This switching transformer is called high frequency transformer because its operating frequency is higher than 50HZ low frequency transformer. Then how to obtain the pulse that drives the switch tube or silicon controlled rectifier? This requires an oscillation circuit to generate. We know that the crystal triode has a characteristic, that is, the base to emitter voltage is 0.65-0.7V, which is the amplification state, and above 0.7V is the saturation conduction state, and -0.1V--0.3V is working in the oscillation state. After its working point is adjusted, it will rely on a deeper negative feedback to generate negative pressure, so that the oscillation tube starts to vibrate, The frequency of the oscillator tube is determined by the charging and discharging time of the capacitor on the base. If the oscillation frequency is high, the output pulse amplitude will be large, otherwise it will be small, which determines the output voltage of the power supply regulator tube. How to stabilize the working voltage of the secondary output of the transformer? Generally, a group of coils are wound around the switching transformer. The voltage obtained at the upper end of the coil is used as the reference voltage after rectification and filtering, and then the reference voltage is returned to the base of the oscillator through the optocoupler to adjust the vibration frequency. If the secondary voltage of the transformer increases, the voltage output by the sampling coil also increases, The positive feedback voltage obtained through the optocoupler also increases. When this voltage is added to the base of the oscillator tube, the oscillation frequency decreases, which stabilizes the secondary output voltage. If the operation is too detailed, it is unnecessary to talk about it in detail or understand it in detail. In this way, the high-power voltage is transmitted by the switching transformer and isolated from the subsequent stage, and the returned sampling voltage is also transmitted by the optocoupler and isolated from the subsequent stage, Therefore, the mains voltage of the front stage is separated from that of the rear stage, which is called cold plate and safe. The power supply in front of the transformer is independent, which is called switching power supply.