The LM2698 is used to design a fully isolated forward converter power supply. The output inductor, L1, is augmented by a secondary winding to make a coupled-inductor (transformer) for creating a second low cost linear output. The linear output uses the LP2986 augmented with an additional pass transistor, Q2, to drive up to 1A of current. The LM2698 is a PWM regulator with internal low-side FET switch. The cascode FET stage, M1, provides a low cost means to protect the regulator IC from the high input voltage while providing very fast switching.
A forward converter architecture is mathematically analogous to a step-down, buck architecture with its Duty Cycle multiplied by the turns-ratio of the transformer, T1. The input voltage is effectively chopped by the switching action of the regulator's internal switch so that the AC signal can be passed through and reduced by the transformer. The AC voltage pulses at the voltage switch node connecting diode D5, flyback diode D4, and the LC filter, L1 & C6 & C12, act to create the correct output voltage as a function of the regulator switch duty cycle.
The LM3411 is a shunt regulator acting as feedback error amplifier plus reference while the opto-coupler provides dc isolation (between the primary and secondary sides supply) for the feedback path of the loop.
Notes: M1 in the schematic is displayed as Q2 in the layout. Q2 in the schematic is Q3 in the layout.
Schematic
Example Schematic Showing Connection for all Components.
Example Schematic Showing Connection for all Components.
