Capacitor bank failures in industrial inverters and power supplies follow a predictable path: ripple current heats the dielectric, the core temperature climbs, ESR rises, and the bank loses capacitance until the DC link collapses or the protection trips. The design decision is rarely about capacitance alone — it is about how quickly the bank can reject heat to the surrounding air. This guide defines the thermal boundary where cooling becomes mandatory, lists the decision thresholds for ripple, ESR, and core temperature, and provides a selection procedure that works with standard manufacturer data.
Define the Thermal Boundary of the Bank
A capacitor bank must be treated as a distributed heat source. Each parallel unit conducts part of the total ripple current, and its internal dissipation is I_rms² × ESR. The boundary between “just pass the ripple” and “must be actively cooled” is set by the per-unit dissipation and the case-to-ambient thermal resistance.
Two common regimes apply:
- Bulk storage banks (DC link, hold-up): ripple current at 100–120 Hz, per-unit dissipation typically below 1 W, natural convection often sufficient.
- High-frequency ripple banks (PFC boost, inverter output): ripple at 10–100 kHz, per-unit dissipation of 2–5 W, forced airflow is usually required unless the bank is significantly oversized.
If the total dissipation exceeds 10 W for a bank of six to ten snap-in electrolytics, a cooling airflow of at least 1 m/s across the cases should be budgeted at the enclosure design stage.
Decision Thresholds for Ripple, ESR, and Temperature
The practical limits, based on data common to electrolytic and film capacitor series, are:
- Ripple current per unit: for a 470–680 µF electrolytic in a 35 × 50 mm snap-in case, a continuous ripple of 1.2–1.5 A at 10 kHz and 85°C ambient is typical. For a 30–50 µF film capacitor in a similar package, 10–20 A is normal.
- ESR at operating frequency: aluminum electrolytics show ESR in the low tens of milliohms (3–5 mΩ per 1 mF at 100 kHz); film capacitors sit in the low milliohm range (1–3 mΩ per 50 µF).
- Core temperature limit: electrolytic hot-spot limit is 105°C; film hot-spot limit is 85–90°C in most series. Above these values, the lifetime falls off rapidly.
- Lifetime derating: for electrolytic capacitors, every 10°C reduction in core temperature below the rated hot-spot doubles the expected service life. For film capacitors, a 10°C reduction typically extends life by a factor of 1.5 to 2, though absolute lifetimes are already much longer.
- Voltage derating: operate electrolytics at 80–90% of rated voltage; film capacitors at 70–85% of rated AC voltage. This also reduces dielectric losses and internal heating.
These thresholds are conservative starting points. Actual values vary with case size, terminal style, and manufacturer, so the datasheet ripple-current curve must be read at the intended ambient temperature and frequency.
Cooling Options for Capacitor Banks
| Cooling method | Typical airflow | Effective Rth (case-to-ambient) | Typical application | Cost / complexity |
|---|---|---|---|---|
| Natural convection | 0.1–0.3 m/s | 10–20 K/W per unit | Low-ripple DC links, hold-up banks | None |
| Axial fan, enclosure-level | 1–2 m/s | 4–8 K/W per unit | Inverter DC links, PFC boost banks | Low; one to two fans |
| Fan with heatsink or ducting | 3–5 m/s | 2–4 K/W per unit | High-ripple banks in compact enclosures | Medium; mechanical design effort |
| Liquid cooling (cold plate) | Not applicable | Below 1 K/W per unit | Traction drives, high-power rectifiers | High; pump and loop system |
The Rth values above are representative ranges for typical snap-in electrolytic cases; cylindrical film capacitors behave similarly. Above an airflow of 5 m/s, the reduction in Rth diminishes, so over-speeding fans is not a substitute for adding parallel capacitance.
Step-by-Step Selection Procedure
- Define the load profile. Collect the RMS ripple current, its frequency, the ambient temperature inside the enclosure, and the ripple duration (continuous or intermittent). A load profile of 20 A RMS at 10 kHz for 30 minutes is a different thermal case than 20 A at 10 kHz continuously.
- Calculate the required capacitance. For a DC link, use C ≈ I_rms / (2 × f_ripple × ΔV), where ΔV is the allowed voltage ripple, typically 5–10% of the rail voltage. For a PFC boost, C is set by the hold-up time requirement.
- Estimate the dissipation budget. Total heat P = N × I_ripple_per_unit² × ESR. Target a core temperature 20–25°C below the hot-spot limit to obtain a realistic lifetime margin.
- Select the capacitor type and parallel count. Divide the total ripple current across units so that each operates at 60–90% of its rated ripple current. This derating keeps individual ESR lower and reduces per-unit temperature rise.
- Choose the cooling method. Use the table above. If natural convection yields a core temperature above the hot-spot limit, move to forced air at 1–2 m/s, or reduce the ripple current per unit by adding more parallel capacitors.
- Validate with a thermal test. Measure the case temperature (not the ambient) at the worst-case operating point and extrapolate the core temperature using the manufacturer’s thermal resistance value. If the result exceeds the hot-spot limit, increase airflow or add parallel units.
This procedure keeps the bank within its thermal envelope without oversizing the capacitance or requiring an expensive cooling architecture. For procurement teams, the practical output is a specification: the required ripple current derating, the core temperature margin, and the minimum airflow, written into the purchase order so the supplied parts are validated for the intended operating condition.
