The so-called over-current may be caused by the following reasons if it refers to the over-current of the hihg frequency transformer input current: 1. high frequency transformer saturation. Have saturation magnetic flux density and magnetic materials, if the high frequency transformer design, improper or bias phenomenon will make transformer work flux density is too high, when is greater than the saturation flux density, permeability is 0, the impedance transformer impedance only basic resistance, the equivalent of the power supply is directly to the negative electrode on a wire, the consequences. In the switching power supply, this phenomenon often occurs in the push-pull topology. 2. Load short circuit!! 3. The insulation of transformer windings is not well done, resulting in inter-turn short circuit or the original side winding short circuit. 4. Some characteristics of magnetic core materials, for example, the influence of dc bias and temperature on magnetic permeability is not considered in the design. Under high temperature and high dc bias, the transformer winding impedance is reduced and overcurrent occurs.