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High-Current Design and Thermal Optimization for Servo Motor Driver PCBAs

Servo motor drivers are key components for industrial automation, which are designed to maintain precise speed and torque for high-current motors. Normally, servo motors operate in very hot and electrically noisy conditions. The controlling PCBA that is used to control the motor can lead to overheating, unnecessary trips, and reduced lifespan.
 
In this technical guide, we discuss the methodology involved in designing the layout of the power stage, which includes the design considerations for placing the IGBTs/MOSFETs and calculation of copper track width. We will also explore the thermal simulation approach, cooling system integration, noise reduction methods, and required protection circuits for high-current servo motor driver PCBA applications.
High-Current Design and Thermal Optimization for Servo Motor Driver PCBAs

High-Current Requirements in Servo Motor Driver PCBAs

Servo motor drivers generally operate to handle rapid current swings and continuous high current. The peak current during the motor acceleration can be much higher than the root-mean-square (RMS) current. This peak current stresses traces, vias, and solder joints in the PCB systems. Thermal cycling from rapid switching accelerates component fatigue. Therefore, the following requirements are needed for the smooth operation of the high-current servo motors.
  • Precise current feedback is required for stable torque and speed control.
  • Ultra-low resistance power pathways to avoid power loss (I²R).
  • The PCBA must withstand mechanical loads and vibrations encountered in industrial environments.
  • Proper sensor placement and robust protection are important for reliable operation.

Power Stage PCB Layout Design for High Current

Placement of IGBTs/MOSFETs and power paths

Power stage layout design ensures the foundation for the safe and efficient operation of the high-current servo motor drivers for PCBAs. Minimizing the loop is for the switching pairs, and separating the power and control returns can improve the operation of the system. Keeping the switching loop as small as possible helps to reduce the parasitic induction. The right placement of bypass capacitors just next to the switch and the gate driver close to the gate pins shortens the gate loops and minimizes the stray inductance. The separation of powers returns from control returns helps to minimize the EMI.
 

PCB trace width calculation

The right selection of trace width or sizing prevents overheating in the PCBAs. It also limits the voltage drop. The copper trace thickness and width affect the temperature rise in PCBAs. The thickness of copper varies from 1 oz to 2 oz, which is equivalent to 35 µm and 70 µm, respectively. The thickest size used is up to 6 oz(210µm). The thicker copper carries the same current over a narrower width; the current is generally estimated by the IPC-2221 standards:
I=k×(∆T)^0.44×A^0.725
Where, I is the current in amps, ∆T is the temperature rise in °C, and A is the area of the copper trace. k is 0.046 for the outer layer and 0.024 for the inner layers.
 

Minimizing the inductance and voltage spikes

The high-current switching generates a significant amount of induction that can create a voltage spike inside PCB systems. This voltage spike can risk the overall PCBA and reduce its performance. The use of the compact loops between the switch, diode, and bus capacitor helps to minimize this effect. Keeping the high-current path shorter and directly routed can also minimize inductance. The use of snubber networks, proper placement of components, and optical trace geometry can reduce EMI and switching stress in high-current PCBAs.

Thermal Management and Cooling System Design

High-current drivers generate significant heat in power semiconductors, shunt resistors, and other components of PCBAs. Proper thermal management is needed to reduce overheating problems. The board-level and system-level cooling system design helps to maintain the temperature of the PCBAs. The use of the copper plane, thermal vias, and large solder pads on the board reduces the heating. The right selection of interface materials, mounting torque, and flatness also ensures good, constant, and predictable thermal performance. In system-level cooling, the optimal size of the heatsink, fin geometry, and cooling fan also help to manage the heat in the servo motor driver PCBA.
 
Thermal simulation using CFD and FEA tools shows a hotspot before PCBA manufacturing, which can also help to detect and solve thermal issues in the board systems.

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Current Sensing and Noise Suppression Techniques

The accurate current sensing is critical for servo motor driver PCBAs. Generally, two methods are used for current sensing: shunt resistors and Hall effect sensors. The shunt resistors provide precise measurement but cause power loss in the circuit. So that shut resistor needs to be carefully placed to avoid overheating problems. On the other hand, Hall sensors offer galvanic isolation and low power losses. However, it requires attention to magnetic coupling and proper layout. That’s why both sensors should be located carefully in the region where current paths are properly defined.
Sensor TypeAdvantagesDisadvantages & Layout Considerations
Shunt ResistorHigh precision, high bandwidth, low cost.Power loss (I²R); must be placed in low-turbulence airflow and use Kelvin connections.
Hall-Effect SensorGalvanic isolation, low power loss.Sensitivity to magnetic coupling; requires careful shielding and routing.
Noise Suppression Techniques for Sense Traces:
To avoid noise in the high-current servo motor driver PCBAs, the following noise suppression methods are suggested:
  • The sense traces need to be shorter and separate from the switching nodes.
  • The addition of an RC filter to the amplifier input helps to reduce the noise.
  • The partition between the analog and power ground can also reduce noise.
  • Avoid vias that open up the loop area on the sense lines.
  • Route differential lines over the continuous reference plane, also minimizing the noise in servo motor Driver PCBAs.

Protection Circuit Design for Half-Bridge Drivers

Half-bridge drivers require fast and reliable detection of overcurrent and overheating events to safeguard the devices. It prevents damage to motors and power supplies from overcurrent. To detect overcurrent and overheating, the comparator or driver fault pin needs to be properly installed. The dedicated protection ICs and firewall-based monitoring system need to be implemented. The PCBA must support these kinds of schemes by ensuring signal integrity. Low-latency paths and robust isolation between the controller side and power side must be ensured. Over-temperature sensing at the heat sink and near the junction is ensured during the design of PCBAs. The placement of the precision logic closes the sensing points and also helps to protect the PCBAs. The longer the feedback path, the more it needs to be avoided. Otherwise, it causes delays, which can damage the system.
Protection Circuit Design for Half-Bridge Drivers in servo motor driver pcba

Design for Manufacturing (DFM) Considerations for High-Current PCBAs

Manufacturability of the servo drive board PCBA is equally important to electrical functionality. As a professional PCBA supplier, our recommendations are as follows for ensuring high yields and good quality:
  • Heavy Copper Capability: Using thick copper (≥3 oz) means using special etching technology. Trace width should be sufficient to avoid any undercutting and acid trap formation during fabrication.
  • Via Plugging and Covering: When designing current-carrying vias, prefer via plugging with epoxy or copper instead of mere covering to avoid any solder wicking effect during the reflow process and provide maximal current-carrying capability.
  • Solder Paste Stencil Design: In case of large thermal pads, design segmented stencils to avoid the effect of “gasing” and voiding control of solder joints.
  • Board Material Selection: Select boards made of high-Tg (Glass Transition Temperature) material (FR-4 with Tg > 170°C) capable of enduring thermal cycling.

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Design Verification and Reliability Testing for Servo Motor Driver PCBAs

Verification and testing of high-current servo motor driver PCBAs ensure their reliability and performance efficiency. The following are the major design verification and testing that need to be done before packaging of the PCBAs for high-current servo motor drivers.
  • Electrical testing: The continuity of the electrical circuit must be tested under real electrical and thermal loads to ensure the maximum working limits of the board system. The insulation resistance must be checked.
  • Thermal and mechanical testing: The board needs to be tested under high humidity, extreme temperature, and vibration.
  • Functional and certification testing: The working of the board is verified under rated and peak load conditions. The devices must be tested and certified by ISO 9001.
To avoid system failure, the measured data can be compared with the simulated results, which ensures the validity of the thermal simulation. This comparison also helps to refine the design of the PCBA before fabrication for next design.

Case Study: Thermal Optimization of a High-Current Servo Driver PCBA

A PCBA design company fabricated a 90A servo driver prototype. This prototype ran too hot during operation after the power supply. The initial layout used 1 oz copper, single-row vias, and a tight switch cluster in the boards. The thermal run showed a clear hot spot at the MOSFET junctions. After the thermal run, the designer team re-evaluated and made changes for the next second prototype fabrication. The changes made by the designers are given below:
  • Prototype copper traces thickness moved to 3 oz copper and increased the width of the main power traces.
  • They added the dense fill via array under the MOSFET pads.
  • They change the MOSFET orientation to spread heat across the heatsink.
  • They improved the thermal pad contacts and mounting pressure.
  • They added a forced cooling fan with directed airflow channels under the closed loop control to ensure the maximum heat rejection from the board system.
After the above changes, the second prototype is fabricated and retested that shows the junction temperature of 25 °C at peak load, which is okay. After ensuring thermal management, the board undergoes vibration and other burn-in testing.

Application Scenarios for High-Power Servo Motor Drivers

The knowledge of the end application enables us to design the PCBA with respect to particular considerations. Our high current servo driver PCBA services include the following applications:
  • Industrial Robotics: Multi-axis arms that require high peak torque and dynamic performance.
  • CNC Machine: Spindle and axis drives that need precision and power controls.
  • Electric Vehicle (EV): Steering and braking systems that require redundancy high current control.
  • Packaging Machines: High-duty cycle continuous motion applications.
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Conclusion

In designing a high current servo motor driver PCBA, there is a need for careful consideration between the performance of the electrical aspects and thermal considerations. Copper weight selection, smart PCB layout design, cooling, and protective circuit design are just some of the considerations. Working closely with the right PCBA manufacturer from the start will be beneficial as well.
 
If you are looking for high-quality, turnkey PCBA manufacturing for your servo motor driver projects, contact us today to discuss your specific requirements.

FAQs

A shunt sensor is typically used in a system where high bandwidth and precision are important. If isolation and low insertion losses matter, then use Hall sensors.
You have to select the copper thickness based on the current requirements, allowed temperature rise, and your budget. After that, you can use the IPC-2221 standards or a trusted calculator to estimate the final value of the copper thickness.
You can use any kind of CFD packages, such as ANSYS or FEA tool, that can run the thermal simulation. You can select the tool that imports your PCB layout data or design. After that you can perform the simulations.
Yes, but we highly recommend using High-Tg FR-4 (Tg > 170°C) to ensure the material can endure the thermal cycling without delamination. For extremely high temperatures, consider materials like Polyimide or Metal Core PCBs (MCPCBs).
Use the Kelvin connection (differential routing). Route the traces as a tightly coupled pair on the same plane, filtered with an RC low-pass network at the amplifier input, and keep them well away from the switching node.

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