Startup circuit, switching power source IC, and switching power source apparatus

摘要

A startup circuit includes a MOSFET that is connected between a startup power source and an auxiliary power source made of a smoothing capacitor and passes a startup current from the startup power source to the smoothing capacitor, a JFET that has a drain terminal connected to a drain terminal of the MOSFET and a source terminal connected through a resistor to a gate terminal of the MOSFET, and a pinch-off voltage controller that controls a pinch-off voltage of the JFET to a first reference voltage at startup and to a second reference voltage, which is lower than the first reference voltage, after startup.