AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 8

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
AN2321/D
There are basically two types of inductors: open-loop and closed-loop. Their
difference is in their magnetic field loop. In the open-loop design, the magnetic
field passes through air to complete its field, while in the closed-loop design,
the magnetic field flows through its core material to complete the magnetic
circuit. This is illustrated in
Figure
4.
a) open loop core (solenoid)
b) closed loop core (toroid)
Figure 4. Magnetic Field in Inductor Core
One advantage of the inductor over the capacitor or resistor is that there are no
parasitic inductance, hence there is very little difference between surface
mounts and the leaded types.
Since the magnetic field of the open-loop inductor passes through air, this
causes radiation and can cause EMI problems. For the choice of open-loop
inductors, the bobbin type is better than the rod or solenoid type because the
magnetic field is controlled by the core (i.e. localized magnetic field within the
core).
a) rod inductor
b) bobbin inductor
Figure 5. Open Loop Inductors
For the closed-loop inductor, the magnetic field is totally controlled by the core.
Therefore, this type of inductor is more ideal in circuit designs, except they are
more expensive. One advantage of the toroid shape of the closed-loop inductor
is that it not only keeps the magnetic loop within the core, it also has a self-
cancelling effect on any incident field radiating into the inductor.
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Designing for Board Level Electromagnetic Compatibility
MOTOROLA
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