Description
Demonstration circuit 1961A features the LT®8309, a
secondary synchronous driver in an isolated, no optocoupler,
flyback converter. It regulates a 12V, 5A output
from a 36V to 72V input source.
Output regulation is handled on the primary side by the
LT3748, a boundary conduction mode flyback controller
which senses output voltage directly from the primary, resulting
in a simple flyback schematic with no opto-coupler.
The LT8309 synchronous rectifier driver replicates the
behavior of a diode by sensing the synchronous MOSFET
drain-to-source voltage to determine its turn on period.
By replacing the diode with a N-channel MOSFET, applications
are no longer restricted by the heat constraints of
the rectifier diode.
On the DC1961A, the LT8309 is biased from the rectified
drain voltage node of secondary side synchronous
MOSFET, not directly connecting to output voltage. This
configuration allows the synchronous MOSFET to remain
conducting at all times, even when output is shorted
to ground, so as to provide a very robust short circuit
performance.
The Performance Summary table summarizes the performance
of the demo board at room temperature. For
thermally critical applications, proper amount of air flow
can help to reduce power components’ temperature rise,
therefore greatly improving circuit reliability.
The LT8309 data sheet gives a complete description of
the part, operation and application information. The data
sheet must be read in conjunction with this quick start
guide for demo circuit 1961A.