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How is gate current calculated?
Since the gate current is constant, the time axis can be expressed in terms of gate charge Qg by multiplying time by constant gate current IG. (The gate charge is calculated as Qg=IG×t.) The gate of a MOSFET starts accumulating electric charge when a voltage is applied to it.
What is total gate charge?
The Total Gate Charge (Qg) is the amount of charge that needs to be injected into the gate electrode to turn ON (drive) the MOSFET. The unit of Qg is the Coulomb (C), and if the total gate charge is large, it will take time to charge the capacitor necessary for turning ON the MOSFET, increasing switching loss.
What is gate current?
The gate current is the equivalent of the base current for the n-p-n transistor; the resulting larger collector current is the base current for the p-n-p transistor.
What does the total gate charge ( QG ) mean?
The Total Gate Charge (Qg) is the amount of charge that needs to be injected into the gate electrode to turn ON (drive) the MOSFET. The unit of Qg is the Coulomb (C), and if the total gate charge is large, it will take time to charge the capacitor necessary for turning ON the MOSFET, increasing switching loss.
What is the minimum charge for a gate?
The figure shows an example of the dynamic input characteristics (Qg-V GS ). In the figure, the drain side supply voltage (V DD) and drain current (I D) are fixed, and the minimum amount of charge necessary for I D =30A (V DD =300V) current to flow is 60nC. At this time, the Gate-Source voltage (V GS) is 6.5V.
How to calculate worst case gate charge times?
From your datasheet… spec: VGS = 4.5V, RGEN = 6 Ω You are driving gate with 1k5 instead of 6 Ω , so we use total Gate Charge equation and datasheet specs instead using worst case. Yet if we simply used T=RC=1k5*195pf=293ns (typ) (0.3us) is only 30% typ of the more accurate worst case gate charge times.
However, the characteristics for Total Gate Charge and ON Resistance are in a trade-off relationship. Generally, the smaller the chip size (surface area) of the MOSFET the smaller total charge but the ON resistance will increase. In other words, there is a conflicting relationship between switching loss and power loss during operation.