Failure analysis of film capacitors

Film capacitors belong to a kind of electrical appliances with a relatively high frequency of use. They are mainly used for cross-connection of analog signals and other power sources. Film capacitors have many excellent features and are a very good capacitor product.

Capacitor failure:

Capacitor breakdown, open circuit, lead break, insulation rupture and other failures that make the capacitor completely incapable of working are fatal failures. Other failures will make the capacitor unable to meet the requirements of use and gradually transition to fatal failure;

Under the combined action of working stress and environmental stress, the capacitor will produce certain failure modes separately or at the same time after working for a period of time. The same failure mode has multiple failure mechanisms, and the same failure mechanism can produce multiple failure modes. The relationship between failure mode and failure mechanism is not completely addressed one by one.

Failure analysis of film capacitors:

Film capacitors are used in diode absorption circuits. The rated voltage of the capacitor is 630VDC, and the normal peak in the circuit is less than 200V, which meets the derating requirements. The appearance of the failed capacitor is good, and there is no trace of damage with the LCR long test. The failed samples have no capacity, and the pins are in an open state. The capacitor plastic package is unlocked. The lead and the lead are welded well to the gold sprayed layer. The gold sprayed layer and the core are connected Darkening traces; unfold the metalized film of the capacitor core, some of the metalized film of the failed capacitor is bright and smooth, and some of the metalized film has been heated and deformed. The preliminary judgment is that excessive dv/dt of the circuit causes the capacitor to fail. The voltage waveform in the test circuit found that the dv/dt is 3000V/u s, but the maximum dv/dt of the film capacitor selected is only 40V/μ s.

In order to verify the failure mode, take a batch of new capacitors, and the initial test is normal. After the machine is aging, 80-90% of the capacitors fail. Eliminate the quality problems of the manufacturer's incoming materials and determine that the capacitor is not suitable for use in the circuit.

solution:

All single boards (including semi-finished products, finished products and shipped products" The capacitor used in this position is replaced with a double-sided metalized polypropylene container with a higher specification dv/dt and the problem is solved.

For film capacitors, in addition to voltage and capacity, the most important parameters for selecting absorption capacitors are dv/dt and ripple current (voltage). Especially the dv/dt parameter, even for capacitors with the same capacity and voltage, different series, and different pin spacing, the rated value can range from a few V/us to a few KV/us. If the capacitor is only selected from the voltage and capacity indicators, the capacitance I will attenuate and disappear after a period of time. In addition, because the switching frequency of power devices is relatively high (up to hundreds of KHz), small voltage fluctuations at high frequencies will lead to relatively high ripple currents (I=WCU), and excessive ripple currents will cause heart capacity temperature Open high, parameter deterioration or burnout.