Easily overlooked MLCC selection tips
Time：2020.08.27
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Although MLCC is relatively simple, it is also a device with a relatively high failure rate. High failure rate, on the one hand, is the inherent reliability problem of the MLCC structure, and there are also selection problems and application problems.
Since capacitors are considered "simple" devices, some design engineers do not understand the unique characteristics of MLCC due to insufficient attention. In an ideal situation, when selecting a capacitor, it is enough to mainly consider the two parameters of capacity and withstand voltage. But for MLCC, it is far from enough to just consider these two parameters.
The impact of different materials of MLCC on performance
When using MLCC, one has to understand the different materials of MLCC and the corresponding performance of these materials. There are many kinds of MLCC materials, and each material has its own unique performance characteristics. If you don't understand these, the selected capacitors may not meet the circuit requirements. For example, common MLCCs are C0G (also called NP0) material, X7R material, and Y5V material. C0G has the best operating temperature range and temperature coefficient, and the temperature coefficient is 0 ±30ppm/°C in the operating temperature range of -55°C to +125°C. X7R comes next, with a capacity change of ±15% in the operating temperature range of -55°C to +125°C. The operating temperature of Y5V is only -30°C to +85°C, and its capacity can vary from -22% to +82% within this operating temperature range. Of course, the costs of COG, X7R, and Y5V are also reduced in sequence. When selecting models, if the requirements for working temperature and temperature coefficient are very low, Y5V can be considered, but X7R should be used under normal circumstances, and COG should be selected when the requirements are higher. In general, MLCC manufacturers design to maximize the capacity of capacitors made of X7R and Y5V at room temperature, but as the temperature rises or falls, their capacity will decrease.
It is not enough to understand the above knowledge. Since the dielectric constants of the C0G, X7R, and Y5V media decrease in sequence, the maximum capacity that can be made under the same size and withstand voltage also decreases in sequence. Some inexperienced engineers think that they have whatever capacity they want, and they will make mistakes when selecting models and choose specifications that do not exist. For example, I want to use 0603/C0G/25V/33000pF capacitors, but 0603/C0G/25V MLCCs generally only achieve 10000pF. In fact, as long as you carefully read the selection manual provided by the manufacturer, you will not make such a mistake. In addition, for design engineers who have just started, they have no idea about the number sequence of component specifications (E12, E24, etc.), and will give out non-existent specifications such as 0.5uF. Even experienced engineers have no idea about the compression of specifications. For example, in the filter circuit, someone originally used a 3.3uF capacitor, and his circuit can also use a 3.3uF capacitor, but he might choose a 4.7uF or 2.2uF capacitor that no one has used before. Those who don’t read the manufacturer’s selection manual will make the following mistakes, such as choosing a 0603/X7R/470pF/16V capacitor. In fact, the general manufacturer’s 0603/X7R/470pF capacitor only produces 50V and its The above voltage does not produce 16V and the like.
Note that there are two representations for the package of chip capacitors, one is British representation and the other is metric representation. American manufacturers use imperial systems, Japanese manufacturers basically use metric systems, while domestic manufacturers use imperial and metric systems. Capacitor packages used by a company can only be expressed in one standard, not the imperial system for the engineer and the metric system for the engineer. Otherwise it will mess up. In extreme cases, mistakes can be made. For example, the inch version has a 0603 package, and the metric version also has a 0603 package, but the two are actually completely different sizes. The British 0603 package corresponds to 1608 for the metric system, while the metric 0603 package corresponds to 0201 for the British system! In fact, the number of the British package is approximately multiplied by 2.5 (the first 2 digits and the last 2 digits are multiplied separately) to become the metric package specification. Now the popular way is to use the imperial package expression. For example, the 0402 package we often say is the expression of the British system, and the corresponding metric package is 1005 (1.0*0.5mm).
In addition to understanding the temperature performance of MLCC, design engineers should also understand more performance. For example, although the capacitance of Y5V dielectric is very large, this ferroelectric ceramic has a shortcoming, that is, its static capacitance decreases with the increase of its DC bias working voltage, and the maximum even decreases by 70%. For example, a Y5V/50V/10uF capacitor may have a capacity of only 3uF under a DC voltage of 50V! Of course, the characteristics of different manufacturers are different, and some declines may not be so severe. If you must use Y5V capacitors, in addition to knowing the graph of its capacity versus temperature, you must also ask the manufacturer for the graph of its capacity versus DC bias voltage (even a comprehensive graph of capacity temperature DC bias) . Use Y5V capacitors to have sufficient voltage derating. The capacity of X7R decreases with the increase of its DC bias operating voltage, but it is not as obvious as that of Y5V. At the same time, the smaller the MLCC size, the more obvious this effect.
