Sharing experience on switching power supply EMI design

The EMI interference sources of the switching power supply are concentrated in the power switch tube, rectifier diode, high-frequency transformer, etc. The interference of the external environment to the switching power supply mainly comes from the jitter of the power grid, lightning strikes, external radiation, etc.

1. EMI sources of switching power supplies

The EMI interference sources of the switching power supply are concentrated in the power switch tube, rectifier diode, high-frequency transformer, etc. The interference of the external environment to the switching power supply mainly comes from the jitter of the power grid, lightning strikes, external radiation, etc.

(1) Power switch tube

The power switch tube works in a state of rapid On-Off cycle conversion, and both dv/dt and di/dt change rapidly. Therefore, the power switch tube is the main interference source of both electric field coupling and magnetic field coupling.

(2) High frequency transformer

The EMI source of high-frequency transformers is mainly reflected in the rapid cycle change 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 discontinuity point of the reverse recovery current will produce 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-mentioned interference sources. The quality of PCB directly corresponds to the quality of suppression of the above-mentioned EMI sources.

2. Classification of switching power supply EMI transmission channels

1. Transmission channel of conducted interference

(1) Capacitive coupling

(2) Inductive coupling

(3) Resistive coupling

a. Resistive conduction coupling caused by the internal resistance of the common power supply

b. Resistive conduction coupling caused by common ground impedance

c. Resistive conduction coupling caused by common line impedance

2. Transmission channel of radiated interference

(1) In a switching power supply, components and wires that can constitute a radiation interference source can be assumed to be antennas, and thus analyzed using the electric dipole and magnetic dipole theory; diodes, capacitors, and power switches can be assumed to be electric dipoles, and inductors can be assumed to be magnetic dipoles;

(2) When there is no shield, the electromagnetic wave transmission channel generated by electric dipoles and magnetic dipoles is air (which can be assumed to be free space);

(3) When there is a shield, the gaps and holes in the shield are considered and analyzed according to the mathematical model of the leakage field.
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3. 9 measures to suppress EMI in switching power supply

In switching power supplies, sudden changes in voltage and current, i.e. high dv/dt and di/dt, are the main causes of EMI. Technical measures for implementing 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 arrange them reasonably;

(2) Suppress the EMI of the power supply and improve the EMS of the power supply through grounding, filtering, shielding and other technologies.

Separately speaking, the nine measures are:

(1) Reduce dv/dt and di/dt (reduce their peak values ​​and slow down their slopes)

(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

4. Control of leakage inductance of high-frequency transformer

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

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

(1) Choose a suitable magnetic core to reduce leakage inductance. Leakage inductance is proportional to the square of the number of turns on the primary side. Reducing the number of turns will significantly reduce 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 and reduce leakage inductance.

5. Shielding of high frequency transformer

In order to prevent the leakage magnetic field of the high-frequency transformer from interfering with the surrounding circuits, a shielding tape can be used to shield the leakage magnetic field of the high-frequency transformer. 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 ring relative to the leakage magnetic field, thereby suppressing the leakage of the leakage magnetic field in a wider range.

In high-frequency transformers, relative displacement occurs between the magnetic cores and windings, which causes noise (howling, vibration) to be generated 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 core (such as EE and EI cores) to suppress the occurrence of relative displacement.

(2) Use "glass beads" adhesive to bond the magnetic core for better results.