It’s common to see fellow engineers asking in groups: How to connect the PCB ground to the product’s metal enclosure?
It’s common to see fellow engineers asking in groups: How to connect the PCB ground to the product’s metal enclosure?
Today we share an article on this topic with key guidelines noted first:
- Connect the metal enclosure to protective earth (GND_EARTH), with a minimum 2mm gap maintained between it and the system GND;
- For the processing of metal ground:
Connect the enclosure ground to the signal ground in series with a 1MΩ resistor, and also connect a 0.01μF capacitor from the enclosure ground to the signal ground.
I. The Function of the Capacitor
From the perspective of EMS (Electromagnetic Susceptibility), on the premise that the protective earth (PE) is well connected to the earth, this capacitor reduces the impact of potential high-frequency interference signals (referenced to the earth level) on the circuit, and is used to suppress the transient common-mode voltage difference between the circuit and the interference source.
In fact, a direct connection between GND and PE is optimal, but such a connection may be unfeasible or unsafe. For example, the GND generated after 220V AC passes through a rectifier bridge cannot be directly connected to PE. Thus, a path that blocks low-frequency signals and allows high-frequency signals to pass is created.
From the perspective of EMI (Electromagnetic Interference), if the metal enclosure is connected to PE, this high-frequency path prevents high-frequency signals from radiating outwards.
Capacitors conduct alternating current and block direct current. Assuming the enclosure is well earthed:
- EMS aspect: The capacitor suppresses the dynamic common-mode voltage between high-frequency interference sources and the circuit;
- EMI aspect: The capacitor forms a high-frequency path, through which high-frequency interference generated inside the PCB flows into the enclosure and then to the earth, avoiding radiation from high-frequency interference acting as an antenna.
In another scenario where the enclosure is not reliably earthed (e.g., no earth wire, dry ground around the earthing rod), the enclosure’s potential may be unstable or carry static electricity. A direct connection between the PCB and the enclosure could damage the PCB’s chips. Adding a capacitor isolates low-frequency high voltage and static electricity, protecting the PCB.
Note: This shunt capacitor should be a Y-capacitor or high-voltage film capacitor, with a capacitance value ranging from 1nF to 100nF.
Summary of Capacitor Functions
- EMS: Suppresses the transient common-mode voltage difference between the circuit and interference sources;
- EMI: Forms a high-frequency path for high-frequency interference inside the PCB to flow into the enclosure and earth.
II. The Function of the Resistor
This resistor prevents damage to the PCB caused by ESD (Electrostatic Discharge).
If only a capacitor is used to connect the PCB ground and enclosure ground, the PCB becomes a floating ground system. During ESD testing or operation in a complex electric field environment, electrostatic charges injected into the PCB have no discharge path and will accumulate gradually. When the accumulated charge exceeds the withstand voltage of the weakest insulation point between the PCB and the enclosure, discharge occurs—generating a current of tens to hundreds of amperes on the PCB in a few nanoseconds, which can cause the circuit to crash due to electromagnetic pulses or damage components near the discharge point.
Connecting this resistor in parallel slowly dissipates the accumulated charges and eliminates high static voltage. According to the IEC61000 ESD test standard (2kV high-voltage charge dissipated in 10 seconds per test), a resistor with a value of 1MΩ to 2MΩ is generally selected. If the enclosure carries high-voltage static electricity, this high-value resistor also effectively reduces the discharge current and prevents damage to circuit chips.
Safety Standard Reference: The leakage current limit for Class II products specified in safety standards is 0.25mA. The human body can feel a tremor when the leakage current exceeds 0.5mA. Calculation: 220V / 0.25mA = 880kΩ, which is why 1MΩ is the commonly used value.
Summary of Resistor Function
Slowly dissipates charges accumulated on the PCB and eliminates high static voltage.
Finally, a question for reflection: Enclosures are divided into metal and non-metal types. So,
how to handle the system ground when the enclosure is non-metallic?
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