Why do MOS tubes need to be connected in parallel with a diode?

[vbsemi]

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MOS has a body diode connected in parallel between the D and S poles, so why is this diode connected in parallel?

This starts with the process and structure of MOS. The diode is composed of a pair of PN junctions. The P-type region corresponds to the positive pole of the diode, the N-type region corresponds to the negative pole of the diode, and the PN junction is in the middle. SiO2 in the MOS tube itself is not conductive, so the driving pole G basically does not carry current.

In addition to the three poles D, G, and S, there is also an intermediate pole, which is connected to the S pole, so in the circuit symbol of MOS, the arrow pointing to the channel N channel inside the MOS is connected to the S pole.

In addition, the drain of the N-type region is connected to the middle P-type region and then to the source, which just forms a diode structure, so a diode is connected in parallel in the MOS symbol.

What is the use of this body diode?

In some scenarios, such as battery protection, after the lithium battery is over-discharged, the protection function will be turned on: turn off the discharge MOS. When the charger is plugged in, the MOS body diode is used to make the circuit conductive and the system work normally. However, in some scenarios, the existence of this diode is undesirable because it may cause leakage between the S pole and the D pole.

[ilianaboone]

MOSFETs are often connected in parallel with a diode, typically a flyback diode or Schottky diode, to protect the MOSFET from voltage spikes and to ensure proper operation in certain circuits, especially in inductive load applications.

1. Inductive Loads: When a MOSFET is used to switch an inductive load (such as a motor or relay), the inductance of the load generates a high voltage spike (back EMF) when the MOSFET turns off. This spike can exceed the MOSFET's voltage rating and potentially damage it. The diode provides a path for the current to safely dissipate, preventing the MOSFET from experiencing excessive voltage.

2. Body Diode of MOSFET: MOSFETs have an intrinsic body diode (built-in diode between drain and source), but in some applications, this diode may not provide adequate protection or may be slow to respond. Adding an external diode can improve performance, reduce losses, and ensure faster response time.

3. Flyback Diode for Switching Circuits: In circuits like DC-DC converters, the diode helps manage the energy stored in the inductor during switching, improving efficiency and ensuring stable operation.

click here：Introduction to the basics of MOSFET