Analysis of actual EMI rectification strategies for switching power supplies
EMC classification and standards:
EMC (Electromagnetic Compatibility) is electromagnetic compatibility, which includes EMI (electromagnetic harassment) and EMS (electromagnetic anti-harassment). EMC is defined as: the ability of a device or system to operate normally in its electromagnetic environment without causing unacceptable electromagnetic disturbance to anything in the environment. EMC is called electromagnetic compatibility. EMP refers to electromagnetic pulse.
EMC = EMI + EMS EMI: Electromagnetic interference EMS: Electromagnetic compatibility (immunity)
EMI can be divided into two parts: conduction and radiation.
Conduction specifications can generally be divided into: FCC Part 15J Class B; CISPR 22 (EN55022, EN61000-3-2, EN61000-3-3) Class B;
National standard IT category (GB9254, GB17625) and AV category (GB13837, GB17625).
The FCC test frequency is 450K-30MHz, the CISPR 22 test frequency is 150K--30MHz, Conduction can be tested with a spectrum analyzer, and Radiation must be tested in a specialized laboratory.
EMI is electromagnetic interference. EMI is a part of EMC. EMI (Electronic Magnetic Interference) electromagnetic interference. EMI includes conduction, radiation, current harmonics, voltage flicker, etc. Electromagnetic interference is composed of three parts: interference source, coupling channel and receiver, which are usually called the three elements of interference. EMI is linearly proportional to the current, current loop area and the square of the frequency: EMI = K*I*S*F2. I is the current, S is the loop area, F is the frequency, and K is a constant related to the circuit board material and other factors.
Radiated interference (30MHz-1GHz) is propagated through space and based on the characteristics and laws of electromagnetic waves. But not any device can radiate electromagnetic waves.
Conducted interference (150K--30MHz) is interference that propagates along conductors. Therefore, the propagation of conducted interference requires a complete circuit connection between the interference source and the receiver.
EMI refers to the external electromagnetic interference of the product. Generally, it is divided into two levels: Class A & Class B. Class A is the industrial grade and Class B is the civilian grade. Those for civilian use are stricter than those for industrial use, because the allowable radiation for industrial use is slightly larger. Regarding the radiation test of the same product in EMI testing, at 30-230MHz, Class B requires that the radiation limit of the product cannot exceed 40dBm, while Class A requires that the radiation limit cannot exceed 50dBm (taking the three-meter anechoic chamber measurement as an example), which is relatively loose. Generally speaking, CLASSA means that under EMI test conditions, without operator intervention, the equipment can continue to work normally as expected, and no performance degradation or functional loss below the specified performance level is allowed.
EMI measures the radiation and conduction of the equipment when it is working normally. During testing, there are two upper limits for EMI radiation and conduction on the receiver, which represent Class A and Class B respectively. If the observed waveform exceeds the B line but is lower than the A line, then the product is Class A. EMS uses test equipment to interfere with the product and observe whether the product can work normally under the interference. If it works normally or does not experience performance degradation exceeding the standards, it is rated A. It can automatically restart and there will be no performance degradation exceeding the standard requirements after restarting, which is Class B. If it cannot restart automatically and needs to be manually restarted, it is classified as level C. If it fails, it is classified as level D. The national standard has D-level regulations, and EN only has A, B, and C. EMI is most difficult to deal with at odd multiples of the operating frequency.
