How can we evaluate the heat dissipation performance and stability of capacitors?

Assessing the thermal performance and stability of capacitors—particularly power-type capacitors such as metallized polypropylene film capacitors and electrolytic capacitors—is crucial for ensuring their long-term reliable operation in power electronic systems. The following provides a systematic overview from four perspectives: testing methods, key parameters, design verification, and practical operation and maintenance.

I. Evaluate thermal performance through temperature rise testing.

Core principle: During operation, capacitors generate heat due to dielectric losses and equivalent series resistance (ESR). If heat dissipation is inadequate, the internal temperature rises, accelerating aging and potentially leading to failure. Therefore, temperature rise is the most direct indicator of a capacitor’s heat-dissipation capability.

II. Analyze Key Electrical Parameters to Determine Stability

1. Tangent of the loss angle (tanδ)

• The smaller the tanδ, the lower the dielectric loss and the less heat generated.

• The tanδ of MKP capacitors is typically less than 0.1% (at 1 kHz). If the measured value increases significantly (e.g., to over 0.3%), it indicates that the material has aged or become damp, leading to a decline in stability.

2. Equivalent Series Resistance (ESR)

• ESR directly affects Joule heating (P = I_{\text{rms}}^2 × ESR);

• At high frequencies, the ESR varies with frequency and must be measured at the actual operating frequency.

• An abnormally elevated ESR often indicates deterioration or drying out of internal connections (electrolytic capacitors).

3. Rate of capacitance change

• A capacity degradation of ≤5% after long-term operation is normal.

• If the capacity declines rapidly (e.g., by more than 10% within one year), it indicates frequent self-healing or medium degradation, resulting in poor stability.

4. Insulation Resistance and Leakage Current

• A decrease in insulation resistance or an increase in leakage current indicates a higher risk of moisture-induced degradation, contamination, or dielectric breakdown.

• Can be tested by comparing results at room temperature and high temperature using a high-resistance meter or a dedicated LCR meter.

3. Evaluate long-term stability through accelerated life testing.

Manufacturers typically use the High-Temperature and High-Humidity Bias (THB) test or the High-Temperature Load Life Test to simulate long-term usage.

• For example: The capacitor is continuously operated at 105℃ under its rated voltage for 2,000 to 10,000 hours;

• Regularly sample and test capacitance, tanδ, and ESR;

• If the parameter variation is within the specified range, the stability is considered to be compliant.

Although it’s difficult for users to conduct full-life-time tests, they can refer to the lifespan estimation formulas provided by manufacturers (e.g., for electrolytic capacitors, the lifespan typically doubles for every 10°C decrease in temperature).

4. Structure and installation method affect thermal performance.

Even if the capacitor itself performs well, external heat dissipation conditions significantly affect the actual temperature rise.

• Layout: Avoid dense stacking and maintain a thermal clearance of ≥5mm.

• Direction: Vertical installation promotes natural convection; for horizontal installation, pay attention to heat dissipation from the bottom.

• PCB Design: Large copper foil ground planes can help with thermal management (especially for SMD capacitors).

• Forced air cooling: In high-power-density systems, installing fans can significantly reduce temperature rise by 10–20℃.

• Keep away from heat sources: Do not place the device directly adjacent to heat-generating components such as MOSFETs and transformers.

V. Stability Monitoring in Actual Operation

During the equipment operation and maintenance phase, indirect assessment can be carried out through the following methods:

• Infrared Inspection: Regularly check whether the surface temperature of capacitors is abnormal.

• Noise and Vibration: Some capacitors may emit a faint “hissing” sound (partial discharge) during the early stages of aging.

• System performance issues: Frequent overheat protection of the inverter and increased fluctuations in bus voltage may be caused by degradation of capacitor performance.

• Preventive Replacement: For critical equipment, replace it ahead of schedule according to the manufacturer’s recommended service life (e.g., 5–10 years) to avoid sudden failures.