AN3994 datasheet(9/53 Pages) STMICROELECTRONICS

9 / 53 page

AN3994

Parasitic capacitances overview

Doc ID 022380 Rev 1

9/53

Cgd is the sum of two contributions: the first one is related to the overlap of the JFET region

and the gate electrode (“1” component in Figure 5 and 6). The second component is the

capacitance of the depletion region under the gate (“2” component in Figure 5 and 6).

The equivalent capacitance Cgd decreases as the drain source voltage applied increases.

Cds capacitance is the junction capacitance of the body-drain diode (“7” components in

Figure 5 and 6). Its value varies as the p-body / n-drift junction thickness changes with the

VDS applied, according to the following formula:

Equation 1

The relevant datasheets report the static equivalent capacitance values in the electrical

characteristics as the following:

Equation 2

Equation 3

Equation 4

Cgd is also called “Miller capacitance”, as it’s placed in the feedback loop between the input

and the output of the device. It's value can be much larger in switching operations,

contributing to the achievement of a dynamic input capacitance of the MOSFET larger than

the sum of the static capacitances.

In order to simplify the switching performance comparison among MOSFETs from different

manufacturers or even different MOSFET technologies of the same brand, it can be useful

to consider the gate charge parameters instead of capacitances. Figure 7 shows a clamped

inductive load switching test circuit which helps to analyze the parasitic capacitance

behavior during the MOSFET switching. The considerations reported in the following

sections are valid if the driving source is supposed to provide any Ig current to the MOSFET

input capacitances and the circuit is ideal with no stray inductances.

)

(

ds ∝

measured @ VGS=0 V, VDS=25 V

iss

⊕

measured @ VGS=0 V, VDS=25 V

rss

measured @ VGS=0 V, VDS=25 V

oss

⊕

AN3994 Datasheet(PDF) 9 Page - STMicroelectronics