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