The DC-link capacitor in an adjustable-speed drive or solar inverter sits between the rectifier and the switching stage, absorbing ripple current at the IGBT or SiC switching frequency and holding the bus voltage steady. When you buy replacement capacitors for this position, capacitance and voltage class are only the starting point. A replacement that fails within six months costs far more than the price savings on the purchase order, in the form of warranty claims and field service trips. This article walks through the cost drivers that matter, a method for comparing bids on equal terms, an incoming inspection checklist, and stocking advice.
Identify the Real Cost Drivers First
The dominant stress at the DC link is ripple current, not average current. As a rule of thumb, the DC-link ripple current of a typical six-pulse rectifier drive is roughly half the rated output current. For a 45 kW drive at 400 V AC, with an output current of about 65 A, the bus ripple current is on the order of 30–35 Arms. A standard aluminum electrolytic snap-in capacitor of 470 µF, 450 V is typically rated for 2.5–4 Arms at 10 kHz and 105 °C. To carry that ripple, you would need ten to fifteen units in parallel. The true cost is not the unit price of one capacitor; it is the number of parallel components, the busbar copper, the heatsink, the mounting hardware, and the assembly labor.
Two families compete here: aluminum electrolytic and metallized polypropylene film. Film capacitors cost more per microfarad but deliver far higher ripple-current density per microfarad and do not have a wear-out mechanism linked to electrolyte evaporation. In practical terms, film becomes the economical choice when the application demands high ripple at modest capacitance — for example, a low-inductance DC link in a high-switching-frequency SiC inverter. Electrolytic remains the volume answer when you need large capacitance at low cost and can live with a shorter lifetime at elevated hotspot temperatures.
Ripple current capability is directly tied to ESR. The power dissipated in the capacitor is Irms2 × ESR, and that heat raises the core temperature. Lifetime follows the Arrhenius rule: every 10 K reduction in hotspot temperature approximately doubles the operating life. When a quote looks cheap, check whether it meets the ripple-current budget at your actual ambient, not at the idealised 25 °C figure printed in marketing pages.
Compare Offers on Equal Terms
At least three parameters are quoted inconsistently between datasheets. Standardize them before comparing prices.
- Ripple current at the same ambient temperature. Some vendors rate Irms at 105 °C, others at 85 °C. A capacitor rated 3.5 A at 105 °C will typically carry roughly 1.5–1.7 times that current at 85 °C, per the manufacturer’s temperature derating curve. Compare at your real operating temperature, which is usually between 60 and 80 °C case temperature.
- ESR at the same frequency. Electrolytic ESR is frequency-dependent. An IGBT inverter switches at 4–20 kHz, so compare ESR at 10 kHz or the switching frequency, not at 100 Hz mains ripple. At 10 kHz, electrolytic ESR is markedly lower than at 100 Hz; quoting one without the other can mislead the thermal calculation.
- End-of-life definition. Check whether the claimed lifetime corresponds to a 20% or 30% capacitance drop and a 2× or 3× ESR rise. The same physical capacitor can be quoted with significantly different hours depending on which criterion is used. The IEC 60384-4 convention is 20% capacitance loss and 2× ESR increase; a vendor quoting 30% loss will show a longer life number for the same product.
Also verify the voltage-derating expectation. For a 400 V AC bus, the DC link peaks near 560 V; a 450 V capacitor is common with about 20% margin for regenerative spikes. If your inverter uses SiC devices with a 750–900 V bus, you need 900 V film capacitors or series strings of 500 V electrolytics with balancing resistors. The series configuration adds cost that the comparator must carry.
Incoming Inspection Checklist
When buying from brokers, surplus stock, or a second-source distributor, verify that received goods match the datasheet before they enter your bin. The table below lists the checks that catch most mismatches and re-marked units. These are quick measurements suitable for a receiving bench with an LCR meter and a DC supply.
| Check item | Method | Acceptance criterion |
|---|---|---|
| Capacitance | LCR meter at 100 Hz, 1 V RMS | Within −20/+20% of marked value (electrolytic), or per datasheet tolerance |
| ESR | LCR meter at 10 kHz (or datasheet frequency) | No more than 1.3× the datasheet maximum for new stock |
| Leakage current | DC supply at rated voltage, 5-minute soak | ≤ 0.01 CV µA (with C in µF, V in volts) for 105 °C types, or datasheet limit |
| Ripple-current temperature rise | Apply rated Irms at 10 kHz, measure case temp at 85 °C ambient | Case-temperature rise ≤ 15 K at rated conditions |
| Marking and date code | Visual under magnification; compare with reel or batch label | Date code within 18 months of manufacture; no double-print, scratched logos, or over-stamped values |
| Terminal integrity | Torque check on screw terminals; snap-in pin alignment | Terminal accepts datasheet torque without rotation; pins align to PCB drill pattern |
Do not skip the temperature-rise test on a statistically meaningful sample. A capacitor with the correct capacitance and ESR at 25 °C can still run hot if its internal electrolyte has dried out, which is exactly the failure mode of aged or improperly stored surplus stock.
Negotiation and Stocking Advice
When negotiating, ask for the lifetime-versus-temperature curve rather than a single headline hour figure. Compare offers on the ripple current each capacitor can carry at your worst-case ambient and airflow. If one supplier quotes a lower price but a 2000-hour life at 105 °C, while another quotes 7000 hours at 105 °C for 15% more money, the latter almost always wins the total-cost calculation for an inverter rated for ten years of service.
For stocking, maintain a buffer of the most commonly consumed DC-link values: 470–2200 µF at 400–500 V for the 5–75 kW inverter range, plus a small quantity of 900 V film capacitors for high-bus-voltage designs. Film capacitors typically carry longer lead times than electrolytics, so plan an eight-to-sixteen-week horizon for those. Polypropylene film capacitors have no electrolyte wear-out mechanism, which simplifies long-term storage; electrolytics should be stored below 40 °C and re-formed after two years on the shelf. When an original electrolytic family is declared obsolete, the usual replacement path is a film capacitor with equivalent voltage and ripple-current ratings, provided the mechanical footprint and terminal pitch fit your existing busbar.