Datasheet

LTC3766
21
3766fa
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applicaTions inForMaTion
Be careful to keep these divider resistors very close to the
FB pin to minimize the trace length and noise pick-up on
the sensitive FB signal. Using a low resistance (<2k) for
the output voltage divider also minimizes noise on the FB
pin. If the remote sense amplifier is used, then the divider
should be placed between the V
SOUT
pin and GND. See
the Remote Sensing section for details.
of power level, choose a family of transformers whose
rated power level exceeds that of the required amount of
output power. Be careful to allow for room to “grow,” as
the power requirements of many electronic systems tend
to increase throughout development.
Once a family of transformers has been selected, the
next step is to choose a suitable transformer from within
that family. This mainly consists of choosing the correct
number of primary and secondary turns (N
P
and N
S
). The
value of N
S
can be calculated from:
N
S
=
10
8
V
OUT
f
SW
A
C
B
M
where A
C
is the cross-sectional area of the core in cm
2
(as
normally given in the transformer data sheet) and B
M
is the
maximum AC flux density desired. For the Pulse PA08xx
series power transformers used in the Typical Applications
section, A
C
= 0.59cm
2
. For the Pulse PA09xx series power
transformers, AC = 0.81cm
2
. Most high frequency trans-
formers use a ferrite core material. Consequently, selecting
a maximum AC flux density of 2000 gauss is normally a
good starting point, provided that the switching frequency
is between 150kHz and 350kHz. This value of B
M
leaves
headroom during transients and avoids excessive core
losses. Note that the choice of B
M
together with switch-
ing frequency will determine the amount of core loss for
a given transformer. Consult the transformer data sheet to
evaluate the resulting core loss and temperature rise. In
some cases, it may be necessar
y to increase N
S
somewhat
in order to reduce B
M
and the associated temperature rise.
In all cases, be sure to stay well below the saturation flux
density of the transformer core.
Once the value of has N
S
been selected, the required
transformer turns ratio can be calculated from
N
P
N
S
=
D
MAX
V
IN(MIN)
V
OUT
where V
IN(MIN)
is the minimum input voltage and D
MAX
is the maximum duty cycle. Although the LTC3766 has a
maximum duty cycle of 79% (D
MAX
= 0.79), normally a
lower value of D
MAX
is chosen in the above equation so
that there is duty cycle headroom to accommodate load
Figure 7. Setting the Output Voltage
V
FB
R
B
R
A
LTC3766
V
OUT
3766 F07
Selecting the Main Transformer
The job of the transformer in a forward converter is to
step the voltage either up or down while providing isola
-
tion between the primary and secondary grounds. Ideally,
this transformer would not store any energy (it would have
infinite magnetizing inductance). Note that this objective
is very different from that of the transformer used in a
flyback converter. The transformer used in a flyback con
-
verter is really a coupled inductor, the purpose of which is
to store energy during the primar
y-side on time and then
deliver
it to the secondary during the off-time. In a forward
converter, by contrast, the power is transferred during the
primary-side on-time, and the off-time is used to recover
the small amount of energy that was inadvertently stored
in the core of the transformer.
For nearly all applications, an off-the-shelf transformer can
be selected. Transformers using planar winding technology
are widely available and are a good choice for minimizing
leakage inductance as well as component height. There
are two basic items to consider in selecting an appropri
-
ate family of off-the-shelf transformers: 1) the isolation
requirements and 2) the power level requirements. If the
application circuit has specific isolation requirements,
choose a family of transformers whose isolation level
satisfies that requirement. In addition to an isolation volt
-
age rating, the application may require a transformer with
certification from a particular agency
, or it may require a
specific type of isolation (e.g., basic or functional). In terms