Capacitors are one of the most commonly used components in electronic circuit design. A typical mobile phone motherboard will use over 3000 electronic devices, of which nearly 1000 are capacitors, accounting for almost 30%. The current flowing through a capacitor is proportional to the change in voltage across the capacitor, as shown in Equation (1-1). In some books or classrooms, only the capacitance parameter is introduced when discussing capacitors. In fact, a capacitor is not just a simple symbol "C". In addition to the basic capacitance value, it has several other very important parameters. In some cases, a capacitor may even become an inductor. New engineers often overlook these parameters, leading to reduced product reliability.
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Ceramic capacitors and electrolytic capacitors are commonly used in engineering, as shown in Figure 1-2. Electrolytic capacitors have polarity (including tantalum capacitors), which means there is a directional requirement when using them. These capacitors need to be connected to the power supply and ground with attention to polarity, otherwise the capacitor may explode. On the other hand, MLCC (Multi-layer Ceramic Capacitors) capacitors do not have polarity requirements. This section introduces the five characteristic parameters of MLCC ceramic capacitors. It is important to understand that in engineering, there are no perfect components in real-world environments, and all components have parasitic parameters.
This is a very important parameter, which refers to the capacitance value decreases with the effective voltage applied between the two terminals. In other words, the higher the two-terminal voltage of the capacitor, the lower the capacitance value. If the bias characteristic is not considered in the design, the capacitor is likely to fail or fail to meet the performance requirements. Figure 1-3 is the bias characteristic of 10 uF capacitor GRM319B31A106KE18, when the voltage is 10 V, the effective capacitance value is reduced by nearly 70%, only about 3 uF. |
| figure 1-3 DC bias characteristic |
In some data sheets, special emphasis is given to the requirements of capacitor bias conditions, such as the requirement in Figure 1-4 that the capacitance C5 10nV should be 1nF under 11V bias. When selecting capacitors, you can not only focus on the capacitance value (10nF) and withstand voltage (16V) of the capacitor itself, but also check the data sheet of the capacitor to ensure that the bias characteristics of the selected capacitor meet the requirements of Figure 1-4.
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| figure 1-4 The requirement for bias characteristic of capacitor in the manual (LSM9DS1) |
- The equivalent model of capacitor
Ideally, the capacitor impedance decreases as the frequency increases, but in reality, the capacitor has ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance), which can form series resonant, as shown in Figure 1-5. Therefore, its impedance will have a transition at a certain frequency, and the impedance will increase with the increase of frequency, and the capacitor will become an inductor instead.
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| figure 1-5 The equivalent model of capacitor |
Due to the existence of ESL and ESR, the actual capacitor presents capacitance at low frequency and inductance at high frequency, as shown in Figure 1-6, where the solid line is the impedance-frequency curve of the actual capacitor and the dotted line is the impedance-frequency curve of the ideal capacitor. At the transition point of the arrow, the reactance is equal to the reactance, and the impedance of the capacitor is minimum (the impedance includes resistance, reactance and inductance). What we usually say is that the high frequency characteristics of small capacitors are good, which means that the transition frequency point should be better. This transition frequency is also called resonance frequency, which is a very important parameter, very important in optimizing the mobile power PDN, and specific optimization operations will be described in detail in nest chapters.
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figure 1-6 Impedance Frequency
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The effective value of the capacitor will also vary with the change of AC voltage, as shown in Figure 1-7, the effective value of the capacitor has about 20% change under different AC voltages.
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| figure 1-7 AC characteristic |
This type of parameter describes the type of electrical medium material used in the capacitor, temperature characteristics and error parameters, which is a manifestation of the stability of the capacitor. For example, in Figure 1-8, X5R means -55℃~+85℃, and the capacity changes by ±15%. Ceramic capacitors are divided into Class I capacitors and Class II capacitors. C0G and NP0 are both Class I ceramic capacitors, which have good stability, high accuracy and are not sensitive to temperature, that is, the capacity change caused by temperature is very small, and the capacitance is usually small, usually pF level. X7R and X5R are Class II capacitors, which have slightly poorer temperature stability, and the capacity deviation caused by temperature is also larger, but the capacity can be larger, up to tens or hundreds of uF.
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| figure 1-8 Class II capacitor |
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