Is it possible to block current flow in one direction like a simple diode using FET transistors to avoid the voltage drop?


simulate this circuit – Schematic created using CircuitLab

I am not talking about reverse polarity protection. With VS>0 and Vbat>0 if Vbat>VS the current flow from battery to power supply. I want that the current only can flow from power supply to battery, so always Ich > 0.

I am reading this document from Texas Instruments: http://www.ti.com/lit/an/slva730/slva730.pdf But I don't understand how figure 4 works. How the gates of P-channel MOSFET are driven? The FET configuration is symmetrical, how is it possible to block only the current in one direction? enter image description here

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    \$\begingroup\$ You can do it like this: Almost a duplicate 1, almost a duplicate 2. \$\endgroup\$ Oct 18, 2017 at 22:18
  • \$\begingroup\$ "3.2 Dual FET / Back-to-Back: (figure 4.) Using back-to-back FETs is a powerful option, since it offers current blocking in both directions" \$\endgroup\$ Oct 18, 2017 at 23:39

1 Answer 1


You ARE talking about reverse polarity current protection, you said:

".. if Vbat>VS the current flow from battery to power supply."

This is a reversed polarity current.

Every MOSFET has a parasitic body diode structure. So even when it is turned OFF, it doesn't block the current if it goes in opposite direction. To completely block any current in OFF state, two transistors are needed, which is illustrated in the Figure 4.

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    \$\begingroup\$ I think that's reverse current protection, not reverse polarity protection. Reverse polarity is when you just invert the "+" and "-" signs of the battery. They might not always be exactly the same thing. Think of a battery charging circuitry: you might want reverse polarity protection (the battery is disconnected if it is mounted reversed) but you might still want direct (charging) or reverse (power delivered to the load) current. \$\endgroup\$
    – next-hack
    Oct 19, 2017 at 5:45
  • \$\begingroup\$ @next-hack, you are right. Will try to edit this in. \$\endgroup\$ Oct 19, 2017 at 6:17
  • \$\begingroup\$ @Ali Chen Yes I am talking about reverse current flow protection, no reverse polarity because VS>0 and Vbat>0. With two transistors of course the current can be blocked in both directions. But I don't want to block both directions, I would like to block only current from battery to power supply, allowing to flow in normal way from power supply to battery (Ich>0). So who decides when the transistors are ON or OFF? \$\endgroup\$
    – D. Arius
    Oct 19, 2017 at 14:24
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    \$\begingroup\$ @D.Arius, the gates are usually controlled either by a primitive comparator between VS and VBAT as Harry Svensson noted electronics.stackexchange.com/q/223935/117785, or controlled by electro-mechancal switch in barrel jack that is usually found on VS side of devices. \$\endgroup\$ Oct 19, 2017 at 18:20

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