AN1262 APPLICATION NOTE
6
INPUT BULK CAPACITOR SELECTION
The input bulk capacitor C
in
, along with the bridge rectifier, converts the AC mains voltage to an unregulated
DC bus, V
in
, which is the input voltage for the downstream flyback converter. C
in
must be large enough to have
a relatively low ripple superimposed on top of the DC level, as shown in fig. 3. At minimum specified mains volt-
age, the value of C
in
determines the absolute minimum, V
inmin
, of the DC input voltage of the converter. The
maximum duty cycle and the maximum peak current allowed by the IC must not be exceeded at this voltage.
However, as to thermal consideration, the bus DC voltage (V
DCmin
@ V
ACmin
) should be considered.
Figure 3. Input voltage waveforms:
a) without holdup capability;
b) with holdup capability.
V
in
V
PKmin
V
inmin
T
C
V
DCmin
1
f
L
a)
V
in
V
PKmin
V
inmin
V
DCmin
V
inmin
after fail
T
C
T
C
after fail
1 missing cycle
b)
Large values of C
in
result in higher V
DCmin
and V
inmin
, lower peak and RMS current through the power MOSFET
(i.e. less power dissipation in the device) and less duty cycle range to achieve regulation but, on the other hand,
also in bigger capacitor size, higher peak and RMS current drawn from the mains (i.e. more power dissipation
in the bridge rectifier). Small values of C
in
give origin to the opposite situation.
Experience shows that a good compromise between these contrasting requirements is a C
in
value that causes
the peak-to-peak ripple amplitude to be 25-30% of the peak mains voltage (@ V
ACmin
), which means that V
inmin
will be 70-75% of the peak value. Anyway, if holdup capability is required, a much larger capacitance values will
be needed: the voltage ripple across C
in
is expected to be 25-30% of the peak value, after 1 mains cycle miss-
ing, which means that in normal operation the ripple will be much less.
Table 5 summarizes the required capacitance per watt of input power for a given value of V
inmin
, with and with-
out holdup requirement, and shows the resulting values of V
DCmin
. This allows to calculate the minimum capac-
itance needed, by multiplying the value taken from the table times P
in
. Then a standard value will be selected,
taking also the tolerance into account.
6/42
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