SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Application Information (Cont.)
MOSFET’s with low Rds(on) are used for the bottom switch.
MOSFET power dissipation consists 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 calculated as
IQ1,rms = Io
D(1 +
δ2
12
).
The conduction losses are then
Qgs2 is the additional gate charge required for the switch
current to
Qgd is the
reach its full-scale
charge needed to
cvhaaluregeIdsg.aanted-to-drain
(Miller)
capacitance when Vds is falling.
Switching losses occur during the time interval [t1, t3].
Defining tr = t3-t1 and 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 R , external resistance R and the gate
gi
ge
resistance R within the MOSFET i.e.
g
Ptc =
I2
Q1,rms
Rds(on).
Rgt = Rgi+Rge+Rg.
Rds(on) varies with temperature and gate-source voltage.
Curves showing Rds(on) variations can be found in
manufacturers’ data sheet. From the Si4860 datasheet,
Rds(on) is less than 8mΩ when Vgs is greater than 10V.
However Rds(on) increases by 50% as the junction
temperature increases from 25oC to 110oC.
Vgsp is the Miller plateau voltage shown in Figure 11.
Similarly an approximate expression for tf is
tf
=
(Qgs2 + Qgd )Rgt .
Vgsp
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.
Only a portion of the total losses P = Q V f is dissipated in
g
g cc s
the MOSFET package. Here Q is the total gate charge
g
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.
Gate 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 losses are 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
In Figure 11,
Qgs1 is the gate charge needed to bring the gate-to-source
voltage Vgs to the threshold voltage Vgs_th,
2004 Semtech Corp.
16
IQ2,rms = Io
(1 −
D)(1 +
δ2
12
).
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