Basic knowledge of soft magnetic materials (Part 3)

Working principle and design points of common mode inductor:

1. Working principle of common mode point sensing:

When the switching power supply is connected to the input end of the AC power grid, a common mode filter is usually connected to reduce interference to the power grid, so that the power supply meets the electromagnetic compatibility requirements.

The common mode filter of the switching power supply (as shown in the figure) makes the magnetic field (differential mode) generated in the coil by the phase line L (incoming) and the neutral line N (returning) equal in magnitude and opposite in direction, and the synthetic magnetic field is approximately zero. The interference source double line or power grid (L and N) presents a large impedance to the in-phase noise (common mode noise) of the earth.

The switching power supply should meet the electromagnetic compatibility standard limits of EN55022 B, VDE 0871B, etc., with a frequency range of 150kHz to 30MHz. The filter has a wide frequency band, requiring the common mode filter to have 1. very high magnetic permeability and wide frequency characteristics, 2. very low high-frequency loss, 3. small stray magnetic field and parasitic capacitance, and 4. high performance stability.

The noise spectrum generated by the switching power supply is usually 10kHz ~ 50MHz. In order to achieve sufficient attenuation, the inductor impedance must be high enough in these frequency ranges. The total impedance Zs of the common-mode inductor consists of two parts: the series inductive reactance Xs and the series resistance Rs. At low frequencies, the resistance is the main component of the impedance, and as the frequency increases, the real permeability of the magnetic permeability begins to decrease, while the loss in the magnetic core increases, and the total impedance increases slightly; when it rises to higher frequencies, the distributed capacitors play a major role, and the total impedance begins to decrease. Therefore, the design of the common-mode inductor is mainly based on the test conditions of the actual circuit to determine the frequency band of noise suppression.

2. Key points of common mode inductor design:

R - ring inner diameter r - inner diameter minus wire diameter circumference = 2πr

The main parameters of common mode inductor design are input current, impedance, and frequency. The effective value of input current determines the conductor size of the inductor coil. The current density can be j=400A/cm2, which is related to the insulation and core materials used in the inductor. Single-strand wire is usually selected because of its low cost. At the same time, the skin effect of high-frequency current increases the AC resistance and also attenuates noise.

The internal impedance of the power grid also provides noise attenuation, but the grid impedance is difficult to determine. Designers can use 50Ω as the load with an impedance balancing network (LISN) based on the test conducted interference, which may be very different from the actual situation.

The design starts with a known inductance (Ls = Xs/2πf), and the core selection is quite arbitrary. Just wind the number of turns that meets the inductance.

The common mode inductor has two coils with equal turns, usually single-layer. In order to meet the safety requirements between lines, the two coils are distributed on opposite sides of the magnetic ring, usually only one layer. If there are two layers, the high-frequency performance is reduced. Each coil occupies 150°~170° of the inner circumference of the ring.

Since the wire size is always determined by the grid current, the inner circumference can be calculated by subtracting the wire diameter from the inner diameter. The maximum number of turns can be calculated by dividing the inner circumference occupied by each coil by the diameter of the wire with insulation. Note that each coil occupies 150°~170° of the inner circumference in order to insulate the two coils. In this way, the maximum number of turns that can be wound at 150°~170° can be calculated by subtracting the wire diameter with paint from the inner diameter of the ring.

After calculating the maximum number of turns, go to the manual to select the material. It is easy to calculate the required number of turns based on the inductance (AL) of the toroidal core material:

Where N is the number of turns; L is the inductance; AL is the inductance coefficient.