Serial | Murata Noise Suppression Small Class: EMI Suppression Filter - Actual Characteristics of Capacitors (Part 2)

This section discusses why the noise reduction characteristics of a simple bypass capacitor differ from the basic characteristics. Understanding why can help you build filters that provide superior noise reduction at a lower cost and choose cost-effective components.

6-5-3. Effect of capacitor parasitic elements

(1) How does the impedance change?

The previous chapter introduced the fact that the impedance frequency characteristic of a capacitor forms a V shape, and that low frequency (left) and high frequency (right) correspond to electrostatic capacitance and ESL, respectively. The electrostatic capacitance of a capacitor can be easily controlled by specifying the part number. How effective is ESL?

Figure 12 Example of impedance change when the installation form is modified (1,000pF)

Figure 12 shows an example of impedance measurements from several types of ceramic capacitors with a nominal electrostatic capacitance of 1,000pF. The diagram shows...

1. MLCC (laminated structure) (not single board)
2. Capacitors with shorter leads
3. SMD capacitors (not leaded capacitors)

Both are closer to ideal capacitors and have less impedance until higher frequencies are reached. This also shows that ESL decreases in this order. This trend is generally present in all capacitors - not just ceramic capacitors. This is because the main factors behind ESL are the internal electrodes and lead shapes.

When capacitors are used to eliminate emitted noise, they are used at frequencies of 30MHz or higher. As shown, even using the same 1,000pF capacitor, there can be a 10x or greater difference at that frequency due to differences in ESL.

(2) What is the ESL value?

So what is the value of ESL now?

Figure 13 shows the results of calculating the impedance after changing the ESL on a 1,000pF capacitor using the equivalent circuit model.

Comparing Figure 12 and Figure 13, we can estimate that the ESL will be:

• About 10nH (for MLCC with 10mm leads) ((2) in Figure 12)
• 1nH or less (for leadless SMD MLCC) ((4) in Figure 12)
• 0.1nH or less (for a three-terminal capacitor) ((5) in Figure 12)

The nH values ​​mentioned here are extremely small values ​​that occur on leads only a few millimeters long. Looking at frequencies above 100MHz in the figure, you will find that even such a weak inductance has a significant effect.

Note that the three-terminal capacitor shown in (5) of Figure 12 is a high-performance capacitor that uses a special structure designed to reduce ESL. Three-terminal capacitors will be discussed further in Chapter 8.

(3) Use capacitor leads as short as possible

It's very good to use capacitors with very low ESL to suppress noise. As shown in (2), (3) and (4) in Figure 12, when using capacitors, the leads should be as short as possible (SMD should be used if possible).

In fact, in the experiment shown in Figure 2 of Section 6-4, the noise reduction effect was changed by the difference in ESL of the capacitor itself and the difference in ESL caused by the presence or absence of leads. If the capacitor is mounted on a lead wire approximately 10 mm long (Section 6-4, Figure 2 (d)), the noise reduction effect will be at least reduced compared to the case without a lead wire (Section 6-4, Figure 2 (c)) 10dB.

(4) Impedance characteristics of electrolytic capacitors

To date, explanations of capacitor characteristics have mostly used MLCCs as examples. For applications requiring large electrostatic capacitance (such as power leveling), use electrolytic capacitors with large electrostatic capacitance per volume. The impedance characteristics of electrolytic capacitors are slightly different from those of MLCCs. Figure 14 shows some comparative examples.

Aluminum electrolytic capacitors are sometimes used for power leveling. Figure 14 shows that the impedance curve of an aluminum electrolytic capacitor forms a bowl (or U-shape). The figure also shows that the self-resonance cannot be clearly seen.

This means that the capacitor losses are relatively large; there will be significant ESR in the equivalent circuit of Figure 7.

Figure 14 Example of comparing impedance of electrolytic capacitors and MLCCs

(5) What effect does ESR have?

Figure 15 shows the results of calculating the impedance when the ESR changes, using a 1μF capacitor as an example. At an ESR of 500 Megohms, characteristics similar to those measured for the aluminum electrolytic capacitor in Figure 14(a) can be obtained. Therefore, the impedance characteristics of electrolytic capacitors can be reproduced by increasing the ESR. The impedance corresponding to the bottom of the bowl-shaped characteristic curve represents the ESR value.

The ESR of aluminum electrolytic capacitors can reach 1Ω or more. Capacitors with higher impedance will not have less than the ESR; this means that capacitors with larger ESR are not suitable for noise suppression.

On the other hand, capacitors used to suppress noise may resonate with surrounding circuits, causing malfunctions. In this case, the ESR can be used as a resonant damping resistor to prevent this failure. Therefore, a capacitor with a slightly larger ESR would be beneficial.

Figure 15 Calculation results of impedance change when ESR changes

(6) Electrolytic capacitors with lower ESR

Some electrolytic capacitors are designed to greatly reduce ESR. Examples include tantalum capacitors and conductive polymer capacitors. The measurements in Figure 14 also include examples using these capacitors. The illustration shows that the impedance around the resonant frequency is smaller than the impedance of the aluminum electrolytic capacitor.

However, this does not extend to MLCCs, even with these capacitors. Even in applications that require large electrostatic capacitance (such as power leveling), when noise reduction is important, a large-capacity MLCC should be selected, or the MLCC should be installed in parallel with an electrolytic capacitor for use.

muRata

MORE CONTENT