Datasheet

LT1950
15
1950fa
APPLICATIO S I FOR ATIO
WUU
U
ability of the V
SEC
pin to follow input voltage and control
maximum switch duty cycle.
Forward Converter Applications
The LT1950 provides sophisticated control of the simplest
forward converter topology (single primary switch, see Q1
Figure 11). A significant problem in a single switch for-
ward converter topology is transformer reset. Optimum
transformer utilization requires maximum duty cycles.
Unfortunately as duty cycles increase the transformer
reset time decreases and reset voltages increase. This
increases the voltage requirements and stress on both
transformer and switch. The LT1950 incorporates an
adaptive maximum duty cycle clamp which controls maxi-
mum switch duty cycle based on system input voltage.
The adaptive clamp allows the converter to operate at up
to 75% duty cycle, allowing 25% of the switching period
for resetting the transformer. This results in greater
utilization of MOSFET, transformer and output rectifier
components. The V
SEC
pin can be programmed from
system input to adaptively control maximum duty cycle
(see Applications Information “Programming Volt-Sec-
ond Clamp” and the Maximum Duty Cycle vs V
SEC
Voltage
graph in the Typical Performance Characteristics section).
Figure 9. LT1950-Based Synchronous Forward
Converter Efficiency vs Load Current
LOAD CURRENT (A)
0
EFFICIENCY (%)
100
95
90
85
80
75
70
5101520
1950 F09
V
IN
= 48V
V
OUT
= 3.3V
f
OSC
= 235kHz
POWER
MODULE
V
OUT
(100mV/DIV)
LT1950
V
OUT
(100mV/DIV)
500µs/DIV
1950 F10
Figure 10. Output Voltage Transient Response
to Load Steps (0A to 3.3A) LT1950 (Trace1)
vs Power Module (Trace 2)
94% Efficient 3.3V, 20A Synchronous Forward
Converter
The synchronous forward converter in Figure 11 is based
on the LT1950 and uses MOSFETs as synchronous output
rectifiers to provide an efficient 3.3V, 20A isolated output
from 48V input. The output rectifiers are driven by the
LTC1698 which also serves as an error amplifier and
optocoupler driver. Efficiency and transient response
are shown in Figures 9 and 10. Peak efficiencies of 94%
and ultra-fast transient response are superior to presently
available power modules. In addition, the circuit in Figure 11
is an all-ceramic capacitor solution providing low output
ripple voltage and improved reliability. The LT1950-based
converter can be used to replace power module converters
at a much lower cost. The LT1950 solution benefits from
thermal conduction of the system board resulting in
higher efficiencies and lower rise in component tempera-
tures. The 7mm height allows dense packaging and the
circuit can be easily adjusted to provide an output voltage
from 1.23V to 15V. In addition, higher currents are achiev-
able by simple scaling of power components. The LT1950-
based solution in Figure 11 is a powerful topology for
replacement of a wide range of power modules.