Datasheet

LTC3766
25
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
completely recovered by the active clamp capacitor. For
most applications, these assumptions are valid and the
above equation is a good approximation.
The active clamp P-channel MOSFET has the same BV
DSS
requirement as that of the N-channel MOSFET. Since the
P-channel MOSFET only handles the magnetizing current,
it is normally much smaller (typically a SOT package).
To accommodate abnormal transients, use a P-channel
MOSFET that has a pulsed drain current rating of 2A or
higher. Also, note that when the N-channel MOSFET turns
off, the leakage inductance will momentarily force the re
-
flected load current into the body diode of the P-channel
MOSFET. Consequently
, the body diode should be rated
to handle a pulsed forward current of:
I
D(MAX)
=
N
S
N
P
⎛
⎝
⎜
⎞
⎠
⎟
I
MAX
In some cases, it may be more practical to add a separate
diode in parallel with the body diode of the P-channel
MOSFET.
The primary-side P-channel MOSFET may be driven by a
simple level-shift circuit that shifts down the drive voltage
on the LTC3765 AG pin. Alternatively, the level-shift circuit
can be omitted if the source of the P-channel MOSFET
is returned to the V
CC
pin of the LTC3765. Refer to the
LTC3765 data sheet for details.
In nonisolated applications where the LTC3766 is used
standalone, it is necessary to use a resonant reset tech
-
nique instead of the active clamp reset. As a result, there
are special considerations in selecting the primary-side
MOSFET
. See the Nonisolated Applications section for
additional information.
Secondary-Side Power MOSFET Selection
On the secondary side, the peak-to-peak drive level for the
N-channel MOSFETs is determined by the V
CC
pin on the
LTC3766. Assuming that one or both of the linear regula-
tors in the LTC3766 are used, the V
CC
regulation voltage
can be set to either 7V or 8.5V as needed for driving the
gates of the MOSFETs.
The first step in selecting the secondary-side MOSFETs is
to determine the needed breakdown voltage. The maximum
voltage seen by the synchronous MOSFET is approximately:
V
DS(SG)
= 1.2•
N
S
N
P
⎛
⎝
⎜
⎞
⎠
⎟
V
IN(MAX)
where the factor of 1.2 has been added to allow for ringing
and overshoot. This assumes that a snubber has been used
on the secondary side of the main transformer (see the RC
Snubbers section). If no snubber is used, the ringing and
peak overshoot will be considerably higher. The maximum
voltage seen by the forward MOSFET is approximately:
V
DS(FG)
=
1.2• V
OUT
1–
V
OUT
V
IN(MIN)
N
P
N
S
⎛
⎝
⎜
⎞
⎠
⎟
where the factor of 1.2 has again been added to allow for
ringing and overshoot.
Having determined the BV
DSS
requirement for the forward
and synchronous MOSFETs, the next step is to choose the
on-resistance. Since both secondary-side MOSFETs are
zero-voltage switched, choose MOSFETs that have a low
R
DS(ON)
and have been optimized for use as synchronous
rectifiers, including a body diode with a fast reverse re-
covery if possible. In most applications, the nominal input
voltage will correspond to approximately 50% duty cycle,
so the for
ward and synchronous MOSFET
s will be selected
to have the same R
DS(ON)
. The power loss associated with
the forward MOSFET can be approximated by:
P
FG
=
N
P
N
S
⎛
⎝
⎜
⎞
⎠
⎟
V
OUT
I
MAX
( )
2
V
IN
1+ δ
( )
R
DS(ON)
+Q
GTOT
V
BIAS
f
SW
where δ is the temperature dependence of the on-resistance
and V
BIAS
is the input to the LTC3766 linear regulator (if
used). The value for Q
GTOT
can be taken from the V
GS
versus Q
G
curve given in the MOSFET data sheet. Q
GTOT
is the value of Q
G
when V
GS
= V
CC
, where V
CC
is the