LTC1871-7
26
18717fd
applicaTions inForMaTion
rating for the inductor should be checked at the minimum
input voltage (which results in the highest inductor cur-
rent) and maximum output current.
SEPIC Converter: Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-to-source breakdown voltage (BV
DSS
), the threshold
voltage (V
GS(TH)
), the on-resistance (R
DS(ON)
) versus gate-
to-source voltage, the gate-to-source and gate-to-drain
charges (Q
GS
and Q
GD
, respectively), the maximum drain
current (I
D(MAX)
) and the MOSFET’s thermal resistances
(R
TH(JC)
and R
TH(JA)
).
The gate drive voltage is set by the 7V INTV
CC
low dropout
regulator. Consequently, 6V rated threshold MOSFETs are
required in most LTC1871-7 applications.
The maximum voltage that the MOSFET switch must
sustain during the off-time in a SEPIC converter is equal
to the sum of the input and output voltages (V
O
+ V
IN
).
As a result, careful attention must be paid to the BV
DSS
specifications for the MOSFETs relative to the maximum
actual switch voltage in the application. Many logic-level
devices are limited to 30V or less. Check the switching
waveforms directly across the drain and source terminals
of the power MOSFET to ensure the V
DS
remains below
the maximum rating for the device.
Sense Resistor Selection
During the MOSFET’s on-time, the control circuit limits
the maximum voltage drop across the power MOSFET to
about 150mV (at low duty cycle). The peak inductor cur-
rent is therefore limited to 150mV/R
SENSE
. The relationship
between the maximum load current, duty cycle and the
sense resistor is:
R
SENSE
≤
V
SENSE(MAX)
I
O(MAX)
•
1
1+
χ
2
•
1
V
O
+ V
D
V
IN(MIN)
+1
The V
SENSE(MAX)
term is typically 150mV at low duty
cycle and is reduced to about 100mV at a duty cycle of
92% due to slope compensation, as shown in Figure 11.
The constant ‘
χ
’ in the denominator represents the ripple
current in the inductors relative to their maximum current.
For example, if 30% ripple current is chosen, then
χ
= 0.30.
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. As a result, some iterative calculation is normally
required to determine a reasonably accurate value. Since
the controller is using the MOSFET as both a switching
and a sensing element, care should be taken to ensure
that the converter is capable of delivering the required
load current over all operating conditions (load, line and
temperature) and for the worst-case specifications for
V
SENSE(MAX)
and the R
DS(ON)
of the MOSFET listed in the
manufacturer’s data sheet.
The power dissipated by the MOSFET in a SEPIC converter
is:
P
FET
= I
O(MAX)
•
D
1– D
2
•R
DS(ON)
•D • ρ
T
+ k • V
IN
+ V
O
( )
2
•I
O(MAX)
•
D
• C
RSS
• f
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%/°C.
Figure
12 illustrates the variation of normalized R
DS(ON)
over temperature for a typical power MOSFET.
