The Characteristics of Communication Switching Power Supply and the Mechanism of Electromagnetic Interference Generation
There are four basic characteristics of switching power supplies:
① The location is relatively clear. Mainly concentrated on power switching devices, diodes, and connected heat sinks and high-frequency transformers;
② The energy conversion device operates in an on/off state. Due to the fact that a switching power supply is an energy conversion device that operates in a switching state, its voltage and current change rates are high, resulting in significant interference intensity;
③ The wiring of power printed circuit boards (PCBs) is usually manually arranged. This arrangement makes it highly arbitrary, increasing the difficulty of extracting PCB distribution parameters and predicting and evaluating near-field interference;
④ The switching frequency is large, ranging from tens of thousands of Hz to several megahertz. The main forms of interference are conducted interference and near-field interference.
Mechanism of electromagnetic interference generation
Electromagnetic interference generated by 1 switch circuit
The switching circuit is the core of the switching power supply, mainly composed of switching tubes and high-frequency transformers. The dv/dt generated by it is a pulse with a large amplitude, a wide frequency band, and rich harmonics. The main reasons for this pulse interference are twofold: on the one hand, the switch tube load is the primary coil of a high-frequency transformer, which is an inductive load. At the moment when the switch tube is turned on, the primary coil generates a large surge current and a high surge peak voltage appears at both ends of the primary coil; At the moment of switch tube disconnection, due to the leakage flux of the primary coil, a portion of energy is not transmitted from the primary coil to the secondary coil. The energy stored in the inductor will form a decaying oscillation with spikes along with the capacitance and resistance in the collector circuit, which will be superimposed on the turning off voltage to form a turning off voltage spike. This type of power supply voltage interruption will generate the same magnetization surge current transient as when the primary coil is connected, and this noise will be transmitted to the input and output terminals, forming conductive interference. On the other hand, the high-frequency switching current loop composed of the primary coil, switch tube, and filter capacitor of the pulse transformer may generate significant spatial radiation, forming radiation interference.
The interference caused by the reverse recovery time of the diode in the high-frequency rectifier circuit is caused by a large forward current flowing through the rectifier diode during forward conduction. When it is turned off due to the reverse bias voltage, due to the accumulation of more carriers in the PN junction, the current will flow in the opposite direction during the period before the carriers disappear, causing a sharp decrease in the reverse recovery current of the carriers disappear and causing a significant current change (di/dt).
Electromagnetic interference suppression measures
The three elements that form electromagnetic interference are the interference source, propagation path, and disturbed equipment. Therefore, suppressing electromagnetic interference should be done from these three aspects.
The purpose is to suppress interference sources, eliminate coupling and radiation between interference sources and disturbed equipment, improve the anti-interference ability of disturbed equipment, and thereby improve the electromagnetic compatibility performance of switching power supplies.
Using filters to suppress electromagnetic interference
Filtering is an important method for suppressing electromagnetic interference, which can effectively suppress electromagnetic interference entering the equipment in the power grid and also suppress electromagnetic interference entering the power grid within the equipment. Installing a switching power filter in the input and output circuits of a switching power supply can not only solve the problem of conducted interference, but also an important weapon for solving radiation interference. The filtering suppression technology is divided into two methods: passive filtering and active filtering.
Passive filtering technology
Passive filtering circuits are simple, cost-effective, and reliable, making them an effective way to suppress electromagnetic interference. Passive filters are composed of inductance, capacitance, and resistance components, and their direct function is to solve conductive emissions.
