Abstract
D.Ing.
Transformer design is an art which spans a century. Although the basic transformer has
changed little over this period, the challenges that face high frequency power transformer
designers today have grown considerably. Increasing frequency and power density and decreasing
size and profile are among the most important. Eddy currents, controlling circuit
behaviour and minimising losses are important aspects of design, and close attention is paid
to heat removal and cooling. Modern transformers are no longer limited to certain shapes
and sizes; choosing the topology and optimising the shape is often part of the design process.
For each aspect of design, numerous modelling techniques exist for analysing transformer
behaviour, with varying degrees of complexity. A common feature of optimisation
techniques is the large number of variables and interdependent functions that relate different
aspects, from the associated behavioural models, to one another.
In this study, this complexity is reduced by integrating the individual analytical
models for transformer behaviour.
Since a convenient thermal model for high frequency transformers does not exist
at present, a new thermal reference model is devised and verified. It is specifically suited
to high frequency transformer applications and design, and practical sets of reference data
are provided for a few ferrite materials and for copper. Transformer losses are considered
next, with special attention given to eddy current analysis techniques. New formulations of
eddy current solutions are given, with extensions of the orthogonality principle for skin- and
proximity effects and superposition thereof. An investigation of leakage impedance design as
a function of frequency scaling follows. The relationship between leakage reactance voltage
drop as a function of frequency scaling by dividing a monolithic transformer into distributed
elements is considered, and the results are applied to two case studies of a 35kVA transformer
for a plasma burner application.
A new model, the generic proportionality model, applies the thermal referenCe model
to scaling of transformer parameters. A case study is also presented, demonstrating the
relationships that exist between design parameters and performance functions.
Another generic model, the scant model, is introduced, which integrates the thermal
reference model into optimisation of transformer shape. It uses a limited number of
functional and form parameters, and is applicable to a wide variety of geometries. Two
case studies, demonstrate the effects of varying the shape of a rectangular configuration on
derating factors.