General EMI response strategy before switching power supply design
Adopting AC input EMI filter
There are usually two ways to transmit interference current on wires: common mode mode and differential mode. Common mode interference is the interference between the carrier fluid and the earth: the interference magnitude and direction are consistent, and it exists between any phase of the power supply and the earth, or between the neutral line and the earth. It is mainly generated by du/dt, and di/dt also generates a certain amount of common mode interference. Differential mode interference is the interference between carrier fluids: the interference is equal in magnitude and opposite in direction, existing between the power supply phase line and the neutral line, as well as between the phase lines. Interference current can occur in both common mode and differential mode when transmitted on wires; But common mode interference current can only interfere with useful signals after becoming differential mode interference current.
There are two types of interference on the input line of AC power supply, usually low-frequency differential mode interference and high-frequency common mode interference. In general, differential mode interference has a small amplitude, low frequency, and causes minimal interference; Common mode interference has a large amplitude and high frequency, and can also generate radiation through wires, causing significant interference. If an appropriate EMI filter is used at the input end of the AC power supply, electromagnetic interference can be effectively suppressed. The basic principle of the power line EMI filter is shown in Figure 1, where the differential mode capacitors C1 and C2 are used to short-circuit the differential mode interference current, while the intermediate grounding capacitors C3 and C4 are used to short-circuit the common mode interference current. A common mode choke is composed of two equally thick coils wound in the same direction around a magnetic core. If the magnetic coupling between two coils is very tight, the leakage inductance will be very small, and the differential mode reactance will become very small within the frequency range of the power line; When the load current flows through the common mode choke, the magnetic field lines generated by the coils connected in series on the phase line and the magnetic field lines generated by the coils connected in series on the middle line are in opposite directions, and they cancel each other out in the magnetic core. Therefore, even under high load currents, the magnetic core will not saturate. For common mode interference current, the magnetic fields generated by the two coils are in the same direction, resulting in a larger inductance and thus attenuating the common mode interference signal. The common mode choke coil here should use ferrite magnetic materials with high magnetic permeability and excellent frequency characteristics.
