Why are PCBs grounded to metal chassis using resistive capacitive connections?

1. Core Objectives of RC Connection

1.1 Electromagnetic Compatibility (EMC) Requirements

• High-frequency noise suppression: Provides a low-impedance path for noise above 30MHz (capacitor function)
• Low-frequency isolation: Blocks 50Hz power interference (high resistance characteristic)
• Typical parameters: 1-100nF/Y-capacitor + 1-2MΩ resistor

1.2 Safety Protection Requirements

• ESD discharge: 1MΩ resistor meets IEC61000-4-2 standard (discharges 2kV static in ≤10 seconds)
• Leakage current control: Limits current to ≤0.1mA (compliant with UL1950)

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2. Key Functions and Selection of Capacitors

2.1 High-Frequency Noise Management

• Y-capacitor selection:
• Medical devices: Prioritize Y1 class (8kV withstand voltage)
• Consumer electronics: Y2 class (5kV) for better cost-performance
• Capacitance impact:
• 1nF capacitor: ~3.2MΩ impedance at 50Hz (effective isolation)
• 100nF capacitor: ~0.016Ω impedance at 100MHz (low-impedance path)

2.2 Hazardous Voltage Isolation

• Safety standards:
• Reinforced insulation: ≥2.5kV withstand voltage (IEC62368-1)
• Creepage distance: ≥6mm (220VAC systems)

3. Resistor Design for Protection Mechanisms

3.1 ESD Protection

3.2 Current Limiting Protection

• When the chassis has 1kV static:
• Direct connection current: Theoretical ∞ (dangerous)
• 1MΩ resistor current: 1mA (safe range)

4. Engineering Practices and Test Verification

4.1 Typical Connection Scheme

PCB_GND ────┬──── 1MΩ ──────┬─── EGND  
             │                │  
             └── 100nF/Y2 ───┘  

4.2 Key Test Items

1. Withstand Voltage Test: 3kVAC/60s (between GND and EGND)
2. Radiated Emission: 30MHz-1GHz band (must be <30dBμV/m)
3. ESD Test: ±8kV contact discharge (Class A criteria)

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5. Industry Applications and Optimization

5.1 Parameter Selection by Application

5.2 Reliability Enhancement

• Advanced solution: Parallel TVS diode (e.g., SMBJ15CA)
• Layout optimization: Use 0201 package to save 30% PCB space

The resistor-capacitor connection achieves a balance between safety isolation and EMC performance through precise parameter design (1MΩ+100nF is the optimal combination). In actual design, it is necessary to comprehensively optimize based on safety standards (such as IEC62368), test data, and cost factors. This solution has become a standard practice in fields such as consumer electronics and medical equipment.