Power supply anti-reverse connection
Each product will have a power supply interface when designing. For these devices that require DC power supply, we must take into account the reverse connection of the power supply when designing. Once the power is connected reversely, it may cause the internal circuit of the terminal device to burn out. In order to prevent misoperation and reverse the positive and negative poles of the power supply, causing damage to the circuit, we generally need to design an anti-reverse connection protection circuit. Here we will introduce the anti-reverse connection protection circuit through diodes, fuses and MOS tubes.
1. String diode
Using diodes for protection, the circuit is simple, low cost and takes up little space. If the power supply is connected in the reverse direction, the diode will not conduct in reverse cutoff, but when the PN junction of the diode is conducting, there will be a voltage drop <= 0.7V, causing unnecessary losses to the circuit, such as battery-powered systems. , circuits with larger currents will have a more obvious impact (in the circuit, power consumption and heat are issues that cannot be ignored).
2. Series fuses and parallel diodes
Place a fuse in series with a diode in parallel with the circuit. This anti-reverse connection circuit has a fuse in series and a diode in parallel at the input. When the power supply is connected correctly, as long as the load current does not exceed the allowable range of the fuse, the fuse will not blow, the diode will be reversely blocked, and there will be no loss in the input voltage, which will not affect the normal operation of the load circuit. When the power supply is connected reversely, the diode conducts, and the current is large and the fuse blows, thus realizing the circuit protection function. But the trouble is that every time the circuit is reversed, the fuse must be replaced, which is very inconvenient. If you want to save time, you can connect a self-restoring fuse, which is more expensive.
3. MOS tube prevents reverse connection
Due to factors such as process improvements and their own properties, the conduction internal resistance of MOS tubes is often in the milliohm level or even smaller. This causes a very small loss in terms of voltage drop and power consumption of the circuit, and can even be ignored. Therefore, It is recommended to choose MOS tube to protect the circuit.
3.1. NMOS tube prevents reverse connection
At the moment of power-on, the parasitic diode of the MOS tube is turned on, and the system forms a loop. The potential of the source S is about 0.6V, and the potential of the gate G is Vbat. The turn-on voltage of the MOS tube is extremely: Vgs = Vbat - Vs, and the gate The pole behaves as a high level, the DS of the NMOS is turned on, the parasitic diode is short-circuited, and the system forms a loop through the DS access of the NMOS. If the power supply is connected reversely, the conduction voltage of NMOS is 0, the NMOS is turned off, the parasitic diode is connected reversely, and the circuit is disconnected, thus forming protection.
3.2. PMOS tube prevents reverse connection
At the moment of power-on, the parasitic diode of the MOS tube is turned on, and the system forms a loop. The potential of the source S is approximately Vbat-0.6V, while the potential of the gate G is 0. The turn-on voltage of the MOS tube is extremely: Vgs = 0 - (Vbat -0.6), the gate shows a low level, the PMOS ds is turned on, the parasitic diode is short-circuited, and the system forms a loop through the PMOS ds access. If the power supply is connected reversely, the turn-on voltage of NMOS is greater than 0, the PMOS is turned off, the parasitic diode is connected reversely, and the circuit is disconnected, thus forming protection.
*Note: The NMOS tube connects ds to the cathode, and the PMOS tube connects ds to the anode. The direction of the parasitic diode is towards the direction of the correctly connected current.
3.3. NMOS tube and PMOS tube expansion connections
Access to the D and S poles of the MOS tube: When using an N-channel MOS tube, the current usually enters from the D pole and flows out from the S pole. For PMOS, S enters and D exits. When used in this circuit, it is just right. On the contrary, the voltage condition for MOS tube conduction is met through the conduction of parasitic diodes. The MOS tube will be fully conductive as long as a suitable voltage is established between the G and S poles. After it is turned on, it is like a switch is closed between D and S, and the current is the same resistance from D to S or S to D.
In practical applications, the G electrode is usually connected in series with a resistor. In order to prevent the MOS tube from being broken down, a Zener diode can also be added. The capacitor connected in parallel with the voltage divider resistor has a soft-start effect. At the moment when current starts to flow, the capacitor is charged, and the voltage of the G electrode gradually builds up.
4. Development summary
1. Diode has a simple circuit, low cost and small space occupation; the disadvantage is that under the condition of large current, the impact on power consumption is very large and the heat generation is large.
2. The fuse has small voltage loss and does not affect the normal operation of the load circuit. The fuse blows when the circuit is reversed. The trouble is that every time the circuit is reversed, the fuse must be replaced, which is troublesome. Using a self-restoring fuse is more expensive.
3. MOS tube. The internal resistance of the MOS tube is very small, which solves the problems of voltage drop and excessive power consumption in the diode power supply anti-reverse connection solution. Generally, PMOS tube circuits are more commonly used. Compared with NOMS, PMOS requires Vgs to be greater than the threshold voltage. Since its turn-on voltage can be 0, the voltage difference between DS is not large, which has more advantages than NMOS.