Still it’s important to understand why a certain pin configuration exists and how you can create your own. But now let’s begin with the actual schematic.

Power

Input Power Protection

As discussed, the board receives VBAT via the ESC connector. The main user error we want to prevent regarding this is reverse voltage when plugging in the ESC. Without protection this would most likely break core components on the board.

Reverse Polarity Protection

To stop current from flowing when the voltage is reversed, we use a P-channel MOSFET setup. Drain connected to VBAT, source to the rest of the board, and gate connected to GND through a $100\text{kΩ}$ resistor.

In correct polarity, the body diode conducts first, pulling source up to VBAT.

As $V_{GS}$ is negative the MOSFET turns on.

In reverse polarity, battery negative is on the drain and battery positive is on the gate.

A P-channel FET needs a negative $V_{GS}$ to conduct, so it stays off and the board is protected.

VGS Clamping

Most standard FETs have a maximum $V_{GS}$ of $±20\text{V}$. When powering the board from a $6\text{S}$ pack ($25\text{V}$), $V_{GS}$ would exceed this without protection. Therefore we add a Zener diode across source and gate in reverse bias to clamp the voltage. If $V_{GS}$ exceeds the Zener voltage, the diode conducts and clamps $V_{GS}$ to a safe value. A $15\text{V}$ Zener clamps $V_{GS}$ to $15\text{V}$ keeping $V_{GS}$ well within the $±20\text{V}$ limit on any cell count.

The gate resistor is essential here. Without it, the gate would be hardwired to GND, and the Zener would create a direct short from source to GND once it conducts. I learned this the hard way on my first board.

Although reverse polarity protection isn’t necessary for the main functionality of the FC it is still highly recommended.

Overvoltage Protection

Since we're powering the flight controller directly from the same battery that powers the ESCs, there's a risk of voltage spikes exceeding the maximum ratings of our components. If space allows, it can make sense to add a TVS diode to clamp the voltage to a safe maximum.

The TVS diode sits between VBAT (after the reverse polarity protection) and GND in reverse bias. To size it correctly, we need two parameters.

$V_{RWM}$ (reverse stand-off voltage) is the maximum voltage the diode can see continuously without meaningful conduction. This must sit above our maximum nominal battery voltage, otherwise the diode would leak current during normal operation.

$V_{BR}$ (breakdown voltage) is where the diode actually starts clamping. This must sit below the absolute maximum rating of the components we're protecting. $V_{BR}$ is usually specified as a range ($V_{BR_{MIN}}$ to $V_{BR_{MAX}}$).

For our board the max nominal VBAT is $25\text{V}$ and the buck's absolute maximum is $32\text{V}$. A TVS with $V_{RWM} ≈ 28V$ and $V_{BR} ≈ 32V$ fits the window. It stays off during normal operation with some margin above $25\text{V}$ , and starts conducting before the buck sees anything dangerous.

An important note is that even with these specs, clamping isn't instant and isn't perfectly at $V_{BR}$. During a fast transient, components downstream may briefly see voltages above $32\text{V}$ before the TVS fully conducts. But the input capacitors of the buck absorb most of this, so the IC itself will most likely not see the peak.

USB-C

The second power input on our FC is USB-C which we use during configuration. Looking at the symbol and footprint of the USB-C connector, you might notice that most pins occur twice which is the case to allow plugging in the connector in both orientations.