A small mistake caused the board to be remade twice, but the problem was not resolved due to insufficient EMC testing experience. Taking a ditch and gaining wisdom, it seems that in the process of circuit design, capital investment is used to solve the deficiencies in experience.
First time: ring ground
In order to isolate the ground network of the circuit board from the ground network of the power supply system, a ground loop connected to the peripheral circuit was drawn during the PCB design, and a common mode inductor and a differential mode inductor were used for the internal ground. connection. I always thought it was a very simple circuit, and I didn't do EMC bottom-up testing at all. After the successful welding and debugging, I will be confident to do the formal test. Who knows that something went wrong, and the rectification was not resolved during the experiment. After thinking about it to find out that it may be caused by the phenomenon of earth loops.
The highest frequency point is the 128Mhz accessory, and the switching power supply on the circuit has a frequency of 500khz, which is impossible to generate such a large frequency multiplication. When the external power supply network is removed, the amplitude is reduced, but the effect is still poor.
The first test failed.
Second test:
After the loop rectification and continuous modification of the plan, the pcb was re-made, and the copper was completely laid. At the same time, the circuit board was changed to four layers, and the power layer was increased. At the same time, the power input port was added with multi-stage π-type filtering and lc filtering, and the circuit design was re-layed out according to the signal. The layout design of the input and output method was changed. This test passed successfully, but there was still a large amplitude at the multiplier of 500Khz. The filtering was not successful, but the test passed.
Experience talk:
Precautions related to handling EMI EMC:
1. Minimize the PCB copper foil area of the noisy circuit node; such as the drain and collector of the switch tube, the node of the primary and secondary windings, etc.
2. Keep the input and output terminals away from noise components, such as transformer coils, transformer cores, heat sinks of switch tubes, and so on.
3. Keep noise components (such as unshielded transformer wire packages, unshielded transformer cores, and switch tubes, etc.) away from the edge of the shell, because the edge of the shell is likely to be close to the grounding wire outside under normal operation.
4. If the transformer does not use electric field shielding, keep the shield and heat sink away from the transformer.
5. Minimize the area of the following current loops as much as possible: the secondary (output) rectifier, the primary switching power device, the gate (base) drive circuit, and the auxiliary rectifier.
6. Do not mix the gate (base) drive feedback loop with the primary switching circuit or auxiliary rectifier circuit.
7. Adjust and optimize the damping resistance value so that it does not produce ringing sound during the dead time of the switch.
8. Prevent saturation of EMI filter inductor.
9. Keep the turning node and the 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 oscillating node and component body of the primary circuit away from the shield or heat sink.
11. Make the high frequency input EMI filter close to the input cable or connector end.
12. Keep the high frequency output EMI filter close to the output wire terminal.
13. Keep a certain distance between the copper foil of the PCB on the opposite side of the EMI filter and the component body.
14. Put some resistors on the line of the rectifier of the auxiliary coil.
15. Connect a damping resistor in parallel with the magnetic rod coil.
16. Connect damping resistors in parallel at both ends of the output RF filter.
17. It is allowed to put 1nF/ 500 V ceramic capacitors or a series of resistors in the PCB design, which is connected across the static end of the transformer primary 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, you can leave the pin for the shield winding and the position for the RC damper on the PCB. The RC damper can be connected across the shield winding.
20. If space permits, place a small radial lead capacitor (Miller capacitor, 10 picofarad/1 kV capacitor) between the drain and gate of the switching power FET.
21. If space permits, put a small RC damper on the DC output.
22. Don't put the AC socket and the heat sink of the primary switch tube close together.
23. The grounding of the filter of the metal shell is best to directly pass the large area overlap between the shell and the ground. Check whether the input and output lines of the filter are close to each other.
24. Properly adjust the capacitance of the X/Y capacitor, the differential mode inductance and the inductance of the common mode choke;
25. Pay attention to safety issues when adjusting the Y capacitor; changing the parameters may improve the radiation of a certain section, but it will cause the other frequency to become worse, so you need to keep trying to find the best combination.
26. It is a good way to appropriately increase the resistance value on the trigger electrode; it can be effectively reduced by connecting a small capacitor to the ground of the secondary output rectifier or the collector of the switching transistor (or the drain of the MOS tube). Small common mode switching noise.
27. The switching power supply board must control the return area of each loop during PCB wiring, which can greatly reduce the differential mode radiation. 28.Add 104/103 capacitors to the PCB power supply traces for power supply decoupling; the power plane and the ground plane are required to be close to each other when wiring multi-layer boards;
29. Put the magnetic ring on the power line for comparison and verification, which can be realized by adding common mode inductance on the single board in the future, or injecting the magnetic ring on the cable.
30. The length of the L line of the input AC line should be as short as possible; whether there is an interference source near the hole in the shielding device;
31. Check whether the grounding screw is sprayed with insulating paint and whether it is well grounded.
