Datasheet

16
LTC1698
1698f
Power Transformer Selection
The forward transformer provides DC isolation and deliv-
ers energy from the primary to the secondary. Unlike the
flyback topology, the transformer in the forward converter
is not an energy storage device. As such, ungapped ferrite
material is typically used. Select a power material rated
with low loss at the switching frequency. Many core
manufacturers have selection guides and application notes
for transformer design. A brief overview of the more
important design considerations is presented here.
For operating frequencies greater than 100kHz, the flux
in the core is usually limited by core loss, not saturation.
It is important to review both criteria when selecting the
trans former. The AC operating flux density for core loss is
given by:
B
VDC
NAf
AC
IN
PeSW
=
••
•••
10
2
8
where:
B
AC
is the AC operating flux density (gauss)
DC is the operating duty cycle
A
e
is the effective cross sectional core area (cm
2
)
f
SW
is the switching frequency
To prevent core saturation during a transient condition,
the peak flux density is:
B
V DC MAX
NAf
PK
IN MAX
PeSW
=
()
•( )•
••
10
8
The minimum secondary turns count is:
NN
VV
V DC MAX
S MIN P
OUT D
IN MIN
()
()
•
•( )
=
+
where:
V
OUT
is the secondary output voltage
V
D
is the voltage drop across the rectifier in the secondary
V
IN(MIN)
is the minimum input voltage
DC(MAX) is the maximum duty cycle
The core must be sized to provide sufficient window area
for the amount of wire and insulation needed. The best
performance is achieved by making each winding a single
layer evenly distributed across the width of the bobbin.
Multiple layers may be used to increase the copper area.
Interleaving the primary and secondary windings will
decrease the leakage inductance.
In a single-ended forward converter, much of the energy
stored in the leakage inductance is dissipated in the
primary-side MOSFET during turn-off. It is good design
practice to sandwich the secondary winding between two
primary windings.
For the 2-transistor forward converter shown in Figure 1,
energy stored in the leakage inductance is returned to the
input by diodes D1 and D2. With this topology, additional
insulation for higher isolation can be used without signifi-
cant penalty.
For a more detailed discussion on transformer core and
winding losses, see Application Note AN19.
Inductor Selection
The output inductor in a typical LTC1698 circuit is chosen
for inductance value and saturation current rating. The
output inductor in a forward converter operates the same
as in a buck regulator. The inductance sets the ripple
current, which is commonly chosen to be 40% of the full
load current. Ripple current is set by:
I
Vt
L
RIPPLE
OUT OFF MAX
=
•
()
where:
t
DC MIN
f
OFF MAX
SW
()
–()
=
()
1
and DC(MIN) is calculated based on the maximum input
voltage.
DC MIN
N
N
V
V
P
S
OUT
IN MAX
() •
()
=
APPLICATIO S I FOR ATIO
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