In the ESD (electrostatic discharge) immunity test, direct discharge and indirect discharge are two core testing methods that simulate different electrostatic discharge scenarios. The following are the definitions, differences, and typical applications of the two methods.
1. Direct Discharge
Definition: Direct discharge refers to the application of discharge from an electrostatic discharge generator directly to a specific part of the equipment under test (EUT), simulating the release of static electricity when a human body or object directly contacts the equipment.
Classification:
Method: The sharp tip of the discharge gun directly contacts the conductive surface of the equipment (such as metal casing, interface terminals).
Applicable scenarios: metal parts, exposed conductors that can be touched. Advantages: Clear discharge path, stable current waveform, high repeatability.
2)Air discharge
Method: The round electrode of the discharge gun approaches the non-conductive surface of the equipment (such as plastic casing, insulation coating) at a certain speed, and discharges air through arc.
Applicable scenarios: Insulation areas that cannot be directly contacted (such as key gaps, coating surfaces).
Challenge: The discharge path is unstable and greatly affected by environmental humidity and surface roughness.
Test purpose:
To verify the anti-interference ability of the equipment against direct contact discharge.
Detect whether semiconductor devices are damaged due to direct discharge (such as breakdown or burning).
2. Indirect discharge
Definition:
Indirect discharge applies discharge to equipment through a horizontal coupling plates (HCP) or vertical coupling plates (VCP), simulating the interference of electrostatic fields generated by human contact with adjacent objects on the equipment.
Characteristics:
Coupling plate material: The same metal plate (such as copper or aluminum) as the ground reference plane.
Discharge method: Only contact discharge is used (the coupling plate is grounded through 470kΩ resistor).
Layout requirements:
Horizontal coupling plate: 0.1m away from the equipment, and the discharge gun discharges vertically to the edge of the plate.
Vertical coupling plate: parallel placed on the side of the equipment at a distance of 0.1m, with the discharge gun discharging perpendicular to the center of the plate surface.
Test objective:
To simulate the electrostatic field coupling interference generated when the human body comes into contact with surrounding objects (such as door handles, tools).
To verify the anti-mis operation ability of the device under electrostatic field radiation.
3. Direct discharge vs. Indirect discharge
|
Item |
Direct Discharge |
Indirect Discharge |
|
Discharge object |
The device itself |
Coupling plate (simulating nearby objects) |
|
Discharge mothed |
Contact discharge, air discharge |
Only contact discharge |
|
Typical scenario |
The operator touches the device |
The human body touches the door handle and approaches the equipment |
|
Current path |
Directly discharge through the electrical path of the device |
Interference through electrostatic field coupling |
|
Test severity |
Higher (direct energy injection) |
Lower (depending on field coupling) |
|
Standard requirements |
Contact discharge preferred |
The coupling plate needs to be grounded through 470kΩ resistor |
4. Key Points of the Test Process
1) Direct discharge step
Pre-scan: Rapidly scan the surface of the device at a rate of 20 times per second to mark sensitive points.
Formal test: discharge each sensitive point 20 times at a rate of 1 time per second (positive and negative polarity 10 times each).
Record: Functional abnormalities (such as restarts, data loss) and hardware damage (such as burn marks).
2) Indirect discharge step
Coupling plate layout: The horizontal/vertical coupling plates are 0.1m away from the equipment and grounded through resistors.
Discharge execution: Contact discharge is carried out on the edge or side of the coupling plate to simulate the discharge of adjacent objects.
Verification: Observe whether the equipment malfunctions due to electrostatic field interference.
5. Typical Application Scenarios
Direct discharge
Contact discharge of the mobile phone charging port (verification interface protection).
Discharge when the car key touches the door handle (simulating the discharge of metal parts).
Indirect discharge
Accidental touch of the display screen after the human body touches the plastic casing (such as screen flickering).
The functional abnormality of the industrial equipment casing after contact with maintenance tools.
6. Standard Basis
IEC/EN 61000-4-2: Direct discharge is preferred to contact discharge, and air discharge is used for non-conductive surfaces.
ISO 10605 (Automotive Electronics): Indirect discharge is required to cover vehicle level and component level testing.
GB/T 17626.2: The domestic equivalent adopts IEC standard, and the discharge level is specified (contact 2~8kV, air 2~15kV)
Through the comprehensive test of direct and indirect discharge, the anti-static capability of the equipment in real use scenarios can be comprehensively evaluated to ensure its functional reliability and safety.
