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LT1950

sn1950 1950fs

APPLICATIO S I FOR ATIO

ability of the VSEC 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 VSEC 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 VSEC Voltage

graph in the Typical Performance Characteristics section).

Figure 9. LT1950-Based Synchronous Forward

Converter Efficiency vs Load Current

LOAD CURRENT (A)

100

510

1950 F09

VIN = 48V

VOUT = 3.3V

fOSC = 235kHz

POWER

MODULE

VOUT

(100mV/DIV)

LT1950

VOUT

(100mV/DIV)

500

µs/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.

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