反激式电路国宽带电话网络-Flyback Circuit P

With the advent of broadband access, many companies have developed high voltage SLICs (subscriber line interface card) which control the ringing and voice transmission on phone systems. The SLICs essenTIally perform two funcTIons. One is ringing the phone at the customer's premises. Another is generaTIng loop current for off-hook operaTIon. Power supplies for SLICs have special requirements. Multiple Line, Multiple Phone Applications Power RequirementsThe equivalent circuit for a US ringer, at the ringer frequency of about 20Hz, essentially presents a resistive load of 8kΩ per phone. The number of parallel phones on a line is called the ringer equivalent number (REN). The North American ringing load requirement of five REN is among the world's most stringent. Ringing a phone requires a minimum phone ring voltage of 45VRMS (40VRMS at the handset). The phone ringing voltage can be either trapezoidal or sinusoidal. The sinusoidal waveform places more stringent demands on the power supply. A sine wave with a 45VRMS requirement needs a peak voltage of 64V(negative). Peak power supply current for one phone is 64/8k = 8mA. The power supply must put out a few additional volts above the 64V peak for protection resistors voltage drop and SLIC output amplifier overhead. However, line resistances can add to the total required voltage. The phone lines are generally longer for a central-office (CO) application as compared to a typical DSL or cable-modem customer. In the application considered here (for Legerity Inc. using AMD79R79 SLIC) this voltage requirement is -90V.

Many applications power multiple lines. Phones may ring sequentially to reduce peak power or some overlap may be allowed which then dictates the power requirements. Peak current requirements for a REN=5 application is 40mA. The AMD79R79 SLIC, developed by Legerity Inc., sets this requirement to 56mA (to account for various tolerances in the SLIC circuit).In an application using four lines and allowing for overlap in two of four lines, peak current requirements are 112mA. Any line-card ringing implementation requires the removal of the ring signal from the subscriber line within a specified time after the subscriber goes off-hook. This prevents the ring signal from causing discomfort to the subscriber. In order to detect this status a DC current path is established between ground and the supply for a specified time (150 msec. for the AMD79R79 SLIC). The SLIC device senses the DC current for ring trip, resulting in the removal of the ring signal. Applications using AMD79R79 typically set the value of this DC current at about 40% higher than the load presented by the phone (both values can be set independently in AMD79R79). This sets the DC ring trip current at 100mA for a REN=5 application. If the application also uses four lines with overlap of two out of four lines, then the ring trip current presents an additional load of 200mA for 150msec. Therefore, the power supply should be designed for a maximum load of at least 312mA at -90V output.

Besides ringing the phone, the power supply must power the voice transmission and reception. In many cases, once voice transmission begins the SLIC requires a lower input voltage of about -24V to establish a 20 to 25mA loop. In a single line, single phone application, designs combine the lower and higher supply voltage and operate the SLIC at a compromise voltage of about -53V. For a multiple line application the ringer and talk portions can be powered by the same supply. A typical load for the 20mA loop is 200Ω to 500Ω. This implies 4V to 12V drops across the load and the remainder across the SLIC. A more efficient way is to use two different supplies for the ringer and the voice transmission, as in the application being considered. For multi-line operation, if one line is off-hook in the talk-state, it is operating on the lower supply voltage and cannot be ringing. The power for the talk-state will then see a maximum load of about 100mA.

These applications use wall-mounted DC supplies that produce 10V to 25V, but generally supply the required 12V. Table 1 below summarizes this discussion about power requirements for an AMD79R79 application using four lines with overlap of two out of four lines.

Table 1. Power Supply Requirements for AMD79R79 Four-Line Application
Parameter