Datasheet

LTC3862
27
3862fc
For more information www.linear.com/LTC3862
Finally, check the MOSFET manufacturer’s data sheet for
an avalanche energy rating (EAS). Some MOSFETs are not
rated for body diode avalanche and will fail catastrophi
-
cally if the V
DS
exceeds the device BV
DSS
, even if only by
a fraction of a volt. Avalanche-rated MOSFETs are better
able to sustain high frequency drain-to-source ringing near
the device BV
DSS
during the turn-off transition.
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be known.
This power dissipation is a function of the duty cycle, the
load current and the junction temperature itself (due to
the positive temperature coefficient of its R
DS(ON)
). As a
result, some iterative calculation is normally required to
determine a reasonably accurate value.
The power dissipated by the MOSFET in a multi-phase
boost converter with n phases is:
P
I
nD
RD
FET
OMAX
MAX
DS ON MAX
=
()






()
()
•–
•••
1
2
ρρ
T
OUT
OMAX
MAX
RSS
kV
I
nD
Cf
+
()
••
•–
••
()
2
1
The first term in the equation above represents the I
2
R
losses in the device, and the second term, the switching
losses. The constant, k = 1.7, is an empirical factor inversely
related to the gate drive current and has the dimension
of 1/current.
The ρ
T
term accounts for the temperature coefficient of
the R
DS(ON)
of the MOSFET, which is typically 0.4%/ºC.
Figure 20 illustrates the variation of normalized R
DS(ON)
over temperature for a typical power MOSFET.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T
J
= T
A
+ P
FET
• R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(CA)
). This value
of T
J
can then be compared to the original, assumed value
used in the iterative calculation process.
It is tempting to choose a power MOSFET with a very low
R
DS(ON)
in order to reduce conduction losses. In doing
so, however, the gate charge Q
G
is usually significantly
higher, which increases switching and gate drive losses.
Since the switching losses increase with the square of
the output voltage, applications with a low output voltage
generally have higher MOSFET conduction losses, and
high output voltage applications generally have higher
MOSFET switching losses. At high output voltages, the
highest efficiency is usually obtained by using a MOSFET
with a higher R
DS(ON)
and lower Q
G
. The equation above
can easily be split into two components (conduction and
switching) and entered into a spreadsheet, in order to
compare the performance of different MOSFETs.
applicaTions inForMaTion
Figure 20. Normalized Power MOSFET R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
3862 F20
0.5
0
0
50
100
2.0