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Sep 30 2019

Analysis of Common Design Errors of High Frequency Magnetic Elements in Switching Power Supply

Catalog


I

Overview

II

Discrimination of Some Wrong Concepts

Filling Core Windows - Optimized Design

"Iron loss = copper loss" -- Optimized Transformer Design

Leakage Inductance = 1% Magnetization Inductance

Leakage Inductance is Related to Core Permeability

The optimal current density of transformer winding is 2A/mm2 ~ 3.1A/mm2

III

Conclusion


Overview

The design of high frequency magnetic elements in switching power supply is very important for the normal operation of the circuit and the realization of various performance indexes.

In order to optimize the design of high frequency magnetic elements, multiple design variables must be considered comprehensively and repeated calculation and adjustment must be made according to the application.

Because of this, the design of high frequency magnetic components has been a headache for designers who are new to the field of power supply, and even for power engineers who have many years of work experience.

The design methods or formulas of magnetic elements given in many literatures and related technical materials often ignore the influence of some design variables directly, So a set of formulas is obtained after simplifying the hypothesis. Or the application conditions of the formula are not clearly stated, and even some of the information conveyed in the literature itself is incorrect.

Many power designers do not realize this point, directly apply the formula in the design manual, or take some words out of context in the design manual, respected as "design guidelines", without thorough analysis and thinking, as well as experimental verification.

The result is that the designed high frequency magnetic elements can not 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 prevent power designers from making the same mistakes in the design process, we summarize some conceptual problems encountered in the study and development, hoping to provide a reference for everyone.


Discrimination of Some Wrong Concepts

Five common misconceptions in the design of high frequency magnetic elements of switching power supply are presented in the form of subheadings and analyzed in detail.

High Frequency Magnetic Elements 

Filling Core Windows - Optimized Design

Many power designers think that filling the core window in the design of high frequency magnetic elements can get the optimal design, but it is not. In the design of several high-frequency transformers and inductors, we can find that adding one or more layers of windings or adopting enameled wires with larger wire diameters will not achieve the optimized effect, but will increase the total loss of windings due to the proximity effect in winding.

Therefore, in the design of high-frequency magnetic elements, it does not matter if the winding does not cover the window of the core, but only 25% of the window area. You don't have to try to fill the whole window.

This kind of false concept is mainly influenced by the design of power frequency magnetic elements. In the design of power frequency transformer, the integrity of iron core and winding is emphasized, so there is no gap between the core and the winding. Generally, the winding is designed to fill the whole window, so as to ensure its mechanical stability. However, the design of high-frequency magnetic elements does not have this requirement.

"Iron loss = copper loss" -- Optimized Transformer Design

Many power designers, even in a lot of magnetic elements design reference books have "iron loss = copper loss" as one of the high frequency transformer optimization design standards, in fact, it is not. In the design of high-frequency transformer, the iron loss and copper loss can be greatly different, sometimes the difference between the two can even reach an order of magnitude, but this does not mean that the high-frequency transformer design is not good.

This concept is also influenced by the design of power frequency transformer. Power frequency transformers often take up a large area due to the large number of turns of the windings. Therefore, from the perspective of thermal stability and uniformity, the empirical design rule of "iron loss = copper loss" is obtained.

But for high frequency transformers, very thin enameled wires are used as windings. This rule of thumb does not hold true. In the design of switching power high-frequency transformers, there are many factors to determine the optimal design, and "iron loss = copper loss" is the least concerned aspect.

Leakage Inductance = 1% Magnetization Inductance

Many power supply designers, after designing the magnetic components, , often need to explain the size of the leakage inductance requirements when they submit the relevant technical requirements to the transformer manufacturer. On many technical lists, such technical requirements as "leakage inductance = 1% magnetization inductance" or "leakage inductance < 2% magnetization inductance" are marked. In fact, this kind of writing or design standard is very unprofessional.

The power designer shall set a numerical limit on the acceptable leakage inductance according to the normal working requirements of the circuit. In the process of making the transformer, the leakage inductance should be reduced as much as possible without making other parameters of the transformer (such as capacitance between turns, etc.) worse, instead of giving the proportional relationship between leakage inductance and magnetization as the technical requirement.

Because the relationship between leakage inductance and magnetization varies greatly with the presence and absence of air gap in the transformer. In the absence of air gap, the leakage inductance may be less than 0.1% of the magnetization inductance, while in the presence of air gap, the proportion of leakage inductance and magnetization inductance may reach 10% even though the transformer windings are tightly coupled.

Therefore, the ratio of leakage inductance to magnetization inductance should not be provided to the magnetic element manufacturer as a transformer design indicator. The correct approach is to specify a clearly acceptable absolute value of leakage, plus or minus of course a certain proportion, the typical value of which is 20%.

Leakage Inductance is Related to Core Permeability

Some power designers think that adding magnetic core to windings will make the windings more tightly coupled and reduce the leakage between windings. And some other power designers believe that the magnetic core will be coupled with the field between the winding, which can increase the leakage inductance.

The fact is that in the switching power supply design, the leakage of two coaxial winding transformers is not related to the presence or absence of magnetic cores. This result may be puzzling because a material with a relative permeability of several thousand is close to the coil and has little effect on leakage.

The measured results of several hundred transformer groups show that the leakage inductance change value is not more than 10%, but only about 2%.

The optimal current density of transformer winding is 2A/mm2 ~ 3.1A/mm2

When designing high-frequency magnetic elements, many power designers often regard the current density in the winding as the standard of optimal design.

In fact, the optimal design has nothing to do with the winding current density. What really matters is how much loss there is in the windings and whether the cooling measures are sufficient to keep the temperature rise within the allowable range.

We can consider two limiting cases of heat dissipation measures in a switching power supply. When liquid immersion and vacuum are used respectively for heat dissipation, the corresponding current density in the winding will be greatly different.

In the actual development of switching power supply, we don't care what the current density is, but how hot is the wire package? Is the temperature rise acceptable?

This kind of wrong concept is the restriction imposed by designers to simplify the number of variables to simplify the calculation process in order to avoid tedious trial calculation. However, this simplification does not indicate the application conditions.


Conclusion

In order to make the power designer make less mistakes in the process of power design, some conceptual problems related to the design of high-frequency magnetic elements in the research and development of switching power supply are summarized. Hope it's helpful to you.


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