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Magnetic material appearance structure shape and material selection

With the rapid development of science and technology, electronic ballasts are widely used in energy-saving lamps. For example: neon lights are widely used in major shopping malls. In order to improve its quality, electronic ballasts are a core component. Therefore, the selection of core components must meet the characteristics and requirements of electronic ballasts.
1 Appearance structure shape
Magnetic core components in electronic ballasts include two major categories of inductors and pulse transformers, so its magnetic core components must be properly selected. The core structure often used in pulse transformers is more ring-shaped, common specifications such as: T12*6*4, T9*5*3, 10*6*4, T8*4*4, etc.; selected in the half-bridge circuit When using MOSFETs for switching, the inner diameter of the magnetic ring should be slightly larger to wind enough turns to get enough voltage to drive the gate. The magnetic cores used in EMI filter inductors, PFC boost inductors, and chokes are mainly UU, EE, EI, PQ, etc. The core types used in 35~100W electronic ballasts are more, such as EE19/16, E25, and EF20. , EF25, PQ26/20 and so on. At present, there are many companies in China which jointly or exclusively produce magnetic materials, and the types and models are relatively complicated. When choosing a core, consider the structure of the core and the material of the material. Cores with the same specification and model but different materials have very different characteristics. When the material of the magnetic material is selected, the size of the structure is determined by parameters such as lamp power and inductance.
2 choice of magnetic material
Different magnetic materials have different characteristics and different application ranges. In general ferrite can be divided into: Mn-Zn ferrite, nickel-zinc-ferrite, amorphous and alloy, soft ferrite commonly used for switching power supply is PC40, PC30, PC44 and so on. Because the initial magnetic permeability of nickel-zinc-ferrite material is relatively low (generally μi <1000), its Curie temperature is higher and the operating frequency is above 0.1 MHz. If the FK1 TC of FERRITE KING reaches 400°C, the operating frequency is 10-150MHz, and μi is only 10-20; on the contrary, the μi of N10J material is 10000, but the TC is about 120°C, and the operating frequency is also lower is only 100KHz, even if Manganese-zinc (Mn-Zn) materials are different in material properties. Table 1 lists several standard Mn-Zn ferrite electrical characteristics.
When the electronic ballast uses a bipolar transistor as a switch, the operating frequency reaches 55KHz; when a MOSFET is used as a switch, the operating frequency is up to 150 KHz, and most magnetic materials can meet the requirements of the electronic ballast. The selection of magnetic materials for electronic ballasts should focus on the following aspects of magnetic materials:
(1) Curie temperature TC should be high enough. Because the temperature of the electronic ballast, especially in the fluorescent lamp housing, often reaches 80° C. or more, the temperature of the magnetic core itself can be more than 90° C. If the Curie temperature of the magnetic core is low, the temperature of the magnetic core will inevitably bring the Curie temperature close to the Curie temperature. Magnetic permeability μi, saturated magnetic flux density BS and sharp drop in inductance and lamp power increase drastically shorten the lifetime of the electronic ballast. Therefore, the temperature inside the electronic ballast housing is much lower than the magnetic core. Curie temperature, the best choice for Curie temperature TC> 180 °C magnetic material.
(2) The initial magnetic permeability μi of the core should be moderate. The core initial conductivity μi has many specifications. From 100 to 10,000, the initial magnetic permeability of the core assembly must meet the Curie temperature TC requirement. Generally, the material whose magnetic permeability μi is above 4,000 has its Curie temperature. Most of them are lower than 150°C and even lower than 130°C, but the Curie temperature of the material whose permeability is less than 3,000 can generally reach 180°C or more. Therefore, choosing cores with μi2000~3000 to make chokes and other inductors is more appropriate. For the pulse transformer magnetic ring itself less heat, the ambient temperature will generally reach 90 °C, so the Curie temperature of the magnetic ring can be appropriately lower, the magnetic permeability as high as possible to obtain a high enough drive signal to be able to push The transistor quickly reaches saturation. At the same time, the initial magnetic permeability μi higher can also reduce the number of winding turns, thereby reducing the leakage inductance and distributed capacitance, which is beneficial to improving the driving signal waveform.
(3) The resistivity ρ should be relatively high. When the operating frequency is fixed, the eddy current loss of the magnetic material is inversely proportional to the resistivity. In order to reduce the loss of the core assembly, a core with a higher resistivity ρ should be used. The resistivity of magnetic materials is mostly between 0.15 and 108 Ω.m, and the resistivity ρ of Mn-Zn ferrite materials is generally between 0.1 and 100 Ω.m. The resistivity of magnetic materials is not necessarily the better, but must take into account the other characteristics of the material. For example, nickel-zinc N3L material ρ can reach 1*107Ω.m, but its Curie temperature is too low (TC100°C). This kind of material is not suitable for use in electronic ballasts. Ferrite materials When considering only several parameters of μi, TC, and BS, N2J and N3J materials can be selected. The material has the highest resistivity (p6.5Ω.m) and thus has a small power loss.
(4) The appropriate temperature coefficient. Different temperature coefficients are required in magnetic assemblies for different applications in electronic ballasts. For pulse transformer magnetic rings, a negative temperature coefficient is required, ie its magnetic permeability or coil inductance decreases with increasing temperature. When the temperature is changed from room temperature to 100°C, the current gain Hef of the power switch transistor increases by about 10% to 15% with a rise in temperature, and the collector current also increases. In this temperature range, as long as the magnetic ring has a magnetic permeability with a negative temperature coefficient, it just cancels or mostly cancels with the positive temperature coefficient of the transistor Hef, and it basically maintains a balance, so that stable operation of the electronic ballast can be ensured.
In the core component of the inductor in the EMI filter, the inductance of the core coil should be as small as possible due to temperature rise, so that the LT characteristic curve is kept flat in the whole; otherwise, if the inductance value changes with temperature, the inductance value is large, then at room temperature The filtered results that have been debugged will also be worse.
For high-frequency choke cores, the magnetic permeability μi preferably has a positive temperature coefficient, that is, the inductance of the choke increases as the temperature increases, so that the power of the energy-saving lamp decreases as the temperature increases, R2K material It has this temperature characteristic. There is no doubt that either the choke coil or the APFC boost inductor is the best choice for cores with a negative temperature coefficient. The power consumption of μi R2.5k materials decreases from 25°C to 80°C as the temperature increases, and the power consumption is minimal at about 80°C. These materials are more in line with the requirements.
Even if the temperature coefficient of the magnetic material is not completely satisfactory, it should at least be able to ensure that the inductance and power consumption and other parameters change as little as possible with the temperature increase.
(5) Saturation flux density BS and hysteresis loop. Magnetic components in electronic ballasts should have a high saturation flux density, and generally require BS: 450-550mT to ensure that the pulse transformer has enough drive power to prevent high frequency chokes or boost inductors from entering magnetic saturation. As the temperature rises, if the BS value is selected too low, a sufficiently high Curie temperature cannot be guaranteed.
Since the hysteresis loss of the core is proportional to the area surrounded by the hysteresis loop, the relatively narrow core of the hysteresis loop is beneficial for reducing power consumption. The pulse transformer magnetic loop must have an approximately rectangular hysteresis loop. To ensure that the two transistors of the half-bridge inverter can generate symmetrical current waveforms, the magnetic loop hysteresis loop is required to have better symmetry.
(6) Core assembly testing, screening and binning. Since the degradation and attenuation of the parameters of the magnetic components are relatively severe during the period of time, if they are installed immediately in the electronic ballast, they may not work properly after a certain period of time. Generally within a month after the core is released, its natural drop is much larger than the falling coefficient of the specification. As long as the core assembly is guaranteed to have a storage time of not less than one month, and then tested for binning, the parameter degradation will be small. The magnetic components will also cause the loss of characteristic parameters after being subjected to shocks, shocks, and squeezing. Therefore, the cores after the test bins should be free from compression, impact, and drop, and should be handled gently during handling and assembly.
The consistency of magnetic cores produced today is poor, and the dispersion of parameters is also greater in the same batch of products produced in the same batch. Therefore, the core must be 100% tested, screened and binned. For example, if a pulse transformer magnetic ring is used in mass production without screening, screening and binning, some electronic ballasts cannot generate oscillations, and some may start but soon fail. Since the number of core coils in mass production is fixed, testing and screening of cores is an indispensable link.

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