Detailed explanation of basic knowledge of high-frequency electronic transformers

High-frequency electronic transformers are electronic transformers with high operating frequency, generally higher than 20kHz.

The precise definition is an electronic transformer with a high operating frequency. Generally, the operating frequency is higher than 20kHz, which is high frequency.

The definition of high-frequency electronic transformer is very clear, but it has been misled by some opinions expressed by some domestic experts.In order to return to the true nature of high-frequency electronic transformers, it is necessary to clarify the concepts of transformers, electronic transformers, and high-frequency electronic transformers in order to eliminate those misunderstandings.

First, let’s find out what is a transformer?A transformer that works on the principle of electromagnetic induction means that alternating voltage is applied to the primary winding of the coil to generate alternating magnetic flux, and an output voltage is induced in the secondary winding, thereby transmitting energy, converting voltage (or signal), and electrically insulating The role of isolation.

To produce electromagnetic induction, alternating voltage must be applied to the primary winding. It is impossible to have a transformer with DC voltage as the working power supply.The statement that DC voltage is used as the working power supply is a misunderstanding that the inverter that converts DC into AC, or the variable frequency power supply is included in the scope of the transformer.

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

Regardless of the operating frequency, the transformer transmits energy through electromagnetic induction.The amount of transmitted energy is related to the material, structure, size and operating frequency of the transformer.If the energy transmitted is a constant value, the operating frequency is high, the energy is transmitted more times within a certain period of time, and the energy transmitted each time can be less, then the transformer uses less material and has a smaller structural size.The idea that transformers have limited transmission energy and that high frequency should be used to increase transmission energy is a misunderstanding that puts the cart before the horse.Using 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.It is also a misunderstanding to think that after PWM control, there are differences in the way high-frequency transformers and low-frequency transformers transmit energy, and there are differences in the ways high-frequency transformers and low-frequency transformers change voltage.

Secondly, find out what is an electronic transformer?Electronic transformers are transformers used in electronic circuits and electronic equipment.If the scope is expanded, it includes all electromagnetic components such as transformers, inductors, and mutual inductors used in all electronic circuits and electronic equipment.Electronics are not limited to power electronics (the more common domestic term is power electronics), but also include industrial electronics, information electronics, wireless electronics and microelectronics. Although electronic transformers are different from power transformers, they are not different from radio frequency signal transformers, nor are they only Limited to 'power transformers in switching power converter circuits'.Power transformer is just one type of electronic transformer.If you only regard electronic transformers as power transformers, you will inevitably draw a trap.Therefore, it is a misunderstanding to limit electronics to power electronics and to limit electronic transformers to power transformers.

Again, clarify what is a high frequency electronic transformer?Now, there is a common saying for the classification of high, medium and low working frequencies of electronic transformers, that is, the working frequency of 50Hz or 60Hz is called power frequency, or the one below it is called 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 no audible noise can be heard.Therefore, the operating frequency exceeds 20kHz, from 20kHz to MHz level and GHz level, which are high frequencies.There are two misunderstandings about the 'application frequency range from tens of kHz to several megakHz' electronic transformer: one is 20kHz, which is different from tens of kHz.One is several GHz, not several megakHz (note: please follow the prefix of the specification, do not make up your own).

High frequency can also be divided into higher frequency (20kHz ~ 50kHz), medium and high frequency (50kHz ~ 200kHz), high frequency (200kHz ~ 1MHz), and ultra-high frequency (above 1MHz), but they are all high frequencies and are not suitable for use. The power is different and the understanding of high frequency is different.It is a misunderstanding to say that high frequencies have different ranges under different powers.

the whole frame

In order to adapt to the increasingly lighter, thinner and smaller electronic equipment, one of the main development directions of high-frequency electronic transformers is from three-dimensional structure to planar structure, chip structure, and film structure, thus forming new generations of high-frequency electronic transformers: planar transformers, Chip transformer, film transformer.The development of the overall structure of high-frequency electronic transformers not only forms new core structures and coil structures and uses 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 the best optimized design, we also need to study various problems with multi-layer structures.In terms of production technology, various new processing methods should be studied to ensure the consistency of performance and realize the mechanization and automation of processing technology.

Among MHz-level high-frequency electronic transformers, air-core transformers are used in more and more application fields.Exploring the structure, design methods, manufacturing processes and application characteristics of air-core transformers is also its research and development direction.In addition, the research on high-frequency electronic transformers with new working principles such as piezoelectric transformers is also a development direction. After nearly ten years of research and development, piezoelectric transformers have been practically used in some fields.

Using computers to optimize and specifically design the overall structural plan is one of the main development directions of various electronic devices now, and of course it is also a main development direction of high-frequency electronic transformers.This can shorten design time, reduce material usage, shorten production cycle, and reduce costs.

Core materials and construction

The magnetic core is the most critical component in high-frequency electronic transformers that use soft magnetic materials and work on the principle of electromagnetic induction.The main development directions of magnetic core materials are to reduce losses, widen the temperature range of use and reduce costs.The main development direction of magnetic core structure is how to form planar cores, chip cores and thin film cores with optimal shape and size (for parameters such as electromagnetic performance, heat dissipation, dosage and cost).

Nowadays, various soft magnetic materials are constantly being improved and developed to compete in the high-frequency electronic transformer market.

Soft ferrite is the main core material used in high-frequency electronic transformers. The development direction is to develop new varieties with better performance and new processes to reduce costs.In terms of new material varieties, Japan's TDK Company developed the wide-temperature low-loss material PC95 in 2003. The loss is less than 350mW/cm3 in the temperature range of 25°C to 120°C (under the condition of 100kHz×200mT).The loss is minimum at 80℃, which is 280mW/cm3. At 25℃, Bs is 540mT, and at 100℃, Bs is 420mT.We have also developed PE33, a high temperature and high saturation magnetic density material, with a Curie point Tc>290°C and a Bs of 450mT at 100°C.Under the conditions of 100℃, 100kHz×200mT, Pc ≤ 1100mW/cm3, Japanese FDK Company, German 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 magnetic permeability materials, such as TDK's H5C5 for pulse transformers, with a μi of about 30,000.The anti-electromagnetic interference inductor uses HS10, which has good frequency characteristics and impedance characteristics. It still has high magnetic permeability at 500kHz, although the initial magnetic permeability is not high, only about 10,000.High magnetic permeability and high saturation magnetic density material DN50, Bs is 550mT at 25℃, Bs is 380mT at 100℃, μi is about 5200, Curie temperature Tc≥210℃.

In terms of new processes, self-propagating high-temperature synthesis (SHS) has been a research hotspot in recent years. Its principle is to use the chemical energy inside the reactants to synthesize materials.The entire process is extremely simple, has low energy consumption, high production efficiency and product purity, and has no pollution to the environment. Mg, MgZn, MnZn, and NiZn ferrites have been successfully synthesized and are being industrialized.The spark plasma sintering method (SPS) can successfully produce a multi-layer MnZn ferrite and permalloy composite soft magnetic material core, which has the high frequency and low loss characteristics of MnZn ferrite and the high magnetic permeability of permalloy. With high saturation magnetic density characteristics, this composite soft magnetic material core will significantly improve the performance of high-frequency electronic transformers.Other processes such as auto-combustion synthesis, rapid combustion synthesis, hydrothermal synthesis, new hydrothermal synthesis, mechanical alloy method, microwave sintering, etc., have carried out a lot of research in recent years, 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, in the higher frequency range of 20kHz to 100kHz, the cost-performance advantage is not as obvious as in the high frequency range above 100kHz. Several other soft magnetic materials have higher frequencies in the range of 20kHz to 100kHz. Within the frequency range, it competes fiercely with soft ferrite.Various soft magnetic materials have their own characteristics. Therefore, how to give full play to the advantages of various soft magnetic materials in specific high-frequency electronic transformer products to achieve better performance-price ratio is the soft magnetic material used in high-frequency electronic transformers. The development direction of magnetic materials.

Silicon steel is characterized by 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 silicon steel, 6.5% silicon steel, gradient silicon steel and chromium-containing silicon steel.In particular, chromium-containing silicon steel has been used in 25kHz and 70kHz electronic transformers.The working frequency used by silicon steel has now reached 325kHz.

High magnetic permeability permalloy is characterized by high magnetic permeability and good environmental adaptability, but it is expensive. The ultra-thin permalloy belt developed in recent years has an operating frequency exceeding 1MHz and is used in places with special requirements and military equipment. used in.Cobalt-based amorphous alloy is the material with the lowest high-frequency loss among existing soft magnetic materials. It is expensive. However, when used at high frequencies above 200kHz, the core weight is small and the price factor is not prominent. Currently, it is used at 200kHz and 1MHz. It is widely used in high-frequency electronic transformers.

Soft magnetic composite materials have now become a major development direction for magnetic core materials for high-frequency electronic transformers. Compared with traditional soft magnetic ferrite and soft magnetic alloys, their magnetic metal particles or films can be distributed in non-conductors and other materials. , which significantly reduces high-frequency losses and increases the operating frequency.At the same time, its processing technology can either use hot pressing to process into powder cores, or use current plastic engineering technology to injection mold into complex-shaped magnetic cores, which have low density, light weight, high production efficiency, low cost, and product repeatability. and good consistency.Different ratios can also be used to change the magnetism.Examples of composite materials composed of soft magnetic ferrite and permalloy have been introduced above. Soft magnetic composite material powder cores with operating frequencies above 10kHz have been developed and can replace soft magnetic ferrite 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.In the past, some planar magnetic cores were modified from the original soft ferrite cores. Now there are various low-height soft ferrite cores specially used for planar transformers.Various low-height soft magnetic composite cores may also be developed in the future.In addition to further compressing the planar core, the core of the chip transformer also has chip cores manufactured by the co-firing method.Thin film cores and magnetic materials are currently one of the most active development directions for high-frequency electronic transformers. They will become the main core materials and structures for high-frequency electronic transformers above MHz. It is possible to reduce the height of thin-film electronic transformers to less than 1mm. Loaded into various cards.Several centers have been established in China to conduct vigorous research.Now we hope to unite material development, electronic transformer manufacturing and application units to turn the domestically developed thin-film soft magnetic materials into high-frequency electronic transformer cores in electronic information products as soon as possible, forming a domestic thin-film transformer with independent intellectual property rights. The authors are working hard to make this happen.

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Coil Materials and Construction

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

For high-frequency transformer coils with a three-dimensional structure, the conductor material is multi-stranded wire (Ritz wire) due to consideration of skin effect and proximity effect, and sometimes flat copper wire and copper tape are also used.The insulating material is made of materials with high heat resistance in order to increase the allowable temperature rise and reduce the size of the coil. Double-layer and triple-layer insulated wires are used to reduce the size of the coil.To give an example, recently, domestically developed C-class insulated electromagnetic wires using nanotechnology to coat mica on copper wires. It has been used in industrial frequency motors and transformers and achieved good results. It is estimated that it will also be used in high-frequency electronic transformers. will be applied.

For planar structure coils, the conductors are made of copper foil. Most are made of single-layer and multi-layer printed circuit boards. Some are made of copper foil with a certain pattern and multiple folds.Insulation materials generally use Class B materials.

Thin film structure coil, the conductors are made of copper, silver and gold films, made into comb-shaped, spiral, sports field-shaped and other patterns, and the insulation materials are made of H-class and C-class materials.There are also multi-layer structures, or several multi-layer coils are combined, or several coils and several cores are crisscrossed and overlapped.In short, thin film transformers are high-frequency electronic transformers that are currently being vigorously developed. Many structures are not finalized, and perhaps many new coil structures will appear.