Mapping Body-Diode Paths in a Reverse-Polarity MOSFET Circuit

A MOSFET used for reverse-polarity protection must be oriented for its intended startup path. The schematic symbol, footprint pin assignment and intrinsic diode all matter. A circuit can look correct after the transistor turns on while behaving incorrectly during insertion or external backfeeding.

Draw the off-state path first

Analog Devices’ reverse-battery example shows a P-channel arrangement in which correct polarity initially forward-biases the body diode, allowing the source voltage to rise and the device to turn on. Reversing the battery blocks that initial path in the stated circuit.

Redraw the selected transistor as its body diode plus an open channel. Label actual drain, source and gate pins from the component data sheet. Then trace correct insertion and reversed insertion using the real connector pinout, including any alternate ground or supply connection.

Calculate gate stress by state

List gate-source and drain-source voltages during normal operation, reversed input, disconnected input and externally energized output. Use voltages measured between the actual device terminals. A gate tied to board ground does not necessarily experience a small gate-source voltage.

For an illustrative 12 V input system, a source near 12 V and gate at ground produce approximately minus 12 V gate-source voltage. Whether that is permitted depends on the selected device and transient conditions. This arithmetic is not a recommended gate-drive level.

Include the other power sources

Identify programming cables, communication connectors, backup supplies and charged output capacitors. Establish whether any can raise the protected output while the main input is absent or reversed. Repeat the diode and channel-state analysis for those combinations.

Do not equate reverse-battery protection with guaranteed reverse-current blocking. Decide which directions of current the product must prevent in each state, then select a topology explicitly intended for those requirements. A single transistor may not provide every required disconnect function.

Verify the board implementation

Check the symbol-to-footprint mapping and inspect the fitted package orientation before powered testing. Use a current-limited setup with representative output loading and capacitance. Capture input, output and gate-source voltage during insertion and removal, observing the applicable ratings throughout.

Retain a state table with the expected diode conduction, gate bias and allowed current direction. Include external cable states in the test record. This gives reviewers a concrete basis for evaluating substitutions and makes it easier to distinguish a reversed footprint, an insufficient gate clamp and a topology that cannot meet the requested power-path behavior. Label the measurement reference on every captured voltage trace.