Stratégie de câblage GND de la prise secondaire

Transformer secondary center tap GND grounding strategy: digital signal ground vs. chassis ground

——Key points of EMC and noise suppression design of servo drive system

introduction

In the design of servo drive system power supply, the grounding selection of the transformer secondary center tap directly affects the system electromagnetic compatibility (EMC), signal integrity and safety. Based on the needs of industrial scenarios, this article analyzes the physical mechanisms of different grounding strategies and compares their effects through measured data.

1. La signification physique du robinet intermédiaire secondaire

The center tap (CT) is often used in full-wave rectification (such as dual-diode rectification) in rectification and filtering circuits. Its grounding method determines the discharge path of high-frequency noise.

Paramètres clés affectant :

• &Common-mode noise path: The center tap acts as a "virtual midpoint" for high-frequency noise, directly affecting the direction of common-mode current flow
• &Ground loop impedance: The choice of grounding point changes the noise loop impedance, which in turn affects conducted emissions (CE) and radiated emissions (RE)

2. Analyse des effets du signal numérique au sol (DGND)

1. Méthode de connexion

Connecting the center tap directly to the digital ground plane of the PCB is common in low-cost or space-constrained designs:

CT → DGND (plan de masse du PCB)
 

2. Avantages

• &Simplified wiring: no need for additional isolation devices, reducing the number of PCB layers
•  Suppress differential mode noise: Good filtering effect for low frequency (<100kHz) switching noise

3. Défauts et risques

• &Common-mode noise coupling: High-frequency noise (such as MHz-level interference caused by IGBT switching) is coupled to the signal loop through the ground plane

Measured data: When a 400V servo drive is connected to DGND, the peak-to-peak value of the CAN bus common-mode noise reaches 1.2V (exceeding the ISO 11898-2 limit)

• &Ground potential rise: Large current transients cause local DGND potential fluctuations, causing ADC sampling errors

Case: Position data jumps due to DGND jitter on a servo encoder interface (error > 5LSB)

3. Analyse des effets du châssis GND

1. Méthode de connexion

The center tap is connected to the metal chassis through a low impedance path and is usually used with a Y capacitor:

CT → Y capacitor → chassis ground (connected to the ground through conductive screws)
 

2. Avantages

• &Blocking common mode loop: High-frequency noise is discharged directly through the housing, reducing pollution to the signal ground

Actual measurement comparison: After connecting the chassis to ground, the EFT immunity of the RS485 interface is improved from Level 2 to Level 4

•  Improved heat dissipation path: The chassis acts as a large capacitance plane to absorb transient energy

3. Remarques

• &Grounding impedance control: The grounding point of the chassis needs to be welded at multiple points or with conductive pads to ensure that the contact resistance is <10mΩ
• &Safety isolation: Reinforced insulation is required (IEC 61800-5-1 requires primary-case withstand voltage > 3000VAC)

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4. Stratégie de mise à la terre hybride et conception d'optimisation

1. Perles de ferrite + mise à la terre composite du condensateur Y

Exemple de topologie :

CT → Ferrite bead (100Ω@100MHz) → Y capacitor (2.2nF/3kV) → Chassis ground
|
+-- Cavalier de résistance 0Ω → DGND (connecté uniquement pendant le débogage)
 

effet :

• &Le bruit basse fréquence est filtré par des billes magnétiques
• &Le bruit haute fréquence est contourné vers le châssis via des condensateurs Y
• &DGND can be temporarily connected for troubleshooting during the debugging phase

2. Conception d'alimentation isolée

Insérez un dispositif d'isolation entre la masse numérique et la masse du châssis :

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5. Comparaison des données mesurées dans des scénarios industriels

Trois schémas de mise à la terre ont été testés sur un certain type de servomoteur :

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VI. Critères de conception et pratiques d'ingénierie

• &Donner la priorité au sol du châssis :
• &Use braided tape with a cross-sectional area of ≥2.5mm² to connect the housing
• &The grounding point of the chassis is less than 5cm from the transformer to reduce the lead inductance
• &Étapes de mise en œuvre de la mise à la terre hybride :
• &Step 1: Measure the noise spectrum using an impedance analyzer (Keysight E5061B is recommended)
• &Step 2: Select the bead impedance curve according to the main noise frequency (such as TDK MPZ1608S101A for 100MHz)
• &Step 3: Use a network analyzer to optimize the Y capacitor value (Smith chart matching)
• &Scénarios interdits :
• &❌ Floating center tap (resulting in no release path for common mode voltage)
• &❌ Connectez le DGND et la masse du châssis en même temps (formant une boucle de masse)

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en conclusion

In industrial servo systems, it is recommended that the transformer secondary intermediate tap be connected to the chassis ground through a Y capacitor, and that a magnetic bead be used to achieve frequency domain selective filtering. This solution can increase the system EMC margin by 6-10dB, while ensuring the sub-millivolt accuracy of the signal link, meeting the dual needs of modern intelligent manufacturing for high reliability and precision control.