LTC3765
18
3765fb
For more information www.linear.com/LTC3765
APPLICATIONS INFORMATION
ripple on the V
CC
capacitor (C
VCC
) due to the magnetizing
current can be approximated by the following equation:
∆V
CC
=
V
OUT
N
P
/N
S
6.8 •C
VCC
•L
MAG
• f
SW
2
1–
V
OUT
N
P
/N
S
V
IN(MAX)
In general, a 4.7µF capacitor is a good choice for most
application circuits when the active clamp current is
returned to V
CC
.
Direct Flux Limit
In active clamp forward converters, it is essential to es
-
tablish an accurate limit to the transformer flux density
in
order to avoid core saturation during load transients or
when starting up into a pre-biased output. Although the
active clamp technique provides a suitable reset voltage
during steady-state operation, the sudden increase in
duty cycle caused in response to a pre-bias output or a
load step can cause the transformer flux to accumulate
or “walk,” potentially leading to saturation. This occurs
because the reset voltage on the active clamp capacitor
cannot keep up with the rapidly changing duty cycle. This
effect is most pronounced at low input voltage, where the
voltage loop demands a greater increase in duty cycle due
to the lower voltage available to ramp up the current in
the output inductor.
The LTC3765 and LTC3766 implement a new unique system
for monitoring and directly limiting the flux accumulation in
the transformer core. During a reset cycle, when the active
clamp PMOS is on, the magnetizing
current is sensed by
a
resistor (R
MAG
) connected to the source of the PMOS.
The voltage across this resistor is sensed by the I
SMAG
pin. Both the traditional and alternative configurations for
the active clamp driver, shown previously in Figures 7a
and 7b, are supported. In the traditional configuration, if
the voltage on the I
SMAG
pin is less than –1V, the active
clamp PMOS is turned off. Similarly for the alternative
configuration of Figure 7b, if the voltage on the I
SMAG
pin
is less than (V
CC
– 1V), the active clamp PMOS is turned
off. The I
SMAG
pin therefore directly monitors and limits
the magnetizing current to prevent core saturation in the
negative direction.
Choose the magnetizing current sense resistor value to
limit the transformer saturation current (I
SAT
):
R
MAG
=
I
SAT
where the saturation current is calculated from the maxi-
mum flux density (B
MAX
), area of the core in cm
2
(A
C
),
number of turns on the primary (N
P
), and typical magne-
tizing inductance
(L
MAG(TYP)
) from the following formula:
I
SAT
=
MAX
C
P
10
8
•L
MAG(TYP)
For a transformer designed for 2000 gauss operating flux
density, which is typical for a ferrite core, set B
MAX
to 2700
gauss to keep sufficiently far from saturation over tempera-
ture. 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, A
C
= 0.81cm
2
.
Be sure to use the typical value for the magnetizing induc-
tance in
this formula. Using a minimum value for L
MAG
,
which is generally specified in transformer data sheets,
artificially limits the flux swing. In general, multiplying
the minimum value by 1.25 gives a good estimate for the
typical value.
