Initially I chose a Infinion PROFET smart high side switch (ITS428L2). In the datasheet it is stated that this device can provide reverse battery protection with external resistor. Frankly, I am not able to figure out how this protection should work: The body diode of the internal MOSFET is placed in a way, that current can flow in case of reverse voltage. So where is the protection?


Therefore i tried to figure out a circuit that provide real reverse voltage protection and on/off-switching capabilities for currents in the ~10A region and as well for high currents (~100A). Is this a suitable approach?


simulate this circuit – Schematic created using CircuitLab

Edit: Changed Source and Drain of PMOS2

Looking at the datasheets of serveral p-channel MOSFETs i wondered about the max. Drain-Current value. Sometimes this current is specified negative, sometimes positive - sometimes there are both values given but of different value. I thought if the MOSFET is conducting there is no direction for the current flow?

Example of SUM110P04-05:



2 Answers 2


It seems to me that in the datasheet of the ITS428L2, reverse battery protection only means that the switch will not be destroyed in case of wrong polarity (see on page 8).

Concerning your circuit: even if it protects against reverse battery, you can never switch off the load because regardless of the MOSFET's gate potential the body diodes will always conduct (given that the battery is connected in correct polarity).

When it comes to the direction ID in a MOSFET datasheet you have to pay attention to the mode of operation. If the gate is driven, the channel is defined by the RDSon. Here the direction of the current doesn't matter, as you correctly said. But suppose an inductive load is driving a current through the body diode into the battery when the MOSFET is off. The body diode is a much lossier conductor that a fully turned on channel and therefore the permissive current is significantly less.

  • \$\begingroup\$ Thanks for your answer! 1. I accidently placed the PMOS2 the wrong way round - I changed it in the above schematic. Does it make sense now? 2. So when the MOSFET is turned on, the direction does not matter. That means it is indifferent weather the Drain-current is declared positive or negative in the Datasheet? \$\endgroup\$
    – tobawo
    Commented Jan 4, 2016 at 20:01
  • \$\begingroup\$ @1: Yes, looks good now! \$\endgroup\$
    – christoph
    Commented Jan 6, 2016 at 10:19
  • \$\begingroup\$ @2: I am not sure if it is defined somewhere in the JEDEC standard, when a current in a datasheet is positive or negative. Sometimes you see a note like "currents flowing into device are positive". So a PMOS in it's "natural" connection (S pos vs. D) would have a negative drain current, flowing out of D. But in case of doubt check the footnotes in the datasheets. In the datasheet of the SUM110P04-05 they really seem to mix up the signs, this is an error on the companies part... \$\endgroup\$
    – christoph
    Commented Jan 6, 2016 at 10:26

With a high-current supply source and capacitive load, don't overlook inrush current. This is often managed by ramping turn-on, and can be achieved in the given circuit as follows...

Apply a series resistor from R2/SW1 junction to the gate of PMOS2. Apply a capacitor (significantly greater than Cgs of PMOS2) between PMOS2 gate and drain.

I'll leave you to experiment or do the maths (based on choice of MOSFET and circuit conditions), though a good starting block might be 100nF and 10k+.


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