20khz frequency transformer and its development direction

       Shenzhen CXWON Technology Co., Ltd. has long been dedicated to the development, production, and sales of various types of power transformers. High frequency power transformers and their development direction have recently become a focus of attention in the power transformer industry. A professional technical magazine (hereinafter referred to as the magazine) has published a "special feature" on this issue, gathering the opinions of several professionals. The author also writes an article to discuss with the readers. If there are any errors, please criticize and correct them.

1. High frequency power transformer

What is a high-frequency power transformer? There is a precise definition, which is a power transformer with a high operating frequency, generally above 20kHz, which is considered high frequency. But after reading the opinions of several professionals published in the magazine, I got confused about the originally clear concept of high-frequency power transformers. As one of the professionals said, "No one in the industry has accurately defined it, nor has anyone done academic integration, so there is still no accurate definition. In fact, the definition of high-frequency power transformer is very clear, but it has been misled by some opinions published in magazines. In order to reveal the true nature of high-frequency power transformers, it is necessary to clarify the concepts of transformers, power transformers, and high-frequency electronic transformers in order to eliminate those misunderstandings.

Firstly, clarify what a transformer is? A transformer that operates on the principle of electromagnetic induction refers to a transformer that generates alternating magnetic flux by applying alternating voltage to the primary winding of the coil, and induces output voltage in the secondary winding, thereby transmitting energy, transforming voltage (or signal), and providing electrical insulation and isolation.

To generate electromagnetic induction, an alternating voltage must be applied to the primary winding, and there cannot be a transformer with a direct current voltage as the working power source. The statement that uses DC voltage as the working power source is a misunderstanding that includes DC to AC inverters or variable frequency power sources within the scope of transformers.

As long as there is electromagnetic induction, the transformer can work without necessarily having a magnetic core. For example, an electronic transformer with a working frequency of MHz is a hollow transformer made of a printed circuit board. The statement that high-frequency power transformers are "magnetic transformer components used in variable frequency circuits" is a double misunderstanding that includes variable frequency circuits within the scope of transformers and believes that transformers must have magnetic cores.

Transformers transmit energy through electromagnetic induction regardless of their operating frequency. The amount of energy transmitted is related to the materials, structure, size, and operating frequency used in the transformer. If the transmitted energy is a constant value, the operating frequency is high, and the number of times energy is transmitted within a certain period of time is high, and the amount of energy transmitted each time can be reduced, then the materials used for the transformer are less and the structural size is smaller. The notion that transformers transmit limited energy and require high frequencies to increase transmission energy is a misconception that puts the cart before the horse. Using pulse width modulation (PWM) to change the energy and voltage transmitted by transformers is just an external control method that can be used not only for high-frequency transformers, but also for low-frequency transformers. The notion that there are differences in the way high-frequency transformers and low-frequency transformers transmit energy and change voltage after PWM control is also a misconception.

Secondly, clarify what a power transformer is? Electronic transformers are transformers used in electronic circuits and power equipment. If the scope is expanded to include all electromagnetic components such as transformers, inductors, and transformers used in electronic circuits and power equipment. Electronics is not limited to power electronics (the more commonly used term in China is power supply), but also includes industrial power supply, information power supply, wireless power supply, and micro power supply. Although power transformers are different from power transformers, they are not different from RF signal transformers, nor are they limited to "power transformers in switch power converter circuits". A power transformer is just one type of power transformer. If we only focus on the success rate of power transformers, it is inevitable to draw a solid picture. Isn't the content about inductors published by a professional in a magazine feature denying oneself? So, limiting power sources to power sources and limiting power transformers to power transformers is a misconception.

Secondly, clarify what a power transformer is? Electronic transformers are transformers used in electronic circuits and power equipment. If the scope is expanded to include all electromagnetic components such as transformers, inductors, and transformers used in electronic circuits and power equipment. Electronics is not limited to power electronics (the more commonly used term in China is power supply), but also includes industrial power supply, information power supply, wireless power supply, and micro power supply. Although power transformers are different from power transformers, they are not different from RF signal transformers, nor are they limited to "power transformers in switch power converter circuits". A power transformer is just one type of power transformer. If we only focus on the success rate of power transformers, it is inevitable to draw a solid picture. Isn't the content about inductors published by a professional in a magazine feature denying oneself? So, limiting power sources to power sources and limiting power transformers to power transformers is a misconception.

Furthermore, clarify what a high-frequency power transformer is? Nowadays, there is a common saying for dividing the working frequency of power transformers into high, medium, and low frequencies, that is, the working frequency of 50Hz or 60Hz is called the power frequency, or the frequency below it is called the low frequency; 60Hz to 20kHz is called intermediate frequency, and 400Hz is intermediate frequency, not power frequency; Above 20kHz is called high frequency. Why choose 20kHz as the limit? Because 20kHz is the upper limit of audio frequency, beyond which audible noise cannot be heard. So, when the operating frequency exceeds 20kHz, the range from 20kHz to MHz and GHz is considered high frequency. There are two misunderstandings about the term "electronic transformers with application frequency range from tens of kHz to several megakHz": one is 20kHz, which is different from tens of kHz. One is a few GHz, not a few megakHz. The power transformer in the switching power conversion circuit with a working frequency of several GHz has not been reported by the author at home and abroad. The author hopes that the magazine can indicate its source.

High frequency can also be divided into higher frequency (20kHz~50kHz), medium high frequency (50kHz~200kHz), high frequency (200kHz~1MHz), and ultra-high frequency (above 1MHz), but they all belong to high frequency and do not have different understandings of high frequency due to different applicable powers. The statement that there are different ranges of high frequency at different powers is a misunderstanding.

2. The development direction of high-frequency power transformers and the efforts made by CXWON Technology Company

The biggest feature of high-frequency power transformers is high-frequency conversion. From the perspective of the working principle of transformers, increasing the operating frequency can reduce the volume and weight of transformers, achieving miniaturization and lightweight, thereby increasing the transmission power per unit volume (or weight), that is, high power density. These are the inherent characteristics and direct results of high-frequency power transformers themselves, and cannot be simply regarded as the development direction of high-frequency electronic transformers, such as high-frequency, short, lightweight, and high power density. Isn't it too easy to propose a few slogans that can provide great guidance for the electronic transformer industry? A series of problems should be brought to power transformers by high-frequency conversion. By solving these problems, performance can be improved and costs can be reduced, that is, the pursuit of the best performance price ratio should be taken as the starting point. A more detailed development direction should be proposed, which may have certain guiding significance for the industry. In the special feature published in the magazine, some professionals also expressed specific opinions, but unfortunately some did not provide further explanation, and some only focused on "output inductors", which is neither in-depth nor comprehensive. Shenzhen Chengxinwang Technology Co., Ltd. has made a more in-depth and comprehensive exposition on the development direction of high-frequency power transformers.

Below, the author provides some suggestions on the development direction of high-frequency power transformers from several aspects, including the overall structure, magnetic core materials and structures, and coil materials and structures, for readers' reference.

2.1 Overall Structure

In order to adapt to the increasingly lightweight and compact electronic devices, a major development direction for high-frequency power transformers is to shift from three-dimensional structures to planar, sheet, and thin film structures, thus forming generation after generation of new high-frequency power transformers: planar transformers, sheet transformers, and thin film transformers. The development of the overall structure of high-frequency power transformers not only forms new magnetic core structures and coil structures, using new materials, but also brings new development directions to design and production processes. In terms of design, in addition to studying the electromagnetic field distribution of various new structures and how to achieve optimal optimization design, it is also necessary to study various problems of multi-layer structures. In terms of production technology, it is necessary to study various new processing methods to ensure consistency in performance and achieve mechanization and automation of processing technology.

In MHz level high-frequency electronic transformers, more and more application areas are using hollow transformers. Exploring the structure, design methods, manufacturing processes, and application characteristics of hollow transformers is also a research and development direction. In addition, research on high-frequency power transformers with new working principles such as piezoelectric transformers is also a development direction. After nearly a decade of research and development, Shenzhen Chengxinwang Technology Co., Ltd. has found that piezoelectric transformers have been practically applied in some fields.

Optimizing and designing the overall structural scheme using computers is one of the main development directions for various electronic devices, and of course, it is also a major development direction for high-frequency power transformers. This can shorten design time, reduce material usage, shorten production cycles, and lower costs.

2.2 Magnetic Core Materials and Structure

The magnetic core is the most critical component in high-frequency power transformers that use soft magnetic materials and operate based on electromagnetic induction principles. The main development direction of magnetic core materials is to reduce losses, broaden the temperature range of use, and reduce costs. The main development direction of magnetic core structures is how to form planar magnetic cores, chip magnetic cores, and thin film magnetic cores with the best shape and size (parameters such as electromagnetic performance, heat dissipation, dosage, and cost).

Various soft magnetic materials are constantly being improved and developed to compete in the market for high-frequency power transformers.

Soft magnetic ferrite is currently the main magnetic core material used in high-frequency power transformers, and the development direction is to develop new varieties with better performance and new processes to reduce costs. In terms of new material varieties, TDK Corporation in Japan developed the wide temperature and low loss material PC95 in 2003, with a loss of less than 350mW/cm3 in the temperature range of 25 ℃ to 120 ℃ (under 100kHz × 200mT conditions). The minimum loss is 280mW/cm3 at 80 ℃. At 25 ℃, Bs is 540mT, and at 100 ℃, Bs is 420mT. High temperature and high saturation magnetic density material PE33 has also been developed, with a Curie point Tc>290 ℃ and Bs of 450mT at 100 ℃. Under the conditions of 100kHz × 200mT at 100 ℃, Pc≤1100mW/cm3， Japan's FDK company, Germany's EPCOS company, and Ferrocube company have also developed similar high-temperature and high saturation magnetic density materials.

There are also many new varieties of high permeability materials, such as H5C5 for TDK's pulse transformer, with a μ i of around 30000. The anti electromagnetic interference inductor uses HS10, which has good frequency and impedance characteristics. It still has a high magnetic permeability at 500kHz, although the initial magnetic permeability is not high, only about 10000. The high permeability and high saturation magnetic density material DN50 has a Bs of 550mT at 25 ℃ and 380mT at 100 ℃, with a μ i of about 5200 and a Curie temperature Tc ≥ 210 ℃.

In terms of new processes, self propagating high-temperature synthesis (SHS) has been a research hotspot in recent years, which utilizes the chemical energy inside the reactants to synthesize materials. The entire process is extremely simple, with low energy consumption, high production efficiency and product purity, and no pollution to the environment. Mg, MgZn, MnZn, and NiZn ferrites have been successfully synthesized and are being industrialized. Spark plasma sintering (SPS) method can successfully produce multi-layer MnZn ferrite and perovskite composite soft magnetic material cores, which have both the high-frequency low loss characteristics of MnZn ferrite and the high permeability and high saturation magnetic density characteristics of perovskite. This composite soft magnetic material core will significantly improve the performance of high-frequency power transformers. In recent years, Shenzhen Chengxinwang Technology Co., Ltd. has conducted extensive research on other processes such as self combustion synthesis, rapid combustion synthesis, hydrothermal synthesis, new hydrothermal synthesis, mechanical alloy synthesis, microwave sintering, etc., all of which are in line with the development direction of improving performance and reducing costs.

Due to the low saturation magnetic density of soft magnetic ferrite, its cost-effectiveness advantage is not as significant in the high-frequency range of 20kHz to 100kHz as in the high-frequency range above 100kHz. Other soft magnetic materials compete fiercely with soft magnetic ferrite in the high-frequency range of 20kHz to 100kHz. Various soft magnetic materials have their own characteristics. Therefore, how to fully utilize the advantages of various soft magnetic materials in specific high-frequency power transformer products to achieve better performance price ratio is the development direction of soft magnetic materials used in high-frequency power transformers.

The characteristics of silicon steel are high saturation magnetic density, stable performance, and low price. In recent years, a series of high-frequency silicon steels have been developed, including ultra-thin strip silicon steel, 6.5% silicon steel, gradient silicon steel, and chromium containing silicon steel. Especially chromium containing silicon steel has been used in electronic transformers at 25kHz and 70kHz. Shenzhen Chengxinwang Technology Co., Ltd. currently uses a working frequency of 325kHz for silicon steel.

The characteristics of high magnetic permeability PoMo alloy are high magnetic permeability and good environmental adaptability, but it is expensive. In recent years, the development of ultra-thin PoMo alloy strips has been used in areas with special requirements and military equipment with operating frequencies exceeding 1MHz.

Cobalt based amorphous alloy is the material with the lowest high-frequency loss among existing soft magnetic materials. It is expensive, but when used in high frequencies above 200kHz, the magnetic core weight is small and the price factor is not prominent. Currently, it is widely used in high-frequency power transformers at 200kHz and 1MHz.

Soft magnetic composite materials have become a major development direction for magnetic core materials used in high-frequency power transformers. Compared with traditional soft magnetic ferrite and soft magnetic alloys, their magnetic metal particles or films can be distributed in non-conductive and other materials, significantly reducing high-frequency losses and increasing operating frequencies. At the same time, its processing technology can be used to process powder cores by hot pressing, or injection molded into complex shaped magnetic cores using current plastic engineering technology. It has the characteristics of low density, light weight, high production efficiency, low cost, good product repeatability and consistency. Different ratios can also be used to change the magnetic properties. The example of composite materials composed of soft magnetic ferrite and Permalloy has been introduced above. Shenzhen Chengxinwang Technology Co., Ltd. has now developed a soft magnetic composite material powder core with a working frequency of over 10kHz, which can replace soft magnetic ferrite in high-frequency filtering inductors.

According to the development requirements of the overall structure of high-frequency power transformers, the development direction of magnetic core structures is flat magnetic cores, chip magnetic cores, and thin film magnetic cores. Previously, some flat magnetic cores were modified with original soft ferrite cores, but now there are various low height soft ferrite cores specifically used for flat transformers. In the future, various low height soft magnetic composite material magnetic cores may also be developed. In addition to further compressing the flat magnetic core, there are also chip magnetic cores manufactured by co firing method for the magnetic core of chip transformers. Thin film magnetic cores and magnetic materials are currently one of the most active development directions of Chengxinwang Technology Company, and will become the main magnetic core materials and structures for high-frequency power transformers above MHz. It is possible to achieve a height of less than 1mm for thin film power transformers, which can be loaded into various cards. Shenzhen Chengxinwang Technology Co., Ltd. has established several centers to vigorously research. Now we hope to unite material development, power transformer manufacturing, and application units to quickly transform domestically developed thin film soft magnetic materials into high-frequency power transformer cores in electronic information products, forming domestically independent intellectual property rights thin film transformers. Chengxinwang Technology is working hard to promote this work.

2.3 Coil Material and Structure

With the development of the overall structure of high-frequency power transformers, the main development directions of coil structures are planar coils, sheet coils, and thin film coils, including multi-layer structures. There are also some new developments in the material selection for various coil structures.

The high-frequency transformer coil with a three-dimensional structure uses multi stranded wire (Ritz wire) and sometimes flat copper wire and copper strip due to the consideration of skin effect and proximity effect. The insulation material is made of materials with high heat resistance to increase the allowable temperature rise and reduce the volume of the coil. Double layer and three-layer insulated wires are used to reduce the size of the coil. For example, recently, China has developed a C-grade insulated electromagnetic wire that uses nanotechnology to coat mica on copper wire, which has been applied in power frequency motors and transformers with good results. It is estimated that it will also be applied in high-frequency power transformers.

Flat structure coils and wires are made of copper foil. Most of them are manufactured using single-layer and multi-layer printed circuit boards, but there are also copper foils with certain patterns that are folded into multiple pieces. Insulation materials generally use B-grade materials.

The thin film structure coil is made of copper, silver, and gold thin films, and the wires are made into comb shaped, spiral shaped, and sports field shaped shapes. The insulation material is made of H-grade and C-grade materials. There are also those that use multi-layer structures, or combine several multi-layer coils, or overlap several coils and magnetic cores. In short, thin-film transformers are high-frequency power transformers that are currently being vigorously developed, and many structures are not yet standardized. Perhaps, many new coil structures will also emerge.

3. Conclusion

The technical personnel of Shenzhen Chengxinwang Technology Co., Ltd. have analyzed the concepts of transformers, power transformers, high-frequency electronic transformers, and low-frequency transformers step by step, hoping to clarify some misunderstandings. From the aspects of overall structure, magnetic core material and structure, coil material and structure, the development direction of high-frequency power transformers is specified, hoping to provide readers with some reference.