Capacitor Characteristics

Equivalent Series Inductance (ESL)

Equivalent Series Inductance (ESL) is the small amount of inductance present inside every real capacitor. Although an ideal capacitor has no inductance, the leads, terminals, electrodes and internal construction introduce a small inductance that becomes increasingly important at high frequencies. For RF circuits, switch-mode power supplies (SMPS), high-speed digital electronics and pulse circuits, ESL is often just as important as capacitance and ESR.

Equivalent Series Inductance

What Is ESL?

Equivalent Series Inductance is the parasitic inductance that appears in series with the ideal capacitance of a real capacitor.

At low frequencies, ESL has little effect. As frequency increases, the inductive behaviour becomes more significant and can reduce the effectiveness of the capacitor.

Equivalent Circuit

A practical capacitor can be represented by the following equivalent circuit: 

        ESL        ESR
----LLLLLL----/\/\/\/\----||----

The inductor represents the Equivalent Series Inductance, the resistor represents the Equivalent Series Resistance and the capacitor represents the ideal capacitance.

What Causes ESL?

Source Description
Lead Wires Long leads increase inductance.
Internal Connections Bond wires and welded joints contribute to ESL.
Electrode Geometry The physical arrangement of the plates affects inductance.
PCB Tracks Long PCB traces add extra inductance.
Package Size Larger packages generally have higher ESL.

Why ESL Matters

  • Limits high-frequency performance.
  • Reduces filtering effectiveness.
  • Affects transient response.
  • Influences switching power supply efficiency.
  • Can increase electromagnetic interference (EMI).
  • Affects RF circuit performance.

Self-Resonant Frequency (SRF)

Every capacitor has a frequency at which its capacitance and inductance resonate. This is called the Self-Resonant Frequency (SRF).

Frequency Behaviour
Below SRF Capacitor behaves mainly as a capacitor.
At SRF Impedance reaches its minimum value.
Above SRF Capacitor behaves increasingly like an inductor.

Typical ESL by Capacitor Type

Capacitor Type Typical ESL
MLCC Ceramic Very Low
SMD Film Low
Silver Mica Low
Tantalum Chip Low
Radial Electrolytic Moderate
Large Screw-Terminal Capacitor High

How to Reduce ESL

  • Use surface-mount (SMD) capacitors where possible.
  • Choose smaller package sizes for high-frequency circuits.
  • Keep PCB tracks short and wide.
  • Place decoupling capacitors close to IC power pins.
  • Use several capacitors of different values in parallel.
  • Avoid unnecessarily long leads.

Applications Where Low ESL Is Important

  • Microprocessor power supplies.
  • Switch-mode power supplies.
  • RF transmitters and receivers.
  • High-speed digital electronics.
  • FPGA and CPU decoupling.
  • Class D audio amplifiers.
  • Pulse power circuits.
  • EMI suppression.

Testing ESL

ESL cannot usually be measured with a standard multimeter.

Specialised equipment is normally required, such as:

  • LCR meter.
  • Vector network analyser (VNA).
  • Impedance analyser.
  • RF test equipment.

ESL vs ESR

Characteristic ESL ESR
Represents Inductance Resistance
Main Effect High-frequency performance. Power loss and heating.
Measured In Henries (usually nH). Ohms (Ω).
Most Important At High frequencies. High ripple current.

Real-World Examples

Equipment Importance of Low ESL
CPU Motherboard Fast power supply decoupling.
Graphics Card High-speed transient response.
5G RF Module High-frequency filtering.
Switch-Mode Power Supply Reduced switching noise.
Class D Amplifier Improved output filtering.

Common Misconceptions

Myth Reality
Only ESR matters. ESL becomes critical at high frequencies.
Large capacitors always filter better. Large capacitors often have higher ESL.
Any capacitor can decouple high-speed ICs. Low-ESL ceramic capacitors are generally preferred.
Lead length is unimportant. Long leads increase inductance.

Key Points

  • Every real capacitor has some Equivalent Series Inductance.
  • ESL limits capacitor performance at high frequencies.
  • Surface-mount ceramic capacitors generally have the lowest ESL.
  • Keeping leads and PCB traces short helps reduce ESL.
  • ESL becomes especially important in RF, SMPS and high-speed digital circuits.

Next Lesson

Continue by learning about Ripple Current, including how AC ripple affects capacitors, how ripple current is specified and why excessive ripple can shorten capacitor life.

Next Lesson → Ripple Current