Practical application and specific parameters of planar transformer
High-frequency, high-power density power conversion modules are widely used and developed in power electronics. To increase the power density of the converter, the key is to reduce the size and weight of the magnetic components. On the one hand, considering the traditional electromagnetism theory, for a certain coil window area and core cross-sectional area, the length of the coil loop and the core loop is required to be short for the Zui superior structure, so as to reduce the total core volume and the coil. On the other hand, from the perspective of thermal design theory, Zui greatly increases the heat dissipation surface area of ​​the magnetic component, and reduces the thermal resistance from the hot spot of the magnetic member to the surface area of ​​the magnetic member, thereby increasing the power density. Pain Relief Patch(Pain Areas)
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The transformer structure is undergoing three updates. *The second is a planar transformer, which is 80% smaller than the three-dimensional transformer (common transformer). It has been formed from 5W to 20KW, 20KHZ to 2MHZ, and the typical efficiency is 98%. The second time is a chip transformer, which is especially suitable for low voltage and high current. The height (thickness) is further reduced. The current can reach more than 100 amps. It is composed of multiple cores with one secondary winding instead of the previous one. . The primary windings of multiple cores are connected in series to achieve the step-down isolation requirements. The internal temperature rise is lower than that of the planar transformer, only about 10 °C. It can be mounted on a substrate with a higher rated temperature rise. The third time is a thin film transformer with a film height of less than 1 mm. The working frequency exceeds 1MHZ and reaches 10~100MHZ. The cost does not increase due to the fabrication using integrated circuit processes. It is the new development direction of the DC switching power supply transformer. The reason why "positive experience" is emphasized is that at this stage, different application ranges and markets, from the performance price ratio, require different transformer structures. Three-dimensional transformers are still used in large quantities. Planar transformers have been formed into series and are being promoted. Chip transformers are in the individual and small batch production stages. Thin film transformers are only a few cases and are still in the research and development stage.
It can be seen that the ferrite planar transformer will play an extremely important role in the future power conversion module, especially in the larger power module.
2. Structural principle Planar transformers usually have two or more cylindrical cores of the same size. Now take a planar transformer with two cores as an example to introduce its structure, as shown in Figure 1. The two corners of each magnetic core column are connected by copper skin on the diagonal, and the copper skin is in close contact with the inner wall of the magnetic core when passing through the magnetic core column. Two magnetic cores are placed side by side, and the adjacent two corners are welded with copper skin. The copper skins on the two outer sides of one magnetic core are welded together with a piece of copper. This is the planar transformer secondary coil. At the center, if the tap is drawn here, it is the center tap of the secondary coil; the copper bumps at the two corners on the outer side of the other core are the ends of the secondary coil of the planar transformer. This basically constitutes the main part of a planar transformer. Its secondary coil is only one turn and can have a center tap. A complete planar transformer also has a preset energy storage inductor. One end is often connected to the center tap. There is a fixed copper plate on the top and bottom. They sandwich the core and the filter inductor, and serve as a rectified power supply. Two poles and a heat sink.
It can be seen that the planar transformer is composed of a copper lead frame and a flat continuous copper spiral instead of a magnetic copper wire wound on a conventional ferrite core, which is a medium coated with copper foil. The material sheets are etched and then stacked on a flat high-frequency ferrite core to form a transformer magnetic circuit. Then, the core material is bonded with a small particle size epoxy resin so that the core loss is small, and the high temperature (130) insulating material in the spiral laminate ensures high insulation between the windings.
2.1 Manufacturing method 2.1.1 Winding type This winding method is the same as that of the conventional transformer, and is suitable for the manufacture of high frequency and high voltage transformers.
2.1.2 Foil-type foil winding Folding planar transformer Firstly, copper foil is used as the winding, and then folded into a multi-layer coil, and the coil is wound by high-frequency stranded wire. In this way, it is suitable for making low-voltage, high-current planar transformers with low leakage inductance.
2.1.3 Multilayer Printed Circuit Board This method uses a manufacturing process of a printed board to form a spiral coil on a multilayer board using a precision thin copper sheet or a plurality of planar copper windings etched on the insulating sheet. It is especially suitable for making high-frequency, high-voltage medium and small power planar transformers.
2.2 Characteristics Table 1 compares the characteristics of conventional transformers, piezoelectric ceramic transformers and planar transformers. The physical and electrical characteristics of the planar transformer are described below [1].
2.3 Physical characteristics Planar transformers have a small size, usually between 0.325 inches and 0.750 inches, which is quite attractive for applications where the internal space of the power supply is severely limited.
The planar transformer printed circuit board structure means that once the circuit board components are set as planar devices, the transformer windings in the subsequent production process should have exactly the same pitch. Therefore, it is allowed to be produced by automatic assembly equipment, which can greatly improve the repeatability and reliability of each device, and avoid the irregularity and instability caused by manual winding of conventional transformers.
In summary, planar transformers have good consistency due to the mechanical processing of the multilayer manufacturing process; reproducibility due to the geometry of the windings and their associated parasitic properties being limited to PCB manufacturing tolerances; due to the high energy density, suitable for It is assembled by surface mount method and has miniaturization characteristics. In addition, the performance consistency and predictability of planar transformers make them easier to model than conventional transformers, which is especially suitable for modeling with computer-aided engineering tools (eg SPICE).
2.4 Electrical characteristics The eddy current effect is the edge current effect caused by the alternating magnetic field of adjacent conductors. The skin effect is a phenomenon in which an induced current such as an induced magnetic field generates a current in a round wire, which concentrates on the outer surface of the wire, especially At higher frequencies, eddy current effects and skin effect are particularly pronounced. As a result, the total current carrying area is smaller than the entire conductor area, making the AC impedance greater than the DC impedance, reducing the effective conduction performance, thereby making the utilization of the round wire winding wound around the ferrite core in the conventional transformer underutilized. . However, the winding of the planar transformer is a copper foil layer etched on the printed circuit board. Although the current concentrates on the outer layer of the copper foil layer due to the skin effect, since the copper foil layer is thin, the current actually flows through almost The entire wire can achieve higher efficiency and smaller volume than conventional transformers. When the operating frequency of the transformer is higher than 300 kHz, the thickness of the copper foil layer is equal to the skin thickness, which also avoids the extra loss caused by the stray current.
The planar transformer structure makes the parasitic reactance (capacitance between the windings and the leakage inductance) small, usually less than 0.5% of the primary inductance. The low leakage inductance is achieved by a separation measure in which one part of the primary winding is placed on top of the stack and the other part is placed on the bottom of the stack, and then the secondary winding is evenly sandwiched on both sides of the stack. The low stray capacitance and leakage inductance of the planar transformer are beneficial for reducing the high frequency transient disturbance of the transformer output voltage. The use of a conductive circuit on the dielectric sheet, this structure can also achieve good electrical insulation between the primary and secondary and secondary and secondary of the planar transformer, the transformer can be applied to a wide range of input voltage, and can be pressed It is required to give one, two or three outputs that also meet or exceed the performance requirements of the offline converter.
In short, the planar transformer has high frequency (1MHz), high efficiency (98% to 99%), low loss, low leakage inductance and other electrical characteristics due to its flat winding; because the conductive circuit and the insulating sheet overlap, it has good Insulation (up to 4KV insulation isolation between primary and secondary). In addition, the planar transformer also has a wide operating temperature range (-40 ~ 130), high current density (up to 200A per layer winding) and high power (single device power up to 5 ~ 25KW).
3. Points to Note 3.1 Parallel Winding Problems Today, planar transformers are widely used in low-voltage, high-current, ultra-thin DC/DC modules. As the output voltage is getting lower and lower, and the output current is getting higher and higher, a parallel multilayer structure is often used to reduce the winding loss. However, there is a current distribution imbalance phenomenon in the parallel winding layer [2], which results in a greatly reduced effect of the parallel winding layer. The main cause of this unbalanced current sharing is the leakage flux of the loop formed by the parallel layers, which in turn depends on the winding distribution and the spatial distance between the parallel layers.
The factors that affect the current sharing are:
(1) Frequency: The higher the frequency, the larger the unbalance current per parallel layer, resulting in a large circulating current, thereby increasing the AC winding loss;
(2) Winding distribution: The winding distribution not only affects the AC impedance and the leakage inductance of the transformer, but also greatly affects the current sharing between the parallel layers. The method of inserting windings using a symmetrical spacer (PSSPPSSP) allows the parallel layers of the primary and secondary windings to be evenly distributed, greatly reducing the AC impedance and thus reducing AC losses. Compared with the method of inserting windings with asymmetric interlayers (PSPSPSPS), the AC loss is low in a certain frequency range, and the critical frequency depends on the thickness of the copper sheet and the winding distribution;
(3) Parallel layer spatial distance: reducing the spatial distance can significantly reduce the number of leakage fluxes, but it also inevitably increases the parasitic capacitance of the winding and the coiled capacitance between the primary and secondary windings. Therefore, the parallel layer space distance should be compromised.
In summary, the main factors affecting the current sharing current and AC winding losses are three aspects: operating frequency, winding distribution and insulator thickness. Generally, the distribution method of the primary winding sandwiched between the primary windings can effectively balance the current sharing current, thereby reducing the AC impedance. However, the method of inserting the symmetrical spacer into the winding can effectively solve the current current imbalance in the critical frequency.
3.2 Iron core zui miniaturization design problem 3.2.1 Core loss model The iron loss of the transformer is mainly caused by hysteresis and eddy current effect, and the hysteresis loss is generally considered to be caused by the magnetic domain motion and friction of the magnetic material. The hysteresis loss is proportional to the frequency, and the eddy current loss is proportional to the square of the frequency. The magnetic loss power density per unit volume is:
Where k is the loss factor, B is the peak-to-peak value of the magnetic induction, f is the alternating frequency of the magnetic field, and k, m, n are related to the characteristics of the magnetic material, which can be derived from the loss curve given by the supplier of the magnetic material.
3.2.2 Winding loss model In high frequency applications, in order to reduce copper loss and increase current capacity, the winding conductors usually use flat copper sheets, and each layer has only one turn of conductor, so that the current can be distributed along the width of the conductor, reducing The loss due to the skin effect is also beneficial to reduce the overall height of the transformer. If the influence of the connection points of the conductors of each layer is neglected, for a winding with a number of turns N, the DC resistance is:
Where tw, dw are the thickness of the conductor and the gap between the winding and the core, respectively.
Due to the high frequency effect, the resistance of the winding will increase significantly. The AC resistance of the winding can be expressed as: RΩ=FrRd, where Fr is the ratio of the AC to the DC resistance, which is related to the geometry and arrangement of the core and the winding. Based on Dowell's calculation model for transformer winding AC resistance [3], it can be seen that when the original negative-side windings are arranged separately, the values ​​are:
among them:
δ is the skin thickness at a frequency f, and N is the number of winding layers from the zero leakage magnetic field.
When the transformer is used in a switching power supply, the current waveform flowing through the winding is not a sine wave and contains higher harmonics, so it is not enough to consider only the influence of the fundamental wave. A suitable approach should be to first obtain the harmonic components of the current waveform and then determine the winding losses of the corresponding current harmonic components.
For a cyclically varying winding current, the total winding loss model is:
Among them, the effective value of the nth harmonic component of the winding current and the frequency are the alternating current resistance of the winding.
3.2.3 iron core zui design [4]
The mathematical model of the small core volume of zui is:
Among them: the effective volume of the magnetic core, the saturation magnetic induction of the magnetic material and the rated transformer efficiency, respectively, the effective cross-sectional area of ​​the magnetic core and the magnetic path length.
3.3 How to reduce costs? The following aspects should be considered:
3.3.1. Design: The raw materials and structural forms used are decisive for cost.
3.3.2. Process: Use tooling and machining as much as possible.
3.3.3. Reduce the additional cost of production: including the selection of raw materials and accessories for the most common use, reduce the variety and reduce inventory, and shorten the delivery time as soon as possible.
3.4 Principles of Use There are three main principles for the use of planar transformers:
3.4.1. Select the corresponding type of planar transformer according to the output voltage;
3.4.2. Determine the number of planar transformers connected in parallel according to the magnitude of the output current;
3.4.3. Determine the ratio and the number of turns of the primary coil according to the magnitude of the input and output voltage.
In addition, in practical applications, it is also necessary to know the ratio of the planar transformer. The ratio can also be calculated by the following formula:
Where K is the coefficient, K = 0.5 when the output of the planar transformer is through the center tap; K = 1 when the planar transformer has no center tap. N is the number of planar transformer units connected in parallel; P is the number of primary turns of the planar transformer.
4. The application of planar transformers has been widely used in communications, notebook computers, automotive electronics, digital cameras and digital TVs for more than 10 years. For example, a DC-DC converter with a power range of 5 to 60 W made of a planar transformer has been applied to a plug-in board power supply for a telecommunication system. Due to the special electrical and mechanical environment in the car, more stringent requirements are placed on the transformer design and process. Planar transformers are used in DC-DC converters for Xenon arc lamp ballasts and have been used in mid-range cars. Secondly, the planar transformer for broadband transmission applications also shows good development prospects. In addition, the product range of planar transformers has involved various aspects of conventional ferrite core transformers, such as power transformers, bandwidth transformers and impedance matching converters. Due to its good consistency and small size, it is especially suitable for use in electronic equipment where energy saving and heat dissipation are required in a small internal space. In areas where defense, aerospace, aerospace and other requirements for weight and stability are extremely high, the application of planar transformers will also open up a new situation for the miniaturization of the system.
In short, miniaturized, planarized inductive ferrite components will be more interesting for people. It is believed that in some high-tech fields, planar transformers will soon replace traditional transformers and gradually realize large-scale production.
5. Summary The development of micro-transformers is the demand of today's electronics and information technology. The miniaturization of transformers is an inevitable trend in the development of transformer technology. At present, the planar transformer with ferrite as the core has small volume and high power density, which is the mainstream of the current miniature transformer. Thin film transformers with micro-manufacturing technology are in the development stage, and the actual application is still an individual case. With the rapid development of electronic technology, ferrite planar transformers will still play a major role in the power supply of larger power modules.
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