Capacitor Selection: Key Factors to Consider

Semiconductor Review | Friday, February 24, 2023

Understanding the application's performance characteristics and operating conditions is crucial in choosing the suitable automotive capacitor.

FREMONT, CA: The various technologies' performance traits and the application's operating circumstances must be understood to select a reliable capacitor for today's automotive electronic applications. The application environment will therefore be crucial in identifying the best-performing and most affordable solution because it might substantially impact the actual in-circuit performance compared to the datasheet specifications.

Important Factors Other Than Capacitance and Voltage

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Low volumetric efficiency is caused by a combination of low K and low dielectric breakdown strength (as with poly film capacitors). The fact that these devices offer exceptionally low losses and steady electrical features at a reasonable price may compensate for their large physical dimensions.

Equivalent series resistance (ESR), which indicates losses for the capacitor in the equivalent circuit, is the actual portion of the impedance when considering capacitors in operation. Temperature, frequency, and dielectric type all affect these values.

For a given applied voltage, the capacitor's insulation resistance (IR) defines how much dc leakage current it will pass; for electrostatic (film and ceramic) capacitors, this leakage current is often significantly smaller. Such leakage is temperature and voltage-dependent, and inductance depends on the electrode designs.

Capacitor Relationships

The capacitor's impedance is crucial to understanding how it influences incoming signals. Low ESR ensures high efficiency, less thermal loss, and dependability during charge/discharge cycles. Inductive reactance (XL) and capacitive reactance (XC) represent the capacitor's inductive field and energy-storage capacity.

Notably, the device reaches its resonance frequency when XC and XL are equal. Keep this in mind when selecting a decoupling capacitor to take the noise from ac components out of a dc signal. A capacitor with a resonant frequency close to the frequency of the undesirable ac noise should be chosen for minimum impedance and maximum decoupling to ground to eliminate ac signal components from a dc power rail effectively.

Automobile-Related Issues

When considering crucial performance, dependability, and accuracy, automotive applications are sometimes separated into power control (ECU and transmission) and safety and comfort (such as airbags and climate control). The in-vehicle location and resulting operational circumstances are other significant variations.

Salt spray, water, fuel contact or immersion, operating temperatures of 125°C or higher, and vibration levels of 15 g, up to 200 Hz, may all be present in under-the-hood applications. In comparison to the passenger compartment, these conditions are very different. In reality, due to its operational temperature restrictions (85°C), another high-capacitance technology, the electric double-layer capacitor, or EDLC, is only suitable for in-cabin use in applications like eLatch power backups.

Selecting a capacitor involves many different decisions. Each capacitor has a unique collection of electrical properties, performance weaknesses, mechanical considerations, and financial factors. The application, the surrounding circumstances, and the actual circuit function all influence each factor's relative relevance. With the wide range of options, it's crucial to consult each manufacturer's specifications to find the ideal capacitor.

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