Estimation of MOS tube drive current and analysis of several special applications of MOS drive
The estimation of MOS tube drive current is the focus of this article, the following parameters:
One might calculate like this:
Turn-on current
Ion=Qg/Ton=Qg/Td(on)+tr, bring in the data to get Ion=105nc/(140+500)ns=164mA
Shutdown current
Ioff=Qg/Toff= Qg/Td(off)+tf, bring in the data to get Ioff=105nc/(215+245)ns=228mA.
So it is concluded that the drive current only needs to be about 300mA. Think carefully about this calculation, right? Here we must pay attention to such a conditional detail, RG=25Ω. So this metric is meaningless.
How should it be calculated? In fact, it should be like this, the switching current is determined according to the switching speed of the product. According to I=Q/t, the specific MOS transistor Qg data and the current capability of our circuit can be obtained, and Ton=Qg/I can be obtained. For example, 45N50, when Vgs=10V, VDS=400V, Id=48A, Qg=105nC. If it is driven with a driving capability of 1A, a switching speed as fast as 105nS can be obtained.
Of course, this can only estimate the value of the drive current, and further testing of the overshoot waveform of the MOS tube is required. When designing the drive circuit, generally a resistor of about 10Ω is connected in front of the MOS tube (adjust the parameters according to the test waveform).
It should be noted here that Qg is used to calculate the turn-on and turn-off speed, not the gate capacitance.
The MOS tube drive current estimation has been discussed, and the MOS tube turn-on process is described below:
Start to charge the MOS tube Cgs, and when the voltage rises to 5V, a certain current flows through Id. Continue to charge, Id is getting bigger and bigger, but not fully turned on. When Id rises to the electric current, Id no longer changes, Cgs also no longer changes.
At this time, the input voltage does not charge Cgs, but charges the Cgd Miller capacitor, and then the MOS tube is completely turned on.
After the MOS tube is fully turned on, the input voltage no longer passes through the Miller capacitor, and continues to charge Cgs until Vgs is equal to the input voltage of 10V.
In the figure, the Vgs input voltage remains unchanged, that is, the Qgd stage, the input voltage does not charge Cgs, but charges the Cgd Miller capacitor. This is the inherent transfer characteristic of MOS tubes. The constant voltage during this period is also called the plateau voltage.
At this time, the current and resistance of the MOS tube, according to P=I I R, the power consumed by the tube at this time, the heat is serious, so try to make the working time of the platform voltage as short as possible.
Generally speaking, the higher the withstand voltage level, the larger the input capacitance of the MOS tube, the smaller the reverse transmission capacitance Crss, and the corresponding reduction of the Miller effect.
Several special applications of MOS drivers
1. Low voltage application
When using a 5V power supply, if the traditional totem pole structure is used at this time, because the be of the triode has a voltage drop of about 0.7V, the actual final voltage applied to the gate is only 4.3V. At this time, there is a certain risk in choosing a MOS tube with a nominal gate voltage of 4.5V. The same problem also occurs when using 3V or other low voltage power supplies.
2. Wide voltage application
The input voltage is not a fixed value, it will change with time or other factors. This change causes the driving voltage provided by the PWM circuit to the MOS tube to be unstable.
In order to make the MOS tube safe under high gate voltage, many MOS tubes have built-in voltage regulator tubes to forcibly limit the amplitude of the gate voltage. In this case, when the provided driving voltage exceeds the voltage of the Zener tube, a large static power consumption will be caused.
At the same time, if the gate voltage is simply reduced by the principle of resistive voltage division, when the input voltage is relatively high, the MOS tube works well, but when the input voltage decreases, the gate voltage is insufficient, resulting in insufficient conduction, thereby increasing power consumption .
3. Dual voltage application
In some control circuits, the logic part uses a typical 5V or 3.3V digital voltage, while the power part uses 12V or even higher voltage. The two voltages are connected in a common way. This puts forward a requirement that a circuit needs to be used so that the low-voltage side can effectively control the MOS tube on the high-voltage side, and the MOS tube on the high-voltage side will also face the problems mentioned in 1 and 2. In these three cases, the totem pole structure cannot meet the output requirements, and many off-the-shelf MOS driver ICs do not seem to include a gate voltage limiting structure.