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Film capacitor failure analysis

by:SMiLer     2021-01-17
Film capacitor is to belong to a kind of use frequency is high, it is widely used in analog signal coupling and other power supply. Film capacitor has very many excellent characteristics, it is a fairly good capacitor products. Capacitor failure capacitor breakdown, open circuit, lead the capacitor such as fracture, insulating the rupture failure is a fatal failure completely lose the ability to work, the rest of the some failure causes capacitance can not meet the requirement of the use, and gradually transition to a critical failure; Capacitor under the action of working stress and environmental stress, work after a period of time, can produce certain failure mode respectively or simultaneously. The same failure mode has a variety of failure mechanism, failure mechanism and can produce various failure modes. The relationship between failure mode and failure mechanism is not one-to-one. Film capacitor failure analysis film capacitor is used for diode absorbing circuit, capacitance, rated voltage 630 VDC circuit normal peak is less than 200 v, derating requirements. Failure capacitor appearance is good, no damage trace using the LCR test, failure samples without the capacity, in open circuit state between pin untied capacitor plastic package, and lead wire and metal spraying welding layer is good, metal spraying layer and the core connection with black marks; Core metallized film capacitor, metallized film capacitor part failure bright perfect condition, on the other. Part of the metallized film has hot deformation. The preliminary judgment for circuit dv/dt, is the one inducing the inefficiency of capacitor. Test circuit of voltage waveform and found the dv/dt is 3000 v/u s, but choose the film capacitor dv/dt Max only 40 v/u s. To verify the failure mode, take new capacitor, measuring the normal at the beginning of computer after aging, capacitor - 80 90% failure, eliminate the manufacturer the incoming quality problem and determine if the capacitor is not suitable for use in the circuit. Solution for all single boards ( Including semi-finished products, finished products and delivery products, the use of the capacitor all change to specifications dv/dt is double-sided metallized polypropylene mountain after container problem solving. For thin film capacitors, select absorption of capacitance parameters in addition to the voltage and capacity, the main index and dv/dt and ripple current, Voltage) 。 Especially the dv/dt this parameter, even the same capacity and voltage, series of pins, distance between different capacitance, the rating can be from a few V/us to a few KV/us. If only from the voltage and capacity index selection capacitance, capacitance I for attenuation after a period of time will let values were all disappear. Because the power device switching frequency is higher ( Up to hundreds of KHz) , smaller voltage fluctuation under the high frequency ripple current will lead to more people ( 我= WCU) , ripple current will cause heart, let people high open, degradation of strength parameters or burned.
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