Design considerations for common mode inductance of switching power transformer
In the design process of power transformers, engineers need to strictly calculate and complete the common mode inductance design and numerical selection, which directly affects the operational accuracy of switching power transformers. In today's article, we will conduct a brief analysis of the common mode inductance design of switching power transformers, and see what issues should be paid attention to in the design and calculation process of common mode inductance of power transformers. In the design and production process of power transformers, engineers need to design common mode inductors, which mainly require three basic parameters: input current, impedance and frequency, and magnetic core selection. Let's take a look at the input current first. This parameter value directly determines the required wire diameter for the winding. When calculating and selecting the wire diameter, the current density is usually taken as 400A/cm ³， But this value must vary with the temperature rise of the inductance.
Usually, the winding is operated with a single wire, which can reduce 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 a small value at the given frequency conditions. The linear impedance in series can provide the generally required noise attenuation. However, in reality, the issue of linear impedance is often overlooked, so designers often use a 50W linear impedance stable network instrument to test common mode inductors, and gradually become a standard method for testing the performance of common mode inductors. But the results obtained usually differ significantly from the actual situation. In fact, the common mode inductance will first produce a frequency of -6dB attenuation per octave increase in angular frequency during normal operation (angular frequency is -3dB produced by the common mode inductance). This angular frequency is usually very low, so that the inductance can provide impedance.
Therefore, inductance can be expressed using this formula, i.e. Ls=Xx/2 π f. There is another issue that engineers need to pay attention to here, which is to pay attention to the material of the magnetic core and the required number of turns when designing a common mode inductor. Firstly, let's take a look at the selection of magnetic core models. If there is a specified inductance space at this time, we will select the appropriate magnetic core model based on this space. If there are no regulations, the selection of magnetic core models is usually arbitrary.
After determining the magnetic core model of the power transformer, the next task is to calculate the number of large turns that the magnetic core can wind. Generally speaking, a common mode inductor has two windings, usually a single layer, and each winding is distributed on each side of the magnetic core, with a certain distance between the two windings. Double layer and stacked windings are also occasionally used, but this approach can improve the distributed capacitance of the winding and reduce the high-frequency performance of the inductance. Due to the fact that the diameter of the copper wire is 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, for larger coils, the diameter of the copper wire with insulation and the circumference occupied by each winding can be calculated.
