A friend sent me a private message asking: What are the overcurrent protection circuits for MOS tubes?

It is often important to sense the load current before shutting down the load.

Today, WeiBi is going to share an overcurrent protection circuit from the Internet. It uses an operational amplifier to sense whether an overcurrent occurs in the circuit .

Circuit Design:

 - A power source of at least 12V

-LM358

-IRF540N

-100uf/25V capacitor

-heat sink

- 50kΩ Potentiometer

- 1% precision 1kΩ and 100kΩ resistors

- 1MΩ resistor

- 1Ω shunt resistor, rated at 2W

These are the components needed for the circuit. The MOSFET model here is IRF540N , which can be better replaced by VBL1104N.

How it works

When the MOS tube is turned on, the load current flows from the drain to the source, and then leads to GND through R1. Generally, the detection current passes through this shunt resistor R1 (1Ω 2W), which will produce a voltage drop.

How is this voltage drop calculated?

This requires the use of Ohm's law (V=I x R): if the load current is 1A, the voltage drop across the shunt resistor is 1V.

In other words, this voltage can be compared with the preset voltage when using the op amp to detect overcurrent conditions, and then the state of the MOS tube can be changed to cut off the load.

How does voltage control the switch of MOS tube?

This LM358 is a comparator, which can compare the size of two values, one is the voltage drop of R1, which is generally imported into the inverting pin of the comparator, and the other is the preset voltage generated by RV1, which is imported into the same direction pin of the comparator.

The main function of RVI here is pressure division.

That is to say, when the induced voltage is lower than the preset voltage, RV1 will generate a positive voltage (close to VCC) at the output, otherwise it will generate a negative voltage (grounded, 0V), which can control the MOS tube.

But be careful:

If the 1A of potentiometer RV1 is set as the threshold for MOS tube shutdown, the voltage drop of R1 detected by the comparator may be 1.01V, which will cause the comparator (LM358) to disconnect the load. The transient response increases the preset voltage and makes the comparator work linearly.

So it is best to use a stable power supply in the comparator so that transient changes will not affect the input voltage and reference voltage of the comparator.

In addition, the comparator needs to add hysteresis , such as the 100kΩ R4 and the voltage regulator LM7809. Does anyone know the role of these two? You can leave a message in the comment area to discuss this circuit design!

Some of the above pictures and information are from the Internet