In power supply design, the role of capacitors is crucial, especially the selection and application of decoupling capacitors, which directly affects the stability and anti-interference ability of the system. Ceramic capacitors, tantalum capacitors, and electrolytic capacitors are suitable for different scenarios due to differences in structure, materials, and performance. The following is an analysis from the core differences, key parameters, application examples, and design techniques.

C: Capacitor capacitance.

ESL: Capacitor equivalent series inductance. The pins of the capacitor have inductance. In low-frequency applications, the inductance is small, so it can be ignored. When the frequency is higher, this inductance must be considered. For example, a 100nf chip capacitor in a 0805 package has an inductance of 1.2nH per pin, so the ESL is 2.4nH. The resonant frequency of C and ESL can be calculated to be around 10MHz. When the frequency is higher than 10MHz, the capacitor exhibits inductance characteristics.

ESR: Capacitor equivalent series resistance. No matter what kind of capacitor it is, there will be an equivalent series resistance. When the capacitor works at the resonant frequency, the capacitive reactance and inductive reactance are equal, so it is equivalent to a resistor, which is ESR.

Ⅰ. The core difference between the three types of capacitors

1. Ceramic capacitors

Principle and structure: Based on physical reaction charging and discharging, the response speed is extremely fast (up to GHz level), non-polar, and the dielectric materials include C0G (best temperature stability), X7R, Y5V, etc.

Features:

Excellent high-frequency performance: low ESR (tens of milliohms), small ESL (equivalent series inductance), suitable for high-frequency decoupling.

Capacitance and temperature/voltage sensitivity: C0G has stable capacitance but small capacity (usually <1μF); Y5V has large capacity (up to tens of μF), but is significantly affected by temperature and DC bias (for example, the capacity of a 50V withstand voltage Y5V capacitor may drop to 30% of the nominal value at 30V).

Mechanical fragility: fragile, need to be away from the deformation area of the circuit board.

2. Tantalum Capacitor

Principle and structure: It is a kind of electrolytic capacitor. The dielectric layer is formed by oxidation of tantalum powder particles inside. The charging and discharging speed is fast, but it has polarity.

Features:

Small size and large capacity: The capacity is higher than that of ceramic capacitors at the same volume, and the ESR is between aluminum electrolytic and ceramic capacitors.

Poor surge resistance: It is easy to break down and short-circuit due to instantaneous large current, and it should be avoided in high surge scenarios (such as the input end of the switching power supply).

The trade-off between withstand voltage and capacity: The particle fineness determines the capacity, and the particle size affects the withstand voltage. It is difficult to have both high withstand voltage and large capacity.

3. Aluminum Electrolytic Capacitor

Principle and structure: Charge and discharge through aluminum foil oxidation and electrolyte chemical reaction, slow response speed (usually <1MHz), polarity.

Features:

Large capacity and low cost: suitable for low-frequency filtering (such as ripple suppression at the power input end), but high ESR (hundreds of milliohms to several ohms)

Life is greatly affected by temperature: the electrolyte is volatile, and the life is halved for every 10°C increase in temperature (for example, a life of 10,000 hours at 27°C will only be 1,250 hours at 57°C).

Ⅱ. Design logic and parameter selection of decoupling capacitors

1. Function of decoupling capacitors

Suppressing common-path coupling interference: When the chip needs a large current instantly, the decoupling capacitor is used as a "temporary power supply" to supply power nearby, avoiding voltage drops due to line inductance and reducing interference to other circuits

Differences between filtering and decoupling:

Filtering: Filtering out external noise (such as switching power supply ripple), requiring large-capacity capacitors (such as aluminum electrolytic capacitors).

Decoupling: Suppressing the leakage of local circuit noise, requiring fast-response capacitors (such as ceramic capacitors)

2. Selection of key parameters

Resonant frequency: The ESL and capacitance of the capacitor determine the resonance point (such as the resonant frequency of a 0.1μF ceramic capacitor is about 10MHz). Above this frequency, the capacitor is inductive and loses its decoupling effect.

ESR and multiple capacitors in parallel: Low ESR can reduce voltage fluctuations. Multiple small capacitors in parallel (such as two 0.01μF) have lower impedance in the high-frequency band than a single large capacitor (such as 0.1μF), and the decoupling effect is better.

Ⅲ. Application Examples and Layout Techniques

1. Example: Power supply design for high-speed chips

Multi-level capacitor combination:

Chip level: Place multiple ceramic capacitors (such as 0.1μF and 0.01μF combination) near the chip power pins to cover the wide frequency band impedance requirements.

Module level: Add tantalum capacitors or aluminum electrolytic capacitors (such as 10μF) at the power input of the functional module to supplement low-frequency energy storage.

Typical configuration: A 500-pin BGA chip requires at least 30 ceramic capacitors and several large capacitors for a 3.3V power supply, with a total capacity of ≥200μF.

2. Layout optimization

Capacitor placement order: The power input is aluminum electrolytic capacitor (filtering low-frequency noise) → tantalum capacitor (medium frequency) → ceramic capacitor (high frequency) in turn, forming a "low impedance channel".

Distance and routing: Decoupling capacitors should be as close to the chip pins as possible to shorten the current loop and reduce the influence of parasitic inductance

IV. Selection and pit avoidance guide

High-frequency scenarios: Give priority to ceramic capacitors made of C0G/NP0 material (good temperature stability) and avoid Y5V.

Tantalum capacitors should be used with caution in the following scenarios: Switching power supply input, circuits with surge current, use aluminum electrolytic or parallel ceramic capacitors instead.

Lifespan and heat dissipation: Aluminum electrolytic capacitors should be kept away from heat sources, and solid electrolytic capacitors can be used in high temperature environments