Flight Controller Board Architecture: Layout & Power
CRITICAL The physical layout of the flight controller determines its electrical noise environment and thermal performance. Choosing between an All-In-One (AIO) and a Stack system is not just about size; it's about physics.
1. Form Factors: AIO vs. Stack
1.1 The AIO (All-In-One) Architecture
An AIO board integrates the Flight Controller (MCU/IMU) and the 4-in-1 ESCs onto a single PCB.
- The Advantage: Compactness and weight. Essential for sub-250g builds and "Whoop" class drones.
- The Thermal Problem:
- Physics: ESC MOSFETs generate significant heat (switching losses). The STM32 processor and IMU are sensitive to heat.
- The Gradient: On an AIO, the heat from the ESCs soaks directly into the IMU. This causes gyro bias drift during the flight as the board heats up.
- The Electrical Noise Problem:
- Physics: High-current switching (40kHz-96kHz) happens millimeters away from sensitive 3.3V logic lines.
- Consequence: AIOs inherently suffer from higher noise floors. They require aggressive software filtering (lower
INS_GYRO_FILTER) which adds latency.
- Repairability: Zero. If you blow one ESC FET, the entire FC is trash.
1.2 The Stack Architecture (FC + ESC)
The industry standard for 5" and larger drones. The FC and ESC are separate boards connected by a ribbon cable.
- The Advantage: Isolation.
- Thermal: The ESC is usually on the bottom, receiving direct airflow. The FC is stacked above it with an air gap. This thermal break keeps the Gyro temperature stable.
- Electrical: The high-current path is confined to the lower board. The upper board remains a "clean" environment for logic and sensors.
- Maintenance: Modular replacement.
2. Power Regulation: The BEC (Battery Elimination Circuit)
The BEC is the heart of the FC's power distribution network. It steps down the main battery voltage (e.g., 25V for 6S) to usable logic voltages (5V, 9V, 3.3V).
2.1 Linear Regulators (LDO)
- How they work: They burn off excess voltage as heat.
- Efficiency: Terrible. Converting 25V to 5V is ~20% efficient.
- Usage: Only used for the final clean 3.3V stage for the MCU and IMU. They are electrically "quiet" (no switching noise), providing the cleanest power for sensors.
2.2 Switching Regulators (Buck Converters)
- How they work: They rapidly switch the input voltage on and off, using an inductor to smooth the output.
- Efficiency: Excellent (>90%). Used for the 5V (RC, GPS) and 9V (VTX) rails.
- The Danger: Switching Noise. A cheap or poorly filtered switching regulator introduces high-frequency noise into the video feed (lines in FPV) and can interfere with sensitive GPS receivers.
- Design Check: High-quality FCs shield the inductor component. Look for a grey square block on the board—if it's shielded, good. If you see exposed copper coils, expect noise.
2.3 The "9V Rail" Myth
Many modern digital VTXs (DJI O3, Walksnail) draw up to 2A.
- The Trap: Many FCs claim a "9V 2A" BEC. In reality, under load and heat, they often voltage-sag, causing the VTX to reboot in mid-air ("blackout").
- The Fix: For high-power builds, run the VTX directly from V-Bat (if supported) or use an external, dedicated BEC (PDB). Do not trust the onboard FC BEC for >15W loads unless verified.
3. Mounting Patterns
- 30.5mm x 30.5mm (M3): The "Standard". Most robust. Large pads for soldering. Fits standard 4-in-1 ESCs. Best for 5" - 10" builds.
- 20mm x 20mm (M2/M3): The "Mini". Now capable of handling high power (60A), but thermal mass is lower. Harder to solder. Good for lightweight 5" or 3-4" builds.
- 25.5mm x 25.5mm (Whoop): Exclusively for AIO boards.