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AN2870 Datasheet(PDF) 21 Page - STMicroelectronics

Part # AN2870
Description  L6585DE combo IC
PDF  41 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2870 Datasheet(HTML) 21 Page - STMicroelectronics

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AN2870
Designing with L6585DE
21/41
Boost choke design
PFC stage operates in transition mode; for a certain value of input voltage the on time (Ton)
is constant over the entire half period of the input voltage.
The frequency changes along the period of the input voltage: in particular the frequency is
the lowest when the input voltage reaches its maximum.
Moreover the frequency is higher if the output power is low and the frequency variation
changes if input voltage changes.
The internal starter requires a minimum PFC frequency equal to around15 kHz.
Equation 4
Using Eq.4 with both minimum and maximum value of Vin, the value of L can be selected as
the minimum obtained value. Even the maximum frequency should be checked to avoid to
absorb too much current from Vcc and to degrade the input performances due to excessive
frequency (> 450 kHz) in correspondence to zero input voltage.
The calculation of the maximum frequency is only a rough evaluation of the real frequency;
in fact the presence of the THD optimizer reduces the frequency near the crossover of the
input voltage.
The maximum current flowing into the choke can be evaluated as twice the maximum input
current:
Equation 5
The ohmic power losses will be evaluated considering the RMS value of the current:
Equation 6
The choke is realized around of a gapped ferrite core; the core shape has to be selected
considering the electrical parameters (Eq.4,5 and 6), the dimensions of the ballast and the
availability of the selected core. The core should be made by a material suitable for high
frequency operation.
(
)
kHz
15
V
V
2
1
L
P
2
V
f
V
t
f
2
sin
V
2
1
L
P
2
V
f
out
in
in
2
in
min
,
PFC
out
mains
in
in
2
in
PFC
≥
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⋅
−
⋅
⋅
⋅
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⋅
⋅
π
⋅
⋅
−
⋅
⋅
⋅
=
min
,
in
in
max
,
L
V
P
2
2
I
⋅
⋅
=
min
,
in
in
RMS
,
L
V
P
3
2
I
=



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