ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 42

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

LTC4268-1
However, other schemes are possible. There are gate drivers
and secondary side synchronous controllers available
that provide the buffer function as well as additional
features.
Capacitor Selection
In a fl yback converter, the input and output current fl ows
in pulses, placing severe demands on the input and output
fi lter capacitors. The input and output fi lter capacitors
are selected based on RMS current ratings and ripple
voltage.
Select an input capacitor with a ripple current rating
greater than:
Continuing the example:
APPLICATIONS INFORMATION
42
RIPPLE WAVEFORM
I
I
RMS PRI
RMS PRI
OUTPUT VOLTAGE
Figure 18. Typical Flyback Converter Waveforms
(
(
SECONDARY
PRIMARY
CURRENT
CURRENT
)
)
=
=
29 5
V
IN MIN
41
P
.
(
IN
V
ΔV
W
COUT
I
)
PRI
1 49 4
ΔV
− 1
−
49 4
ESR
DC
DC
. %
MAX
. %
MAX
=
I
PRI
N
0 728
.
DUE TO ESL
RINGING
A
42681 F18
Keep input capacitor series resistance (ESR) and inductance
(ESL) small, as they affect electromagnetic interference
suppression. In some instances, high ESR can also
produce stability problems because fl yback converters
exhibit a negative input resistance characteristic. Refer
to Application Note 19 for more information. The output
capacitor is sized to handle the ripple current and to ensure
acceptable output voltage ripple.
The output capacitor should have an RMS current rating
greater than:
This is calculated for each output in a multiple winding
application.
ESR and ESL along with bulk capacitance directly affect the
output voltage ripple. The waveforms for a typical fl yback
converter are illustrated in Figure 18.
The maximum acceptable ripple voltage (expressed as a
percentage of the output voltage) is used to establish a
starting point for the capacitor values. For the purpose
of simplicity we will choose 2% for the maximum output
I
I
Continuing
RMS SEC
RMS SEC
(
(
SECONDARY
WINDING
)
)
FROM
=
=
I
5 3
the exa a mple:
OUT
.
A
+
− 1
1 49 4
C1
47μF
0.1μH
DC
3
−
49 4
L1
Figure 19.
DC
MAX
. %
+
MAX
. %
C
470μF
OUT
=
5 24
.
V
OUT
A
C
1μF
OUT2
42681 F19
R
LOAD
42681fa