LT1103_15 Datasheet, PDF (26/32 Pages) Linear Technology – Offline Switching Regulator

LT1103/LT1105
APPLICATIONS INFORMATION
V_OUT in continuous mode is defined as:
V_OUT = V_IN • N • DC
The secondary voltage charges up L1 through D1 when S1
is on. When S1 is off, energy in L1 is transferred through
free-wheeling diode D2 to C1. The extra transformer
winding and diode D3 are needed in a single switch
Simplified Forward Converter
V_IN
D3
1:N
D1
L1
D2
S1
V_OUT
C1
COMMON
LT1103 AI02
forward converter to define the switch voltage when S1 is
off. This “reset” winding limits the maximum duty cycle
allowed for the switch. This topology trades off reduced
transformer size for increased complexity and parts count.
A separate isolated feedback path is required for full
isolation from input to output because voltages on the
primary are no longer related to the DC output voltage
during switch off time.
The isolated feedback path can take several forms. A
second transformer in a modulator/demodulator scheme
provides the isolation, but with significant complexity. An
optoisolator can be substituted for the transformer with a
savings in volume to be traded off with component
variations and possible aging problems with the
optoisolator transfer function. Finally, an extra winding
closely coupled to the output inductor L1 can sense the
flux in this element and give a representation of the output
voltage when S1 is off.
TYPICAL APPLICATIONS
26
LT1103 FET Connection
15V
LT1103
V_SW
LT1103 TA03
LT1105 FET Connection
15V
LT1105 ^VSW
I_LIM
LT1103 TA04