A colleague asked VBsemi's editor for an article on 'How to Quickly Turn Off MOSFETs', so here it is. Today we will learn why MOSFETs need to speed up their turn-off process, and how to do it.

There are many ways to drive MOSFETs. Previously, we had some understanding of this, one of which is to use a dedicated driver chip for driving. We will mainly focus on this driver design.

When a MOSFET is turned off, the voltage is generally higher than when it is turned on, and the losses generated during turn-off are greater than those generated during turn-on, so we always hope that the circuit can turn off faster.

So, how can we speed up the turn-off of MOSFETs?

Let's take a look at the diagram below:

When the MOSFET is turned on, resistors R1 and R4 limit the current, and then charge the capacitance of the MOSFET;

Here, it is important to note that the resistance parameters of R1 and R4 are not fixed, and R1 is smaller than R4.

When the MOSFET is turned off, the voltage on resistor R4 is clamped at 0.7V. When it exceeds 0.7V, the current passes through the diode, and then discharges in series with R1, reducing the driving resistance, so that the MOSFET can achieve fast turn-off, thereby reducing turn-off losses.

Here's a question: What if the chip's driving capability is insufficient?

The answer is: We can achieve this by adding a PNP transistor.

When the MOSFET is turned on, let the chip directly drive it, but when it is turned off, discharge can be achieved through the transistor.

At this time, the transistor's be current controls the ec current, thereby discharging the charge on the gate capacitance of the MOSFET. When the transistor's ce current meets certain conditions, the ec current can be amplified through the be current, reducing the turn-off capability required by the power IC.

This not only speeds up the slow turn-off of the MOSFET, but also solves the problem of insufficient IC driving capability.

Well, that's about it for this issue. Follow me, next time VBsemi's editor will talk to you about the situation of "PMOS as a power switch with the power supply voltage being momentarily pulled down".