FA5502P/M
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So that the voltage inputted to IDET pin will not be
beyond -1V, whereby the overcurrent limiting circuit
will not operate at a normal operation, calculate Rs by:
(3) Designing a boost converter
Fig.19 shows a basic circuit of boost converter used
as an PFC converter. The following describes how to
determine each values of the circuit.
Vin(min)
RS
≤
[Ω] • • • • • (15)
D
2 ×Pin(max)
L
Where,
Vin (min): Minimum AC input voltage [Vrms]
Pin (max): Maximum input position [W]
Q
Vo
Vin
Co
As a matter of fact, the peak current changes with
switching ripple current contained in the inductor
current, circuit efficiency, etc. Definitely determine it by
evaluation on a actual circuit.
Rs
Fig.19 Boost converter circuit
(3-1) Output voltage
(3-4) Smoothing capacitor
Set the output voltage of boost converter at least
10V higher than the peak value of maximum input
voltage to ensure a stable operation. When it is used
as PFC converter, the input voltage has a sinusoidal
waveform. Therefore, set the output voltage Vo by:
PFC converter output contains ripple voltage of
twice the line frequency as shown in Fig.21.
Instantaneous value Vo(t) of output voltage is
approximated by:
Io
Vo(t) = Vo −
× sin(2ω0 × t) • • • • • (16)
Vo ≥ 2 × Vin(max) + 10 [V]
Vin(max) : Maximu AC input voltage [Vrms]
2ω0 ×Co
• • (12)
Where,
Io: Output current [A]
ω0 = 2πf0 (f0: AC line frequency [Hz])
Co: Output smoothing capacitance [F]
(3-2) Inductor
When PFC converter operates in the continuous
current mode, select an approximate inductance
considering the ratio of inductor ripple current to the
peak input current by:
Therefore, output ripple voltage Vrp (p-p) is:
Io
Vrp =
• • • • • (17)
Vin2
γ × fs×Pin× Vo
Vo − 2 × Vin
ω0 ×Co
L ≥
• • • • • (13)
Using formula (17), determine the necessary value.
The overvoltage limiting circuit of FA5502 monitors the
instantaneous output voltage. Therefore, determine
the capacitance of smoothing capacitor Co so that the
instantaneous output voltage including the ripple at a
normal operation will not reach the overvoltage limit.
Where,
Vin: AC input voltage [Vrms]
γ: Ratio of ripple content to peak input current.
(Set to approx. 0.2, see Fig.20)
fs: Switching frequency [Hz]
Pin: Maximum input power [W]
V
rp
Inductor current
γ = Ir /Iac(peak)
Vo
Ir
2xfac
Fig.21 Output ripple voltage
line current
Iac(peak)
t
Fig.20 Outline of inductor and AC line current
(3-3) Current detecting resistance Rs
Rs is a resistor which allows to detect an inductor
current to control the line current into sinusoidal.
Because the threshold voltage for overcurrent limiting
circuit is -1.1V (typ.), peak inductor current limit Ip is
calculated by:
1.1
RS
Ip =
[A] • • • • • (14)
19
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