ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 40

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

LTC4268-1
• The external feedback resistive divider ratio directly
• Leakage inductance on the transformer secondary
• The transformer secondary current fl ows through the
If the output load current is relatively constant, the feedback
resistive divider is used to compensate for these losses.
Otherwise, use the LTC4268-1 load compensation circuitry.
(See Load Compensation.) If multiple output windings are
used, the fl yback winding will have a signal that represents
an amalgamation of all these windings impedances. Take
care that you examine worst-case loading conditions when
tweaking the voltages.
Power MOSFET Selection
The power MOSFETs are selected primarily on the criteria of
“on” resistance R
breakdown voltage (BV
and maximum drain current (ID
APPLICATIONS INFORMATION
40
affects regulated voltage. Use 1% components.
reduces the effective secondary-to-feedback winding
turns ratio (NS/NF) from its ideal value. This increases
the output voltage target by a similar percentage. Since
secondary leakage inductance is constant from part to
part (within a tolerance) adjust the feedback resistor
ratio to compensate.
impedances of the winding resistance, synchronous
MOSFET R
equivalent current for these errors is higher than the
load current because conduction occurs only during
the converter’s “off” time. So divide the load current
by (1 – DC).
DS(ON)
Figure 17. Gate Charge Curve
V
GS
DS(ON)
Q
and output capacitor ESR. The DC
a
A
GATE CHARGE (Q
, input capacitance, drain-to-source
DSS
MILLER EFFECT
), maximum gate voltage (V
(MAX)
G
Q
b
)
B
).
42681 F17
GS
)
For the primary-side power MOSFET, the peak current
is:
where X
For each secondary-side power MOSFET, the peak current
is:
Select a primary-side power MOSFET with a BV
than:
where N
primary winding. L
tance and C
the drain capacitance (C
MOSFET). A snubber may be added to reduce the leakage
inductance as discussed.
For each secondary-side power MOSFET, the BV
be greater than:
Choose the primary side MOSFET R
gate drive voltage (7.5V). The secondary side MOSFET
gate drive voltage depends on the gate drive method.
Primary side power MOSFET RMS current is given by:
BV
I
I
I
BV
PK PRI
PK SEC
RMS PRI
DSS
DSS
(
(
(
MIN
SP
)
≥ V
≥
)
=
)
=
P
I
is peak-to-peak current ratio as defi ned earlier.
refl ects the turns ratio of that secondary-to
PK
=
is the primary-side capacitance (mostly from
V
OUT
1
IN MIN
−
V
IN MIN
(
I
DC
OUT
L
+ V
(
C
LKG
LKG
MAX
P
P
)
IN(MAX)
IN
•
P
)
DC
+
is the primary-side leakage induc-
IN
•
DC
V
OSS
MAX
⎛
⎝ ⎜
IN MAX
1
MAX
(
+
) of the primary-side power
• N
•
X
⎛
⎝ ⎜
SP
MIN
2
)
1
+
+
⎞
⎠ ⎟
DS(ON)
V
X
OUT MAX
MIN
2
N
SP
(
⎞
⎠ ⎟
at the nominal
)
DSS
DSS
greater
should
42681fa