Design Principles and Considerations for High-Frequency, Low-Impedance Electrolytic Capacitors

High-frequency, low-impedance electrolytic capacitors are a new type of capacitor developed to meet the needs of high-frequency, high-speed switching power supplies and digital circuits. They feature low loss, excellent high-frequency characteristics, and lower impedance at high frequencies (about 1/3 to 1/2 of standard products). They also allow for a higher ripple current. The performance grade of these capacitors is typically defined by their impedance (Z) or equivalent series resistance (ESR) at 100 kHz, as well as the maximum allowable ripple current.

1. The Practical Significance of High-Frequency, Low-Impedance Electrolytic Capacitors

1.1 The Equivalent Series Circuit of a Real Capacitor

An actual electrolytic capacitor can be modeled as a series circuit comprising capacitance (C), equivalent series resistance (ESR), and equivalent series inductance (ESL).

1.2 The Relationship Between Frequency and Impedance

• A capacitor's ESR decreases as frequency increases, but its dissipation factor (

  tanδ

  ) rises. Lowering the ESR helps to mitigate the increase in

  tanδ

  with frequency.

• The impedance (Z) of a capacitor fully reflects the effects of ESR and ESL on frequency.

• At the resonant frequency (

  f0​

  ), Z is at its minimum.

  f0​

  represents the highest frequency at which the component functions effectively as a capacitor. Beyond this point, it loses its capacitive properties.

• At high frequencies, if ESL is negligible, Z is approximately equal to ESR. In this case, Z has a strong correlation with

  tanδ

  .

Therefore, a lower Z and ESR at high frequencies results in a smaller

, which helps increase the allowable ripple current, reduce temperature rise, lessen heat generation, and extend the capacitor's lifespan.

1.3 Filtering Efficiency

Filtering efficiency is directly related to ripple voltage:

. For the same ripple current, a lower Z leads to a lower ripple voltage and better filtering performance.

2. Design and Considerations for High-Frequency, Low-Impedance Electrolytic Capacitors

2.1 Basic Requirements

• Miniaturization: Must align with the trend of smaller electronic devices.

• High-Frequency Performance: Must aim to maximize the resonant frequency (

  f0​

  ) and improve high-frequency characteristics, primarily by reducing ESL while also lowering

  tanδ

  .

• Low Z/ESR: Minimizing Z or ESR not only improves filtering but also helps reduce temperature rise (heat power

  P=I 2​⋅ESR

  ) and increases ripple current capability.

2.2 Low-Voltage, High-Frequency, Low-Impedance Electrolytic Capacitors

1. Miniaturization

To achieve a smaller size, high-capacitance foil should be used. However, high capacitance increases the foil's resistance, which works against a low ESR. A balance must be found. Specialized foils for high-frequency, low-impedance capacitors have been developed to tackle this issue. They aim for high capacitance with minimal foil resistance by focusing on both etching and forming techniques. This involves improving the quality of the oxide film and its voltage coefficient, which in turn increases capacitance for a given voltage.

2. High-Frequency Performance

Lowering ESL is key to improving high-frequency performance. A capacitor's inductance comes from its leads and its internal windings (the core).

• Lead Inductance: For leaded capacitors, thick, short leads help reduce inductance. For capacitors with internal tabs, the tab's inductance depends on its length, width, and thickness, as well as the number of tabs. A longer tab means higher inductance; more tabs lead to lower inductance.

• Core Inductance: The core inductance is proportional to the ratio of the foil's length (l) to its width (w). When l/w = 23, the core inductance is at its minimum. Thus, a long, narrow winding core generally has lower inductance than a short, wide one.

Additionally, winding methods such as multi-lead designs, stacked constructions, or protruding-foil cylindrical windings can be used to lower inductance.

3. Lowering Z or ESR

ESR primarily originates from lead resistance, the contact resistance between the foil and the tab, the electrolyte's resistance, and the separator paper's resistance.

• Leads: While not a significant source of resistance, the leads should be high-quality, with a smooth surface and a diameter that fits the sealing plug well.

• Tabs: High-grade tabs should be used in a thick, short, or multi-pair configuration to reduce inductance, resistance, and prevent chloride ion corrosion.

• Foil Contact Resistance: Laser welding or ultrasonic welding should be employed to ensure a solid bond between the lead and the foil, reducing contact resistance.

• Foil: Foil resistance is dominated by its length. Heavily etched foils have higher resistivity, but this effect is minimized with a multi-tab structure.

• Electrolyte and Separator Paper: To lower ESR, the main focus for high-frequency, low-impedance capacitors is to achieve high-conductivity electrolytes and low-density separator paper.

  • Electrolyte: Water-based electrolytes have much higher conductivity than non-aqueous ones.

  • Paper: Lower-density paper can affect its strength, leading to the development of patterned paper. This also necessitates stricter requirements for burr-free foil cutting.

2.3 Medium- to High-Voltage, High-Frequency, Low-Impedance Electrolytic Capacitors

1. Necessity

High-voltage capacitors (400-450V) used for input filtering in switching power supplies also need to handle some high-frequency ripple. Using high-frequency, low-impedance high-voltage capacitors helps reduce high-frequency impedance, which improves the input filtering effect, reduces temperature rise, and increases ripple current capability and lifespan. This has driven the continuous development of this type of capacitor.

2. Design Considerations

The basic requirements are the same as for low-voltage products, but the approach to solving them differs. Since it's difficult to increase the conductivity of high-voltage electrolytes by adding water, other methods like using branched electrolyte salts must be employed. Progress in increasing the capacitance-to-volume ratio of high-voltage foils is also slower. As a result, the requirements for miniaturization in medium- to high-voltage products must be more flexible. When electrolyte conductivity is insufficient, a lower-capacitance foil might be used to achieve lower Z and ESR. To improve lifespan and heat resistance, foils with a higher forming voltage may also be utilized.