Switching power supply EMI interference source is embodied in the power switch, rectifier diodes, high-frequency transformers, etc., the external environment on the switching power supply interference mainly from the grid jitter, lightning, external radiation.
(1) power switch tube
Power switch is working in the On-Off fast cycle of the state, dv / dt and di / dt are in the rapid transformation, therefore, the power switch is the main interference source of the electric field coupling, but also the main source of interference with the magnetic field.
(2) high-frequency transformer
The EMI source of the high-frequency transformer is embodied in the di / dt fast cyclic transformation corresponding to the leakage inductance, so the high-frequency transformer is an important source of interference for the magnetic field coupling.
(3) rectifier diodes
Rectifier diode EMI source is reflected in the reverse recovery characteristics, the reverse recovery current intermittent point in the inductor (lead inductance, stray inductance, etc.) produce high dv / dt, resulting in strong electromagnetic interference.
Specifically, PCB is the interference source of the coupling channel, the pros and cons of PCB, directly corresponding to the EMI source of the above is good or bad.
2, switching power supply EMI transmission channel classification
(A) conduction interference transmission channel
(1) capacitive coupling
(2) Inductive coupling
(3) resistance coupling
A. Resistance transmission caused by internal power supply resistance
B. The resistance of the common ground impedance produces the coupling
C. Resistance of the common line impedance generated by the coupling
(B) radiation interference transmission channel
(1) in the switching power supply, can constitute a radiation interference source components and wires can be assumed to be the antenna, so the use of electric dipole and magnetic dipole theory for analysis; diodes, capacitors, power switch can be assumed to be The electric dipole, the inductance coil can be assumed to be a magnetic dipole;
(2) there is no shield, the electric dipole, magnetic dipole, the resulting electromagnetic wave transmission channel for the air (which can be assumed to be free space);
(3) with a shield, consider the cracks and holes in the shield, according to the leakage field of the mathematical model for analysis and processing.
4, high-frequency transformer leakage inductance control
High-frequency transformer leakage inductance is the power switch off the spike voltage is one of the important reasons, therefore, control leakage inductance to solve the high-frequency transformer to bring EMI first to face the problem.
Reduce the leakage of high-frequency transformer two points: electrical design, process design!
(1) select the appropriate core, reduce the leakage inductance. The leakage inductance is proportional to the square of the original turns, and reducing the number of turns will significantly reduce the leakage inductance.
(2) reduce the insulation between the windings. Now there is a known as the "gold film" insulation layer, the thickness of 20 ~ 100um, pulse breakdown voltage up to several thousand volts.
(3) to increase the coupling between the winding, reducing leakage inductance.
5, high-frequency transformer shielding
In order to prevent the leakage of high-frequency transformer on the surrounding circuit interference, shielding can be used to shield the high-frequency transformer leakage magnetic field. The shield tape is typically made of copper foil and wound around the transformer for one week and grounded. The shield tape is a short circuit ring relative to the leakage field, thereby suppressing a greater leakage of the leakage magnetic field.