Solution of EMI interference to switching power supply
Generally speaking, the interference from external radiation, lightning strike, or power grid jitter, etc. to the external environment of related components of power switch, such as rectifier diode, high-frequency transformer, power switch tube, etc., is the main manifestation of EMI interference sources of switching power supply.
Firstly, the transmission channel of radiation interference is introduced.
(1) In the switching power supply, the components and wires that can constitute radiation interference sources can be assumed as antennas, so that the analysis can be made by using the theory of electric dipole and magnetic dipole; Diodes, capacitors, power switches can be assumed as electric dipoles, and inductance coils can be assumed as magnetic dipoles.
(2) When there is no shield, the electromagnetic wave transmission channel generated by electric dipole and magnetic dipole is air (which can be assumed as free space);
(3) When there is a shield, considering the gaps and holes of the shield, the leakage field is analyzed and treated according to the mathematical model.
Secondly, it is the transmission channel that conducts interference.
(1) capacitive coupling
(2) Inductive coupling
(3) Resistance coupling
A. resistance conduction coupling caused by internal resistance of public power supply
B. resistive conductive coupling caused by common ground impedance
C. resistive conduction coupling caused by common line impedance
The following suppression schemes are related to EMI interference sources:
1. Shielding of high frequency transformer
In order to prevent the magnetic leakage of high-frequency transformer from interfering with the surrounding circuits, shielding tape can be used to shield the leakage magnetic field of high-frequency transformer. The shielding tape is generally made of copper foil, which is wound around the outside of the transformer and grounded. The shielding tape is a short-circuit ring relative to the leakage magnetic field, thus suppressing the leakage of the leakage magnetic field in a wider range.
Relative displacement will occur between cores and windings of high-frequency transformer, which will lead to noise (howling and vibration) in the operation of high-frequency transformer. To prevent this noise, it is necessary to take reinforcement measures for the transformer:
(1) Bonding three contact surfaces of magnetic cores (such as EE and EI cores) with epoxy resin to suppress relative displacement;
(2) It is better to bond the magnetic core with "Glass beads" adhesive.
Separately, there are nine measures to suppress EMI in switching power supply:
(1) reasonable PCB design
(2) Reasonable application of varistor to reduce surge voltage.
(3) Reduce dv/dt and di/dt (reduce its peak value and slow down its slope)
(4) Damping network suppresses overshoot.
(5) Adopt a reasonably designed power line filter.
(6) Diodes with soft recovery characteristics are used to reduce high-frequency EMI.
(7) Active power factor correction and other harmonic correction technologies.
(8) Effective shielding measures
(9) Reasonable grounding treatment
