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I am working on updating a PCB design by adding ESD protection to the power supply. I see that an experienced engineer at my company has placed a PMOS transistor rather than a series diode, and I'm not sure why.

The PMOS has the drain connected to the power supply, gate connected to ground, and source connected to the load.

What would be the benefits to doing one over the other (PMOS vs. diode)?

Am I way off base thinking that they are interchangeable?

PMOS circuit

Diode circuit

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  • \$\begingroup\$ Lower voltage drop across the device. An on-state FET has a small resistance, while an on-state diode has a (comparatively) large (relatively) constant voltage drop. \$\endgroup\$ – Hearth Oct 22 at 18:09
  • \$\begingroup\$ Could I reasonably use a Schottky diode then for a smaller voltage drop? \$\endgroup\$ – jhandy Oct 22 at 18:11
  • \$\begingroup\$ Yes, this is common. Bear in mind that Schottky diodes have a higher reverse leakage current, too. \$\endgroup\$ – Hearth Oct 22 at 18:12
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    \$\begingroup\$ That is not for ESD protection. It is for reverse polarity protection. \$\endgroup\$ – DKNguyen Oct 22 at 18:22
  • \$\begingroup\$ Doesn't PMOS imply it being used in digital logic circuits, that is, it should instead be p-channel MOSFET (possible shorthand pMOS)? (From the datasheet: "40 V P-CHANNEL ENHANCEMENT MODE MOSFET") \$\endgroup\$ – Peter Mortensen Oct 23 at 11:02
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The purpose of the diode and MOSFET are for reverse polarity protection, not ESD. The diode is more simplistic, but it is lossy as it will drop some voltage across it (0.4 V to 1.0 V, depending on the type). The p-channel MOSFET should have a much lower voltage drop if chosen properly.

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The main reason is just to obtain a lower voltage drop and thus lower power loss in the device. An on-state FET can be modelled as a resistor, ideally a small one (a few mΩ is typical of power devices), while an on-state diode is more like a voltage source.

For instance, say your device draws 100 mA from the supply. A diode with 0.7 V forward voltage at this current would waste 70 mW of power. A FET with a 10 mΩ on-state resistance would waste 0.1 mW.


Worth noting, also, is that this is reverse polarity protection, not ESD protection. ESD protection involves different topologies entirely.

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The MOSFET or diode are probably not being used for ESD protection. ESD protection usually uses a diode (TVS) in a configuration like this:

Enter image description here

Source: https://www.semtech.com/technology/esd-protection

Both circuits provide reverse polarity protection. The diode has more loss (the smallest diodes have 0.2 V, and silicon diodes have 0.7 V. This means the loss is quite high as it is the diode voltage drop multiplied by the current running through the diode.

In many applications this loss is unacceptable (and leads to diode heating) which means a MOSFET is better as it is very low loss (10s of mΩ or less).

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