Impact of output capacitor on feedback loop stability of a power supply

In a switching power supply, the output capacitors are typically selected based on output voltage ripple and transient response requirements. However, they also play an important role in the stability of the feedback control loop of the converter.

As the output capacitors are part of the converter’s power stage, parameters like their capacitance and ESR (equivalent-series-resistance) appear in the plant transfer function of the control system. Therefore, once the compensator has been designed to stabilize the control loop for a specific plant characteristic, changes in the output capacitors will reshape the frequency response of the plant and in turn, of the open-loop transfer function from which stability is studied, with consequences ranging from just a degradation in transient performance up to full-blown instability in a worst-case scenario. Therefore, it is important to reassess feedback loop stability whenever changes in the output capacitors are implemented.

In this application note, the role of output capacitors’ parameters on the plant transfer function of a voltage-mode buck converter is studied. The analysis is supported by experimental results on a DC-DC converter prototype, showing how even small parameter variations caused by changes in the output capacitors may be enough to cause instability of the control loop, requiring a redesign of the compensator.

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