Datasheet

R
T
+
ǒ
1
f
SW
17.82 E * 06
* 23
Ǔ
kW + 408 kW, use 412 kW
( )
55 3.3 3.3
L 11.9 H
55 2 130 kHZ
- ´
= = m
´ ´
( )
J SR JA A
T P T 0.644 40 85 111 C= ´ q + = ´ + = °
SR RR COND DC
P P P P 0.107 0.485 0.052 0.644 W= ´ ´ = + + =
P
RR
+ 0.5 Q
RR
V
IN
f
SW
+ 0.5 30 nC 55 V 130 kHz + 0.107 W
DC O FD DELAY SW
P 2 I V t f 2 5 A 0.8 V 50 ns 130 kHZ 0.052 W= ´ ´ ´ ´ = ´ ´ ´ ´ =
( )
( )
2
COND
P 4.85 0.011 1 0.007 150 25 0.485 W= ´ ´ + - =
I
RMS
+ I
O
1 * d
Ǹ
+ 5 1 * 0.0588
Ǹ
+ 4.85 A
RMS
T
J
+
ǒ
P
COND
) P
SW
Ǔ
q
JA
) T
A
+
(
0.324 ) 0.715
)
40 ) 85 + 127
O
C
TPS40060
TPS40061
www.ti.com SLUS543F –DECEMBER 2002–REVISED JUNE 2013
(50)
5. Calculate synchronous rectifier losses
The synchronous rectifier MOSFET has two loss components, conduction, and diode reverse recovery losses.
The conduction losses are due to I
RMS
losses as well as body diode conduction losses during the dead time
associated with the anti-cross conduction delay.
The I
RMS
current through the synchronous rectifier from Equation 51
(51)
The synchronous MOSFET conduction loss from Equation 29 is:
(52)
The body diode conduction loss from Equation 35 is:
(53)
The body diode reverse recovery loss from Equation 36 is:
(54)
The total power dissipated in the synchronous rectifier MOSFET from Equation 37 is:
(55)
The junction temperature of the synchronous rectifier at 85°C is:
(56)
In typical applications, paralleling the synchronous rectifier MOSFET with a Schottky rectifier increases the
overall converter efficiency by approximately 2% due to the lower power dissipation during the body diode
conduction and reverse recovery periods.
6. Calculate the Inductor Value
The inductor value is calculated from Equation 12.
(57)
A standard inductor value of 10-µH is chosen. A Coev DXM1306-10RO or Panasonic ETQPF102HFA could be
used.
7. Setting the switching frequency
The clock frequency is set with a resistor (R
T
) from the RT pin to ground. The value of R
T
can be derived from
following Equation 58, with f
SW
in kHz.
(58)
8. Programming the Ramp Generator Circuit
The PWM ramp is programmed through a resistor (R
KFF
) from the KFF pin to V
IN
. The ramp generator also
controls the input UVLO voltage. For an undervoltage level of 14.4V (20% below the 18 V
IN(min)
), R
KFF
is
calculated in Equation 59.
R
KFF
= (80%xV
IN(min)
– 3.5)(65.27 ×R
T
+ 1502) Ω = 309 kΩ, use 301 kΩ (59)
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