High frequency transformer is the main component of switching power supply. There are many topologies in switching power supplies. For example, the half-bridge power conversion circuit turns on two switching triodes in turn to generate a high-frequency pulse wave of 100kHz. Then, voltage transformation is carried out through the high-frequency transformer to output alternating current. The turns ratio of each winding coil of the high-frequency transformer determines the output voltage. A typical half bridge type transformer circuit in the most conspicuous is three high frequency transformer, the main transformer, transformer and auxiliary transformer (standby transformer), each transformer in the regulations of the state has its own measure, such as main transformer, as long as it is more than 200 w power supply, its core diameter (level) shall not be less than 35 mm. The auxiliary transformer, when the power supply does not exceed 300W, has a magnetic core diameter of 16mm.