AND8099 Datasheet, PDF (3/8 Pages) ON Semiconductor – 5.0 V, 2.0 A Flyback Converter

AND8099/D
In order for the regulator to operate in discontinuous mode
under worse case conditions and to maximize power, the
maximum on time is 48% of the full period, therefore the the
maximum primary inductance is calculated based on a
maximum duty cycle of 48%. Using a larger inductance than
calculated will cause the power supply output to fall out of
regulation.
Lpri +
Vin(low)
Ipeak
·
·
ē _max
fop
,
where ē _max
duty cycle
is
the
maximum
Lpri
+
80.2 · 0.48
0.667 * 100 · 103
+
0.577
mH
Primary flyback voltage:
VFB
+
Vin(low)
toff
·
ton
+
80.2 · 4.8 · 10-
5.2 · 10- ⁶
6
+ 74.03 V, where
ton is 4.8 ms
Primary to secondary turns ratio:
Npri
N _sec
+
VFB
Vout )
VF
+
5
74.03
) 0.525
+
13.4
[
13
turns
By rearranging the above equation and solving for N_sec
yields ≈1 turn.
Energy entering the core during on-time (when the power
switch is conducting):
Estored
+
Lpri
·
Ipeak2
2
,
where the
measured
stored energy
in Joules.
is
Estored
+
0.577
·
10- ³
2
·
0.6672
+ 1.28 10- ⁴ Joules
One can double check if the power capability of the
transformer is large enough to supply enough power to the
output with the following equation:
Pin(core)
+
Lpri
·
Ipeak2
2
·
fop
u
Pout
Pin(core) + 1.28 10- ⁴ · 100 kHz + 12.8 W u Pout
+ 10 W
Input Snubber
Because of the high dv/dt characteristic of the power
transistor drain voltage and of the transformer leakage
inductance, voltage spikes and ringing occur at the drain
when the power switch is turned off. Resistor R1, C3, D5
compromise an RCD snubber. In parallel to the primary
winding are R2 and C4 which compromise an RC ringing
damper which slows down the dv/dt and reduces the
peak voltage therefore decreasing the ringing due
to high frequency noise. Since i + C · ^ddvt , increasing the
capacitance will also reduce the magnitude of the voltage
ripple. The snubber and ringing damper act together to
protect the IC from voltage transients greater than 700 V and
reduce radiated noise.
Output Block
The output consists of a diode rectifier, a pi-filter, and a
voltage regulator. For a flyback converter with an output
voltage less than 7.5 V, a Schottky rectifier provides the
maximum efficiency and is therefore the best choice. The
Schottky rectifier used is the ON Semiconductor 1N5822, in
which V_R = 40 V, I_F = 3.0 A, and V_F = 0.525 V. The main
purpose of this rectifier is to take the secondary voltage and
convert it to a DC voltage. The following equations are used
in selecting the Schottky rectifier:
Maximum reverse peak voltage (calculated at high line):
ƪ ƫ VRout u Vout )
Vpeak(max)
·
N _sec
Npri
ƪ ƫ _{VRout u 5 )}
375 ·
1
13
+ 33.85 V
For discontinuous mode, the maximum forward peak
current can be approximated using:
IFout + 4 · Iout + 8 A
Diode D6 along with C7, C8, C9, L2, and C11 rectify the
transformer secondary and filter the output in order to
provide a tightly regulated DC output . Capacitor C7, C8 and
C9 are placed in parallel in order to reduce ESR. In addition,
the voltage rating of C7, C8 and C9 should be high enough
for them to withstand the voltage spikes and the output
voltage. L2 and C11 form a low pass filter that attenuates
high frequency noise.
Output filter capacitor:
Cout
+
Iout(max) · Toff(max)
Vripple(desired)
,
where Toff(max)
+
ē _min
·
1
fop
and Vripple(desired) + 40 mV
Cout
+
8
·
0.52 · ¹
100 ·
0.040
10³
+
1040
mF
Output filter choke (designed for a break frequency of
4.0 kHz):
ƪ ƫ L +
1
2·p·f·
Ǹ_C
2
, where C + C11 + 330 mF
is the corner frequency.
and
f
ƪ ƫ L +
1
2 · p · 4k · ^Ǹ330 mF
2
+ 4.8 mH
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