Measures to prevent EMI when designing switch mode power supplies
Time:2023-12-26
Views:202
As an energy conversion device working in a switching state, the voltage and current change rate of a switching power supply is very high, and the interference intensity generated is relatively high; The interference sources are mainly concentrated during the power switch period, as well as the heat sink and high level transformer connected to it, and the position of the interference sources in the digital circuit is relatively clear; The switching frequency is not high (from tens of KHz to a few MHz), and the main forms of interference are conducted interference and near-field interference; The wiring of printed circuit boards (PCBs) usually adopts manual wiring, which has greater randomness, making it more difficult to extract PCB distribution parameters and estimate near-field interference.
Within 1MHz
Mainly focusing on differential mode interference, increasing the X capacitance can solve the problem.
1MHz --5MHz
Differential mode common mode mixing, using input terminals and a series of X capacitors to filter out differential interference and analyze which type of interference exceeds the standard and solve it; For frequencies above 5MHz, common touch interference is mainly used, and methods of suppressing common touch are adopted. For shell grounding, winding a magnetic ring around the ground wire for 2 turns will have a significant attenuation of interference above 10MHz; For 25-30MHz, it is possible to increase the ground Y capacitance, wrap a copper skin on the outside of the transformer, change the PCBLAYOU, connect a small magnetic ring with a dual wire winding in front of the output line, reduce the winding by 10 turns, and install an RC filter at both ends of the output rectifier tube.
30MHz --50MHz
It is generally caused by high-speed turn-on and off of MOS transistors, which can be solved by increasing the MOS driving resistance, using 1N4007 slow transistors for RCD buffer circuits, and 1N4007 slow transistors for VCC power supply voltage.
100MHz --200MHz
It is generally caused by the reverse recovery current of the output rectifier tube, and magnetic beads can be connected in series on the rectifier tube.
Most of the frequencies between 100MHz and 200MHz are due to PFC MOSFETs and PFC diodes. Now, MOSFETs and PFC diodes have magnetic beads in series, which can basically solve the problem in the horizontal direction, but it is very helpless in the vertical direction.
The radiation of switch mode power supplies generally only affects the frequency band below 100MHz, and corresponding absorption circuits can also be added to MOS and diodes, but the efficiency may be reduced.
Measures to prevent EMI when designing switch mode power supplies
1. Reduce the PCB copper foil area of noise circuit nodes to a maximum extent, such as the drain and collector of switch tubes, and the nodes of the primary winding.
2. Keep the input and output terminals away from noisy components such as transformer wire packages, 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 edges of the casing, as under normal operation, the edges of the casing are likely to be close to the external grounding wire.
4. If the transformer does not use electric field shielding, keep the shielding body and heat sink away from the transformer.
5. Minimize the area of the following current loops as much as possible: secondary (output) rectifier, primary switching power device, gate (base) drive circuit, auxiliary rectifier.
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 to prevent ringing during the dead time of the switch.
8. Prevent EMI filtering inductance saturation.
9. Keep the turning 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. Bring 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 terminals.
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. Parallel damping resistor 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 it at the end of the winding.
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 the two ends of the shielding winding.
20. If space permits, place a small radial lead capacitor (Miller capacitor, 10 picosecond/1kV capacitor) between the drain and gate of the switching power field-effect transistor.
21. If space permits, 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.
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