Electrification is increasing the number of high-power conversion stages in transport, renewable energy, industrial drives and grid-support equipment.
A Knowles technical white paper examines how this shift raises the electrical, thermal and lifetime demands placed on film DC-link capacitors, ceramic decoupling capacitors and supercapacitor energy buffers.
Rather than introducing a new capacitor family, the paper describes a change in selection priorities: capacitance must now be assessed together with impedance over frequency, ESR, ESL, ripple-current heating, operating temperature and the required duration of energy support.
Key features and benefits
- DC-link capacitors combine local energy storage with switching-ripple filtering in converter DC buses.
- Film capacitors and supercapacitors cover different functions: film technology supports DC-link energy buffering and ripple-current handling, while supercapacitors provide short-duration high-power energy storage.
- Faster SiC and GaN switching increases the influence of parasitic inductance and capacitor placement.
- A hybrid capacitor network can divide duties by frequency, with bulk film capacitance handling energy storage and ripple current while low-ESL ceramic capacitors suppress the fastest transients near switching devices.
- Electrified grid equipment can require an energy buffer that responds within milliseconds and sustains output through a short disturbance.
Technical highlights
The paper identifies four familiar capacitor functions in power electronics: electrostatic energy storage, impedance-based filtering, resonant energy exchange, and signal coupling or transfer-function shaping. Electrification places particular pressure on energy storage and filtering because conversion stages operate at higher voltage, switching frequency and power density.
A DC-link capacitor sits across the internal DC bus between a power source and a switching bridge. It supplies rapidly changing switching current locally and reduces voltage ripple imposed on the upstream source. Ripple current flowing through ESR produces heat according to:
The white paper uses an 800 V traction inverter as an example of a demanding DC-link position, citing continuous ripple-current requirements above 100 A and a 15-year service target. These values are application examples in the paper, not ratings for a specific Knowles capacitor series or part number.
| Selection factor | Why it matters | System consequence |
|---|---|---|
| DC voltage | Bus and transient stress | Insulation coordination |
| RMS ripple current | Drives internal heating | Thermal design |
| ESR | Sets ripple loss | Temperature rise |
| ESL | Causes switching overshoot | Layout sensitivity |
| Energy storage | Supports bus stability | Voltage excursion |
| Service life | Sets maintenance exposure | System availability |
The document describes metallized polypropylene film capacitors as a DC-link technology with self-healing behaviour. It also contrasts them with aluminium electrolytic capacitors, whose liquid electrolyte and higher ESR can constrain use under high ripple-current and long-life conditions. Those are technology-level comparisons in the white paper; capacitor ratings, endurance and permissible ripple current must be taken from the datasheet of the selected series and verified at the actual voltage, temperature and cooling conditions.
From capacitance to impedance
Nominal capacitance does not fully describe a capacitor’s behaviour in a fast-switching converter. The paper notes that wide-bandgap switches can produce voltage transitions above 50 V/ns, with some devices exceeding 100 V/ns. At these edge rates, a capacitor’s equivalent series inductance contributes directly to transient voltage:
A physically large bulk capacitor can provide substantial energy storage but may not be electrically close enough to a switching device to control the highest-frequency current loop. Conversely, a small ceramic capacitor can offer lower inductance but insufficient stored energy or ripple-current capability for the bulk DC bus.
This supports a frequency-banded decoupling approach:
| Capacitor function | Primary characteristic | Typical placement |
|---|---|---|
| Bulk DC-bus energy storage | Capacitance and ripple capability | Across DC link |
| Converter ripple filtering | Low ESR and thermal capability | DC-link loop |
| Fast switching transient control | Low ESL | Close to switches |
| Short-duration energy support | Stored energy and power capability | DC bus or storage interface |
Film capacitors can provide the principal DC-link function, while MLCC and ceramic capacitor technologies can address high-frequency decoupling close to the semiconductor switching loop. The effectiveness of this arrangement depends on busbar geometry, PCB layout, mounting inductance, conductor spacing and the switching-loop area, not on capacitance value alone.
Supercapacitors for fast energy buffers
Supercapacitors, also called electric double-layer capacitors, take position between conventional capacitors and batteries. Their purpose is not long-duration energy storage. Instead, they can rapidly absorb or deliver power in applications where battery response, cycle life or both are limiting factors.
The application examples include:
- UPS hold-up and ride-through energy.
- Regenerative-braking energy capture.
- Wind-turbine blade-pitch backup power.
- Burst-power buffering in drones.
- Short-duration grid-support energy storage.
- Virtual-inertia and frequency-response functions.
The paper cites an E-STATCOM installation at Mehrum, Germany, commissioned in December 2025. It identifies the installation as a Siemens Energy SVC PLUS FS® system operated with TenneT, combining a 300 MVA modular multilevel converter with supercapacitor banks specified in the paper for ±200 MW active-power delivery within milliseconds. This is a system-level case example, not a component-level supercapacitor specification.
A supercapacitor bank requires more than the cells alone. Series-connected cells need voltage balancing, while the complete storage system requires protection, monitoring, pre-charge control, thermal assessment and a control strategy that defines power limits and the permitted depth of discharge. Supercapacitor balancing methods are therefore part of the electrical design, not an optional implementation detail.
Application fit
The source supports the following capacitor functions and circuit locations. It does not identify a new Knowles series, part number, case size, datasheet, qualification report or model for a newly released component family.
| Application | Capacitor role | Supporting condition |
|---|---|---|
| EV traction inverter | DC-link buffer | High-voltage bus, ripple current |
| Onboard charger | DC-link filtering | Switching converter bus |
| Motor drive | Local energy storage | Fast load-current changes |
| Renewable inverter | Bus stabilization | Power-conversion stages |
| Solid-state transformer | Multiple DC links | Multi-stage conversion |
| STATCOM / E-STATCOM | DC bus and energy buffer | Voltage and real-power response |
| Virtual inertia system | Fast energy storage | Millisecond power response |
| UPS hold-up | Ride-through buffer | Short interruption support |
In solid-state transformers, each conversion stage can introduce another DC bus that needs local energy storage and filtering. The source describes medium-voltage, isolation-stage and output-side DC links as potential capacitor locations. Actual voltage, insulation, creepage, clearance, fault-energy and safety requirements must be established at system level.
For high-frequency power converters, ripple current and its effects on capacitor performance should be evaluated together with ESR, hot-spot temperature, cooling conditions and the capacitor manufacturer’s endurance model. A low nominal ESR alone does not establish acceptable field lifetime.
Design-in notes for engineers
- Define the DC-bus operating voltage, tolerable ripple voltage, surge and fault-transient envelope before selecting a voltage rating.
- Calculate capacitor RMS ripple current over the real switching spectrum, including load profile, modulation method and worst-case operating conditions.
- Assess ESR loss, hot-spot temperature and cooling together. The capacitor thermal environment is often set by nearby semiconductors, magnetics and enclosure airflow.
- Treat ESL as a layout parameter as well as a component parameter. Minimise the switching-current loop area and place low-inductance decoupling capacitors close to the relevant switching devices.
- Separate bulk-energy, intermediate-frequency ripple and high-frequency transient duties where one capacitor technology cannot meet the full impedance target.
- Check the actual capacitance under applied voltage, temperature, frequency and ageing conditions, particularly when ceramic capacitors form part of the local decoupling network.
- For supercapacitor banks, validate maximum cell voltage, balancing method, leakage, temperature range, available energy over the intended voltage window, protection coordination and end-of-life behaviour.
- Do not transfer system-level examples from the white paper directly into a new design. Final capacitor selection should be verified under the converter’s intended electrical, thermal, mechanical and fault conditions before schematic, layout or production release.
Further reading
- DC-Link Capacitors
- Capacitors Selection for DC/DC Converters
- ESL Capacitor Parasitic Inductance
- Energy Density, Power Density and Capacitor Energy Content
Source
This article is based on a Knowles technical white paper and its system-level examples of film DC-link capacitors, supercapacitors and hybrid capacitor networks in electrified power-conversion applications. Engineers should consult current manufacturer datasheets, application documentation and system-level requirements before component qualification and design release.
