Characteristics of switching power supplies and how electromagnetic interference occurs
There are four basic characteristics of a switching power supply:
① The location is clearer. It mainly focuses on the power switching devices, diodes, and the heat sinks and high frequency transformers connected to them;
② energy conversion device working in the switching state. Because the switching power supply is working in the switching state of the energy conversion device, so its voltage, current rate of change is very high, resulting in greater intensity of interference;
③ power supply printed circuit board (PCB) alignment is usually arranged manually. This arrangement makes it very arbitrary, increasing the difficulty of PCB distribution parameter extraction and near-field interference prediction and evaluation;
④ The switching frequency is large, which can be from tens of thousands of Hz to several trillion Hz, and the main forms of interference are conducted interference and near-field interference.
2 Electromagnetic interference generation mechanism
The switching circuit is the core of the switching power supply, mainly composed of switching tubes and high-frequency transformers, which produces dv/dt with a large amplitude of the pulse, the frequency band is wide and rich in harmonics. There are two main reasons for this pulse interference: on the one hand, the switching tube load is the primary coil of the high-frequency transformer, which is an inductive load. In the switch on moment, the primary coil generates a large inrush current, and in the primary coil at both ends of the high surge voltage spike; in the switch off instant, due to the leakage flux of the primary coil, resulting in a part of the energy is not transferred from the primary coil to the secondary coil, stored in the inductance of this part of the energy will be and the collector circuit capacitance, resistance with a spike of the formation of the attenuation of the oscillations, superposed on the shutdown voltage, the formation of shutdown voltage spike. This will be superimposed on the turn-off voltage, forming a turn-off voltage spike. This power supply voltage interruption will produce the same magnetisation impact current transient as when the primary coil is switched on, and this noise will be conducted to the output of the input, forming a conducted interference. Another aspect of the pulse transformer primary coil, switching tubes and filter capacitors constitute a high-frequency switching current loop may produce a large space radiation, forming radiation interference.
Diode reverse recovery time caused by interference in the high-frequency rectifier circuit of the rectifier diode forward conduction when there is a large forward current flow, in its reverse bias voltage and turn to cut-off, due to the PN junction in the accumulation of more carriers, and thus in the carriers before the disappearance of the period of time, the current will be reversed flow, resulting in the disappearance of carriers in the reverse recovery current decreases drastically and the occurrence of a large change in the current (di) /dt).
Electromagnetic interference suppression measures
The three elements of electromagnetic interference are interference source, propagation path and disturbed equipment. Thus, the suppression of electromagnetic interference should start from these three aspects.
The purpose of suppressing the source of interference, eliminating the coupling and radiation between the source of interference and the disturbed equipment, and improving the immunity of the disturbed equipment so as to improve the EMC performance of the switching power supply.
Use of filters to suppress electromagnetic interference
Filtering is an important method of suppressing electromagnetic interference, which can effectively inhibit electromagnetic interference in the power grid into the equipment, but also inhibit electromagnetic interference within the equipment into the power grid. The installation of switching power supply filters in the switching power supply input and output circuits can not only solve the problem of conducted interference, but also an important weapon to solve the radiation interference. Filter suppression technology is divided into passive filtering and active filtering in two ways.
Passive filtering technology
Passive filter circuit is simple, low cost, reliable working performance, is an effective way to suppress electromagnetic interference. Passive filter consists of inductive, capacitive and resistive components, and its direct role is to solve the conduction emission.
