Electromagnetic compatibility design of switching power supply
Electromagnetic compatibility (EMC) of switching power supply refers to the coexistence of various electrical devices in a limited space, time and spectrum range without causing performance degradation, which includes electromagnetic disturbance (EMD) and electromagnetic sensitivity (EMS). EMD refers to the noise emitted by electrical products, while EMS refers to the ability of electrical products to resist external electromagnetic disturbance. A device with good electromagnetic compatibility should neither be affected by the surrounding electromagnetic environment nor cause electromagnetic disturbance.
The power switch tube in the switching power supply has a large voltage and current jump during the on-off process at high frequency, thus generating strong electromagnetic disturbance, but the frequency range of the disturbance (< 30MHz) is relatively low. The geometric size of most low-power switching power supplies is much smaller than the corresponding wavelength of 30MHz electromagnetic field (about 10m in air medium), and the electromagnetic disturbance phenomenon studied by switching power supply systems belongs to the range of quasi-stationary field. When studying their electromagnetic disturbance problems, the main consideration is conduction disturbance.
Electromagnetic disturbance
The discussion of electromagnetic disturbance is generally carried out from three aspects: the characteristics of disturbance source, the characteristics of disturbance coupling channel and the characteristics of disturbed object.
1 switching power supply in the main electromagnetic interference source
Electromagnetic interference source in switching power supply mainly includes switching devices, diodes and nonlinear passive components; In switching power supply, improper wiring of printed boards is also a major factor causing electromagnetic disturbance.
Electromagnetic disturbance caused by switching circuit
For switching power supply, the electromagnetic disturbance generated by switching circuit is one of the main disturbance sources of switching power supply. Switching circuit is the core of switching power supply, which is mainly composed of switching tube and high frequency transformer. The dv/dt generated by it is a pulse with large amplitude, wide frequency band and rich harmonics. The main reasons for this impulse disturbance are
1) The switch tube load is the primary coil of high-frequency transformer, which is an inductive load. At the moment when the switch tube is turned on, the primary coil produces a great inrush current, and a high surge peak voltage appears at both ends of the primary coil. At the moment when the switch tube is turned off, a part of energy is not transmitted from the primary coil to the secondary coil due to the leakage flux of the primary coil, and this part of energy stored in the inductor will form an attenuation oscillation with spikes with the capacitors and resistors in the collector circuit, and will be superimposed on the turn-off voltage to form a turn-off voltage spike. This interruption of power supply voltage will produce the same magnetization impulse current transient as when the primary coil is turned on, and this noise will be conducted to the input and output terminals, resulting in conduction disturbance, and in the worst case, it may breakdown the switch tube.
2) The high-frequency switching current loop composed of pulse transformer primary coil, switching tube and filter capacitor may produce large space radiation and form radiation disturbance. If the filtering capacity of the capacitor is insufficient or the high-frequency characteristics are not good, the high-frequency impedance on the capacitor will lead the high-frequency current to the AC power supply in a differential mode to form conduction disturbance.
Electromagnetic disturbance caused by diode rectifier circuit
The |di/dt| of the reverse recovery current generated by the rectifier diode in the main circuit is much smaller than that of the flywheel diode. As a electromagnetic interference source, the disturbance intensity caused by the reverse recovery current of rectifier diode is large and the frequency band is wide. The voltage jump generated by the rectifier diode is much smaller than that generated when the power switch tube in the power supply is turned on and off. Therefore, it is possible to study the rectifier circuit as a part of the electromagnetic interference coupling channel, regardless of the influence of |dv/dt| and |di/dt| produced by rectifier diodes.
Relationship between dv/dt and load size
Dv/dt generated when power switch tube is turned on and off is the main disturbance source of switching power supply. Theoretical analysis and experiments show that the value of |dv/dt| generated by turn-off increases with the increase of load, but the change of load has little effect on |dv/dt| turned on. Because the |dv/dt| generated when turning on and off is different, the disturbance pulse generated externally is also different.
