Introduction to Switching Power Supply EMI Technology Solutions

1. EMI sources of switching power supplies

The EMI interference sources of switching power supplies are mainly reflected in power switch tubes, rectifier diodes, high-frequency transformers, etc. The interference of the external environment on switching power supplies mainly comes from grid jitter, lightning strikes, external radiation, etc.

(1) Power switch tube

The power switch tube works in the 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) High-frequency transformer
The EMI source of high-frequency transformer is mainly reflected in the rapid cycle conversion of di/dt corresponding to leakage inductance. Therefore, high-frequency transformer is an important interference source of magnetic field coupling.

(3) Rectifier diode
The EMI source of rectifier diode is mainly reflected in the reverse recovery characteristics. The discontinuity point of reverse recovery current will generate high dv/dt in the inductor (lead inductance, stray inductance, etc.), resulting in strong electromagnetic interference.

(4) PCB
To be precise, PCB is the coupling channel of the above interference sources. The quality of PCB directly corresponds to the quality of suppression of the above EMI sources.

2. Classification of EMI transmission channels of switching power supplies

(I). Transmission channels of conducted interference
(1) Capacitive coupling
(2) Inductive coupling
(3) Resistive coupling
a. Resistive conduction coupling generated by the internal resistance of the common power supply
b. Resistive conduction coupling generated by the impedance of the common ground line
c. Resistive conduction coupling generated by the impedance of the common line

(II). Transmission channels of radiated interference (1) In a switching power supply, the components and wires that can constitute the source of radiated interference can be assumed to be antennas, and thus analyzed using the electric dipole and magnetic dipole theory; diodes, capacitors, and power switching tubes can be assumed to be electric dipoles, and inductors can be assumed to be magnetic dipoles; (2) When there is no shielding body, the electromagnetic wave transmission channel generated by the electric dipole and magnetic dipole is air (which can be assumed to be free space); (3) When there is a shielding body, the gaps and holes in the shielding body are considered and analyzed according to the mathematical model of the leakage field.

3. 9 measures to suppress EMI in switching power supplies

In switching power supplies, sudden changes in voltage and current, i.e. high dv/dt and di/dt, are the main causes of EMI. The technical measures to achieve EMC design of switching power supplies are mainly based on the following two points:
(1) Minimize the interference source generated by the power supply itself, use interference suppression methods or components and circuits that generate less interference, and make reasonable layout;
(2) Suppress the EMI of the power supply and improve the EMS of the power supply through grounding, filtering, shielding and other technologies.

Separately, the 9 measures are: (1) Reduce dv/dt and di/dt (reduce its peak value and slow down its slope) (2) Reasonable application of varistors to reduce surge voltage (3) Damping network to suppress 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 reasonably designed power line filters (7) Reasonable grounding treatment (8) Effective shielding measures (9) Reasonable PCB design

4. Control of leakage inductance

of high-frequency transformer The leakage inductance of high-frequency transformer is one of the important reasons for the generation of peak voltage when the power switch is turned off. Therefore, controlling leakage inductance becomes the primary problem to solve the EMI caused by high-frequency transformer.

There are two entry points to reduce the leakage inductance of high-frequency transformer: electrical design and process design!

(1) Select a suitable magnetic core to reduce leakage inductance. The leakage inductance is proportional to the square of the number of turns on the primary side. Reducing the number of turns will significantly reduce the leakage inductance.
(2) Reduce the insulation layer between windings. There is now an insulation layer called "gold film" with a thickness of 20 to 100 um and a pulse breakdown voltage of several thousand volts.
(3) Increase the coupling between windings to reduce leakage inductance.

5. Shielding of high-frequency transformers

To prevent the leakage magnetic field of high-frequency transformers from interfering with surrounding circuits, shielding tape can be used to shield the leakage magnetic field of high-frequency transformers. The shielding tape is generally made of copper foil, wrapped around the outside of the transformer and grounded. The shielding tape is a short-circuit loop relative to the leakage magnetic field, thereby suppressing the leakage of the leakage magnetic field over a larger range.

In high-frequency transformers, relative displacement will occur between the magnetic cores and between the windings, causing the high-frequency transformer to generate noise (howling, vibration) during operation. To prevent this noise, reinforcement measures need to be taken for the transformer:

(1) Use epoxy resin to bond the three contact surfaces of the magnetic core (such as EE, EI magnetic core) to suppress the generation of relative displacement;
(2) Use "glass beads" adhesive to bond the magnetic core for better results.