Unveiling the secrets of clever grounding in electronic design: Why is the PCB ground connected to the metal casing using resistors and capacitors?
Latest update time：2024-10-12
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The grounding problem of electronic products is a common topic. This article only talks about a small part of it, mainly the grounding problem of metal shell and circuit board. We often see that in some system designs, a high-voltage capacitor C1 (1~100nF/2KV) is usually connected in parallel with a large resistor R1 (1M) between the ground (GND) of the PCB board and the metal shell (EGND). So why is it designed like this?
Figure 1 Schematic diagram
Figure 2 Actual PCB
The role of capacitors
From the perspective of EMS (electromagnetic interference immunity), this capacitor is designed to reduce the impact of possible high-frequency interference signals that use the earth potential as a reference on the circuit on the basis of ensuring that PE is connected to the ground, so as to achieve the purpose of suppressing the instantaneous common-mode voltage difference between the circuit and the interference source. In fact, it is ideal to connect GND directly to PE, but direct connection may cause operational difficulties or safety hazards. For example, the GND generated after the rectifier bridge cannot be directly connected to PE, so a path is designed that prevents low-frequency signals from passing but allows high-frequency signals to pass. From the perspective of EMI (electromagnetic interference), if there is a metal shell connected to PE, the existence of this high-frequency channel also helps prevent high-frequency signals from radiating to the external environment.
Capacitors pass AC but block DC. Assuming the chassis is well connected to the ground, from the perspective of electromagnetic interference immunity, the capacitor can suppress the dynamic common-mode voltage between the high-frequency interference source and the circuit; from the perspective of EMI, the capacitor forms a high-frequency path, and the high-frequency interference generated inside the circuit board will flow into the chassis and into the ground through the capacitor, avoiding antenna radiation formed by high-frequency interference. In another case, assuming that the chassis is not reliably connected to the ground (such as no ground wire, the ground rod environment is dry), the shell potential may be unstable or have static electricity. If the circuit board is directly connected to the shell, the circuit board chip will be damaged. Adding capacitors can isolate low-frequency high voltage, static electricity, etc. to protect the circuit board. This parallel capacitor should use a Y capacitor or a high-voltage film capacitor with a capacitance value between 1nF and 100nF.
The role of resistance
This resistor can effectively prevent ESD (electrostatic discharge) from damaging the circuit board. If only a capacitor is used to connect the circuit board ground to the shell ground, the circuit board will form a floating ground system. When conducting ESD testing or using it in a complex electromagnetic field environment, the charge injected into the circuit board is difficult to be effectively released, and will accumulate; when it accumulates to a certain extent, it exceeds the voltage value that the weakest point of the insulation between the circuit board and the shell can withstand, and it will cause a discharge phenomenon-in a very short time, tens to hundreds of amperes of current can be generated on the circuit board, which may cause the circuit to stop running due to electromagnetic pulses, or damage the connected components near the discharge site. If this impedance is added, the charge can be gradually released and the high voltage can be eliminated. According to the ESD test standard of IEC61000, each discharge must complete the release of 2 kilovolts within 10 seconds, so it is generally recommended to use a resistor of 1 megohm to 2 megohm. If the shell carries high-voltage static electricity, this high-impedance component can also effectively reduce the current, thereby preventing damage to the circuit chip.
Issues to note
1. If the device casing is well grounded, then the PCB should also be well grounded at a single point with the casing. At this time, the power frequency interference will be eliminated through the casing grounding and will not interfere with the PCB.
2. If there may be safety issues in the use of the equipment, the equipment casing must be well grounded;
3. In order to achieve better results, it is recommended that the device casing is well grounded as much as possible, and the PCB and the casing are well grounded at a single point; of course, if the casing is not well grounded, it is better to float the PCB, that is, not connect it to the casing, because if the PCB is isolated from the earth (the so-called floating ground), the impedance of the power frequency interference loop is extremely large, but it will not cause any interference to the PCB;
4. When multiple devices need to be connected to each other, try to ensure that the shell of each device is well grounded to the earth at a single point, and the PCB inside each device should also be grounded to its shell at a single point;
5. However, if the device housing cannot be well grounded when multiple devices are connected to each other, it is more appropriate to turn it into a floating state, and the internal PCB does not need to be grounded to the housing;
6. The chassis ground may not be an ideal grounding choice. For example, relevant safety regulations are not followed in the distribution network and there is no ground wire; or the soil around the grounding rod is too dry, and the grounding bolt is rusted or loose.
7. There is electromagnetic interference in the environment. There are high-power transformers, high-power motors, electromagnetic furnaces, high-voltage power grid harmonics, etc. in the working environment.
8. High-frequency noise will be generated inside the PCB, such as high-frequency switching tubes, diodes, energy storage inductors, high-frequency transformers, etc. These interference factors will cause the potential fluctuations of the signal ground of the PCB and the casing (containing both high-frequency and low-frequency components), or static electricity between the two, so good and reliable grounding of them is necessary and is also required by product safety regulations.
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