50W DC-DC converter — Cascaded Buck current fed Push-Pull topology with Average Current Mode Control

Abstract

With the future Satellite Power Bus switching to 70V range and at the same time, to meet the compatibility with present 40V bus, there was a need to develop a DC/DC converter with a design robust enough to easily switch to either (22V–45V) or (36V–80V) range bus without any significant change required in the circuit and without any significant increase in the voltage/ current stress experienced by the semiconductor devices. This converter was designed based on a new topology, ‘Cascaded Buck current fed Push Pull converter’ (rather than commonly used Forward converter topology in space industry) and Average Current Mode Control scheme, which meets the above mentioned requirements and also provides ease of testing & production. This converter stands apart from other DC-DC converters in having 04 nos. of O/Ps (+5V, +15V, −15V & +40V) and fourth O/P being +40V provides a major advantage of total isolation of the 70V bus from mainframe systems requirement of 40V. Same converter can be configured for single output of +40V/3A or +28V/6.5A by just changing the transformer winding details. Because of the current fed DC/DC topology, optimized transformer winding technique and HKB voltage feedback, the converter exhibits very good cross coupling between the outputs. Hence the use of LDOs (Low Drop out regulators) or cross coupled inductors at the outputs is eliminated. The converter can also operate at No Load (i.e. with open circuit at outputs) unlike other topologies which demand minimum load current to keep the output inductor in continuous conduction mode.