Abstract
Placing fly-back converters in a parallel-input-parallel-output configuration (PIPO) is a
well-established technique for increasing the power handling capability beyond the maximum
operating power of 150W, for a single fly-back converter. Additionally, this allows
for the distribution of thermal stresses between individual components. However, rather
than simply placing converters in a direct-parallel fashion, paralleling converters can be
extended to be used in a multi-phase configuration. In multi-phase converters, converters
are placed in parallel with a relative phase-shift between the pwm on-time to one another.
By doing so, the size of the bulk output capacitors of a converter can be reduced due to
capacitor ripple cancellation effect.
Even though the use of a multi-phase configuration allows for the power handling capability
to be increased, fly-back converters suffer from the same limitations prominent
in all converters. That is, when a converter is placed under light-load operation its efficiency
will noticeably be decreased. This limitation has not gone unnoticed in industry
and academia. Subsequently, several methods have been developed in order for PIPO
converters to maintain maximum efficiency over a wide load range. One such method,
and the main focus of this work, is a phase-shedding technique for converters operating
under light-load conditions. In a multi-phase fly-back converter, consisting of two flyback
converters in parallel, it is typical for both converters to be operated with uniform
current distribution for the complete load range. The phase-shedding technique, however,
involves completely turning off one fly-back converter when the load is below 50% of
full-load power. This is referred to as ”shedding a phase” or ”dropping a phase”. By removing
a phase, advantage is taken of the maximal efficiency point of a single converter
rather than making use of the diminished efficiency of two converters operated at 50% of
full-load power.
In this dissertation, a comparison between fly-back converters employing a directparallel,
multi-phase without phase-shedding, and a multi-phase with phase-shedding
configuration is offered. It is shown that a multi-phase fly-back converter making use
of a phase-shedding technique provides a superior efficiency rating, over a wide load
range, when compared to the other configurations. Additionally, the terminology used
in parallel configurations is enlightened to prevent some of the ambiguity present in the
existing literature.
M.Ing. (Electrical Engineering)