Why is the PCB ground connected to the metal case with a resistor and capacitor? Is there a better solution?

btty038

Why is the PCB ground connected to the metal case with a resistor and capacitor? Is there a better solution? [Copy link]

 

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
1. Function of capacitors
From the perspective of EMS (electromagnetic interference immunity), the capacitor aims to reduce the impact of possible high-frequency interference signals with the ground 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 and block DC
. Assuming that 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, if the case 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 there may be 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 between 1nF and 100nF.
2. The role of the resistor
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 then it 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 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 IEC61000 ESD test standard, 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 has high voltage static electricity, this high impedance component can also effectively reduce the current, thereby preventing damage to the circuit chip.
3. Issues that need attention
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 occasion where the equipment is used, the device 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 is well grounded at a single point with the casing; 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 power frequency interference loop impedance is extremely large, but it will not cause any interference to the PCB;
4. When multiple devices need to be connected to each other, it should be ensured that the casing of each device is well grounded to the earth at a single point, and the PCB inside each device should also be grounded at a single point with its casing;
5. However, if the device casing 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 casing;
6. The chassis ground may not be an ideal grounding choice. For example, the relevant safety regulations are not followed in the distribution network, and there is no ground wire; or the soil around the ground rod is too dry, and the ground bolt is corroded or loose.
7. There is electromagnetic interference in the environment. There are high-power transformers, high-power motors, electromagnetic furnaces, high-voltage 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 chassis (containing high-frequency and low-frequency components at the same time), or there will be static electricity between the two, so good and reliable grounding treatment for them is necessary, and it is also required by product safety regulations.

maychang

[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;]

I don't agree. Look at the motherboard of a desktop PC. It is well connected to the metal shell, but it is definitely not a "single point" connection, but a multi-point connection, using at least 4 to 6 screws.

吾妻思萌

[Power frequency interference will be eliminated by grounding the shell, and will not interfere with the PCB]

It's a 50Hz 220V power supply, and then there's radiation crosstalk or something, it doesn't seem to be industrial frequency.

qwqwqw2088

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;

It's like saying it again.

btty038

maychang posted on 2024-2-3 15:25 [1. If the device casing is well grounded, then the PCB should also be well grounded to the casing. At this time, the power frequency interference will be eliminated through the casing grounding, which is good for...

Can you tell me about the requirements for this host experiment?

btty038

Azuma Simeng published on 2024-2-3 20:57 [The power frequency interference will be eliminated through the shell grounding, and will not cause interference to the PCB] Is it a 50Hz 220V power supply and then radiation crosstalk or something, I don’t feel...

It is the experimental requirement, the insulation resistance after recovery under various harsh conditions,

btty038

qwqwqw2088 posted on 2024-2-4 20:09 5. However, if the device casing cannot be well grounded when multiple devices are connected to each other, then it can be turned into a floating state, and the internal PCB does not need to be connected to the casing...

For example, if...

qwqwqw2088

btty038 posted on 2024-2-4 22:14 For example, if...

There's no need to give examples for this.

That is to say, in front

4. When multiple devices need to be connected to each other, try to single-point grounding. At the same time, each device should be single-point grounded to its housing.

5 said, however, that multiple,connections,,floating state, PCB does not need to be grounded to the shell,,suitable,,

Nubility

Personal understanding:

1. The 1M resistor is because the leakage current of Class II products specified in the IEC60335-1 safety standard is 0.25mA. 220V/0.25mA = 880K. For safety reasons, choose a larger resistor, 1M.

2. The function of the 100nF capacitor is to provide an AC path to conduct various interference signals in the circuit with PGND as the reference level to the ground.

btty038

Nubility posted on 2024-2-5 16:34 Personal understanding: 1. The 1M resistor is because the leakage current of the second-class product specified in the IEC60335-1 safety standard is 0.25mA. 220V/ ...

Analog requires good grounding, digital needs to be distinguished, which is equivalent to protecting the digital as a whole.

btty038

qwqwqw2088 posted on 2024-2-5 13:07 There is no need to give an example. That is to say, in 4 above, when multiple devices need to be connected to each other, try to single-point grounding. At the same time, each device...

How are the individual board grounding and single-point grounding in the module chassis coordinated with safety regulations, safety codes, and six properties?

btty038

Nubility posted on 2024-2-5 16:34 Personal understanding: 1. The 1M resistor is because the leakage current of the second-class product specified in the IEC60335-1 safety standard is 0.25mA. 220V/ ...

I asked a professional board designer about this and his reply was to take measures to prevent lightning and electrostatic high voltage isolation.

Gen_X

This connection method is designed to address weaknesses and is also called DC virtual ground.

The main purpose is to prevent static electricity accumulation and interference or discharge between devices or between devices and the earth (or people). Many sensors have this design. In addition, adding a capacitor obviously achieves a high-frequency "grounding" effect. These two simple components achieve the design purpose and avoid harmful voltages between devices or between devices and people.

If the switching power supply we use now is really DC floating, it will generally cause 70-1000V DC static electricity accumulation to the entire system being powered in a few minutes or even hours. The speed depends on the PCB design, circuit design, and the process and form of the isolation transformer used in the power supply. It is difficult to explain it in one sentence. But prevention is necessary.

This grounding is especially important for improving the resistance of operational amplifiers to "common mode" interference. Many foreign "modules" must have single-point or multi-point grounding and dispersed signal grounding because they cannot withstand the harm caused by poor "floating ground" measures in the system, and then put forward many "standard" requirements to users.