Common Misconceptions in the Design of High-Frequency Magnetic Components in Switching Power Supplies

1 Introduction

The design of high-frequency magnetic components in switching power supplies is critical to the normal operation of the circuit and the realization of various performance indicators. In addition, the design of high-frequency magnetic components includes many detailed knowledge points, and these details are difficult to be listed one by one in one or several so-called "design encyclopedias" [1-3]. In order to optimize the design of high-frequency magnetic components, it is necessary to comprehensively consider multiple design variables according to the application scenario and repeatedly calculate and adjust. Because of this, the design of high-frequency magnetic components has always been a headache for designers who are new to the power supply field, and even a problem that troubles power supply engineers with many years of work experience.

The magnetic component design methods or formulas given in many literatures and related technical materials often directly ignore the influence of certain design variables, make assumptions and simplify them to obtain a set of formulas; or do not explain the application conditions of the formulas clearly, and even some of the information conveyed by the literature itself is incorrect. Many power supply designers do not realize this, and directly apply the formulas in the design manual, or take some words in the design manual out of context and regard them as "design guidelines". , without thorough analysis and thinking, as well as experimental verification. The result is often that the designed high-frequency magnetic components cannot meet the requirements of the application, affecting the progress of research and development and the completion of the project on schedule.

In order to enable power designers to avoid making the same mistakes in the design process, we have summarized some conceptual problems encountered in learning and research and development, hoping to provide a reference for everyone. 2 Analysis of some wrong concepts Here are 8 common wrong concepts in the design of high-frequency magnetic components of switching power supplies in the form of subheadings, and detailed analysis. 1) Fill the core window - optimized design Many power designers believe that in the design of high-frequency magnetic components, filling

the core window can obtain the optimal design, but this is not the case. In the design of many high-frequency transformers and inductors, we can find that adding one or more layers of windings, or using larger diameter enameled wires, not only can not achieve the optimization effect, but will increase the total loss of the winding due to the proximity effect in the winding. Therefore, in the design of high-frequency magnetic components, even if the winding does not fill the core window, only 25% of the window area is filled. %. It does not matter. You do not have to try to fill the entire window area. This misunderstanding is mainly affected by the design of power frequency magnetic components. In the design of power frequency transformers, the integrity of the core and winding is emphasized. Therefore, it is not desirable to have a gap between the core and the winding. Generally, the winding is designed to fill the entire window to ensure its mechanical stability. However, there is no such requirement in the design of high-frequency magnetic components. 2) "Iron loss = copper loss" - optimized transformer design Many power supply designers, even in many magnetic component design reference books, list "iron loss = copper loss" as one of the standards for optimizing the design of high-frequency transformers. In fact, this is not the case. In the design of high-frequency transformers, the difference between iron loss and copper loss can be large, and sometimes the difference between the two can even reach an order of magnitude. However, this does not mean that the design of the high-frequency transformer is not good [4]. This misunderstanding is also affected by the design of power frequency transformers. Power frequency transformers often have more turns and occupy a larger area. Therefore, from the perspective of thermal stability and thermal uniformity, the conclusion is "iron loss = copper loss". This rule of thumb is used for design. However, for high-frequency transformers, this rule of thumb does not hold true when very thin enameled wire is used as windings. In the design of high-frequency transformers for switching power supplies, there are many factors to determine the optimal design, and "iron loss = copper loss" is actually the least concerned aspect. 3) Leakage inductance = 1% magnetizing inductance After designing magnetic components, many power designers submit relevant technical requirements to transformer manufacturers, and often explain the leakage inductance requirements. On many technical sheets, there are similar technical requirements such as "leakage inductance = 1% magnetizing inductance" or "leakage inductance <2% magnetizing inductance". In fact, this writing or design standard is very unprofessional. Power designers should design the power supply according to the normal working requirements of the circuit. Set a numerical limit on the acceptable leakage inductance value. In the process of manufacturing transformers, the leakage inductance value should be reduced as much as possible without deteriorating other parameters of the transformer (such as inter-turn capacitance, etc.), rather than giving the ratio of leakage inductance to magnetizing inductance as a technical requirement. This is because the relationship between leakage inductance and magnetizing inductance varies greatly depending on whether the transformer has an air gap. When there is no air gap, the leakage inductance may be less than 0.1% of the magnetizing inductance, while when there is an air gap, even if the transformer windings are tightly coupled, the ratio of leakage inductance to magnetizing inductance may reach 10% [5]. Therefore, do not provide the ratio of leakage inductance to magnetizing inductance as a transformer design indicator to magnetic component manufacturers. Otherwise, it will show that you do not understand the knowledge of leakage inductance or do not really care about the actual leakage inductance value. The correct approach is to clearly specify the acceptable absolute value of leakage inductance, of course, you can add or subtract a certain ratio, the typical value of this ratio is 20% . ) Leakage inductance is related to the magnetic permeability of the magnetic core Some power supply designers believe that adding a magnetic core to the winding will make the winding coupling tighter and reduce the leakage inductance between the windings; some power supply designers believe that after adding a magnetic core to the winding, the magnetic core will couple with the field between the windings, which can increase the leakage inductance. The fact is that in the design of switching power supplies, the leakage inductance of two coaxial winding transformers has nothing to do with the presence of a magnetic core. This result may be incomprehensible, because a material with a relative magnetic permeability of several thousand has little effect on the leakage inductance when it is close to the coil. The actual measurement results of hundreds of sets of transformers show that the leakage inductance change value will basically not exceed 10% with or without a magnetic core, and many changes are only about 2%. 5) The optimized value of the transformer winding current density is 2A/mm2~3.1A /mm2Many power supply designers often regard the current density in the winding as the standard for optimal design when designing high-frequency magnetic components. In fact, the optimal design has nothing to do with the current density of the winding. What really matters is how much loss there is in the winding and whether the heat dissipation measures are sufficient to ensure that the temperature rise is within the allowable range. We can imagine two extreme cases of heat dissipation measures in switching power supplies. When liquid immersion and vacuum are used for heat dissipation respectively, the corresponding current density in the winding will differ greatly. In the actual development of switching power supplies, we do not care about how large the current density is, but only how hot the coil is? Is the temperature rise acceptable? This erroneous concept is an artificial restriction imposed by designers to avoid tedious repeated trial and error, to simplify the number of variables and thus simplify the calculation process, but this simplification does not explain the application conditions. 6) Primary winding loss = secondary winding loss" - Optimized transformer design Many power designers believe that the optimized transformer design corresponds to the transformer primary winding loss and secondary winding loss being equal. Even in many magnetic component design books, this is used as a standard for optimized design. In fact, this is not a standard for optimized design. In some cases, the iron loss and copper loss of the transformer may be similar. But if the primary winding loss is much different from the secondary winding loss, it does not matter much. It must be emphasized again that for high-frequency magnetic component design, what we care about is how hot the winding is under the heat dissipation method used? Primary winding loss = secondary winding loss is just an empirical rule for power frequency transformer design. 7) Winding diameter is smaller than penetration depth—— High-frequency loss will be very small The fact that the winding diameter is smaller than the penetration depth does not mean that there is no large high-frequency loss. If there are many layers in the transformer winding, even if the winding uses enameled wire with a wire diameter much thinner than the penetration depth, it may produce large high-frequency losses due to the strong proximity effect. Therefore, when considering the winding loss, the loss size cannot be judged only from the thickness of the enameled wire. The arrangement of the entire winding structure must be considered comprehensively, including the winding method, the number of winding layers, the thickness of the winding wire, etc.












































8) The open-circuit resonant frequency of the transformer in the forward-type circuit must be much higher than the switching frequency

Many power supply designers believe that the open-circuit resonant frequency of the transformer must be much higher than the switching frequency of the converter when designing and testing the transformer. In fact, the open-circuit resonant frequency of the transformer has nothing to do with the switching frequency. We can imagine the extreme case: for an ideal magnetic core, its inductance is infinite, but there will also be a relatively small inter-turn capacitance, and its resonant frequency is approximately zero, which is much smaller than the switching frequency.

What really matters to the circuit is the short-circuit resonant frequency of the transformer. In general, the short-circuit resonant frequency of the transformer should be more than two orders of magnitude higher than the switching frequency.

3 Conclusion

In order to prevent power supply designers from making the same mistakes in the power supply design process, we have summarized some conceptual issues related to the design of high-frequency magnetic components encountered in the research and development of switching power supplies, hoping to play a role in inspiring others.