  In this paper, a novel adaptive-gain Second Order Sliding Mode (SOSM)
observer is proposed for multicell converters by considering it as a class of
hybrid systems. The aim is to reduce the number of voltage sensors by
estimating the capacitor voltages only from the measurement of load current.
The proposed observer is proven to be robust in the presence of perturbations
with \emph{unknown} boundary. However, the states of the system are only
partially observable in the sense of observability rank condition. Due to its
switching behavior, a recent concept of $Z(T_N)$ observability is used to
analysis its hybrid observability, since its observability depends upon the
switching control signals. Under certain condition of the switching sequences,
the voltage across each capacitor becomes observable. Simulation results and
comparisons with Luenberger switched observer highlight the effectiveness and
robustness of the proposed observer with respect to output measurement noise
and system uncertainties (load variations).
