If Crss is large, the rise in drain current is delayed even after the gate is turned on, and the fall in current is delayed after the gate is turned off. If Coss is large, a current arising due to Coss flows at the output even when the gate is turned off, and time is required for the output to turn off completely.Ĭrss is the gate-drain capacitance Cgd itself, and is called the feedback capacitance or the reverse transfer capacitance. Qg is the amount of charge necessary to drive (charge) Ciss.Ĭoss is the output capacitance, obtained by adding the drain-source capacitance Cds and the gate-drain capacitance Cgs, and is the total capacitance on the output side. This capacitance must be driven (charged) in order to cause the MOSFET to operate, and so is a parameter of importance when studying the drivability of an input device or input losses. These are important parameters affecting switching performance.Ĭiss is the input capacitance, and is the capacitance obtained by totaling the gate-source capacitance Cgs and the gate-drain capacitance Cgd it is the capacitance of the MOSFET as a whole, as seen from the input. On data sheets which provide separate descriptions of static characteristics and dynamic characteristics, these are classified as dynamic characteristics. The three parameters Ciss, Coss, Crss appearing on MOSFET data sheets in general relate to these parasitic capacitances. The drain-source capacitance Cds is the junction capacitance of the parasitic diode. The gate-source capacitance Cgs and gate-drain capacitance Cgd in the diagram below are determined by the capacitance of the gate oxide film.
A PN junction is formed between the drain and source with substrate intervening, and a parasitic ("body") diode is present. The drain and source of a MOSFET are insulated from the gate by the gate oxide film. In the power MOSFETs we are here considering that handle large amounts of power, the parasitic capacitance must be regarded as a parameter that limits the usage frequency and switching speed. The diagram below is for an example of an N-channel MOSFET, but the situation is much the same for P-channel devices. In continuation of the previous discussion of Si transistor types, features and basic characteristics, we here provide an additional explanation of the characteristics of Si MOSFETs that are at present widely used as power switches.ĭue to their structure, MOSFETs have a parasitic capacitance, as indicated in the diagram below.
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