BLDC Motor Driver Board with FOC

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Overview

BLDC Motor Driver
Figure 1: BLDC Motor Driver Complete

This project is a full-featured BLDC (Brushless DC) motor driver board designed around the Texas Instruments DRV8353RSRGZT smart gate driver. The system integrates all peripheral blocks needed for closed-loop motor control, including a UART debug interface, differential encoder inputs, Hall-effect position sensing, and a 3.3V regulation rail for logic. The project highlights my ability to design robust high-current switching hardware, manage system-level signal integrity, and integrate mixed-signal...

Objective

To develop a standalone motor driver PCB capable of handling three-phase commutation for BLDC motors up to 48V, while supporting external control via PWM/SPI and providing accurate feedback using Hall and encoder sensors. The system was optimized for modularity, noise resilience, and future expandability for use in robotics, mechatronics, and mobility systems.

Role

Key Features

BLDC Motor Driver
Figure 2: Complete Schematic

DRV8353RSRGZT Smart Gate Driver

BLDC Motor Driver
Figure 2: DRV8353RSRGZT Motor Driver Schematic

The DRV8353 was selected as the core of the gate drive subsystem after an extensive review of competing three-phase gate drivers from Infineon, STMicroelectronics, and Monolithic Power Systems (MPS). The key reasons for choosing this part were:

Other options considered included:

IRLZ44N N-Channel Power MOSFETs

The IRLZ44NPBF FETs were chosen for initial prototyping due to the following:

Alternatives explored included CSD18540Q5B and PSMN2R6-40YS, but these SMD FETs would have required reflow assembly and had longer lead times during supply chain constraints.

AMS1117-3.3 Linear Regulator

BLDC Motor Driver
Figure 3: AMS1117-3.3 Schematic

The AMS1117 was chosen over switch-mode regulators (like LM3671) for simplicity and EMI cleanliness. Key considerations:

CP2102N USB-UART Bridge

BLDC Motor Driver
Figure 4: CPS2102N USB Bridge Schematic

This chip enables host PC debug access without needing a separate USB debugger. Advantages included:

SN65HVD75 RS-422 Receiver

BLDC Motor Driver
Figure 5: SN65HVD75 Schematic

Encoder lines were routed differentially to maximize noise immunity. The SN65HVD75 supports:

Chosen over basic line receivers like 74HC14 due to superior line termination and ESD protection.

Hall Sensor Input Block

BLDC Motor Driver
Figure 6: Hall Sensor Schematic

Passive RC filters and series diodes were used to protect against voltage overshoot from back-EMF coupling. Signal levels were tuned to STM32-compatible 3.3V logic, with fast edge rates preserved for accurate edge timing on digital inputs.

PCB Layout Strategy

BLDC Motor Driver
Figure 2: Complete PCB

Placement Philosophy

Power and Signal Domain Separation

The most fundamental layout decision was to physically and electrically separate the high-power switching stage (VM, phase outputs, FETs, DRV gate paths) from the sensitive low-voltage digital subsystem (UART, Hall, encoder, 3.3V logic). This reduced noise coupling between high di/dt transitions and slow digital edges, minimizing the potential for false triggering or EMI radiation.

Loop Area Minimization

Minimizing the loop area of high-current paths is essential for both electromagnetic compatibility and efficiency. The switching loop formed between high-side FET, phase node, low-side FET, and GND return was routed as compactly as possible by placing FETs close to the DRV IC and directly connecting them using wide polygon pours. The return path was mirrored on the second layer with extensive stitching vias to complete the loop and minimize inductance.

Polygon Pour Hierarchy

BLDC Motor Driver
Figure 2: Complete PCB with Combined Pours

Placement by Functional Zone

This separation facilitated focused routing, minimized net crossover, and ensured each subsystem had clean ground and power return paths.

Gate Trace Routing

Gate traces were kept as short as possible and directly routed from the DRV outputs to the MOSFET gates. Gate resistors were placed within 2mm of the gate pin to suppress ringing and overshoot. Layout included:

SW Node Shielding

The SW node (phase switch point) is a major source of EMI. To prevent its radiation from coupling into feedback lines or MCU I/O, the SW node was:

Differential Pair Integrity (Encoder Lines)

The A/B encoder lines are routed as matched-impedance differential pairs from the SN65HVD75 input header to the MCU header. While this isn’t a true controlled-impedance board, traces were routed symmetrically, with equal lengths (within 20 mils), and referenced to continuous GND on the lower layer to suppress common-mode conversion.

Decoupling and Bypass Cap Layout

Every power input to the DRV8353, CP2102, AMS1117, and encoder receiver includes both bulk and high-frequency MLCCs. Decoupling caps were placed as close as possible to the power pins (within 1.5mm), and return directly to the nearest GND via. Bypass cap paths were verified for low ESL with direct routing to copper pours.

Silkscreen and DFM Considerations

Connectors are clearly labeled by function and polarity to assist integration. Designators do not overlap with via arrays or copper pour edges. Mechanical mounting holes were left with a soldermask ring and pullback clearance for potential standoff hardware.

PCB Stackup and Materials

2-layer FR-4 1.6mm board with ENIG finish. Copper thickness: 1oz by default, with selected regions (FET outputs, power plane) enlarged to 2oz via custom fab specification for higher current handling. Next version will move to 4-layer with internal GND and VM planes to further isolate return paths and boost thermal spread.

Challenges

Developing a high-performance motor driver board involves navigating a range of challenges — electrical, mechanical, layout, and systems integration. This section presents a deep analysis of the issues encountered throughout the design and build process and how they were methodically resolved or mitigated. The goal was not just to “get it working,” but to build a scalable, stable system grounded in sound engineering principles.

Switching Noise Coupling into Signal Traces

Problem: Initial tests revealed erratic behavior on Hall sensor outputs and encoder lines during high-speed PWM switching. The suspicion was cross-coupling from high dv/dt phase transitions into signal traces.

Root Cause: Poor spacing between high-side FET drain (SW nodes) and low-speed digital lines, along with a lack of shielding.

Solution:

Ground Bounce and Shared Returns

Problem: Noise was seen on the 3.3V rail under load. The DRV’s gate current return was shared with the CP2102 UART ground plane, causing UART disconnections at startup.

Root Cause: Mixed analog and switching ground domains tied too closely without segmentation.

Solution:

Power Dissipation in FETs During Stall

Problem: Under test stall conditions (low PWM frequency), MOSFET temperatures rose rapidly.

Root Cause: Gate drive strength was not optimized for switching speed, leading to increased overlap losses and conduction heating.

Solution:

Debug Interface Instability

Problem: UART messages were occasionally corrupted or dropped, especially during commutation events or rapid SPI updates.

Root Cause: CP2102 shares power rail with noisy DRV logic current and analog front end; voltage dips created brownouts.

Solution:

DRV Fault Response Tuning

Problem: DRV8353 entered fault mode intermittently under load without clear diagnostic reason.

Root Cause: Overcurrent detection was set too aggressively and did not account for motor inrush at spin-up.

Solution:

Silkscreen and Assembly Errors

Problem: During initial assembly, polarity errors were made due to ambiguous silkscreen on power and Hall sensor headers.

Solution:

Design Review and Iteration Strategy

Each board revision was reviewed against a checklist for thermal performance, functional isolation, testability, and integration. Key changes across versions:

Tools

Future Work

PPO Deployment for BLDC Motor Driver

I have also deployed a Proximal Policy Optimization (PPO) policy onto a custom-designed high-voltage BLDC Motor Driver PCB. The system was built to enable real-time closed-loop motor control using reinforcement learning — with a focus on robotic applications such as humanoid locomotion, dynamic actuation, and torque adaptation.

Both the paper and the corresponding model repository can be found in the above links.

System Overview

Training Pipeline

The PPO agent was trained in simulation using Stable-Baselines3 in a custom MuJoCo environment that modeled BLDC motor dynamics and a reference trajectory task. The reward function penalized tracking error, overshoot, and power inefficiency — enabling robust adaptation to varied load conditions and dynamic setpoint changes.

Deployment

After policy convergence, the trained model was quantized and converted to .tflite format. It was deployed to the STM32 microcontroller running a custom embedded interpreter. The system executed inference in real time, taking sensor feedback as input and adjusting torque commands at millisecond intervals.

Key Results

Significance

Deploying PPO directly to an embedded motor driver opens new possibilities for real-time learning-based control in robotics. Unlike traditional PID or FOC loops, the PPO policy adapts to disturbances, load shifts, and changing trajectories without re-tuning. This design enables in embedded robotic systems — a critical capability for next-generation actuators in humanoids, manipulators, and mobile robots.