A new soft-start method with abnormal over current protection function for switching power supplies

The present paper relates to the SMPS (switching mode power supply) used fly-back converter. In conventional built-in soft start up, it normally applied to the pulse-width-modulated converter to reduce stress of system and the inrush current on the input circuits. During the soft start up, it exists uncontrollable time which consists of leading edge blanking (LEB) time, internal circuit delay and power MOSFET on/off time. Consequently the primary side transformer current (I _ds) can´t be controlled by feedback loop, and then the drain voltage (V_ds) is increased higher than BV_dss level of the power MOSFET. Therefore power MOSFET is seriously stressed. To overcome this problem, this paper proposes a new soft-start method with AOCP (abnormal over current protection) function. The uncontrollable time of the power MOSFET is minimized 46% by AOCP function. Therefore V _ds ^max rating can be reduced less 33% than conventional soft-start method, because minimum turn on time is minimized with the clamped current of primary side transformer