High frequency transformer design principle requirements and procedures
High frequency transformer design principle requirements and procedures
The high-frequency transformer is a power transformer with a working frequency exceeding the intermediate frequency (10 kHz). It is mainly used for high-frequency switching power supply transformers in high-frequency switching power supplies, and also for high-frequency inverters in high-frequency inverter power supplies and high-frequency inverter welding machines. Variable power transformer. According to the working frequency, it can be divided into several grades: 10 kHz to 50 kHz, 50 kHz to 100 kHz, 100 kHz to 500 kHz, 500 kHz to 1 MHz, and 1 MHz or more. The transmission power is relatively large, the operating frequency is relatively low; the transmission power is relatively small, and the operating frequency is relatively high. In this way, there are differences in operating frequency and transmission power. The design method of the power transformer with different operating frequencies is different, and it should be self-evident.
Starting from the design principle, four design requirements can be proposed for high-frequency transformers: use conditions, complete functions, improve efficiency, and reduce costs.
1, the conditions of use
Electromagnetic compatibility means that the high-frequency transformer neither generates electromagnetic interference to the outside world and can withstand external electromagnetic interference. Electromagnetic interference includes audible audible noise and unspeakable high frequency noise. One of the main causes of electromagnetic interference from high-frequency transformers is the magnetostriction of the magnetic core. The soft magnetic material with large magnetostriction produces large electromagnetic interference. For example, the MnZn soft ferrite has a magnetostriction coefficient λS of 21×10-6, which is more than 7 times that of the oriented silicon steel, and is 20 times or more higher than that of the high magnetic permalloy and the amorphous alloy. More than 10 times the alloy. Therefore, the electromagnetic interference generated by the manganese-zinc soft ferrite core is large.
The main cause of electromagnetic interference in high-frequency transformers is the suction between the cores and the repulsive force between the winding wires. The frequency of these forces varies with the operating frequency of the high frequency power transformer. Therefore, a high-frequency transformer operating at a frequency of about 100 kHz has no special reason for not generating audio noise below 20 kHz. Since the audio noise frequency of a single-chip switching power supply below 10W is proposed, it is about 10 kHz to 20 kHz, which must be the reason.
Since the noise spectrum is not drawn, the specific reason is not clear, but it is unlikely to be generated by the high-frequency power transformer itself. It is not necessary to use a glass bead glue to bond the core. As for this bonding process, the audio noise can be reduced by 5dB. Please give examples and data and a detailed explanation of the cause of the noise, which will be credible.
Shielding is a good way to prevent electromagnetic interference and increase the electromagnetic compatibility of high-frequency transformers. However, in order to prevent the electromagnetic interference propagation of high-frequency transformers, corresponding measures should be taken in designing the core structure and designing the winding structure. It is not necessarily the best solution to add the outer shielding tape, because it can only prevent radiation interference, and cannot Block conducted interference.
2, complete the function
There are three High frequency transformer completion functions: power transfer, voltage conversion and insulation isolation. There are two ways to transfer power. The first type is the transmission mode of the transformer power. The voltage applied to the primary winding generates a magnetic flux change in the magnetic core, so that the secondary winding induces a voltage, so that the electric power is transmitted from the primary side to the secondary side. In the power transmission process, the magnetic core is divided into two working modes: magnetic flux single direction change and double direction change. In the unidirectional change mode of operation, the magnetic flux density changes from a maximum value Bm to a residual magnetic flux density Br, or from Br to Bm. The magnetic flux density change value ΔB = Bm - Br.
In order to increase ΔB, it is desirable that Bm is large, and the Br mode changes the working mode magnetic flux from +Bm to -Bm, or from -Bm to +Bm. The magnetic flux density change value ΔB=2Bm, in order to increase ΔB, it is desirable that Bm is large, but Br is not required to be small, whether it is a unidirectional change operation mode or a bidirectional change operation mode, the transformer power transmission mode is not directly related to the magnetic core permeability. related. The second type is the inductor power transmission mode. The electric energy input from the original winding causes the magnetic core to be excited and becomes magnetic energy storage. Then, the demagnetization causes the secondary winding to induce a voltage and turn it into electric energy to be released to the load. The transmission power is determined by the energy storage of the inductor core, which in turn is determined by the inductance of the original winding. The inductance is related to the magnetic permeability of the magnetic core, the magnetic permeability is high, the inductance is large, and the energy storage is large, and is not directly related to the magnetic flux density. Although the power transfer method is different and the required core parameters are different, in the high-frequency transformer design, the choice of the material and parameters of the magnetic core is still a main content of the design.
In the article "Design Essentials" of power transformers, it is regrettable to lack this main content. Just in the "AC loss" section, the typical BAC value is 0.04 ~ 0.075T. Obviously, the high-frequency transformer in this paper uses the inductive power transmission method. Why not mention the magnetic permeability, and the BAC is unclear. After reviewing, in the "Power Technology Application" 2003 1/2 issue, the "main points of design" of the switching power supply written by the same main author, listed "the choice of magnetic core", did not mention the magnetic permeability, but raised the maximum The magnetic flux density Bm is 0.275T. Since there is no waveform for changing the magnetic flux density, it is unclear whether the BAC in the previous paragraph and the Bm in the following are consistent: Why is the difference between BAC and Bm 6.8 to 3.7 times? What is more unclear, which soft ferrite material is selected? Why choose this model? There is no explanation in the two articles, so I have to let the readers guess.
The voltage conversion is done by the turns ratio of the primary and secondary windings. Regardless of the power transmission method, the voltage conversion ratio of the primary side and the secondary side is equal to the turns ratio of the primary winding and the secondary winding, and the voltage conversion is not affected as long as the turns ratio is not changed. However, the number of winding turns is related to the leakage inductance of the high frequency power transformer. The leakage inductance is proportional to the square of the turns of the original winding. Interestingly, can the leakage inductance specify a value? The two articles published in the sixth issue of Power Technology Applications in 2003 have different opinions. The article "Design Essentials" states: "For a high-frequency transformer that meets insulation and safety standards, the leakage inductance should be 1% to 3% of the primary inductance when the secondary is open." "Discrimination" said: "On many technical sheets, the magnetizing inductance or leakage inductance marked with leakage inductance = 1%.
The above is the design principle requirements and procedures for high frequency transformers.