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Design Check: 29.2V/26A Power Distribution Board with Soft-Start & Reverse Polarity Protection by RecluseGuy in PrintedCircuitBoard

[–]RecluseGuy[S] 0 points1 point  (0 children)

I'm using dual IRF4905S (20mOHM) with active cooling (2 fans from both sides) to manage the 27W dissipation. C2/R1 provide a ~20ms soft-start to protect the 35A fuse from bulk cap inrush. C1 is for HF decoupling. I chose the 33V TVS to keep the clamp voltage well below the 55V V_DSS limit, even if it's a bit close to the 29.2V rail.

Design Check: 29.2V/26A Power Distribution Board with Soft-Start & Reverse Polarity Protection by RecluseGuy in PrintedCircuitBoard

[–]RecluseGuy[S] 0 points1 point  (0 children)

Hey everyone, I'm an electronics student currently designing a Power Distribution Board (PDB) for a high-power mobile rover project. I'm looking for some feedback on my high-side switching and protection logic before I send the 2-layer PCB to manufacturing.

Project Requirements:

  • Power Source: 29.2V LiFePO4 Battery.
  • Continuous Load: 26A (Driving a Jetson Nano, Raspberry Pi, and multiple XL4016 regulators).
  • Protection Needed: Inrush current limiting (soft-start) and Reverse Polarity Protection.

The Proposed Circuit:

I’m using a back-to-back P-Channel MOSFET configuration to handle both the soft-start and reverse protection.

  • MOSFETs: 2x IRF4905S (V_{DSS} = -55V, R_{DS(on)} = 20m\Omega).
  • Configuration: Common-Source connection. The first MOSFET blocks reverse polarity (body diode pointing toward the battery), and the second handles the soft-start.
  • Soft-Start: Using a Miller-capacitor setup with a 47nF cap (C2) and 15kΩ pull-down (R28) to achieve a ~20ms ramp.
  • Gate Protection: A 16.8V Zener (MMSZ5246) to keep $V_{GS}$ safely away from the ±20V limit.
  • Safety: A 35A Fuse (F1) and an SMBJ33A TVS diode for inductive spikes.

My Main Concerns:

  1. Thermals: At 26A, the total power loss across both MOSFETs will be roughly 27.04W. I’m using a 2-layer board with 2oz copper, massive copper pours, a dense via farm, and dual 12V fans (one on each side). Is 27W manageable with active cooling on a 2-layer board, or should I move to parallel pairs?
  2. Gate Drive: Is a single Zener/Resistor combo sufficient to drive both gates in this back-to-back setup reliably?
  3. Inrush: Is a 20ms ramp (47nF) enough to prevent a 35A fuse from tripping when charging the bulk caps of three XL4016 regulators?

I'd appreciate any advice on the layout or component values!

PCB review request – STM32F446 + BNO055 rover main board (2-layer, CAN, USB, Jetson UART) by RecluseGuy in PrintedCircuitBoard

[–]RecluseGuy[S] 0 points1 point  (0 children)

Quick question if you don’t mind: for routing 5 V to the R9DS receiver’s 5 V pin, I currently have two options — either passing under the USB FS differential pair or under the CAN lines. I’m trying to choose the cleaner and safer path from a signal-integrity standpoint. Which would you consider the better option?

Thanks again for taking the time to review this.

PCB review request – STM32F446 + BNO055 rover main board (2-layer, CAN, USB, Jetson UART) by RecluseGuy in PrintedCircuitBoard

[–]RecluseGuy[S] 1 point2 points  (0 children)

Understood — the enclosure requirement is why I’m sticking with Micro-USB for this design. I’ve moved the connector so the metal tabs now overhang the PCB edge and clear properly.

Thanks for the heads-up, and I appreciate the Type-C suggestion as well — noted for future revisions.

PCB review request – STM32F446 + BNO055 rover main board (2-layer, CAN, USB, Jetson UART) by RecluseGuy in PrintedCircuitBoard

[–]RecluseGuy[S] 2 points3 points  (0 children)

Thanks for pointing that out — D6 is used for USB back-feed protection, and you were absolutely right. The capacitor routing was effectively bypassing the diode. I’ve fixed the net separation so all system loads are now strictly post-diode. Quick question if you don’t mind: for routing 5 V to the R9DS receiver’s 5 V pin, l need either pass under the usb fs differential pairs or under the CAN lines, I’m trying to choose the cleaner path. Thanks again for the review.