“The design of high-frequency magnetic components in the switching power supply is very critical for 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 list clearly in one or several so-called “design books”.[1－3].
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1 Introduction
The design of high-frequency magnetic components in the switching power supply is very critical for 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 list clearly in one or several so-called “design books”.[1－3].
In order to optimize the design of high-frequency magnetic components, multiple design variables must be considered comprehensively according to the application, and the calculation and adjustment must be repeated.
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 for 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 ignore the influence of some design variables directly, and make a set of formulas after making assumptions and simplifying; or do not explain the application conditions of the formulas clearly, and even some literature The information conveyed is itself 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, thinking, and experimental verification.
The result is often that the designed high-frequency magnetic components cannot meet the requirements of the application, which affects the progress of research and development and the completion of the project on schedule.
In order to make the power supply designers avoid making the same mistakes in the design process, we have summarized some conceptual problems encountered in the study and research and development, hoping to provide you with a reference.
2. Discrimination and Analysis of Some Misconceptions
Here, 8 common misconceptions in the design of high-frequency magnetic components of switching power supplies are given in the form of subtitles, and they are analyzed in detail.
1), fill the core window – optimized design
Many power supply designers believe that filling the core window is the optimal design for high-frequency magnetics, 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 enameled wires with larger wire diameters will not only fail to obtain the optimal effect, but will cause problems due to the proximity effect in the windings. Increase the total winding loss.
Therefore, in the design of high-frequency magnetic components, it does not matter even if the winding does not fill the iron core window, but only fills 25% of the window area. You don’t have to figure out how to fill the entire window area.
This misconception is mainly influenced by the design of power frequency magnetic components. In the design of power frequency transformers, the integrity of the iron core and the winding is emphasized, so there is no gap between the iron core and the winding. Generally, the winding is designed to fill the entire window to ensure its mechanical stability. But high-frequency magnetic component design does not have this requirement.
2), “iron loss = copper loss” – optimized transformer design
Many power supply designers even listed “iron loss = copper loss” as one of the criteria for optimal design of high-frequency transformers in many reference books of magnetic component design, but it is not. In the design of high-frequency transformers, the difference between iron loss and copper loss can be large, and sometimes the difference can even reach an order of magnitude, but this does not mean that the design of the high-frequency transformer is not good.
This misconception is also influenced by the design of power frequency transformers. Power frequency transformers often occupy a large area because of the large number of winding turns. Therefore, from the perspective of thermal stability and thermal uniformity, the empirical design rule “iron loss = copper loss” is obtained.
But for high-frequency transformers, using very thin enameled wire as the winding, this rule of thumb does not hold. 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
Many power supply designers often need to explain the leakage inductance requirements when submitting the relevant technical requirements to the transformer manufacturer after designing the magnetic components.On many technical sheets, it is marked “leakage inductance = 1% magnetizing inductance” or “leakage inductance”
The power supply designer should make a numerical limit on the acceptable leakage inductance value according to the normal working requirements of the circuit. In the process of making the transformer, the leakage inductance value should be reduced as much as possible without making other parameters of the transformer (such as inter-turn capacitance, etc.) worse, instead of giving the proportional relationship between the leakage inductance and the magnetizing inductance as a technique Require.
Because the relationship between the leakage inductance and the magnetizing inductance varies greatly with whether the transformer has an air gap or not. Without the air gap, the leakage inductance may be less than 0.1% of the magnetizing inductance, while with the air gap, even if the transformer windings are tightly coupled, the proportional relationship between the leakage inductance and the magnetizing inductance may reach 10%.
Therefore, do not provide the ratio of leakage inductance to magnetizing inductance as a transformer design specification to magnetics manufacturers. Otherwise, it will show that you don’t understand the leakage inductance knowledge or don’t really care about the actual leakage inductance value. The correct way is to specify the absolute value of the leakage inductance that is clearly acceptable. Of course, a certain ratio can be added or subtracted. The typical value of this ratio is 20%.
4) The leakage inductance is related to the magnetic permeability of the magnetic core
Some power supply designers believe that adding a magnetic core to the windings will make the windings more tightly coupled and reduce the leakage inductance between the windings; some power supply designers believe that after adding a magnetic core to the windings, the magnetic field between the magnetic core and the windings will be reduced. Coupling with each other can increase the leakage inductance.
The fact is, in switching power supply designs, the leakage inductance of the two coaxial winding transformers has nothing to do with the presence or absence of a magnetic core. This result may be incomprehensible because a material with a relative permeability of several thousand has little effect on the leakage inductance when placed close to the coil.
The measured results of hundreds of groups of transformers show that the leakage inductance change value basically does not exceed 10% with or without the existence of a magnetic core, and many changes are only about 2%.
5) The optimal value of the transformer winding current density is 2A/mm²～3.1A/mm²
When designing high-frequency magnetic components, many power supply designers often regard the current density in the windings as a criterion for optimal design.
In fact, the optimal design has nothing to do with the winding current density. What really matters is how much losses are in the windings and whether the heat dissipation measures are sufficient to keep the temperature rise 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, the corresponding current densities in the windings will be quite different.
In the actual development of switching power supply, we do not care how much the current density is, but only how hot the wire package is? Is the temperature rise acceptable?
This erroneous concept is that the designers artificially limit the number of variables in order to avoid tedious trial and error to simplify the number of variables, thereby simplifying the calculation process, but this simplification does not explain the application conditions.
6), primary winding loss = secondary winding loss” – optimized transformer design
Many power supply designers believe that the optimal transformer design corresponds to the transformer primary winding losses equal to the secondary winding losses. This is even used as a criterion for optimal design in many design books of magnetic components. In fact, this is not a standard for optimal design.
In some cases the iron and copper losses of the transformer may be similar. But it doesn’t matter much if the primary winding loss is quite different from the secondary winding loss.
It must be emphasized again that for high frequency magnetics design we are concerned with how hot the windings are given the heat dissipation method used? Primary winding loss = secondary winding loss is just a rule of thumb for power frequency transformer design.
7), the winding diameter is smaller than the penetration depth – the high frequency loss will be very small
A winding diameter smaller than the penetration depth does not mean that there will be no significant high frequency losses. If there are many layers in the transformer winding, even if the winding is made of enameled wire with a much thinner wire diameter than the penetration depth, there may be large high frequency losses due to strong proximity effects.
Therefore, when considering the winding loss, the size of the loss cannot be judged only from the thickness of the enameled wire. The arrangement of the entire winding structure should be comprehensively considered, including the winding method, the number of winding layers, and the thickness of the winding.
8) The open-circuit resonant frequency of the transformer in the forward circuit must be much higher than the switching frequency
Many power supply designers design and test transformers with the assumption that the open-circuit resonant frequency of the transformer must be much higher than the switching frequency of the converter. In fact, the open-circuit resonant frequency of the transformer has nothing to do with the switching frequency.
We can imagine the limit case: for an ideal magnetic core, its inductance is infinite, but there is also a relatively small inter-turn capacitance, and its resonant frequency is approximately zero, 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 of the switching frequency.
3. Conclusion
In order to make the power supply designers less likely to make the same mistakes in the process of power supply design, some conceptual problems related to the design of high-frequency magnetic components that we encountered in the research and development of switching power supplies are summarized. effect.
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