MOS tube acceleration and deceleration switch circuit principle

[vbsemi]

https://www.youtube.com/watch?v=czRugPWSqx0

How does the MOS acceleration and deceleration switch work?

Take NMOS as an example. When the current flowing into the gate is increased, the MOS will be accelerated when it is turned on. When the current flowing into the gate is reduced, the MOS will be decelerated when it is turned on; when the MOS tube needs to be turned off quickly, the gate charge needs to be released faster, and the deceleration shutdown will decelerate the release. PMOS is the reverse process of the above.

We understand the switching process of the MOS tube. When an NMOS is turned on, the gate is charged. When the Cgs charge is full, the gate will turn on the threshold. When the NMOS is turned off, the GS charge needs to be discharged. When the charge is discharged, the gate will turn off the threshold. This explains the fast switch and deceleration switch we talked about earlier.

So what is its fundamental mechanism?
In essence, it is related to the gate resistance of the MOS tube. Because the gate resistance directly controls the size of the gate current, the switching speed varies with the gate resistance value. And choosing the right one is very important. Different gate resistors can be used for different switching speed requirements of MOSFET.

When the MOS tube is turned on, the gate resistors: R1 and R2 are connected in parallel, and R2 is closed when it is turned off, so that it can be turned on faster and turned off slower; or like this circuit, the MOS tube passes through the gate resistor R1 when it is turned on, and R1 and R2 are connected in parallel when it is turned off, so that it can be turned on slower and turned off faster.

The turning on and off of MOS is a dynamic process. The control end and the gate level are the same during continuous off or on, but the control end and the gate level are different at the moment of turning on and off. The gate level changes slower than the control end level, so the different levels will cause the gate current to flow into or out of the gate.

Increasing the gate resistance value will slow down the switching speed of the MOSFET and increase its switching loss. Reducing the gate resistance value will increase the switching speed of the MOSFET.

This has actually been mentioned in the previous section about the selection of the size of the resistor. It is not recommended to use K-level resistors when controlling the switching speed. Common resistance values are 3.3Ω/10Ω/33Ω, etc. Some relatively large discharge currents can be achieved by selecting low output impedance MOSFETs or voltage devices, such as using accelerating diodes.