1 Design Overview
The Power Distribution Board (PDB) is the central power management hub for the planting mechanism on the Canopy reforestation robot. Powered from a MARBERO portable power station (110 VAC, 200 W continuous / 270 W surge), an isolated AC/DC converter steps the input down to a 24 VDC main bus. The PDB routes this bus to the NEMA 23 stepper motor driver and the BLDC motor driver, while also generating regulated 9 V and 5 V rails for the microcontroller and encoder logic respectively. Reverse-polarity protection, fusing, and e-stop integration are incorporated on-board to ensure safe field operation.
2 Power Architecture
2.1 Input and Main Bus
The 110 VAC inverter output is converted to 24 VDC via an isolated AC/DC converter. After accounting for downstream conversion losses, the practical continuous DC power budget is estimated at 150–170 W. The 24 V rail is the primary distribution bus and serves as the supply for both high-power motor loads.
2.2 Subsystem Power Rails
| Rail | Load | Regulation | Peak Current |
|---|---|---|---|
| 24 V | NEMA 23 Stepper Motor Driver | Direct from bus (no additional regulation) | 7.0 A |
| 24 V | BLDC Motor Driver | Direct from bus (driver-capped at 30 A) | 30 A |
| 9 V | ELEGOO Mega R3 (MCU) | Mean Well N7809-1C DC/DC buck (8–36 V → 9 V) | 0.1–0.3 A |
| 5 V | Stepper encoder logic | XP Power isolated DC/DC (6.5–36 V → 5 V, 2 A) | 1–2 A |
The ELEGOO Mega R3 is isolated on its own 9 V rail (via the N7809-1C buck) to protect I/O pins from noise on the high-current 24 V bus. The 5 V rail provides a clean logic supply for the stepper’s built-in encoder and may be extended to future sensors (e.g., flex force, soil moisture) for closed-loop control.
2.3 Operating Modes
Because the combined peak draw of both motors exceeds the 200 W source capacity, the system enforces mutually exclusive operating modes. Simultaneous high-power operation is prohibited until power-station overload behavior is verified under stall conditions.
Stepper Mode
Stepper motor enabled, BLDC disabled. Used during horizontal positioning of the planting mechanism via the ball screw actuator.
BLDC Mode
BLDC driver enabled, stepper disabled. Activated once the auger is aligned with the planting location and drilling begins.
3 Protection Circuitry
3.1 Reverse-Polarity Protection
A comparative thermal analysis was performed between a high-side P-MOSFET and an ideal-diode controller (LM5050MK-1/NOPB) driving a low-RDS(on) N-MOSFET (NVMFS5C604NLAFT1G). At the worst-case continuous current of 4.2 A:
P-MOSFET (RDS(on) = 75 mΩ)
Pcond = (4.2)² × 0.075 ≈ 1.32 W
ΔT = 1.32 × RθJA → 66 °C (best case) to 99 °C (worst case)
TJ = 91–124 °C @ 25 °C ambient → unacceptably high
N-MOSFET via LM5050 (RDS(on) = 10 mΩ)
Pcond = (4.2)² × 0.01 ≈ 0.18 W
ΔT ≈ 0.18 × 50 ≈ 9 °C @ 25 °C ambient → substantial thermal margin
The LM5050 ideal-diode controller was selected. It generates the necessary gate overdrive for high-side N-MOSFET switching, achieving robust reverse-polarity protection with significantly reduced steady-state conduction losses compared to a P-channel solution of equivalent footprint. 60 V P-channel devices inherently exhibit higher RDS(on) than comparable N-channel devices in the same package.
3.2 Fusing
An 80 V / 15 A panel-mount fuse holder (F1) is placed in series with the 24 V fused output path to the NEMA 23 stepper connector (J1). A dedicated test point (J4, TP_NEMA_FUSED) provides probe access at this node. A 40 V Schottky diode (D4) provides back-EMF clamping on the fused 24 V rail to protect upstream circuitry during motor deceleration.
3.3 E-Stop Integration
A two-position terminal block (J6, ESTOP_Block) interrupts the 24V_SAFE net when the e-stop circuit is opened. This net feeds the gate drive path of Q1 (N-MOSFET) and the NEMA motor connector, ensuring both high-power loads are de-energized on e-stop assertion. Ring-terminal connectors are required for reliable e-stop wiring; these were pending delivery as of the March 26 bringup milestone.
3.4 Status Indicators
- D1 (LED_24BAT) — 2 kΩ series resistor; indicates 24 V battery input present.
- D2 (LED_NEMA) — 2 kΩ series resistor; indicates 24 V fused rail active.
- D3 (LED_9V) — 680 Ω series resistor; indicates 9 V MCU rail active.
4 Key Components
| Reference | Part | Function | Key Spec |
|---|---|---|---|
| U2 | LM5050MK-1/NOPB | Ideal-diode controller | 60 V, high-side N-FET gate drive |
| Q1 | NVMFS5C604NLAFT1G | Reverse-polarity N-MOSFET | 60 V, RDS(on) = 10 mΩ |
| U1 | Mean Well N7809-1C | MCU supply regulation (9 V buck) | 8–36 V in, 9 V out |
| — | XP Power DC/DC Converter | Isolated encoder 5 V supply | 6.5–36 V in, 5 V / 2 A out, 10 W |
| F1 | FuseHolder_80V/15A | Stepper overcurrent protection | 80 V, 15 A |
| D4 | 40V Schottky | Back-EMF clamp | 40 V |
| C1 | 0.1 µF ceramic | Gate drive decoupling | 50 V, X7R, 0805 |
| R1 | 100 Ω | Gate resistor — Q1 inrush limiting | — |
| R2, R4 | 2 kΩ | LED current-limiting (24 V rails) | — |
| R3 | 680 Ω | LED current-limiting (9 V rail) | — |
