DC Reverse polarity protection using P-MOSFET

In summary: Using a zener diode to clamp the gate-source voltage makes sense. With a generic FET, it's possible to find one with lower on resistance.
  • #1
Jdo300
554
5
Hello All,

I'm designing a circuit board that needs to run on a 12VDC supply and will need to supply a fair amount of power to drive a bunch of 12V relay coils, in addition to some electronics. Since I want to keep the voltage drop on the supply a small as possible, I thought about using a P-CH MOSFET to make an active diode on the input of the board to prevent damage if the user plugs the power in backwards.

Using this method, I would have the circuit setup like the attached picture, however, the limitation here is that the input voltage is limited to 20V because of the MOSFET gate. Under normal operating conditions, going over 20V should never be a problem for this particular design, but I'm curious to know if there are any clever ways to allow the reverse voltage to go above 20V without harming the gate (using zener diodes and resistors or something). I've tried to come up with a few ideas but so far nothing obvious sticks out. Does anyone know of any simple solutions to this problem that don't involve using any fancy ICs?

Thank you,
Jason O
 

Attachments

  • Reverse Protection Circuit.PNG
    Reverse Protection Circuit.PNG
    1.8 KB · Views: 1,440
Engineering news on Phys.org
  • #2
Something like this should work.
 

Attachments

  • testcircuit.png
    testcircuit.png
    2.1 KB · Views: 4,758
  • #3
Why make it more complicated than it needs to be. Just put a full wave rectifier on the input.
 
  • #4
skeptic2 said:
Why make it more complicated than it needs to be. Just put a full wave rectifier on the input.

Hi Skeptic. The OP stated that the object of the mosfet was to avoid the diode drop and increase efficiency. A full wave rectifier would add two diode drops, the exact opposite of OPs stated purpose.

Of course the op could have used a schottky diode to make some improvement in efficiency, but the mosfet used as an an "active" diode can be even more efficient, and can actually have a forward bias volt drop very close to zero.
 
  • #5
uart said:
Something like this should work.

It seems like it would work without the zener and 10 K resistor.
 
  • #6
Hi uart,

Thank you for your reply, I actually drew a circuit very similar to this one but missed one small detail. This is exactly what I was looking for though.

Thank you!
Jason O
 
  • #7
skeptic2 said:
It seems like it would work without the zener and 10 K resistor.

The point of adding the zener diode is to keep the magnitude of the gate source voltage from exceeding 20V if a higher supply voltage is desired for the load.

- Jason O
 
  • #8
skeptic2 said:
It seems like it would work without the zener and 10 K resistor.

But w/o the 10k resistor & Zener diode, the P-FET may never turn on, & the conduction would be via the body diode, which has a forward voltage drop as large as a p-n junction device. The 2 parts in the above diagram insure that the P-FET gate is biased on, the Zener clamps Vgs, & the resistor limits Zener current to a safe value. Conduction is via the drain-source channel of the P-FET, & the forward voltage drop is typically well below that of a diode, even below that of a Schottky. I've measured well below 0.100 volts, & in some cases even as low as tens of millivolts depending on Rdson & forward current. This approach is well proven, & I've used it many times.

Claude
 
  • #9
I ran it in LTSpice both ways, by putting the gate-source across the load and also putting it across the supply. I like putting it across the supply better but it works both ways. Good idea about using the zener to protect the gate-source voltage. The voltage drop across the PMOS FET was about 0.78 or about the same as a single diode. However I used a generic FET and it should be possible to find one with lower on resistance.
 

1. What is DC Reverse polarity protection using P-MOSFET?

DC Reverse polarity protection using P-MOSFET is a circuit protection mechanism that prevents damage to electronic devices by blocking current flow in the opposite direction. It uses a P-MOSFET (positive-channel metal-oxide-semiconductor field-effect transistor) to act as a switch and prevent reverse current from flowing through the circuit.

2. How does DC Reverse polarity protection using P-MOSFET work?

The P-MOSFET is connected in series with the electronic device and is normally turned off. When the polarity of the DC input is correct, the P-MOSFET allows current to flow through the circuit. However, if the polarity is reversed, the P-MOSFET will automatically turn on and block the reverse current, protecting the device.

3. What are the advantages of using P-MOSFET for reverse polarity protection?

P-MOSFETs have a lower resistance when turned on, which means they can handle higher currents without overheating. They also have a faster response time compared to other types of transistors, making them more effective in protecting devices from reverse current.

4. Can DC Reverse polarity protection using P-MOSFET be used for all electronic devices?

Yes, DC Reverse polarity protection using P-MOSFET can be used for any electronic device that operates on DC power. It is commonly used in devices such as power supplies, motors, and LED lights to protect them from reverse current.

5. Are there any limitations to using P-MOSFET for reverse polarity protection?

One limitation of using P-MOSFET for reverse polarity protection is that it can only block reverse current up to a certain voltage, called the breakdown voltage. If the reverse voltage exceeds this limit, the P-MOSFET may fail and the device could still be damaged. Additionally, P-MOSFETs can only block reverse current in one direction, so multiple P-MOSFETs may be needed for bidirectional protection.

Similar threads

  • Electrical Engineering
Replies
10
Views
345
Replies
39
Views
3K
  • Electrical Engineering
Replies
1
Views
767
  • Electrical Engineering
Replies
12
Views
1K
Replies
4
Views
776
  • Electrical Engineering
Replies
11
Views
858
Replies
55
Views
3K
Replies
3
Views
271
  • Electrical Engineering
Replies
8
Views
1K
  • Electrical Engineering
Replies
14
Views
2K
Back
Top