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I'm trying to achieve reverse polarity protection for the TPL7404LADR chip which is basically an updated version of the ULN2003 darling transistor array. To do this I want to use a P Channel MOSFET but I'm worried about the effects this will have for fly back current protection. If an inductive load (say a solenoid) is switched from ON to OFF will the P Channel MOSFET effectively allow the chip's internal free-wheeling diode to carry the fly back current through to the +24V source?

Thanks

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  • \$\begingroup\$ Any reason you can't connect the load to the output side of the MOSFET, rather than directly to the power supply? \$\endgroup\$ – Bruce Abbott Apr 18 at 21:01
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To be completely safe you could pick a MOSFET that can handle the load (solenoid) currents, then the free wheeling diodes will be connected across the loads.

Aside from the effect of the inductance under pathological conditions, reversing the power will energize the solenoid through the body diode of the NMOS transistors. Maybe I missed it, but I don't see a specification for the maximum reverse current through an output (the 0.3V negative voltage will be exceeded). Since the voltage drop of the body diode is considerably higher than the NMOS MOSFET when on, most likely, the current handling capability may be considerably less in the reverse direction.

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Theoretically, the FET will be ON and conductive in both directions as long as the 24V is present (with correct polarity). So it should allow the current to pass through the flyback diode and channel of the FET to the 24V.

If the 24V were shorted during a fault and the inductor still had energy, it could avalanche the FET, so maybe look for an avalanche energy rated FET that can handle the max energy in the inductor.

From your diagram it's not clear how and when the inductor is switched in and out, since it's right across the supply when the switch is closed and would saturate in the DC condition, but presumably you're switching with some duty cycle to avoid that.

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