Why do metallized polypropylene film capacitors bulge or rupture?

Metalized polypropylene film capacitors (commonly referred to as MKP capacitors) exhibit high reliability, long service life, and excellent self-healing capability when designed and manufactured properly. However, once bulging or even bursting occurs, it usually indicates that serious internal abnormalities have taken place. The primary causes can be attributed to the following categories:

1. Long-term operation under overvoltage or overcurrent conditions

When the voltage applied to a capacitor exceeds its rated value or when the ripple current flowing through it is too high, the internal electric field strength increases, making the polypropylene film dielectric prone to localized breakdown. Although MKP capacitors possess a “self-healing” function, frequent breakdowns will gradually erode the metal coating and generate heat. If this heat cannot dissipate in time and the temperature continues to rise, the film material may undergo thermal decomposition, releasing gases such as hydrogen and methane, which in turn cause the internal pressure to increase. Eventually, this could lead to bulging or even rupture of the capacitor’s outer casing.

2. Excessive high-frequency harmonics or switching noise

In applications such as variable-frequency drives, inverters, and switch-mode power supplies, capacitors often have to withstand high-frequency pulsed currents. These high-frequency components can significantly increase dielectric losses (manifesting as an increase in the loss tangent tanδ), and the power dissipation is proportional to both frequency and the square of the voltage. If heat dissipation is inadequate, the internal temperature of the capacitor will rise rapidly, accelerating material aging and potentially triggering thermal runaway, which in turn can lead to gas generation and subsequent swelling of the enclosure.

3. Ambient temperature too high or poor heat dissipation

Although MKP capacitors are rated to withstand temperatures up to 105°C or 110°C, this rating refers to the temperature at their internal hot spots, not the ambient temperature. If the capacitor is installed in a confined space, close to heat-generating components (such as power transistors or transformers), or in an environment with poor air circulation, its surface temperature could easily exceed the safe operating range. High temperatures not only reduce the self-healing efficiency but also accelerate the oxidation and decomposition of the polypropylene film, generating gases that can cause bulging.

4. The self-healing process repeatedly occurs or fails.

The “self-healing” mechanism of MKP capacitors relies on the instantaneous vaporization of the metal layer at the breakdown point, which forms an insulating isolation zone. This process itself generates trace amounts of gas and heat. In systems characterized by frequent voltage fluctuations or persistent interference, self-healing may occur repeatedly, leading to a gradual accumulation of heat and gas. If the energy of a particular breakdown event is excessively high, self-healing may fail, potentially resulting in a sustained electric arc that violently decomposes the dielectric material. This rapid decomposition produces large quantities of gas within a short period, ultimately causing an explosive rupture.

5. Manufacturing defects or poor encapsulation

Although less common, capacitors can also fail prematurely under normal operating conditions if they have film impurities, uneven plating,虚焊 of leads, or inadequate potting/sealing processes—such as residual moisture or trapped air. For example, tiny internal air gaps may undergo partial discharge under high voltage, gradually eroding the dielectric material and eventually leading to thermal accumulation and gas generation.