Capacitors may be the best tool in your arsenal for solving electromagnetic compatibility (EMC) issues. In the simplest terms, the function of a capacitor is to block DC signals while providing a low impedance path for AC signals and providing a local source of energy for ICs. However, these components are anything but simple, and adding them to a design can have unintended side effects.
So far, the “EMC Mitigation” sub-series has explored how to leverage inductors, resistors, and ferrite beads to improve EMC performance in electronic circuit designs. But when it comes to managing the flow of unwanted noise in your system, I’ve been saving the best component for last. This post will cover some basic effects of capacitors in EMC design and how to choose the right capacitor for EMI suppression.
In an ideal scenario, a capacitor would possess a singular value of reactance such that, as the frequency increases, the reactance would decrease (Eq1).
Equation 1. Capacitor reactance
The concept of a perfect capacitor is, unfortunately, a theoretical one. Just like ferrite beads in the previous blog, real-world capacitors are far from perfect and can introduce unexpected that are less than desirable, such as Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). ESR and ESL are considered 'parasitic' elements due to their undesirable effect on the capacitor's performance. Figure 1 shows an equivalent circuit for a capacitor.
Figure 1. An equivalent circuit for a capacitor
To further complicate matters, the values of ESR and ESL are not constants. They fluctuate based on factors like the size of the capacitor package, the type of dielectric material used, and the leads. Therefore, when dealing with capacitors in real-world applications, it's crucial to account for these parasitic elements to ensure the efficient operation of the overall circuit. So how can you do that?
The secret lies in the capacitor’s resonant frequency. The impedance at the resonant frequency is the lowest impedance the capacitor will have. Remember that the goal is often to provide a low impedance path for AC signals—an easy path through which to direct EMI noise.
Resonant frequency is determined by the capacitance C and the ESL. At this frequency, the capacitive and inductive reactance essentially nullify each other, and the impedance of the capacitor is determined by the ESR. This is demonstrated in Figure 2 for a typical 100nF, 0603 capacitor.
Figure 2. Impedance curve for a 0603, 100nF Capacitor (GCJ188R71C104KA01) [1]
Below self-resonance, the capacitor looks capacitive and has an impedance that decreases with frequency. Above self-resonance, the capacitor looks inductive and has an impedance that increases with frequency. You can maximize the capacitor’s performance by selecting a component with a series self-resonant frequency corresponding to the unwanted noise frequency. Make sure that the highest noise frequency of interest in your design is equal to or less than the resonant frequency of the capacitor.
When designing for EMC, you may consider using surface mount capacitors to filter EMI. Capacitors are often used in EMC mitigation to shunt energy to ground and reduce the size of current loops, and in this respect, the surface mount type is no different from any other capacitor.
Due to their compact size and lack of leads, surface mount capacitors carry considerably lower inductance than their leaded counterparts, enhancing their efficiency as high-frequency capacitors—in other words, they have a higher resonant frequency. Generally, the smaller the package or case of the capacitor, the lower the inductance. We know that RF noise typically follows the path of least inductance; therefore, choosing the optimal capacitor means minimizing the series inductance at the noise frequencies and providing that “path of least resistance” for EMI noise to leave the system.
In this blog post, I chose to focus on one aspect of capacitors as an introduction to their use in EMC mitigation. Capacitors of course have many uses, even just within EMC design, and I look forward to exploring more of those in future posts. Please don’t hesitate to contact me if you need further help with capacitor selection for EMI suppression.
