High efficiency is a common requirement for power supplies and electronic devices. Improving the working efficiency of high frequency transformers can save power. It also has double social and economic benefits of environmental protection. Therefore, improving efficiency is a major design requirement.

Generally speaking, the working efficiency should be increased to more than 95%, and the loss should be reduced to less than 5%. There will be core loss (iron loss) and winding loss (copper loss) in the high frequency transformer.

You may care about the ratio of iron loss and copper loss of transformer. The value varies with the operating frequency of the transformer. If the applied voltage of the transformer remains the same, the lower the operating frequency, the more winding turns and the greater the copper loss. Therefore, at a power frequency of 50 Hz, the copper loss far exceeds the iron loss.

For example: 50Hz, 100kVAS9 type three-phase oil-immersed silicon steel power transformer, the copper loss is about 5 times of the iron loss. 50Hz, 100kVASH11 three-phase oil-immersed amorphous alloy power transformer, the copper loss is about 20 times 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.

As for iron loss, it includes hysteresis loss and eddy current loss. The loss increases rapidly as the operating frequency increases. In a certain operating frequency, the copper loss may be equal to the iron loss. What if it is beyond this operating frequency? The iron loss is greater than the copper loss. That is the reason why the iron loss is not equal to the copper loss.

Although the selection of wire thickness is affected by the surface effect, it is mainly determined by the transmission power of the high frequency transformer. There is no direct relationship with the operating frequency.

Moreover, the selection of very thin enameled wires as windings may increase copper loss and delay the downward trend of copper loss. In the condition of 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 high frequency transformers is around 100 kHz, and the iron loss is greater than the copper loss, which has become the main part of the loss.

Iron loss is the real problem. The selection of the magnetic core material according to the iron loss is a main content of the high frequency transformer design. Iron loss has also become a major parameter for evaluating soft magnetic core materials.

The iron loss is related to the working frequency of the magnetic flux density of the core. When talking about 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. The unit of the working magnetic flux density B is T (Tesla), and the unit of the working frequency f is Hz. For example, P0.5/400 represents the loss when the working magnetic flux density is 0.5T and the working frequency is 400 Hz. For another example, P0.1/100k represents the loss when the working magnetic flux density is 0.1T and the working frequency is 100 kHz.

Iron loss is also related to the working temperature. When introducing the iron loss of the soft magnetic core material, the working temperature must be specified. It is worth mentioning the soft ferrite material, which is sensitive to temperature changes. Iron loss in the range of 25 °C to 100 °C should be listed in the product manual.

The saturation magnetic flux density of soft magnetic materials does not fully represent the upper limit of the working magnetic flux density used. It is often the iron loss that limits the upper limit of the working magnetic flux density.

Therefore, in the new classification standard for soft ferrite materials for power transformers, the product of the allowable operating magnetic flux density and operating frequency B×f is used as the performance factor of the material. The allowable loss value under the condition of the performance factor is explained.

The new classification standard divides soft ferrite materials into five categories according to performance factors: PW1, PW2, PW3, PW4, and PW5. The higher the performance factor, the higher the operating frequency and the higher the limit frequency.

For example, for PW3 type soft ferrite materials, the operating frequency is 100 kHz, the limit frequency is 300 kHz. The performance factor B×f is 10000mT×kHz, that is, at 100 mT (0.1T) and 100 kHz, the loss a level at 100°C is less than 300 kW/ m (300 mW/cm3), class b≤150 kW/m3 (150 mW/cm3).

At a certain operating frequency, when the winding loss (copper loss) of the high-frequency power transformer is close to the iron loss, for example, within the range of copper loss/iron loss=100% to 25%, the copper loss can‘t be ignored. Just 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 equal to the copper loss of the secondary winding in the design, in order to simplify the design calculation process.

It is not possible to design high frequency transformers only based on temperature rise. It is not easy to determine thermal resistance accurately. The design calculation is quite troublesome. Therefore, in order to simplify calculations, it is sometimes necessary to pre-recommend some principles and data based on experience.

In the same way, to simplify the calculation, it is also necessary to recommend different winding current densities for high frequency transformers with different operating frequencies and powers, but not limited to a certain current density value, for example, 2A/mm2 - 3A/mm2. It should be noted that the method to realize the design requirements of high-frequency power transformer is not limited to one.