In this paper, the loop instability issue regarding the inner current loop used in the interleaved parallel-series active clamp forward (ACF) has been addressed, which occurs mostly under the condition of zero output ripple current when D approaches 0.5. To have a full investigation, the small-signal model regarding the inner current loop is proposed for the analysis, based on which the criterion and the design tips targeting to achieve the loop stability have been proposed and elaborated. Finally, the measurement based on LM5034 related application has been conducted for the verification. It is shown that the proposed solution agrees well with measurement.