AN605 データシートの表示（PDF） - Vishay Semiconductors

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AN605

Power MOSFET Basics: Understanding MOSFET Characteristics Associated With The Figure of Merit

Vishay Semiconductors 

AN605

Vishay Siliconix

CDG

CDG(on)

Turn-Off

CDG(off)

Turn-On

VGS = VDS

VDS

FIGURE 5. Gate-drain capacitance over the drain-source

voltage range

VDS

Gate

Voltage

Miller

Plateau

Current

FIGURE 6. Gate voltage and respective voltage and current

during turn-on

However, it should be noted that because the equivalent circuit

is much more complex than a resistor and three capacitors,

these capacitances may only be used for understanding the

behavior of the switching transients.

The gate-drain capacitance Cgd and the gate-source

capacitance Cgs are voltage-dependent capacitances, and

therefore the capacitance value changes depending on the

voltage that appears across the drain source and also across

the gate source of the device. The change in Cgd is more

significant than Cgs, simply because the voltage that appears

across it is much larger than that seen across Cgs. The change

in Cgd4, shown in Figure 5, can be as large as a factor of 100

and usually is approximated to two static values. These

changes in capacitance have an influence on the voltage that

appears at the gate of the device, called the Miller Plateau4.

This causes the turn-off and turn-on rise and fall times on the

switching transients and causes the gate voltage to “flatten

out” as shown in Figure 6.

CONCLUSIONS

This application note is the first in a series of technical

documents describing the basic characteristics and operating

performance of the power MOSFET when implemented in a

switched-mode power supply. It is intended to give the reader

a thorough background on the device technology behind

Vishay Siliconix MOSFETs.

The FOM does not in itself enable the power supply designer

to choose the ideal device, but does give an overview of the

device technology and possible performance. To give a

definitive and subjective analysis, every FOM would have to

be modified to include information on the application in which

the MOSFET was to be used. Therefore the application note

defines the principle characteristics (Table 1), which need to be

taken into account when choosing the correct device for a

specific application.

4. “Power Electronics, converters, Applications and Design,” Mohan,

Underland and Robbins. ISBN 0-471-58404-8.

www.vishay.com

4

Document Number: 71933

08-Sep-03

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