12

LT1508

The total equivalent 120Hz ripple in the output capacitor

can be calculated by:

1.43

120Hz ripple current is:

1.79

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

°C ambient is

1.72A for the 470

µF KMH 35mm × 50mm capacitor. The

expected life of the output capacitor may be calculated

from thermal stress analysis:

(105

10

where

L = Expected life time

ambient temperature

condition.

rent, R is capacitor ESR and KA is a volume constant.

condition

Table 1. PFC Capacitor RMS Ripple Current

100W

200W

300W

100

0.2

0.6

0.41

1.18

0.62

1.79

120

0.2

0.5

0.41

0.97

0.62

1.45

230

0.2

0.53

0.41

0.87

0.62

1.26

rated 1.72A.

1.72A

= 5

°C

1.72A

Assuming the operating ambient temperature is 60

°C, the

approximate lifetime is:

(105

°C + 5°C) – (60 + 3.3°C)

10

L = (2000)(2)

= 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

control voltage pin sets duty factor. An additional current

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’s 50% maximum

duty cycle.