1、 preamble
When we design PCB boards, we design a reference potential, which is normally what we call “ground”. The grounding in EMC rectification design is to change the path of common mode current injected from outside, so as to avoid common mode current flowing into the circuit. Therefore, when common mode noise exceeds the standard, finding the right location of grounding point and multi-point grounding are effective EMC rectification measures.
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2、Example analysis
As shown above, the test records for a motor after the addition of BDL and filter capacitor, vertical and horizontal exceeded the standard, there are some garbled waveforms between 100MHz and 400MHz, the highest exceeded 7.99dB, after checking the circuit board, found that the board only a long thin wire grounded, and later in the BDL and filter capacitor ground terminal were connected to a thick ground wire to the motor housing The following figure shows.
After connecting two ground wires, it was found that the excessive noise was all reduced, by at least 17dB, and could pass the test.
3、Other case studies
As shown above the left picture is a virtual diagram of a product, the right picture is a virtual diagram of a product PCB board with the shell metal connected to a low-impedance metal.
As shown above, the left picture for the PCB board is not connected to the metal enclosure ground, the right picture for the PCB board using a longer wire connected to the metal enclosure ground, we can see that the noise at the mark M1 and M2 points have been reduced, but more than 200MHz frequency have significantly increased, which is what causes it?
PCB ground and metal shell connection can effectively reduce the PCB in the high-frequency signal transfer to the external wiring harness and the resulting radiation, PCB and metal shell because of the potential difference generated by the common mode current would have been passed on the PCB board changed to pass from the external metal ground, thereby reducing the product PCB internal common-mode noise generated from the wire harness emitted. But many times we will use a multimeter buzzer file to measure the grounding point to ring the position is considered to be through, and ignore the consideration of the ground impedance, when our ground impedance is large, PCB board ground and metal shell ground will produce a larger parasitic inductance, PCB board and metal shell circuit will produce a LC series resonance, resonance, PCB board and metal shell Loop current becomes larger, the inductance becomes larger, the voltage across the capacitor also becomes larger, resulting in the common mode current flowing through the harness also becomes larger, the resonant frequency point corresponding to the highest radiation, which is why the PCB board ground and the metal shell connected to some high frequency point radiation becomes high.
We want to reduce the radiation of these high-frequency points, we must eliminate resonance, we must reduce the parasitic inductance of the PCB board and metal shell, we can use the shortest possible metal conductor to connect the PCB and metal shell, thus playing the role of grounding, reduce the noise value of the product.
4、summary
PCB design grounding needs to consider the grounding point should be closer to the harness port, PCB in connection with the metal shell need to use a relatively short wire or screw connection, which can make the grounding impedance value is lower, so as to ensure that the connection after the PCB and metal shell potential difference of 0.
