I. Solution Background and Design Boundaries

The neck joint of a humanoid robot serves as the core support unit for the robot's vision and environmental perception. It integrates servo drives, high-precision encoders, IMU inertial sensors, multi-axis torque sensors, and high-speed communication buses, while also handling three-degree-of-freedom motion control pitch, yaw, and roll. With its compact space, integration of high and low power, dense concentration of sensitive components, and high frequency of human-robot interaction, the neck joint is a high-risk area for electrostatic discharge (ESD) and surge events.

This solution from Leiditech is designed based on the industry's mainstream 48V DC bus humanoid robot platform and fully complies with IEC 61000-4-2 (ESD), IEC 61000-4-5 (Surge), GB/T 17626.2, and other relevant standards. It achieves protection capabilities of no performance degradation under ±8kV contact discharge and ±15kV air discharge, as well as no hardware damage under basic surge events, covering the entire link of power supply, drive, sensing, and communication.

The key electrical specifications of the neck joint are as follows:

II. Core Sources of ESD and Surge Failure Risks

1. Core Risks of Electrostatic Discharge (ESD)

· Introduction through human-robot interaction: The neck is the core area for visual interaction between the robot and humans. Electrostatic discharge generated by human touch and clothing friction (HBM model: 150pF + 330Ω) can reach peak voltages of over 15kV, directly invading the circuit through housing gaps and interface cables.

· Internal frictional electrification: During rotational motion of the joint, friction from the reducer, cables, and slip rings generates CDM (Charged Device Model) electrostatic discharge, which directly damages sensitive chips such as encoders and sensors.

Space-coupled interference: High-frequency radiation generated by motor PWM switching couples into sensitive signal lines, forming equivalent ESD pulses, which cause sensor data glitches and MCU crashes.

2. Core Risks of Surge Events

· Motor back EMF: During emergency stops or direction changes of the neck joint, voltage spikes generated by inductive loads can reach 2 to 3 times the bus voltage, directly breaking down MOSFETs and driver chips.

· Power bus disturbance: During coordinated multi-joint operation of the entire robot, bus surges caused by sudden load changes invade the joint control board through the power path.

External power injection: During charging or debugging, grid surges introduced by the external power supply are conducted through the bus to the joint module.

III. Complete Link Protection Solution Design

(I) 48V Main Power Port ESD, Surge, and Reverse Polarity Protection Circuit

Leiditech selects the SMBJ58CA for basic ESD and surge protection at the 48V DC power interface. It supports a wide voltage input range of 39V to 54V and complies with IEC 61000-4-2 Level 4 (8kV contact discharge, 15kV air discharge). For high-level IEC 61000-4-5 surge testing, high-power devices are required. The front-end PTC provides    overcurrent protection, while D1 and D2 provide back EMF energy discharge for the motor.

(II) Power Drive and MOSFET Protection Circuit

The neck joint servo drive adopts a three-phase full-bridge inverter topology. MOSFETs are the core power components and are high-risk units for surge events. The protection design of this solution covers the entire path of the gate, drain-source, and drive circuit.

1.Power Drive and MOSFET Protection Circuit

For the neck joint power level of 100–300W, the recommended MOSFET selection parameters are as follows:

· Withstand voltage: ≥100V, providing more than twice the voltage margin for the 48V bus;

· On-resistance Rds(on): <30mΩ, reducing conduction losses and heat generation;

· Package: DFN5x6 / TO-252, suitable for the compact space of the joint while providing excellent thermal dissipation performance;

Leiditech has launched N+P co-packaged MOSFETs specifically optimized for robot joint drives, offering significant advantages in integration, consistency, and reliability. The parameters and application recommendations for some models are as follows:

2. MOSFET Gate ESD and Surge Protection:

· Gate-source parallel TVS diode: Select the SMBJ18CA, a bidirectional TVS with a clamping voltage lower than the MOSFET gate's maximum withstand voltage of 30V, directly discharging ESD and surge events at the gate to prevent gate oxide breakdown.

· Layout requirement: The gate drive trace length should be less than 5mm, and the TVS device should be placed as close as possible to the MOSFET gate and source pins to minimize parasitic inductance.

(III) Sensor and Signal Interface Protection Circuit

The encoder, IMU, and torque sensor in the neck joint are mV-level weak signal devices. The core challenge of ESD protection is balancing protection performance with signal integrity. Ultra-low capacitance protection devices must be used to avoid signal distortion.

1. SPI Interface ESD Protection

Leiditech recommends the 2-channel ESD array SMC12. It features a per-channel junction capacitance of less than 50pF and supports IEC 61000-4-2 ±15kV air discharge and ±8kV contact discharge, providing ESD protection without affecting signal edges or integrity.

2. IMU and Torque Sensor Protection

· Power protection: At the 3.3V/5V power input of the sensors, connect SD03CW/SD05C ESD diodes in parallel to provide ESD and surge protection for the power rails.

· Shielding design: Shielded twisted pair (STP) cables are used for sensor wiring. The shield is grounded at a single point (at the main controller side) to avoid ESD interference introduced by ground loops.

(IV) Communication Bus Interface Protection Circuit

The communication between the neck joint and the main controller is primarily based on the CAN FD bus. The specific protection design is as follows:

· Device selection: Leiditech recommends the integrated CAN-FD bus protection device SMC24LV / SMC27LVQ. With a junction capacitance of <5pF, it ensures signal integrity while filtering out noise and passing ESD testing. By connecting this device in parallel between CAN_H-GND and CAN_L-GND, it achieves ESD protection levels of IEC 61000-4-2 ±15kV air discharge and ±8kV contact discharge.

· Interference suppression: Select the LDW43T-513T common-mode choke to suppress common-mode interference on the bus and improve communication stability.

IV. Key Points for PCB Layout and Engineering Implementation

1. Follow the shortest protection path principle: All ESD and surge protection devices must be placed as close as possible to the interface, with a discharge path length of less than 3mm, to avoid parasitic inductance from long traces that would degrade protection effectiveness.

2. Implement strict isolation between high- and low-voltage domains: The power drive area (high voltage) and the control and sensing area (low voltage) must be strictly separated. A single-point grounding scheme should be adopted to prevent surge currents from the power ground from coupling into the control ground, which could otherwise cause MCU crashes and sensor data anomalies.

3. Optimize structural shielding design: The metal housing of the joint must be reliably connected to the system ground to form a Faraday cage, shielding against radiated ESD interference. Gaps in the housing and cable outlets should be sealed to prevent electrostatic discharge from directly entering the internal circuitry.

V. Core Device Selection List

Shanghai Leiditech has always believed that high-performance component selection and rigorous PCB layout are the two core pillars of EMC design, and neither can be omitted. Only by deeply aligning component parameters with system voltage withstand limits, combined with极致 (meticulous) layout techniques, can product survivability be ensured in increasingly complex electromagnetic environments. Moving forward, Shanghai Leiditech will continue to leverage its over 20 years of technical expertise to provide customized EMC circuit protection solutions and technical support for the humanoid robotics industry.