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.