Electromagnetic Compatibility Design for Switching Power Supplies
Electromagnetic compatibility of switch mode power supplies refers to the coexistence of various electrical devices within a limited space, time, and frequency spectrum without causing a decrease in performance. It includes two aspects: electromagnetic interference (EMD) and electromagnetic sensitivity (EMS). EMD refers to the external noise emitted by electrical products, while EMS refers to the ability of electrical products to resist external electromagnetic interference. A device with good electromagnetic compatibility performance should neither be affected by the surrounding electromagnetic environment nor cause electromagnetic interference to the surroundings.
The power switching transistor in a switching power supply generates significant voltage and current jumps during the on/off process at high frequencies, resulting in strong electromagnetic interference, but the frequency range of the interference (<30MHz) is relatively low. The geometric dimensions of most low-power switching power supplies are much smaller than the wavelength corresponding to a 30MHz electromagnetic field (about 10m in air medium). The electromagnetic interference phenomena studied in switching power supply systems belong to the range of quasi steady fields. When studying their electromagnetic interference problems, the main consideration is conducted interference.
2 Electromagnetic harassment
The discussion of electromagnetic interference is generally conducted from three aspects: the characteristics of the interference source, the coupling channel characteristics of the interference, and the characteristics of the disturbed body.
2.1 Main sources of electromagnetic interference in switch mode power supplies
The electromagnetic interference sources in switch mode power supplies mainly include switching devices, diodes, and nonlinear passive components; Improper wiring of printed circuit boards is also a major factor causing electromagnetic interference in switch mode power supplies.
2.1.1 Electromagnetic interference generated by switch circuits
For switching power supplies, electromagnetic interference generated by the switching circuit is one of the main sources of interference. The switch circuit is the core of a switching power supply, mainly composed of switch tubes and high-frequency transformers. The dv/dt generated by it is a pulse with a large amplitude, a wide frequency band, and abundant harmonics. The main reason for this pulse disturbance is
1) The load of the switch tube is the primary coil of the high-frequency transformer, which is an inductive load. At the moment when the switch tube is turned on, a large surge current is generated in the primary coil, and high surge peak voltage appears at both ends of the primary coil; At the moment when the switch tube is disconnected, due to the leakage flux of the primary coil, a portion of the energy is not transmitted from the primary coil to the secondary coil. The energy stored in the inductor will form a decay oscillation with a peak with the capacitance and resistance in the collector circuit, which will be superimposed on the turn off voltage to form a turn off voltage peak. This type of power supply voltage interruption will generate the same transient magnetizing impulse current as when the primary coil is connected. This noise will be conducted to the input and output terminals, forming conductive disturbances, and in severe cases, it may break down the switch tube.
2) The high-frequency switching current loop composed of the primary coil, switching tube, and filtering capacitor of the pulse transformer may generate significant spatial radiation, forming radiation disturbance. If the filtering capacity of the capacitor is insufficient or the high-frequency characteristics are poor, the high-frequency impedance on the capacitor will cause high-frequency current to conduct to the AC power supply in a differential mode, forming conduction disturbance.
