The active clamp feature is shown in Figure 5. During the off-state, the power MOSFET is turned back on when Vds > Vzener +Vf,diode + V threshold,MOSFET. Also, there are two large resistances, the resistor and the MOSFET, within this current path to help dissipate energy. (Note that during the active clamp the FET is in a linear, or high-resistance, mode.) The load demagnetizes quickly during active clamp because the stored energy in the load is dissipated across a large po- tential [Vcc - Vclamp]. The larger the difference, the faster the demagnetization. The energy dissipated by the IPS during the Active Clamp sequence is Vclamp x ?Ids(t)dt.The energy stored by the inductance is [Vclamp - Vcc] x ?Ids(t) dt. Thus, it is important to notice that during the active clamp sequence, the device dissipates more power than the load. (The current through the load and the IPS is the same but the voltage across the IPS is higher than the one across the load).
The disadvantage is that the current in the inductive load decays more slowly, which can be a problem in some applications.