AND8039 View Datasheet(PDF) - ON Semiconductor
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AND8039 Datasheet PDF : 12 Pages
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AND8039/D
This results in a maximum RDS(ON) of 3.5 ohms. So a
summary of the MOSFET ratings are:
VDSS > 450 V
ID > 2.24 A
RDS(on) < 3.5 ohms
To further reduce the heat, an MTB8N50E was chosen.
Output Rectifier
The output rectifier will also be a surface mount D2PAK.
This efficiently couples the heat to the copper pad on the
PCB.
The maximum reverse voltage is:
Vr(min) + Vin(max)(N sec ńNpri) u102 V
The Peak output current is:
Iout(pk) + 2.8 Iout(max) or 11.2 A
The selected rectifier is the MURB1620CT.
Design of the Output Filter Section
As in all forward–mode converters, the output is
converted back to DC by the use of an L–C filter. A
two–stage filter is going to be used which is a much more
efficient output filter than a single stage filter. The
abbreviated schematic is shown in Figure 3.
L1
T1
D3
L2
+VOUT
N1
N2
+
C9
+
C10
+
C11
GND
Figure 3. Schematic of the Two–Stage Output Filter
Below the voltage feedback crossover frequency
(fxo ≈ about 8.0 kHz) all of the output capacitors appear to
be essentially in parallel (i.e., C9, C10 and C11). The first
stage inductor should be calculated such that it does not
enter the discontinuous–mode at light load. The
second–stage filter has its corner frequency at about 22 kHz
and provides an additional 15–20 dB of ripple attenuation
with little additional phase lag and no additional output
capacitance.
The first stage inductor should be sized to allow 20 percent
of the AC ripple current through to the capacitor. This is a
little more than is typically allowed, but the existence of the
second filter provides a more pronounced effect, thus
allowing the first filter to be smaller.
Lo [ (Vsec(min) * Vout) toff(min)ń1.4 Iout(min) (eq. 4)
where: Vsec(min) is 1.1 Vin(min)(Ns/Npri)
The resulting minimum inductance is 88 uH. Lets round
this up to 100 uH which will give us a more standard value
off–the–shelf inductor and extend the minimum current
capabilities of the supply. Now one must choose an
inductor whose core can be driven with 4+ amps on its
winding without the fear of core saturation. Coiltronics P/N
CTX100–2–52.
Next the output filter capacitor is calculated. In
forward–mode converters, the roles of the output capacitor
are transient hold–up voltage and output ripple reduction.
The output filter inductor greatly reduces the RMS ripple
current to the output capacitor(s) thus relaxing their ratings
somewhat. The transient load hold–up function is typically
shared with other filter capacitors outside of the power
supply. So the common method of calculating the value of
the output filter capacitance is by the ripple–reduction
function. Assuming a very benign load (resistor) and so that
only the ripple is considered, one then calculates:
Co + Iout(max) (1 * DC max)ńVripple
(eq. 5)
where: Vripple is the desired p–p ripple voltage on the
output.
This results in a total output capacitance of 533 uF. If one
allocates about one–third of this value to the first–stage
filter and two–thirds to the output, and rounding–up to the
next standard value, one gets C9, C10 and C11 as 220 uF,
50 VDC or Nichicon Part number EVR2E470MPA which
has a 430 mArms ripple current rating.
The second–stage filter inductance is determined by
setting its pole above the crossover frequency of the closed
feedback loop so that it will not contribute significant
additional phase shift, but will further reduce the ripple
voltage. If we set the output filter’s filter pole at no more than
25 percent of the switching frequency and at least three times
the filter pole of the first–stage filter, then the nominal corner
frequency of the second–stage filter is around 20–25 kHz.
The second–stage filter inductor can then be found by:
Lo(2) + (2pfp)2ń(C10 ) C11)
(eq. 6)
Setting the second–stage filter pole at 22 kHz, the resulting
second–stage inductor value is 0.1 uH. This can easily be
done as an air–core inductor or a spiral PCB inductor,
which is what I will do.
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