Electromagnetic compatibility (EMC) and electrostatic discharge (ESD) protection are two crucial considerations in the design and manufacture of electronic equipment, EMC covers both electromagnetic immunity (EMS) and electromagnetic interference (EMI), which are directly related to the stable operation of the equipment in complex electromagnetic environments and the control of interference with other equipment. ESD protection is the key to ensure that the equipment is not damaged in the electrostatic environment. This article will delve into the role of capacitors in enhancing EMS performance and the key function of resistors in preventing ESD damage.
From the perspective of EMS (immunity in electromagnetic compatibility), capacitors play a role in reducing the potential impact of earth-based high-frequency interference signals on circuits based on the assumption of a stable connection between PE (protective earth) and the earth, aiming to mitigate the transient common-mode voltage difference between circuits and the source of the interference. Theoretically, directly connecting GND (ground) to PE is the optimal solution, but in practice it may not be feasible due to safety or technical constraints, for example, GND after the rectifier bridge is not suitable for directly connecting to PE. therefore, a low-frequency blocking and high-frequency pass-through channel is used as an alternative. From the EMI (Electromagnetic Interference) level, if the equipment has a metal enclosure connected to PE, this high-frequency channel can also effectively prevent high-frequency signals from being radiated to the outside.
Capacitors have the property of allowing AC power to pass through and blocking DC power. Under the consideration of electromagnetic immunity, if the chassis is well connected to the earth,the capacitor can inhibit high-frequency interference sources and the dynamic common-mode voltage fluctuations between circuits; from the EMI point of view,the capacitor acts as a conduction path for high-frequency signals, so that the high-frequency interference generated within the circuit board through the capacitance of the inflow of capacitance into the chassis and ultimately leaked into the earth, to avoid these high-frequency interference radiated outwards in the form of an antenna. In the case of the chassis can not be reliably grounded (for example, the lack of ground wire or grounding environment is poor), the shell potential may become unstable or with static electricity, at this time, if the circuit board is directly connected to the shell, the circuit board may be damaged on the chip. By adding capacitors, you can effectively isolate low-frequency high-voltage and static electricity and other unfavourable factors to protect the circuit board from damage.In this scenario,Y-capacitors or high-voltage film capacitors should be used as parallel components, and their capacitance range is usually between 1nF and 100nF.
Functions of Resistors
Resistors play a key role in preventing ESD (electrostatic discharge) damage in circuit boards. If you rely solely on capacitors to connect board ground to enclosure ground, the board creates a suspended ground system.
When subjected to ESD testing or used in complex electromagnetic environments, the charge once injected into the board is difficult to dissipate quickly and builds up over time. When the accumulated charge reaches the limit of the voltage that can be withstood at the weak point in the insulation between the board and the enclosure, a discharge is triggered.This process can generate currents of tens to hundreds of amperes across the board in a very short period of time, which can short out circuits or damage nearby connected components. By adding a resistor, an impedance element, the charge can be gradually released and the high voltage eliminated.According to the IEC61000 standard for ESD testing, a release of 2 kilovolts is required within 10 seconds, so resistors of 1 megohm to 2 megohm are recommended. In addition, this high impedance element effectively limits the current when the enclosure is charged with high voltage static electricity, preventing damage to the circuit chips.
Capacitors and resistors have their own roles to play in electronic equipment. Capacitors enhance electromagnetic compatibility(EMC), while resistors effectively protect against damage from electrostatic discharges, and together they ensure the stable operation of the equipment.
