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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The use of small-sized noise-suppressing magnetic circuits made of amorphous metal alloys. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer A third of a century ago, experiments on the rapid cooling of metal melts, which were carried out in order to obtain a submicroscopic structure of the metal, showed that in some cases there is no crystal lattice in the metal at all, and the arrangement of atoms is characteristic of a structureless, amorphous body. It turned out that the amorphous metal has completely different properties, not similar to the crystalline metal. It becomes several times stronger, its resistance to corrosion increases, electromagnetic characteristics change, and even one of the most stable constants - the modulus of elasticity. Amorphous alloys are called metallic glasses. Interest in them is growing rapidly. First of all, researchers were interested in the ferromagnetic properties of alloys based on iron, nickel and cobalt, which turned out to be higher than those of permalloys, and these properties are more stable. Today we will talk about some areas of application of magnetic cores made of amorphous metal alloys. Magnetic cores made of amorphous metal alloys are wound from thin (average 25 µm) tapes (Fig. 1). By selecting the material and heat treatment mode, you can get unique properties that are optimal for a particular application of products.
The above fragment of the functional diagram of the converter shows four types of magnetic cores (see the Mstator advertisement on p. 33): 1 - for power factor correctors. Due to the high saturation induction (1,45 T), low losses and the ability to work at elevated temperatures, the use of such magnetic circuits allows to reduce the size and weight of the device; 2 - toroidal with saturation mode for magnetic amplifiers (magnetic switches). These magnetic circuits have unique properties: high hysteresis loop squareness (0,96...0,98), low losses and low coercive force at high frequency. A typical application of magnetic switches is multi-channel power supplies, in which feedback to the PWM controller comes from one of the outputs, and voltage stabilization in the remaining channels is ensured by the use of magnetic switches. Such a construction of power supplies eliminates the dependence of the voltage in one of the channels on the degree of loading of the others, increases the stability and reduces the output voltage ripple, makes it easy to implement separate external control, separate protection of channels for current with different thresholds. Similar magnetic circuits are also used to stabilize the output current, for example, in chargers. In addition, these products can improve the efficiency and reliability of the device; 3 - noise suppression. They are often used with a single-turn winding: they are simply put on the output of an element - a diode, a transistor. Such magnetic circuits provide effective suppression of radio interference and reduction of high-frequency output voltage ripples; 4 - small-sized toroidal for power chokes (inductors). These magnetic circuits are characterized by a high level of DC bias while maintaining high permeability. They have a high saturation induction (1,45 T) and low losses, allow to reduce the dimensions of the device and provide operation at a higher level of DC bias than when using magnetic cores made of traditional materials. In addition, magnetic circuits made of amorphous metal alloys are used in common-mode filters of switching power supplies. It uses materials with a narrow hysteresis loop, high initial magnetic permeability (up to 150000), low losses at high frequency. To obtain the necessary inductance, a small number of turns is required, which, in addition to reducing the size, provides a small parasitic capacitance of the winding and a high coefficient of common mode noise suppression. Next, we will dwell in more detail on the use of miniature noise-suppressing magnetic cores. These products prevent rapid changes in electrical current that could otherwise lead to electrical noise and interference. Unlike others, this method eliminates the very cause of interference. Due to the rectangular shape of the hysteresis loop, noise-suppressing magnetic cores have a very large inductance at the moment of current zero crossing, which effectively dampens any rapid current changes. After the rated current is established, the magnetic circuit is saturated, its inductance decreases and does not affect the operation of the device. For example, such products simply and effectively reduce the noise caused by reverse recovery current in semiconductor switching elements at the time of turn-off. Single-turn interference suppressors (based on cylindrical magnetic cores) are structurally optimized for use with a single-turn winding, which is usually the component lead. They are put on the output of the element (transistor, diode) before mounting on a printed circuit board (Fig. 2).
Multi-turn interference suppression devices ("spike killers" or "emission killers") are small saturation magnetic circuits with a winding of several turns. The advantage of the described devices, compared to other methods, is higher efficiency (due to the elimination of the cause of interference - rapid current changes), lower losses (total losses are lower than in a conventional RC circuit, especially at high frequency), saving PCB area (they are put on directly on the terminals of semiconductors, without requiring additional space on the printed circuit board). This class of magnetic circuits is widely used in switching power supplies, DC-to-DC converters, motor control units, switching semiconductor devices, and small-sized common-mode filters. In addition to noise suppression, noise suppression chokes are used to protect semiconductors, as they eliminate potentially dangerous voltage surges. The principle of operation of the noise-suppressing magnetic circuit is illustrated in Fig. 3.
During the flow of direct direct current (region "I" in Fig. 3, a), the magnetic circuit is saturated and its magnetization remains almost constant (region "D" in Fig. 3, b), so the inductor has a very low inductance. After turning off, when the forward current of the diode decreases, the magnetic circuit is still saturated and the inductance of the inductor is still small (region "II" in Fig. 3). The diode current continues to decrease and changes its direction (region "III" in Fig. 3a). The reverse recovery period of a diode is characterized by a high di/dt value, which is the main cause of noise. At this time, the magnetic circuit begins to remagnetize (region "III" in Fig. 3, b), the inductance of the inductor increases rapidly, which leads to a decrease in the diode reverse current surge. When the diode closes, the magnetic circuit will remain practically in a demagnetized state (region "IV" in Fig. 3). As soon as the next pulse arrives, the diode turns on again, and the magnetic circuit, being magnetized, quickly enters saturation (region "V" in Fig. 3) and the above process is repeated. On fig. Figure 4 shows examples of the use of interference-suppressing magnetic cores (interference-suppressing chokes are highlighted in red, storage chokes based on MD magnetic cores made of an amorphous alloy with a DC bias mode are shown in yellow): a - pulse stabilizer; b - push-pull converter; c - flyback converter; g - motor control unit; e - forward converter; e - bridge motor control unit.
On fig. Figure 5 shows comparative oscillograms that clearly demonstrate the advantages of noise suppression devices made of amorphous metal alloys using a forward converter as an example: a, b - output voltage ripple, frequency f = 150 kHz, output voltage Uout = 15 V, load current IН = 10 A: a - amplitude ripples 67 mV (RC circuit and ferrite magnetic core), b - ripple amplitude 45 mV (MP4-2-4.5AP); c, d - voltage at the rectifier input (above - voltage at the anode of the diode, below - current through the diode), f = 500 kHz, Uout = 5 V, lH = 20 A: c - without damping measures, d - MP4- 2-4.5; e, f - voltage on the switching MOSFET transistor, frequency 250 kHz: e - maximum voltage 715 V (ferrite magnetic circuit 4-2-4), e - maximum voltage 690 V (MP4-2-4.5); g, h - corresponding e, f ripples of the output voltage of the converter, f=250 kHz, Uout=5 V, 1n=15 A: g - ripple amplitude 140 mV (ferrite magnetic core 4-2-4), h - ripple amplitude 87 mV (MP4-2-4.5).
In table. 1 gives general recommendations when choosing noise-suppressing magnetic cores used in pulsed sources. After the group is determined, a specific type is selected based on the following ratios.
To effectively suppress the front of the diode recovery reverse current using single-turn devices, it is necessary to fulfill the condition 2Фm≥(Ucxtrr), where 2Фm is the maximum (double peak-to-peak) flux in the magnetic circuit, Wb; Uc - diode reverse voltage, V; trr - diode reverse recovery time, s. As an example, consider a discharge (switching) diode (Fig. 4, e) of a forward converter with an output voltage of 12 V. The reverse recovery time of the diode is 35 ns, the duty cycle is 0,3 (30%). According to the table 1 select a cylindrical noise-suppressing magnetic core. Then we calculate the right side of the expression: 2Фm≥((12/0,3)х35х10-9)=1,4 мкВб. From Table. 2, we select the smallest magnetic circuit that satisfies this condition - MPZx2x4.5AP.
For multi-turn devices, the condition (2ФmxAw)≥(1,5Ucxl0xtrr), where 2Фm is the maximum magnetic flux in the magnetic circuit, Wb; Аw is the area of the window (winding) along the inner diameter of the magnetic circuit housing, mm2; Uc - voltage on the element, V; l0 - element current, A; trr - reverse recovery time, s. As an example, consider the discharge (switching) diode of a forward converter with an output voltage of 24 V and a load current of 2 A. The reverse recovery time of the diode is 60 ns, the duty cycle is 0,3 (30%). According to the table 1 select a multi-turn choke. Then we calculate the right side of the expression: (2ФmxAw≥(1,5х(24/0,3)х2х60х10-9)= =14,4 мкВб·мм2 . From Table. 3 select the smallest magnetic circuit that satisfies this condition - МН080704.5А.
The wire diameter (in mm) and the number of turns of the winding for the selected magnetic circuit are calculated according to the following relationships: dnp≥(0,5√I0 0,7 = mm; N≥(3Ucxtrr/(2Фm))=(3x(24/0,3)x60x10-9 /(1,96х10-6))=7,35 витка. We choose an integer value N=8 turns. The final optimal selection of the noise suppression choke is made by practical testing of a real device. Indicative recommendations for the use of cylindrical noise-suppressing magnetic cores are given in Table. 4 (for forward converters) and in table. 5 (for flyback converters).
Author: E. Fochenkov, Borovichi, Novgorod region
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