Understanding the switching process of MOSFET based on the characteristics of the drain conduction region

　　MOSFET gate charge characteristics and switching process

　　Although MOSFET is widely used in some electronic systems such as switching power supplies and motor control, many electronic engineers do not have a clear understanding of the MOSFET switching process and the state of the MOSFET during the switching process. Generally speaking, electronic engineers usually understand the MOSFET turn-on process based on the gate charge, as shown in Figure 1. This figure can be found in the MOSFET data sheet.

　　The voltage applied to the D and S poles of the MOSFET is VDD. When the driving turn-on pulse is applied to the G and S poles of the MOSFET, the input capacitor Ciss is charged, and the voltage Vgs of the G and S poles rises linearly and reaches the threshold voltage VGS(th). Before Vgs rises to VGS(th), the drain current Id ≈ 0A, and no drain current flows, and the voltage of Vds remains unchanged at VDD.

　　When Vgs reaches VGS(th), the drain begins to flow with current Id, and then Vgs continues to rise, Id also gradually rises, and Vds still maintains VDD. When Vgs reaches the Miller platform voltage VGS(pl), Id also rises to the maximum load current ID, and the voltage of Vds begins to drop from VDD.

　　During the Miller platform, the Id current maintains ID and the Vds voltage continues to decrease.

　　At the end of the Miller platform, the Id current still maintains ID, and the Vds voltage decreases to a lower value. After the Miller platform ends, the Id current still maintains ID, and the Vds voltage continues to decrease, but the slope of the decrease is very small at this time, so the amplitude of the decrease is also very small, and finally stabilizes at Vds = Id × Rds(on). Therefore, it can usually be considered that the MOSFET is basically turned on after the Miller platform ends.

　　The difficulty in understanding the above process is why the voltage of Vgs is constant in the Miller platform area? The driving circuit still provides driving current to the gate and still charges the gate capacitance, so why does the gate voltage not rise? Moreover, the gate charge characteristics are not intuitive for a vivid understanding of the MOSFET turn-on process. Therefore, the following will explain the MOSFET turn-on process based on the drain conduction characteristics.

　　MOSFET drain conduction characteristics and switching process

　　The drain conduction characteristic of MOSFET is shown in Figure 2. MOSFET is the same as triode. When MOSFET is used in amplifier circuit, this curve is usually used to study its amplification characteristic. The only difference is that triode uses base current, collector current and amplification factor, while MOSFET uses gate voltage, drain current and transconductance.

　　The transistor has three working areas: cut-off area, amplification area and saturation area, while the corresponding areas for MOSFET are the turn-off area, constant current area and variable resistance area. Note that the constant current area of ​​MOSFET is sometimes also called the saturation area or amplification area. When the driving turn-on pulse is applied to the G and S poles of MOSFET, the voltage of Vgs gradually increases, and the turn-on trajectory ABCD of MOSFET is shown in the route of Figure 3.

　　Before turning on, the MOSFET's starting operating point is at point A in the lower right corner of Figure 3. The VDD voltage of AOT460 is 48V, the Vgs voltage gradually increases, the Id current is 0, the Vgs voltage reaches VGS(th), and the Id current gradually increases from 0.

　　AB is the process of Vgs voltage increasing from VGS(th) to VGS(pl). In the process from point A to point B, it can be found very intuitively that this process works in the constant current region of MOSFET, that is, the process of Vgs voltage and Id current automatically finding balance, that is: the change of Vgs voltage is accompanied by the corresponding change of Id current, and the change relationship is the transconductance of MOSFET:

　　When the Id current reaches the maximum allowable current ID of the load, the corresponding gate voltage at this time. Since the Id current is constant at this time, the gate Vgs voltage is also constant, as shown in BC in Figure 3. At this time, the MOSFET is in a relatively stable constant current region and works in the amplifier state.

　　Before turning on, the voltage of Vgd is Vgs-Vds, which is a negative voltage. When it enters the Miller platform, the absolute value of the negative voltage of Vgd keeps decreasing and turns to a positive voltage after passing 0. Most of the current of the drive circuit flows through CGD to sweep the charge of the Miller capacitor, so the voltage of the gate remains basically unchanged. After the Vds voltage drops to a very low value, the charge of the Miller capacitor is basically swept away, that is, point C in Figure 3. Therefore, the voltage of the gate begins to rise again under the charge of the drive current, as shown in CD in Figure 3, making the MOSFET further fully turned on.

　　CD is a variable resistance area, and the corresponding Vgs voltage corresponds to a certain Vds voltage. When the Vgs voltage reaches the maximum value, the Vds voltage reaches the minimum value. Since the Id current is constant with ID, the Vds voltage is the product of ID and the on-resistance of the MOSFET.

　　in conclusion

　　Based on the MOSFET drain conduction characteristic curve, we can intuitively understand the process of crossing the turn-off region, constant current region and variable resistance region when the MOSFET is turned on. The Miller platform is the constant current region, the MOSFET works in the amplification state, and the Id current is the product of the Vgs voltage and the transconductance.