CS1600_1006 Datasheet, PDF (12/18 Pages) Cirrus Logic – Low-cost PFC Controller for Electronic Ballasts

CS1600
5.1.3 PFC Boost Inductor
Equation 3 can be rewritten to calculate the PFC boost
Inductor, L_B, as follows:
α
=


-^V-----^l-ⁱ--ⁿ--^k--
400V
×
V^-----_i^9-_n^---0₍^--_m^-V---_i^--_n^---₎
2
×
-^V----^l-ⁱ-ⁿ---^k----–-----^---^4---V----⁰----^l--ⁱ⁰---^n-----V-^k-------×----⁹----⁰---^V------^×---------²---^-
V_link – V_in(min) × 2
[Eq.6]
α
=


--^V----^l-ⁱ-ⁿ---^k--
400V
×
V^-----_i^9-_n^---0₍^--_m^-V---_i^--_n^---₎
2_×^V_-----l_-i_-n_---k_{----–--------4}^---V_----0----^l_--ⁱ_0----ⁿ_----V-^k-------×_----⁹_----⁰_---^V_------^×_---------²_{----}
V_link – V_in(min) × 2
= 0.937
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.6]
L_B
=
0.937
×
0.95×
108²
×
----------⁽---⁴---⁶----⁰----–-----¹---⁰----⁸----^×---------²----⁾-----------
2 × 70 × 10³ × 115 × 460
=
431μ
H
The RMS current rating for the inductor is estimated using an
scaling factor used to account for variations in the input
current shape across the AC line cycle, over and above the
nominally calculated value. The nominal value before using
the scaling factor is as follows:
I_LB(rms)
=
-------------------^P----^O-------------------- × β
V_in(min) × 2 × η
I_LB(rms)
=
----------------¹----¹---⁵------------------ × 1.35
108 × ₂ × 0.95
[Eq.7]
I_LB(rms) = 1.07A
where
β = inductor scaling factor
The peak inductor current, I_LB(pk), may be estimated using the
following equation:
I_LB(pk)
=
--------------4-----^×----^P-----^O---------------
η × V_in(min) × 2
I_LB(pk)
=
------------4----^×-----¹---¹----⁵-------------
0.95 × 108 × 2
I_LB(pk) = 3.17 A
[Eq.8]
Inductor tolerances should be considered when estimating the
peak currents present in the application.
The internal control algorithm of the controller dictates that the
peak inductor current seen in the application could be as high
as a pre-defined threshold of 0.001984 times the inverse of
the inductor, which in this example amounts to 4.72 A. Care
needs to be taken to ensure that the saturation current rating
of the PFC boost inductor factors in this threshold used for the
protection schemes.
For a 40 V ripple and minimum line frequency of 45 Hz, the
5.1.4 PFC MOSFET
The peak voltage stress on the PFC MOSFET is a diode drop
above the output voltage. Accounting for leakage spikes, for
the 460 V output application, a 600 V FET is recommended.
The FET should be able to handle the same peak current as
that seen through the inductor. This would amount to 3.96 A.
The scaling factor to determine the RMS current through the
MOSFET for a 108 V input is about 1.15, and the minimum
RMS current rating, I_FET(rms), required for the FET is
calculated as follows:
I_FET(rms)
=
-------------------^P----^O-------------------- × γ
V_in(min) × 2 × η
[Eq.9]
I_LB(rms)
=
----------------¹----¹---⁵------------------ × 1.15
108 × ₂ × 0.95
I_LB(rms) = 0.91A
where
γ = FET scaling factor
5.1.5 PFC Diode
The PFC diode peak current is equal to the inductor peak
current:
I_D(pk) = I_LB(pk)
[Eq.10]
I_D(pk) = 3.17 A
The PFC diode average current is calculated as follows:
I_D(avg)
=
---^P----^O----
V_link
[Eq.11]
I_D(avg)
=
1_----1_---5_--
460
I_D(avg) = 0.25 A
5.1.6 PFC Output Capacitor
The output capacitor needs to be designed to meet the voltage
ripple and hold-up time requirements. In the case of a cost-
sensitive ballast application, the hold-up requirement is not a
key requirement.
To address the output ripple requirements, the following
equation may be used as a guide:
C_out
=
----------------------------------------^P----^O------------------------------------------
2π × f_line(min) × V_link × ΔV_link(rip)
[Eq.12]
where
C_out = Output Capacitance value
P_o = Output Power
f_line(min) = Minimum Line Frequency
V_link = PFC Output Voltage
ΔV_link = Peak-Peak Voltage Ripple on the PFC Output
12
DS904A6