2
AN1288.1
October 30, 2008
The total power loss in MOSFET consists of conduction loss
and switching loss, as shown in Equation 5:
In this relatively small duty cycle design, the low-side
MOSFET conducts current most of the time. To optimize the
converter efficiency, select the high-side MOSFET with low
gate charge for fast switching transition and low-side
MOSFET with low r
DS(ON)
.
To achieve the target efficiency, the budget power losses in
high-side and low-side MOSFETs are 0.5W and 1W,
respectively.
LOW-SIDE MOSFET SELECTION
The low-side MOSFET’s RMS current is approximated in
Equation 6:
Therefore, the ON-resistance of the low-side MOSFET must
be less than 5m
Ω
. Infineon’s BSC030N03LS is employed in
the ISL8105BEVAL1Z, ISL8105BEVAL2Z evaluation board.
The conduction loss in the low-side MOSFET is calculated
using Equation 7:
The switching loss in the low-side MOSFET is dominated by
the loss in body diode which can be calculated using
Equation 8:
Where t
D
is the total dead time in each switching period
(~60µs) and V
F
is the forward voltage drop of MOSFET’s
body diode.
The total power dissipation in the low-side MOSFET is
calculated using Equation 9:
HIGH-SIDE MOSFET SELECTION
For the high-side MOSFET selection, first we assume that
the conduction loss and the switching loss contribute evenly
to the total power dissipation.
The high-side MOSFET’s RMS current is approximated
using Equation 10:
Hence, the required ON-resistance of the high-side MOSFET is
7.3m
Ω
. Infineon’s BSC080N03LS is selected. The conduction
loss in the high-side MOSFET is calculated using Equation 11:
The switching loss in the high-side MOSFET can be
approximated using Equation 12:
where t
tr
is the combined ON and OFF MOSFET transition
times.
The total power dissipation in high-side MOSFET is shown in
Equation 13:
Overcurrent Protection Setting
The overcurrent function protects the converter from a shorted
output by using the low-side MOSFET’s r
DS(ON)
to monitor
the current. A resistor, R
BSOC
, programs the overcurrent trip
level. If overcurrent is detected, the output immediately shuts
off, it cycles the soft-start function in a hiccup mode (2 dummy
soft-start time-outs, then up to one real one) to provide fault
protection. If the shorted condition is not removed, this cycle
will continue indefinitely.
The overcurrent function will trip at a inductor current (I
trip
) is
determined using Equation 14:
where I
OCSET
is the internal 21.5µA (typ.) OCSET current
source.
The OC trip point varies mainly due to the MOSFET’s r
DS(ON)
variations. To avoid overcurrent tripping in the normal operating
load range, calculate the R
BSOC
resistor from Equation 14
using:
1. The maximum r
DS(ON)
at the highest junction temperature.
2. The minimum I
OCSET
from the specification table of the
datasheet.
Determine I
trip
for I
trip
> I
OUT(MAX)
+ (
Δ
I)/2, where
Δ
I is the
output inductor ripple current.
P
MOSFET TOT
(
)
P
cond
P
sw
+
=
(EQ. 5)
I
L RMS
(
)
I
OUT
1
D
–
1
1
12
------
Δ
I
L
I
OUT
-------------
⎝
⎠
⎜
⎟
⎛
⎞
2
⋅
+
⋅
⋅
13.9A
≈
=
(EQ. 6)
P
LFET cond
(
)
I
L RMS
(
)
2
r
DS ON
(
)
LFET
⋅
0.58W
=
=
(EQ. 7)
P
diode
I
O
t
D
V
F
F
SW
⋅
⋅
⋅
0.3W
=
=
(EQ. 8)
P
LFET TOT
(
)
0.88W
=
(EQ. 9)
I
H rms
(
)
I
OUT
D
1
1
12
------
Δ
I
L
I
OUT
-------------
⎝
⎠
⎜
⎟
⎛
⎞
2
⋅
+
⋅
⋅
5.85A
≈
=
(EQ. 10)
P
HFET cond
(
)
I
H RMS
(
)
2
r
DS ON
(
)
HFET
⋅
0.27W
=
=
(EQ. 11)
P
HFET SW
(
)
1
2
---
I
O
V
IN
t
tr
F
SW
⋅
⋅
⋅
⋅
1
2
---
C
OSS
V
IN
2
F
SW
⋅
⋅
⋅
+
=
0.17W
=
(EQ. 12)
P
HFET TOT
(
)
0.44W
=
(EQ. 13)
I
trip
2
I
•
OCSET
R
BSOC
•
r
DS ON
(
)
---------------------------------------------------------
=
(EQ. 14)
Application Note 1288
