Power Distribution Board Sub-circuit Design

From a previous post, here are some more detailed descriptions of the circuits I described.

Input Protection:

The first step in creating this circuit is to choose a MOSFET that is suitable for the job. In this case, I needed to find a MOSFET that is tolerant of at least 48VDC, has a very low drain-source resistance to minimize power loss and heat generation, and can allow up to 30A to pass through it continuously. Switching speed is not important because the input on the gate is a step input. This meant that gate capacitance and furthermore, gate charge, isn’t particularly important because it only has to switch on and off once during the operation of the rest of the ROV. This would be a different story if the input was meant to be switching at 1MHz.

The zener diode was then chosen based on the gate voltages tolerable by the P-channel MOSFET, -2 to -20V. To be somewhere in the middle of this range, I chose a 12V zener diode so that the gate voltage would always be -12V.

Physically, that is, in describing the physics within the operation of the circuit, it relies on the voltage potential between pin 3 (the drain of the MOSFET), and pin 1 (the gate of the MOSFET). Here’s what a simulation of that looks like: (modeled on falstad.com/circuit/circuitjs.html)

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In the first version, everything is working because there is a negative voltage across the drain and gate of the MOSFET, allowing it to function. In the second version, there is no negative potential across the drain and gate, and the MOSFET cannot turn on. This forces an open circuit condition, and no current can flow meaning that the electronics connected to the output won’t be damaged.

Alternatively, a “sister” circuit can be used on the low side that utilizes an N-Channel MOSFET and a very similar topology.

5V Converter:

Here, I’ll go through the variable feedback resistor circuit consisting of R1,R2, and RV1. (It’s a simple voltage divider)

Pulling an equation from the datasheet:

I used this, and standard resistor values to create a workable, and adjustable voltage range. Using readily available 2.2kOhm for Rfbt and 470-Ohm for Rfbb, the output becomes 4.5446V. This is not as important as the higher side. Adding in a 500-Ohm resistor on Rfbt makes the output voltage 5.3957V. This is under the absolute maximum voltage for many of the chips on the board, and will be set to about 5.25V for powering the microprocessor on the ROV to prevent undervoltage issues that have been noted before when using a 5.0V rail.