What you should know about the switching characteristics of Mosfet

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What you should know about the switching characteristics of Mosfet [Copy link]

Recently, a friend encountered a problem about MOSFET switches. Although the problem has been easily solved, after careful consideration, the knowledge behind it is relatively basic and important. Considering that young engineers are seeing more and more integrated switching devices, they may not care much about the characteristics of these basic components. Today, just on this issue, let's briefly analyze the switching characteristics of MOSFET.

MOSFET switch power on leakage problem

The background of the problem is as follows: this is an equal voltage conversion switch. The microcontroller controls the pmos switch from 3V3STBY to 3V3SW.

As shown in the figure below, during the power-on process, there is a voltage dip on the 3V3SW power rail. Comparing with 3V3SW_EN (blue), it is found that there is voltage on 3V3SW before the turn on control signal arrives.

Oscilloscope measurements show that before the 3V3SW_EN control signal is pulled high, there is a gap of about 1.1V between the mosfet gate voltage (green) and 3V3. In principle, before the control voltage arrives, Vgate=3V3, so Vgs<Vth and the mosfet is not turned on.

According to the mosfet manual, the minimum Vth is 0.4 V. It can be seen that the voltage difference between Vgs causes leakage.

After removing C986 and C985 from the circuit, the problem was solved (the final solution was to replace C985 with a small 1nf capacitor).

Switching characteristics of Mosfet

The above problem is easy to solve, but the knowledge behind it is more important. This is the switching characteristics of MOSFET.

The following is the equivalent model of MOSFET, there are parasitic capacitors Cgd and Cgs between Gate and Drian and Source respectively. These two parasitic capacitors directly affect the switching characteristics of MOSFET.

In some MOSFET manuals, these two parasitic capacitors are represented by Q.

The following figure shows the entire process of Mosfet trun on:

Total is divided into 4 areas

Region 1, VGS starts to increase, but has not yet reached Vth, so VSD remains unchanged and ID is still zero. At t1, VGS=Vth

Region2, after VGS reaches Vth, MOSFET starts to conduct, and ID starts to have current. Due to the existence of parasitic capacitance between gate and source, the gate voltage starts to charge Cgs. When t2 is reached, Cgs is fully charged, VGS reaches a stable value, and ID reaches a maximum.

In Region 3, VGS continues to remain unchanged, Cgd starts to charge, and the voltage difference between VSD starts to decrease. When t3 is reached, Cgd is fully charged and the voltage difference between VSD almost reaches the minimum value. At this moment, the MOSFET is fully turned on.

Region4, VGS continues to increase to the driving voltage, the voltage difference between VSD = Rdson*ID.

From this process, we can see that if we want to control the slew rate of VSD, we can control the time of region 3. The larger the Cgd, the smaller the VSD slow rate, and of course the smaller the in-rush current. Of course, this is also the reason why there is a capacitor between the drain and gate of the top circuit. Considering that the parasitic Cgd of the mosfet itself may be relatively small, adding such a capacitor can control the slew rate of the switch.

Back to the above problem, since C985 and C986 in the circuit are both 0.1uf, which is relatively large, the Gate level does not quickly reach 3.3V during the 3V3STBY rising process, resulting in leakage. Reducing the capacitance can solve the problem.

In some circuits, in order to avoid the above problems, a diode can be added to quickly conduct so that the gate voltage quickly reaches the same level as the source.

summary

Understanding the MOSFET's run-on process is the key to understanding the various applications of MOSFETs. Both in switching circuits and DCDC circuits, the switching characteristics of MOSFETs need to be considered.

statement:

The above is just my personal opinion.

Relevant information comes from the Internet.