Capacitor selection is a crucial step in circuit design, as improper selection may lead to poor circuit performance, decreased reliability, or even failure. The following are the key rules and considerations that need to be followed:
1、 Clarify the core function of capacitors in circuits
This is the starting point for selection, determining which parameters to focus on.
Power filtering/decoupling:
Objective: To stabilize the power supply voltage, filter out high-frequency noise, and provide transient current.
Key parameters: Equivalent Series Resistance (ESR), Equivalent Series Inductance (ESL), Capacity (C), Self Resonant Frequency (SRF), Rated Ripple Current, Frequency Response.
Type preference: Multilayer ceramic capacitors (MLCC - high-frequency decoupling), aluminum electrolytic capacitors (large capacity energy storage), tantalum capacitors (medium frequency, medium capacity), polymer capacitors (low ESR, high ripple current).
Signal coupling:
Objective: To block direct current and allow alternating current signals to pass through.
Key parameters: Capacity (C), Accuracy/Tolerance, Voltage Coefficient/DC Bias Characteristics, Leakage Current, ESR/ESL (High Frequency Applications), Temperature Stability.
Type preferences: MLCC (NPO/C0G for high stability), film capacitors (polyester, polypropylene - high precision, low distortion), tantalum capacitors (low frequency large capacity).
Timing/oscillation circuit:
Goal: Determine the time constant or oscillation frequency together with resistance/inductance.
Key parameters: high precision/tolerance, extremely low temperature coefficient, low aging rate, low loss (high Q value), low voltage coefficient, low ESR/ESL.
Type preference: MLCC (preferred for NPO/C0G), film capacitor (polypropylene, polystyrene - highest precision and stability).
Energy storage:
Goal: Store charges and release them when needed (such as flashlights, backup power).
Key parameters: large capacity (C), low leakage current, appropriate ESR (affecting discharge efficiency).
Type preference: Aluminum electrolytic capacitor, supercapacitor (ultra large capacity), lithium-ion capacitor.
EMI/RFI filtering:
Objective: Bypass high-frequency noise to ground.
Key parameters: high-frequency characteristics, low ESL, suitable withstand voltage, safety certification (X capacitor, Y capacitor).
Type preference: MLCC (low ESL), specialized safety ceramic capacitors (X1/Y1, X2/Y2), thin film capacitors.
2、 Key electrical parameter rules
Rated voltage (Vrated):
Core rule: It must be higher than the maximum possible operating voltage in the circuit (including ripple voltage, transient voltage, anti peak voltage, etc.), and leave sufficient safety margin.
Reduced usage: It is strongly recommended to reduce usage to improve reliability and lifespan. Common derating rules:
Aluminum/tantalum electrolytic capacitors: 50% -80% derating. For example, if the maximum voltage of the circuit is 12V, at least 16V or 25V capacitors should be selected.
Ceramic capacitors: For types with significant DC bias effects (X5R, X7R, Y5V), the effective capacity will significantly decrease at high operating voltages. Please refer to the manufacturer's specifications for the DC bias characteristic curve to ensure that the capacity still meets the requirements under actual operating voltage. It is generally recommended to choose models with a rated voltage of 2 times or more the operating voltage, especially in high voltage or high-precision situations.
Thin film capacitors: usually have good voltage stability, but they also need to be appropriately rated (such as 20% -50%).
Transient voltage: If there are transient high voltages such as switch spikes and surges in the circuit, the rated voltage of the capacitor must be able to withstand these peak voltages, and protective devices such as TVS should be used if necessary.
Capacity (C):
Meet functional requirements: Calculate the minimum capacity required based on circuit theory (such as filtering cutoff frequency, time constant, energy storage requirements).
Tolerance: Understand the requirements of the application for capacity accuracy. Coupling and timing circuits typically require high precision (± 5%, ± 10%), while power decoupling has relatively loose tolerance requirements (± 20%).
Actual effective capacity:
DC bias effect: The actual capacity of MLCC (especially X7R, X5R, Y5V) will significantly decrease with the increase of applied DC voltage. When selecting, the actual capacity under operating voltage must be considered.
Temperature effect: Capacity will vary with temperature (see temperature coefficient).
Aging: The capacity of certain types of capacitors (such as X7R) will slowly decrease over time.
Equivalent series resistance (ESR):
Core impact: causing self heating (power loss=I ² ESR), affecting filtering effect (especially output ripple of switching power supply), and affecting discharge efficiency.
Selection rule: Select low ESR capacitors according to application requirements.
Switching power supply output filtering: extremely low ESR is crucial and directly affects the magnitude of output ripple voltage (Vripple=Iripple ESR).
High frequency decoupling: Low ESR helps provide a lower AC impedance path.
Large ripple current application: Low ESR can reduce self heating, improve reliability and lifespan.
Equivalent series inductance (ESL):
Core impact: Together with capacitors, it forms a series resonant circuit (self resonant frequency SRF). When the frequency is higher than SRF, the capacitor exhibits inductance and loses its decoupling/filtering effect.
Selection rules:
Choose low ESL packaging: Typically, small-sized packaging (such as 0402, 0201) has lower ESL than large-sized packaging (such as 1206). Three terminal and array capacitors can significantly reduce ESL.
Optimize PCB layout and routing: reduce the loop inductance from capacitor pads to vias/power planes.
For high-frequency applications (>10MHz), ESL is often more important than capacity itself.
Self resonant frequency (SRF):
Core rule: Capacitors exhibit capacitance only in the frequency range below their SRF. For decoupling capacitors, their effective decoupling frequency band is near their SRF (where the impedance is lowest).
Selection rule: The SRF of the selected capacitor should be higher than the target noise frequency that needs to be filtered/decoupled. High frequency noise requires small capacitors with high SRF (low ESL); Low frequency energy storage/filtering requires large capacity capacitors, but their SRF is relatively low, often requiring parallel connection of large and small capacitors.
Rated Ripple Current (Iripple):
Core impact: Heating caused by ESR inside the capacitor. Exceeding the rated value will accelerate aging and even thermal failure (bulging, explosion).
Selection rules:
It is necessary to calculate or simulate the RMS of the ripple current flowing through the capacitor in the circuit.
It is necessary to ensure that the rated ripple current of the selected capacitor is greater than the RMS value of the ripple current in the actual circuit.
Consider the environmental temperature and heat dissipation conditions, and further reduce the rating if necessary.
Leakage current:
Core impact: consumes energy and affects the accuracy and stability of high impedance circuits such as sample and hold, timers, and signal coupling.
Selection rules: For high resistance circuits, long cycle timing, precision coupling, and low-power applications, choose capacitor types with extremely low leakage current (such as film capacitors, C0G/NPO ceramics, specific polymer capacitors). Avoid using aluminum electrolytic capacitors with high leakage current.
Temperature coefficient:
Core impact: The degree to which capacity changes with temperature.
Selection rules:
Refer to the temperature coefficient indicated in the specification sheet (e.g. X7R: ± 15%, C0G: ± 30ppm/° C).
Evaluate the operating temperature range of the circuit (-40 ° C to+85 ° C? -55 ° C to+125 ° C?) and confirm whether the change in capacitance is within the acceptable range of the circuit throughout the entire temperature range.
For high stability requirements (oscillation, filtering, precision coupling), capacitors with low temperature coefficients (C0G/NPO, polypropylene film) must be selected.
Dielectric type (ceramic capacitors only):
Core impact: It determines the temperature stability, DC bias characteristics, aging characteristics, and dielectric loss of capacitors.
Selection rules: Choose according to application requirements:
C0G/NPO: Super stable, almost zero temperature coefficient and voltage coefficient, low loss. Used for high-frequency, timed, high-Q value, high-precision circuits. The capacity is usually small.
X7R: Medium capacity, moderate stability (± 15% temperature variation). Universal type, widely used for decoupling and coupling. Pay attention to the voltage coefficient.
X5R: Similar to X7R, but with an upper operating temperature range of 85 ° C.
Y5V: Large capacity, but extremely poor temperature and voltage stability (± 22% to+82%/-82%). Only used in situations where capacity and stability requirements are extremely low.
3、 Physical and Environmental Rules
Size and packaging:
Core rule: Must comply with PCB layout space and height restrictions.
Encapsulation affects ESL, heat dissipation capability, and mechanical strength.
Common packaging: MLCC, lead type (electrolytic, thin film), surface mount (SMD aluminum electrolytic, polymer, tantalum), bolt type (large electrolytic).
Working temperature range:
Core rule: The rated operating temperature range of the selected capacitor must fully cover the expected operating ambient temperature range of the circuit, taking into account its own heating.
High temperatures can accelerate the drying of electrolytes (electrolytic capacitors), reduce their lifespan, and alter parameters.
Low temperature may cause electrolyte freezing (electrolytic capacitance) and an increase in ESR.
Lifespan and reliability:
Core rule: Evaluate the lifespan requirements of the application scenario.
Electrolytic capacitor: Life is a key parameter (such as 2000 hours @ 105 ° C). The actual life follows Arrhenius' law, where the life is halved for every 10 ° C increase in temperature. The expected lifespan needs to be calculated based on the actual operating temperature and ripple current.
Ceramic/film capacitors: typically have a very long lifespan (several decades), with a focus on failure rate (FIT).
Environmental stress: Considering vibration, impact, humidity, chemical pollution, etc., select capacitors that meet corresponding standards or take protective measures.
Safety requirements:
Core rule: If the capacitor is connected across a safety isolation barrier (L-N, L/N-PE), a capacitor (X capacitor, Y capacitor) that has been certified according to relevant safety regulations must be selected.
X capacitor: Connected across the L-N line, the failure mode is mainly short circuit. Authentication is required (such as X1, X2).
Y capacitor: Connected across L/N and ground (PE), the failure mode must be open circuit. Certification is required (such as Y1, Y2). There are strict requirements for leakage current.
4、 Cost and Supply Chain Rules
Cost:
Choose the most cost-effective solution while meeting all electrical, physical, environmental, and reliability requirements.
Compare prices across different brands and channels.
Consider usage and bulk discounts.
Accessibility and Supply Chain:
Core rule: Prioritize selecting capacitors from mainstream brands, universal models, stable supply, and multiple reliable suppliers.
Avoid using obscure and soon to be discontinued (EOL) models.
Consider stocking cycle and minimum order quantity (MOQ).
Evaluate the qualifications and reputation of suppliers.
Summary of Selection Process
Clearly define requirements: functionality? Voltage? Capacity? Temperature range? environment life? Cost?
Preliminary screening type: Based on functional and key parameter requirements, narrow down to several possible capacitor types.
Key parameter calculation and verification:
Calculate the working voltage and determine the rated voltage (strictly derating).
Calculate the required capacity/accuracy/temperature stability.
Calculate the ripple current and verify the rated ripple current.
Evaluate the impact of ESR, ESL, and SRF on performance.
Consider the DC bias effect (MLCC).
Verify the temperature range.
Physics and environment matching: size? Encapsulation? Safety regulations? life? Environmental tolerance?
Refer to the specification sheet: Carefully read the detailed specification sheet of the potential candidate model and confirm that all parameters meet the requirements.
Cost and Supply Chain Assessment: Price? Supply situation? Brand reputation?
Sample testing and verification: Conduct actual circuit testing (especially power ripple, temperature rise, high-frequency characteristics) in the final design to confirm that the performance meets the requirements.
Remember: there is no absolute one size fits all answer for capacitor selection, it requires a balance between performance, size, cost, reliability, and availability. Careful reading and understanding of the specification sheet is a crucial step towards successful selection.