Switching power supply transformer common mode inductor design considerations
During the design process of the power transformer, engineers need to strictly calculate and complete the common mode inductor design and numerical selection, which is directly related to the operating accuracy of the switching power supply transformer. In today's article, we will briefly analyze the design of common mode inductors of switching power supply transformers to see what issues should be paid attention to during the design and calculation process of common mode inductors 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, namely input current, impedance and frequency, and core selection. Let’s look at the input current first. This parameter value 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 operated using a single wire, which reduces high-frequency noise and skin effect losses. During the calculation process, the impedance of the common mode inductor of the switching power supply transformer is generally specified as the minimum value under the given frequency conditions. Linear impedances in series provide the generally required noise attenuation. But in fact, linear impedance issues are often the most overlooked. Therefore, designers often use a 50W linear impedance stabilized network instrument to test common mode inductors, and it has gradually become a standard method for testing the performance of common mode inductors. But the results obtained are often quite different from reality. In fact, when the common mode inductor is normal, the corner frequency will first produce a frequency that increases -6dB attenuation per octave (the corner frequency is -3dB produced by the common mode inductor). 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, when designing common-mode inductors, they must pay attention to the core material and the required number of turns. First, let’s look at the selection of the magnetic core model. If there is a specified inductance space, 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 model of the power transformer, the next step is to calculate the maximum number of turns the core can make. Generally speaking, a common mode inductor has two windings, usually a single layer, and each winding is distributed on each side of the core. The two windings must be separated by a certain distance. Double-layer and stacked windings are also occasionally used, but this approach will increase the distributed capacitance of the winding and reduce the high-frequency performance of the inductor. Since 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 core. Therefore, the maximum number of turns can be calculated by the diameter of the copper wire plus insulation and the circumference occupied by each winding.
