RC5041
PRODUCT SPECIFICATION
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 importance, are:
• MOSFET I
2
R Losses
•
•
•
•
•
•
•
Inductor 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.
Efficiency of the converter under heavy loads can be calculated as follows:
P
OUT
I
OUT
×
V
OUT
-
Efficiency = ------------- = -------------------------------------------------------
,
I
OUT
×
V
OUT
+ P
LOSS
p
IN
where
P
LOSS
= PD
MOSFET
+ PD
INDUCTOR
+ PD
RSENSE
+ PD
GATE
+ PD
DIODE
+ PD
TRAN
+ PD
CAP
+ PD
IC
Preliminary Information
Design Equations:
(1) PD
MOSFET
= I
OUT
× (
R
DS
(
ON
)
×
1.5
) ×
DutyCycle
where 1.5 is the temperature multiplier
OUT
D
-
where
DutyCycle = -----------------------------------------
2
+V
V
IN
+ V
D
– V
SW
V
(2) PD
INDUCTOR
= I
OUT
×
R
INDUCTOR
(3) PD
RSENSE
= I
OUT
×
R
SENSE
(4) PD
GATE
= q
GATE
×
f
×
5V
, where
q
GATE
is the gate charge and f is the switching frequency
(5) PD
DIODE
= V
f
×
I
OUT
(
1 – Dutycycle
)
V
IN
×
C
RSS
×
I
LOAD
×
f
= -------------------------------------------------------------
, where C
RSS
is the reverse transfer capacitance of the MOSFET.
I
DRIVE
2
2
2
2
(6) PD
TRAN
(7) PD
CAP
= I
RMS
×
ESR
(8) PD
IC
= V
CC
×
I
CC
Example:
3.3 + 0.5
DutyCycle = ----------------------------- = 0.70
-
5 + 0.5 – 0.1
PD
INDUCTOR
= 10
×
0.010 = 1W
2
PD
MOSFET
= 10
× (
0.010
×
1.5
) ×
0.70 = 1.05W
PD
RSENSE
= 10
×
0.0065 = 0.65W
2
2
PD
GATE
= CV
×
f
×
5V = 1.75nf
× (
9 – 1
)V ×
300Khz
×
5V = 0.021W
PD
DIODE
= 0.5
×
10
(
1 – 0.70
)
= 1.5W
5
×
400pf
×
10
×
300khz
-
PD
TRAN
= ---------------------------------------------------------------
∼
0.074W
0.7A
PD
CAP
=
(
7.5 – 2.5
) ×
0.015 = 0.37W
PD
IC
= 0.2W
PD
LOSS
= 1.05W + 1.0W + 0.65W + 0.021W + 1.50W + 0.074W + 0.37W + 0.2W = 4.865W
3.3
×
10
-
∴
Efficiency = --------------------------------------
∼
87%
3.3
×
10 + 4.865
2
2
10
RAYTHEON [ RAYTHEON COMPANY ]
