Why are Three-Terminal Capacitors Highly Favored in Mobile Smart Products?

Capacitors are classified into types such as ceramic capacitors and electrolytic capacitors. Ceramic capacitors are widely used in mobile intelligent products and can be divided into two-terminal capacitors and three-terminal capacitors. It is commonly said that three-terminal capacitors have good high-frequency characteristics. As a hardware engineer, do you understand three-terminal capacitors? Figure 1-9 is a physical comparison of two-terminal capacitors and three-terminal capacitors, and it can be seen that three-terminal capacitors have a few more pins. Due to this design difference, three-terminal capacitors have significant performance advantages but also come with a high price tag. It's hard to have your cake and eat it too.

 

Figure 1-9 two-terminal capacitor and three-terminal capacitor

An ideal capacitor has an impedance that decreases as the frequency increases. In other words, the higher the frequency, the lower the capacitor's "resistance" and the closer it gets to a short circuit state, which makes it easier for the signal to pass through. This is the reason why capacitors "block DC and pass AC." However, actual capacitors have parasitic parameters. Due to the presence of series equivalent resistance (ESR) and series equivalent inductance (ESL), the impedance-frequency characteristics of the capacitor undergo significant changes. Figure 1-10 shows the impedance-frequency characteristics of an actual two-terminal capacitor. We can see that in the low-frequency range, capacitance dominates, and impedance decreases as the frequency increases. However, in the high-frequency range, inductance dominates, and impedance increases as the frequency increases, which is something we don't want to see.

 

Figure 1-10 Actual Capacitance Impedance Frequency Curve

The so-called good high-frequency characteristics of three-terminal capacitors are due to their low ESL. Figure 1-11 shows the difference in impedance between a 22μF two-terminal capacitor and a three-terminal capacitor. The two-terminal capacitor has an impedance of approximately 3mΩ at 1MHz, while the three-terminal capacitor has a higher resonant frequency, and its impedance is only about 2mΩ at 3MHz. Looking at the high-frequency part, the impedance of the two-terminal capacitor even exceeds 1Ω at 1GHz, while the impedance of the three-terminal capacitor is only 110mΩ. To reduce the high-frequency impedance of the two-terminal capacitor, more capacitors need to be connected in parallel (with smaller inductance). In other words, multiple two-terminal capacitors need to be connected in parallel to achieve the impedance characteristics of a single three-terminal capacitor at high frequencies. The high-frequency characteristics of three-terminal capacitors are excellent!

 

Figure 1-11  Comparison between Two-Terminal Capacitors and Three-Terminal Capacitors.

Because of their good high-frequency characteristics and small package size, three-terminal capacitors are sometimes used in power delivery network optimization to replace two-terminal capacitors, which not only improves the system's power stability but also reduces the area of capacitor layout. 

Why do three-terminal capacitors have good high-frequency characteristics? (Readers should develop the habit of asking questions.) The same question: why is the ESL of three-terminal capacitors small? That's because the unique structure of three-terminal capacitors shortens the current path, making ESL have parallel characteristics, thereby reducing ESL, as shown in Figure 1-13. Regular capacitors have an ESL in the direction of the current, but the unique structure of three-terminal capacitors causes these ESLs to be connected in parallel, resulting in a reduction of ESL (with smaller inductance) and better high-frequency characteristics.

 

Figure 1-13 Low ESL of Three-Terminal Capacitors.

Although three-terminal capacitors have good high-frequency characteristics and smaller packages, they come with a high price tag. If you ask how to balance price, the answer is: in designs where area is not a concern, one word "disassemble."