  We present a search for charged Higgs bosons in top quark decays. We analyze
the \eplus, \muplus, $ee$, $e\mu$, $\mu\mu$, \etau and \mutau final states from
top quark pair production events, using data from about 1${\text{fb}}^{-1}$ of
integrated luminosity recorded by the \dzero experiment at the Fermilab
Tevatron Collider. We consider different scenarios of possible charged Higgs
boson decays, one where the charged Higgs boson decays purely hadronically into
a charm and a strange quark, another where it decays into a $\tau$ lepton and a
$\tau$ neutrino and a third one where both decays appear. We extract limits on
the branching ratio $B(t\to H^+ b)$ for all these models. We use two methods,
one where the $t\bar{t}$ production cross section is fixed, and one where the
cross section is fitted simultaneously with $B(t\to H^+b)$. Based on the
extracted limits, we exclude regions in the charged Higgs boson mass and $\tan
\beta$ parameter space for different scenarios of the minimal supersymmetric
standard model.
