Datasheet

LTC3766
23
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
choose a capacitor whose voltage rating is greater than
this maximum V
CL
voltage by 50% or more. Typically, a
good quality (X7R) ceramic capacitor is a good choice for
C
CL
. Also, be sure to account for the voltage coefficient of
the capacitor. Many ceramic capacitors will lose as much
as 50% of their value at their rated voltage.
The value of the clamp capacitor should be high enough
to minimize the capacitor ripple voltage, thereby reducing
the voltage stress seen by the MOSFETs. However, a larger
clamp capacitor will ultimately result a slower transient
response to avoid transformer saturation during load
transients. While Direct Flux Limit will automatically limit
the PWM on-time only as needed to prevent saturation, a
larger clamp capacitor will require a longer time to charge
or discharge in response to a load transient. Consequently,
the value of the clamp capacitor represents a compromise
between transient response and MOSFET voltage stress. A
reasonable value for the clamp capacitor can be calculated
using the following:
C
CL
=
1
2L
M
•
4
2πf
SW
⎛
⎝
⎜
⎞
⎠
⎟
2
An additional design constraint on C
CL
occurs because of
the resonance between the magnetizing inductance L
M
of
the main transformer and the clamp capacitor C
CL
. If the
Q of this resonance is too high, it will result in increased
voltage stress on the primary-side MOSFET during load
transients. Also, a high Q resonance between L
M
and C
CL
complicates the compensation of the voltage loop, and can
cause oscillations under certain conditions. To avoid the
problems associated with this resonance, always use
an RC snubber in parallel with the clamp capacitor as
shown in Figure 9. The values for this RC snubber can
then be calculated using:
R
CS
=
1
1–
V
O
V
IN(MIN)
•
N
P
N
S
⎛
⎝
⎜
⎞
⎠
⎟
L
M
C
CL
and
C
CS
= 6C
CL
Figure 9 shows a typical arrangement of the active clamp
capacitor with an RC snubber. Be careful to account for
the effect of voltage coefficient for both C
CS
and C
CL
to
ensure that the above relationship between C
CS
and C
CL
is maintained.
DUTY CYCLE (%)
20
ACTIVE CLAMP VOLTAGE
NORMALIZED TO 50% DUTY CYCLE
1.3
1.4
1.5
1.2
1.1
40 60
30
50 70 80
1.0
0.9
1.6
3766 F08
Figure 8. V
CL
Voltage vs Duty Cycle
V
IN
L
M
SWP
SNUBBER
C
CL
ACTIVE
CLAMP
PMOS
C
CS
3766 F09
R
CS
Figure 9. Active Clamp Capacitor and Snubber
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. The L
TC3765
and L
TC3766 implement a new unique system for moni
-
toring and directly limiting the flux accumulation in the
transformer core. Unlike previous methods, the direct flux
li
mit directly measures and monitors flux accumulation and
guarantees that the transformer will not saturate in either
direction, even when starting into a pre-biased output.
This technique also provides the best possible transient
response, as it will temporarily allow very high duty cycles,
only limiting the duty cycle when absolutely necessary.
Moreover, this technique prevents overcurrent damage to