Parasitic Effects How Three-Terminal Capacitors help minimise unwanted Inductances

Working with ideal components allows engineers to be clear about their design intent. However, the physics of reality demands that they also consider the parasitic effects created by real currents flowing in real devices. This article shows, how three-terminal capacitors help to minimise unwanted inductances.

High-school physics teaches us that an electromagnetic field is generated when charge passes through a conductor. In many circuits designs this basic fact is set aside as an inconvenience, so that designers can work with ”ideal” passive components that only have, for example, their specified resistance, capacitance, or inductance. When it comes to implementation, though, the physics of reality cannot be denied, and designers must work with or around secondary effects in the components they use. Now, new passive component architectures such as three-terminal capacitors are being introduced to meet the challenging requirements of emerging applications such as advanced driver assistance systems (ADAS).

Take the capacitor shown in Figure 2. It has a specified capacitance of C, plus three other characteristics: R_ISOL, R_ESR and L_ESL.

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R_ESR is the equivalent series resistance of the capacitor’s terminals and electrodes. L_ESL is the equivalent series inductance, effectively the parasitic inductance created by current flowing through the capacitor’s terminals and electrodes. This is undesirable, especially if it becomes relatively large. A high R_ESR will turn electrical energy flowing through the capacitor into heat, which will accelerate the device’s ageing and shorten its operating lifetime.

High L_ESL values contribute to unwanted current flows in the device that generate electromagnetic interference (EMI). This issue is becoming more acute as current flows rise with increased functional integration. High L_ESL values also mean that the capacitor’s inductance, in series with its capacitance, forms a reactive circuit with frequency-dependent impedance characteristics. Reducing the capacitor’s L_ESL therefore helps it retain its capacitive behaviour to higher operating frequencies than would otherwise be possible.

Reducing Equivalent Series Inductance

There are several ways of reducing L_ESL in capacitors. One way is to make the capacitors smaller, by using more sophisticated materials to increase their charge storage density. This approach shortens the current’s path through the capacitor, and so reduces the electromagnetic field within the device.

Another way is to ensure that capacitors are mounted on the printed circuit board (PCB) to minimise the length of the current loop between their electrodes, and so that any related vias are also as close to them as possible. The fourth, and most profound way to reduce unwanted inductances is to change the geometry of the capacitor by introducing a third terminal.

Shrinking Capacitors to cut Equivalent Series Inductance

Making capacitors smaller reduces L_ESL, at the cost of reducing their capacitance and therefore requiring the use of multiple devices in parallel to achieve the same performance. This approach also demands changes to the PCB, which takes design time and could affect the functionality of other parts of the circuit.

Another approach is to change the aspect ratio of the capacitor, so that its terminals become wider and closer together. This shortens the current path through the device and reduces the resistance of its terminals. It also demands minimal changes to the design’s PCB, since the device’s capacitance remains the same and its geometry is like the original version. Figure 3 shows the standard geometry of multilayer ceramic capacitors (MLCCs), whose tall, narrow aspect ratio creates long current paths.

In the ”reverse-geometry” capacitor (Figure 4), the terminals are wider, reducing their resistance, and closer together, shortening the current path and so reducing the induced field.

The Advantages of Adding a Third Terminal to MLCCs

Murata has developed a range of MLCCs with a third terminal (implemented as a pair of connections (Figure 5), which help reduce the device’s L_ESL. The NFM series MLCCs retain the advantages of the reverse-geometry capacitor, such as its short current path and large sectional area for current flow. Adding the third terminals also creates four parallel current paths through the device, which suppresses noise currents (Figure 5).

Using these devices does demand minor changes to PCB layouts, and Murata has a design team that can help with the necessary adjustments. The relatively high capacitance of the NFM parts also means that designers can use fewer of them than would be necessary if designers tried to limit L_ESL by using multiple smaller-value devices in parallel.

The third terminal of these devices also allows for some novel noise reduction strategies.

The usual approach with a three-terminal capacitor is to break the PCB trace for the signal that you want to protect from noise, insert the capacitor inline, and then connect the third terminal to ground. This reduces the EMI current passed through the circuit trace and reduces its LESL because there are now two parallel paths for current to ground.

A more unconventional approach has been developed to stabilise voltage fluctuations in an IC by using a three-terminal device as a bypass capacitor on its power supply. The three-terminal capacitor is connected to the power supply line without cutting its trace, effectively in parallel with it. This halves the device’s LESL, cuts the impedance of the bypass route, and so reduce any variations in the voltage presented to the IC.

It is helpful to ensure that the power and ground vias serving each IC on the PCB are next each other, so that the magnetic fluxes generated by the current through them cancel out. This approach is less effective at reducing EMI emissions because some of the noise passes through the power line without going through the three-terminal capacitor.

Working with ideal components allows engineers to be clear about their design intent. However, the physics of reality demands that they also consider the parasitic effects created by real currents flowing in real devices. Careful component design, materials, process, and topology innovations such as Murata’s three-terminal NFM capacitors can all help designers of sophisticated systems such as ADAS controllers to develop the most effective solutions. (tk)

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