AN-9738 Datasheet, PDF (20/25 Pages) Fairchild Semiconductor – Design Guideline on 150W Power Supply for LED Street Lighting Design Using FL7930B and FAN7621S

AN-9738
Meanwhile, the ripple current flowing through output
capacitor is given as:
I RMS
Co
=
( ^{π Io} )²
22
− I_o²
=
π
2_−
8
8
I
o
(64)
The voltage ripple of the output capacitor is:
∆V_o
=
π
2
I_o
⋅
R_C
(65)
where R_C is the effective series resistance (ESR) of the
output capacitor and the power dissipation is the output
capacitor is:
P_Loss.Co = (I_CoRMS )² ⋅ R_C
(66)
APPLICATION NOTE
(Design Example) The voltage stress and current stress of
the rectifier diode are:
V_D = 2(V_o +V_F ) = 2(103+ 0.9) = 207.8V
I RMS
D
=
π
4
I_o
= 1.14 A
The 600V/8A ultra-fast recovery diode is selected for the
rectifier, considering the voltage overshoot caused by the
stray inductance.
The RMS current of the output capacitor is:
I RMS
C_o
=
( ^πIo )²
22
− I_o²
=
π
2 _−8
8 ^Io
=
0.584 A
When two electrolytic capacitors with ESR of 100mΩ are
used in parallel, the output voltage ripple is given as:
∆V_o
=
π
2
I_o
⋅ R_C
=
π
2
⋅1.46 ⋅ ( ^0.1)
2
=
0.114V
The loss in electrolytic capacitors is:
P_Loss,Co = (I_Co ^RMS )² ⋅ R_C = 0.584² ⋅ 0.05 = 0.017W
[STEP-19] Control Circuit Configuration
Figure 38 shows the typical circuit configuration for the RT
pin of FAN7621S, where the opto-coupler transistor is
connected to the RT pin to control the switching frequency.
The minimum switching frequency occurs when the opto-
coupler transistor is fully tuned off, which is given as:
f _min
=
5.2kΩ
R_min
× 100 (kHz )
(67)
Assuming the saturation voltage of opto-coupler transistor is
0.2V, the maximum switching frequency is determined as:
f _max
=
( ^{5.2k Ω}
R_min
+
4.68kΩ _{) ×100(kHz)}
R_max
(68)
Figure 38. Typical Circuit Configuration for RT Pin
Soft-start prevents excessive inrush current and overshoot of
output voltage during startup, increases the voltage gain of
the resonant converter progressively. Since the voltage gain
of the resonant converter is reversely proportional to the
switching frequency, soft-start is implemented by sweeping
down the switching frequency from an initial high
frequency (f ^ISS) until the output voltage is established, as
illustrated in Figure 39. The soft-start circuit is made by
connecting RC series network on the RT pin as shown in
Figure 38. FAN7621S also has an internal soft-start for 3ms
to reduce the current overshoot during the initial cycles,
which adds 40KHz to the initial frequency of the external
soft-start circuit, as shown in Figure 39. The actual initial
frequency of the soft-start is given as:
f ^ISS = (5.2^kΩ + ^5.2kΩ ) ×100 + 40 (kHz)
(69)
R_min
R_SS
It is typical to set the initial frequency of soft-start (f ^ISS) at
2~3 times of the resonant frequency (f_o).
The soft-start time is determined by the RC time constant:
t_SS = 3 ~ 4( R_SS • C_SS )
(70)
Figure 39. Frequency Sweep of the Soft-Start
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 4/20/11
20
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