Reverse protection circuit design

Reverse protection is very important in the designed circuit. Reverse protection is in two ways. Enabling the circuit not to accept reverse voltage at the input and disabling any reverse current to flow into the input power supply terminals. This is achieved by two ways. Using a simple Schottky diode or using a MOSFET. Both techniques have their inherent advantages and disadvantages and it is at the discretion of the designer to choose a particular technique. It is always understood that at the system level special care has been taken from the visual point of view and mechanical point of view to avoid any such reverse power supply connectivity. At the debugging phase and validation phase most of the damage happens and in some systems where special care is not taken at system level because of cost constraints, it is always important to have this protection. Also, for the system to pass through rigorous standards it is always important to have these all protections. For example, in automotive industry, ISO16750-2 standard specifies the reverse polarity test. The standard talks about system withstanding reverse voltage for a specific period.

We will look at the approach to using a MOSFET to design a reverse polarity protection circuit:

The primary intention of reverse protection circuit can be understood by the below block diagram:

To overcome the two scenarios in the above circuit, we have to design a reverse polarity circuit. 

Step 1:

The first choice in designing MOSFET based protection is to come up with a conclusion on the type of FET to be used, whether it is p-channel or n-channel. P-channel must be the preferred one here as P-channel becomes ON when the VGS is negative. So, a negative terminal of power supply connected to gate and positive terminal connected to source gives a simply powered ON MOSFET. When the battery is in reverse, the VGS becomes positive and can't drive the p-channel MOSFET thus cutting off load to the circuit. So, from above analysis, definitely, p-channel mosfet is the choice. The only disadvantage of p-channel is that the RDSON of it is always more than n-channel mosfets. So, n-channel mosfets used with LM74610 is another design approach.

Step 2:

If in the above block diagram, load current in IL and resistance of RDS(ON) for the mosfet, then voltage drop of RDS*IL happens across the MOSFET which becomes significant only if the RDS is very high.

Step 3:

From steps 1,2 above, it is preferable to use a n-channel FET with desirable drive circuit. In case, where the drop across FET is not a criteria following is the circuit that can be designed.

You can see from the above analysis that voltage of 5V is seen as the FET "T1" is ON.

With battery reversed, the voltage is not seen across the load R1.

Main advantages of MOSFET based reverse polarity protection:

• Mosfets with a lower RDS(ON) are available in the market.