How to Design EMI for Switching Power Supplies
EMC's classification and standards:
EMC (Electromagnetic Compatibility) is electromagnetic compatibility, which includes EMI (Electromagnetic Disturbance) and EMS (Electromagnetic Immunity). EMC is defined as the ability of a device or system to function normally in its electromagnetic environment without causing unbearable electromagnetic interference to any device or thing in that environment. EMC is referred to as electromagnetic compatibility. EMP refers to electromagnetic pulses.
EMC=EMI+EMS EMI: Electromagnetic interference EMS: Electromagnetic compatibility (immunity)
EMI can be divided into two parts: Conduction and Radiation,
The Conduction specification can generally be divided into: FCC Part 15J Class B; CISPR 22 (EN55022, EN61000-3-2, EN61000-3-3) Class B;
GB IT (GB9254, GB17625) and AV (GB13837, GB17625).
The FCC testing frequency is between 450K-30MHz, and the CISPR 22 testing frequency is between 150K-30MHz. The Conduction can be tested using a spectrum analyzer, while the Radiation must be tested in a specialized laboratory.
EMI refers to electromagnetic interference, which is a part of EMC. Electronic Magnetic Interference (EMI) refers to electromagnetic interference, which includes conduction, radiation, current harmonics, voltage flicker, and so on. Electromagnetic interference is composed of three parts: interference source, coupling channel, and receiver, commonly referred to as the three elements of interference. EMI is linearly proportional to the current, the area of the current loop, 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 circuit board material and other factors.
Radiated interference (30MHz 1GHz) propagates through space and follows the characteristics and laws of electromagnetic waves. But not every device can radiate electromagnetic waves.
Conducted interference (150K-30MHz) is interference that propagates along a conductor. So the propagation of conducted interference requires a complete circuit connection between the interference source and receiver.
EMI refers to the external electromagnetic interference of a product. Generally, it is divided into two levels: Class A and Class B. Class A is industrial grade, and Class B is civilian grade. Civil use is stricter than industrial use, as industrial use allows for slightly higher radiation levels. In terms of radiation testing for the same product in EMI testing, at 30-230MHz, Class B requires that the radiation limit of the product should not exceed 40dBm, while Class A requires that the radiation limit should not exceed 50dBm (using the three meter anechoic chamber measurement as an example), which is relatively loose. Generally speaking, CLASSA refers to the ability of the equipment to continue working normally as expected under EMI testing conditions without the intervention of operators, No performance degradation or functional loss below the specified performance level is allowed.
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 on the receiver, representing Class A and Class B. If the observed waveform exceeds the line of B but falls below the line of A, the product is classified as Class A. EMS is the use of testing equipment to interfere with a product and observe whether it can function normally under interference. If it operates normally or does not experience performance degradation exceeding the standard, it is classified as Grade A. It can automatically restart without exceeding the performance degradation specified in the standard, and is classified as Class B. If automatic restart is not possible, manual restart is required to be level C, and if it is suspended, it is level D. The national standard has D-level regulations, and EN only has A, B, and C. EMI is the worst at odd times the operating frequency.
