Column: EMC
The importance of capacitors in the design of EMI filters
By Dr Min Zhang, EMC consultant at Mach One Design, and design engineers at REO UK C
apacitors are energy storage devices that store electrical charge without a continuous current flow. By using a charge and discharge
method, capacitors reduce magnetically- driven electrical impulses, limiting the likelihood of electromagnetic interference (EMI). The simplest form of a capacitor is
two conductors separated by a dielectric material like air. For example, when you see a power transmission line over the earth, each cable forms a capacitance with the earth. As an energy storage device, capacitors
are made with a high dielectric material to store more energy. Capacitors can be electrolytic, film or ceramic.
Electrolytic capacitors Capacitance is the measured value of the ability of a capacitor to store an electric charge. Compared with other types,
electrolytic capacitors can achieve a relatively large capacitance value. As a prime energy source, electrolytic
capacitors are mostly found in the DC link of a grid-tied inverter design or the input side of a DC-DC converter. The selection of electrolytic capacitors
The selection of electrolytic capacitors depends on several factors: capacitance value, ESR and ESL, along with ageing, temperature
extremes, humidity stress and leakage current
depends on a variety of factors: one is the capacitance value, which determines the low frequency ripple of the circuit and its ESR (equivalent series resistance) and ESL (equivalent series inductance), which determine the capacitor’s differential-mode noise ripple. Temperature is also important since it affects the capacitor’s ESR, along with with ageing, temperature extremes, humidity stress and leakage current. Sometimes, increasing the ESR of an
electrolytic capacitor is a good move. In certain applications we need some resistive components to improve the system’s damping. Resonance in a system can be caused by input cable inductance and the input ceramic
10 November 2022
www.electronicsworld.com
capacitors, which generally have a very small ESR value. One effective way of preventing this is to add an electrolytic capacitor in between, since its ESR will make the system more stable. Figure 1 shows a 2m cable between
a voltage source and the circuit. We simulated two scenarios, one with and one without an electrolytic capacitor. The step response shows that the system is damped by the electrolytic capacitor.
Ceramic capacitors Ceramic capacitors are small devices that can deliver energy quickly. One of the most frequently asked questions about them is: “What capacitance do I get when I buy a multilayer ceramic capacitor (MLCC)?” This might sound odd, but with
MLCCs the capacitance value you get is not the one stated in the datasheet, since the actual capacitance value depends on tolerance, temperature coefficient, dielectric class, and more. The DC voltage applied to the
capacitor also has a big impact on capacitance. It is not surprising that the effective capacitance value is only 50% of the value stated in the datasheet. Perhaps another question worth
asking is: “How much capacitance do I need for a ceramic capacitor?” The answer is that the capacitance value shouldn’t matter that much. The important consideration here is to work out the frequency at which the speed of the energy delivery would be sufficient for the application. If a conducted emission failed at 100MHz, then a capacitor with the least impedance at 100MHz would be a good option. Here is another misconception of
MLCCs. Engineers often spend great effort selecting a ceramic capacitor with the lowest ESR and ESL, only to connect
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