  Exchange bias training effect in ferromagnetic/antiferromagnetic bilayers is
investigated. In some systems the evolution of the exchange bias field $H_E$
with the number of cycle $n$ cannot be fitted by the empirical $1/\sqrt{n}$
function. A unified expression is derived from a discretized Landau-Khalatnikov
equation in the framework of the thermodynamics model which is proposed by Ch.\
Binek. This generalized model describes well training effect independent of the
magnetization reversal mechanism in the ferromagnetic layers.
