Selection of components – FUJITSU MB39A105 User Manual
Page 19
MB39A105
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SELECTION OF COMPONENTS
• Nch MOS FET
The N-ch MOSFET for switching use should be rated for at least 20% more than the maximum output voltage.
To minimize continuity loss, use a FET with low R
DS(ON)
between the drain and source. For high output voltage
and high frequency operation, on/off-cycle switching loss will be higher so that power dissipation must be
considered. In this application, the SANYO MCH3405 is used. Continuity loss, on/off switching loss, and total
loss are determined by the following formulas. The selection must ensure that peak drain current does not exceed
rated values.
Continuity loss : P
C
On-cycle switching loss : P
S (ON)
Off-cycle switching loss : P
S (OFF)
Total loss : P
T
P
T
=
P
C
+
P
S (ON)
+
P
S (OFF)
Example: Using the SANYO MCH3405
Input voltage V
IN (Max)
=
2.4 V, output voltage V
O
=
9 V, drain current I
D
=
0.94 A, Oscillation frequency
f
OSC
=
500 kHz, L
=
6.8
µ
H, drain-source on resistance R
DS (ON)
:= 160 m
Ω
, tr
=
18 ns, tf
=
8 ns.
Drain current (Max) : I
D (Max)
Drain current (Min) : I
D (Min)
P
C
=
I
D
2
×
R
DS(ON)
×
Duty
P
S (ON)
=
V
D (Max)
×
I
D
×
tr
×
f
OSC
6
P
S (OFF)
=
V
D (Max)
×
I
D (Max)
×
tf
×
f
OSC
6
I
D (Max)
=
V
O
×
I
O
+
V
IN(Min)
ton
ton
=
V
O
−
V
IN(Min)
t
V
IN(Min)
2L
V
O
=
9
Ч
0.25
+
2.4
Ч
(9
−
2.4)
Ч
1
2.4
2
Ч
6.8
Ч
10
−
6
Ч
9
500
Ч
10
3
:=
1.20 (A)
I
D (Min)
=
V
O
×
I
O
−
V
IN(Min)
ton
V
IN(Min)
2L
=
9
Ч
0.25
−
2.4
Ч
(9
−
2.4)
Ч
1
2.4
2
Ч
6.8
Ч
10
−
6
Ч
9
500
Ч
10
3
:=
0.68 (A)