Anti-Reverse Protection Circuit
1. Under normal operating conditions, a DC power input anti-reverse protection circuit typically uses the unidirectional conductivity of a diode to provide protection against reverse polarity. As shown in Figure 1:
This configuration is simple and reliable, but it results in significant power loss when large currents are involved. For example, with an input current rating of up to 2A, using a fast recovery diode like Onsemi's MUR3020PT, which has a rated voltage drop of 0.7V, the power dissipation would be at least Pd = 2A × 0.7V = 1.4W. This leads to low efficiency and excessive heat generation, requiring a heatsink for proper operation.
2. Another common approach is to use a diode bridge rectifier to ensure correct polarity regardless of the input orientation (as shown in Figure 2). However, this method also suffers from energy loss due to the voltage drop across the diodes. When the input current is 2A, the power consumption in the circuit shown in Figure 1 is 1.4W, while the power consumption in the bridge rectifier circuit in Figure 2 is double that, at 2.8W.
Figure 1: A series diode protection system is unaffected by reverse polarity. The diode has a voltage drop of 0.7V.
Figure 2: A bridge rectifier works fine regardless of polarity, but with two diodes conducting, the power consumption is twice that of Figure 1.
MOSFET-Based Anti-Reverse Protection Circuit
Figure 3: An NMOS transistor-based anti-reverse protection circuit.
Figure 3 illustrates how the switching characteristics of a MOSFET can be used to design an effective anti-reverse protection circuit. Unlike traditional diode-based solutions, the MOSFET has a very low on-resistance (Rds(on)), often in the milliohm range. This significantly reduces voltage drop and power loss, solving the inefficiency issues seen in conventional diode-based systems.
The protection FET can be either a PMOS or an NMOS transistor. In the case of a PMOS, its gate is connected to the ground of the protected circuit, while the source is connected to the power supply terminal. The drain is connected to the substrate of the protected circuit. For an NMOS, the gate is connected to the power supply, the source to ground, and the drain to the circuit’s load.
When the polarity of the power supply is reversed, the FET turns off, creating an open circuit and preventing damage to the components. In a normal connection, the resistor R1 provides the necessary VGS voltage to fully turn on the MOSFET, allowing current to flow through the load.
In a reverse connection, the MOSFET cannot conduct, thus acting as a barrier to reverse current. With an Rds(on) as low as 20mΩ, the power loss at 2A is only (2A)^2 × 0.02Ω = 0.08W, eliminating the need for a heatsink. This makes the MOSFET-based solution much more efficient and compact compared to traditional diode-based methods.
Figure 4: Schematic of the NMOS-based anti-reverse protection circuit.
Figure 5: Detailed layout of the protection circuit.
The Zener diode VZ1 is used to protect the gate-source voltage of the MOSFET from overvoltage. Generally, NMOS transistors have lower on-resistance than PMOS, making them a better choice for this application. The NMOS is connected between the power supply and the load, with the gate controlled to turn on at a high level. The PMOS, on the other hand, is connected to the positive supply terminal and turned on at a low gate voltage.
By replacing traditional diodes with MOSFETs, the anti-reverse protection circuit becomes more efficient, cooler, and suitable for high-current applications without the drawbacks of excessive power loss.
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