Fairchild SEMICONDUCTOR RC5040 - Manual (Page 9)

APPLICATION NOTE AN42
9
Converter Efficiency
Losses due to parasitic resistance in the switches, inductor,
and sense resistor dominate at high load-current levels. The
major loss mechanisms under heavy loads, in order of
importance, are:
• MOSFET I
2
R Losses
• Inductor 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.
Efficiency of the converter under heavy loads can be calculated as follows:
,
where
Design Equations:
, where
, where q is the gate charge and f is the switching frequency
Efficiency
P
OUT
p
IN
--------------
I
OUT
V
OUT
×
I
OUT
V
OUT
P
LOSS
+
×
--------------------------------------------------------- = =
P
LOSS
PD
MOSFET
PD
INDUCTOR
PD
RSENSE
PD
GATE
PD
DIODE
PD
TRAN
PD
CAP
PD
IC
+ + + + + + + =
(1) PD
MOSFET
I
OUT
2
R
DS,ON
×
DutyCycle
×
= DutyCycle
V
OUT
V
D
+
V
IN
V
D
V
SW
– +
------------------------------------------ =
(2) PD
INDUCTOR
I
OUT
2
R
INDUCTOR
×
=
(3) PD
RSENSE
I
OUT
2
R
SENSE
×
=
(4) PD
GATE
q
GATE
f 5V
Ч Ч
=
GATE
, where C RSS is the reverse transfer capacitance of the high-side MOSFET.
Example:
(5) PD
DIODE
V
f
I
D
×
1 DutyCycle –
( )
=
(6) PD
TRAN
V
IN
2
C
RSS
×
I
LOAD
×
f
×
I
DRIVE
------------------------------------------------------------- =
(7) PD
CAP
I
RMS
2
ESR
×
=
(8) PD
IC
V
CC
I
CC
×
=
DutyCycle
3.3 0.5 +
5 0.5 0.3 – +
------------------------------ 0.73 = =
PD
MOSFET
10
2
0.030
Ч
0.73
Ч
2.19W = =
PD
INDUCTOR
10
2
0.010
×
1W = =
PD
RSENSE
10
2
0.0065
×
0.65W = =
PD
GATE
CV f
×
5V
×
1.75nf 9 1 –
( )
V 650Khz 5V
Ч Ч Ч
0.045W = = =
PD
DIODE
0.5 10 1 0.73 –
( ) ×
1.35W = =
PD
TRAN
5
2
400pf
Ч
10
Ч
650khz
×
0.7A
---------------------------------------------------------------- 0.010W
∼
=
PD
CAP
7.5 2.5 –
( ) 2
0.015
×
0.37W = =
PD
IC
0.2W =