Capacitance measurement method using sinusoidal voltage injection in isolating phase-shifted full-bridge DC–DC converter output stage

When it comes to the reliability of power electronics, wear-out caused by ageing of electrolytic capacitors is a major concern. The capacitor ageing presents indicators such as a decrease in the capacitance and an increase in the equivalent series resistance, the detection of which can be used for capacitor health monitoring. In this study, the measurement of the output stage capacitance in an isolating DC–DC converter output stage is shown, which is used to assess the capacitor condition. In addition to presenting the method, fundamental issues related to the traditional capacitor ageing detection are discussed. The proposed method uses a sinusoidal voltage injection into the output voltage, which generates a current through the output stage capacitor. The capacitance is evaluated by the capacitor impedance, which is defined by the amplitudes of the applied voltage and the resulting capacitor current with the injection signal frequency. The method does not require hardware modifications to the converter or the measurement system, and it is executed online during the converter operation. The results show that the method is able to detect changes in the output stage capacitance regardless of the load.