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APPLICATION NOTE AN50
9
Converter Efficiency
Losses due to parasitic resistance in the switches, coil, and
sense resistor dominate at high load-current level. The major
loss mechanisms under heavy loads, in usual order of impor-
tance, are:
• MOSFET I
2
R Losses
• Coil Losses
• Sense Resistor Losses
• gate-charge losses
• diode-conduction losses
• transition losses
• Input Capacitor losses
• losses due to the operating supply current of the IC.
Calculation of Converter Efficiency Under Heavy Loads
where , where
, where q
GATE
is the gate charge and f is the switching frequency
, where C
RSS
is the reverse transfer capacitance of the high-side MOSFET.
Example
Efficiency
P
OUT
p
IN
-------------
I
OUT
V
OUT
×
I
OUT
V
OUT
P
LOSS
+×
--------------------------------------------------------= =
P
LOSS
PD
MOSFET
PD
COIL
PD
SENSER
PD
GATE
PD
DIODE
PD
TRAN
PD
CAP
PD
IC
+ + + + + + +=
PD
MOSFET
I
OUT
2
R
DS,ON
× DutyCycle×= DutyCycle
V
OUT
V
D
+
V
IN
V
D
V
SW
–+
------------------------------------------=
PD
COIL
I
OUT
2
R
COIL
×=
PD
SENSER
I
OUT
2
R
SENSE
×=
PD
GATE
q
GATE
f 5V××=
PD
DIODE
V
f
I
D
× 1 Dutycycle–( )=
PD
TRAN
V
IN
2
C
RSS
× I
LOAD
× f×
I
DRIVE
-------------------------------------------------------------=
PD
CAP
I
RMS
2
ESR×=
PD
IC
V
CC
I
CC
×=
DutyCycle
3.3 0.5+
5 0.5 0.3–+
------------------------------ 0.73= =
PD
MOSFET
10
2
0.030× 0.73× 2.19W= =
PD
COIL
10
2
0.010× 1W= =
PD
SENSER
10
2
0.0065× 0.65W= =
PD
GATE
CV f× 5V× 1.75nf 9 1–( )V 285Khz 5V××× 0.019W= = =
PD
DIODE
0.5 10 1 0.73–( )× 1.35W= =
PD
TRAN
5
2
400pf× 10× 285khz×
0.7A
----------------------------------------------------------------
0.010W∼=
PD
CAP
7.5 2.5–( )
2
0.015× 0.37W= =
PD
IC
0.2W=
PD
LOSS
2.19W 1.0W 0.65W 0.019W 1.35W 0.010W 0.37W 0.2W+ + + + + + + 5.789W= =
Efficiency
3.3 10×
3.3 10 5.815+×
---------------------------------------
85%≈=∴