Considerations for designing common-mode inductance of switching power supply transformer
In the design process of power transformers, engineers need to strictly calculate and complete the design and numerical selection of common mode inductance, which directly affects the operational accuracy of switching power transformers. In today's article, we will provide a brief analysis of the common mode inductance design of switching power supply transformers, to see what issues should be noted in the design and calculation process of common mode inductance for power transformers. In the design and manufacturing process of power transformers, engineers need to carry out common mode inductance design, which mainly requires three basic parameters: input current, impedance and frequency, and magnetic core selection. Let's first take a look at the input current. This parameter value directly determines the required wire diameter for the winding. When calculating and selecting the line diameter, the current density is usually taken as 400A/cm ³, but this value must vary with the temperature rise of the inductor. Normally, windings are operated using 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 the minimum value under the given frequency conditions. A series linear impedance can provide the required noise attenuation. However, in reality, the issue of linear impedance is often overlooked, so designers often use a 50W linear impedance stabilization network instrument to test common mode inductance and gradually become a standard method for testing the performance of common mode inductance. But the results obtained usually differ significantly from reality. In fact, under normal circumstances, the common mode inductor will first produce a frequency of -6dB attenuation for every octave increase in angular frequency (angular frequency is -3dB produced by the common mode inductor). This angular frequency is usually very low so that the inductance can provide impedance. Therefore, inductance can be expressed using the formula Ls=Xx/2 π f. There is another issue that engineers need to pay attention to, which is the magnetic core material and required number of turns when designing common mode inductors. Firstly, let's take a look at the selection of magnetic core models. If there is a specified inductance space, we will choose the appropriate magnetic core model based on this space. If there is no regulation, the selection of magnetic core models is usually arbitrary. After determining the magnetic core model of the power transformer, the next step is to calculate the maximum number of turns the magnetic core can be wound. Generally speaking, common mode inductors have 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 layered and stacked windings are occasionally used, but this approach can increase the distributed capacitance of the winding and reduce the high-frequency performance of the inductance. Since the diameter of copper wire is determined by the magnitude of linear current, the inner circumference can be calculated by subtracting the copper wire radius from the inner radius of the magnetic core. Therefore, the maximum number of turns can be calculated based on the diameter of the copper wire with insulation and the circumference occupied by each winding.
