Looking at the solution to common mode interference in switching power supplies from the three-circuit model
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Looking at the solution to common mode interference in switching power supplies from the three-circuit model

Posted Date: 2024-01-22

The common mode current model of the switching power supply can be briefly explained using the following three loops. Driven by the common mode voltage of the switch tube, an input loop, an output loop and an input-output large loop are formed. From the perspective of conducted emission, we hope to reduce the common mode current of the input loop and the large input-output loop. From the perspective of radiated emission, we hope to reduce the common mode current on the one hand, and reduce the area of ​​each loop on the other. .

On the other hand, the size of the common mode current is closely related to the grounding status of the product. Because the common mode current generally flows from the PCB to the chassis ground through the distributed capacitance or the ground wire between the PCB and the chassis, then flows to the ground through the distributed capacitance between the chassis and the earth or the chassis ground wire, and finally returns to the common mode voltage source. , The grounding status of the product's chassis has a great impact on the distributed capacitance and grounding impedance, which will affect the size of the common mode loop impedance and ultimately the size of the common mode current.

We use two examples to illustrate the impact of product grounding status on common mode current.

Example 1 is as follows:

Phenomenon: When a piece of equipment was undergoing CE102 testing, the following phenomenon was discovered. If the equipment under test is placed directly on the metal plate of the test bench, the conducted emission will exceed the standard; but if an insulator is placed between the equipment under test and the metal plate, , the test result will be qualified.

The common-mode voltage of a product is generally based on a larger metal object as a reference point. When an insulating object is placed between the product and the metal table, the loop impedance of the common-mode current is increased, thus reducing the size of the common-mode current. This reduces the conducted emissions of the product. The return path of the differential mode conduction emission current does not pass through the ground wire of the product, so the grounding status has no effect on the differential mode conduction current. This shows that the product has insufficient common mode filtering capabilities. After inspection, it was found that the power filter and the metal chassis were overlapped. is not good and reduces the common mode filtering capability of the filter.

Since the grounding point of the common mode filter capacitor is the chassis ground, when the overlapping effect between the filter and the chassis is not good, the filtering effect of the common mode filter will be reduced, which will cause more common mode current to flow to LISN. As a result, the conducted emission exceeds the standard. When an insulator is placed between the product, the equipment under test, and the metal plate, the common mode impedance flowing to the LISN loop is increased, thereby reducing the common mode current of the loop and reducing the common mode conducted emission.

Example 2 is as follows:

Phenomenon: For a CNC machine tool, it was found that when the metal shell of the control unit was connected to the machine tool, the control unit would not work properly. If an insulating object was placed between the control unit and the machine tool, the interference phenomenon would disappear.

picture Electromagnetic interference caused by common mode current

We know that the IGBT switch tube of the motor driver is constantly on and off driven by the PWM signal, thus generating a large dV/dt. The red part of the pulse width modulation variable speed motor drive circuit in the figure below is the main large dV/dt. The common mode current generated by this interference source will be coupled to other modules through the cable. Motor drives are therefore a strong source of interference.

The above phenomenon shows that there is a strong common mode current on the cable connecting the motor driver and the control unit, causing the control unit to be interfered. When the metal shell of the control unit is connected to the machine tool, the impedance of the common mode current loop is small. At this time, a strong common mode current flows into the control unit and causes interference; when an insulating object is placed between the control unit and the machine tool frame, interference occurs. The phenomenon will disappear. This is because the impedance of the common mode loop is increased at this time, and the common mode current flowing into the control unit is reduced, thereby reducing interference.

So how to solve this problem? We should increase the common mode filtering of the I/O port of the control unit to enhance its anti-interference capability, and at the same time take some measures at the source motor driver to reduce its common mode interference.

These two examples seem to give us the illusion that it is better if our metal casing is not grounded, but in fact this is not the case. The low-impedance connection between the metal casing and the ground plate is very important for our lightning and static electricity protection. , the smaller the grounding impedance, the better the energy will be discharged to the earth, reducing the energy coupling into the product and reducing the interference to the PCB.

Therefore, through the above two examples, we realize that common mode interference is related to the grounding status of the product, and grounding is also crucial to the electromagnetic protection performance of the product. For the impact of grounding, we must consider both EMI and EMS.

Review Editor: Huang Fei


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