[P-Channel MOSFET as polarity protection diode with gate open[1]

I am using MOSFET as diode for polarity protection of system. Vehicle supply of 12V is applied to drain & system is connected to source. Gate kept open purpose. I have simulated attached schematic on LTSPICE with following graph as output. It works as diode as expected even if gate is floating.

I want to know what will happen in real case if gate is kept open / floating?

I have modified circuit referred in Application Note to check how circuit works in simulation

Application Note: Infineon - enter link description here

Part No. (RRS140P03FRA) - P Channel Enhancement type MOSFET

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  • \$\begingroup\$ What is the purpose of doing this? A MOSFET is not optimized to be a diode even if it includes a parasitic diode in its design. Leaving a sensitive floating gate is never a good idea. \$\endgroup\$ – Edgar Brown Nov 14 '18 at 13:52
  • \$\begingroup\$ @EdgarBrown I just wanted to know if MOSFET will act as optimized diode if gate kept floating. What will be consequences of gate kept floating in this case? \$\endgroup\$ – Akky Nov 14 '18 at 13:55
  • \$\begingroup\$ Why not just use a diode? \$\endgroup\$ – Finbarr Nov 14 '18 at 13:59
  • \$\begingroup\$ @Finbarr Diode have high losses compared to MOSFET \$\endgroup\$ – Akky Nov 14 '18 at 14:01
  • \$\begingroup\$ Depends on the diode. \$\endgroup\$ – Finbarr Nov 14 '18 at 14:01

Leaving the gate open makes the MOSFET highly susceptible to noise due to the extremely high input impedance.

If you want to use the internal body diode of the MOSFET then you should short gate to source. Please note that this configuration is used in ICs as an ESD protection.

If you want to make a reverse polarity protection then that configuration is not the correct way. Here's the correct way instead:


simulate this circuit – Schematic created using CircuitLab

How this circuit works? When the system is energized "properly", current first flow through the body diode until the voltages induced across the source and gate. Then the MOSFET turns on and shorts the body diode. From then on, current flows through the MOSFET. If the supply is applied in reverse then the MOSFET will never turn on and body diode will prevent the current to flow. Thus the circuit is protected.

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