  This paper provides sufficient conditions for stability of switched linear
systems under dwell-time switching. Piece-wise quadratic functions are utilized
to characterize the Lyapunov functions and bilinear matrix inequalities
conditions are derived for stability of switched systems. By increasing the
number of quadratic functions, a sequence of upper bounds of the minimum dwell
time is obtained. Numerical examples suggest that if the number of quadratic
functions is sufficiently large, the sequence may converge to the minimum
dwell-time.
