Voltage Control of the Boost Converter: PI vs. Nonlinear Passivity-based Control
Leyan Fang, Romeo Ortega, Robert Griñó
TL;DR
This work analyzes direct voltage control of a Boost converter with a resistive load, revealing that classical voltage-fed PI control can create multiple equilibria and instability, especially when the inductor parasitic resistance is negligible. The authors adopt a passivity-based control framework and present three nonlinear voltage-feedback strategies, including two simple static rules and a PID-PBC variant with a current observer, all deriving from energy-shaping and dissipation concepts to achieve asymptotic regulation with straightforward gain tuning. They show that with inductor parasitics ($d_1>0$) the PI controller yields two equilibria, one unstable and one stable under specific gain conditions, while the practical, low-current equilibrium is associated with instability; the PBC approaches offer robust performance and simple domain-of-attraction guarantees, even in the presence of parameter uncertainty. The results provide practical guidance for robust voltage regulation in DC-DC converters and illustrate how nonlinear PBC methods can outperform conventional PI control in non-minimum phase systems; future work targets extending these strategies to other converter topologies and experimental validation.
Abstract
We carry-out a detailed analysis of direct voltage control of a Boost converter feeding a simple resistive load. First, we prove that using a classical PI control to stabilize a desired equilibrium leads to a very complicated dynamic behavior consisting of two equilibrium points, one of them always unstable for all PI gains and circuit parameter values. Interestingly, the second equilibrium point may be rendered stable -- but for all tuning gains leading to an extremely large value of the circuit current and the controller integrator state. Moreover, if we neglect the resistive effect of the inductor, there is only one equilibrium and it is always unstable. From a practical point of view, it is important to note that the only useful equilibrium point is that of minimum current and that, in addition, there is always a resistive component in the inductor either by its parasitic resistance or by the resistive component of the output impedance of the previous stage. In opposition to this troublesome scenario we recall three nonlinear voltage-feedback controllers, that ensure asymptotic stability of the desired equilibrium with simple gain tuning rules, an easily defined domain of attraction and smooth transient behavior. Two of them are very simple, nonlinear, static voltage feedback rules, while the third one is a variation of the PID scheme called PID-Passivity-based Control (PBC). In its original formulation PID-PBC requires full state measurement, but we present a modified version that incorporates a current observer. All three nonlinear controllers are designed following the principles of PBC, which has had enormous success in many engineering applications.