S

#### [email protected]

- Jan 1, 1970

- 0

D said:I'm currently trying to find an effective way to select a mosfet with

the lowest power dissipation for the following circuit values:

Id=2A

Vd=270V

f=100khz

D=40%

Mosfet Driver Specs

Claims up to 2A peak gate drive

Tf = Tf = 14nS with 1nF load

Rdson is just a conduction loss.

Gate capacitance is the main cause of switching loss due to slowing

down driver rates. <<Not sure about that..

Rdson and gate capacitance are inversely proportional for mosfets?

<<Not sure about that too..

If so...

By finding the balance between Rdson and gate capacitance, I could get

less total power dissipation.

In that case...

Is there a figure of merit...Maybe like Ciss/Rdson? I just go for the

smallest number?

I've been looking at the SPP21N50

Rds(on) 0.19 ohms

Ciss = 2400pF

This mosfet has a great Rdson but I wonder if I could get less

dissipation with more Rdson and less Ciss?

What's a good selection procedure?

I''m hoping for some hints while I google for answers.

D from BC

After reading the book "Switching Power Supplies A to Z" by Sanjaya

Maniktala, I derived the following expression for the switching loss

(Pm) in the Miller region. All the parameters can be found on a

typical mosfet data sheet.

//Back-of-envelope calculation of Miller-plateau crossover loss.

//The Miller loss omits the loss as the drain current swings at

VDS=Vin constant.

//The Miller loss is the largest switching loss which occurs as VDS

swings at IL constant

theta=Qgs/(Qg-Qgd);//Ubiquitous gate-charge factor appearing in Miller

loss formula

Pm=(IL*Vin*Qgd*Rdrive*f/(2*Vdrive))*(1/(1-theta)+1/(theta-Vsat/

Vdrive));

where,

//Parameters (SI units)

f=500.0e3;//Switching frequency

Vdrive=4.5;//Gate driver voltage high voltage

Vsat=0.0;//Gate driver logic low voltage

Rdrive=2.0;//Lumped drive impedance plus mosfet gate resistance Rg

Vin=15.0;//Input voltage for the buck VDS(t=0) for all topologies

IL=22.0;//Free-wheeling diode current

//Mosfet parameters at a typical VDS

Qg=36.0e-9;//Gate charge factor Si4442DY, VDS=15V, VGS=4.5V, ID=22A

Qgs=8.0e-9;//Gate charge factor Si4442DY, VDS=15V, VGS=4.5V, ID=22A

Qgd=10.5e-9;//Gate charge factor Si4442DY, VDS=15V, VGS=4.5V, ID=22A

Stephen

http://www.stebla.pwp.blueyonder.co.uk