Abstract:
A telephone line-powered power supply, a method of operating the same and telephone line-powered ancillary equipment. In one embodiment, the power supply includes: (1) a transformer having a primary winding and a secondary winding and (2) an energy storage device (which may, but is not required to, be a capacitor) series-coupled to the primary winding, the primary winding and the energy storage device adapted to be coupled to, and provide a terminating resistance for, a telephone line. A node between the primary winding and the energy storage device provides DC power. Another embodiment employs a control switch to regulate an energy storage device, thereby dispensing with a need for the transformer.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to telecommunications and, more specifically, to a telephone line-powered power supply for ancillary equipment and a method of operating the same. 
     BACKGROUND OF THE INVENTION 
     The addition of ancillary equipment to telephone systems has been increasing in both requirements and complexity primarily due to customer demands. Initially, the ancillary equipment included options such as automatic dialer circuits which typically provided memory for frequently called telephone numbers. The power supply requirements for these types of circuits are usually well within the capability of even plain old telephone systems (POTS) to provide. 
     Certain telephony applications, such as answering machines and speaker phones may require AC power to supply sufficient energy to the integrated circuits involved. In many earlier systems, the telephone circuits were entirely separate from the answering machine circuit. The two were joined only at the telephone line. Subsequent products used battery power as back-up for times when the AC power failed. Conventional line interface circuits provided little power to be used by any integrated circuit. 
     However, it is desirable in the U.S. and required in many foreign countries that if the AC power fails, the basic telephone functions of speech (speaking and hearing), network address, and alerting be maintained. This could technically be done by switching to an alternate set of circuits that are capable of providing these functions using only power from the telephone line. This approach, however, is obviously not economical. Also, conventional telephone line interface circuits which require at least a 600 ohm AC impedance do not allow much power for use in ancillary circuits. 
     In basic telephone line powered circuits, a common configuration is to use a diode bridge (polarity guard circuit) to guarantee voltage polarity, a switch hook (electronic or mechanical), a termination impedance as mentioned and a DC voltage-current characteristic circuit. This later circuit often consists of a bipolar transistor with an emitter degeneration resistor and a DC bias path for the base of the transistor. This DC bias path often includes the AC termination resistance of 600 to 1000 ohms, and a large capacitor to prevent attenuation of voice band signals. In this conventional circuit, starting with the required voltage-current system restriction of less than 6 volts across the telephone line with a loop current of 20 mA, we may calculate a DC power availability across the large capacitor of approximately 2.5 mA at 1.8 volts. 
     These values are insufficient to operate digital signal processor (DSP) or coder-decoder (CODEC) equipment. CODEC equipment converts voice signals from their analog form into digital signals acceptable to more modern digital PBXs and digital transmission systems. It then converts those digital signals back to analog so that the voice signal may be understood by the person who hears it. In some phone systems, the CODEC is in the PBX and shared by many analog phone extensions. In other phone systems, the CODEC is actually in the phone. 
     The maintaining of POTS capability encompasses several requirements. These include the setting of the specific DC voltage-current characteristic for the telephone line, as mentioned, while active. The AC line termination resistance of approximately 600 ohms and a bandpass frequency range of 300 to 3300 Hertz are typically required. Network addressing with dual tone multifrequency (DTMF) and pulse capability must be provided as well as alerting, the detecting of a “ringing” signal and the sounding of an “alerter”. 
     Speech capability for POTS includes full duplex coupling to a telephone line for transmitting and receiving voice signals. The amplification of microphone signals for transmission and the amplification of the receive signal for the handset speaker is necessary. In the U.S., the speech hybrid function is defined by average loudness templates referred to as TOLR, ROLR, and SOLR which are Transmit, Receive and Sidetone Objective Loudness Ratings respectively. Transmit and Receive also have normalized frequency templates as mentioned earlier. Sidetone is a part of the design of a telephone handset which allows one to hear their own voice while speaking. Too much sidetone result in an echo and too little renders the channel unerring. Therefore, controlling the amount of microphone signal that is audible in the handset speaker is critical. 
     Accordingly, what is needed in the art is an economical way for a POTS architecture, during active speech mode, to provide additional current and voltage for ancillary circuits. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides a telephone line-powered power supply, a method of operating the same and telephone line-powered ancillary equipment. In one embodiment, the power supply includes: (1) a transformer having a primary winding and a secondary winding and (2) an energy storage device (which may, but is not required to, be a capacitor) coupled to the primary winding, the primary winding and the energy storage device adapted to be coupled to, and provide a terminating resistance for, a telephone line. A node between the primary winding and the energy storage device provides DC power. 
     The present invention therefore introduces the broad concept of substituting a conventional terminating resistor with a power supply to increase the ability of a telephone line to provide power to ancillary equipment. In alternate embodiments, the power supply can include a transformer or be transformerless. “Ancillary equipment” is defined, for purposes of the present invention, as equipment that requires electric power. “Ancillary equipment” includes, but is not limited to, signal processing or other electronic circuitry located within a telephone and answering, caller identification or other equipment that a user may want to locate near a telephone. “Ancillary equipment” may be powered by the telephone line only as required or may be powered regardless of the availability of alternative power. 
     In one embodiment of the present invention, the power supply further includes a line transformer interface coupled to the secondary winding. The line transformer interface, which is not necessary to the present invention, provides a signal interface for signals derived from or supplied to the secondary winding. Ancillary equipment, such as signal processors, may employ a line transformer interface to communicate via the telephone line. 
     In one embodiment of the present invention, the power supply further includes a diode series-coupled to the primary winding. The diode can protect other components in the power supply from receiving discharges from the energy storage device. 
     In one embodiment of the present invention, the power supply further includes an activation switch couplable to the telephone line and functioning based on a hookstate. A “hookstate” is literally the state of the telephone&#39;s hook: “off-hook or “on-hook.” The ancillary equipment senses the voltage of an alternative source of electric power and enables the power supply with the activation switch when the alternative source of electric power is interrupted. 
     In one embodiment of the present invention, the power supply further includes a line voltage sense circuit couplable to the telephone line. The voltage circuit allows the voltage of the telephone line to be determined. The line voltage sense circuit is used by the ancillary equipment to bias and control other circuits necessary for proper telephone operation. 
     In one embodiment of the present invention, the power supply further includes a filter coupled to the secondary winding. The filter, while not necessary to the present invention, is desirable in some applications to smooth signals associated with the secondary winding. 
     In one embodiment of the present invention, the power supply further includes a power fail ringing circuit couplable to the telephone line. The power fail ringing circuit, if present, provides ringing signals in the event of an interruption of an alternative source of electric power. 
     The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a schematic diagram of a conventional telephone circuit which may be used to provide power to some types of ancillary equipment; and 
     FIG. 2 illustrates a schematic diagram of a telephone circuit showing an embodiment of an improved telephone line-powered power supply for ancillary equipment; and 
     FIG. 3, illustrates a schematic diagram of a telephone circuit showing an alternate embodiment of an improved telephone line-powered power supply for ancillary equipment. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, illustrated is a schematic diagram of a conventional telephone circuit  100  which may be used to provide power to some types of ancillary equipment. The telephone circuit  100  includes a polarity guard circuit  110 , a DC voltage-current (DC V-I) circuit  120 , an AC terminating resistor  130  and a filter capacitor  140  which provides a DC voltage (Vout) as shown. 
     The polarity guard circuit  110  includes diodes  111 ,  112 ,  113 , and  114  arranged in a diode bridge configuration. Tip and ring DC voltage inputs to the polarity guard circuit  110  may adopt either polarity (e.g., tip positive and ring negative, or tip negative and ring positive) and the guarded output voltage (GT, GR) polarity of the polarity guard circuit  110  remains substantially constant, as shown. Originally, tip had a positive polarity and ring had a negative polarity. As wiring installations became more complex, maintaining this convention became more difficult. Additionally, some current systems now reverse the tip and ring polarity between the dial and talk modes of operation. Maintaining a proper polarity has therefore become critical for electronic telephones. 
     The DC V-I circuit  120  includes a transistor  121 , first and second resistors  122 ,  123  and first and second diodes  124 ,  125 . The DC V-I circuit  120  provides a required POTS voltage versus current characteristic waveform. The first and second diodes  124 ,  125  and the second resistor  123  form a DC bias network for the transistor  121  employing the first resistor  122 . The DC current flowing through the transistor  121  is basically the voltage applied to its base terminal divided by the resistance of the first resistor  122  (neglecting the transistor&#39;s base-to-emitter voltage drop). 
     An output voltage (Vout) is provided by the junction of a third resistor  130  with the series circuit elements of the first and second diode  124 ,  125  and the second resistor  123 . A capacitor  140  constitutes a filter element for the voltage Vout. Typically, the capacitor  140  has a value of at least 470 microfarads. The third resistor  130  also functions as the AC terminating resistance for the telephone line and typically has a value of 600 to 1000 ohms. A terminating resistance of this value limits the amount of DC current that may be provided to ancillary equipment, since many types of ancillary equipment require that Vout be no less than 3 volts to function properly. 
     Turning now to FIG. 2, illustrated is a schematic diagram of a telephone circuit  200  showing an embodiment of an improved telephone line-powered power supply for ancillary equipment. The telephone circuit  200  is couplable to a telephone line (which is environmental and therefore not shown) and includes a polarity guard circuit  210 , a DC V-I circuit  220 , a telephone line-powered power supply  230 , a signal processor  250 , a telephone handset  260  and an alternate power source  270 . The polarity guard circuit  210  and the DC V-I circuit  220  perform the same functions as described in FIG.  1 . However, bias-setting and control for the DC V-I circuit  220  is provided by the ancillary equipment, as will be described below. 
     The telephone line-powered power supply  230  includes an AC to DC energy transfer device (shown as a transformer  231  having a primary winding  231   a  and a secondary winding  231   b ) and an energy storage device  232  (shown as being a capacitor in this embodiment, but is not required to be a capacitor) coupled to the primary winding  231   a.  The primary winding  231   a  provides an AC terminating resistance for the telephone line via a reflected resistance from the transformer secondary  231   b.  This allows the DC resistance of the transformer winding  231   a  to be made as low as practical, providing increased capability to deliver DC power supply current to the energy storage device  232  coupled to a node A between the primary winding  231   a  and the energy storage device  232  and thereby providing DC power to the ancillary equipment. The power supply  230  also includes a diode  235  which is series-coupled to the primary winding  231   a.  The diode  235  can protect other components in the power supply  230  from receiving discharges from the energy storage device  232 . A diode  237  further isolates the power supply  230  from the alternate power source  270  which normally provides DC power to the ancillary equipment signal processor  250 . 
     The present invention therefore introduces the broad concept of substituting a conventional terminating resistor with a power supply to enhance the ability of a telephone line to provide power to ancillary equipment. Ancillary equipment, in general, may perform as many additional functions as may be required or desired, subject to the availability of appropriate technology and the power required for its proper operation. In this embodiment, the signal processor  250  provides analog and digital signal conditioning between the telephone line and the telephone handset functions  260  of hearing (via a speaker  261 ) and speaking (via a microphone  262 ). For hearing, the signal processor  250  may provide a programmable gain amplifier (PGA)  251  coupled to a speaker driver amplifier  252  which then drives the speaker  261 . For speaking, the microphone  262  may drive a microphone amplifier  253  coupled to a line driver circuit  254  provided by the signal processor  250 , as shown. Additionally, the signal processor  250  provides other functions in the illustrated embodiment, as will be discussed. 
     In this embodiment of the present invention, the power supply  230  further includes a line transformer interface  233  coupled to the secondary winding  231   b.  The line transformer interface  233 , while not necessary to the present invention, provides a signal interface for signals derived from or supplied to the secondary winding  231   b.  Ancillary equipment, such as the signal processor  250 , may employ a line transformer interface  233  to communicate via the telephone line. The power supply  230  further includes a filter  234  coupled to the secondary winding  231   b.  The filter, while also not necessary to the present invention, is desirable in some applications to smooth signals associated with the secondary winding. 
     In the illustrated embodiment, the telephone circuit  200  includes an activation switch  236  couplable to the telephone line and functioning based on a hookstate supplied by a first reference amplifier  256  of the signal processor  250 . Again, a “hookstate” is defined literally as the state of the telephone&#39;s hook: “off-hook” or “on-hook.” Of course, the hookstate signal could alternately be supplied from a mechanical hook switch. At initial activation, the activation switch  236  may be in its closed position, to assure proper power-up of the ancillary equipment, until the state of the alternate power source  270  is determined. The activation switch  236  would then typically open if the alternate power source  270  is found to be operational. 
     The power supply  230  also includes a line voltage sense circuit  240 , if needed, that is couplable to the telephone line. The line voltage sense circuit  240  includes first and second resistors  241 ,  242  and a filter capacitor  243 . The voltage circuit  240  allows the voltage of the telephone line to be determined by providing a portion of the total telephone line voltage (which is resident at node B) to a second reference amplifier  257  in the signal processor  250 . The second reference amplifier  257  is used to provide the bias-setting and control for the DC V-I circuit  220  by comparing the voltage at node B to a reference voltage and adjusting the bias for the DC V-I circuit  220  as required. 
     In the illustrated embodiment, the power supply  230  includes an optional power fail ringing circuit  245  couplable to the telephone line. The power fail ringing circuit  245  provides ringing signals in the event of an interruption of an alternative source of electric power. The power fail ringing circuit  245  includes a piezoelectric transducer (PZT)  246 , a switching device  247  and a diode  248 . A driver circuit  258  in the signal processor  250  provides an input to the switching device  247 , causing it to turn ON and OFF. This action applies a repetitive voltage across the PZT  246 , causing it to “ring.” The diode  248  suppresses unwanted transients. In this embodiment, the diode  248  may be a conventional device or a light-emitting diode (LED) that also provides for visual, as well as audible, ringing via the PZT transducer  246 . 
     Turning now to FIG. 3, illustrated is a schematic diagram of a telephone circuit  300  showing an alternate embodiment of an improved telephone line-powered power supply for ancillary equipment. The telephone circuit  300  is couplable to a telephone line (which is environmental and therefore not shown) and includes a polarity guard circuit  210 , a DC V-I circuit  220 , a telephone line-powered power supply  330 , a line voltage sense circuit  240 , a power fail ringing circuit  245 , a signal processor  250 , a telephone handset  260  and an alternate power source  270 . 
     The polarity guard circuit  210 , the DC V-I circuit  220 , the line voltage sense circuit  240  the power fail ringing circuit  245 , the signal processor  250 , the telephone handset  260  and the alternate power source  270  perform the same functions as described in FIG.  1  and FIG. 2 respectively. The structure and operation of the alternate embodiment of the telephone line-powered power supply  330  will now be described. The telephone line-powered power supply  330  includes a power switch  339 , a control switch  331 , first, second and third resistors  332 ,  333 ,  334 , a filter capacitor  335 , a hybrid network  336  and a terminating impedance  337 . 
     Initially, the filter capacitor  335  is discharged and both the power switch  339  and the control switch  331  are OFF (not conducting). When an activation switch  236  coupled to the power switch  339  closes, DC voltage from the point GT causes the power switch  339  to turn ON by forward biasing its base-emitter junction through the third resistor  334 . This action causes the voltage across the filter capacitor  335  to increase and supplies power to the signal processor  250 . This voltage increase continues until the voltage across the filter capacitor  335  reaches a voltage Vreg, which is the desired value of the output voltage. At this point the control switch  331  turns ON, causing the power switch  339  to turn OFF. As the output voltage decreases below the voltage Vreg, a voltage point is reached which turns the control switch  331  OFF and allows the power switch  339  to again turn ON thereby completing a regulating cycle. 
     The value of the third resistor  334  may be chosen to be sufficiently large enough not to affect the AC line impedance, which may then be determined by the hybrid network  336  and selection of the terminating impedance  337 . Additionally, values for the first and second resistors  332 ,  333  may also be chosen to be sufficiently large enough to divert only a small amount of the output current of the telephone line-powered power supply  330  from the load. 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.