A soft-starting and soft-turning-off circuit of DC solid-state switch

Solid-state switch is a no-contactor switch. It is used in connecting and disconnecting loads, fast restricting the increasing current in the case of over-load and short-circuit. However, surge-current will be produced when solid-state switch is turned on especially in capacitive loads and spike-voltage is induced between the drain and source of MOSFET in inductive loads when solid-state switch is turned off. Surge-current and spike-voltage are the main reasons resulting in the malfunctioning of MOSFET and the reducing solid-state switch´s reliability. In this paper, a soft-starting and soft-turning-off circuit of DC solid-state switch is introduced. Soft-starting is used to restrain the impact of the surge-current when solid-state switch is turned on. Soft-turning-off is used to suppress the spike-voltage of inductive loads between the drain and source of MOSFET when solid-state switch is turned off. Based on RC nets, this circuit is simple and practical. The experimental results show that the soft-starting and soft-turning-off circuit improves the ability of carrying loads and has a strong controlling ability. The designing idea and circuit structure presented in this paper can be applied to any switch-controlled circuit based on solid-state device.