The size of LLC converters magnetics: Frequency dependence
by Sam Ben-Yaakov, YouTube, Mar 29 2018
Focus:
In this 14-min. video, the presenter refutes a claim made on LinkedIn that an increase in
frequency does not lead to a reduction in the size of the magnetics component(s) for an LLC
resonant converter. After explaining how the input inductors and couple inductors required in
this topology may be implemented as integrated magnetics on a single core, he works through the
math for core selection for the coupled inductors. He derives expressions for core cross
sectional area, total winding area, and the product of these two, area product, which can be
used to select the core given a desired core loss density. He explains how inductance and Bmax
are the frequency dependent terms in these expressions, which ultimately leads to a figure of
merit (area product ratio = area product at high frequency/area product at low frequency),
which can be used to gauge the potential size reduction achievable for the coupled inductors
when frequency is increased. He applies this area product ratio in a design example using
vendor core data, demonstrating a 4X reduction in area product at higher frequencies. He adds
that the size reduction will be slightly offset by a reduction in packing factor k due to use
of litz wire but that leakage inductance will be absorbed in the resonant element and is
therefore not a factor.
What you’ll learn:
- How to evaluate the variation in magnetics size versus frequency in an LLC resionant converter
- How to determine core size for the coupled inductors in an LLC resonant converter
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