Series of articles catalog
Base element 1.
The circuit design 3.PCB design 4. The welding element 6. programming 9. testing standards
Many projects of hardware engineers are completed on the hole-hole board, but there is the phenomenon of accidentally connecting the positive and negative poles of the power supply, causing many electronic components to be burned, and even the entire board is scrapped, and another piece of welding is required. , I don’t know if there is any good way to solve it?
First of all, carelessness is inevitable. Although it is only to distinguish between the positive and negative wires, one red and one black, we may wire it once, and we will not make a mistake; we will not make a mistake if we connect the wire 10 times, but 1000 times? How about 10,000? It's hard to say at this time. Due to our carelessness, some electronic components and chips are burnt out. The main reason is that the components are broken down due to excessive current. Therefore, measures must be taken to prevent reverse connection.
The following methods are commonly used:
1. Diode series anti-reverse connection protection circuit
A forward diode is connected in series with the positive power input terminal to make full use of the forward conduction and reverse cutoff characteristics of the diode. Under normal circumstances, the diode is turned on and the circuit board works.
When the power supply is connected reversely, the diode is cut off, the power supply cannot form a loop, and the circuit board does not work, which can effectively prevent the problem of reverse power connection. However, there will be a diode voltage drop, and it is also important to select the corresponding diode according to the power supply current.
2. Rectifier bridge type anti-reverse connection protection circuit
Use the rectifier bridge to change the power input into a non-polar input, no matter whether the power is connected or reversed, the circuit board works normally.
The above uses diodes for anti-reverse treatment. If silicon diodes have a voltage drop of about 0.6 0.8V, germanium diodes also have a voltage drop of about 0.2 0.4V. If the voltage drop is too large, you can use MOS tubes for anti-reverse treatment. The voltage drop is very small, up to a few milliohms, and the voltage drop is almost negligible.
3. MOS tube anti-anti-protection circuit
Due to factors such as process improvement and its own nature, most of the internal resistance of the MOS tube is in the milliohm level or even smaller. This will cause the voltage drop and power consumption of the circuit to be extremely small, or even negligible, so Choosing a MOS tube to protect the circuit is a more recommended way.
1) NMOS protection
As shown in the figure below: 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, and the turn-on voltage of the MOS tube is extremely: Ugs = Vbat-Vs , The gate shows 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 of the NMOS.
If the power supply is connected reversely, the turn-on voltage of the NMOS is 0, the NMOS is cut off, the parasitic diode is reversed, and the circuit is disconnected, thereby forming protection.
(2) PMOS protection
As shown in the figure below: At the moment of power-up, the parasitic diode of the MOS tube is turned on, and the system forms a loop. The potential of the source S is about Vbat-0.6V, and the potential of the gate G is 0, and the turn-on voltage of the MOS tube is extremely: Ugs = 0 -(Vbat-0.6), the gate is shown as low level, the ds of the PMOS is turned on, the parasitic diode is short-circuited, and the system forms a loop through the ds of the PMOS.
If the power supply is connected reversely, the on-voltage of the NMOS is greater than 0, the PMOS is cut off, the parasitic diode is reversed, and the circuit is disconnected, thereby forming protection.
Note: The NMOS tube ds is connected to the negative pole, and the PMOS tube ds is connected to the anode. The direction of the parasitic diode is toward the current direction of the correct connection.
The connection of D pole and S pole of MOS tube: When using N-channel MOS tube, the current usually enters from the D pole and flows out from the S pole, and the PMOS is S in and D out. It is just right when used in this circuit. On the contrary, through the conduction of the parasitic diode, the voltage condition for the conduction of the MOS tube is satisfied. The MOS tube will be completely turned on as long as a suitable voltage is established between the G and S poles. After turning 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 pole is generally 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 has a soft-start function. At the moment when the current starts to flow, the capacitor is charged, and the voltage at the G pole is gradually established.
For PMOS, compared to NOMS, the turn-on 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 is more advantageous than NMOS.
4. Fuse protection
Many common electronic products can be seen to have a fuse added to the power supply after disassembly. When the power supply is connected reversely and there is a short circuit in the circuit, the fuse will be blown to protect the circuit due to high current. It is troublesome to repair and replace. But you can choose to buy a self-recovery fuse.