Fairchild SEMICONDUCTOR RC5040 User Manual
Page 7
APPLICATION NOTE
AN42
7
MOSFET Selection
This application requires the use of N-channel, Logic Level
Enhancement Mode Field Effect Transistors. The desired
characteristics of these components are:
• Low Static Drain-Source On-Resistance
R
DS,ON
< 37 m
Ω
(lower is better)
• Low gate drive voltage, V
GS
≤
4.5V
• Power package with low thermal resistance
• Drain current rating of 20A minimum
• Drain-Source voltage > 15V.
The on-resistance (R
DS,ON
) is the main parameter for MOS-
FET selection. It determines the MOSFET’s power dissipa-
tion, thus significantly affecting the efficiency of the
converter. Several suitable MOSFETs are shown in Table 5.
Note:
1. R
DS(ON
) values at Tj = 125
°
C for most devices were extrapolated from the typical operating curves supplied by the manufac-
turers and are approximations only.
Table 5. MOSFET Selection Table
Manufacturer & Model #
Conditions
1
R
DS, ON
(m
Ω
)
Package
Thermal
Resistance
Typ.
Max.
Fuji
2SK1388
V
GS
= 4V
I
D
= 17.5A
T
J
= 25
°
C
25
37
TO-220
Φ
JA
= 75
T
J
= 125
°
C
37
—
Siliconix
SI4410DY
V
GS
= 4.5V
I
D
= 5A
T
J
= 25
°
C
16.5
20
SO-8
(SMD)
Φ
JA
= 50
T
J
= 125
°
C
28
34
National Semiconductor
NDP706AL
V
GS
= 5V
I
D
= 40A
T
J
= 25
°
C
13
15
TO-220
Φ
JA
= 62.5
Φ
JC
= 1.5
NDP706AEL
T
J
= 125
°
C
20
24
National Semiconductor
V
GS
= 4.5V
I
D
= 10A
T
J
= 25
°
C
31
40
TO-220
Φ
JA
= 62.5
NDP603AL
T
J
= 125
°
C
42
54
Φ
JC
= 2.5
National Semiconductor
V
GS
= 5V
I
D
= 24A
T
J
= 25
°
C
22
25
TO-220
Φ
JA
= 62.5
NDP606AL
T
J
= 125
°
C
33
40
Φ
JC
= 1.5
Motorola
V
GS
= 5V
I
D
= 37.5A
T
J
= 25
°
C
6
9
TO-263
Φ
JA
= 62.5
MTB75N03HDL
T
J
= 125
°
C
9.3
14
(D
2
PAK)
Φ
JC
= 1.0
Int. Rectifier
V
GS
= 5V
I
D
= 31A
T
J
= 25
°
C
—
28
TO-220
Φ
JA
= 62.5
IRLZ44
T
J
= 125
°
C
—
46
Φ
JC
= 1.0
Int. Rectifier
V
GS
= 4.5V
I
D
= 28A
T
J
= 25
°
C
—
19
TO-220
Φ
JA
= 62.5
IRL3103S
T
J
= 125
°
C
31
Φ
JC
= 1.0
Two MOSFETs in Parallel
We recommend two MOSFETs used in parallel instead of a
single MOSFET. The following significant advantages are
realized using two MOSFETs in parallel:
• Significant reduction of power dissipation.
Maximum current of 14A with one MOSFET:
P
MOSFET
= (I
2
R
DS,ON
)(Duty Cycle) =
(14)
2
(0.050
*
)(3.3+0.4)/(5+0.4-0.35) = 7.2 W
With two MOSFETs in parallel:
P
MOSFET
= (I
2
R
DS,ON
)(Duty Cycle) =
(14/2)
2
(0.037*)(3.3+0.4)/(5+0.4-0.35) = 1.3W/FET
*
Note: R
DS,ON
increases with temperature. Assume R
DS,ON
= 25m
Ω
at 25
°
C. R
DS,ON
can easily increase to 50m
Ω
at high temperature
when using a single MOSFET. When using two MOSFETs in
parallel, the temperature effects should not cause the R
DS,ON
to rise
above the listed maximum value of 37m
Ω
.
• No added heat sink required.
With the power dissipation down to around one watt and
with MOSFETs mounted flat on the motherboard, no
external heat sink is required. The junction-to-case
thermal resistance for the MOSFET package (TO-220) is
typically at 2
°
C/W and the motherboard serves as an
excellent heat sink.
• Higher current capability.
With thermal management under control, this on-board
DC-DC converter can deliver load currents up to 14.5A
with no performance or reliability concerns.