(1) The high-frequency switching current loop composed of the primary L1 of the high-frequency transformer, the switching tube V5, and the filtering capacitor C1 may generate significant spatial radiation. If the capacitor filtering is insufficient, high-frequency current will still be conducted to the input AC power supply in a differential mode.
(2) The secondary L2 of the high-frequency transformer, rectifier diode V6, and filtering capacitor C2 also form a high-frequency switching current loop that generates spatial radiation. If the capacitor filtering is insufficient, the high-frequency current will be mixed in the form of differential mode and transmitted outward on the output DC voltage.
(3) There is distributed capacitance Cd between the primary and secondary of the high-frequency transformer, and the high-frequency voltage of the primary is directly coupled to the secondary through these distributed capacitors, generating common mode noise of the same phase on the two output DC power lines of the secondary. If the impedance of two wires to ground is unbalanced, it will also transform into differential mode noise.
(4) The output rectifier diode V6 will generate reverse surge current. When a diode is conducting in the forward direction, the charge accumulates in the PN junction. When a reverse voltage is applied to the diode, the accumulated charge disappears and a reverse current is generated. Because the switching current needs to be rectified by a diode, the time for the diode to transition from conduction to cutoff is very short. In a short period of time, a surge of reverse current is generated to make the stored charge disappear. Due to the distributed inductance, distributed capacitance, and surge in the DC output line, high-frequency attenuation oscillation is caused, which is a type of differential mode noise.
(5) The load on switch V5 is the primary coil L1 of the high-frequency transformer, which is an inductive load. Therefore, when the switch is turned on or off, there will be a high surge peak voltage at both ends of the transistor, and this noise will be transmitted to the input and output terminals.
(6) There is a distributed capacitance CI between the collector of the switching tube V5 and the heat sink K, so high-frequency switching current will flow through CI to the heat sink K, then to the chassis ground, and finally to the protective ground wire PE of the AC power line connected to the chassis ground, thereby generating common mode radiation. The power lines L and N have a certain impedance to PE, and if the impedance is unbalanced, common mode noise will also transform into differential mode noise.
