How to EMI design for switching power supplies
EMC classification and standards:
EMC (Electromagnetic CompaTIbility) is Electromagnetic Compatibility, which includes EMI (Electromagnetic Molestation) and EMS (Electromagnetic Molestation Susceptibility).EMC is defined as the ability of a device or a system to function properly in its electromagnetic environment without causing intolerable electromagnetic disturbances to anything of any equipment in that environment.EMC is referred to in its entirety as EMP is Electromagnetic Pulse.
EMC = EMI + EMS EMI: Electromagnetic Interference EMS: Electromagnetic Compatibility (Immunity)
EMI can be divided into Conducted ConducTIon and Radiated RadiaTIon.
ConducTIon specification can be generally divided into: FCC Part 15J Class B; CISPR 22 (EN55022, EN61000-3-2, EN61000-3-3) Class B;
National standard IT class (GB9254, GB17625) and AV class (GB13837, GB17625).
FCC test frequency in 450K-30MHz, CISPR 22 test frequency in 150K-30MHz, Conduction can be tested by spectrum analyser, Radiation must be tested in special laboratory.
EMI is Electromagnetic Interference, EMI is a part of EMC, EMI (Electronic Magnetic Interference) Electromagnetic Interference, EMI including conduction, radiation, current harmonics, voltage flicker and so on. Electromagnetic interference is composed of three parts: the interference source, the coupled channel and the receiver, usually called the three elements of interference. EMI is linearly proportional to the current, the current loop area, and the square of the frequency, i.e.: 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 board material and other factors.
Radiated interference (30MHz-1GHz) travels through space and propagates with the characteristics and laws of electromagnetic waves. However, not any device can radiate electromagnetic waves.
Conducted interference (150K - 30MHz) is interference that propagates along a conductor. Therefore, the propagation of conducted interference requires a complete circuit connection between the interference source and the receiver.
EMI is the external electromagnetic interference of a product. Generally, it is classified as Class A & Class B. Class A is the industrial class, Class B is the industrial class, and Class B is the industrial class. Class A is the industrial class and Class B is the civil class. Class A is for industrial use and Class B is for residential use. Residential use is more stringent than industrial use because industrial use allows for a slightly higher level of radiation. The same product in the test EMI radiation test, in 30-230MHz, Class B requires that the radiation limit of the product can not exceed 40dBm and Class A requires that the radiation limit can not exceed 50dBm (three-metre method of measurement in the darkroom as an example) is relatively more lenient, generally speaking, CLASSA refers to the EMI test conditions without the need for the operator to intervene, the equipment can continue to work as expected normal, and does not allow the performance below the specified level. In general, CLASSA means that under EMI test conditions, without operator intervention, the equipment will continue to function as expected, with no degradation or loss of functionality below the specified performance level.
EMI is the radiation and conduction of the equipment measured during normal operation. In the test, EMI radiation and conduction in the receiver has two upper limits, respectively, on behalf of Class A and Class B, if the observed waveform exceeds the line of B but lower than the line of A, then the product is a class A. EMS is to use the test equipment to interfere with the product, to observe the product in the interference under the normal operation, if the normal operation or does not appear to exceed the performance degradation of the standard provisions, for the A class. EMS is to use test equipment to interfere with the product to observe whether the product can work normally under interference. If the product works normally or does not show performance degradation exceeding the standard, it is class A. If the product can restart automatically and does not show performance degradation exceeding the standard after restarting, it is class B. If it cannot restart automatically and needs to be restarted by human, it is level C. If it hangs up, it is level D. The national standard has the provision of level D, EN only A, B, C. EMI is the worst at odd multiples of the operating frequency.
