http://en.wikipedia.org/wiki/Power_factor_correcti on
To comply with current EU standard EN61000-3-2, all switched-mode power supplies with output power more than 75 W must include passive PFC, at least.
PFC musi odstranit harmonicke, t.j. mal by sposobit ze vstupny prud je velmi podobny sinusovke. Ostava fazovy posun - ten by sposobil len to ze mera viac ako kolko to v skutocnosti zere.