An aluminum electrolytic capacitor does not behave like an ideal capacitor. Its internal foil, electrolyte, tabs, and contacts add resistance, represented in a simple model as equivalent series resistance, or ESR. When AC ripple current flows through that resistance, the capacitor dissipates real power as heat.

For power-supply filtering, this mechanism can matter as much as capacitance and voltage rating. A capacitor may have the expected number of microfarads and still run too hot if its ESR is too high for the ripple current imposed by the circuit.

ESR adds a resistive loss inside the capacitor

A useful first-order model is an ideal capacitor in series with a resistor:

terminal ── ESR ── ideal capacitor ── terminal

The ESR is not a separate physical resistor installed inside the can. It is an equivalent parameter that represents several internal losses.

When current flows through ESR, the resistive loss is approximately:

P = Irms² × ESR

where:

  • P is internal power dissipation,
  • Irms is the RMS ripple current,
  • ESR is the equivalent series resistance at the relevant operating conditions.

This square relationship is important. Doubling ripple current increases the ESR-related heating by a factor of four if ESR remains unchanged.

Nichicon’s application guidance for aluminum electrolytic capacitors uses the same relationship for AC loss and notes that ripple-current heating raises the capacitor’s internal temperature.

A small ESR can still produce meaningful heat

Consider a capacitor carrying 1.5 A RMS of ripple current with an ESR of 80 mΩ:

ESR = 0.08 Ω
Irms = 1.5 A

P = 1.5² × 0.08
  = 0.18 W

A loss of 180 mW may look small compared with the power handled by the rest of a converter, but that heat is generated inside a compact component whose life is strongly affected by temperature.

If ripple current rises to 3 A while ESR remains 80 mΩ:

P = 3² × 0.08
  = 0.72 W

The current doubled, but internal dissipation quadrupled.

This is one reason a capacitor that appears electrically adequate by capacitance and voltage alone can become hot in a switching power supply.

Ripple current is not the same as load current

The load may draw several amperes without the capacitor carrying exactly the same RMS current. Capacitor ripple current is the alternating component that repeatedly charges and discharges the capacitor.

Its waveform depends on the topology.

In a rectifier followed by a reservoir capacitor, the capacitor can receive narrow charging pulses near the peaks of the AC waveform and then discharge into the load between those peaks.

In a buck converter, the output capacitor carries the difference between inductor current and load current:

Icapacitor = Iinductor - Iload

The average capacitor current in steady state can be near zero while its RMS ripple current is still substantial.

That is why capacitor selection should use the expected capacitor-current waveform or a validated converter design calculation rather than simply copying the DC load current.

ESR changes with frequency and temperature

ESR is not one fixed number under every condition.

Aluminum electrolytic capacitor datasheets commonly specify impedance or ESR at a particular frequency and temperature. The value can change when either condition changes.

Manufacturer application guidance notes that ESR generally rises at lower temperature. Frequency also affects the equivalent resistance and the allowable ripple-current calculation.

This has two practical consequences.

First, a capacitor that is acceptable at room temperature may behave differently during a cold start.

Second, an ESR value measured at one frequency should not automatically be used for a ripple component at a very different frequency unless the datasheet or impedance curve supports that assumption.

Switching converters can also contain multiple ripple-frequency components. The capacitor therefore sees a composite current waveform rather than a single ideal sine wave.

ESR also contributes to output ripple voltage

ESR does more than produce heat. Ripple current flowing through ESR produces a voltage component:

Vesr = Iripple × ESR

For a switching converter, output ripple is therefore not determined by capacitance alone. A simplified view includes at least:

capacitive ripple + ESR ripple + inductive/parasitic effects

Panasonic notes that in some switching-power-supply outputs using aluminum electrolytic capacitors, ESR can dominate the ripple voltage when capacitance is already large.

This explains why replacing a failed capacitor with another part of the same capacitance and voltage rating does not guarantee equivalent performance. A replacement with significantly higher ESR can increase ripple voltage and internal heating.

Ripple-current rating is a thermal limit

Datasheets usually specify a rated ripple current under defined conditions such as temperature and frequency.

That number should not be treated as an arbitrary current-handling specification. It is closely connected to internal heating and allowable temperature rise.

Exceeding the rated ripple current can raise the element temperature beyond the conditions used to establish the capacitor’s expected life.

Even operation within the nominal ripple-current rating does not make temperature irrelevant. Ambient temperature, airflow, nearby hot components, PCB layout, enclosure temperature, and the capacitor’s own ESR loss all contribute to its internal temperature.

A capacitor mounted next to a heatsink or power semiconductor can therefore operate under much harsher thermal conditions than the same part on a cooler board.

Heating and aging reinforce each other

Aluminum electrolytic capacitors age as their electrolyte and internal structure change over time. Elevated temperature accelerates this aging process.

As a capacitor ages, ESR can increase. Higher ESR then causes more heat for the same ripple current:

higher ESR
   ↓
higher I²R loss
   ↓
higher internal temperature
   ↓
faster degradation

This feedback is one reason aging electrolytic capacitors often show symptoms such as increased power-supply ripple, unstable converter behavior, or visible swelling in severe cases.

Panasonic’s application guidance specifically warns that long-term degradation of ESR can increase internal heating caused by ripple current.

Capacitance and voltage rating are only part of the selection

For an electrolytic capacitor used in a power path, at least these parameters can be relevant:

  • capacitance,
  • rated voltage,
  • ESR or impedance,
  • rated ripple current,
  • operating temperature,
  • expected endurance or lifetime,
  • package size and thermal environment.

Polarity also remains fundamental. Most conventional aluminum electrolytic capacitors are polarized and must be used according to their specified polarity and voltage limits.

The peak voltage across the capacitor, including any ripple component, also has to remain within the manufacturer’s stated limits.

A practical replacement check

Suppose a power supply originally uses a 1000 µF, 25 V capacitor. Replacing it with any other 1000 µF, 25 V capacitor is not automatically equivalent.

A more useful comparison is:

capacitance:          compatible?
rated voltage:        equal or higher as appropriate?
ESR/impedance:        suitable for the circuit?
ripple-current rating: sufficient at the operating frequency and temperature?
temperature rating:   suitable for the environment?
mechanical size:      compatible?

For a switching supply, the ESR and ripple-current entries can determine whether the replacement stays cool and keeps output ripple within the intended range.

An electrolytic capacitor therefore does more than store charge and smooth voltage. In a real circuit, its internal resistance converts ripple current into heat. That thermal mechanism connects ESR, output ripple, capacitor life, and failure behavior, so all four need to be considered together when selecting or diagnosing the component.

Electronics Capacitor ESR Ripple Current