Diode vs MOSFET: How to Prevent Reverse Polarity?
In electronic circuit design and applications, reversing the power supply polarity is a common potential risk that can instantly damage expensive core components and cause unnecessary financial losses. Therefore, implementing effective reverse polarity protection is a step every design must consider. This article will take an in-depth look at diode- and MOSFET-based reverse polarity protection solutions.
Catalog
III. Diode-Based Reverse Polarity Protection
V. MOSFET-Based Reverse Polarity Protection
I. What is a diode?
A diode is the most basic semiconductor component in electronic circuits, and its most important feature is its one-way conductivity. That means it allows current to pass when forward-biased, but blocks current under reverse voltage. This characteristic makes traditional diode-based reverse polarity circuits simple and effective. In a typical design, a diode is placed in series with the positive input of the power supply. When the supply is connected correctly, the diode conducts forward current and powers the downstream circuit. When the power is connected in reverse, the diode blocks current flow, effectively preventing reverse current from entering the circuit and reducing the risk of component damage. A standard rectifier diode usually causes about a 0.7-volt forward voltage drop, while a Schottky diode, thanks to its material properties, has a lower drop—typically around 0.3 to 0.4 volts. This reduces energy loss and improves efficiency, but there's still some voltage drop and power loss to consider, so the choice must match the specific current and voltage requirements.
II. What is a MOSFET?
A MOSFET, or Metal-Oxide-Semiconductor Field-Effect Transistor, is a commonly used power control device, usually found in power management and switching circuits. Unlike diodes, a MOSFET's conduction is controlled by the gate voltage, and it includes an intrinsic body diode internally. For reverse polarity protection, engineers often use P-channel or N-channel MOSFETs. With proper connection and gate drive, the MOSFET conducts under normal polarity, offering very low on-resistance (Rds(on)), and shuts off under reverse polarity, blocking current flow. The biggest advantage of a MOSFET-based reverse polarity circuit is its extremely low voltage drop when on, greatly reducing power loss, making it ideal for low-voltage, high-current systems or applications where efficiency is critical.
III. Diode-Based Reverse Polarity Protection
There are several circuit topologies for reverse polarity protection. The simplest method is the series diode approach mentioned earlier. To reduce power loss, engineers often use Schottky diodes with lower forward voltage drop, around 0.3V, which, under a 2A current, can cut losses to roughly 0.6W, improving performance. However, even Schottky diodes can cause a significant voltage drop in low-voltage systems, potentially affecting the operating voltage margin for downstream circuits. Therefore, diode selection should consider the power supply voltage, operating current, and acceptable power loss.
IV. Fuse + Reverse Diode
Combining a fuse with a reverse diode is a cost-effective protection method. Under normal operation, current flows through the fuse while the diode is reverse-biased, producing minimal loss. If the power is connected backward, the reverse diode becomes forward-biased, allowing a near-short-circuit current that quickly blows the fuse, cutting off the power completely. The downside is that this protection is one-time use; after a fault, both the fuse and diode must be replaced, increasing maintenance costs. Also, just before the fuse blows, the circuit briefly experiences a reverse voltage pulse, which may threaten sensitive downstream components, so additional protection measures are often needed.
V. MOSFET-Based Reverse Polarity Protection
PMOS-based reverse polarity circuits are currently the preferred choice for high-performance applications. The clever trick is leveraging the intrinsic body diode inside the PMOS. When the supply is connected correctly, current first flows through this body diode, raising the PMOS source voltage and creating a negative gate-source voltage that fully turns on the MOSFET. At this point, the main current channel switches from the body diode to the MOSFET channel. Because the channel’s on-resistance can be as low as a few milliohms, the resulting voltage drop and power loss are far lower than any diode solution. When the supply is reversed, the body diode is reverse-biased, the gate-source voltage is zero, and the PMOS remains off, leaving the downstream circuit at zero volts and fully protected. This approach combines low loss with high reliability.
VI. Precautions
When designing a reverse polarity circuit, several key points must be considered:
· To ensure effective protection, choose a PMOS with appropriate Vds and Vgs ratings. Also, select a PMOS with low on-resistance to further reduce conduction losses.
· Power dissipation and heat dissipation area are critical for circuit stability and reliability. The PMOS model and specifications should be chosen to prevent overheating or other issues during normal operation. Consider proper heat dissipation in the design, such as increasing the thermal area, to maintain circuit stability and reliability.
VII. Conclusion
In conclusion, reverse polarity protection is essential for the safe operation of electronic devices. Diode-based solutions are simple and low-cost, suitable for applications where efficiency is not a priority. Fuse-based methods offer one-time protection, while MOSFET-based circuits provide near-zero loss conduction and are ideal for high-performance, high-reliability systems. Engineers should weigh cost, power loss, reliability, and maintainability when selecting the most suitable protection scheme to build robust and reliable electronic products.
