Texas Instruments SLVP089 - Manual (Page 23)

Design Procedures
2-5 Design Procedure
The power dissipation (conduction + switching losses) can be approximated
as:
P
D
+
ǒ
I
2
O
r
DS(ON)
D
Ǔ
)
ǒ
0.5 V
I
I
O
t
r ) f
f
Ǔ
Assuming total switching time, t
r+f
, = 100 ns, a 55
°
C maximum ambient tem-
perature, and r
DS(ON)
adjustment factor = 1.6, then:
P
D
+
ƪ
3
2
(0.04 1.6) 0.64
ƫ
)
ƪ
0.5 5.5 3
ǒ
0.1 10
–6
Ǔ ǒ
100 10
3
Ǔ ƫ
+ 0.45 W
The thermal impedance for Q1 R q JA
= 90
°
C/W for FR-4 with 2-oz. copper and
a one-inch-square pattern, thus:
T
J
+
T
A
)
ǒ
R
q
JA
P
D
Ǔ
+
55
)
(90 0.45)
+
96
°
C
2.3.4 Synchronous Switch and Rectifier
The synchronous switch calculations follow the same path as the power switch
except that the duty cycle is 1–D. Then r
DS(ON)
should be less than 0.012 V
3A = 40 m
W
. Selecting an IRF7201 with an r
DS(ON)
= 30 m
W
, then:
P
D
+
ƪ
3
2
(0.03 1.6) 0.36
ƫ
)
ƪ
0.5 5.5 3
ǒ
0.1 10
–6
Ǔ ǒ
100 10
3
Ǔ ƫ
+
0.238 W
T
J
+
T
A
)
ǒ
R
q JA
P
D
Ǔ
+
55
)
(90 0.238)
+
76
°
C
The catch rectifier serves as a backup device for the synchronous switch and
conducts during the time interval when both devices are off. The 30BQ015 is
a 3-A, 15-V rectifier in an SMC power surface-mount package. If the synchro-
nous switch were not used, the power dissipation for the catch diode would be:
P
D
+
I
O
V
D
ǒ
1 – D
Min
Ǔ
+
3 0.7 0.71
+
1.491 W
However, since the catch diode actually conducts only during the deadtime
and switching time, the power dissipation is:
P
D
+
I
O
V
D
t
r ) f
f
+
3 0.7
ǒ
0.1 10
–6
Ǔ ǒ
100 10
3
Ǔ
+
2.1 mW
2.3.5 Snubber Network
A snubber network is usually needed to suppress the ringing at the node where
the power switch drain, output inductor, and synchronous switch drain con-
nect. This is usually a trial-and-error sequence of steps to optimize the net-
work, but as a starting point, select a snubber capacitor with a value that is
4–10 times larger than the estimated capacitance of the synchronous switch
and catch rectifier. Then, measuring a ringing time constant of 3 ns, R is:
R
+ 3 10 –
9
C
+ 3 10 –
9
1000 10 –
12
+
3
W