Core selection

(1) Concentration and dispersion of air gaps. The magnetomotive force of the inductor (current multiplied by the number of turns) is used entirely to generate magnetic flux. In order to generate an appropriate magnetic flux density in the magnetic core, Bw can be selected from two options.

① Option 1: Use a core material with a higher μ value (the μ value is not important) commonly used in high-frequency transformers, and add an appropriate centralized air gap (paper or cardboard pad) in the magnetic circuit to prevent the magnetic flux from entering a saturation state; the stray magnetic field of the centralized air gap is larger.

②Scheme 2: Use a constant magnetic permeability ferromagnetic powder core toroidal core with low initial magnetic permeability μi. Its appearance has no air gap. In fact, the adhesive used to bond the magnetic powder material forms many tiny air gaps, which are evenly distributed inside the magnetic material. The stray magnetic field of this distributed air gap core is small; the equivalent air gap reduces the equivalent μi value of the magnetic material. The more relative content of adhesive in the core, the smaller the equivalent μi value of the material. Products with different μi values ​​can be made, such as μi in the range of 14 to 350, which can be divided into multiple specifications for selection.

(2) High-frequency loss and saturation magnetic flux density. The alternating components of the three types of inductor current and magnetic flux are different in size and should be treated differently.

① High-frequency AC inductor: For example, the resonant inductor used in the LC resonant circuit in the soft switching circuit has the following characteristics: the current has only high-frequency AC components, no DC components, and the magnetic flux is also bidirectionally magnetized. Bw = 2Bm. When Bm is large, the core loss is also large. Bm and materials with low loss should be appropriately selected. When magnetic powder core materials are selected, the loss is also small when μ is small.

② DC filter inductor: The inductor current is mainly composed of DC current components, and the high-frequency AC component is small. Usually, the peak value of the AC component only accounts for 20% of the DC rated current; the high-frequency loss is relatively small. In order to reduce the volume, a magnetic core material with a larger Bs should be selected, such as an iron powder core.

③ Energy storage inductor: It is divided into two types: continuous current type (CCM) and discontinuous current type (DCM). The continuous energy storage inductor is the same as the DC filter inductor mentioned above; the AC current component of the discontinuous energy storage inductor is equivalent to the DC current component, and the high-frequency AC loss is larger, which is smaller than the high-frequency AC inductor.

(3) Selection of inductor core type and size: The size of the inductor core is related to the magnetic field energy. For the power volume design method, it is selected by Aw·Ac, and for the adjustment rate volume design method, it is selected by Kd.