Telephone Line Circuit DC-To-DC Converter.

A unique, low-power, dc-to-dc switching converter has been developed to provide dc power for analog telephone lines. The converter employs a flyback configuration with a nonlinear element in one of two separate feedback loops. The feedback provides a dual mode of operation with constant voltage output for low-load currents and limited power output for higher load currents. This allows a simple, versatile circuit configuration that is well suited to telephone line feed applications. Its output characteristics cannot be reproduced with conventional linear techniques or switching converter techniques. This paper provides background information about the design of the converter and describes the operation of the converter at the functional block level. It describes the overall circuit features and characteristics of the line-feed application as well as circuit implementation and practical considerations. Finally, laboratory test results and general conclusions are presented.