LTC3862_15 Datasheet, PDF (27/42 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
Applications Information
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:
( ) ^PFET
=



n
•
I_O(MAX)
1– D_MAX
²
 ^{• R}_DS(ON) ^{• D}_MAX ^{• ρ}_T

( ) ⁺
k
•
V_OUT2
•
n
•
I_O(MAX)
1– D_MAX
• C_RSS • f
The first term in the equation above represents the I²R
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.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3862 F20
Figure 20. Normalized Power MOSFET R_DS(ON) vs Temperature
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.
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