AN-9732 Datasheet, PDF (5/22 Pages) Fairchild Semiconductor – LED Application Design Guide Using BCM Power Factor Correction (PFC) Controller for 200W Lighting System

AN-9732
[STEP-1] Define System Specifications
 Line Frequency Range (V_LINE,MIN and V_LINE,MAX)
 Line Frequency (f_LINE)
 Output-Voltage (V_OUT)
 Output Load Current (I_OUT)
 Output Power (P_OUT =V_OUT  I_OUT)
 Estimated Efficiency ()
To calculate the maximum input power, it is necessary to
estimate the power conversion efficiency. At universal input
range, efficiency is recommended at 0.9; 0.93~0.95 is
recommended when input voltage is high.
When input voltage is set at the minimum, input current
becomes the maximum to deliver the same power compared
at high line. Maximum boost inductor current can be
detected at the minimum line voltage and at its peak.
Inductor current can be divided into two categories; one is
rising current when MOSFET is on and the other is output
diode current when MOSFET is off, as shown in Figure 10.
APPLICATION NOTE
(Design Example) Input voltage range is universal input,
output load is 500mA, and estimated efficiency is selected
as 0.9.
V_LINE,MIN  90V_AC , V_LINE,MAX  265V_AC
f _LINE  50Hz
V_OUT  400V , I_OUT  500mA
  0.9
I _L,PK


4  P_OUT
 ^{4  400V  0.5A}  6.984A
2 V_LINE,MIN 0.9  2  90
I _IN,MAX
 ^{I L,PK}
2
 ^6.984A  3.492A
2
I _IN,MAXRMS  ^{I IN,MAX}  ^3.492A  2.469A
2
2
Figure 10. Inductor and Input Current
Because switching frequency is much higher than line
frequency, input current can be assumed to be constant
during a switching period, as shown in Figure 11.
Figure 11. Inductor and Input Current
With the estimated efficiency, Figure 10 and Figure 11
inductor current peak (I_L,PK), maximum input current
(I_IN,MAX), and input Root Mean Square (RMS) current
(I_IN,MAXRMS) are given as:
I _{L ,PK}


4  P_OUT
2  V _{LINE ,MIN}
[ A]
(3)
I_IN,MAX  I_L,PK / 2 [ A]
(4)
I_IN,MAXRMS  I_IN,MAX / 2 [ A ]
(5)
[STEP-2] Boost Inductor Design
The boost inductor value is determined by the output power
and the minimum switching frequency. The minimum
switching frequency must be higher than the maximum
audible frequency band of 20kHz. Minimum frequency near
20kHz can decrease switching loss with the cost of
increased inductor size and line filter size. Too-high
minimum frequency may increase the switching loss and
make the system respond to noise. Selecting in the range of
about 30~60kHz is a common choice; 40~50kHz is
recommended with FL7930.
The minimum switching frequency may appear at minimum
input voltage or maximum input voltage, depending on the
output voltage level. When PFC output voltage is less than
405V, minimum switching appears at the maximum input
voltage, according to application note AN-6086. Inductance
is obtained using the minimum switching frequency:
L
 ²
  2V_LINE
[H]
4  f_{SW ,MIN}
 P_OUT
 _{}1 
2V_LINE
V_OUT  2V_LINE


(6)
where L is boost inductance and f_SW,MIN is the minimum
switching frequency.
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 3/23/11
5
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