AN-1001 datasheet(2/4 Pages) IRF

Part #	AN-1001

Description	Application Note

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Manufacturer	IRF [International Rectifier]

Direct Link	http://www.irf.com

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A More Realistic Characterization Of Power MOSFET

Output Capacitance Coss

Introduction:

The

Power

MOSFET

has

gained

popularity and become the dominant

switching device in power

electronics

since 1975. Its fast switching speed has

extended power conversion switching

frequencies from the 20kHz range of

bipolar transistor to beyond 100kHz. in

hard switching. With soft switching

techniques

such

as

zero

voltage

switching

(ZVS)

and

zero

current

switching

(ZCS),

the

switching

frequency can exceed Mega Hertz.

As switching frequency moves upward,

power MOSFET parasitic parameters

such as inductance and capacitance

should be well defined and understood in

order to optimize the particular power

conversion

design.

This

design

tip

focuses

on

explaining

the

power

MOSFET output capacitance Coss and

how

it

actually

affects

the

power

conversion circuit. Other capacitance’s

such as input capacitance Ciss, and

reverse transfer capacitance Crss, and the

related gate charges have been well

explained

in

Rectifier publications.

In hard switching circuits, Coss is used

to

calculate

the

additional

power

dissipation of power MOSFET due to

discharging this output capacitor every

switching

cycle.

In

soft

switching

circuits, Coss may be used to calculate

the resonant frequency or transition time,

which is critical in establishing ZVS and

/ or ZCS conditions. Unfortunately the

value of Coss varies non-linearly as a

function of drain to source voltage Vds.

The value of Coss specified in most

manufacturer’s data sheets are at 25V

Vds which is not really useful in actual

circuit application.

In

an

effort

to

better

aid

circuit

designers, International Rectifier has

taken the extra steps to specify the

effective Coss (Coss at Vds of 1V and

80%

Vdss)

and

also

extend

the

capacitance curves to 80% of Vdss ,

instead of 50V, for IRs new range of

high voltage HEXFETs. An example of

these curves for a new 600V HEXFET is

shown in Fig. 1 below :

Fig. 1:. Typical Capacitance Vs. Drain to Source

Voltage

Listed below are the definitions of Coss

effective and Coss @ 80% of Vdss, how

they are measured and how to apply

them in circuit design calculations.

Coss effective:

Coss effective is defined as a fixed

capacitance that would give the same

1

10

100

1000

10000

100000

1

10

100

1000

DS

V

, Drain-to-Source Voltage (V)

A

V

= 0V,

f = 1MHz

C

= C

+ C

, C

SHORTED

C

= C

C

= C

+ C

GS

iss

gs

gd

ds

rss

gd

oss

ds

gd

Ciss

Coss

Crss

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