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Can you explain more about MOS FET's Qg, Qgs, and Qgd parameters?

Latest Updated:10/01/2006

Question:

I would like to ask about a MOS FET's Qg, Qgs, and Qgd parameters.

For example, in the case of the 2SK3510, the data sheet states the following:
Qg = Typ. 150 nC
Qgs = Typ. 30 nC
Qgd = Typ. 52 nC

Why is this not Qg = Qgs + Qgd?

Answer:

Please look at the following DYNAMIC INPUT/OUTPUT CHARACTERISTICS graph from the 2SK3510 data sheet.



Qgs indicates the charge until the FET enters the ON state.
The capacitor between the gate and the source charges until the FET enters the ON state.
In the graph, the initial ramp-up period of VGS is equivalent to this state.
From the graph, this figure equates to about 30 nC.

Qgd indicates the charge from when the FET enters the ON state to when the voltage between the drain and source starts to drop. In this period, the capacitor between the gate and the drain is being charged. In the graph, the period of flat VGS after initial VGS ramp-up is equivalent to this state.
From the graph, this figure in this flattening out period equates to about 50 nC.

Once the capacitor between the gate and the drain is charged, the gate voltage ramps up again.
As described in the specification conditions, Qg indicates the total gate charge required to obtain a gate-to-source voltage of 10 V.
In other words,Qg = Qgs + Qgd + (Charge necessary for VGS to reach 10 V during ramp up after flattening).
However, from the graph, this figure equates to about 150 nC when VDD = 60 V.

For this reason, Qg in the actual specifications does not equal Qgs + Qgd.
Suitable Products
Power MOSFETs