Plug-and-play realization of the virtual infinite capacitor

Abstract

We propose a plug-and-play realization of the virtual infinite capacitor (VIC), which is an electronic circuit recently proposed in the literature for DC voltage filtering. The VIC is meant to replace large, expensive and unreliable electrolytic capacitors, and is mainly intended for applications where random fluctuations of the DC bus voltage may occur (i.e., the VIC is not tuned to one particular ripple frequerncy). Connecting a VIC into a larger circuit, such as a power supply based on a power factor compensator (PFC) is relatively complicated, because the VIC charge controller needs access to the voltage control loop of the power source (e.g., the PFC). The plug-and-play realization of the VIC, proposed in this paper, can be connected directly to the DC bus like a usual filtering capacitor, no extra connections are needed. This is achieved by automatically adjusting the reference voltage of the VIC so that it will be equal to the equilibrium voltage of the DC bus (i.e., the constant DC bus voltage at which current supply and current demand are equal). We investigate carefully the output impedance of the plug-and-play VIC. This impedance must behave like a relatively small capacitor both at very low and at very high frequencies, but must be very small (ideally, zero) at an intermediate range of frequencies (the range where the ripple is expected). For the stability of complex interconnected systems, it is recommended that the output impedance of the plug-and-play VIC should be positive-real. We discuss how to achieve these properties by very careful choice of controllers and filters. The results are tested by simulation and experiment using a 300V demo system with a 270W load.