STMicroelectronics A 200 W ripple-free input current PFC pre-regulator with the L6563S EVL6563S-200ZRC EVL6563S-200ZRC Fiche De Données
Codes de produits
EVL6563S-200ZRC
Capacitor selection
AN3180
18/39
Doc ID 17273 Rev 1
5 Capacitor
selection
It is obvious that the capacitance of the smoothing transformer capacitor should be as large
as possible and its ESR as low as possible to minimize the impressed voltage mismatch.
Although this assertion is always true, the use in a PFC pre-regulator to minimize the input
current ripple poses an important limitation to the capacitance value that can be used.
as possible and its ESR as low as possible to minimize the impressed voltage mismatch.
Although this assertion is always true, the use in a PFC pre-regulator to minimize the input
current ripple poses an important limitation to the capacitance value that can be used.
In
it is shown that in the zero-ripple input current configurations there is a DC path
between the capacitor and the input port of the converter. This means that, at low frequency,
the capacitor appears as if directly connected to the input rectifier bridge; therefore, in a
PFC stage it adds to the capacitor normally placed just after the bridge and contributes to
the increase in total harmonic distortion (THD) of the low-frequency line current as well as to
lowering the power factor (PF).
the capacitor appears as if directly connected to the input rectifier bridge; therefore, in a
PFC stage it adds to the capacitor normally placed just after the bridge and contributes to
the increase in total harmonic distortion (THD) of the low-frequency line current as well as to
lowering the power factor (PF).
Actually, the capacitor placed after the bridge can be reduced because of the ripple
reduction caused by the smoothing transformer; but the value of the smoothing capacitor
must, in any case, be selected trading off the filtering performance of the smoothing
transformer against the need for a low THD of the line current. The peak-to-peak voltage
ripple
reduction caused by the smoothing transformer; but the value of the smoothing capacitor
must, in any case, be selected trading off the filtering performance of the smoothing
transformer against the need for a low THD of the line current. The peak-to-peak voltage
ripple
Δv appearing across the capacitor, which is essentially twice the voltage mismatch, is
related to the AC ripple amplitude
ΔI:
Equation 19
which is a maximum at the top of the sinusoid at minimum line voltage, where the amplitude
of the AC current is maximum and the switching frequency is close to its minimum value.
Substituting the values that can be derived by the PFC relationships [
of the AC current is maximum and the switching frequency is close to its minimum value.
Substituting the values that can be derived by the PFC relationships [
], it is possible to find
the following expression for the relative voltage mismatch:
Equation 20
Typically, the values normally used for the capacitor after the bridge (5
÷15 nF/W) lead to a
good compromise. Polypropylene capacitors are recommended for their low ESR.
s
sw
pk
pk
C
f
8
I
v
Δ
=
Δ
−
min
2
2
min
min
max
s
1
max
1
Vin
2
Vout
Vout
Vin
Pin
C
4
L
)
t
(
V
)
t
(
v
−
⎟
⎟
⎠
⎠
⎞
⎜
⎜
⎝
⎝
⎛
=
Δ