When designing electronic products, the anti-reverse connection design of the power interface is a common problem. Once the power supply of the product is reversed,
if there is no anti-reverse connection mechanism, it will cause irreversible damage to the product.
Here are several anti-reverse connection design ideas:
1. The interface adopts an anti-reverse connection plug
This method is low-cost, simple, and effective, but the reverse connection still easily occurs during product production, on-site installation, debugging, etc;
2. Insert a diode in series with the circuit
This method has a simple circuit and low cost, but there is a voltage difference between the diodes and the output voltage may be lower than required. At the same time, if the current is large, the diode will seriously heat up. These are all issues to consider.
3. In order to solve the voltage drop problem of the diode, the following circuit can be used
When the power supply is connected normally, the diode does not conduct. Only when the power supply is reversed,
does the diode conduct and the large current causes the fuse to blow out quickly.
Although this circuit solves the problem of the diode voltage difference,
the moment of reverse connection will still cause damage to the back of the diode. Shocks caused by the circuit may damage some components.
The circuit below replaces the blown fuse with a resettable fuse, eliminating the trouble of returning the fuse.
4. Use a bridge to prevent reverse connection
The advantage of using a bridge is that the circuit can work normally regardless of whether the power supply is connected forward or reverse. The disadvantage is also the diode voltage difference, and the voltage difference between two diodes is prone to serious heat generation in high current applications.
5. Use N-channel MOS tubes to prevent reverse connection
The on-resistance of the MOS tube is very small, and the voltage drop of the DS pole will be very small, which is especially suitable for high-power applications. Pay attention to the connection between the D and S poles in the circuit. Normally, the NMOS current flows from the D pole to the S pole, but it is just the opposite in this circuit. The reason is that the field effect transistor will be completely turned on as long as a suitable voltage is established between the G and S poles. Rds and Rsd have the same resistance, so the current can flow in both directions.
The working principle of the circuit: When the polarity of the power supply is correct, Vgs>0, the NMOS conducts, and the circuit works normally. When the power supply is reversed, when Vgs<0, the NMOS does not conduct, and the circuit does not work.
Vgs must be controlled within 20V, otherwise, it will be broken down, so add a voltage regulator tube D1 for protection. The parallel capacitor C1 provides a soft start for the MOS tube.
Also, note that this circuit cannot be used for battery charging.