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How to design a high-efficiency high-frequency transformer?
Apr 21, 2018


            How to design a high-efficiency high-frequency transformer?

High-frequency transformers are energy storage and transmission in switching power supplies. A high-efficiency high-frequency transformer should have low DC loss, low AC loss, small leakage inductance, distributed capacitance of the winding itself, and small coupling capacitance of each winding.


The DC loss of the high frequency transformer is caused by the copper loss of the coil. In order to increase the efficiency, the thicker wire should be selected as much as possible, and the current density J=4~10A/mm2 should be taken.


AC losses in high-frequency transformers are caused by the skin effect of high-frequency currents and the loss of the magnetic core. High-frequency currents tend to flow from the surface when passing through the wire, which reduces the effective flow area of the wire and makes the AC equivalent impedance of the wire much higher than the copper resistance. The penetrating ability of the high-frequency current to the conductor is inversely proportional to the square root of the switching frequency. To reduce the AC copper resistance, the radius of the conductor must not exceed twice the reachable depth of the high-frequency current. The curve of the available wire diameter and switching frequency is shown in Figure 1. For example, when f=100kHz, the diameter of the wire may theoretically be φ0.4mm. But to reduce the skin effect, finer wires are stranded and wound without a thick wire.


When designing high-frequency transformers, leakage inductance is minimized. The larger the leakage inductance is, the higher the peak voltage amplitude is, and the greater the loss of the drain clamp circuit, this will inevitably reduce the power efficiency. A high frequency transformer conforming to insulation and safety standards should have a leakage inductance of 1% to 3% of the primary inductance of the secondary open circuit.

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