Datasheet

LTC3766
22
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
transients when operating at minimum input voltage. A
value for D
MAX
of 0.65 to 0.70 is appropriate for most
applications.
Having selected a particular transformer, calculate the
copper losses associated with the transformer winding.
These losses are highest when operating at maximum
duty cycle and full load. However, it is better to evaluate
copper losses at the nominal operating point of 50% duty
cycle, where the losses are approximately:
P
CU
=
I
MAX
( )
2
2
R
SEC
+
N
S
N
P
⎛
⎝
⎜
⎞
⎠
⎟
2
R
PRI
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
where R
PRI
and R
SEC
are the primary and secondary
winding resistances respectively, and I
MAX
is the maxi-
mum output current. An optimal transformer design has
a reasonable balance between copper and core losses. If
they are significantly different, then adjust the number of
secondary turns (and recalculate the needed turns ratio)
to achieve such a balance.
Inductor V
alue Calculation
The selection of an output inductor is essentially the same
as for a buck converter. For a given input and output volt
-
age, the inductor value and operating frequency determine
the ripple current. The ripple current
∆
I
L
increases with
higher V
IN
and decreases with higher inductance:
ΔI
L
=
V
OUT
f
SW
L
1–
V
OUT
V
IN
•
N
P
N
S
⎛
⎝
⎜
⎞
⎠
⎟
Accepting larger values of ∆I
L
allows the use of low in-
ductances, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
the ripple current is ∆
I
L
= 0.3(I
OUT(MAX)
) for nominal V
IN
.
The maximum ∆I
L
occurs at the maximum input voltage.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of the more expensive ferrite cores. Actual
core loss is essentially independent of core size for a fixed
inductor value but it is very dependent on the inductance
selected. As the inductance increases, core losses decrease.
Unfortunately, increased inductance requires more turns
of wire and therefore copper losses will increase.
Ferrite designs have very low core losses and are pre
-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
in
duct
ance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Active Clamp Capacitor
The active clamp capacitor, C
CL
, stores the average reset
voltage of the transformer over many cycles. The voltage
on the clamp capacitor is generated by the transformer
core reset current, and will intrinsically adjust to the optimal
reset voltage regardless of other parameters. The voltage
across the capacitor at full load is approximately given by:
V
CL
=
V
IN
2
V
IN
–1.15 V
OUT
•
N
P
N
S
⎛
⎝
⎜
⎞
⎠
⎟
N
P
/N
S
is the main transformer turns ratio. The factor of
1.15 accounts for typical losses and delays. When PG and
AG on the LTC3765 are low, the bottom side of the clamp
capacitor is grounded, placing the reset voltage, V
CL
, on
the SWP node. When PG and AG are high, the top side of
the capacitor is grounded, and the voltage on the bottom
side of the capacitor is –V
CL
. Therefore the voltage seen
on the capacitor is also the voltage seen at the drains of
the PG and AG MOSFETs.
As shown in Figure 8, the V
CL
voltage has a minimum when
the converter is operating at 50%. For a given range on
V
IN
, therefore, the maximum clamp voltage (V
CL(MAX)
) will
occur either at the minimum or maximum V
IN
, depending
on which input voltage causes the converter to operate
furthest from 50% duty cycle. The maximum V
CL
voltage
can be determined by substituting the maximum and
minimum values of V
IN
into this equation and selecting
the larger of the two. In order to leave room for overshoot,