Switching power supply EMI design experience
1. EMI source of switching power supply
The EMI interference sources of the switching power supply are mainly reflected in the power switch tube, rectifier diode, high-frequency transformer, etc. The interference of the external environment on the switching power supply mainly comes from the jitter of the power grid, lightning strikes, and external radiation.
(1) Power switch tube
The power switch tube works in the state of On-Off rapid cycle switching, and the 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 the high-frequency transformer is concentrated in the di/dt rapid cycle transformation corresponding to the leakage inductance, so the high-frequency transformer is an important interference source of magnetic field coupling.
(3) Rectifier diode
The EMI source of the rectifier diode is mainly reflected in the reverse recovery characteristics. The discontinuous point of the reverse recovery current will generate high dv/dt in the inductance (lead inductance, stray inductance, etc.), resulting in strong electromagnetic interference.
(4) PCB
To be precise, the PCB is the coupling channel for the above-mentioned interference sources, and the quality of the PCB directly corresponds to the suppression of the above-mentioned EMI sources.
2. EMI transmission channel classification of switching power supply
(1) Transmission channel for conducted interference
(1) Capacitive coupling
(2) Inductive coupling
(3) Resistive coupling
a. Resistive conduction coupling generated by the internal resistance of the public power supply
b. Resistive conduction coupling generated by common ground impedance
c. Resistive conduction coupling generated by common line impedance
(2) Transmission channel of radiation interference
(1) In the switching power supply, the components and wires that can constitute the radiation interference source can be assumed as antennas, so that the electric dipole and magnetic dipole theory can be used for analysis; diodes, capacitors, and power switch tubes can be assumed as Electric dipoles, inductance coils can be assumed to be magnetic dipoles;
(2) When there is no shield, the transmission channel of electromagnetic waves generated by electric dipoles and magnetic dipoles is air (which can be assumed to be free space);
(3) When there is a shielding body, consider the gaps and holes of the shielding body, and analyze and process according to the mathematical model of the leakage field.
3. Nine major measures for EMI suppression of switching power supply
In a switching power supply, sudden changes in voltage and current, namely high dv/dt and di/dt, are the main causes of EMI. The EMC design technical measures to realize the switching power supply are mainly based on the following two points:
(1) Minimize the source of interference generated by the power supply itself, use the method of suppressing interference or generate components and circuits with less interference, and make a 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 speaking, the nine major measures are:
(1) Reduce dv/dt and di/dt (reduce its peak value, slow down its slope)
(2) Reasonable application of varistors to reduce surge voltage
(3) Damping network suppresses overshoot
(4) Diodes with soft recovery characteristics are used to reduce high-frequency EMI
(5) Active power factor correction, and other harmonic correction techniques
(6) Use a reasonably 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 turn-off peak voltage of the power switch tube. Therefore, controlling the leakage inductance becomes the primary problem to solve the EMI caused by the high-frequency transformer.
There are two entry points for reducing the leakage inductance of high-frequency transformers: electrical design and process design!
(1) Choose 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 the windings. Now there is an insulating layer called "gold film" with a thickness of 20-100um and a pulse breakdown voltage of several thousand volts.
(3) Increase the coupling between windings and reduce leakage inductance.
5. Shielding of high-frequency transformers
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 magnetic field leakage of the high-frequency transformer. The shielding tape is generally made of copper foil, wound around the outside of the transformer, and grounded. The shielding tape is a short-circuit ring relative to the leakage field, thereby suppressing the leakage of the leakage field in a larger range.
For high-frequency transformers, relative displacement will occur between the magnetic cores and between the windings, which will cause noise (howling, vibration) in the operation of the high-frequency transformer. To prevent this noise, the transformer needs to be hardened:
(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" (Glass beads) glue to bond the magnetic core, the effect is better.
