Inductors for power circuits

Inductors used in circuits that convert energy, such as switching power supplies and DC-DC converters. They are often used as "choke coil". For power supply equipment, it is important to design so it disperses the generated heat, but it is also important to select a low loss transformer and inductor, which generate heat. The figure below shows the circuit diagram of the AC adapter used as a battery charger for smartphones.

Both L1 and L2 are inductors for noise reduction. Since AC adaptors are plugged into a household outlet, it is necessary to remove the noise to keep the electromagnetic noise generated inside the AC adapter from being transmitted to other devices.

In general, a low-pass filter is configured by combining an across-the-line capacitor Cx and a line-by-pass capacitor Cy. In the figure above, both L1 and L2 are used as normal mode inductors. C1 is a film capacitor that works as an “across-the-line capacitor”.

L3 is a smoothing choke coil, which smooths the output current, including the ripple rectified by the diode and smoothed by the capacitor, using the impedance. Since the output current of the AC adapter flows through the choke coil, the required inductance value should be determined by calculation, and a choke coil with as low a direct current resistance as possible should be selected with a consideration of the mounting space given for the choke coil.

Selection of high frequency inductors

Inductors with excellent high-frequency characteristics should be selected for RF circuit resonance inductors, impedance matching, and harmonic traps.

Selecting a high Q inductor

Q is an index to measure loss at high frequencies. As with the concept of copper loss in inductors for power supply circuits, it is important to have a low resistance value. In this case, however, the resistance value does not refer to the DC resistance, but to the electrical resistance at high frequencies.

At high frequencies, the skin effect becomes noticeable, so it is important to increase the surface area, not the cross-sectional area of the conductor. Since the same phenomenon occurs in the magnetic flux in the iron core, the effective magnetoresistance is reduced by increasing the coil diameter.

Selecting an inductor with high self-resonant frequency

Common mode choke coil

Anti-EMI (electromagnetic interference) component that has a ferrite core with two windings, and is mainly used in power supply lines and interface circuits that use differential signals such as USB, PCI, and LVDS. It plays an important role as an EMI countermeasure for electronic devices.

The feature of the common mode choke coil is that noise is removed by taking advantage of the difference between the noise and signal transmission modes. Therefore, common mode choke coil dose not adversely affect on the signal having normal mode, and selectively remove the common mode noise that become the source of radiation noise.