This article will introduce in detail five experiences in open source power supply design, namely: EMI source of switching power supply, classification of switching power source EMI transmission channel, 9 measures of switching power supply E?MI suppression, high frequency transformer, leakage inductance control , shielding of high frequency transformers.
1. EMI source of switching power supplyThe EMI interference source of the switching power supply is 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, lightning strike and external radiation of the power grid.
(1) Power switch tube
The power switch tube operates in the On-Off fast cycle transition state, and both dv/dt and di/dt are sharply changed. Therefore, the power switch tube is both the main interference source for electric field coupling and the main interference source for magnetic field coupling.
(2) High frequency transformer
The EMI source of the high-frequency transformer is concentrated in the di/dt fast cycle conversion corresponding to the leakage inductance, so the high-frequency transformer is an important interference source for the magnetic field coupling.
(3) Rectifier diode
The EMI source of the rectifier diode is concentrated in the reverse recovery characteristic. The discontinuous point of the reverse recovery current will generate high dv/dt in the inductor (lead inductance, stray inductance, etc.), resulting in strong electromagnetic interference.
(4) PCB
Accurately speaking, the PCB is the coupling channel of the above interference source, and the advantages and disadvantages of the PCB directly correspond to the suppression of the above EMI source.
2. Switching power supply EMI transmission channel classification(1) Transmission channel for conducted interference
(1) Capacitive coupling
(2) Inductive coupling
(3) Resistance coupling
a. Conductive conduction coupling generated by the internal resistance of the common power source
b. Resistance conduction coupling generated by common ground impedance
c. Resistance conduction coupling generated by common line impedance
(two). Radiated interference transmission channel
(1) In a switching power supply, components and wires that can constitute a source of radiated interference can be assumed to be antennas, and thus analyzed by electric dipole and magnetic dipole theory; diodes, capacitors, and power switches can be assumed to be Electric dipole, the inductor coil can be assumed to be a magnetic dipole;
(2) When there is no shield, the electric dipole and magnetic dipole are generated by the electromagnetic wave transmission channel (which can be assumed to be free space);
(3) When there is a shield, consider the gaps and holes of the shield, and analyze and process according to the mathematical model of the leak field.
3. 9 measures for EMI suppression of switching power supplyIn switching power supplies, sudden changes in voltage and current, ie high dv/dt and di/dt, are the main causes of EMI. The EMC design technical measures for implementing switching power supply are mainly based on the following two points:
(1) Minimize the interference source 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 techniques such as grounding, filtering, and shielding.
Separately speaking, the nine major measures are:
(1) Decrease dv/dt and di/dt (reduce its peak value and slow its slope)
(2) Reasonable application of varistor to reduce surge voltage
(3) Damping network suppresses overshoot
(4) Diodes with soft recovery characteristics to reduce high frequency EMI
(5) Active power factor correction, and other harmonic correction techniques
(6) Adopt 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 transformerThe leakage inductance of the high-frequency transformer is one of the important reasons for the turn-off spike voltage of the power switch. Therefore, controlling the leakage inductance has become the primary problem to solve the EMI caused by the high-frequency transformer.
Reduce the leakage inductance of high-frequency transformers two entry points: electrical design, process design!
(1) Select a suitable core to reduce leakage inductance. The leakage inductance is proportional to the square of the primary edge, and reducing the number of turns significantly reduces the leakage inductance.
(2) Reduce the insulation between the windings. There is now an insulating layer called "golden film" with a thickness of 20 to 100 um and a pulse breakdown voltage of several thousand volts.
(3) Increase the coupling degree between windings to reduce leakage inductance.
5. Shielding of high frequency transformerIn order to prevent the leakage of the high-frequency transformer from interfering with the surrounding circuit, the shielding can be used to shield the leakage magnetic field of the high-frequency transformer. The shielding tape is generally made of copper foil, wound around the outside of the transformer for one week, and grounded. The shielding tape is a short-circuit ring with respect to the leakage magnetic field, thereby suppressing a wider leakage of the leakage magnetic field.
High-frequency transformers, relative displacement between the cores and between the windings, resulting in high-frequency transformers generating noise (howling, vibration) during operation. To prevent this noise, it is necessary to take reinforcement measures for the transformer:
(1) Bonding three contact faces of a magnetic core (for example, EE, EI core) with an epoxy resin to suppress the generation of relative displacement;
(2) It is better to bond the core with "Glass beads" glue.
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