The broad understanding of magnetic devices for network communication refers to the magnetic devices needed for network communication equipment. First, it includes the power supply part (including the power transformer used on the power supply, input and output inductors, current transformers, and differential common mode filtering. Device, etc.): Second: Data transmission part. Relative to the former, the requirement for power density is replaced by the requirement for broadband and reliable transmission of data signals. Therefore, the corresponding requirements for magnetic devices are very different, and the design theories of the two are also completely different. Third: With the deteriorating electromagnetic noise and the mandatory implementation of related standards, how to solve the electromagnetic compatibility problem with the best cost performance and minimum space occupation has become one of the focuses of engineers’ consideration, in addition to requiring interference to the system In addition to accurate diagnosis and positioning of the source, reasonable device design and selection from the perspective of the device level have also become an important part of solving electromagnetic compatibility. Therefore, the reasonable choice of magnetic materials becomes the key to device design. Below, we will discuss the above three aspects.
Part 1: POWER part
With the development of communication, the requirements for power supply are developing in the direction of higher power density, lower voltage and larger current. The main factors that limit the miniaturization of switching power supplies and high frequency are magnetic devices such as inductance transformers, active switching tubes and diodes. From the perspective of magnetic components, due to the increase in operating frequency, the design of the main transformer of the power supply puts forward new requirements for the selection of the magnetic core. For the DC-DC module, the switching frequency has been above 400KHLz. This frequency has been used in the past. The commonly used PC40 material can no longer meet the requirements of reduced performance. It is necessary to use cores made of materials such as P44, 47, 95 to achieve low power consumption. At the same time, for the output inductor, due to the large DC bias characteristic requirements, the same Compared with powder core materials, the saturation magnetic induction (Bs) of ferrite is low, and the ability of DC-BIAS is poor. When the requirements of reducing the height and volume of the device cannot be met, APP must be used. Alloy), SENUAT (Silcon Aluminum), HIGH FLUX high flux core) and IRONPODER (iron powder core), etc. At the same time, flat spiral wire winding technology is adopted to reduce winding space and increase reliability. At present, well-known foreign transformer companies such as pluse, coilraft, VISHAY, NECATOKON of Japan, Panasonic, SUMTDA, and BI technology of the United Kingdom have mass supply of high-current output planar inductors. Did not see similar products. Among these products, there are composite magnetic cores, insulated NiZn bases, and sendust flat spiral inductors on the top. A NEC&TOKIN product is shown in Figure 1: Figure 1: A NEC&TOKIN
inductor, the largest Rated current up to 30A
There are also inductors with plastic skeletons and sendust as magnetic cores. The crimping process of the lead joints not only meets the requirements of large currents for small contact resistance and high reliability, but also meets the requirements of environmental protection and lead-free. VISHAY uses the powder core material for insulation treatment to form an integrated inductor that is once stamped and formed. Since no frame is needed, the coil is wound separately and then formed again. Not only is the height lower, but also the noise is lower, the reliability is higher, and the transient The current saturation resistance is stronger. The maximum rated current can reach 60A, and the transient core saturation current can reach 120A. At the same time, the eddy current loss is very low, and the operating frequency can work up to 5MHz, as shown in Figure 2:
Figure 2 yISAY's high current inductor
In terms of transformers, planar transformers should be the trend in the development of transformers in the communications industry. At present, Japan TDK, Flying Magnet (formerly Philip), and British G are now acquired by British TT Group. They have developed various planar magnetic cores to meet market demand. , Let aunt, board mounted power supply (BMP) (that is, the transformer winding is directly designed under the winter PC of the power supply main board, and the magnetic core can be directly glued to the main board), the International Electrotechnical Commission (IEC ) In the standard 61860, the magnetic core column is similar to an ellipse, which not only reduces the opening area of the main board, but also reduces the length of each turn of the coil, and the allowable width of the wiring is also increased. Therefore, Rdc/L. is smaller, making the transformer The copper loss is lower and the response is better. As shown in Figure 3:
Figure 3
Part 2: Network Transformers
At present, the development of the Internet is changing with each passing day. The Internet in our country is relatively late compared with the European customs. This is not a good thing for our country and can reduce many unnecessary detours. The network structure involving electrical basic inductance transformers includes transmission network, switching network, and access network. Especially this year, the access network has developed very fast. From the perspective of the transmission layer medium, it can be divided into optical fiber access technology, hybrid optical fiber and HFC access technology, copper wire access technology. At present, because copper wire access technology can use the telephone line of the traditional public switched telephone network, it has a better cost performance, which is more suitable for China’s national conditions and develops rapidly. The development speed of ADSL access network is very fast. It is foreseeable that the large-scale development of VDSL is not far away, and the broadband transformer involved is the most critical component. The network transformer is different from the traditional power transformer, and its design theory is based on Transmission line theory requires higher transmission bandwidth, but not high power requirements. The following is
a brief discussion on the application of magnetic cores in the design of broadband transformers .
Basic theory
Broadband transformers are magnetic devices with wire-wound design, which can transmit energy in a wide frequency range. Most broadband transformers are widely used in various low-power telecommunication equipment.
Figure 4
Figure 4 shows the typical characteristics of the insertion loss-frequency curve of a broadband transformer. The bandwidth of the transformer is the frequency interval between f2 and f1, or the frequency interval between f2' and f1'. As can be seen from the figure, with The bandwidth (f2'-f1') of the cutoff frequency characteristic curve of the line breaking is narrower than the flat and steep frequency characteristic (f2-f1). It can also be seen from the figure that the three frequency bands are represented separately.
The cut-off of the broadband transformer The frequency is determined according to the design requirements of the specific transformer. Therefore, the lower limit frequency f can be higher than 10MHz or lower than 300Hz. The bandwidth may also be from several hundred Hz to hundreds of MHz. A typical indicator of broadband transformer design is in the mid-band The maximum insertion loss and the maximum allowable insertion loss at the cut-off frequency. Figure 2 is an equivalent schematic diagram of the lumped parameters of a transformer. The circuit is regarded as an ideal transformer, including the parasitic resistance and inductance. The secondary components have been converted To the primary side, including parasitic and load impedance.
Figure 5: Transformer equivalent circuit of lumped parameters
Among them: Ea-indicating the excitation source Ra-indicating the internal resistance of the source Lp-indicating the
primary side inductance under no load (open circuit) L11-indicating the primary leakage inductance
Rp-indicating the parallel resistance of the core loss The
following are the components converted from the secondary side to the primary side Parameters:
C2'-indicating the distributed capacitance between turns of the secondary winding
R2'-indicating the resistance of the secondary winding Rb'-indicating the load resistance L12'-indicating the leakage inductance of the secondary side
Figure 6: Simplified transformer equivalent circuit
Where: Cd=C1+C2' Rc=R1+R2' L1=L11+L12'
Please refer to Figure 2
for other circuit parameters. In order to simplify the circuit, the original and secondary components are combined. The simplified equivalent circuit is shown in Figure 3. The physical meaning of the parameters is listed under the equivalent circuit. In the low frequency area, the deterioration of the transmission characteristics is due to the low frequency area. Caused by lower excitation impedance. The excitation impedance decreases as the frequency decreases, resulting in increased signal attenuation. In the excitation impedance, the primary excitation inductance XLP accounts for the main part, ignoring the equivalent parallel loss resistance that generates leakage current. Therefore, The insertion loss is expressed by the primary side parallel magnetizing inductance as follows:
Here, R=Ra×Rb'/Ra=Rb'
In the design of most broadband transformers, the coil resistance is the main factor that affects the transmission performance in the center passband. The insertion loss due to the coil resistance is expressed as:
Here, Rc=R1+R2'In
the high frequency band, the transmission characteristics are mainly affected by the leakage inductance of the coil and the distributed capacitance between the turns. At this time, under normal circumstances, both the coil excitation inductance and the coil resistance must be considered, depending on the impedance characteristics of the resistance. In a low impedance resistor, the attenuation of high frequency signals due to leakage inductance is expressed as follows:
In a high impedance circuit, the attenuation of high frequency signals due to distributed capacitance is expressed as follows:
Reviewing the insertion loss characteristics of the above three frequency bands, the following conclusions can be drawn: In the design of the transformer, the material characteristics and shape of the ferrite core determine the highest inductance per turn of f1 at the lowest cut-off frequency. It also determines the low frequency. The minimum number of coil turns required to achieve the inductance required by the design. Fewer coil turns are exactly what is desired for the central frequency band to achieve low insertion loss requirements, and it is also conducive to meeting the high frequency f: low winding required for good frequency response Parasitic parameter requirements.
In the application design of broadband transformers in low frequency and intermediate frequency bands , the more suitable magnetic core is MnZn material with the highest initial permeability at the lower limit frequency of the low end, such as 5k or 7k, which is very suitable Used in the design of low-frequency and intermediate-frequency broadband transformers. Generally speaking, the parallel excitation inductance of the transformer is not the most critical parameter. As long as the frequency increases, the permeability of the magnetic core material is constant or decreases faster than the frequency. To be sure, when designing a transformer, as long as the lower limit frequency f1 is in the flat part of the ui-f curve of MnZn ferrite, it is enough. Although in the entire pass band of the transformer, the permeability of the magnetic material It has been reduced, but in fact it has no effect on the pass-band characteristics of the transformer. In the design process of the broadband transformer, the geometric size of the MnZn ferrite should minimize the ratio of coil resistance to inductance, that is, Rdc/L, in other words , The ratio of the DC resistance and the inductance of one turn on the base core must be as small as possible. The International Electrotechnical Commission has designed and defined the smallest Rdc/L value pot core in the IEC60133 document. Other shapes such as EP type And PQ type broken cores can also be used in the design of broadband transformers. Under normal circumstances, the final choice of core will also be restricted by constraints such as the difficulty of coil winding, coil end processing and other mechanical design constraints.
Broadband transformer with static DC bias magnetic field
When designing a transformer with static DC bias current, open the air gap magnetic core to overcome the drop of excitation inductance. The Hanna curve provided by the manufacturer can help design engineers evaluate the DC bias The effect on inductance.
Although the high- frequency broadband transformer does not have a clear division for the high, medium and low frequency bands, the following mainly recommends the use of NiZn material as the magnetic filter of the high-frequency broadband transformer, which mainly refers to the design of broadband transformers with a bandwidth of more than 500kHz. In this paragraph In the frequency range, the complex permeability characteristics of the Kaozhong core material become particularly important. Unlike the design of transformers in low frequency bands, only simple magnetic constants of the core are considered, such as the inductance factor AL. It is
also very important. One point must be considered, that is, high-frequency transformers are usually used in low-impedance circuits. Therefore, the required excitation impedance is also relatively low, which means that fewer coil turns are required, and therefore, the coil resistance becomes smaller. The impact on device performance becomes no longer important. The design criterion is to minimize Rdc/L. At this time, the focus of the design is mainly on the core shape and at the lower limit frequency f1, while reaching the required excitation impedance as much as possible. The magnetic material characteristics required to reduce the leakage inductance of the winding.
Since the magnetic permeability characteristics of the material and the core loss directly affect the size of the excitation impedance, the design process of the high-frequency band broadband transformer must take these parameters into consideration. Influence, Figure 4, Figure 5 and Figure 6 are the magnetic core impedance, equivalent parallel inductance Xp and equivalent parallel loss resistance Rp frequency characteristics.
Figure 7
Figure 8
Figure 9
For broadband transformers in the high frequency band, the toroidal core is the best choice. It requires fewer turns to achieve the required inductance, and winding is easier. However, fewer turns are necessary to obtain the desired impedance ratio. To a certain degree of difficulty. In order to minimize the leakage inductance of the coil, it is recommended that the primary and secondary sides take the form of twisted pair to achieve tight coupling between the primary and secondary sides.
It is also possible to use a porous magnetic core to replace two adjacent magnetic rings to improve the performance of the magnetic ring Compared with a single magnetic ring with the same size factor C1, the porous core has a shorter coil length per turn, so the designed transformer has a higher bandwidth, and many broadband transformers have achieved good results using NiZn ferrite. If the required bandwidth cannot be achieved with a single magnetic ring, a porous NiZn ferrite core can be used to design.
Summarizing
the lower limit frequency f1 characteristics of the transformer is the most important factor in the selection of ferrite, and the highest initial permeability is required at the f1 frequency. The design of the transformer with the lower limit frequency f1 of MnZn material lower than 500kHz. Above this frequency, NiZn must be used material. In the low frequency and intermediate frequency bands, the rule of choosing the shape of the magnetic core is to make the DC resistance of each turn of the coil as small as possible. If the circuit requires a DC bias circuit, you can refer to the Hanna curve to select the open air gap magnetic core. In the high frequency band, Select NiZn ferrite material small magnetic ring and porous magnetic core as the preferred magnetic core shape.
The number of turns of the coil is as small as possible to reduce leakage inductance and inter-turn distributed capacitance. The primary and secondary windings are tightly coupled through twisted pairs. Reduce leakage inductance.
For ADSL network port transformers, EP (EP13/EP10/EP7) cores are currently widely selected. In signal transmission, impedance matching must be required to reduce signal reflection and oscillation. At the same time, due to the nonlinearity of the magnetic core magnetization, higher harmonics will be generated. How to reduce higher harmonics is a key parameter to improve the quality of network transmission. Therefore, the total harmonic distortion THD (Totle harmonic distrion) of the magnetic core must be as small as possible. When the magnetic core works in a small signal, the fine material characteristics meet the Rayleigh equation. Therefore, it is necessary to use high permeability and Open the air gap appropriately. From theoretical derivation, it can be known that the even harmonics in the harmonics just cancel out, only the odd harmonics, and the third harmonics account for the majority. As long as the amplitude of the third harmonic is reduced, THD can be significantly reduced. Therefore, for magnetic core material manufacturers, how to adjust the material formula and sintering process to reduce the third harmonic becomes very important. The third harmonic calculation formula is as follows:
![THD = V / V; or20.10og (V / V1) [dB]](https://img-blog.csdnimg.cn/20201019111754618.png#pic_center)
The test circuit of THD is as follows:
Figure 10
At the same time, in order to further reduce THD, the core manufacturer has also optimized the shape and structure of the core. As shown in Figure 11a, the center column is similar to an elliptical EPO or EPX core. Through this improvement, THD has been further improved (Figure 11b) Where CDF stands for harmonic distortion factor).
Figure 11a
Figure 11b
In broadband transformers, in addition to the commonly used 1:1, there are other impedance transformation ratios. The following are the commonly used transformation ratios:
Note:
l is the length of the transmission line of a winding, or the length of the magnetic ring,
L is the inductance of a winding, or the inductance of a single wire of the magnetic ring,
Z is the characteristic impedance of the transmission line,
Z0 is the best characteristic impedance, and
Z00 is odd The model impedance
Z0e is the even model impedance
Rg is the internal resistance of the power supply.
Rb' is the equivalent load converted to the primary (=Rb/n2, n is the turns ratio)
β is the phase constant (2π/λg)
Lp is the load when the load is open The primary inductance
T0 is the best transmission coefficient
R0=RgRb'/ (Rg+Rb')
T1 Parallel inductance transmission coefficient
T2 Transformation circuit's own transmission coefficient
Part 3: Electromagnetic compatibility
With the mandatory implementation of the national CCC certification, the mandatory implementation of the safety regulations for exports from Europe, America and Japan, and the strengthening of inspection levels (from A to B), the electromagnetic compatibility design difficulty and design methods of electronic products must be paid attention to by lead engineers. In the process of implementing electromagnetic compatibility, the correct selection of EMC components and materials is the last important step. Therefore, the characteristics of magnetic materials must be fully understood.
As we all know, interference is divided into conduction interference and radiation interference. Conducted interference is divided into common mode (CM) and differential mode (DM) interference. This article mainly discusses conducted interference (radiation interference will be discussed in a later article ), EMC commonly used magnetic cores are divided into several categories: including MnZn ferrite; NiZn ferrite; iron powder core; sendust, etc., amorphous ultra-microcrystalline materials.
The characteristics of the above materials are introduced one by one below. :
Common mode inductors (CMC) often use high-conductivity MnZn materials to maximize inductance (especially in low frequency bands, only by increasing the permeability can the impedance of the coil be increased and the effect of suppressing low-frequency noise is increased). However, due to The permeability of MnZn is usually about 7000, and the highest is about 12000. Therefore, it is not enough to provide high enough impedance at low frequency. Amorphous or ultra-microcrystalline materials have a very high initial permeability of tens of thousands, as shown in Figure 12. Therefore, they have higher impedance at low frequencies and were not commonly used in the past. There are several reasons: On the one hand, the price is high, on the other hand, electromagnetic compatibility is not enforced. In addition, due to the poor high-frequency characteristics of amorphous ultrafine crystals, it cannot cross the use frequency band of MnZn, which limits its usability. Currently, the German VAC company develops Materials that are particularly suitable for common mode inductors have been developed. The impedance curve is
shown in Figure 13 and Figure 12.
Figure 13a
Figure 13b
It can be seen from Figures 13a and 13b that the ultra-microcrystalline material VITROPERM covers a wider frequency range than MnZn, especially in the low frequency band, which can better suppress low-frequency noise, and provides higher impedance under the same number of coil turns.
Meanwhile, it can be seen from FIG. 14, the impedance corresponding to the same core volume is greatly reduced, for which very restricted space communication power, the very attractive.
FIG. 14
Differential mode inductors need to avoid the saturation of the magnetic core. Therefore, for large currents, materials that can withstand DC bias are often used, including iron powder cores, sendust, high flux cores (high flux), MPP, etc. The specific material to choose is selected according to actual needs and cost performance. The following is a comparison of various material parameters.
| Comparison of Inductor Core Materials | Iron Powder | Hi-Flux high flux core | Super-MSS Sendust | Molybdenum Permalloy Molybdenum Permalloy | Ferrite |
|---|---|---|---|---|---|
| Basic Magnetic Material Composition | Iron | 50% Nickel, 50% Iron Alloy | 85% Iron, 9% Silicon, 6% Aluminum Alloy | 81% Nickel, 17% Iron, 2% Molybdenum Alloy | Manganese-Zinc Oxides Joined with Iron Oxides |
| Permeability Range | 3 to 85 | 14 to 160 | 60 to 125 | 14 to 350 | Single Air Gap |
| Maximum Saturation Flux Density (Teslas) | 2.0 | 1.4 | 1.0 | 0.7 | 0.4 |
| Typical Core Loss at 50 kHz, 0.05 Tesla (mW/cm3) | 330 (75-Perm.) | 170 (125-Perm.) | 80 (125-Perm.) | 55 (125-Perm.) | 5 (TDK PC40") |
| Typical Core Loss at 100 kHz, 0.1 Tesla (mW/cm3) | 3400 (75-Perm.) | 1500 (125-Perm.) | 800 (125-Perm.) | 550 (125-Perm.) | 70 (TDK PC40") |
| Curie Temp. ( C) | 750 | 500 | 600 | 400 | 200 |
| Maximum Operating Temperature ( C) | 130 | 130 to 200 | 130 to 200 | 130 to 200 | 130 to 200 |
| Core Shapes | Various | Rings (Toroidal) Only | Rings (Toroidal) Only | Rings (Toroidal) Only | Various |
| Relative Price | Low | High | Medium | High | Medium |
At the same time, in order to solve the noise interference in data transmission, foreign companies (especially Japan) have developed various three-terminal filters and two-end band stop filters, as shown in Figure 15a, b, and c. The appearance, structure, and insertion loss comparison with magnetic beads and three-terminal capacitors.
Figure 15a
Figure 15b
Figure 15c
It can be seen from Figure 16 that the band stop filter adopts a parallel composite structure of R, L, and C, which generates high impedance through parallel resonance, and absorbs the interference signal energy through the resistance. It has sharp insertion loss characteristics at the frequency point of the characteristic, and its center frequency Divided into 820MHz, 1000MHz 1500 MHz, 2200MHz, the frequency is just in the mobile communication frequency band, which has a good effect on solving the interference of specific frequencies in 3G communication equipment.
Figure 16a
Figure 16b