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LT1508I Datasheet

  • LT1508I

  • Linear Technology [Power Factor and PWM Controller(Voltage ...

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LT1508
APPLICATIONS INFORMATION
The total equivalent 120Hz ripple in the output capacitor
can be calculated by:
2
2
+ I
HF
Table 1. PFC Capacitor RMS Ripple Current
100W
V
INRMS
100
120
230
I
120HZ
0.2
0.2
0.2
I
HF
0.6
0.5
0.53
0.41
0.41
0.41
200W
I
120HZ
I
HF
1.18
0.97
0.87
0.62
0.62
0.62
300W
I
120HZ
I
HF
1.79
1.45
1.26
I
RMS
=
I
120HZ
( )
1.43
I
HF
= 100kHz Ripple Current.
For I
LOAD(DC)
= 0.88A, 1
120Hz
= 0.62A and the equivalent
120Hz ripple current is:
I
RMS
=
2
2
+ 1.79
0.62
= 1.4A
RMS
1.43
( )
Table 1 lists the ripple current components from lab
measurements for various output powers and line volt-
ages. The 120Hz ripple current rating at 105擄C ambient is
1.72A for the 470碌F KMH 35mm
脳
50mm capacitor. The
expected life of the output capacitor may be calculated
from thermal stress analysis:
(105擄C +
鈭員
K
) 鈥?(T
A
+
鈭員
O
)
10
L = (L
O
)2
where
L = Expected life time
L
O
= Hours of load life at rated ripple current and rated
ambient temperature
鈭員
K
= Capacitor internal temperature rise at rated
condition.
鈭員
K
= (I
2
R)/(KA), where I is the rated cur-
rent, R is capacitor ESR and KA is a volume constant.
T
A
= Operating ambient temperature
鈭員
O
= Capacitor internal temperature rise at operating
condition
12
U
W
U
U
In our example, L
O
= 2000 hours assuming
鈭員
K
= 5擄C at
rated 1.72A.
鈭員
O
can then be calculated from:
I
1.4A
鈭員
O
=
鈭員
K RMS
= 5擄C
= 3.3擄C
1.72A
1.72A
( ) ( )
2
2
Assuming the operating ambient temperature is 60擄C, the
approximate lifetime is:
L = (2000)(2)
(105擄C + 5擄C) 鈥?(60 + 3.3擄C)
10
= 50,870 Hours
For longer life a capacitor with a higher ripple current rating
or parallel capacitors should be used.
PWM Comparators
The LT1508 includes two comparators in the PWM section
which implement voltage mode PWM control. The V
C
or
control voltage pin sets duty factor. An additional current
limit comparator turns GTDR2 off in the event the I
LIM
pin
voltage exceeds 1.1V. On-chip blanking avoids reset due
to leading edge noise.
Typical Application
Figure 9 shows a 24VDC, 300W power factor corrected,
universal input supply. The 2-transistor forward converter
offers many benefits including low peak currents,
nondissipative snubber, 500VDC switches and automatic
core reset guaranteed by the LT1508鈥檚 50% maximum
duty cycle.

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