The failure of electrolytic capacitors is the cause of most breakdowns of
static converters. The function of these capacitors is to filter and to
store electrical
energy. Accelerated aging tests showed that the increase of the internal
resistance
ESR of the capacitors is a good indicator of their faulty state.
The filter function is
affected by the capacitors wearout on the one hand. At high frequency,
the voltage
ripple at the terminals of the capacitors increases according to ESR rise. On the other
hand, the storage function is not much influenced by the capacitors aging. As static
converters work most of the time at variable load, high transient values of the voltage
ripple occur when output current changes that can induce false alarms. These
transients are verified theoretically and experimentally. In order to avoid these
transients effects, we suggest to monitor the fundamental component of the voltage
ripple. This latter waveform is the best signature of the capacitors state. The ESR of
the capacitors and the time before their failure are deduced from the processing of
this waveform with other converter parameters such as input voltage, output current
and ambient temperature.