flybackinductiveprotectionreverse-polarity
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
Consider using an LM311 comparator powered from the available +/- supplies and driving an SSR.
Divide the input voltage down by (say) 11:1 as shown. That gives a 45mV to 4.5V signal at the inverting input of U1 for a 0.5 to 50V input. The non-inverting terminal is held at about 23mV, so input voltages greater than +360mV (allowing for 10mV \$V_{OS (MAX)}\$) will turn the SSR on. Zero or negative input voltages leave the SSR off.
The circuit shown presents a small resistive (11K) load across the input. If that's objectionable, an additional high-impedance buffer could be added ahead of the comparator.
simulate this circuit – Schematic created using CircuitLab
A suitable SSR might be the 60V-rated CPC1907B, which will look like a ~0.06 ohm resistor when it is on and is okay for 3A (6A rating) (a bit more resistance worst-case as it warms up).
The same principle could be used with the comparator driving a heftier SSR, or a mechanical relay, which would allow even lower series resistance. It's also permissible to parallel a few CPC1907Bs with the input LEDs either driven from individual 1K resistors or connected in series with single a lower value series resistor.
Schottky diode- a hefty one such as this. The 1N914 can't conduct enough current to protect the MOSFETs. There are cheaper 10A ones- do a parametric search.
But it would be nicer to use a series MOSFET (or a unipolar TVS and a polyfuse) and protect the product against reverse voltage. Every customer who has a warranty claim denied is a potential source of a lot of bad publicity, even if it was entirely their negligence that caused the issue.
Best Answer
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.