Datasheet

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SLVS612 − APRIL 2006
22
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DESIGN EXAMPLE
4. Calculate the power losses
Power losses in the high-side MOSFET (Si7860DP) at 14.4-V
IN
where switching losses dominate can be
calculated from equation (27).
I
RMS
+ I
O
d
Ǹ
+ 8 0.086
Ǹ
+ 2.35 A
substituting (27) into (26) yields
P
COND
+ 2.35
2
0.008
(
1 ) 0.007
(
150 * 25
))
+ 0.083 W
and from equation (28), the switching losses can be determined.
P
SW(fsw)
+
ǒ
V
IN
I
O
t
SW
Ǔ
f
SW
+ 14.4 V 8A 20 ns 170 kHz + 0.39 W
The MOSFET junction temperature can be found by substituting equation (30) into equation (29)
T
J
+
ǒ
P
COND
) P
SW
Ǔ
q
JA
) T
A
+
(
0.083 ) 0.39
)
40 ) 85 + 90
O
C
5. Calculate synchronous rectifier losses
The synchronous rectifier MOSFET has two loss components, conduction, and diode reverse recovery losses.
The conduction losses are due to I
RMS
losses as well as body diode conduction losses during the dead time
associated with the anti-cross conduction delay.
The I
RMS
current through the synchronous rectifier from (31)
I
RMS
+ I
O
1 * d
Ǹ
+ 8 1 * 0.126
Ǹ
+ 7.48 A
RMS
The synchronous MOSFET conduction loss from (26) is:
P
COND
+ 7.48
2
0.008
(
1 ) 0.007
(
150 * 25
))
+ 0.83 W
The body diode conduction loss from (32) is:
P
DC
+
2
I
O
V
FD
t
DELAY
f
SW
+
2
8.0 A
0.8 V
100 ns
170 kHz
+
0.218 W
The body diode reverse recovery loss from (33) is:
P
RR
+ 0.5 Q
RR
V
IN
f
SW
+ 0.5 30 nC 14.4 V 170 kHz + 0.037 W
The total power dissipated in the synchronous rectifier MOSFET from (34) is:
P
SR
+ P
RR
) P
COND
) P
DC
+ 0.037 ) 0.83 ) 0.218 + 1.085 W
The junction temperature of the synchronous rectifier at 85°C is:
T
J
+ P
SR
q
JA
) T
A
+
(
1.085
)
40 ) 85 + 128
o
C
In typical applications, paralleling the synchronous rectifier MOSFET with a Schottky rectifier increases the
overall converter efficiency by approximately 2% due to the lower power dissipation during the body diode
conduction and reverse recovery periods.
(44)
(45)
(46)
(47)
(48)
(49)
(50)
(51)
(52)
(53)