Datasheet

LTC4278
36
4278fc
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(PRI)
=
P
IN
V
IN(MIN)
1–DC
MAX
DC
MAX
Continuing the example:
I
RMS(PRI)
=
29.5W
41V
1– 49.4%
49.4%
= 0.728A
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 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 current
and to ensure acceptable output voltage ripple. The output
capacitor should have an RMS current rating greater than:
I
RMS(SEC)
=I
OUT
DC
MAX
1–DC
MAX
Continuing the example:
I
RMS(SEC)
= 5.3A
49.4%
1– 49.4%
= 5.24A
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 17.
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
OUTPUT VOLTAGE
RIPPLE WAVEFORM
SECONDARY
CURRENT
PRIMARY
CURRENT
I
PRI
∆V
COUT
4278 F17
RINGING
DUE TO ESL
I
PRI
N
∆V
ESR
Figure 17. Typical Flyback Converter Waveforms
ripple, divided equally between the ESR step and the
charging/discharging
ΔV. This percentage ripple changes,
depending on the requirements of the application. You can
modify the following equations.
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
The other 1% is due to the bulk C component, so use:
C
OUT
≥
I
OUT
1% • V
OUT
• f
OSC
In many applications, the output capacitor is created from
multiple capacitors to achieve desired voltage ripple,
reliability and cost goals. For example, a low ESR ceramic
capacitor can minimize the ESR step, while an electrolytic
capacitor satisfies the required bulk C.
Continuing our example, the output capacitor needs:
ESR
COUT
≤1% •
5V • 1– 49.4%
( )
5.3A
= 4mΩ
C
OUT
≥
5.3A
1% • 5 • 200kHz
= 600µF
These electrical characteristics require paralleling several
low ESR capacitors possibly of mixed type.