1 With the advancement of society and the rapid development of science and technology, everyone knows that neon lights are widely used in major shopping malls. In order to improve its quality, electronic core ballasts are a core component, so the core components must be selected in accordance with electronics. The characteristics and requirements of ballasts.
2 Appearance structure shape
The core components in the electronic ballast include two types of inductors and pulse transformers, so the core components must be properly selected. The core structure often used for pulse transformers is more ring-shaped. Common specifications such as: T12*6*4, T9*5*3, 10*6*4, T8*4*4, etc.; elected in the half-bridge circuit When using a MOSFET tube as a switch, the inner diameter of the magnetic ring is slightly larger so that a sufficient number of turns can be wound to obtain a sufficient voltage to drive the gate. The cores used in EMI filter inductors, PFC boost inductors and chokes are mainly UU, EE, EI, PQ, etc. For cores of 35~100W electronic ballasts, such as EE19/16, E25, EF20 , EF25, PQ26/20 and so on. There are many companies that have joint ventures or sole proprietorships to produce magnetic core materials, and the variety models are quite complicated. When selecting a core, the structure of the core should be considered together with the material of the material. Magnetic cores with the same specifications and different materials have quite different characteristics. When the material of the core material is selected, the size of the structure is determined by parameters such as lamp power and inductance.
3 core material selection
Different magnetic materials have different characteristics and different application ranges. In general, soft ferrite can be divided into: manganese zinc ferrite, nickel zinc ferrite, amorphous and alloy. Generally, soft ferrites for switching power supplies include PC40, PC30, and PC44. Since the nickel-zinc ferrite material has a low initial magnetic permeability (generally μi < 1000) but its Curie temperature is high, the operating frequency is above 0.1 MHz. For example, FERRITE KING's FK1 TC reaches 400°C, the operating frequency is 10-150MHz, and μi is only 10-20; on the contrary, the μi of N10J material reaches 10000, but the TC is about 120°C, and the operating frequency is also lower than 100KHz, even if the same It is a material of manganese-zinc (Mn-Zn). The properties of different materials are also very different. Table 1 lists the standard electrical characteristics of several common manganese-zinc ferrites.
When the electronic ballast uses a bipolar transistor as the switch, the operating frequency reaches 55KHz; when the MOSFET is used as the switch, the operating frequency can reach up to 150 KHz, and most of the core materials can meet the requirements of the electronic ballast. The selection of core materials for electronic ballasts should focus on the following requirements for core materials:
(1) Curie temperature TC should be high enough. Because the temperature inside the electronic ballast, especially the fluorescent lamp cover, often reaches above 80 °C, the temperature of the core itself can be above 90 °C. If the Curie temperature of the core is low, the temperature of the core itself will be close to the Curie temperature, resulting in an initial The magnetic permeability μi, the saturation magnetic flux density BS and the inductance value drop sharply and the power of the lamp increases sharply, resulting in shortened life of the electronic ballast, thus ensuring that the temperature inside the electronic ballast housing is much lower than that of the core. For the Curie temperature, it is preferable to use a core material having a Curie temperature of TC > 180 °C.
(2) The initial magnetic permeability μi of the core should be moderate. The initial conductivity μi of the core has many specifications, from 100 to 10000. The initial permeability of the core component must meet the requirements of the Curie temperature TC. The material with a general permeability μi of 4000 or higher has a Curie temperature. Most of them are below 150 ° C, even lower than 130 ° C, and the material Curie temperature of the magnetic permeability μi below 3000 is generally above 180 ° C. Therefore, it is more suitable to use a magnetic core of μi2000~3000 to make an inductor such as a choke coil. For the pulse transformer, the magnetic ring itself generates less heat, and the ambient temperature generally reaches 90 ° C. Therefore, the Curie temperature of the magnetic ring can be suitably lower, and the magnetic permeability is as high as possible to obtain a sufficiently high driving signal to drive. The transistor quickly reaches saturation. At the same time, the initial permeability μi is higher, the number of winding turns can be reduced, thereby reducing the leakage inductance and the distributed capacitance, which is beneficial to improving the driving signal waveform.
(3) The resistivity Ï should be relatively high. When the operating frequency is constant, the eddy current loss of the core material is inversely proportional to the resistivity. In order to reduce the loss of the core component, a core having a higher resistivity Ï should be selected. The resistivity of the core material is mostly between 0.15 and 108 Ω.m, and the resistivity Ï of the MnZn ferrite material is generally between 0.1 and 100 Ω.m. The resistivity of the core material is not as high as possible, and the other properties of the material must be taken into consideration. For example, although the nickel-zinc N3L material Ï reaches 1*107Ω.m, but the Curie temperature is too low (TC100°C), the sample material is not suitable for use in electronic ballasts. For the various materials in Table 1, manganese zinc When considering only the parameters of μi, TC, and BS, the N2J and N3J materials can be selected. The material has the highest resistivity (p6.5 Ω.m) and thus has a small power loss.
(4) A suitable temperature coefficient. Different temperature coefficients are required for magnetic components in different applications in electronic ballasts. For pulse transformer magnetic rings, a negative temperature coefficient is required, that is, the magnetic permeability or coil inductance decreases with increasing temperature. When changing from room temperature to 100 ° C, the current gain Hef of the power switching transistor increases by about 10% to 15% with increasing 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 largely cancels the positive temperature coefficient of the transistor Hef, and basically maintains the balance, thereby ensuring stable operation of the electronic ballast.
In the core assembly of the inductor in the EMI filter, the inductance of the core coil should be as small as possible due to the temperature rise, so that the LT characteristic curve remains flat in the whole. Otherwise, if the inductance value changes relatively with temperature, then at room temperature The filtered effect that has been debugged will also be worse. For the high-frequency choke core, the magnetic permeability μi preferably has a positive temperature coefficient, which means that the inductance of the choke increases with increasing temperature, so that the power of the energy-saving lamp decreases with increasing temperature, and the material of R2K It has this temperature characteristic. There is no doubt that it is best to use a negative temperature coefficient core for both the choke and the APFC boost inductor. Like μi R2.5k materials, the power consumption decreases from 25 ° C to 80 ° C with increasing temperature, and the power consumption is the lowest at around 80 ° C. Such materials are more suitable.
If the temperature coefficient of the magnetic material is not completely satisfactory, at least the parameters such as inductance and power consumption should be kept as small as possible with increasing temperature.
(5) Saturated magnetic flux density BS and hysteresis loop. The magnetic components in the electronic ballast should have a high saturation magnetic flux density, and generally require BS: 450-550mT to ensure that the pulse transformer has sufficient driving power to prevent the high frequency choke or boost inductor from entering the magnetic saturation. The temperature rise is intensified, and if the BS value is too low, a sufficiently high Curie temperature cannot be guaranteed.
Since the hysteresis loss of the core is proportional to the area enclosed by the hysteresis loop, the magnetic core with a narrow hysteresis loop is advantageous for reducing power consumption. The pulse transformer magnetic ring must have an approximately rectangular hysteresis loop. It is guaranteed that the two transistors in the half-bridge inverter can generate a symmetrical current waveform, which requires better symmetry of the magnetic hysteresis loop.
(6) Testing, screening and binning of core components. Since the parameter degradation and attenuation of the magnetic component is relatively serious for a period of time, if it is installed in the electronic ballast immediately, it may not work properly after a period of time. Generally, the natural drop in the magnetic core within one month is much larger than the drop factor of the specification. As long as the core assembly has a storage time of not less than one month, and then the test is binned, the parameter degradation is small. The magnetic components will also cause the drop of the characteristic parameters after being subjected to vibration, impact and crushing. Therefore, the core after the test is divided should avoid pressurization, impact and drop, and should be handled gently during handling and assembly.
The consistency of today's domestic magnetic cores is poor, and the dispersion of parameters is also large in the same batch of products of the same batch. Therefore, the core must be 100% tested and screened. For example, a pulse transformer magnetic ring is installed in mass production without detection, screening and binning. Some electronic ballasts cannot generate oscillations, and some can start but will soon fail. Since the number of core coils in the mass production is fixed, the test screening and binning of the core is an indispensable part.
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