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For some products for daily use, this problem will be considered when the product is designed. The customer simply uses the plug to connect the power supply, so the reverse plug is generally used, which is a simple, low-cost and effective method.
However, when the product is in the production stage of the factory, it may be inconvenient to use an error-proof connector, which may cause a reverse connection due to the negligence of the production personnel, resulting in losses.
Therefore, it is sometimes necessary to add an anti-reverse circuit to the circuit, although it increases the cost.
The following are the commonly used anti-reverse circuits:
1. The easiest way is to put a diode in series in the circuit
Advantages: simple circuit and low cost. It is suitable for products with low current and strict cost requirements.
Disadvantage: When the PN junction of the diode is turned on, there is a voltage drop, generally below 0.7V. This voltage drop makes this circuit unsuitable for use in circuits with large currents. If the circuit has a current of 10A, the power consumption of the diode is 0.7*10=7W, and the heat generation is still considerable. In a product with a compact structure and limited space, the impact on the stability of the product or the user's feeling of use is still relatively large.
2. Is there any way to overcome the voltage drop problem of the diode mentioned above? See the circuit below.
The above anti-reverse circuit uses a fuse and an anti-parallel diode. The polarity of the power supply is correct. When the circuit is working normally, due to the small current of the load, the diode is in the reverse blocking state, and the fuse will not be blown.
When the power supply is reversed, the diode is turned on, and the current is relatively large at this time, and the fuse will be blown, thereby cutting off the power supply and protecting the load.
Advantages: The voltage drop of the fuse is very small, and there is no heating problem. The cost is not high.
Disadvantage: Once the connection is reversed, the fuse needs to be replaced, which is troublesome to operate.
3. The circuit that can work normally in both forward and reverse connections:
Advantages: No matter how the input terminal is connected, the circuit can work normally.
Disadvantage: There is a voltage drop across the two diodes. Suitable for small current circuits.
4. N-channel enhanced FET anti-reverse circuit
Determined by the FET manufacturing process, the on-resistance of the FET is relatively small. It is a very commonly used switching device, especially in high-power applications. Taking the IRFR1205 packaged in TO-252 as an example, its main parameters are as follows: Vdss=55V, Id=44A, Rds=0.027 ohms; it can be seen that its on-resistance is only 27 milliohms.
The following figure is an anti-reverse circuit composed of N-channel field effect transistors
The biggest feature of this circuit is the connection of the D and S poles of the FET. Usually, when we use an N-channel enhancement type MOS tube, the current generally enters from the D pole and flows out from the S pole. When applied in this circuit, the opposite is true.
I once saw this circuit in a forum, and the landlord who posted this circuit was criticized by many netizens. It is impossible to say that there is a diode between DS. The landlord did not specify the pin name of the FET. Due to the inertial thinking of applying FET, the landlord was wronged.
In fact, the FET will be fully turned on as long as a suitable voltage is established between the G and S poles. After conduction, between D and S is like a switch is closed, and the current is the same resistance from D to S or S to D.
When the polarity of the power supply is correct, the current is turned on through the voltage regulator tube of the FET at the beginning, and the voltage of the S pole is close to 0V.
After the two resistors are divided, a voltage is provided to G to make the FET conduct. Because its on-resistance is small, the diode inside the FET is replaced.
When the power supply is reversed, the diode in the FET does not conduct until the breakdown voltage is reached. There is no current flowing through the divider resistor, which cannot provide the G-pole voltage and is not conducting. thus play a protective role.
For the Zener diode connected in parallel with the voltage dividing resistor in the circuit, because the input resistance of the FET is very high,
It is a voltage-controlled device. The voltage of the G pole should be controlled within 20V. Too high voltage pulse will cause the breakdown of the G pole. This Zener diode is to protect the FET to prevent breakdown.
For the capacitor connected in parallel with the voltage divider resistor, there is a soft-start effect. At the moment when the current begins to flow, the capacitor is charged, and the voltage of the G pole is gradually established.
For the resistance-capacitor series circuit connected in parallel between the FET D and S, I feel that it is still debatable. Resistor-capacitor series circuits are generally used for pulse absorption or delay. The use here depends on the load situation, and it may not be good to add it. After all, this results in a brief on-pulse when the power supply is reversed.
P-channel field effect transistors can also be used, but the device is only connected to the input terminal of the positive electrode. It is no longer described here.
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