4 Conclusions

Clearly from the scope shots, the soft-stop feature lowers the drain-to-source voltage for all four MOSFETs of an active clamp forward. This completely eliminates the two potential considerations that could cause MOSFET damage, thus allowing lower voltage rated parts to be used. Without soft-stop, designers had to consider the high-voltage spikes in their transformer design. One way to address the spikes is to change the transformer turns ratio, but that sacrifices performance. These conditions are caused by a combination of the input bulk capacitance, the output capacitance, the load during shutdown, and the transformer design. This leads to unpredictability in the design, which causes adjustments to be made after the initial build. Soft-stop eliminates this design consideration because it makes the shutdown predictable, independent of the load. The TPS2373X part allows for the programming of the soft-stop time through the SST and I_STP pins. This feature enables the active clamp forward topology for 12-V output designs, which were previously either costly or unreliable. This topology is highly efficient; the TPS23730EVM-093 can get 92.6% at 5-A load. The TPS2373X gives designers the ability to choose the best topology for their design by overcoming cost and damage barriers.