AN2644 datasheet(48/64 Pages) STMICROELECTRONICS

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Power MOSFET driving energy in ZVS operation

AN2644

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Appendix A

Power MOSFET driving energy in ZVS

operation

When power MOSFETs are switched on under ZVS conditions, i.e. when their drain-to-

source is already zero, Miller effect due to drain-to-source voltage modulation is absent.

This reduces the gate charge required to turn the power MOSFET fully on, the turn-on

energy, as well as the total driving energy. This can be conveniently examined on the gate-

charge characteristic of the device (see Figure 29).

The black curve is the normal piecewise-linear gate-charge curve. The first portion is that

related to the charge of the input capacitance CGS+CGD, and Qgs is the charge needed to

bring the gate voltage up to the plateau value VM that lets the power MOSFET carry the

specified current. The flat portion is the so-called "Miller plateau", where the gate voltage

does not increase because the drain-to-source voltage is falling and CGD is wiping out the

charge supplied to the gate until the charge Qgd has been supplied. The third rising portion

is related to the overcharge of CGS+CGD needed to minimize RDS(on). Note that the slope of

this line is lower than that of the first portion. This is due to the modulation of CGD, which is

now considerably larger (CGS is essentially constant). The total gate charge supplied to the

gate to bring its voltage up to the final value VGS is Qg. The values of Qgs, Qgd, Qg are

specified on the datasheet for given VGS, VDS, ID.

The grey-hatched area included between the gate-charge curve and the VGS horizontal line

is the energy lost in the gate driver to charge the gate up to the final value VGS and turn the

power MOSFET fully on. Its value can be found with simple geometric considerations:

Equation 19

Figure 29.

Comparison of gate-charge characteristics with and without ZVS

ONHS

--- QgsV

Qgs

Qgd

() V

–

()

[]

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