Our lecturer posed us this problem. I just can't get it solved.

Design a circuit using a MOSFET which provides reverse polarity protection and ALSO over current protection. Over current protection should activate when current through a load is more than 2A.

Designing reverse polarity protection is easy, adding the over current protection... I can't get it :(


I will give you a big hint. You will need more than one MOSFET to perform both functions (assuming you wish to block reverse polarity inputs) since the body diode precludes using a MOSFET that blocks reverse polarity from being used to limit current.

  • \$\begingroup\$ I've been faffing around with two MOSFETs (both P and N type) in a single circuit for almost the entire day though and still can't get to grips with it. \$\endgroup\$ Sep 9 '17 at 17:49
  • \$\begingroup\$ A small current sense R with say 50mV drop can be used with some Vref to compare and drive the output FET which has suitable low RdsOn. Reverse protection uses another FET. \$\endgroup\$ Sep 9 '17 at 17:55
  • \$\begingroup\$ I figured that if I used a 1ohm resistor, and a voltage source ramping up from 0-3V (I know my polarity protection works), I would want the second MOSFET to become an open circuit when the input voltage reaches 2V (creating the 2A current through the load). Selecting a P-type made sense to me as it is normally closed between source and drain. Where to connect the source and gate is the issue I have not solved in an entire day haha. I used a Pfet with a threshold voltage of -2V. \$\endgroup\$ Sep 9 '17 at 18:02
  • \$\begingroup\$ You can do it with only a NMOS but it will require a boosted gate voltage, via charge pump for example. \$\endgroup\$
    – bobflux
    Sep 9 '17 at 19:15
  • \$\begingroup\$ Try two back-to-back N-channel. If you're completely stuck peek at LTC's hot swap controllers. \$\endgroup\$ Sep 9 '17 at 21:06

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