What are the measures to prevent EMI in the design of switching power supplies
1MHZ -5MHZ - Differential mode common mode mixing, using input and a series of X capacitors to filter out differential interference and analyze which interference exceeds the standard and solve it; 5M - above mainly uses co touch interference and adopts the method of suppressing co touch. For the case grounded, using a magnetic ring for 2 turns on the ground wire will have a significant attenuation of interference above 10MHz (didiu 2006); For 25-30MHz, it is possible to increase the Y capacitance to the ground, wrap a copper skin outside the transformer, change the PCBLAYOUT, and connect a small magnetic ring with a double wire winding in front of the output line, with a minimum of 10 turns, and install an RC filter at both ends of the output rectifier tube.
30-50MHZ is generally caused by high-speed opening and closing of MOS transistors, which can be solved by increasing MOS driving resistance, using 1N4007 slow transistors for RCD buffer circuits, and 1N4007 slow transistors for VCC power supply voltage.
100-200MHz is generally caused by the reverse recovery current of the output rectifier tube, and magnetic beads can be strung on the rectifier tube
Most of the frequencies between 100MHz and 200MHz are due to PFCMOSFET and PFC diodes. Nowadays, MOSFET and PFC diode string magnetic beads have an effect, and the horizontal direction can basically solve the problem, but the vertical direction is very helpless
The radiation of switching power supplies generally only affects the frequency band below 100M. It is also possible to add corresponding absorption circuits on MOS and diodes, but the efficiency will be reduced.
Measures to prevent EMI when designing switching power supplies
1. Minimize the PCB copper foil area of noisy circuit nodes to the greatest extent possible; Such as the drain and collector of the switch tube, the nodes of the primary winding, etc.
2. Keep the input and output terminals away from noisy components such as transformer wire bundles, transformer cores, heat dissipation fins of switch tubes, etc.
3. Keep noise components (such as unshielded transformer wire packages, unshielded transformer cores, and switch tubes, etc.) away from the edge of the enclosure, as the edge of the enclosure is likely to be close to the external grounding wire under normal operation.
4. If the transformer does not use electric field shielding, keep the shielding body and heat dissipation fins away from the transformer.
5. Minimize the area of the following current loops as much as possible: secondary (output) rectifiers, primary switching power devices, gate (base) drive circuits, and auxiliary rectifiers.
6. Do not mix the drive feedback loop of the gate (base) with the primary switch circuit or auxiliary rectifier circuit.
7. Adjust and optimize the damping resistance value so that it does not produce a ringing sound during the dead time of the switch.
8. Prevent EMI filtering inductance saturation.
9. Keep the bending nodes and components of the secondary circuit away from the shielding body of the primary circuit or the heat sink of the switch tube.
10. Keep the swinging nodes and component bodies of the primary circuit away from shielding or heat dissipation fins.
11. Place the EMI filter for high-frequency input close to the input cable or connector end.
12. Keep the EMI filter with high-frequency output close to the output wire terminal.
13. Maintain a certain distance between the copper foil on the PCB board opposite the EMI filter and the component body.
14. Place some resistors on the rectifier circuit of the auxiliary coil.
15. Connect damping resistors in parallel on the magnetic rod coil.
16. Connect damping resistors in parallel at both ends of the output RF filter.
17. In PCB design, it is allowed to place 1nF/500V ceramic capacitors or a series of resistors, which can be connected between the primary static end of the transformer and the auxiliary winding.
18. Keep the EMI filter away from the power transformer; Especially avoid positioning at the end of the wrapping.
19. When the PCB area is sufficient, a foot position for placing the shielding winding and a position for placing the RC damper can be left on the PCB. The RC damper can be connected across both ends of the shielding winding.
20. If space permits, place a small radial lead capacitor (Miller capacitor, 10 pF/1kV capacitor) between the drain and gate of the switching power field-effect transistor.
21. If space allows, place a small RC damper at the DC output end.
22. Do not lean the AC socket against the heat sink of the primary switch tube.
