LTC3721-1
13
sn37211 37211fs
The duty cycle of these square waves is guaranteed to
never exceed 50% by the LTC3721-1. In steady state
operation, the duty ratio is given by:
D = V
OUT
/(2 • V
IN
• N)
To calculate the transformer turns ratio, first determine
the minimum input voltage (V
IN(MIN)
) and the maximum
duty ratio (D
(MAX)
) of the controller IC. This will be the
worst case condition. An example is provided below:
V
IN
= 32V to 75V, use V
IN(MIN)
= 30V to account for
system voltage drops.
V
OUT
= 7V
Maximum duty cycle (D
MAX
) = 47% (per datasheet),
use 45% for margin.
The required transformer turns ratio is given by:
Turns ratio (Ns/Np) = V
OUT
/(V
IN(MIN)
• 2 • D
(MAX)
)
= 7V/(30V • 2 • 0.45)
Ns/Np = (1/3.86)
OPERATIO
U
Note that this is a simplified equation that does not take
into account primary and secondary side voltage drops
due to diodes, power MOSFETs, and resistive elements in
the power paths. By margining down V
IN(MIN)
and D
MAX
as
suggested above, the equation becomes closer to reality.
An alternative secondary winding configuration uses a
single non-center tapped winding and two filter inductors.
Each end of the secondary winding alternately drives an
inductor with <50% duty cycle square wave. The two
inductors are connected together at the opposite ends to
common output filter capacitor(s). This configuration is
also called the current doubler rectifier. The current dou-
bler utilizes half of the secondary windings compared to
the center tapped case. The two out of phase inductors
reduce the ripple current seen by the output and input
capacitors, possibly allowing fewer capacitors in some
applications. In addition, each output inductor carries half
of the total load current, making them physically smaller,
which can help to optimize the power stage layout. How-
ever, the total combined size may be slightly larger than
the single inductor configuration.
