery  1  to another, which duration is for example from 0 ms to 30 ms. 
     In order to permit this temporary drop, the input from the battery  1  to the comparator  3  is delayed by means of a delay circuit  4 . 
     As shown in the drawing, the delay circuit  4  can be implemented as a R-C circuit, comprising a resistor R in series with the input voltage V i  and thus connected to an entry of the comparator  3 , and a capacitor C in parallel with this input. 
     This capacitor C may be very small, for example have a capacitance of 1 μF. Therefore it is relatively inexpensive and needs relatively little space and volume compared to the big capacitors used with the prior art distribution methods which use very big capacitors at the input of equipment  2 . 
     The output voltage V o  may temporarily drop to zero volt, but a lower limit underneath the lowest voltage of the operating range may be determined. 
     As the lower voltage and also the higher current are only temporary, there is no need to adapt the dimensioning of the equipment  2 . 
     Hereafter a practical example is given: 
     A DC/DC converter, constituting the equipment  2  of a Central office Power distribution, normally operates continuously within the predetermined range from 38 V to 56 V, and is supplied by batteries  1  having a nominal voltage of 48 V. 
     The voltage V o  of the distributed power is allowed to drop underneath the lower limit of 38 V to a lower voltage of 28 V during 2 ms, the duration of a drop of the input voltage V i  due to switching between two batteries  1 . 
     In the comparator  3 , the input voltage V i  is continuously compared with a comparison voltage V c  of 38 V. The input from the battery  1  is fed to the comparator  3  through the delay circuit  4  having a resistor R of 10 kohm in series with the battery  1 , while a capacitor C of 1 μF is connected in parallel with the input. The capacitor C needs only about 2 cm 2  surface and about 3 cm 3  volume per 20 W, what permits to distribute power more lines per distribution rack.