Multi-layer chip ceramic capacitors are commonly known as MLCC, and Yirong.com has made a summary of the classification of MLCC characteristics. MLCC is divided into Class 1 and Class 2 according to materials. Class 1 is temperature compensated and Class 2 is temperature stable and common.
It is well known that Class 1 capacitors are usually composed of a mixture of titanates in which barium titanate does not account for the majority. They have predictable temperature coefficients and usually have no aging characteristics. So they are the most stable capacitors available.
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Figure 1: X7R capacitor temperature curve
Class 2 capacitors are also usually composed of barium titanate compounds. It has great capacitance and temperature stability. The most commonly used Class 2 capacitor electrolytes are the X5R and X7R. The X5R and X7R provide only ±15% change in capacitance over the temperature range. Suitable for applications where the temperature range is wide and the capacitance requirements are stable.
According to the research, LED power supply is currently very popular in the world, and the ratio is getting bigger than that of incandescent lamps. The power is between 0.4w and 300w. Household lighting is mainly concentrated below 15w, and most commercial lighting is also below 15w. With the pursuit of aesthetics, the size of LED power supplies is getting smaller and smaller (see Figure 2), so the integration of LED power chips is getting higher and higher. Therefore, the amount of peripheral MLCC usage is small. At present, the demand for MLCC for LED power supply below 15w is mainly concentrated on Vcc, MULT, COMP, RC absorption and bypass and decoupling capacitors.
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Figure 2: Miniaturization of LED power supplies
At the same time, it is found that as IC integration becomes higher (such as integrated MOS), IC is basically a patch SOP-8 package because of volume requirements. The LED itself is a huge heat source, which leads to high ambient temperature of the LED power supply. The high integration of the chip also makes the temperature of the IC as a heat source high, and some even reach 110° and the temperature rises to 85°.
LED power supply generally has a small space and poor heat dissipation conditions. It is also thinking about how to ensure the quality and life of LED power supply. The research found that starting from the design, to avoid the LED power supply quickly failed, it can be said that LED power supply life is the key to restrict the development of LED. This is a comprehensive question that requires system design and consideration. The peripheral factors of LED power supply life are mainly the working environment temperature, grid stability, and switching times. The internal factors are mainly the choice of current protection resistors, electrolytic capacitors and MLCC capacitors.
Class 1 capacitors are generally small in capacitance, high in accuracy and high in cost, so there is less demand on LED power supplies. The LED power supply is mainly Class 2 X5R and X7R capacitors. The MULT and COMP capacitors in the LED power supply are mainly required to connect the impedance between the inverting input terminal and the compensating output end of the chip's internal operational amplifier, forming a negative feedback form, and the bypass capacitor jointly controls the stability of the power supply loop. Therefore, the stability of such capacitors essentially determines the quality of the power supply. Often such capacitors need to be placed close to the chip to ensure the stability of the chip operation. However, the high-temperature characteristics and small size of the LED power supply, and the IC integrated MOS itself is a heat source, so the ambient temperature is very high. The heat is transferred to the capacitor through the PCB, resulting in a high temperature of the capacitor, some up to 90°. The X5R temperature range is -55°--85°, while the X7R temperature range is -55°--125° (temperature profile is shown in Figure 1) and varies by about 1% ΔC every 10 years. Therefore, considering the quality of the LED power supply, the capacitors that need to be placed close to the chip such as MULT, COMP, and bypass capacitors should use X7R capacitors, and others can use X5R.
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