Introduction to high frequency transformer design

High frequency probe high frequency line can be customized other specifications

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The high-frequency chain inverter technology replaces the bulky power frequency transformer in the traditional inverter with a high-frequency transformer, which greatly reduces the size and weight of the inverter. In the hardware circuit design of the high frequency chain, the high frequency transformer is an important part.

Designing a high frequency transformer should start with the core. The magnetic core of the switching power supply transformer is mostly a soft magnetic material used in a low magnetic field, which has high magnetic permeability, low coercive force and high electrical resistivity. The magnetic permeability is high, and when a certain number of turns of the coil is used, a high applied voltage can be withstood by a small exciting current, so that the core volume can be reduced under the requirement of outputting a certain power. The magnetic core has low coercive force and small magnetic hysteresis area, and the iron consumption is also small. The high resistivity makes the eddy current small and the iron consumption small. The ferrite material is a composite oxide sintered body, which has a high electrical resistivity and is suitable for use at a high frequency, but the Bs value is relatively small and is often used in a switching power supply.

The design of high-frequency transformers usually adopts two methods [3]: the first is to find the product AP of the core window area AW and the effective core cross-sectional area Ae (AP=AW×Ae, called the core area product). According to the AP value, look up the table to find the number of the required magnetic material; the second is to first find the geometric parameters, look up the table to find the core number, and then design.

note:

1) In design, at the maximum output power, the magnetic induction in the core should not be saturated, so as to avoid distortion in large signals.

2) During the transient process, the leakage inductance and distributed capacitance of the high-frequency chain will cause surge current and spike voltage and pulse top oscillation, which will increase the loss and cause damage to the switch tube in severe cases. At the same time, when the number of turns of the output winding is large and the number of layers is large, the influence of the distributed capacitance should be considered, and reducing the distributed capacitance is beneficial to suppress the interference of the high-frequency signal on the load. It is difficult to reduce the distributed capacitance and leakage inductance of the same transformer at the same time. The proper capacitance and inductance should be ensured according to different working requirements.

Design points of single-chip switching power supply high-frequency transformers High-frequency transformers are the core components of monolithic switching power supplies. In view of the special design of such high-frequency transformers, the method of reducing their losses and suppressing audio noise is specifically described. Available for high frequency transformer designers.

The monolithic switching power supply integrated circuit has the advantages of high integration, high cost performance, minimal peripheral circuit, and best performance index, and can form an isolated switching power supply with high efficiency and no power frequency transformer. In 1994-2001, various series of single-chip switching power supplies such as TOtch, TOtch-II, TOtch-FX, TOtch-GX, Tinht, and Tintch-II were introduced internationally, and now they have become medium and small power switches. Preferred integrated circuits for power supplies, precision switching power supplies, and switching power supply modules.

The high-frequency transformer is an important component for energy storage and transmission in the switching power supply. The performance of the high-frequency transformer in the single-chip switching power supply not only has a great influence on the power efficiency, but also directly relates to other technical specifications of the power supply and electromagnetic compatibility. Sex (EMC). To this end, a high-efficiency high-frequency transformer should have conditions such as low DC loss and low AC loss, low leakage inductance, distributed capacitance of the winding itself, and small coupling capacitance between the windings.

The DC loss of a high frequency transformer is caused by the copper loss of the coil. In order to improve efficiency, we should try to choose a thicker wire and take the current density J=4~10A/mm2.

The AC loss of a high-frequency transformer is caused by the skin effect of high-frequency current and the loss of the core. High-frequency currents tend to flow through the surface as they pass through the wire, which reduces the effective flow area of ​​the wire and makes the wire's AC equivalent impedance much higher than the copper resistance. The penetration capability of the high-frequency current to the conductor is inversely proportional to the square root of the switching frequency. To reduce the impedance of the AC copper, the radius of the wire must not exceed twice the depth of the high-frequency current. The relationship between the available wire diameter and the switching frequency is shown in Figure 1. For example, when f = 100 kHz, the wire diameter can theoretically be φ 0.4 mm. However, in order to reduce the skin effect, it is practical to use a thinner wire to wrap around without a thick wire.

The leakage inductance must be minimized when designing high frequency transformers. Because the larger the leakage inductance, the higher the peak voltage amplitude generated, the greater the loss of the drain clamp circuit, which inevitably leads to a decrease in power supply efficiency. For a high frequency transformer that meets the insulation and safety standards, the leakage inductance should be 1% to 3% of the primary inductance at the secondary open circuit. In order to achieve the index below 1%, it will be difficult to achieve in the manufacturing process. The following measures can be taken to reduce leakage inductance:

Decreasing the number of turns NP of the primary winding;
Increase the width of the winding (for example, select the EE core to increase the width b of the skeleton);
Increase the height to width ratio of the windings;
Reducing the insulation between the windings;
Increase the degree of coupling between the windings.
Design method of high-frequency transformer of power supply Designing high-frequency transformer is a difficult point in the design process of power supply. The following is a design method of high-frequency transformer of power supply with the feedback current-discontinuous power supply high-frequency transformer as an example.

Design goal: The power input AC voltage is between 180V~260V, the frequency is 50Hz, the output voltage is DC 5V, 14A, the power is 70W, and the power supply working frequency is 30KHz.

Design steps:

1. Calculate the primary peak current Ipp of the high frequency transformer

2, find the minimum duty cycle coefficient Dmin

3. Calculate the primary inductance value of the high frequency transformer Lp

4. Calculate the product Aw*Ae of the winding area Aw and the core effective area Ae, and select the core size.

5, calculate the air gap length Lg

6, calculate the transformer primary coil Np

7, calculate the transformer secondary coil Ns

High-frequency transformer: rectification and transformation In the design of traditional high-frequency transformers, due to the limitation of magnetic core materials, the operating frequency is low, generally around 20 kHz. With the continuous development of power supply technology, the miniaturization, high frequency and high power ratio of power supply systems have become an eternal research direction and development trend. Therefore, the study of the use of higher frequency power transformers is a key factor in reducing the size of the power system and increasing the power output ratio.

As the most important component of the switching power supply, the high-frequency transformer has the following advantages over the traditional power frequency transformer: the high-frequency transformer made of ferrite material has the characteristics of high conversion efficiency and small size; and the traditional power frequency transformer Working at 50Hz, the output of the same power requires a large cross-sectional area, resulting in a large transformer, which is not conducive to the miniaturization of the power supply, and the power conversion efficiency is lower than the switching power supply.

The switching power supply used in the computer generally adopts a half-bridge power conversion circuit. When working, two switching transistors are turned on to generate a high-frequency pulse wave of 100 kHz, and then stepped down by a high-frequency transformer to output a low-voltage alternating current. In this circuit, the maximum current of the switch tube has a certain limit on the output power of the power supply (usually the MOS tube used for the 300W power supply is bulky, and some power supplies even use a switch tube with a current resistance of 10A), and high. The ratio of the turns of each winding of the frequency transformer determines the output voltage. Because of the high frequency, the requirements for component quality and the matching of the lines are very high.

Suppressing the audio noise of the high-frequency transformer The attractive force between the high-frequency transformer EE or the EI-type core can cause the two cores to be displaced; the gravitational or repulsive force between the winding currents can also cause the coil to shift. In addition, it can cause periodic deformation when subjected to mechanical vibration. All of the above factors will cause the high frequency transformer to emit audible noise during operation. The audio noise frequency of a single-chip switching power supply below 10W is about 10 kHz to 20 kHz.

In order to prevent relative displacement between the cores, the epoxy resin is usually used as a glue to bond the three contact faces of the two cores (including the center column). However, the effect of this rigid connection is not ideal. Because this does not minimize audible noise, and the glue is too much, the core is easily broken when subjected to mechanical stress. Recently, a special "glass beads" adhesive has been used abroad to bond ferrite cores such as EE and EI, which is very effective. The glue is a mixture of glass beads and a binder in a ratio of 1:9, which is cured by being placed in a temperature environment of 100 ° C or more for 1 hour. Its role is similar to that of ball bearings. After curing, each core can still be deformed or displaced independently in a small range, and the overall position is unchanged, which inhibits the deformation. The inside of a high frequency transformer bonded with a glass bead glue. This process reduces audible noise by 5dB.