If the power source of the circuit is reversed, for example, connecting the positive wire into the ground and the negative wire into the circuit’s Vcc. Two bad things could happen, either the circuit that we designed can get burned along with all the expensive components in it, or the power source by itself can get destroyed. Things get even more dangerous if the circuit is powered by a battery. Reversing the polarity of a battery is the worst thing that can happen in a circuit because it will not only damage the circuit but can also cause smoke and fire making it a potential threat.
But human error can occur and hence it is the responsibility of the designer to ensure his circuit can handle reverse polarity conditions safely. This is why almost all circuits have an additional safety circuit on its input side called reverse polarity protection circuit. In this article, we will discuss a MOSFET reverse polarity protection circuit that is very efficient to protect the circuit from reverse polarity related damages. The circuit can also act as a battery polarity protection circuit, so the same design guide can be used for protecting your circuits even if it is powered by an external DC adapter or a Battery.
Protecting Circuits from Reverse Polarity
There are several options to protect the circuit from reverse polarity. Most of the time, battery-operated devices use special types of battery connectors that do not allow the battery connector to connect in reverse order. This is a mechanically possible reverse polarity protection for the battery. Another choice is to use a Schottky diode in the power rail but that is the most inefficient way to protect the circuit from reverse polarity.
Using Schottky Diode for Polarity Protection and its Disadvantages
In the below image, a Schottky diode is used in series with the power rail that will get reverse biased during reverse polarity condition and disconnect the circuit. We have also previously discussed this in the Application of Diodes section in a previous article.
The left image is the proper connection of the polarity but the right image is the reverse polarity condition. During the reverse polarity connection, the Schottky diode blocks the flow of current.
But, the above circuit is inefficient due to the constant flow of load current through the Schottky diode. Also, the voltage across the output of the Schottky diode is less than the input voltage due to the forward voltage drop of the diode. So, by using the above method, it will protect the circuit from reverse polarity protection but not in an efficient way.
The proper way to make a reverse polarity protection circuit is by using a simple PMOS MOSFET or NMOS MOSFET. It is advisable to use PMOS because PMOS cuts off the positive rails and the circuit will not get any voltage and there are fewer chances of harmful consequence if the circuit works at high DC voltages.
PMOS MOSFET for Reverse Voltage Protection
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current through it. FETs are devices with three terminals that are source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source. This is the basic thing that is used in P-MOSFET as a switch in reverse polarity protection.
The below figure shows the PMOS reverse polarity protection circuit.
The PMOS is used as a power switch that connects or disconnects the load from the power supply. During the proper connection of the power supply, the MOSFET turns on due to the proper VGS (Gate to Source Voltage). But during the Reverse polarity situation, the Gate to Source voltage is too low to turn on the MOSFET and disconnects the load from the input power supply.
The 100R resistor is the MOSFET gate resistor connected with the Zener diode. The Zener diode protects the gate from overvoltage.
Actual Simulation in Orcad PSPICE
The above circuit has all the required components for reverse polarity protection. The V1 is the source that is in perfect polarity. The P-Channel MOSFET is getting biased from the Resistor 100R and the 6.8V Zener diode 1N4099. The Load is a 10R resistor.
The simulation shows the circuit is working properly in the right polarity of the power supply. The Zener diode protects the gate from the overvoltage and the load is getting 1.3A with 13.9V.
In the above image, the source is reversed. The Load is turned off completely and the circuit is acting as a reverse polarity protector. You can also check out the video below which explains the working of the circuit with simulation:
Selecting MOSFET for Reverse Polarity Protection
It is advisable to use PMOS over NMOS. This is because PMOS is used in the positive rail of the circuit rather than the Negative rail. Therefore, PMOS cuts off the positive rails and the circuit will not have any positive voltage. But, NMOS is used in negative rails, thus disconnecting the negative rail do not disconnect the circuit from the batteries positive rail. Therefore, in the case of high voltage DC, disconnecting the positive rail is much safer than disconnecting the negative rail and there are fewer chances of harmful consequences like short circuits, electrocution, etc. will not happen.
The component selection is an important part of this circuit. The main component is the P channel MOSFET.
A MOSFET has the following specs that are crucial for the circuit.
- Drain Source Resistance (RDS)
- Drain Current
- Drain to source voltage
Drain Source Resistance (RDS):
RDS is the Drain to source resistance. Use a very low RDS (Drain to source resistance) for low heat dissipation and the very low voltage drop across the output. Higher RDS will produce higher heat dissipations.
Drain Current:
This is the maximum current that will pass through the MOSFET. Therefore, if the load circuit requires 2A of current, choose a MOSFET that will withstand this current. In such a case, Mosfet with a drain current of 3A will be a good choice. Choose this parameter larger than the actual required.
Drain to Source Voltage:
Drain to source voltage of the MOSFET needs to be higher than the circuit voltage. If a circuit requires a maximum of 30V, a MOSFET with a drain to source voltage of 50V is required for safe operations. Always choose this parameter larger than the actual required.
During reverse polarity, the MOSFET will be turned off due to the insufficient Vgs, and there will be no effect on the load circuit as well as the MOSFET. The above parameters are required during the normal condition and need to be carefully selected.
Selection of the Zener Diode Voltage:
Each MOSFET comes with a Vgs (gate to source voltage). If the gate to source voltage increases than the maximum rating, this can damage the MOSFETs gate. Therefore, choose a Zener diode voltage that will not exceed the gate voltage of the MOSFET. For a Vgs of 10V, 9.1V Zener diode will be sufficient. Make sure that the gate voltage should not cross the maximum voltage rating.
100R Resistor in the Circuit:
The resistor value needs to be chosen in a way that it should not be high enough to not overheat the Zener, but low enough to provide adequate Zener bias current and to discharge the Gate rapidly if the supply voltage is suddenly reversed. Therefore, here is a tradeoff between the Gate discharge time and Zener biasing. In most cases, 100R-330R is good if there are chances for the appearance of sudden reverse voltage in the circuit. But if there are no chances of sudden reverse voltage during the continuous working of the circuit, anything from the 1k-50k resistor value can be used.
Part Number Suggestion:
The most popular MOSFETs that are used for a wide range of reverse polarity protection-related circuits-
- IRF9530
- IRF9540
- Si2323 (Low Voltage Low Current Operations)
- ILRML6401 (Low Voltage Low Current Operations)
Drawbacks of MOSFET Reverse Polarity Protection Circuit
The main drawback of this circuit is the power dissipation across the MOSFET. However, that can be solved using a P channel MOSFET that has RDS on resistance in milliohms.
FAQs
How does MOSFET select reverse polarity protection? ›
Selecting MOSFET for Reverse Polarity Protection
It is advisable to use PMOS over NMOS. This is because PMOS is used in the positive rail of the circuit rather than the Negative rail. Therefore, PMOS cuts off the positive rails and the circuit will not have any positive voltage.
First get yourself a p-channel MOSFET. Like this FQ P 47 p 0 6. Pay very close attention to which
Which component used for reverse voltage protection? ›Protecting reverse voltage using a diode
If the voltage is reversed, it blocks the voltage and the reversed power does not reach the load. Using the diode is the simplest method and has the advantage of low cost.
Reverse polarity protection is an internal circuit that ensures that the device is not damaged if the power supply polarity is reversed. The reverse polarity protection circuit cuts off power to the sensitive electronic circuits in the transmitter or transducer.
Why MOSFET has no reverse voltage blocking capability? ›As a result the drain current becomes independent of VDS and determined solely by the gate – source voltage VGS. This is the active mode of operation of a MOSFET. Due to the presence of the anti-parallel “body diode”, a MOSFET cannot block any reverse voltage.
How is reverse polarity calculated? ›Anyone can test electrical outlets for reverse polarity. Although there are different ways to test this, one of the easiest methods is using a standard outlet tester with a voltage chart. To test for reverse polarity, you will have to plug in the outlet tester into the receptacle you wish to check.
What is the simplest way of protecting power supplies from reverse polarity reverse current flow? ›Diodes Provide the Simplest Protection
The simplest form of battery-reversal protection is a diode in series with the positive supply line (Figure 1a). The diode allows current from a correctly installed battery to flow to the load and blocks current flow to a backward-installed battery.
Choose a Schottky diode if you can. Schottky diodes have lower voltage drops and are usually better suited for low voltage, low current demand circuits – the kinds of circuits that makers gravitate towards. Choose a diode that it is rated for the voltage and current requirements of your circuit.
How can we protect mosfet from overvoltage? ›Using a Resistor Between Gate and Source
The mentioned resistor ensures that as soon as the switching signal is removed the mosfet is able to quickly turn OFF, and prevent a possible damage. This resistor value could be anywhere between 1K and 10K, however lower values would provide better and more effective results.
The importance of peak inverse voltage is shown in the reverse bias condition of the diode. Because, if this value exceeds the diode might get damaged. When the diode is non-conducting, the reverse voltage gets across the diode.
Why Schottky diodes are used in reverse voltage protection section? ›
To protect the downstream circuits from negative polarity resulting from accidental reverse connection of battery terminals, a few different methods are used. The simplest method is to use a Schottky diode. When the reverse voltage is applied, the diode becomes open circuit and protects other components.
Do cars have reverse polarity protection? ›Reverse polarity is the typical protection required in an automotive environment. During maintenance or service, the battery of the car are typically detached and reconnected. There is a probability of connecting the wires to wrong terminals of the battery.
Why does reverse polarity happen? ›This happens when the hot and neutral wires get flipped around at an outlet, or upstream from an outlet. Reversed polarity creates a potential shock hazard, but it's usually an easy repair.
What happens if you reverse polarity on DC? ›In a reverse polarity situation the diode creates a short circuit causing the internal or external fuse to blow thus cutting the power and saving the equipment's circuitry.
What happens if you reverse polarity on a plug? ›An outlet with reverse polarity can cause some items to be electrically charged at all times. In a correctly wired outlet, electricity will flow to the switch; with reversed polarity, it will be present in the item itself even when it is not turned on.
Is vGS negative for PMOS? ›PMOS, vGS and vDS are both negative and the current flows from source to drain.
Can MOSFET block reverse voltage? ›Yes it does conduct in either direction. Due to the body diode, most discrete MOSFETs cannot block in the reverse direction, but the channel will conduct in either direction when the gate is biased "on".
Can MOSFET conduct in reverse direction? ›MOSFETs have an extremely useful property; that is, when VGS=0, they still conduct in reverse. They do this because they have a parasitic diode between source and drain called an intrinsic body diode.
Which is an example of reverse polarity? ›Earth's magnetic field lines up in a direction opposite of the present magnetic field an example of reverse pWhich is an example of reverse polarity. Imagine, for instance, that your toaster was connected to an outlet that had reverse wiring.
Is reverse polarity positive or negative? ›In straight polarity, the electrode is negative, and the base plates are positive. In reverse polarity, the electrodes are positive, and the base plates are negative. Straight polarity offers high penetration, while reverse polarity offers a higher deposition rate.
What is normal polarity reverse polarity? ›
The polarity can be "normal" or "reversed." Normal polarity is where the magnetic north points (roughly) towards the geographic north pole. This is how the magnetic field is aligned today. Reversed polarity is in the opposite direction, and the north end of the magnetic field is close to the present-day south pole.
How do you select a reverse protection diode? ›Choose a Schottky diode if you can. Schottky diodes have lower voltage drops and are usually better suited for low voltage, low current demand circuits – the kinds of circuits that makers gravitate towards. Choose a diode that it is rated for the voltage and current requirements of your circuit.
Can MOSFET conduct in reverse direction? ›MOSFETs have an extremely useful property; that is, when VGS=0, they still conduct in reverse. They do this because they have a parasitic diode between source and drain called an intrinsic body diode.
How do reverse polarity switches work? ›Reverse Polarity Switching / DPDT switch wiring by VOG (VegOilGuy)
Can MOSFET block reverse voltage? ›Yes it does conduct in either direction. Due to the body diode, most discrete MOSFETs cannot block in the reverse direction, but the channel will conduct in either direction when the gate is biased "on".
What diode is used for reverse polarity? ›To protect the downstream circuits from negative polarity resulting from accidental reverse connection of battery terminals, a few different methods are used. The simplest method is to use a Schottky diode. When the reverse voltage is applied, the diode becomes open circuit and protects other components.
What is the maximum safe reverse voltage rating of a diode? ›Maximum DC reverse voltage = VR or VDC, the maximum amount of voltage the diode can withstand in reverse-bias mode on a continual basis. Ideally, this figure would be infinite.
What will happen if diode is connected in reverse polarity? ›Normally a diode conducts in one direction and it does not conduct in the other direction of applied voltage (polarity). If you reverse the diode from its intended direction, it will conduct when you don't want it to and it won't conduct when you do want it to.
How is MOSFET bidirectional? ›The MOSFET transistor is a bidirectional device, but the current can only flow through the source-drain if the voltage in the source is greater than the drain. The MOSFET is a bidirectional device because the terminals referred to as “source” and “drain” can be replaced.
How can MOSFET be used as bidirectional switch? ›In this post we learn about MOSFET bidirectional power switches, which can be used for operating a load across two points bidirectionally. This is simply done by connecting two N-channel, or P-channel MOSFETs back to back in series with the specified voltage line.
What is the direction of current in PMOS? ›
For a PMOS transistor, the source is always by definition the terminal at the higher voltage so current always flow from source to drain.
Is reverse polarity positive or negative? ›In straight polarity, the electrode is negative, and the base plates are positive. In reverse polarity, the electrodes are positive, and the base plates are negative. Straight polarity offers high penetration, while reverse polarity offers a higher deposition rate.
Is there a switch that reverses polarity? ›When you need to control a DC motor (such as a DC linear actuator) you usually need to be able to swap the polarity on the wires going to the motor. A double pole, double throw switch is used for this purpose but you have to wire it up correctly to reverse the polarity going to the linear actuator.
Is it easy to fix reverse polarity? ›Reversed polarity creates a potential shock hazard, but it's usually an easy repair. Any $5 electrical tester will alert you to this condition, assuming you have a properly grounded three-prong outlet.
What happens if you reverse polarity on an MOSFET? ›During reverse polarity of the battery, the diode in the ground line of the charge pump blocks the voltage. No voltage supplies the Gate and the MOSFET will be switched off. The diode protects as well the charge pump against reverse battery. Otherwise a short via the two transistors would occur.
Do MOSFETs need gate resistors? ›You do not strictly need a base resistor. Not only do MOSFETs not have bases (they have gates), but the gate is (very) high impedance. Except when the MOSFET is changing states, the gate current is essentially zero.
How do you prevent reverse current flow? ›A device to prevent reverse current flow includes a diode connected between a power supply and a load. A switching device, connected in parallel with the diode, has a power loss that is smaller than that of the diode.