Magnetic ring inductors are divided into two categories: common mode inductors and differential mode inductors. The common mode inductors have been introduced in the previous article. This paper introduces magnetic ring differential mode inductors.
Differential mode inductors are mainly used to filter out differential mode interference. Differential mode interference is a kind of interference in the differential signal transmission system, which often appears between two wires. This interference mode is mainly caused by the inconsistency of the length, width and spacing of the two differential signal lines on the circuit board.
Differential mode inductors are usually combined with capacitors to form LC filters to reduce power supply noise. In the EMC part of switching power supply, differential mode inductors are often used to suppress noise from switching power supply into the power grid. Under normal circumstances, the differential mode inductor has only one winding, and the more common core materials are iron powder core, alloy powder core, nickel zinc and so on. The iron core is made of high purity iron powder particles with very small particles and insulation between each other, and has a uniform distributed air gap. The initial permeability of the iron powder core is between 10 and 75, the saturated magnetic flux density is high, the iron loss is large, the application frequency is not high, mostly below 100KHZ, but the material cost is relatively low.
The material of iron powder core is -2 (red/transparent), -8 (yellow/red), -18 (green/red), -26 (yellow/white), -28 (gray/green), -33 (gray/yellow), -38 (gray/black), -40 (green/yellow), -45 (black), -52 (green/blue), etc. At present, the most commonly used is -26 yellow white ring. Alloy core is more commonly used iron silicon aluminum (Sendust), iron nickel molybdenum (MPP), iron nickel (highflux), the permeability of 14~500, the more commonly used is iron silicon aluminum, cost-effective, is currently the most widely used in civilian products.
Nickel-zinc magnetic ring, the permeability is generally less than 1500. The material has low temperature coefficient of magnetic ring, high Curie temperature and good high-frequency characteristics, and is mostly used in middle and high frequency circuits. The two winding magnetic flux directions of the common mode inductor are opposite, so the two kinds of magnetic flux can cancel each other and there will be no saturation phenomenon. The differential mode inductor is different, there is only one winding, it needs to focus on whether there will be saturation after the current. In general, manufacturers also focus on evaluating and testing the saturation current (also known as superposition current) of differential mode inductors. It is worth mentioning that in addition to using magnetic ring inductors as differential mode inductors, I-inductors are also often used as differential mode inductors and are widely used in circuits.