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AN3112 Datenblatt(PDF) 12 Page - STMicroelectronics

Teilenummer AN3112
Bauteilbeschribung  Solution for designing a fixed off-time controlled PFC pre-regulator
PDF  36 Pages
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Hersteller  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
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AN3112 Datenblatt(HTML) 12 Page - STMicroelectronics

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Designing a fixed-off-time PFC
AN3112
12/36
Doc ID 16820 Rev 3
Equation 14
Inductor peak current:
Equation 15
It is also possible to calculate the RMS current flowing into the switch and into the diode,
needed to calculate the losses of these two elements.
RMS switch current:
Equation 16
RMS diode current:
Equation 17
It is worth remembering that the accuracy of the approximate energetic relationships
described here is quite good at maximum load for low values of parameter k, that is, at low
line voltage, but worsens at high line and as the power throughput is reduced. Since, in the
design phase, current stress is calculated at maximum load and minimum line voltage, their
accuracy is acceptable for design purposes.
4.3
Power section design
4.3.1
Bridge rectifier
The input rectifier bridge can use standard slow recovery, low-cost devices.
Typically a 600 V device is selected in order to obtain a good margin against mains surges.
An NTC resistor, limiting the current at turn-on, is required to avoid overstress to the diode
bridge.
The rectifier bridge power dissipation can be calculated using equations
Equation 18,Equation 19,Equation 20. The threshold voltage (Vth) and dynamic resistance
(Rdiode) of a single diode bridge can be found in the component datasheet.
max
PK
r
r
pk
I
k
3
8
k
6
IL
⋅
⋅
−
⋅
=
∆
A
04
.
2
A
98
.
6
34
.
0
3
8
34
.
0
6
IL
pk
=
⋅
⋅
−
⋅
=
∆
max
PK
r
max
pk
I
k
3
8
8
IL
⋅
⋅
−
=
A
01
.
8
A
98
.
6
34
.
0
3
8
8
IL
max
pk
=
⋅
⋅
−
=
π
⋅
−
⋅
⋅
=
3
k
16
2
V
k
P
ISW
min
out
min
in
rms
A
22
.
4
3
32
.
0
16
2
V
400
32
.
0
W
400
ISW
rms
=
π
⋅
−
⋅
⋅
=
π
⋅
⋅
=
3
k
16
V
k
P
ID
min
out
min
in
rms
A
57
.
2
3
32
.
0
16
V
400
32
.
0
W
400
ID
rms
=
π
⋅
⋅
⋅
=



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