EMC design of switching supplies
Switching power supply Electromagnetic compatibility refers to the limited space, time and spectrum range, a variety of electrical equipment coexistence without causing performance degradation, which includes electromagnetic harassment (EMD) and electromagnetic sensitivity (EMS) two aspects of the content of the EMD refers to the electrical products to the outside noise, EMS refers to the electrical products to resist the ability of external electromagnetic harassment. A device with good EMC performance should be neither affected by the surrounding electromagnetic environment nor cause electromagnetic harassment to the surrounding.
Power switching tubes in switching power supplies produce large voltage and current jumps during the on/off process at high frequencies, thus generating powerful electromagnetic nuisance, but the frequency range of the nuisance (<30MHz) is relatively low. Most of the geometry of the low-power switching power supply is much smaller than 30MHz electromagnetic field corresponding to the wavelength (about 10m in the air medium), switching power supply system to study the electromagnetic harassment phenomenon belongs to the scope of the seemingly steady field, the study of their electromagnetic harassment, the main consideration is the conduction harassment.
2 electromagnetic harassment
Discussion of electromagnetic harassment is generally from the characteristics of the source of harassment, harassment of the coupling channel characteristics and the characteristics of the disturbed body in three aspects.
2.1 Switching power supply in the main electromagnetic nuisance sources
Switching power supply in the electromagnetic harassment sources are mainly switching devices, diodes and nonlinear passive components; in the switching power supply, printed circuit board wiring is also a major factor causing electromagnetic harassment.
2.1.1 Electromagnetic nuisance generated by the switching circuit
For switching power supply, the electromagnetic harassment generated by the switching circuit is one of the main sources of harassment of switching power supply. 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 the switching circuit is a pulse with large irradiance, wide bandwidth and rich in harmonics. The main reasons for this pulse harassment are
1) The switching tube load is the primary coil of the high-frequency transformer, which is an inductive load. In the switching tube conduction moment, the primary coil produces a large inrush current, and in the primary coil at both ends of the high surge spike voltage; in the switching tube disconnection moment, due to the leakage flux of the primary coil, resulting in a portion 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 in the formation of capacitance, resistance with a spike of the attenuation of the oscillator, superimposed on the shutdown voltage 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 with the primary coil on, this noise will be transmitted to the input and output terminals, the formation of conducted nuisance, the heaviest possible breakdown of the switching tube.
2) Pulse transformer primary coil, switching tube and filter capacitors constitute a high-frequency switching current loop may produce a large space radiation, the formation of radiation harassment. If the capacitor filter capacity is insufficient or the high frequency characteristics are not good, the capacitor on the high frequency impedance will make the high frequency current in the form of differential mode conduction to the AC power supply to form a conducted nuisance.
