Choosing a DC-link film capacitor comes down to four numbers: voltage rating and derating, ripple current, ESR, and the size that fits your inverter. Get these right and the capacitor rides along quietly; miss one and it becomes the failure point in the drive.
In our look at eVTOL DC-link film capacitors, we covered why film is the right choice for electric aviation and high-power drives. This article walks through how to pick the part.
Start with Voltage Rating and Derating
DC-link capacitors sit on a bus that can swing above nominal during load steps and regeneration. A common rule is to rate the capacitor at 1.5-2x the nominal bus voltage to leave margin for overshoot. For an 800 V bus, that points to parts rated at 1,000 V or higher.
- Check the rated DC voltage and the surge or transient capability.
- Leave derating margin for switching overshoot and operating temperature.
Then Ripple Current and ESR
The DC-link capacitor must survive the ripple current at your switching frequency. Higher ripple heats the part from inside; low ESR keeps that heat down.
- Ripple current rating – the part must handle the RMS ripple it will actually see.
- ESR at the switching frequency – lower ESR means less self-heating and less ripple voltage on the bus.
- Temperature rise – derate the ripple rating to your real ambient.
Energy, Capacitance and Size
For eVTOL and high-power drives, the DC-link also stores energy that helps ride through brief load steps. Capacitance and voltage set the stored energy (1/2 x C x V2), and that drives the physical size:
- Capacitance – pick for the ripple current and hold-up the drive needs.
- Voltage – higher rating increases size; match to the bus with derating.
- Form factor – box, cylindrical or module; weight matters on an aircraft.
Our DC-link film capacitor application guide covers the sizing logic in detail, and the film capacitor guide explains the types.
Selection Quick Reference
| Parameter | How to pick | Why it matters |
|---|---|---|
| Voltage rating | 1.5-2x the bus voltage | Margin for overshoot |
| Ripple current | Match actual RMS at switching freq | Avoid overheating |
| ESR | Lowest at switching frequency | Less self-heating, longer life |
| Size / form factor | Fit the inverter and weight budget | Critical on aircraft |
In practice, the datasheet is the contract: check the ripple current rating at your switching frequency, not just at the reference frequency. If the part is borderline on temperature rise, moving to a larger package or a lower ESR series often solves it before you build. For eVTOL especially, the weight and volume budget makes this a real design trade-off, not an afterthought.
A good selection process ends with a cross-check against the inverter operating envelope – the worst-case voltage, the ripple at full load, and the ambient temperature in the enclosure.
Common Selection Mistakes
Two mistakes come up most often. The first is picking capacitance first and voltage second – the reverse order leaves the part vulnerable to overshoot. The second is ignoring ripple current at the actual switching frequency, which causes premature heating even when the capacitance looks right. Start with voltage and ripple, then size for energy.
Xuansn publishes ripple and lifetime data for every DC-link film series, so the numbers in the datasheet match the part you receive.
Common Questions About Choosing DC-Link Film Capacitors
What voltage rating do I need for an 800 V bus?
Typically 1,000 V or higher, to give derating margin against overshoot during switching and regeneration.
How do I size the capacitance?
Size for the ripple current and the hold-up energy the drive needs; higher capacitance and voltage store more energy but increase size.
Why does ESR matter for DC-link film capacitors?
ESR converts ripple current into heat. Lower ESR at the switching frequency keeps the capacitor cool and extends its life.
What to Read Next
For the wider AI power picture, see why AI server power demand is exploding and our server capacitors for AI power.
Need DC-link film capacitors for eVTOL or high-power drives? Xuansn Capacitor manufactures film capacitors for DC-link duty – our engineers will reply within 24 hours with feasibility and a quote.
✉ sales1@xuanxcapacitors.com | ☎ +86-769-8166 8821 | Send your specification
Sources: Industry practice for DC-link capacitor selection (voltage derating, ripple, ESR); eVTOL powertrain requirements.
