Failure prediction using low stability phenomenon of digitally controlled SMPS by electrolytic capacitor ESR degradation

Abstract

In this paper, we report the low stability phenomenon of digitally controlled switching mode power supply (SMPS) caused by Equivalent Serial Resistance (ESR) degradation of electrolytic capacitor. The ESR of the output capacitor stack of SMPS is known as the dumping factor in a control loop that enhances SMPS stability. However, while studying electrolytic capacitor degradation in digitally controlled SMPS, we found the low stability phenomenon of SMPS with ESR degradation. Unlike an analog controlled SMPS, a time delay between an analog-digital (AD) sampling and a pulse width modulation (PWM) duty update creates ringing on a voltage waveform at the step load change response. The amplitude and frequency of this ringing becomes larger and higher with a higher ESR and is easily evaluated with a Micro-Processing Unit (MPU) used for digital control. We propose a new failure prediction method for a digitally controlled SMPS using this low stability phenomenon.