High efficiency ringing generator
BACKGROUND OF THE INVENTION 
     This invention relates in general to telecommunications systems and more particularly to a DC to AC power converter for use as a ringing generator in telephone subscriber line circuits. 
     Ringing generators have been used for signaling calls between subscriber telephone instruments since the hand crank magneto generator of Alexander Graham Bell&#39;s day. In fact, magneto generators powered by -50 volt DC motors are still used for signaling calls today. 
     Today however, there is a need to reduce the power consumption of large telecommunication systems and therefore a need for increasing the efficiency of ringing generators. 
     To ensure continuity of service within a telecommunication system, all ringing generators are powered from a DC battery source. A battery voltage of -50 volts is generally used as the input voltage and an AC voltage of 85 to 100 volts RMS is generated at the output. The most common battery voltage is -50 volts for local subscriber lines and trunk circuits. Carrier systems for long distance toll communications will typically use +130 volts and -130 volts to power repeaters along the carrier span. The +/-130 voltages are usually derived from DC to DC converters powered from the -50 volt battery. 
     For example, current subscriber line circuitry powered by carrier spans that use repeater links are powered by the aforementioned +/-130 volts with the current limited to 200mA. Therefore, the power consumption of the subscriber line circuitry at the remote terminal is strictly limited. Since the ringing generator is the most power hungry piece of equipment at the remote site there is substantial interest in developing a ringing generator which is highly efficient. 
     One efficient ringing generator presently known is the suppressed carrier double sideband generator. This ringing generator uses a microprocessor which contains a look-up table. The converter uses the data in the look-up table to create a sinusoidal output. A digital-to-analog converter is used to create a dynamic signal which controls a pulse width modulator. The pulse width modulator has two phases which are controlled by the microprocessor. The phase information is fed through an isolation transformer where the original AC waveform is reconstructed by using a synchronous detector and filter. This particular ringing generator can achieve efficiencies of 85% however, these efficiencies are gained at the expense of complexity (and thus reliability) as well as cost. Additionally, the timing of the microprocessor, switches and synchronous detector of this converter is quite critical, and can present a number of design problems. 
     Another efficient ringing generator presently known is the Cuk converter. The Cuk converter uses a much simpler converter topology than the suppressed carrier double sideband generator described above and has attained efficiencies of over 90%. The Cuk converter includes a transistor which is fed a pulse width modulated square wave that varies according to the amplitude of the sine wave that the converter is trying to simulate. The disadvantage to this type of converter is the large size of the capacitors necessary, (typically 4,700uF or larger) to achieve high efficiencies. The Cuk converter is considered a capacitive energy transfer converter and the value of the capacitors is intrinsic to its design. 
     It therefore becomes the object of the present invention to provide a new and efficient means for generating ringing voltages to subscriber lines without any of the disadvantages mentioned above. 
     SUMMARY OF THE INVENTION 
     In accomplishing the object of the present invention there is provided an improved ringing generator for producing and transmitting an AC ringing voltage waveform to a subscriber line from a negative and positive DC voltage source. 
     The ringing generator of the present invention comprises, coupling means including a first coupling section and a second coupling section. The first coupling section includes a plurality of inputs dividing the first coupling section into a plurality of coupling segments. The second coupling section is normally connected to the subscriber line via a filter network. 
     The present invention further includes a plurality of switching means. Each switching means includes an input connected to the DC voltage source, an output connected to a respective one of the first coupling means inputs and an enable means connected to a source of control signals. 
     The AC ringing voltage waveform is produced in response to the control signals alternatively enabling a selected pair of switching means. The selected pair of switching means are enabled for a predetermined period of time thereby, connecting an associated coupling segment to the DC voltage source. The switching in and out of selected coupling segments by the switching means produces and couples to the second coupling section an AC ringing voltage waveform which is transmitted to the subscriber line via the filter network. 
    
    
     A BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the ringing generator in accordance with the present invention. 
     FIG. 2 is a schematic representation of a typical buffer-driver shown in FIG. 1. 
     FIG. 3 is a representation of the waveform representative of the output signal generated by the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The ringing generator of the present invention uses switch mode technology in a rather unique way. Instead of using a high frequency pulse width modulated voltage through a filter, as is more common in prior art ringing generators, the present invention switches transformer windings to provide a stepped voltage that can be bidirectional. The advantage of such a switching topology is its simplicity, small size and low cost of the components that it uses. 
     Turning now to FIG. 1 it can be seen that the invention is comprised of three distinct component groups. A multi-tapped transformer 10, a set of drivers 20, and an output filter 30. 
     The Transformer provides isolation and the capability to increase or decrease the magnitude of the output voltage according to its winding ratio. Traditionally, the need for isolation has been based on the application of a -50 volt DC bias to one side of the secondary (or output) winding of a transformer. The result appeared on the other side of the secondary winding. The zero crossing point of the complete ringing waveform then occurs at -50 volts. The winding ratio of a transformer is usually fixed. If 1 volt, for example, is applied to the input of the primary winding, then a fixed voltage can be expected across the secondary winding. This invention, however, uses a variable winding ratio where the primary winding 11 has several taps. Each individual tap is driven by a separate switching buffer-driver 21a-21f and therefore the output voltage across terminals 41, 42 will vary according to which buffer-driver is conducting. 
     The output filter 30 is comprised of inductors and capacitors which are represented by components 31 and 32 respectively of the simple circuit of FIG. 1. The output filter 30 is used to reduce the high frequency by-products of the switching waveform. The present invention does not require or use diodes, transistors or other active components on the secondary side of the transformer 10 and therefore, presents a completely passive network at its output. 
     Turning now to FIG. 2, the implementation of the buffer-drivers 20 is shown. Essentially each buffer-driver 21a-21f acts as a high voltage tri-state buffer. The enable lead 22 via a logic network comprised of gates 23, 24 and 25 will turn on one of two power transistors 26 or 27. The output present on lead 29 will change from a high impedance state to either a high or low voltage depending on the voltage being applied to input lead 28. The complementary transistors 26 and 27 are in a so-called totem pole configuration and can be either bipolar or MOSFET types. 
     As mentioned earlier the buffer-driver must simulate three state logic, that is, it must be able to switch to the positive supply rail (usually ground), the negative supply rail (usually -50 volt battery) and to a high impedance state. When the buffer-driver is either at the positive or negative rail only one transistor is on, when the buffer-driver is in the high impedance state neither transistor is on. 
     The diodes 35 and 36 connected to transistors 26 and 27 respectively, allows current to flow in the direction opposite to the direction that the transistor would normally conduct. This ensures the capability of the drivers to be current bidirectional and therefore, making the output of the ringing generator current bidirectional. 
     It should be noted that even though each buffer-driver 21a-21f of the present invention is shown constructed from discrete components, it will be well understood by those skilled in the art that the components shown comprising each buffer-driver could be packaged in an integrated circuit. Furthermore, because switching transistors ordinarily dissipate little power i.e. heat, it is quite possible to put several high voltage transistors in a dual-in-line package at a reasonable cost. 
     With renewed reference to FIG. 1, an example of the operational use of the present invention will be given. In this example the invention will be used as a square wave generator using just two of the buffer-drivers 20. The rest of the drivers are disabled via respective leads 22 and remain in a high impedance state and therefore isolated and floating. While one buffer-driver is alternating between positive and negative supplies applied via lead 28, a second complimentary buffer-driver is also alternating between positive and negative supplies simultaneously, but with opposite polarity. The resulting switched voltage waveform appears at the secondary winding 12 where it is filtered by filter components 31 and 32 to reduce its harmonic content and is output across terminals 41 and 42. Given a constant positive and negative supply voltage, it is possible to vary the amplitude of the square wave output voltage by using a different set of drivers. In effect, the winding ratio of the transformer is changed, with the output voltage changing as a result. 
     The change in the winding ratio is perhaps the best descriptive explanation of how the invention derives a varying voltage from a DC source. The square wave generator described previously wasted too much power in generating harmonics. Approximately 20% of the power of a square wave is in higher harmonics. Since a subscriber instrument&#39;s mechanical ringer will operate only at the fundamental frequency the higher harmonics represent wasted energy. 
     Therefore, by using a stepped voltage source, it is possible to reduce the percentage of harmonic content and provide more useful energy to the subscribers mechanical ringer. The stepped voltage is produced by first turning on the buffer-drivers 21a and 21f on the outermost set of windings. Then, as the buffer-drivers on the inner windings are progressively turned on (21b and 21e, etc.), the output voltage will step up to a higher voltage. It should be noted that since only two buffer-drivers are active at any one time, the buffer-driver that was turned on first is in a high impedance state at the most positive incursion of the voltage waveform. The turning on and off of each buffer-driver can be readily accomplished with the aid of a control device which can be programmed with the proper sequence required for the AC waveform generation. 
     To step the voltage down, the drivers on the innermost set of windings 21c and 21d are turned on first with the remainder of the buffer-drivers progressively turned on until the outermost buffer-drivers 21a and 21f are activated. The routine of the just described sequence will produce a switched output voltage of one polarity only. The stepped voltage for the opposite polarity can now be generated by using the switched enabling sequence described earlier. FIG. 3 illustrates the final AC ringing waveform after filtering. 
     A more practical approach to sequencing the buffer-drivers is to let one buffer-driver at either end of the outermost windings be active at all times thereby, letting the other buffer-drivers turn on and off in a sequence that changes the winding ratio one step at a time. 
     Although the preferred embodiment of the invention has been illustrated, and that form described in detail, it will be readily apparent to those skilled in the art that various modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.