ADDC02808PB

REV. A

–17–

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

µF 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

µF capacitor with its 1 Ω series resistance is required to

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

µH at low frequencies,

giving a total differential inductance of 100

µH. 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

µF capacitor and 1 Ω resis-

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

µH

built into the LISNs. For those systems, a much smaller input

capacitor could be used.

The 2

µF 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

ADDC02805SA converter.

2. Its differential input filter inductors are smaller in value by a

factor of two compared to those in the ADDC02805SA

converter to accommodate input stability at the higher power

level.

3. A repetitively pulsed load will cause large input currents at

the fundamental frequency (and harmonics) of the pulse

waveform.

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

µF ceramic (low

parasitic inductance) capacitor placed across the input of the

converter in Figure 15 to 30

µF or, a small 0.5 µH, 16 A induc-

tor could be placed in series between the top of the 2

µF capaci-

FREQUENCY – Hz

100

10

1

0.1

0.01

0.001

ADDC02808PB

ADDC02805SA

Figure 38. Comparison of Transfer Functions for the Input

EMI Filters in ADDC02805SA and ADDC02808PB