LT8708
35
Rev 0
It is very important to consider power dissipation when
selecting power MOSFETs. The most efficient circuit will
use MOSFETs that dissipate the least amount of power.
Power dissipation must be limited to avoid overheating
that might damage the devices. In forward conduction,
the M1 and M3 switches will have the highest power dis-
sipation, while M2 and M4 will have the highest power
dissipation in reverse conduction. In some cases it can be
helpful to use two or more MOSFETs in parallel to reduce
power dissipation in each device. This is most helpful when
power is dominated by I
2
R losses while the MOSFET is
“on”. The additional capacitance of connecting MOSFETs
in parallel can sometimes slow down switching edge rates
and consequently increase total switching power losses.
The following sections provide guidelines for calculating
power consumption of the individual MOSFETs. From a
known power dissipation, the MOSFET junction tempera-
ture can be obtained using the following formula:
T
J
= T
A
+ P • R
TH(JA)
where:
T
J
is the junction temperature of the MOSFET
T
A
is the ambient air temperature
P is the power dissipated in the MOSFET
R
TH(JA)
is the MOSFET’s thermal resistance from the
junction to the ambient air. Refer to the manufacturer’s
data sheet.
R
TH(JA)
normally includes the R
TH(JC)
for the device plus
the thermal resistance from the case to the ambient tem-
perature R
TH(CA)
. Compare the calculated value of T
J
to
the manufacturer’s data sheets to help choose MOSFETs
that will not overheat.
The power dissipation of the external N-channel MOSFETs
comes from two primary components: (1) I
2
R power when
the switch is fully “on” and inductor current is flowing
between the drain and source connections and (2) power
dissipated while the switch is turning “on” and “off”. The
MOSFET switching power consists of (A) a combination
of high current and high voltage as the switch turns “on”
and “off” and (B) charging and discharging the SW1 or
SW2 node capacitance, which is dominated by the out-
put capacitance of the external MOSFETs. Use Table 7 to
determine which power components are applicable in the
various regions of operation.
Table 7. NMOS Power in Various Operating Regions
OPERATING
REGION
M1
M2
M3
M4
Pos.
I
L
Buck
P
I
2
R
+ P
SW
P
I
2
R
0
P
I
2
R
Boost
P
I
2
R
0
P
I
2
R
+ P
SW
Buck-Boost P
I
2
R
+ P
SW
P
I
2
R
P
I
2
R
+ P
SW
Neg.
I
L
Buck
P
I
2
R
P
I
2
R
+ P
SW
0
P
I
2
R
Boost
0
P
I
2
R
P
I
2
R
+ P
SW
Buck-Boost
P
I
2
R
+ P
SW
P
I
2
R
P
I
2
R
+ P
SW
APPLICATIONS INFORMATION
The MOSFET power components listed above can be ap-
proximated using the following equations. Note that I
IN
can be substituted for I
OUT
using:
I
IN
≅
V
OUT
V
IN
• I
OUT
where necessary.
I
2
R Component Equations:
P
I2R[M1,BUCK]
or P
I2R[M4,BOOST]
≅
V
OUT
V
IN
• I
OUT
2
• R
DS(ON)
•
ρ
τ
W
P
I2R[M1,BOOST]
≅
V
OUT
V
IN
• I
OUT
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
2
• R
DS(ON)
•
ρ
τ
P
I2R[M2,BUCK]
≅
V
IN
– V
OUT
V
IN
• I
OUT
2
• R
DS(ON)
•
ρ
τ
W
P
I2R[M3,BOOST]
≅
V
OUT
– V
IN
V
IN
2
• V
OUT
• I
OUT
2
• R
DS(ON)
•
ρ
τ
W
P
I2R[M4,BUCK]
≅
I
OUT
2
• R
DS(ON)
•
ρ
τ
W
