Design Principle of High Frequency Transformer

Label:High Frequency Transformer, Design Principle

Sep 5, 20235700

Design Principle of High Frequency Transformer

When designing the high-frequency transformer, the leakage inductance and distributed capacitance must be minimized, because the transformer in the switching power supply transmits a high-frequency pulse square wave signal. During the transient process of transmission, leakage inductance and distributed capacitance will cause surge current and peak voltage, as well as top oscillation, resulting in increased loss. Usually the leakage inductance of the transformer is controlled as 1% to 3% of the primary inductance.


When you design high-frequency transformers, have you understood the design principles, requirements and procedures of high-frequency transformers? Have you ever thought about why the high-frequency transformer is designed in this way, and what are the benefits of this design? What are its related design procedures? These are very helpful for a deep understanding of high frequency transformers.


High-frequency transformer is a power transformer whose operating frequency exceeds intermediate frequency (10kHz). It is mainly used as a high-frequency switching power transformer in high-frequency switching power supplies, and is also used as a high-frequency inverter in high-frequency inverter power supplies and high-frequency inverter welding machines. According to the working frequency, it can be divided into several grades: 10kHz-50kHz, 50kHz-100kHz, 100kHz-500kHz, 500kHz-1MHz, and above 1MHz.


If the transmission power is large, the operating frequency is low. If the transmission power is small, the operating frequency is high. In this way, there are both differences in operating frequency and differences in transmission power. It should be self-evident that the design methods of power transformers with different grades of operating frequency are different.


Starting from design principles, four design requirements can be put forward for high-frequency transformers: use conditions, complete functions, improve efficiency, and reduce costs.


Use conditions


Electromagnetic compatibility means that the high-frequency transformer neither produces electromagnetic interference to the outside world, but also can withstand the electromagnetic interference from the outside world. Electromagnetic interference includes audible audible noise and inaudible high-frequency noise.


One of the main causes of electromagnetic interference in high-frequency transformers is the magnetostriction of the magnetic core. Soft magnetic materials with large magnetostriction will generate large electromagnetic interference. For example, manganese zinc soft magnetic ferrite is more than 20 times that of high magnetic permeability permalloy and amorphous alloy, and more than 10 times that of microcrystalline nanocrystalline alloy. Therefore, the electromagnetic interference generated by the manganese zinc soft ferrite core is large.


The main causes of electromagnetic interference in high-frequency transformers are the attraction between the magnetic cores and the repulsion between the winding wires. These forces vary at a frequency consistent with the operating frequency of the high-frequency power transformer. Therefore, a high-frequency transformer with an operating frequency of about 100kHz will not produce audio noise below 20kHz without special reasons. Since it is proposed that the audio noise frequency of a single-chip switching power supply below 10W is about 10kHz to 20kHz, there must be a reason.


Since the noise spectrum diagram is not drawn, the specific reason is not clear, but it is unlikely to be generated by the high-frequency power transformer itself. There is no need to use glass bead glue to bond the magnetic core. As for the reduction of audio noise by 5dB by using this bonding process.


Shielding is a good way to prevent electromagnetic interference and increase the electromagnetic compatibility. However, in order to prevent the electromagnetic interference propagation of high-frequency transformers, corresponding measures should also be taken in the design of the magnetic core structure and the design of the winding structure. It is not necessarily the best solution to add an outer shielding tape, because it can only prevent radiation interference and cannot Prevent conducted interference.


Complete functions


The high-frequency transformer has three functions: power transmission, voltage conversion and insulation isolation. There are two ways of power delivery. The first one is the power transmission method of the transformer. The voltage applied to the primary winding produces a change in magnetic flux in the magnetic core, which induces a voltage in the secondary winding, so that the electric power is transmitted from the primary side to the secondary side.


In the process of power transmission, the magnetic core is divided into two working modes: one-way change of magnetic flux and two-way change of magnetic flux. Single-direction changing working mode, the magnetic flux density changes from the maximum value Bm to the residual magnetic flux density Br, or changes from Br to Bm. The magnetic flux density change value is ΔB=Bm-Br.


In order to improve ΔB, it is hoped that Bm is large, and the magnetic flux of the Br-school bidirectional change working mode changes from +Bm to -Bm, or from -Bm to +Bm. Magnetic flux density change value ΔB=2Bm, in order to increase ΔB, Bm is expected to be large, but Br is not required to be small. Whether it is a single-direction change operation mode or a two-way change operation mode, the power transmission mode of the transformer is not directly related to the magnetic core permeability. related.


The second one is the power transmission method of the inductor. The electric energy input by the primary winding excites the magnetic core, which is stored as magnetic energy. Then the auxiliary winding induces a voltage through demagnetization, which becomes electric energy and is released to the load. The transmitted power depends on the energy storage of the inductor magnetic core, which in turn depends on the inductance of the primary winding.


Inductance is related to the magnetic permeability of the magnetic core. High magnetic permeability, large inductance, and large energy storage are not directly related to the magnetic flux density. Although the power transmission methods are different and the required magnetic core parameters are different, in the design of high-frequency transformers, the selection of magnetic core materials and parameters is still a main content of the design.


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