High frequency electronic transformer and its development direction

Abstract: The concept of transformer, electronic transformer and high-frequency electronic transformer is analyzed in detail, and the development direction of high-frequency electronic transformer is embodied in several aspects such as the overall structure, magnetic core and coil.

0 Introduction The high-frequency electronic transformer and its development direction have recently become a focus of attention in the electronic transformer industry. A professional technical magazine (hereinafter referred to as the magazine) has published a “special feature feature” on this issue, bringing together a number of professionals. Some views. The author also wrote an article and discussed it with the reader. If there are any errors, please criticize and correct me.

1 High Frequency Electronic Transformer What is a high frequency electronic transformer? An accurate definition is that an electronic transformer with a high operating frequency has a higher operating frequency than 20 kHz. However, after reading the opinions of several professionals published by the magazine, I confused the concept of the original high-frequency electronic transformer. For example, one of the professionals said: "No one in the industry has defined precisely, and no one has done academic integration. Therefore, there is no accurate definition so far." In fact, the definition of high-frequency electronic transformers is very clear, but it was misled by some of the opinions published in the magazine. In order to restore the original features of high-frequency electronic transformers, it is necessary to clarify the concept of transformers, electronic transformers, and high-frequency electronic transformers in order to eliminate those misunderstandings.

First of all, what is a transformer? The transformer working on the principle of electromagnetic induction means that an alternating voltage is applied to the primary winding of the coil to generate an alternating magnetic flux, and an output voltage is induced at the secondary winding, thereby transmitting energy, transforming a voltage (or signal), and electrically insulating. The role of isolation.

To generate electromagnetic induction, the primary winding must be subjected to alternating voltage, and it is not possible to have a DC voltage as the transformer for the operating power supply. The idea of ​​using a DC voltage as a working power source is a misconception that DC-to-AC inverters, or variable-frequency power sources, are included in the scope of transformers.

As long as there is electromagnetic induction, the transformer can work without having a magnetic core. For example, an electronic transformer operating at a MHz level is a hollow transformer made from a printed circuit board. The claim that the high-frequency electronic transformer is "a magnetic transformer used in a frequency conversion circuit" is a dual misunderstanding that the frequency conversion circuit is included in the scope of the transformer and that the transformer must have a magnetic core.

Regardless of the operating frequency, transformers transmit energy through electromagnetic induction. The size of the transmitted energy depends on the material, structure, size, and operating frequency of the transformer. If the transmitted energy is a fixed value, the operating frequency is high, the number of energy transmissions in a certain period of time is large, and the energy for each transmission may be small, the material used for the transformer is small, and the structure size is small. The argument that transformers have limited transmission energy and use high frequencies to increase transmission energy is a misunderstanding of the cart before the horse. The use of pulse width modulation (PWM) to change the transmission energy and voltage of the transformer is just an external control method. Not only high frequency transformers can be used, but low frequency transformers can also be used. After the PWM control is considered, there is a difference in the energy transmission method between the high-frequency transformer and the low-frequency transformer, and there is a difference in the way the high-frequency transformer and the low-frequency transformer change the voltage. This is also a misunderstanding.

Second, find out what is an electronic transformer? Electronic transformers are transformers used in electronic circuits and electronic devices. If you expand the scope, include all the electromagnetic components such as transformers, inductors, and transformers used in electronic circuits and electronic devices. Electronics is not limited to power electronics (the more common terms in China are power electronics), but also includes industrial electronics, information electronics, wireless electronics, and microelectronics. Although electronic transformers are distinguished from power transformers, they are not different from radio frequency signal transformers. Limited to "power transformer in switching power converter circuit". The power transformer is just one of the electronic transformers. If we look at electronic transformers only as power transformers, we will inevitably put them in prison. In the magazine feature feature, there is a professional published about the inductor. Doesn't he deny himself? Therefore, it is a misunderstanding to limit electrons to power electronics and limit electronic transformers to power transformers.

Second, find out what is a high-frequency electronic transformer? Nowadays, there is a prevailing view of the high, medium, and low divisions of the operating frequency of electronic transformers. The operating frequency is 50Hz or 60Hz called the industrial frequency, or below it is called the low frequency; 60Hz to 20kHz is called the intermediate frequency, 400Hz is the 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, beyond which you can't hear the audible noise. Therefore, the operating frequency exceeds 20 kHz, from 20 kHz to MHz, and GHz is a high frequency. There are two misconceptions about the use of an electronic transformer with an application frequency ranging from tens of kHz to several mega kHz: one is 20 kHz, which is different from several tens of kHz. One is a few GHz, not a few mega kHz. (Author's Note: Please use the prefix of the specification and do not make it yourself.) The author of the power transformer in the switching power conversion circuit with a working frequency of several GHz has not seen reports from home and abroad and hopes that the magazine can identify its source.

High frequency can also be divided into higher frequency (20kHz ~ 50kHz), high frequency (50kHz ~ 200kHz), high frequency (200kHz ~ 1MHz), UHF (above 1MHz), but all belong to high frequency, not because of the application The power is different, and there are different understandings of high frequencies. It is a misunderstanding that there are different ranges of high frequencies for different powers.

2 The development direction of high-frequency electronic transformers The most important feature of high-frequency electronic transformers is high frequency. From the working principle of the transformer, increasing the operating frequency can reduce the volume and weight of the transformer, that is, achieving a short and thin, thereby increasing the transmission power per unit volume (or weight), that is, high power density. These are the inherent characteristics of the high-frequency electronic transformer itself and the direct result, and can not simply be high-frequency, short and thin, high power density, as the development direction of high-frequency electronic transformers. If you mention a few "chemical" slogans, you can "have great guiding significance for the electronic transformer industry." Isn't it easy? A series of problems should be brought from the high frequency to the electronic transformer. By solving these problems, the performance can be reduced and the cost can be reduced. That is to say, the pursuit of the best performance-cost ratio is the starting point, and a more detailed development direction is required before the industry can be certain. The guiding significance. In the magazine's featured feature, some professionals also made some specific comments. Unfortunately, some did not give any further explanations, and some focused only on “output inductors,” that is, they were not in-depth and comprehensive, and they hoped that the magazine could make in-depth comparisons. A comprehensive explanation of the development direction of high-frequency electronic transformers.

The following authors put forward some suggestions on the direction of development from the aspects of the overall structure of the high-frequency electronic transformer, core material and structure, coil material and structure, for the reader's reference.

2.1 Overall Structure In order to adapt to the increasing thinness and shortness of electronic devices, a major development direction of high-frequency electronic transformers has evolved from a three-dimensional structure to a planar structure, a chip structure, and a thin-film structure, thereby forming a new high-frequency electronic transformer from generation to generation: Planar transformers, chip transformers, thin film transformers. The development of the overall structure of the high-frequency electronic transformer not only forms a new magnetic core structure and coil structure, but also adopts new materials, and it also brings new directions of development to design aspects and production processes. In terms of design, in addition to studying the distribution of electromagnetic fields of various new structures, how to achieve optimal optimization design, we must also study various problems of multi-layer structures. In the production process, various new processing methods should be studied to ensure the consistency of performance and to realize the mechanization and automation of the processing technology.

In MHz-class high-frequency electronic transformers, more and more applications use hollow transformers. The structure, design method, manufacturing process and application characteristics of the hollow transformer are also the research and development direction. In addition, the research of high-frequency electronic transformers such as piezoelectric transformers is also a development direction. After nearly a decade of research and development, piezoelectric transformers have been practically applied in some fields.

The use of computers for optimization and specific design of the overall structure scheme is one of the major development directions of various electronic devices. Of course, it is also a major development direction of high-frequency electronic transformers. This can shorten design time, reduce material usage, shorten production cycles, and reduce costs.

2.2 Core material and structure Cores are the most critical components in high-frequency electronic transformers that use soft magnetic materials and operate on the principle of electromagnetic induction. The main development direction of magnetic core materials is to reduce losses, widen the temperature range used and reduce costs. The main development direction of the magnetic core structure is how to form planar magnetic cores, chip magnetic cores, and thin film magnetic cores with the best shape and size (parameters for electromagnetic performance, heat dissipation, usage, and cost).

Nowadays, all kinds of soft magnetic materials are constantly being improved and developed to compete with the market of high-frequency electronic transformers.

Soft ferrites are the main core materials used in high-frequency electronic transformers. The development direction is to develop new varieties with better performance and new processes that reduce costs. In terms of new material varieties, Japan's TDK Corporation developed a wide-temperature, low-loss material PC95 in 2003, with losses of less than 350mW/cm3 (at 100kHz x 200mT) in the 25°C to 120°C temperature range. The loss was the lowest at 280mW/cm3 at 80°C, Bs was 540mT at 25°C, and Bs was 420mT at 100°C. Also developed a high temperature and high saturation magnetic dense material PE33, Curie point Tc> 290 °C, at 100 °C, Bs is 450mT. Under conditions of 100°C and 100kHz×200mT, Pc≤1100mW/cm3, FDK Corporation of Japan, EPCOS Corporation of Germany, and Ferrocube Company also developed similar high-temperature and high-saturation magnetically dense materials.

High permeability materials also have many new varieties, such as TDK's pulse transformer H5C5, μi is about 30,000. Anti-electromagnetic interference inductor HS10, while having good frequency characteristics and impedance characteristics, still has a high permeability at 500kHz, although the initial permeability is not high, only about 10,000. High permeability high saturation magnetic dense material DN50, Bs is 550mT at 25°C, Bs is 380mT at 100°C, μi is about 5200, Curie temperature Tc≥210°C.

In the new process, self-propagating high-temperature synthesis (SHS) is a research hotspot in recent years. The principle is to use the chemical energy inside the reactants to synthesize the material. The entire process is extremely simple, low energy consumption, high production efficiency and product purity, no pollution to the environment, has been successfully synthesized Mg, MgZn, MnZn, NiZn ferrite, is to achieve industrialization. Spark Plasma Sintering (SPS) can successfully fabricate multi-layer magnetic cores of MnZn ferrite and permalloy composite soft magnetic materials. It also has high-frequency low-loss characteristics of MnZn ferrites and high permeability of permalloy. With high saturated magnetic density, this composite soft-magnetic material core will significantly improve the performance of high-frequency electronic transformers. Other processes such as self-combustion synthesis, rapid combustion synthesis, hydrothermal synthesis, novel hydrothermal synthesis, mechanical alloying, microwave sintering, etc., have all undergone extensive research in recent years, all in line with the development direction of improving performance and reducing costs. .

Due to the low saturation magnetic flux density of soft ferrites, in the higher frequency range of 20kHz to 100kHz, the advantage of cost performance is not as obvious as the high frequency range above 100kHz, and other soft magnetic materials are higher in the range of 20kHz to 100kHz. In the frequency range, fierce competition with ferrite occurs. All kinds of soft magnetic materials have their own characteristics. Therefore, how to make full use of the advantages of various soft magnetic materials in specific high-frequency electronic transformer products in order to achieve a better performance-cost ratio is the use of high-frequency electronic transformers soft The development direction of magnetic materials.

Silicon steel is characterized by high saturation magnetic density, stable performance, and low price. In recent years, it has developed a series of high-frequency silicon steels, including ultra-thin silicon steel, 6.5% silicon steel, gradient silicon steel, and chromium-containing silicon steel. In particular, chromium-containing silicon steels have been used in electronic transformers at 25 kHz and 70 kHz. Now the operating frequency of silicon steel has reached 325kHz.

High-permeability permalloy is characterized by high magnetic permeability and good environmental adaptability, but it is expensive. In recent years, permalloy ultra-thin belts have been developed, and the operating frequency has exceeded 1 MHz. The special requirements of the place and military equipment Used in.

Cobalt-based amorphous alloys are the most expensive material for high-frequency loss in existing soft magnetic materials, and they are expensive. However, they are used at high frequencies above 200 kHz. The weight of the magnetic core is small, and the price factor is not prominent. It is currently at 200 kHz and 1 MHz. High frequency electronic transformers are used in large quantities.

Soft magnetic composite materials have now become a major development direction for magnetic core materials for high-frequency electronic transformers. Compared with conventional soft ferrites and soft magnetic alloys, magnetic metal particles or thin films can be distributed on non-conductors and other materials. In this way, the high-frequency losses are significantly reduced and the operating frequency is increased. At the same time, its processing technology can be processed into a powder core by hot-pressing, or it can be injected into a complex-shaped magnetic core using current plastic engineering technology. It has a small density, light weight, high production efficiency, low cost, and product reproducibility. With good consistency and other characteristics. Different proportions can also be used to change the magnetic properties. Examples of composite materials consisting of soft ferrite and permalloy have been introduced above. Soft magnetic composite powder cores with an operating frequency of 10 kHz or more have been developed, and soft magnetic ferrites can be replaced in high-frequency filter inductors.

According to the development requirements of the overall structure of high-frequency electronic transformers, the development direction of magnetic core structures is planar magnetic cores, chip magnetic cores, and thin-film magnetic cores. Previously, planar magnetic cores were modified with the original soft magnetic ferrite core. Now there are various low-profile ferrite magnetic cores that are specially used for planar transformers. In the future, it may also develop a variety of low-grade soft magnetic composite cores. In addition to the planar magnetic core, the magnetic core of the chip transformer has a chip core manufactured by the co-firing method. Thin-film magnetic cores and magnetic materials are currently one of the most active development directions of high-frequency electronic transformers, and will become the main magnetic core materials and structures of high-frequency electronic transformers above MHz, and it is possible to reduce the height of thin film electronic transformers to 1 mm or less. Load in various cards. Several centers in China have been vigorously researched. Now it is hoped that the material development, electronic transformer manufacturing and application units will be united to transform the thin film soft magnetic material developed in China into a high frequency electronic transformer magnetic core in electronic information products as soon as possible to form a domestic thin film transformer with independent intellectual property rights. The author is working hard to promote this work.

2.3 Coil material and structure With the development of the overall structure of high-frequency electronic transformers, the main development direction of the coil structure is planar coils, chip coils and thin film coils, which in turn include multilayer structures. There are also some new developments in the selection of materials for various coil structures.

The three-dimensional structure of the high-frequency transformer coils, wire materials due to the skin effect and proximity effect, the use of multiple strands (Ritz wire), sometimes using flat copper wire and copper tape. Insulation materials are made of high heat-resistant grades to increase the allowable temperature rise and reduce the coil volume. Double and triple insulated conductors can be used to reduce coil size. As an example, recently, domestically developed C-level insulated magnet wires coated with mica swimming on copper wires using nanotechnology have been applied in industrial frequency motors and transformers, and have achieved good results, and are estimated to be also in high-frequency electronic transformers. Will be applied.

Plane structure coils, the use of copper wire, most of the use of single-layer and multi-layer printed circuit board manufacturing, but also use a certain pattern of copper foil, a plurality of folded. Insulation materials generally use Class B materials.

Film structure coils, copper, silver, and gold films are used for the wires to form patterns such as combs, spirals, and field shapes. Insulation materials use H and C grade materials. There are also multi-layer structures, either a combination of several multilayer coils, or several overlapping coils and several cores. In short, thin-film transformers are high-frequency electronic transformers that are being vigorously developed. Many structures are not fixed, and perhaps many new coil structures will emerge.

3 Conclusions The concept of transformers, electronic transformers, and high-frequency electronic transformers was analyzed step by step, hoping to clarify some misunderstandings. From the aspects of the overall structure, core material and structure, coil material and structure, the development direction of high-frequency electronic transformers is embodied, hoping to provide readers with some references.

As the only magnetic element professional journal that integrates market information and technology in the Asia-Pacific region, it proposes to discuss the development direction of high-frequency electronic transformers. It should play a guiding role in the electronic transformer industry, but it must be appropriate to publish various opinions. Edits and choices to avoid unnecessary misunderstandings. If you make some additional instructions, it's even better. This requirement may be higher. It is regrettable that mistakes in the basic concepts of "mega kHz" power transformers and "hysteresis coefficient" appear in professional magazines.