Datasheet

LT3825
24
3525fe
APPLICATIONS INFORMATION
Capacitor Selection
In a flyback converter, the input and output current flows
in pulses, placing severe demands on the input and output
filter capacitors. The input and output filter capacitors are
selected based on RMS current ratings and ripple voltage.
Select an input capacitor with a ripple current rating
greater than:
I
RMS
=
P
IN
V
IN(MIN)
1– DC
MAX
DC
MAX
Continuing the example:
I
RMS
=
44.4W
36V
1– 52.6%
52.6%
= 1.17A
Keep input capacitor series resistance (ESR) and induc-
tance (ESL) small, as they affect electromagnetic interfer-
ence suppression. In some instances, high ESR can also
produce stability problems because flyback converters
exhibit a negative input resistance characteristic. Refer
to Application Note 19 for more information.
The output capacitor is sized to handle the ripple cur-
rent and to ensure acceptable output voltage ripple.
The output capacitor should have
an RMS current
rating greater than:
I
RMS
= I
OUT
DC
MAX
1– DC
MAX
Continuing the example:
I
RMS
= 8A
52.6%
1– 52.6%
= 8.43A
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
flyback converter are illustrated in Figure 7.
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
ripple, divided equally between the ESR step and the
charging/discharging ∆V. This percentage ripple changes,
depending on the requirements of the application. You can
modify the equations below.
For a 1% contribution to the total ripple voltage, the ESR
of the output capacitor is determined by:
ESR
COUT
≤1% •
V
OUT
• 1– DC
MAX
( )
I
OUT
OUTPUT VOLTAGE
RIPPLE WAVEFORM
SECONDARY
CURRENT
PRIMARY
CURRENT
I
PRI
∆V
COUT
3825 F07
RINGING
DUE TO ESL
I
PRI
N
∆V
ESR
Figure 7. Typical Flyback Converter Waveforms