In the design process of the power transformer, engineers need to strictly calculate and complete the design and numerical selection of the common mode inductance, which is directly related to the operating accuracy of the switching power transformer. In today's article, we will briefly analyze the common mode inductance design of switching power transformers, and see what problems should be paid attention to in the design and calculation of the common mode inductance of power transformers. In the design and production process of power transformers, engineers need to design common mode inductance, and there are three basic parameters required, namely input current, impedance and frequency, and magnetic core selection. Let's first look at the input current. The value of this parameter directly determines the wire diameter required for the winding. When calculating and selecting the wire diameter, the current density is usually 400A/cm³, but this value must change with the temperature rise of the inductor. Typically, the windings are run with a single wire, which reduces high frequency noise and skin effect losses. In the calculation process, the impedance of the common mode inductance of the switching power supply transformer is generally specified as the minimum value under the given frequency conditions. A linear impedance in series provides the generally required noise attenuation. But in fact, the problem of linear impedance is often overlooked, so designers often use a 50W linear impedance stabilization network instrument to test common mode inductors, and it has gradually become a standard method to test the performance of common mode inductors. However, the results obtained are usually quite different from the actual ones. In fact, the corner frequency of the common mode inductor will first produce an increase of -6dB attenuation per octave at normal time (the corner frequency is the frequency that the common mode inductor produces -3dB). This corner frequency is usually low so that the inductive reactance can provide impedance. Therefore, the inductance can be expressed by this formula, namely: Ls=Xx/2πf. There is another issue that engineers need to pay attention to, that is, the core material and the required number of turns must be paid attention to when designing the common mode inductor. First of all, let's look at the selection of the magnetic core model. If there is a specified inductance space at this time, we will select the appropriate magnetic core model according to this space. If there is no regulation, the magnetic core model is usually selected at will. After determining the core type of the power transformer, the next job is to calculate the maximum number of turns that the core can make. Generally speaking, a common mode inductor has two windings, generally single-layer, and each winding is distributed on each side of the magnetic core, and the two windings must be separated by a certain distance. Double and stacked windings are also occasionally used, but this approach increases the distributed capacitance of the winding and reduces the high frequency performance of the inductor. Since the wire diameter of the copper wire has been determined by the magnitude of the linear current, the inner circumference can be calculated by subtracting the radius of the copper wire from the inner radius of the magnetic core. Therefore, the maximum number of turns can be calculated by the wire diameter of the copper wire plus insulation and the circumference occupied by each winding
