New Power Feed Techniques for Telephone Subscribers

This paper describes power feed techniques for telephone subscribers in a metallic two-wire digital subscriber loop system and optical fiber subscriber loop systems. In a metallic two-wire digital subscriber loop system, Digital Service Unit (DSU) and Digital Telephone (DTEL) are installed on the subscriber premises. Since the equipment has additional new service functions compared with the conventional analog telephone, the power consumption of the equipment is increased. Therefore, it is difficult to feed the equipment sufficiently using a conventional 48V constant-voltage power supply. A constant current supply method from the central office is suitable because constant power can be supplied with little energy loss, even if subscriber's line distance varies. It is also possible to simplify the DC-DC converter circuit in DSU. A high conversion frequency (150kHz) multi-output constant current converter has been developed for power feeding in NTT. The converter can feed eight subscribers and has good constant current output characteristics. In optical fiber subscriber loop systems, it is difficult to feed the customer's equipment with power from the central office. Therefore, an uninterruptible DC power supply, which includes batteries, has to be installed on the subscriber's premises. The power supply is composed of sealed lead-acid batteries and a multi-input converter which has rectifier and DC-DC converter functions. This low-cost converter is very small compared with the conventional rectifier and DC-DC converter. Sealed lead-acid batteries are used in the power supply due to low cost and easy maintenance.