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© 2020 ROHM Co.,
Ltd.
No. 62AN097E Rev.001
March.2020
Application Note
Polarity of power inductors in DC-DC converter
Considering Polarity of Power Inductor to
Reduce Radiated Emission of DC-DC converter
Introduction
Leakage flux of power inductors
A power inductor consist of an insulated wire wounded
around a core made of a high permeability material.
Electrical current flowing through a wire causes a
magnetic field surrounding it orientated clockwise with
direction of the current. When winding on a magnetic core
and applying current, the magnetic flux refluxes the inside
of the core and causes a magnetic stray field surrounding
the coil.
Unshielded coils have open drum core with an open
magnetic path. In the case of using unshielded coils, the
magnetic flux leaves the core and becomes a leakage
flux, which forms a loop that returns to the core. The
magnetic flux widely spreads around the coil like shown
in Figure 1.
Shielded coils that are more expensive cover and hide
the drum core and windings with a box made by the same
magnetic material used for the core.
This allows magnetic flux passing an air gap between the
pot core and the shielding box to let the magnetic flux flow
through the shielding box. This is shown in Figure 2.
Power inductors are a part of the power stage of switch mode power supplies (SMPS) like none isolated DC-DC
buck converter. Power traces from printed circuit board (PCB), power supply lines and power inductor causing
electromagnetically interferences (EMI) noise like radiated emission. Leakage filed of power inductors influences
overall radiated emission of a SMPS. Manufacturer of power inductors defining start of winding and specifying
wiring of coils, also developing shielded coils to reduce radiated emission of SMPS. It is important to take care
about coil orientation during the mounting process of a coil on a PCB. This application note is about radiated
emission caused by a power inductor in a DC-DC converter dependent to the polarity of a coil.
FIGURE 2: MAGNETIC FLUX IN SHIELDED DRUM CORE COILS
FIGURE 1: MAGNETIC FLUX IN UNSHIELDED DRUM CORE
COLS
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2017.4
© 2020 ROHM Co.,
Ltd.
No. 62AN097E Rev.001
March.2020
Application Note
Since the magnetic flux refluxes inside of the drum core
and the shield core, the magnetic path inside the coil is
closed and the leakage flux is reduced.
However, magnetic shielding does not always provide
complete encapsulation, and flux can leak from the air
gap or terminal of the junction between the shield material
and drum core.
Magnetic flux dependent to start of winding
Direction of magnetic flux in the core and surrounded
resulting H-field is dependent on current which flows
through a wire from a coil.
The leakage flux and H-field increases in power inductors
with increasing current.
Figure 3 shows coil with orientation of 0°. Resulting
magnetic field direction as well magnetic flux. Current flow
is through inductor from “start” to ���end” of winding.
I
H
H
H
H
H
PCB trace
PCB
coil orientation 0°
V
Start of winding
FIGURE 3: MAGNETIC FLUX AND H-FIELD WITH COIL
ORIENTATION OF 0°
In this case, magnetic flux leaves the drum core from
bottom of coil and becomes a leakage flux, which forms
a loop that returns to the top of drum core.
Rotating coil in circuit on PCB by 180°, changes current
flow through inductor. This changes H-field surrounding
wire and coil. Resulting magnetic flux through drum core
of coil changes direction, too.
Figure 4 shows coil with orientation of 180° and resulting
magnetic field direction as well magnetic flux. Current flow
is through inductor from “end” to “start�� of winding.
I
H
H
H
H
H
coil orientation 180°
V
Start of winding
PCB trace
PCB
FIGURE 4: MAGNETIC FLUX AND H-FIELD WITH COIL
ORIENTATION OF 180°
Picture with coil orientation of 180° shows different
direction of H-field and flux by changed direction of DC
current flow though coil wire.
Here, magnetic flux leaves the drum core from top of coil
and becomes a leakage flux, which forms a loop that
returns to the bottom of drum core.