DC-DC Converter Design and
“ Magnetics ” , a term that strikes fear in the heart of the average electronics engineer, along
with terms such as feedback loop compensation, and electromagnetic interference. Before you
throw up you hands in a nervous sweat, consider this, we all had to go through this before you
and survived with a deepened understanding.
Magnetism can also be quite analogous to the flow of water through a pipe. B (flux density)
can be seen as the number of water molecules flowing through a crossection of pipe. H (field
strength) can be seen as the pressure causing the flow. Hysteresis loss is the amount of work
done to reorient the magnetic domains within the core. Saturation is when there are no more
magnetic domains remaining to reorient. Eddy current loss is the localized circulating flux dis-
turbed by obstacles such as corners in the core just as water does in streams around rocks.
The terms, units and equations may be unusual. Be careful to not mix the units between the
MKS and the CGS systems, or you will have results that will extraordinarily large.
Understanding the operation of the magnetics within a switchmode power supply and their
design is fundamental to the design of a switchmode converter. The relationships between cur-
rent, voltage and power are all controlled by the magnetic elements.
Their operation can sometimes seem counter-intuitive. The design of a transformer or inductor
not only involves the electrical design but also its physical design. A poorly designed trans-
former, for instance will ring excessively, increase the radiated noise, and lower the overall
efficiency of the switching power supply.
All the results from these calculations are “ calculated estimates ” . That is, the results are
midpoint results that get you in the area you wish to operate, but the resulting values can be
increased or decreased to enhance a particular feature such as size, cost, etc. Changing the
value from its midpoint, though, will also affect other operational factors, such as heat, and
efficiency, which must also be considered.
Sanjaya Maniktala does a very good job in describing the various factors in designing the mag-
netic elements within a switchmode power supply.