Emced differential mode and common mode interference rectification tips

1、 Differential mode interference and common mode interference

In the field of electromagnetic compatibility, there are two forms when voltage and current changes are transmitted through wires. One is that two wires are used as round-trip lines, which we call "differential mode"; The other way is to use two wires as the destination and the ground wire as the return transmission. We call it "common mode".

1.1 Differential mode interference: between L-N lines, the current enters from L, flows through the positive pole of the rectifier diode, then flows through the load, passes through the thermal earth current diode, and then returns to the rectifier diode N. On this path, there are high-power devices with high-speed switches and diodes with short reverse recovery time. The high-frequency interference generated by these devices will pass through the whole circuit, and these circuits will be detected by the receiver, resulting in conduction exceeding the standard.

1.2 Common mode interference: due to the parasitic capacitance between the earth and the equipment cable, common mode current will be generated between the earth and the cable, resulting in high-frequency interference noise.

The following figure shows the conducted FALL data caused by differential mode interference. The front end of the test data exceeds the standard, which is caused by differential mode interference:

The following figure shows the EMI principle of switching power supply:

In the figure, CX2001 is a safe film capacitor (when the capacitor is broken or damaged, it is shown as an open circuit). When it crosses between the L-line and the N-line, the current between them will bring high-frequency clutter into the circuit when the current passes through the load. At this time, the role of X capacitor is to form a circuit between the load and X capacitor, so as to consume high-frequency shunt in the circuit, rather than enter the market power supply, that is, the interference circuit will not be connected to the outside through the short circuit AC power supply of the capacitor capacitor.

Countermeasures for rectification of differential mode interference:

1. Increase X capacitance value

2. Add common-mode inductors and use their leakage sense to suppress bad mold noise (because common-mode inductors have multiple winding modes, double-wire and winding or double-wire separate winding). In the field of electromagnetic compatibility, no matter what winding method is used, magnetic leakage will occur due to the loose winding and different line length, that is, the magnetic wire generated by one coil cannot completely pass through the other coil, which makes the induction potential between L-N lines, equivalent to the series connection of L-N inductance).

The following figure shows the common mode interference test FALL data:

The parasitic capacitance between the power cable and the earth makes the common-mode interference have a circuit, the interference noise flows to the earth through the capacitance, and the common-mode interference current is formed between LISN-cable-parasitic capacitance-ground on the earth, which is detected by the receiver, Lead to excessive conduction (This can also explain why some motherboards do not pass the ground during the conduction test, and a ground wire exceeds the limit. When the USB mode is not grounded, the current circuit can only pass through L-diode - load - hot ground - diode - N, common-mode current common-mode current LISN, and the noise detected by LISN is very small. When the cold ground of the motherboard is directly connected to the ground, there is a circuit between the cable and the ground. At this time, if the common-mode noise is not detected by the front end, LC if it passes If the filter circuit is absorbed, the conduction will exceed the limit)

Countermeasures for rectification of common mode interference:

1. Increase common-mode inductance

2. Adjust LC filter on L-GND and N-GND to filter common mode noise

3. The main board should be grounded as much as possible to reduce the impedance to the ground, thus reducing the parasitic capacitance between the cable and the earth.

2、 EMC disturbance sources of the product include:

1. Rectification circuit of equipment DC power supply: the upper limit of power frequency rectification noise frequency of power frequency linear power supply can be extended to hundreds of kHz; The upper limit of high-frequency rectification noise frequency of switching power supply can be extended to dozens of MHz.

2. Brush noise of DC motor of electric equipment: the upper limit of noise frequency can be extended to hundreds of MHz.

3. Disturbance emission of variable frequency speed regulation circuit: the frequency of disturbance source of switch speed regulation circuit varies from dozens of kHz to dozens of MHz.

4. Electromagnetic disturbance emission of electromagnetic induction heating equipment: the main frequency of the disturbance source is tens of kHz, and the advanced harmonics can be extended to tens of MHz.

5. Switch noise: the upper limit of noise frequency generated by mechanical or electronic switch action can be extended to hundreds of MHz.

6. Switching circuit of equipment switching power supply: the main frequency of the disturbance source ranges from dozens of kHz to more than 100 kHz, and the advanced harmonics can extend to dozens of MHz.

7. Intelligent control equipment for crystal vibration and digital circuit electromagnetic disturbance: the main frequency of disturbance source ranges from tens of kHz to tens of MHz, and the advanced harmonics can extend to hundreds

8. Operating noise of AC motor of electric equipment: higher harmonics can extend to dozens of MHz. MHz。

9. Microwave leakage of microwave equipment: the main frequency of the disturbance source is GHz.

10. Information technology equipment and digital processing circuit of various automatic control equipment: the main frequency of the disturbance source is tens of MHz to hundreds of MHz (the main frequency can be word GHz through internal frequency doubling), and the high harmonic can be extended to more than ten GHz.

11. The local oscillator and its harmonics in the high-frequency tuning circuit of the TV electro-acoustic receiving equipment: the main frequency of the disturbance source ranges from tens of MHz to hundreds of MHz, and the high harmonics can extend to several GHz.