Transmission channel for switching power supply EMI technology
The EMI interference sources of switching power supplies are concentrated in power switches, rectifier diodes, high-frequency transformers, etc. The external environment's interference to switching power supplies mainly comes from the jitter of the power grid, lightning strikes, external radiation, etc.
(1).Transmission channel of conductive interference
(1) Capacitive coupling
(2) Inductive coupling
(3) Resistive coupling
a. Resistive conduction coupling caused by the internal resistance of the public power supply
b. Resistive conduction coupling caused by the impedance of the common ground wire
c. Resistive conduction coupling caused by common line impedance
Switching power supply EMI technology suppression
(1) Reduce dv/dt and di/dt (reduce their peak value and slow down their slope)
(2) Reasonable application of varistor to reduce surge voltage
(3) Damping network suppresses overshoot
(4) Use diodes with soft recovery characteristics to reduce high-frequency EMI
(5) Active power factor correction and other harmonic correction technologies
(6) Use a properly designed power line filter
(7) Reasonable grounding treatment
(8) Effective shielding measures
(9) Reasonable PCB design
Switching power supply EMI technology interference sources
(1) Power switch tube
The power switch tube works in a state of On-Off rapid cycle conversion, and both dv/dt and di/dt are changing rapidly. Therefore, the power switch tube is not only the main interference source of electric field coupling, but also the main interference source of magnetic field coupling.
(2)
The EMI source of high-frequency transformers is concentrated in the rapid cyclic transformation of di/dt corresponding to leakage inductance. Therefore, high-frequency transformers are an important interference source of magnetic field coupling.
(3) Rectifier diode
The EMI source of the rectifier diode is concentrated in the reverse recovery characteristics. The discontinuous point of the reverse recovery current will produce high dv/dt in the inductance (lead inductance, stray inductance, etc.), resulting in strong electromagnetic interference.
(4) PCB
To be precise, pCB is the coupling channel of the above-mentioned interference sources. The quality of pCB directly corresponds to the quality of the suppression of the above-mentioned EMI sources.
Control of leakage inductance of high frequency transformer
The leakage inductance of high-frequency transformers is one of the important reasons for the turn-off peak voltage of power switches. Therefore, controlling leakage inductance has become the primary problem to solve the EMI caused by high-frequency transformers.
