  We report on a non-conventional temperature behavior of the upper critical
field ($H_{c2}(T)$) which is found for the Boson-Fermion (BF) model. We show
that the BF model properly reproduces two crucial features of the experimental
data obtained for high-$T_c$ superconductors: $H_{c2}(T)$ does not saturate at
low temperatures and has an upward curvature. Moreover, the calculated upper
critical field fits very well the experimental results. This agreement holds
also for overdoped compounds, where a purely bosonic approach is not
applicable.
