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Original text link: https://blog.csdn.net/m0_46582811/article/details/125633906
Several common power supply anti-reverse design
1. Diode anti-reverse connection design
A single diode is connected in series to the input terminal of the power supply. The principle of the anti-reverse connection circuit is to use the unidirectional conductivity of the diode, conduct forward conduction, and reverse cut off. When the power supply is reversed, the diode will not conduct and will not damage any devices. But this circuit has a flaw. During normal operation, we have to consider the voltage drop of 0.7V generated on the diode, and sometimes it can reach about 1V. Circuits that have strict requirements on the supply voltage are not recommended. A voltage drop of 0.7V may cause the circuit not to work. (When choosing a diode, pay attention to choosing a Schottky diode with a small voltage drop)
Disadvantages: The disadvantage of this circuit will be loss, and the power loss is P=I*VF (the size of the conduction voltage drop of D1).
2. Fuse + parallel diode anti-reverse connection design
The principle of this anti-reverse connection also utilizes the unidirectional conductivity of the diode. When the power supply is connected normally, the diode does not work, and the current flows into the circuit through the fuse. When the power supply is reversed, the diode conducts instantaneously, and the positive and negative poles of the power supply are approximately short-circuited. At this time, the short-circuit current generated by the short circuit will blow the fuse to achieve the effect of reverse polarity protection. It should be noted that the selection of the fuse should match its own circuit characteristics (the fuse should be a resettable fuse).
Disadvantages: The disadvantage of this circuit is that there is one more fuse, which increases the cost. If there are multiple loads behind the power supply, you need to pay attention to using the above circuit.
3. MOS tube anti-reverse connection design circuit
The anti-reverse connection circuit of power supply realized by MOS tube, when the power supply is connected correctly, the power supply will supply power to the load normally. When the positive and negative poles of the power supply are reversed, the load circuit is disconnected to protect the load.
1. The power supply is connected correctly
When the power supply is connected normally, the power supply supplies power to the load normally. Assuming that the resistor R1 of the g-pole of the MOS transistor is removed, the MOS transistor will not be turned on at this time, but Vin can supply power to the load through the body diode of the MOS transistor. The drop across the body diode is about 5V - 4.3V = 0.7V.
In fact, the g pole of the MOS tube has a resistor R1, and the g pole of the MOS tube is connected to the GND of the negative pole of the power supply through the resistor R1. Before the MOS transistor is turned on, the voltage of Vin is still connected to Vout (that is, the voltage of the s pole of the MOS transistor) through the body diode of the MOS transistor. And when Vout rises from 0 to a high enough level (often not to 4.3V), there is already enough Vgs voltage to turn on the MOS tube, and the final voltage of each point is as shown in the figure below.
At this time Vgs= Vg - Vs = 0 - 5V = -5V.
2. The positive and negative poles of the power supply are reversed
Since the g-pole voltage of the MOS transistor is 5V, the Vgs voltage is greater than 0, the MOS transistor is not turned on, and the body diode is also reversed, the current cannot form a loop, and the load is protected.
3. Extended application design
The above circuit uses a "P-type" MOS tube, and an "N-type" MOS tube can also be used. The circuit is as follows.
The working principle is that when the power supply is connected normally, the MOSFET is turned on, and when the power supply is reversed, the MOSFET is cut off to achieve the effect of reverse connection protection. What needs to be explained here is that both N-MOS and P-MOS can achieve the anti-reverse connection function of the power supply, but the connection methods of the two tubes are different. When the power is turned on normally, the circuit works normally, and the voltage drop of the MOS tube is small. When the power supply is reversed, the circuit does not work, and the power supply reverse connection protection function is achieved. However, when selecting a MOS tube, it is necessary to pay attention to the conduction voltage of the MOS tube.
Note: 1. In addition to using components to prevent reverse connection, the simplest is to use an anti-reverse connection power interface. There are many types of anti-reverse connectors, which can be selected according to the power supply.
2. Note that polarized capacitors cannot be used between Vin and GND of the input power supply, otherwise the capacitor will be damaged when the positive and negative connections of the power supply are reversed.
4. Full-bridge rectifier anti-reverse connection circuit
The working principle is relatively simple. Regardless of the polarity of the power input to the full-bridge rectifier, the output is always fixed. So the circuit works regardless of positive or negative connection.
Disadvantages: During operation, there will be a voltage drop of around 1.5V across the full bridge rectifier.