如果开关变压器的铁芯在这之前从来没有被任何磁场磁化过，并且开关变压器的伏秒容量足够大，那么，当第一个交流脉冲的正半周电压加到变压器初级线圈a、b两端时，在变压器初级线圈中将有励磁电流流过，并在变压器铁芯中产生磁场;在磁场强度H的作用下，变压器铁芯中的磁通密度B将会按o-a磁化曲线上升;当脉冲电压的正半周将要结束时，磁场强度到达最大值Hm，同时对应的磁通密度也被磁化到最大值Bm。磁通密度在增加，表示流过变压器初级线圈中的励磁电流产生的磁场正在对变压器铁芯进行充磁。
If the core of the switching transformer has never been magnetized by any magnetic field before, and the volt-second capacity of the switching transformer is large enough, when the positive half-cycle voltage of the first AC pulse is applied to both ends of the primary coil A and B of the transformer, the excitation current will flow through the primary coil of the transformer and the core of the transformer. Under the action of magnetic field intensity H, the magnetic flux density B in transformer core will rise according to O-A magnetization curve; when the positive half cycle of pulse voltage is about to end, the magnetic field intensity reaches the maximum Hm, and the corresponding magnetic flux density is also magnetized to the maximum Bm. The increasing flux density indicates that the magnetic field generated by the excitation current flowing through the primary coil of the transformer is magnetizing the core of the transformer.