  We study the associated production of a charged Higgs boson with a bottom
quark and a light quark at the LHC via p p \to H^\pm\,b\,j in the Two Higgs
Doublet Models (2HDMs). Using the effective W approximation, we show that there
is exact cancellation among various Feynman diagrams in high energy limit. This
may imply that the production of charged Higgs can be significantly enhanced in
the presence of large mass differences among the neutral Higgs bosons via
W^\pm-Higgs fusion in the p p \to H^\pm\,b\,j process. Particularly, we
emphasize the potential enhancement due to a light pseudoscalar boson $A$,
which is still allowed by the current data by which we explicitly calculate the
allowed regions in (M_A,\,\tan\beta) plane, and show that the production cross
section can be as large as 0.1 pb for large $\tan\beta$. We also show that the
transverse momentum distribution of the b quark can potentially distinguish the
W^\pm-A fusion diagram from the top diagram. Finally, we point out further
enhancement when we go beyond the 2HDMs.
