Fairchild SEMICONDUCTOR RC5051 - Manual (Page 9)

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
Ч Ч
=
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
Ч
0.73
Ч
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
Ч Ч Ч
0.019W = = =
PD
DIODE
0.5 10 1 0.73 –
( ) ×
1.35W = =
PD
TRAN
5
2
400pf
Ч
10
Ч
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
Ч
3.3 10 5.815 +
Ч
--------------------------------------- 85%
≈
=
∴