On the QCD corrections to the charged Higgs decay of a heavy quark
  Using dimensional regularization for both infrared and ultraviolet divergences, we confirm that the QCD corrections to the decay width $\Gamma(t\to H^+b)$ are equal to those to $\Gamma(t\to W^+b)$ in the limit of a large $t$ quark mass. 
Andrzej Czarnecki and Sacha Davidson