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The electronic device access to the positive and negative power fear is reversed. If there is no anti-reverse circuit, then I do not know what will happen, and component damage that's for sure. Therefore, a general circuit will increase reverse circuit, following several common circuit.
1, with one anti-reverse diode circuit
Typically the anti-DC power input protection circuit using a diode is conductive to achieve unidirectional protection against reverse. As shown in Figure 1:
This connection is simple, reliable, low cost, but the input power affecting the large current is very large. If the input current rating. 3A reached, typically diode drop is 0.7V, then the power must achieve at least: Pd = 3A × 0.7V = 2.1W, such a large loss, so that efficiency must be low, and heat, to add heat sink. This is not does not pay. Therefore, this can only be used in small current, undemanding circuit.
2, using a bridge rectifier circuit made anti-reverse
Done by a diode bridge rectifier input, this circuit would always have the correct polarity. FIG circuit 2
A bridge rectifier composed of four diodes, regardless of how the positive and negative power supply input connected to the output polarity is normal. Principles and Methods 1, are unidirectional continuity of the diodes, but at the same time there are two bridge rectifier diode conduction, the power consumption is twice FIG. When the input current is 3A, Pd = 3A × 0.7V × 2 = 4.2W, but also to increase the heat sink. This higher cost is not practical.
3, MOS tube type anti-reverse protection circuit
Utilizing the switching characteristics of MOS transistors, the conduction control circuit and disconnection protection against reverse circuit design, resistance is very small since the power MOS now MOSFET Rds (on) milliohm been able to do to solve the using conventional anti-reverse power diode voltage drop and power dissipation is present embodiment too large.
Reverse polarity protection with protective FET connected in series with the circuit to be protected. Once the reverse polarity protection circuit, the protection will form an open circuit with FET to prevent burning current FET circuit elements, the overall circuit protection.
N-channel MOS transistor against reverse polarity protection circuit 3 shown in FIG.
N-channel MOS transistor through pin D and S pins connected in series between the power supply and the load resistor R1 to provide a voltage bias MOS transistor, the MOS transistor by the switching characteristic of the control circuit on and off, thereby preventing the power supply reverse damage caused to the load. Then when n, R1 to provide the VGS voltage, MOS saturated conduction. MOS can not reverse when turned on, the anti-reverse play a role. Power MOS transistors Rds (on) only 20mΩ actual loss is small, 3A current, power consumption (3 × 3) × 0.02 = 0.18W do not have external fins. It solves the problem of excessive use of anti-reverse power diode voltage drop and power consumption present embodiment. VZ1 of the zener diode is too high to prevent the gate-source voltage breakdown mos tube.
P-channel MOS transistor against reverse polarity protection circuit 4 shown in FIG.
Since the ON resistance of the NMOS transistor is smaller than the PMOS and relatively cheaper, preferably selected NMOS.