Switching power supplies can be divided into full bridge, half bridge, push-pull, and other types according to the main circuit type. However, regardless of the type of switching power supply, it produces strong noise during operation. They conduct outward through power lines in a common or differential mode manner, while also radiating into the surrounding space. Switching power supplies are also sensitive to external noise intruded by the power grid and transmit it to other electronic devices to generate interference.
After the AC power is input into the switching power supply, the bridge rectifier V1-V4 is organized into a DC voltage Vi and applied to the primary L1 and switch V5 of the high-frequency transformer. The base input of the switch tube V5 is a high-frequency rectangular wave ranging from tens to hundreds of kHz, and its repetition frequency and duty cycle are determined by the requirements of the output DC voltage VO. The pulse current amplified by the switch tube is coupled to the secondary circuit by a high-frequency transformer. The ratio of the first turn of a high-frequency transformer is also determined by the requirements of the output DC voltage VO. The high-frequency pulse current is rectified by diode V6 and filtered by C2 to form a DC output voltage VO. Therefore, the switching power supply will generate noise and form electromagnetic interference in the following aspects.
(1) High frequency switching current loop composed of high frequency transformer primary L1, switch tube V5 and filter capacitor C1 may generate large space radiation. If the capacitor filtering is insufficient, high-frequency current will also be transmitted to the input AC power supply in a differential mode manner.
(2) The secondary L2 of high-frequency transformer, rectifier diode V6 and filter capacitor C2 also form the high-frequency switching current loop, which will generate space radiation. If the filter of the capacitor is insufficient, the high-frequency current will be mixed on the output DC voltage in the form of differential Modular form to conduct outward.
(3) There is a distributed capacitor 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 in 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 the diode conducts in the forward direction, the charge accumulates within the PN junction. When the diode applies a reverse voltage, 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, and in a short period of time, the storage charge needs to disappear, resulting in a surge of reverse current. Due to the distributed inductance, 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 of switch tube 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 tube, and this noise will be transmitted to the input and output terminals.
(6) There is a distributed capacitance CI between the collector of switch tube V5 and the heat sink K, so high-frequency switching current will flow through CI to the heat sink K, then to the casing ground, and finally to the protective ground wire PE of the AC power line connected to the casing 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 can also transform into differential mode noise.
