I understand how to use P-Channel and N-Channel FETs for reverse polarity protection. However, this really only works on a 2 wire (V+ and V-) design. Here's my situation.
I have Multiple V+ lines (V1+ and V2+) and a single V- (GND) line.
Voltage input range is up to 35 Volts in.
Max current will be 4 Amps (@ 10V) . (So I cannot use DIODES for my reverse polarity - Too much heat loss and too expensive for low reverse leakage current at elevated temperatures above 85C)
Both V1+ and V2+ can be turned on independently and can both be on at the same time if so desired.
If V1+ is ON with 30V, I cannot have the 30V voltage leak onto V2+.
I must maintain reverse polarity protection of course.
Here's the difficult part: Both V1+ and V2+ come together to a common node AFTER the reverse polarity protection circuits. (simple with diodes, but again I cannot use two diodes.)
Is this even possible with FETs? Any help would be greatly appreciated.
If each of your input voltages has a max current rating then use shunt connected reverse bias diodes to protect the inputs.
If an input is reverse connected the fuse blows.
Positive inputs will have an input fuse.
On the load side of the fuse you connect a diode, cathode to positive and anode to zero volts.
This way the diodes are reverse biased when polarity is correct, if a polarity is reversed then the diode will be forward biased and conduct, this will blow the fuse and protect the load.
The diodes are only conducting in fault mode so no losses.
Unfortunately, there is no access to the circuitry once built. It is potted with epoxy. I should have mentioned. So no fuses.
The reason I have two different input power wires is to activate a different mode depending on which wire is supplying the power. If I turn switch A on, product behaves in "A" mode. If I turn on switch B, the product behaves in "B" mode. There is a divider before the reverse polarity circuit for a microcontroller to differentiate which power wire is being used. The problems I am having in my FET design is that both FETs turn on when I turn on switch A, which then causes the microcontroller to think both switches (A and B) are on simultaneously.
This hasn't been a problem with my lower powered product because "current" was minimal and I could just use diodes as my reverse polarity protection. But now I am dealing with much higher currents (up to 4 Amps) and to use a diode with a Vf of say 700 mV would need to dissipate 2.8 Watts. That's unacceptable for this design and the cost of a diode that can handle that much current and have a very low reverse current leakage of less than 200 uA at 85 degrees C breaks the budget. Not to mention the size of the diode. This product is only a couple inches big with 60 components already on the one sided PCB.
So I need to figure out a FET solution for two input power wires that then come together to a common node after the reverse polarity protection circuit. simple right? I'm hoping I am just missing something, like a magic component that uses sorcery to work.
In the meantime, here is a nifty circuit I recently saw, that automatically switches the power supply polarity if incorrect. The voltage across the load is always positive, regardless of the polarity of the voltage applied to IN1 and IN2. Of course, the thresholds of the mosfets must be chosen appropriately. The original circuit used the Fairchild FDS8958A, which includes a matched pair of PMOS and NMOS transistors in one package.
jremington:
In the meantime, here is a nifty circuit I recently saw, that automatically switches the power supply polarity if incorrect. The voltage across the load is always positive, regardless of the polarity of the voltage applied to IN1 and IN2. Of course, the thresholds of the mosfets must be chosen appropriately. The original circuit used the Fairchild FDS8958A, which includes a matched pair of PMOS and NMOS transistors in one package.
Any idea what the advantage is over using several diodes to achieve the same thing?
That FET circuit looks neat but I've never been a fan of connecting FET gates (or any pin really) directly to the big bad world.
Not sure how PTCs would help.
Anyhow, there are TVS diodes, Ferrites, and Inductors upstream of the reverse polarity circuit so there is some protection to the big bad world. Also, the existing FET solution I currently use on other designs that use only a 2 wire input (V+ and V-) have a resistor on the gate as well as a 12V zener so my Vgs does not exceed ±20V.
A FET solution for a 3 wire input (V1+, V2+, and V-) would be nice because I need high efficiency and using the DIODES would completely destroy efficiency. Currently I would need to use two $0.70 diodes to achieve the current handling and reverse leakage requirements. I'm using a POWER DI5 package. It works, but gets over 100 degrees C when more than 3 Amps are being applied. There's a processor monitoring temperature and will regulate power so the circuit does not overheat. Using the Diodes is just not a good idea in my design.
It seems that since I have not been told I am an idiot that this is not an easy solution using the FETs. I just wasn't sure if there was a design out there that was a common solution for this predicament. I may still be an idiot however
Instead of the fuses mentioned above, so no access is required for resetting.
Sounds like you have a working solution with the FETs though, may as well stick with that unless a FET expert come out of the woodwork with a reason it's no good.
HI, it looks from here that for the FET system to work, IN1 and IN2 must be a floating supply as the description says the FET array switches the more negative of the IN1 and IN2 to the load GND.
Tom... nifty arrangment if that degree of protection needed.
If space was not a problem a polarity sense circuit and relay, but we can't have it all.
Graynomad:
Instead of the fuses mentioned above, so no access is required for resetting.
Sounds like you have a working solution with the FETs though, may as well stick with that unless a FET expert come out of the woodwork with a reason it's no good.
Rob
I have a working solution for a 2 Wire design. That is, V+ (Positive Terminal of power supply up to 30V DC) and V- (Negative terminal). So a P-Channel or N-Channel FET with resistor and zener works just fine. However, my conundrum is that I need a reverse polarity protection design (using FETS) for a 3 wire design. That is, V1+ (Positive Terminal of power supply "1"), V2+ (Positive Terminal of Power Supply "2"), and V- (Negative Terminal of Power Supply).
So far I know of no FET solution that provides reverse polarity protection while isolating V1+ from V2+.
