AN OFF-LINE APPLICATION
The 50 Watt off-line supply shown in Figure 4 is presented
to illustrate a typical use for this controller. This
application is intended for highly cost-sensitive markets
requiring operation over world-wide line voltages. Its
forward topology provides good regulation with low output
ripple and it takes advantage of the low current requirement
of the UCC3570 to allow simple interfacing
to the high voltage line with minimal power loss. The
switching frequency is 225kHz with the allowable dutycycle
range set to a maximum of 67% - a value which
keeps the peak switch current low while still insuring a
finite reset time for the transformer. The design steps
which follow, while perhaps less rigorous than some
might wish, should still provide a check list of items for
the designer to consider when applying the UCC3570,
regardless of the specific application.