Filter capacitors play a critical role in power supply conditioning for circuit boards. They are used for decoupling, bypass and bulk energy storage purposes to suppress noise, stabilize voltage and guarantee reliable circuit operation.
Nevertheless, real world capacitors deviate from ideal behaviour due to equivalent series resistance (ESR) and equivalent series inductance (ESL).
Self-resonance and anti-resonance effects greatly alter amplitude frequency performance and may degrade filtering results.
This article introduces three functional categories of filter capacitors, analyses the influences of ESR and ESL, and provides practical guidance for capacitor selection as well as layout rules for decoupling, bypass and energy storage capacitors in PCB design.
Types of Filter Capacitors
Capacitors can be classified into three types based on their functions:
Decoupling: Breaks the coupling between ports in a system or circuit to ensure normal operation.
Bypass: Provides a low-impedance path to ground where transient energy is generated; this is one of the essential conditions for effective decoupling.
Bulk: Ensures that the voltage does not drop when the load rapidly reaches its maximum.
Capacitor Self-Resonance Issues
Capacitors used in actual applications are not ideal components;
Rather, they behave as a series combination of an ideal capacitor, inductance, and resistance. When multilayer capacitors are mounted on a PCB, they generate approximately 5 nH of parasitic inductance.
Combined with approximately 30 mΩ of lead resistance, this causes the filter capacitor to exhibit a bandpass filter characteristic centered on the self-resonance point, rather than the ideal low-pass filter response.
When two capacitors are connected in parallel, the presence of ESL and ESR causes anti-resonance issues, resulting in impedance peaks at specific frequencies that impair the bypass effect.
Effect of ESR and ESL on the Amplitude-Frequency Characteristics of Parallel Capacitors
ESR Effect: As ESR decreases, the impedance at the resonance point decreases, but the impedance at the antiresonance point increases;
When multiple identical capacitors are connected in parallel, the minimum impedance may be less than ESR/n.
ESL Effect: ESL varies depending on the capacitor’s packaging and structure.
For example, the ESL of a surface-mount ceramic capacitor (0805 package) is approximately 0.7–0.9 nH, while that of a leaded capacitor is approximately 1.63 nH.
During design, capacitors with low ESL should be selected whenever possible.
Capacitor Selection
For RF designs, ceramic capacitors, polyester capacitors, and polystyrene film capacitors are all excellent choices.
For EMI filters, the dielectric material requirements for capacitors are not stringent; loose dielectrics such as X7R, Y5V, and Z5U are all suitable.
It is recommended that single-board systems operating at frequencies above 50 MHz use 0.01 μF filter capacitors exclusively, rather than the widely used 0.1 μF capacitors.
Design Recommendations for Decoupling and Bypass Capacitors
Select capacitors based on the self-resonant characteristics specified in the supplier’s product documentation to meet the requirements of the design’s clock rate and noise frequency.
Add as many capacitors as possible within the required frequency range.
Place at least one decoupling capacitor as close as possible to each power supply pin of the IC to minimize parasitic impedance.
Place bypass capacitors on the same PCB plane as the IC whenever possible.
For multi-clock systems, divide the power plane and use capacitors with the correct capacitance values for each section.
For systems with a wide range of clock frequencies, place two capacitors with capacitance values in a ratio of approximately 2:1 in parallel to provide a wider low-impedance region.
Design of Energy Storage Capacitors
Board-level energy storage capacitors:
To ensure that the supply voltage across the board does not drop when the load suddenly reaches its maximum, it is recommended to evenly distribute high-capacitance tantalum capacitors (1 μF, 10 μF, 22 μF, 33 μF).
Component-Level Energy Storage Capacitors: These ensure stable supply voltages around components.
For components with high operating frequencies and high power consumption, it is recommended to place 1 to 4 high-capacitance tantalum capacitors around them.
The design of energy storage capacitors should be distinguished from that of decoupling capacitors.
Use a single capacitance value for energy storage capacitors connected to the same supply voltage, and distribute them evenly across chip clusters with different supply voltages.
Conclusion
Decoupling, bypass and bulk energy storage capacitors serve distinct circuit functions.
Parasitic ESL and ESR produce self-resonance in individual capacitors and anti-resonance impedance peaks for parallel combinations, which limit effective filtering bandwidth.
Proper component selection must consider application scenarios:
different dielectric materials and capacitance values are chosen for RF circuits, EMI suppression and high-frequency single-board systems.
Good layout practice requires placing decoupling capacitors extremely close to IC power pins, minimizing parasitic impedance and partitioning power planes for multi-clock hardware.
For energy storage requirements, high value tantalum capacitors should be evenly distributed at board level and component level, and energy storage design must be clearly differentiated from decoupling design.
Fully understanding resonant characteristics and following layout recommendations can optimise power supply noise suppression and improve overall circuit stability.
