Some partners asked VBSEMI to come out with a phase of 'how to quickly shut down the MOS tube', which is coming. Today, let's take a look at why MOS tubes need to speed up the shutdown and how to speed up the shutdown.
There are many kinds of MOS tube drivers, we have known before, one of them is to use a special driver chip for driving, we mainly focus on this driver design.
When the MOS tube is turned off, the voltage will be higher than when it is turned on, and the loss caused by the shutdown is also greater than the loss caused by the switch-on, so we always hope that the circuit can be turned off faster.
So, how do we speed up the shutdown of the MOS tube?
Let's take a look at the following picture.
When the MOS tube is turned on, the resistors R1 and R4 limit the current, and then charge the MOS tube capacitor
It is important to note here that the resistance parameters of R1 and R4 are not fixed, and R1 will be smaller than R4.
When the MOS tube is turned off, the voltage on the R4 resistor is clamped at 07V when greater than 0After 7V, the current passes through the diode and is then discharged in series with R1, which reduces the drive resistance, so that the MOS transistor can be switched off quickly, which in turn reduces the turn-off losses.
Here's the question, what if the chip driver power is insufficient?
The answer is: we can do it by adding a PNP transistor.
When the MOS transistor is turned on, the chip is still directly driven, but it can be discharged through the triode when it is turned off.
In this case, the BE current of the transistor controls the EC current, which in turn discharges the charge of the gate capacitor of the MOS. When the CE current of the triode satisfies certain conditions, the current of the EC can be amplified by the BE current, so that the shutdown capability required by the power supply IC is much smaller.
This not only accelerates the problem of slow MOS tube shutdown, but also solves the problem of insufficient IC drive capability.
Well, that's all for this issue, pay attention to me, and in the next issue of VBSEMI, we will talk to you about the situation of "PMOS as a power switch, and the power supply voltage is instantly pulled down".