–16–

ADDC02812DA/ADDC02815DA

REV. A

Figure 16 shows test results for the vertical measurement and

compares them against the most stringent RE102 requirement;

the horizontal measurement (30 MHz and above) was similar.

As can be seen, the emissions just meet the standard in the

18 MHz–28 MHz range. This component of the emissions is

due to common-mode currents flowing through the input power

leads. As mentioned in the section on CE102 above, the level of

common-mode current that flows is dependent on how the load

is connected. This measurement is therefore a good indication

of how well the converter will perform in the final configuration,

but the user should confirm RE102 testing in the final system.

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

requirement. 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 electro-

magnetic fields generated by antenna transmissions. The con-

verter is designed to meet this requirement. Consult factory for

more information.

Circuit Setup for EMI Test

Figure 17 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

connected to the metal ground plane in the test chamber. For

the RE102 test, a metal screen box was used to cover both the

converter 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 induc-

tance 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 resistor provide

the stabilization required.

It should be noted that the values of these stabilization compo-

nents 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 applications 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 common-

mode capacitors were added to achieve the results shown in the

EMI measurement figures described above.

RELIABILITY CONSIDERATIONS

MTBF (Mean Time Between Failure) is a commonly used

reliability concept that applies to repairable items in which

failed elements are replaced upon failure. The expression for

MTBF is

MTBF = T/r

where

T = total operating time

r = number of failures

In lieu of actual field data, MTBF can be predicted per

MIL-HDBK-217.

MTBF, Failure Rate and Probability of Failure: A proper

understanding of MTBF begins with its relationship to lambda

( ), which is the failure rate. If a constant failure rate is assumed,

then MTBF = 1/ , or

= 1/MTBF. If a power supply has an

MTBF of 1,000,000 hours, this does not mean it will last

1,000,000 hours before it fails. Instead, the MTBF describes the

failure rate. For 1,000,000 hours MTBF, the failure rate during

any hour is 1/1,000,000, or 0.0001%. Thus, a power supply

with an MTBF of 500,000 hours would have twice the failure

rate (0.0002%) of one with 1,000,000 hours.