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ANP029, 2015-06-15, RaB Page 1 of 10
APPLICATI ON N OTE
Accurate Inductor Loss Determination
Using Würth Elektronik’s REDEXPERT
ANP029 RANJITH BRAMANPALLI
1. Introduction_________________________________________________
In a Switch Mode Power Supply (SMPS) the majority of any power losses that occur are in the form of switching
and magnetic losses. Magnetic loss occurs from the core and the windings in the storage/coupled Inductor.
Determination of inductor power loss accurately has become more important to design reliable and efficient
systems, especially in the era of green technology. Estimation of core losses in SMPS can require complex
measurement set-ups, yet cannot be guaranteed whether the estimation is relevant to the particular
application. Historically, core losses are calculated using Steinmetz equation and recently with the extension
of Steinmetz equations. These equations can estimate losses reliably only for certain conditions or materials.
Hence, Würth Elektronik eiSos have developed a state of the art new model to determine core losses
effectively and accurately. This model has now implemented in our new design tool REDEXPERT.
2. Energy Storage in Magnetics __________________________________
In a SMPS, the Inductor acts as storage component. It stores energy in the form of a magnetic field during the
switching-cycle on time and delivers that energy to the load during the off time. Usually, an inductor consists
of a coil pre-dominantly made of copper wire and a core which has magnetic properties. In terms of
electromagnetic physics when a magneto-motive force with respect to the time is applied to the coil, it induces
magnetic flux . An important point is that at any location, the magnetic flux density B is always proportional
to field intensity H..
Where B is the magnetic flux density,
is the permeability of the material,
is the permeability of air
and H is the magnetic field Intensity.
The coil is wound around or placed inside the core with an air gap to tap the magnetic field effectively. The
core contains air gap. The core is usually a ferrite material which has ferro-magnetic properties and much
higher permeability than air. Hence, the air gap places a high reluctance element of air in series with low-
reluctance ferrite material, thereby locating the bulk of the energy in the air gap.
Inductors operate according to the laws of Ampere and Faraday. Ampere’s Law relates current in the coil or
turns of wire to the magnetic field in the core of the inductor. As an approximation, one assumes the magnetic
field in the inductor’s core is uniform throughout the core length (
). That assumption lets us write Ampere’s
law as
Where N is the number of turns of the coil around the inductor core and is the inductor current.
According to Faraday’s Law, voltage applied across the Inductor is
(or)
From above theories Inductor value is calculated as
