14

LT1508

An LT1431 reference/amplifier coupled to a low cost

optoisolator closes the loop from secondary side to pri-

mary side. Efficiency versus power and line voltage is

shown in Figure 10. The PFC preregulator alone has

efficiency numbers between 90% and 97% over line and

load.

A 3-turn secondary added to the 70-turn primary of T1

requirement as well as about 39mA to cover the gate

current of the three FETs and high side transformer. A

0.15

Ω sense resistor is used to sense input current and

servo to the command created by the outer voltage and

multiplier. Thus the input current follows the input line

voltage, and changes as necessary, in order to maintain

constant bank voltage. The forward converter sees a

voltage input of 382VDC unless the line voltage drops out,

in which case the 330

µF main capacitor discharges to

240VDC before the PWM stage is shut down. Compared to

a typical off-line converter, the effective input voltage

range of the forward converter is much smaller, simplify-

ing the design. Additionally, the higher bus voltage pro-

vides greater hold-up times for given capacitor size.

90

80

85

250

1508 • F1O

75

70

132

180

90

200W/300W

100W

30W

Figure 10