I am working on a PMOS-based reverse polarity protection circuit for a lithium thionyl chloride cell, and I have a few doubts about the necessity and sizing of certain components — specifically, a TVS diode and a resistor (R1) in the gate drive path.
From my LTspice simulations, the circuit seems to function correctly even without R1 and the TVS diode. However, I have seen designs include them, and I want to understand their purpose better.
1. Will the circuit function properly without R1 and the TVS diode?
In simulation, the PMOS reverse polarity protection circuit works fine without them. Are these components necessary for real-world reliability or protection?
2. If R1 is required, how should its value be calculated?
The resistor is often seen in series with the PMOS gate. Is it used for limiting inrush current, protecting against gate-source ESD, or something else?
3. What is the purpose of the TVS diode, and how should its breakdown voltage be chosen?
My assumption is that the TVS diode clamps the gate-source voltage in case of an overvoltage or reverse polarity scenario.
If we're protecting against accidental insertion of a higher voltage battery (e.g., above 3.6 V), should the TVS breakdown voltage be around 3.6 V? However in this circuit the TVS breakdown voltage is 10V.
How should I go about selecting the appropriate TVS diode specifications (standoff voltage, clamping voltage, power rating)?
Any guidance on best practices and design considerations for these components in such a circuit would be highly appreciated.
R1 can help in switching noise reduction, however not a problem here.
R1 can be 100R to 200R with no detrimental effects.
If Vgs Maximum is an issue, an appropriate zener from source to top of gate resistor can be added.
Used to snub out high voltage noise spikes/ESD, to prevent damage do to . . .
You want the Stand-off Voltage to be higher than the applied voltage.
Clamping voltage less than the voltage where your circuit might be damaged.
Current, 10-30A will probably be okay.
For use in something like an automotive environment, you would certainly choose a TVS diode to replace the low power Zener diodes D1 and D2. Selection guide
You would only need the gate resistor if you are using a zener diode to clamp the G/S voltage if your incoming supply voltage is greater than the maximum G/S voltage of the mosfet, as shown at the end of the video. Otherwise that resistor serves no purpose.
The back-to-back 10V zeners have nothing to do with reverse polarity protection. You would use them or something like them if your power supply is spikey, which it probably isn't if your supply is a battery.
Here is a classic reverse polarity protection circuit which uses a PMOS.
Is there anything fundamentally wrong with the concept or has there been some misunderstanding here ?
