SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441ITSTRT
Semtech Corporation
SC2441ITSTRT Datasheet PDF : 35 Pages
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SC2441
POWER MANAGEMENT
Applications Information (Cont.)
The losses in power MOSFET’s consist of
a) conduction loss due to the channel resistance Rds(on),
b) switching loss due to the switch rise time tr and fall time
tf and
c) the gate loss due to the gate resistance RG.
Top Switch:
The RMS value of the top switch current is
IQ1,rms = Io
D(1 +
δ2
12
).
Its conduction loss is then
Ptc =
I2
Q1,rms
Rds(on).
Rds(on) varies with temperature and gate-source voltage.
Curves showing Rds(on) variations can be found in
manufacturers’ data sheet. From the Si7882DP datasheet,
Rds(on) is less than 4.5mΩ when Vgs is greater than 5V.
However Rds(on) increases by nearly 40% as the junction
temperature increases from 25°C to 125°C.
The switching losses can be estimated using the simple
formula
Pts
=
1
2
(
t
r
+ t f )(1+
δ
2
)Io
Vin
fs
.
where t is the rise time and t is the fall time of the switching
r
f
process. Different manufactures have different definitions
athnedtytepsictacloMnOdSitiFoEnTssfworitctrhainngdcthf.aTroacctlearriisftyictsheusned,ewr celasmkeptechd
inductive mode in Figure 11.
In Figure 11,
Q is the gate charge needed to bring the gate-to-source
gs1
voltage V to the threshold V ,
gs
gs_th
Qgs2 is the additional gate charge required for the switch
current to
Q is the
reach its full-scale
charge needed to
cvhaaluregeIdsg.aanted-to-drain
(Miller)
gd
capacitance when V is falling.
ds
Switching losses occur during the time interval [t1, t3].
Defining tr = t3-t1. tr can be approximated as
tr
=
(Qgs2 + Qgd )Rgt
Vcc − Vgsp
.
where Rgt is the total resistance from the driver supply rail
to the gate of the MOSFET. It includes the gate driver internal
impedance Rgi, external resistance Rge and the gate
resistance Rg within the MOSFET i.e.
R = R +R +R .
gt
gi ge g
V is the Miller plateau voltage shown in Figure 11.
gsp
Similarly an approximate expression for tf is
tf
=
(Qgs2 + Qgd )Rgt .
Vgsp
Only a portion of the total losses Pg = QgVccfs is dissipated in
the MOSFET package. Here Qg is the total gate charge
specified in the datasheet. The power dissipated within
the MOSFET package is
Ptg
=
Rg
R gt
QgVcc fs.
The total power loss of the top switch is then
Pt = Ptc+Pts+Ptg.
charge
Figure 11. MOSFET switching characteristics
If the input supply of the power converter varies over a
wide range, then it will be necessary to weigh the relative
importance of conduction and switching losses. This is
because conduction loss is inversely proportional to the
input voltage. Switching loss however increases with the
input voltage. The total power loss of MOSFET should be
calculated and compared for high-line and low-line
cases. The worst case is then used for thermal design.
Bottom Switch:
The RMS current in bottom switch can be shown to be
IQ2,rms = Io
(1 −
D)(1 +
δ2
12
).
2005 Semtech Corp.
18
www.semtech.com
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