How to Solve the Switching Power Supply Overradiation Problem
EMI is the measurement of the radiation and conduction of a device during normal operation. During testing, there are two upper limits for EMI radiation and conduction at the receiver, representing Class A and Class B. If the observed waveform exceeds the line of B but falls below the line of A, then the product is classified as Class A. EMS uses testing equipment to interfere with products and observe whether they can work normally under interference. If they work normally or do not experience performance degradation beyond the standard requirements, it is classified as Class A. It can automatically restart without any performance degradation beyond the standard requirements, and is classified as B-level. Unable to automatically restart, manual restart is considered C-level, and failure is considered D-level. The national standard has D-level regulations, while EN only has A, B, and C. EMI is the most difficult at odd multiples of the operating frequency.
EMS (Electromagnetic Suseepibilkr) electromagnetic sensitivity is commonly known as "electromagnetic immunity", which refers to the ability of equipment to resist external disturbances and interference, while EMI refers to the external disturbances of equipment.
The level in EMS refers to Class A, where the equipment is still working normally after testing is completed; Class B, Can work normally after testing is completed or needs to be restarted during testing; Class C, Can restart and work normally after manual adjustment; Class D, The device is damaged and cannot be started no matter how it is adjusted. The strictness level of EMI is B>A, and EMS is A>B>C>D.
Turn off the power input
EMI circuit:
The function of X capacitor:
Suppressing differential mode noise, the larger the capacitance, the better the effect of suppressing low-frequency noise.
The function of Y capacitor:
Suppressing common mode noise, the larger the capacitance, the better the effect of suppressing low-frequency noise. The Y capacitor provides a low impedance loop from the secondary to the primary ground, causing the current flowing to ground and then returning through LISN to short-circuit directly. Due to the incomplete ideal of the Y capacitor, there is also impedance between the various parts of the secondary, so it is impossible for all of them to return. Some of it still flows to the ground. The Y capacitor must be directly connected to the cold ground of the primary and secondary using the shortest possible straight line. If the dv/dt of the MOS when turned on is greater than the dv/dt when turned off, then the Y capacitor is connected to the ground of the primary; Conversely, connect to V+.
The function of common mode inductance:
Suppressing common mode noise, the larger the inductance, the better the effect of suppressing low-frequency noise. Increase the impedance of the common mode current section and reduce the common mode current.
The function of differential inductance:
Suppressing differential mode noise, the larger the inductance, the better the effect of suppressing low-frequency noise.
General EMI response strategies before designing switch mode power supplies
Adopting AC input EMI filter
