LT1509 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1509' PDF : 16 Pages View PDF
LT1509
APPLICATIONS INFORMATION
An LT1431 reference/amplifier coupled to a low cost
optoisolator closes the loop from secondary side to pri-
mary side. Unity loop frequency is a conservative 3kHz.
Figure 10 shows the output voltage’s response with a 2A
to almost 10A current step. Output voltage is maintained
to within 0.5V during the load step. Efficiency versus
power and line voltage is shown in Figure 11. 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
bootstraps VCC to about 15V supplying the chip’s 13mA
requirement as well as about 39mA to cover the gate
current of the three FETs and high side transformer. A
0.15Ω 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 470µF 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.
Because the high side transformer effectively delays the
turn-on reverse recovery spike past the end of the built-in
blanking time, an external blanking transistor is needed.
Controlling the output current during an output short
circuit depends on the duty cycle reducing to a small
fraction of steady state. An additional transistor disables
blanking during turn-on and output short circuit.
5A/DIV
0.5V/DIV
Figure 10
90
200W/300W
85
100W
80
75
70
90
132
180
VRMS
Figure 11
30W
250
LT1509 • F11
14
Share Link: 
