CS1500 Datasheet, PDF (13/22 Pages) Cirrus Logic – Digital Power Factor Correction IC

May ?$shortyear>
CONFIDENTIAL
CS1500
4.1.2 PFC Input Filter Capacitor
To achieve unity power factor, a DCM PFC circuit needs an
input filtering circuit to bypass the high-frequency current so
that the input current consists of the low-frequency portion
only. There are two main factors on PFC input filter capacitor
selection: its voltage ripple and phase lag, which both will
worsen power factor. The filtering capacitance is proportional
to P_o and it is suggested as follows:
C₁
≥
3.3
n_----F_--
W
×
P_o
[Eq.10]
C₁ ≥ 3.3 × 120
C₁ ≥ 390nF = 0.39μF
Use 0.47 μF for tolerance.
If a PI filter used for suppression of conducted EMI is located
on the DC side of the input rectifier, the V_rect sense point has
to be moved to the second capacitor.
4.1.3 PFC Boost Inductor
The value of the inductor in normal mode can be calculated by
the following equation, with α = 0.9 as a derating factor to
ensure the inductor is sized to guarantee DCM operation and
provide a slightly higher power than required by the load:
L_B
=
α
×
η
×
(V_in(min))²
×
V_{-----l-i--n--k----–-----}(_--V_{----i--n---(--m----i-n---)---}×_---------2_----)_-
2 × f_max × P_o × V_link
[Eq.11]
L_B
=
0.9
×
0.95
×
902
×
-------------⁴----⁰---⁰-----–----⁽---⁹---⁰-----^×---------²----⁾-------------
2 × (70 × 103) × 90 × 400
L_B = 374μH
Choose a 360 μH inductor.
I_LB(pk)
=
-------------------⁴-----^×-----^P----^o--------------------
α × η × V_in(min) × 2
4 × 90
I_LB(pk)
=
----------------------------------------------------
0.9 × 0.95 × 90 × ₂
I_LB(pk) = 3.3A
[Eq.12]
η is the efficiency.
The inductor should be designed so that its saturation current
meets the following requirement, where 0.001126 is a pre-
defined threshold for the current protection algorithm:
I_s
at
≤
0_----._--0_---0_---1_----1_---2_---6_--
L
[Eq.13]
where L is the inductance in Henrys.
4.1.4 PFC MOSFET
In normal mode, the PFC MOSFET peak current is equal to
the peak current in the PFC boost inductor:
I_FET(pk) = I_LB(pk)
[Eq.14]
I_FET(pk) = 3.3A
4.1.5 PFC Diode
The PFC diode peak current in normal mode is the equal to the
inductor peak current:
I_D(pk) = I_LB(pk)
I_D(pk) = 3.3A
[Eq.15]
The PFC Diode average current is calculated as follows:
I_D(avg)
=
---^P----^o----
V_link
I_D(avg)
=
--⁹---⁰----
400
I_D(avg) = 0.225A
[Eq.16]
4.1.6 PFC Output Capacitor
The value of the output capacitor is determined by several
requirements. It must meet the voltage ripple and hold-up time
requirements and the RMS current in the capacitor should not
exceed its RMS current rating.
The following equation defines the size of the output capacitor
to meet the output voltage ripple requirements:
C_out(rip)
=
----------------------------------------^P----^O------------------------------------------
2π × f_line(min) × V_link × ΔV_link(rip)
[Eq.17]
f_line(min) is the minimum line frequency the design is required
to support, V_link is the output voltage from the PFC, ΔV_link(rip),
is the output voltage ripple requirement in volts peak-to-peak.
The equation will provide the value of the output capacitor
needed to meet the ripple requirement.
For 10 V of ripple and minimum line frequency of 45 Hz, the
equation becomes:
C_out(rip)
=
----------------------⁹---⁰-----------------------
2π × 45 × 400 × 10
=
80μFuuseu100μF
A second requirement that the output capacitor may be
required to meet is hold-up time. The value of the capacitor
DS849A5
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