1. EMI source of switching power supply
The 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) The EMI source of the high-frequency transformer 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 of the magnetic field coupling.
(3) The EMI source of the rectifier diode 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 inductor, 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) Conduction interference transmission channel (1) capacitive coupling (2) inductive coupling (3) resistance coupling a. resistance conduction coupling generated by common source internal resistance b. resistance conduction coupling generated by common ground impedance c. Resistance conduction coupling
(ii). Transmission channel for radiated interference
(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 supply
In 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 to suppress overshoot (4) Diode with soft recovery characteristics to reduce high frequency EMI
(5) Active power factor correction, and other harmonic correction techniques (6) Reasonable design of 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 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 transformer
In 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.
Product Features:
1. Our SMD LED use 99.99% gold thread, pure copper bracket coated with silver, long life span and low Luminous Decay.
2. Our flexible PCB are all high quality double layers PCB, at least 2 ounces, or 3 ounces in particular models.
3. We always require same bin number SMD LEDs for different production batches for the same customer.
4. We have Everfine integrating sphere to test the color temperature of incoming SMD LEDs and put it in our QC records for every batch.
5. CRI>80
6. 3M 300LSE tape attached.
Warranty: 3 years.
Color Temperature: 2300k, 2700k, 3000k, 4000k, 6500k.
Application: City lighting, landscape lighting, commercial lighting and house contour lighting, etc.
About us:
LBY is a professional manufacturer specialized in developing, manufacturing and marketing of LED light for worldwide sign industry. Since 2004, we have been devoted to designing and producing more efficient and easily-installed LED products. At the same time, we also offer customized design, OEM and work instruction service. Our general target is saving total cost for customers.
The company is improved all the time by working together with many industrial leading partners, today, LBY, the name means reliable quality, good consistency, high light efficiency and cost effective. We now have 22 domestic offices and maintain very good relationships with our distributors from more than 50 countries. The local service cover most products users no matter where they locate. The company is also very actively providing its products and service to national projects and government signs.
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