Technology / Technical guide
DC Capacitor Ripple Current, ESR & Frequency Dependence
A DC capacitor can have a changing current even when its average terminal voltage looks nearly steady. The current’s changing components interact with nonideal resistance and inductance. This explains why capacitance alone does not describe ripple heating.
Separate ideal stored energy from nonideal loss.
Ripple-current behaviour depends on the waveform and frequency-dependent component model, as well as the capacitance.
01 / Current and voltage
Changing charge creates capacitor current.
For an ideal constant capacitor, i = C dv/dt with consistent reference directions.
For an ideal constant capacitor, i = C dv/dt with consistent reference directions. A rapidly changing voltage can therefore create substantial current even when the voltage excursion is small. A steady voltage has zero current in this ideal model.
Ripple refers to a changing component superimposed on an average quantity. Voltage ripple and current ripple are different signals with different units. One cannot be converted into the other without a model that includes the relevant frequency content.
02 / Equivalent model
Real capacitors include resistance and inductance.
Equivalent series resistance, ESR, represents a dissipative part of the capacitor’s behaviour.
Equivalent series resistance, ESR, represents a dissipative part of the capacitor’s behaviour. Equivalent series inductance, ESL, represents an inductive contribution. These terms augment the ideal capacitive element rather than replacing its stored-energy role.
TDK’s technical information explains that the effective behaviour depends on frequency and temperature. An equivalent circuit is useful over a stated range; the same scalar ESR should not be silently applied to every frequency or operating condition.
03 / Ripple heating
Use an RMS current with an applicable ESR.
For one frequency component and approximately constant ESR at that condition, resistive loss is represented by P ≈ Irms² ESR.
For one frequency component and approximately constant ESR at that condition, resistive loss is represented by P ≈ Irms² ESR. Doubling the RMS current would quadruple this modelled loss if ESR and the other conditions were unchanged.
For separate stationary harmonic components in a linear model, the average loss can be approximated by summing I_h,rms² ESR(f_h). Each frequency receives its applicable resistance. Combining all current into one RMS value requires care when ESR varies substantially across the spectrum.
04 / Frequency response
Capacitance is not the only impedance at high frequency.
Ideal capacitive reactance decreases as frequency rises.
Ideal capacitive reactance decreases as frequency rises. Parasitic inductance behaves in the opposite direction, so an actual component can have a self-resonant region. Its impedance does not remain purely capacitive at every frequency.
The LC resonance guide explains reactive energy exchange at network level. Component self-resonance and the response of a complete connected filter must retain their distinct circuit boundaries.
05 / Ratings
A current rating comes with thermal and frequency conditions.
A specified ripple-current rating is tied to the component’s stated conditions, such as reference frequency and temperature.
A specified ripple-current rating is tied to the component’s stated conditions, such as reference frequency and temperature. It should not be interpreted as a universal current allowance independent of the waveform or environment.
An average DC voltage, a ripple-current rating and a thermal result answer different questions. Temperature rise also depends on heat flow from the component. This page does not turn a calculated ESR loss into a service-life prediction or replacement recommendation.
06 / Energy boundary
Keep ripple loss distinct from bus energy balance.
The DC bus energy guide treats ideal stored energy and power imbalance.
The DC bus energy guide treats ideal stored energy and power imbalance. The ESR model explains why a real component additionally dissipates energy while changing current passes through it.
No MARSTON capacitance, ESR, ESL, ripple rating or capacitor life is asserted here. The explanation concerns nonideal component behaviour, without specifying an installed capacitor technology, bus threshold or component-selection procedure.
Continue the review with LC Filter Resonance: Natural Frequency and Damping.
Continue the review with Semiconductor Conduction & Switching Loss Mechanisms.
Continue the review with PWM Switching Frequency, Carrier & Fundamental.
Continue the capacitor component review with capacitor dielectrics and construction and dielectric loss tangent and equivalent models.
Extend the DC capacitor discussion with capacitor charging and inrush current and the separate guide to instantaneous power, storage and dissipation.
General background: TDK Electronics: capacitor equivalent parameters and ripple-current rating conditions. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Ideal current | i = C dv/dt in A | Constant C and consistent voltage/current directions. |
| ESR | Effective series resistance in Ω | Frequency and temperature conditions apply. |
| ESL | Effective series inductance in H | Relevant to the component impedance model. |
| Ripple loss | Irms² ESR in W | Applicable single-frequency or constant-ESR approximation. |
| Ripple rating | Current plus reference conditions | Rating is not independent of thermal and frequency context. |
Engineering questions.
Is ripple current the same as voltage ripple?
No. Their relationship depends on the component and frequency-dependent circuit model.
Does a nearly steady average DC voltage imply no capacitor current?
No. Changing components can still produce current.
Can one ESR value represent all frequencies?
Not generally. The applicable equivalent resistance can depend on frequency and temperature.
Does doubling current double ESR heating?
With unchanged ESR, the RMS-current loss model increases by a factor of four.
Is self-resonance the same as the complete filter response?
No. A component and its surrounding network have different modelling boundaries.
Does this establish an installed capacitor rating or lifetime?
No. No actual MARSTON capacitor parameter or life prediction is supplied.
Technical enquiry
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