Switching power supply unit

摘要

A switching power supply unit comprises a DC power supply (2); a transformer (T) having a primary winding (N1), a secondary winding (N2) and a feedback winding (NB), a main switching element (Q1) connected in series to the primary winding (N1) to be turned on by a voltage generated at the feedback winding (NB), and a voltage dropping circuit (6, 7) for dropping the output voltage obtained form the secondary winding (N2) to reduce a fly-back voltage so that the voltage of a control terminal of the main switching element (Q1) is maintained at a lower value than a threshold voltage of the main switching element (Q1). <IMAGE>