Tips for Design: Improving Working Efficiency of High Frequency Transformers

Label:High Frequency Transformers, Working Efficiency

Aug 1, 20236000

Tips for Design: Improving Working Efficiency of High Frequency Transformers

Improving working efficiency is a common requirement for power supplies and electronic devices. Improving the efficiency of high-frequency power transformers can save electricity. It also has double social and economic benefits of environmental protection. Therefore, improving working efficiency is a major design requirement for high frequency transformers. Generally, the value should be increased to more than 95%, and the loss should be reduced to less than 5%. There are two common issues, core loss (iron loss) and winding loss (copper loss).


You may be concerned with the ratio of iron loss to copper loss. They vary with the operating frequency. If the applied voltage remains the same, the lower the operating frequency, the more winding turns and the greater the copper loss. Therefore, at a power frequency of 50Hz, the copper loss far exceeds the iron loss.

As the operating frequency increases, the number of winding turns decreases. Although the winding loss increases due to the existence of the surface effect and the proximity effect, the general trend is that the copper loss decreases as the operating frequency increases. Iron loss, including hysteresis loss and eddy current loss, increases rapidly as the operating frequency increases.


In a certain operating frequency, copper loss may be equal to iron loss. And iron loss is greater than the copper loss beyond this operating frequency. Why is the iron loss not equal to the copper loss? Although the choice of wire thickness is affected by the surface effect, it is mainly determined by transmission power, and has no direct relationship with the operating frequency.


Moreover, the selection of very thin enameled wires as windings will increase copper loss and delay the downward trend of copper loss. Maybe at the operating frequency selected by the design, there may be a situation where the copper loss is equal to the iron loss. The working frequency of small and medium power transformers is around 100 kHz. The iron loss is already greater than the copper loss, which has become the main part of the component loss.


Just because iron loss is the main part of the loss of high frequency power transformer, the selection of the magnetic core material according to the iron loss is a main content of the high-frequency power transformer design. Iron loss has also become a major parameter for evaluating soft magnetic core materials. This loss is related to the working frequency of the magnetic flux density of the core.


When introducing the iron loss of the soft magnetic core material, it is necessary to explain the loss at what working magnetic flux density and at what working frequency. When expressed in symbols, PB/f must also be indicated [where the unit of the working magnetic flux density B is T (Tesla), and the unit of the working frequency f is Hz.


The iron loss is also related to the working temperature. When talking about the iron loss of the soft magnetic core material, the working temperature must be specified, especially the soft ferrite material, which is sensitive to temperature changes. The saturation magnetic flux density of soft magnetic materials does not fully represent the upper limit of the working magnetic flux density. It is often the iron loss that limits the upper limit of the working magnetic flux density.


At a certain operating frequency, when the winding loss (copper loss) is close to the iron loss. For example, within the range of copper loss/iron loss = 100% to 25%, the copper loss cannot be ignored and should also be considered to take steps to reduce copper loss.


Since the power borne by the primary winding and the secondary winding is similar, the copper loss of the primary winding is often taken to be equal to the copper loss of the secondary winding in the design. This simplifies the design calculation process, and the transformer cannot be designed only based on the temperature rise. Due to thermal resistance, it is not easy to determine accurately. And the design calculation is quite troublesome.


Therefore, it is sometimes necessary to pre-recommend some principles and data based on experience to simplify calculations. Similarly, it is also necessary to recommend different winding current densities for high-frequency power transformers with different operating frequencies and powers, but it is not limited to a certain current density value.


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