Analysis of Electromagnetic Compatibility Design Methods for Switching Power Supplies
Switching power supplies are widely used in communication, control, computer and other fields due to their advantages such as small size and high power factor. However, due to the generation of electromagnetic interference, its further application is limited to some extent. This article will analyze the various mechanisms of electromagnetic interference in switching power supplies, and based on this, propose an electromagnetic compatibility design method for switching power supplies.
Analysis of Electromagnetic Interference in Switching Power Supplies
The structure of the switching power supply is shown in Figure 1. Firstly, the power frequency AC is rectified into DC, then inverted into high-frequency, and finally output through a rectification and filtering circuit to obtain a stable DC voltage. Unreasonable circuit design and layout, mechanical vibration, poor grounding, etc. can all form internal electromagnetic interference. At the same time, the leakage inductance of the transformer and the peak caused by the reverse recovery current of the output diode are also potential sources of strong interference.
Internal interference source
● Switching circuit
The switching circuit mainly consists of switching tubes and high-frequency transformers. There is a distributed capacitance between the switch tube and its heat sink, the shell, and the leads inside the power supply. The du/dt generated by it has a large amplitude of pulses, a wide frequency band, and rich harmonics. The switch tube load is the primary coil of a high-frequency transformer and is an inductive load. When the originally conducting switch is turned off, the leakage inductance of the high-frequency transformer generates a back electromotive force E=- Ldi/dt, which is proportional to the current change rate of the collector and the leakage inductance. It is superimposed on the turning off voltage to form a turning off voltage peak, thereby forming conductive interference.
● Rectifier diodes for rectifier circuits
When the output rectifier diode is cut off, there is a reverse current, and the time it takes to recover to zero is related to factors such as junction capacitance. It will generate significant current changes di/dt under the influence of transformer leakage inductance and other distribution parameters, resulting in strong high-frequency interference, with a frequency of up to tens of megahertz.
● Stray parameters
Due to working at higher frequencies, the characteristics of low-frequency components in switching power supplies may change, resulting in noise. At high frequencies, stray parameters have a significant impact on the characteristics of the coupling channel, and distributed capacitance becomes the channel for electromagnetic interference.
2 External interference sources
External interference sources can be divided into power interference and lightning interference, while power interference exists in both "common mode" and "differential mode" modes. At the same time, due to the direct connection of the AC power grid to the rectifier bridge and filter circuit, only the peak time of the input voltage has input current within half a cycle, resulting in a very low input power factor (approximately 0.6) of the power supply. Moreover, this current contains a large amount of harmonic components, which can cause harmonic "pollution" to the power grid.
