ADDC02808PB Analog Devices, Inc., ADDC02808PB Datasheet - Page 17

• Manufacturer Part Number: ADDC02808PB
• Description: 28 V, 200 W Pulsed Dc/dc Converter With Integral Emi Filter
• Manufacturer: Analog Devices, Inc.

REV. A
RS101: This requirement is specialized and is intended to check
for sensitivity to low frequency magnetic fields in the 30 Hz to
50 kHz range. The converter is designed to meet this require-
ment. Consult factory for more information.
RS103: This test calls for correct operation during and after the
unit under test is subjected to radiated electric fields in the 10 kHz
to 40 GHz range. The intent is to simulate electromagnetic
fields generated by antenna transmissions. The converter is
designed to meet this requirement. Consult factory for more
information.
Circuit Setup for EMI Test
Figure 15 shows a schematic of the test setup used for the EMI
measurements discussed above. The output of the converter is
connected to a resistive load designed to draw full power. There
is a 0.1 F capacitor placed across this resistor that typifies by-
pass capacitance normally used in this application. At the input
of the converter there are two differential capacitors (the larger
one having a series resistance) and two small common-mode
capacitors connected to case ground. The case itself was con-
nected to the metal ground plane in the test chamber. For the
RE102 test, a metal screen box was used to cover both the con-
verter and its load (but not the two meters of input power lead
cables). This box was also electrically connected to the metal
ground plane.
With regard to the components added to the input power lines,
the 100 F capacitor with its 1
achieve system stability when the unit is powered through the
LISNs, as the MIL-STD-461D standard requires. These
LISNs have a series inductance of 50 H at low frequencies,
giving a total differential inductance of 100 H. As explained
earlier in the System Instability section, such a large series
source inductance will cause an instability as it interacts with the
converter’s negative incremental input resistance unless some
corrective action is taken. The 100 F capacitor and 1
tor provide the stabilization required.
It should be noted that the values of these stabilization components
are appropriate for a single converter load. If the system makes
use of several converters, the values of the components will need
to be changed slightly, but not such that they are repeated for
every converter. It should also be noted that most system appli-
cations will not have a source inductance as large as the 100 H
built into the LISNs. For those systems, a much smaller input
capacitor could be used.
The 2 F differential-mode capacitor and the two 82 nF com-
mon-mode capacitors were added to achieve the results shown
in the EMI measurement figures described above.
ADDC02808PB EMI Performance
The EMI performance of the ADDC02808PB power converter
will be different from the ADDC02805SA baseline previously
discussed for several reasons:
1. Its maximum power is 200 W, or twice that of the
2. Its differential input filter inductors are smaller in value by a
ADDC02805SA converter.
factor of two compared to those in the ADDC02805SA
converter to accommodate input stability at the higher power
level.
series resistance is required to
resis-
–17–
3. A repetitively pulsed load will cause large input currents at
The result of Items 1 and 2 is that the ADDC02808PB con-
verter will have higher conducted and radiated emissions in the
1/2 MHz to 10 MHz range. The emissions in this range are
dominated by differential currents. These currents are propor-
tional to power, so we would expect a factor of two increase in
emissions due to the 200 W operating level. It does not matter
that the average power of this pulsed unit is 100 W or lower.
MIL-STD-462D calls for measurements to be made with peak
detectors that will determine the emissions during the 200 W
pulse, and not average them in any way with the lower power
part of the cycle.
In addition, the differential EMI filter in the ADDC02808PB
converter is less effective at attenuating the ripple currents than
is the filter in the ADDC02805SA converter due to smaller
value inductors. Figure 38 shows the transfer functions of these
two filters in the frequency range of interest.
Combining the factor of two and the reduced filter attenuation,
Figure 39 shows the ratio, in dB, by which the emissions of
Figures 12 and 14 should be increased to estimate the emissions
of the ADDC02808PB converter in this frequency range. From
this curve the 1/2 MHz component should increase by 25 dB,
and the 1 MHz and higher components should increase by 22 dB.
For both conducted and radiated tests, this increase would
require some additional differential filtering to meet the most
stringent MIL-STD-461D levels shown in the figures. This
could be done, for example, by increasing the 2 F ceramic (low
parasitic inductance) capacitor placed across the input of the
converter in Figure 15 to 30 F or, a small 0.5 H, 16 A induc-
tor could be placed in series between the top of the 2 F capaci-
tor and the +V
Figure 38. Comparison of Transfer Functions for the Input
EMI Filters in ADDC02805SA and ADDC02808PB
the fundamental frequency (and harmonics) of the pulse
waveform.
0.001
10
10
10
10
10
0.01
100
0.1
10
–4
–5
–6
–7
–8
1
1 • 10
4
IN
pin. Figures 40 and 41 show the ratios, in
1 • 10
FREQUENCY – Hz
5
ADDC02805SA
ADDC02808PB
1 • 10
6
ADDC02808PB
1 • 10
7