Practical rectification strategy for EMI after switching power supply design-conduction part
Within 1MHz, differential mode interference is predominant
1. 150KHZ-1MHz, mainly differential mode, 1-5MHz, differential mode and common mode work together, and after 5MHz, it is basically common mode. The capacitive coupling and inductive coupling of differential mode interference. Generally, interference above 1MHz is common mode, while low-frequency interference is differential interference. Connect a resistor in series with capacitors and then connect them to the Y capacitor pin. Using an oscilloscope to measure the voltage on both pins of the resistor can estimate common mode interference;
2. Add differential mode inductance or resistance after insurance;
3. Small power supplies can be processed using PI type filters (it is recommended to use larger electrolytic capacitors near transformers).
4. The differential mode inductor in the π - type EMI component of the front-end is only responsible for low-frequency EMI. Do not choose a too large volume (DR8 is too large, it is better to use a resistor or DR6), otherwise the radiation will be difficult. If necessary, magnetic beads can be added in series, because high frequencies will fly directly to the front-end and will not follow the wire.
5. When the conduction cooler exceeds the standard at 0.15-1MHz, there is a 7DB margin when the heat engine is running. The main reason is that the DF value of the primary BULK capacitor is too large. The ESR is relatively high when cooling down and relatively low when heating up. The switching current forms a switching voltage on the ESR, which flows between a current LN line and is called differential mode interference. The solution is to use electrolytic capacitors with low ESR or add a differential mode inductor between two electrolytic capacitors.
6. Solution for testing the total exceedance of 150KHZ: Increase the X capacitor to see if it can come down. If it does, it indicates differential mode interference. If it doesn't have much effect, then it's common mode interference, or if you wrap the power cord around a large magnetic ring a few times, it means it's common mode interference. If the interference curve is good at the back, reduce the Y capacitance and check if there is a problem with the layout, or add a magnetic ring in front.
7. The inductance of the single winding inductor in the PFC input section can be increased.
8. Adjust the main frequency of the components in the PWM circuit to around 60KHZ.
9. Attach a copper sheet tightly to the transformer core.
10. The inductance on both sides of a common mode inductor is asymmetric, and one less turn on one side can also cause the conduction to exceed 150KHZ-3MHZ.
11. There are two main points for the generation of general conduction: around 200K and 20M, which also reflect the performance of the circuit; Around 200K, the main spikes are caused by leakage sensation; About 20M is mainly due to the noise of circuit switches. Poor handling of transformers can increase a significant amount of radiation, and shielding is useless as the radiation cannot pass.
