Sensor less capacitor deterioration estimation in a buck converter using ripple voltageIntroductionElectrolytic capacitor uses electrolyte as one of its plates and thereby achieving larger capacitance per unit volume. It has high volumetric efficiency because of its enhanced plate surface area and very thin dielectric layer. Electrolytic capacitors uses cost efficient case sizes and are capable of giving high capacitance value and usability at high voltages as shown in Fig 1. They are widely used in power electronic circuits, filtering circuits, timing networks, by-pass, coupling and other applications. Electrolytic capacitors are generally reliable, however they are subjected to age related deterioration and eventual failure.

Fig. 1. Types of capacitors for different voltage and capacitance Mostly the capacitors will be replaced on periodic basis, when the capacitor deteriorates. However, within this periodic based maintenance there is the possibility of unexpected shutdown and failure. Hence, online condition monitoring of electrolytic capacitors is important for critical high performance applications. Early detection of incipient faults will enable controlled shutdowns of power converters and thereby reducing the outage time and repairs. Hence, in this project a cost effective sensor less capacitor deterioration estimation using already deployed sensors in a synchronous buck converter has been proposed. This project focusses on identifying ESR and capacitance deterioration in an electrolytic capacitor. This project involves developing a Simulink model for closed loop synchronous buck converter with cascade controller. The developed buck converter is simulated for different load, reference voltage and input voltage condition to study the reliability of the closed loop controller. The feasibility study of different parameters like duty, output voltage and inductor current of the developed buck converter for deterioration detection is carried out. Finally, the methodology for capacitance and ESR deterioration using output ripple voltage has been proposed along with estimation of deterioration value. The feasibility of implementing the proposed algorithm in online and its pros and cons has been analyzed and discussed in this project report.Research scope The criteria for defining failures in the capacitor involves the following standards,

1. Change in the capacitance (20% to 30%)

2. Change in the tangent of loss angle (1.5 to 3.0 times the initial value)

3. Change in the leakage current (excess of the specified value)

The most common failure in electrolytic capacitor is reduction in the capacitance due to the evaporation or leakage of electrolyte. The computer motherboards with failed capacitors due to electrolyte leakage and evaporation are shown in Fig 2. The evaporation may occur due to sudden surge or long term usage. The reduction in the content of electrolyte increases the ESR (Equivalent Series Resistance) and decrease in the capacitance value. ESR is also influenced by corrosion of lead which connects the plates in the capacitor.

Fig. 2. Failed capacitors in computer motherboards

In this project increase in the ESR value and decrease in the capacitance value are considered for deterioration detection. As the change in the capacitance beyond 20% is easily identifiable, this research work involves deterioration detection within 10%. It is imperative that, if the proposed methodology works for 10% for more than 10% it will work. A feasibility study of capacitor deterioration detection using modulated gate pulse to the switch, capacitor current and output voltage of the buck converter has been carried out. From the study, it was identified that output ripple voltage serves as best indicator of change in the capacitor value. Hence output voltage of the buck converter is used as key signal for condition monitoring of capacitors in buck converter. As the method is based on pre-existing sensor on buck converter, the methodology can be adopted only for a particular temperature. The Fig 3 shows the variation of ESR with respect to temperature and frequency. Hence, at the expense of an extra temperature sensor the reliability of the proposed methodology can be increased significantly. The prognosis of the capacitor value is based on the deviation of the dielectric angle in the converter output voltage and the manufacturer characteristic curve for dielectric angle Vs estimated life remaining. Hence, in this research work the deviation of dielectric angle with respect to time will be estimated. The consolidated information of the research scope of this project is given in Table I.

Fig. 3. Variation of capacitance with temperature and frequency

Table I. Research scope of this project

Converter Buck converter

ControlVoltage and current control

ConstraintConstant temperature

Capacitor deterioration detection Increase in ESR and decrease in Capacitance

Key signalOutput voltage

Deterioration limit