Datasheet

LT1249
9
V
P-P
= 2 × 0.78A × 7.4Ω = 11.5V. If less ripple is desired,
higher capacitance should be used.
The selection of the output capacitor should also be based
on the operating ripple current through the capacitor.
The ripple current can be divided into three major compo-
nents. The first is at 120Hz whose RMS value is related to
the DC load current as follows:
I
1RMS
≈ 0.71 × I
LOAD
DC
The second component contains the PF switching fre-
quency ripple current and its harmonics. Analysis of this
ripple is complicated because it is modulated with a 120Hz
signal. However, computer numerical integration and Fou-
rier analysis approximate the RMS value reasonably close
to the bench measurements. The RMS value is about
0.82A at a typical condition of 120VAC, 200W load. This
ripple is line voltage dependent, and the worst case is at
low line.
I
2RMS
= 0.82A at 120VAC, 200W
The third component is the switching ripple from the load,
if the load is a switching regulator.
I
3RMS
≈ I
LOAD
DC
For United Chemicon KMH 400V capacitor series, ripple
current multiplier for currents at 100kHz is 1.43. The
equivalent 120Hz ripple current can then be found:
II
II
RMS RMS
RMS RMS
=
()
+






+






1
2
2
2
3
2
143 143..
For a typical system that runs at an average load of 200W
and 385V output:
I
LOAD
DC = 0.52A
I
1RMS
≈ 0.71 × 0.52A = 0.37A
I
2RMS
≈ 0.82A at 120VAC
I
3RMS
≈ I
LOAD
DC = 0.52A
APPLICATIONS INFORMATION
WUU
U
IA
AA
A
RMS
=
()
+






+






=037
082
143
052
143
077
2
22
.
.
.
.
.
.
The 120Hz ripple current rating at 105°C ambient is 0.95A
for the 180µF KMH 400V capacitor. The expected life of the
output capacitor may be calculated from the thermal
stress analysis:
LL
O
CT T T
K AMB O
=×
°+
()
−+
()
2
105
10
∆∆
where
L = expected life time
L
O
= hours of load life at rated ripple current and rated
ambient temperature
∆T
K
= capacitor internal temperature rise at rated
condition. ∆T
K
= (I
2
R)/(KA), where I is the rated current,
R is capacitor ESR, and KA is a volume constant.
T
AMB
= operating ambient temperature
∆T
O
= capacitor internal temperature rise at operating
condition
In our example, L
O
= 2000 hours and ∆T
K
= 10°C at rated
0.95A. ∆T
O
can then be calculated from:
∆∆T
I
A
T
A
A
CC
O
RMS
K
=






×=






×°= °
095
077
095
10 6 6
22
.
.
.
.
Assuming the operating ambient temperature is 60°C, the
approximate life time is:
L
O
CC C C
≈× ≈
°+ ° − °+ °
2000 2 57 000
105 10 60 6 6
10
()(.)
, Hrs.
For longer life, capacitor with higher ripple current rating
or parallel capacitors should be used.