The paper listed below provides the guidelines on ZVS design of an active clamp forward converter, and explains when ZVS makes efficiency benefit and when not.
Hong Huang, "Design guidelines on the effect of resonant transitions of forward converter on efficiency with active clamp",
Applied Power Electronics Conference and Exposition, 2008. APEC 2008. Twenty-Third Annual IEEE, 24-28 Feb. 2008 Page(s):600 - 606
Although the below is in updating, the existing one is still a good reference for designing an active clamp forword converter.
More literatures on active clamp forward converters from TI web:
http://focus.ti.com/lit/ml/slup108/slup108.pdf
http://focus.ti.com/lit/ml/slup112/slup112.pdf
http://focus.ti.com/lit/ml/slup175/slup175.pdf
http://focus.ti.com/lit/an/slua322/slua322.pdf
