Abstract:
A power supply for providing full time electrical power to a customer premises telecommunications hub. The supply includes an AC to DC power converter for converting power from the AC power grid to a DC voltage selected to maintain a backup battery at float voltage. The converter includes a first rectifier section for generating an unregulated DC voltage. This voltage is switched through the primary of an isolation transformer by a pulse width modulated voltage controller. The output of the transformer is connected to a second rectifier circuit and filter to produce a regulated DC output voltage. The regulated voltage is connected to a voltage correction circuit through a divider including a temperature compensator so that the feedback to the voltage controller causes the regulated output voltage to follow the battery float voltage at all temperatures. The unregulated DC voltage is coupled to the voltage controller current limiting input to protect the power supply at high input voltages.

Description:
This is a divisional application of U.S. patent application Ser. No. 09/675,585, filed Sep. 29, 2000, now U.S. Pat. No. 6,297,620 hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to combination power supplies and battery chargers, and more particularly to power supplies for telecommunications hubs located on business or customer premises which provide power to operate the hub while maintaining backup batteries at float charge level compensated for ambient temperature changes. 
     BACKGROUND OF THE INVENTION 
     Traditionally, telephony communications within the United States were handled by the public switched telecommunications network (PSTN). The PSTN can be characterized as a network designed for voice communications, primarily on a circuit-switched basis, with full interconnection among individual networks. The PSTN network is largely analog at the local loop level, digital at the backbone level, and generally provisioned on a wireline, rather than a wireless, basis. The PSTN includes switches that route communications between end users. Circuit switches are the devices that establish connectivity between circuits through an internal switching matrix. Circuit switches set connections between circuits through the establishment of a talk path or transmission path. The connection and the associated bandwidth are provided temporarily, continuously, and exclusively for the duration of the session, or call. While developed to support voice communications, circuit switches can support any form of information transfer (e.g., data and video communications). 
     In a traditional PSTN environment, circuit switches include central office (CO) exchanges, tandem exchanges, access tandem exchanges, and international gateway facilities. Central offices, also known as exchanges, provide local access services to end users via local loop connections within a relatively small area of geography known as an exchange area. In other words, the CO provides the ability for a subscriber within that neighborhood to connect to another subscriber within that neighborhood. Central offices, also known as end offices, reside at the terminal ends of the network. In other words, COs are the first point of entry into the PSTN and the last point of exit. They are also known as class 5 offices, the lowest class in the switching hierarchy. A class  5  telephone switch communicates with an analog telephone using the analog telephony signals in the well-known analog format. The class 5 telephone switch provides power to the telephone; detects off-hook status of the telephone and provides a dial tone in response; detects dual-tone multi-frequency signals from the caller and initiates a call in the network; plays a ringback tone to the caller when the far-end telephone is ringing; plays a busy tone to the caller when the far-end telephone is busy; provides ring current to the telephone on incoming calls; and provides traditional telephone services such as call waiting, call forwarding, caller ID, etc. 
     In an effort to increase the amount and speed of information transmitted across networks, the telecommunications industry is shifting toward broadband packet networks which are designed to carry a variety of services such as voice, data, and video. For example, asynchronous transfer mode (ATM) networks have been developed to provide broadband transport and switching capability between local area networks (LANs) and wide area networks (WANs). The Sprint ION network is a broadband network that is capable of delivering a variety of services such as voice, data, and video to an end user at a residential or business location. The Sprint ION network has a wide area IP/ATM or ATM backbone that is connected to a plurality of local loops via multiplexors. Each local loop carriers ATM over ADSL (asymmetric digital subscriber line) traffic to a plurality of integrated service hubs (ISHs), which may be at either residential or business locations. 
     An ISH is a hardware component that links business or residential user devices such as telephones and computers to the broadband, wide area network through a plurality of user interfaces and at least one network interface. A suitable ISH is described in co-pending U.S. patent application Ser. No. 09/226,575 entitled “Multi-Services Communications Device,” filed on Jan. 7, 1999 (Sprint docket number 1246), which is incorporated by reference herein in its entirety. The network interface typically is a broadband network interface such as ADSL, T1, or HDSL-2. Examples of user interfaces include telephone interfaces such as plain old telephone system (POTS) ports for connecting telephones, fax machines, modems, and the like to the ISH; computer interfaces such as ethernet ports for connecting computers and local area networks to the ISH; and video ports such as RCA jacks for connecting video players, recorders, monitors, and the like to the ISH. 
     In providing telephony services over a broadband network, the ISH connects a telephone in the customer&#39;s premises to a network element such as a service manager. This connection between the telephone and the network element is typically an ATM connection, which is much different than the traditional analog line to the local switch. ATM connections usually do not support analog telephony signals, such as off-hook, dial tone, and busy signals. Therefore, the ISH must provide many of the telephony functions traditionally provided by the telephone provider central office such as detect off-hook conditions, on-hook connections, and digits as well as provide the telephones with dial tone, ring current, ringback, and busy signals. The terms off-hook and off-hook condition as used herein are generic terms meaning that a user device (whether telephone, facsimile machine, modem, etc.) connected to a telephone line is attempting to access and use the line. 
     Another example of such a central office function being provided by the ISH is backup power. Traditionally in cases of power grid failure, the central office provides backup power to customers&#39; telephones through use of an industrial-strength, petroleum-fueled backup generator. Since it is not economical to equip each customer with a backup generator, an ISH must be equipped with a back-up power supply, which is typically a battery pack, to maintain power to the system in cases of power grid failure. 
     The ISH must include a power supply to support the telephony functions (off hook, dial tone, etc.) and to keep the battery pack in charged condition so that it can provide backup power for as long as possible in the event of power grid failure. The power supply of the ISH should be as simple as possible to be cost effective; and yet it is desirable that the power supply be able to operate continuously, use as little power as possible when the power grid fails, and provide high voltage isolation of the user equipment from the power grid. 
     SUMMARY OF THE INVENTION 
     A power supply according to the present invention includes an AC to DC power converter for converting AC power from the power grid to DC power for use in the ISH and a sealed lead acid battery connected directly to the DC output of the power converter. The power converter includes a first rectifier section for converting AC power from the power grid into an essentially unregulated first DC voltage. An isolation transformer has a primary coil connected to the first DC voltage and to a pulse width modulated voltage controller. The secondary of the isolation transformer is connected to a second rectifier section for producing a regulated second DC voltage at an output which is connected to the battery. A voltage comparator has an input connected to the second DC voltage by a divider circuit which includes a temperature sensitive device which adjusts the feedback voltage in proportion to battery temperature. The output of the voltage comparator drives the voltage control input of the voltage controller to maintain the regulated DC output voltage at the level needed to maintain the battery at float voltage over the operating temperature range. A current limit input to the voltage controller is provided with an input which is a combination of the primary winding current level and the first DC voltage level, to maintain maximum power availability level without overheating the power supply. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a power supply according to the present invention; and 
     FIGS. 2A and 2B together provide an electrical schematic diagram of the circuitry of an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, there is provided a block diagram illustrating the primary elements of a power supply according to the present invention. Block  10  represents a source of AC power, which for many user applications will be 117 volt 60 Hertz AC power from a residential power grid which is standard in the United States. The preferred embodiment, however, is adapted for receiving input voltages from 90 to 275 volts and at frequencies from 40 to 440 Hertz, to accommodate power grids in essentially all countries. The input power passes through a primary protection and EMI control section  12  to reduce power spikes and noise. The power then passes through block  14  where additional protection and rectification is provided. A first DC voltage is provided between output lines  16  and  18 . This first DC voltage is not closely regulated and will vary depending on input voltage. An high frequency isolation transformer  20  is connected to the outputs  16  and  18  and to a pulse width modulated voltage controller  22 . The secondary of transformer  20  is connected to a rectification and filtering section  24  which provides a closely regulated DC voltage across its outputs  26 . The regulated DC voltage is fed back to a voltage comparator  28 . Comparator  28  provides a voltage control signal through optical feedback  30  to voltage controller  22 . 
     Voltage controller  28  also receives a temperature compensation input on line  32 . This signal is generated by a thermistor  34  located near battery  36 . Battery  36  is connected through backplane interface  38  to the regulated voltage on lines  26 . Thermistor  34  is likewise connected through the backplane  38  to the control circuitry. Backplane  38  is a printed circuit board with a number of sockets for receiving and interconnecting the various printed circuit boards which comprise a telecommunications hub. Every board connected to the backplane requires electrical power. The power supply of the present invention provides this power from the input  10  so long as the power grid is working. If the power grid fails, the battery  36  is hard wired to the backplane and continues to provide the needed power to allow continued operation of the telecommunications hub. 
     Prior art systems such as that shown in U.S. Pat. No. 4,663,580 typically provide switches and necessary control circuitry to isolate a back up battery from a system requiring power until the power grid actually fails. Other systems such as that shown in U.S. Pat. No. 5,623,195 provide additional circuitry to avoid thermal runaway or overcharging of batteries which have been discharged. The present invention provides very simple cost effective circuitry in place of the more complicated prior art systems. 
     Under normal conditions, that is, when the power grid is working, it is important to keep the battery  36  fully charged, but not overcharged. This requires a closely regulated voltage which is compensated for battery temperature. The float, or fully charged voltage of lead acid batteries is known to vary with temperature. As temperature increases, the battery voltage decreases. If a constant supply voltage is applied to a battery, the charging current will increase as temperature rises. In a sealed lead acid battery, it is important to prevent overcharging because of its limited capacity to recombine oxygen and hydrogen which are produced by excess current. In the present invention, the thermistor  34  is placed near the battery  36 . Its resistance changes with temperature and provides a feedback signal to adjust the regulated output voltage so that it matches the float voltage of the battery  36 . As explained below, by proper choice of components the voltage control can be accurate within one percent. 
     If the “unusual” condition of power grid failure occurs, then power is supplied by the battery  36 . The battery will of course discharge during the power outage. Upon restoration of power from the power grid, the battery  36  will begin recharging. Depending on the state of discharge, the battery could draw significant currents, especially if the input source  10  is a high voltage source. To prevent overstressing components in this situation, the present invention provides a simple current limiting protection arrangement. In FIG. 1, resistor R 28  is an in-line current sensing resistor used to provide a current limiting signal to voltage controller  22 . An additional resistor R 2  is connected from the positive unregulated DC voltage line  16  to the current sense input. As the voltage on line  16  increases, the signal from resistor R 2  limits current through the primary of transformer  20  to protect components from overheating and possible failure. 
     A detailed electrical schematic diagram of an embodiment of the present invention is provided in FIGS. 2A and 2B. Components which are also individually shown in FIG. 1 are identified with the same numbers in FIGS. 2A and 2B. 
     In FIG. 2A, the AC power grid input  10  is illustrated with the standard US power plug configuration. The primary protection (box  12  of FIG. 1) is provided by capacitors C 1 , C 2  and C 3 . Secondary protection and rectification (box  14 , FIG. 1) is provided by transformer T 1 , fuse F 1 , thermistor R 1  and full wave diode bridge D 1 . The first DC voltage appears across lines  16  and  18  and is smoothed by capacitor C 4  and peak limited by zener diodes D 3  and D 7 . 
     Primary winding  40  of isolation transformer  20  is connected between power lines  16  and  18  in series with power transistor Q 3  and current sensing resistor R 28 . A secondary winding  42  together with diodes D 6  and capacitors C 10  and C 16  provide operating voltage to pulse width modulated voltage controller  22 . The driver output of controller  22  is connected through resistor R 12  to transistor Q 3 . Secondary windings  44  of transformer  20  are connected to diode D 8  and capacitors C 6  and C 5  to provide rectification and filtering (block  24 , FIG. 1) for the regulated output DC power on line  26 . 
     FIG. 2B illustrates the voltage control section (block  28 , FIG. 1) and other parts of the power supply. Line  26  connects the regulated DC power to the backplane interface  38 , and through it to battery  36  and all other systems which are operated by this power. The thermistor  34  is connected through the backplane interface to a resistor divider string comprising resistors R 7 , R 5 , R 4 , and R 15  connected in series between line  26  and ground. Thermistor  34  is connected in parallel with resistor R 5 . The voltage at the junction of resistors R 4  and R 15  is applied to the positive input of op-amp, operational amplifier,  46 . This amplifier compares the input voltage to an internal reference and provides a control signal through transistor Q 6  and optical isolator ISO 1  to the control input of voltage controller  22 . The resistance of thermistor  34  changes with the temperature of battery  36 , which in turn changes the feedback signal to op-amp  46 , which causes the controller  22  to adjust the output voltage on line  26  to match the float voltage of battery  36 . With the components specified in FIG. 2B, the output voltage is compensated at the rate of minus 3 millivolts per degree centigrade per cell. For the 12 volt battery of this embodiment, the compensation is therefore minus 18 millivolts per degree centigrade. The parallel combination of the fixed resistor R 5  causes some desirable deviation from this compensation rate at the high end. 
     In this embodiment, op-amp  46  is a part number LTC1541 manufactured by Linear Technology Corporation. Use of this part, or an equivalent part, is important for two reasons. It contains a voltage reference with a 0.4% accuracy. When this is combined with the resistor string R 7 , R 5 , R 4  and R 15  having a total accuracy of 0.5%, the voltage control is accurate within 1%. With this level of accuracy, the battery  36  can be permanently connected to the backplane interface and kept at full charge without overcharging which would shorten its lifetime and reduce its capacity. The LTC 1541 device uses very little power, requiring only about 5 microamps of current. The resistor string specified in FIG. 2B uses about 15 microamps. At these low power levels, there is no need to disconnect the voltage control circuitry when the power grid fails, even if the following system circuits are switched off, such as for storage. 
     As discussed above, the present invention also includes a current limiting circuit to protect the power supply as shown in FIG.  2 A. Resistor R 28  is connected in series with power transistor Q 3  to sense the current levels in primary winding  40  of transformer  20 . The voltage across resistor R 28  is coupled to the current sensing input of voltage controller  22  through resistor R 30 . In the present invention a second input is provided to the current sense input. Resistor R 2  is connected from the current sense input to line  16 . As the voltage on line  16  increases, the peak current levels in primary  40  of transformer  20  are decreased. This prevents damage which might otherwise occur at high input voltages, while allowing use of full supply power capacity over the full operational range. 
     In FIG. 2B there is also illustrated a low voltage shutdown circuit. It includes a voltage comparator  48  which is physically part of the same LTC1541 device which contains op-amp  46 . Comparator  48  has a positive input connected to the junction of resistors R 4  and R 5  and a negative input connected to a reference voltage. With the values shown, comparator  48  will generate a discharge shutdown signal when the battery voltage drops to about nine volts. At this level, at least one cell of battery  36  is fully discharged. Further discharging would probably cause permanent damage to the battery. The discharge shutdown signal is coupled by the backplane interface to the other devices plugged into the backplane. In response to the shutdown signal the other devices should go into an inactive state and essentially stop drawing power from the battery. 
     While the present invention has been illustrated and described with reference to specific circuits and methods of operation, it is clear that various modifications thereof and substitution of parts may be made within the scope of the invention as defined by the appended claims.