Re: Selecting Mosfets for Lowest Power Dissipation

On 12 Feb 2007 13:13:26 -0800, stebla@xxxxxxxxxxxxxxxx wrote:

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 ***.

//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

So...the switching loss calculation includes Qgd, Qgs and Qg...or in
other words, every pin to pin capacitance.

Like someone posted...
I'm probably going to select a bunch of mosfets with the right Vds and
Id ratings. ( Price will be a factor at some point.)
Then try'm all in LTSPICE.
(.step mosfetlibrary 1 100 1???) <<Fictional directive but purpose
should be understood... I might make this a new post topic since I'm
new to LTspice.
I estimate about 100 modern mosfets total from multiple sources.
(Vdsmin=300, Idmin=2A)
Say 1min/sim.
So.. I should get my results in about 2 hours..
I'll get the winning mosfet as it applies in my app...
But the set up is a PITA and I'm guessing it'll take all day to
collect models and figure how to configure ltspice.
D from BC
.