ADDC02808PB
30
The peak in the radiated emissions in the 20 MHz–30 MHz
range of Figure 14 is dominated by common-mode noise. This
common-mode noise emission is changed only slightly between
the ADDC02808PB and the ADDC02805SA converters since it
does not depend on the power level or the differential input
filter. The turns ratio on the transformer has been changed, so
we expect the common-mode emissions might be 2–4 times
larger. This increase could be countered by increasing the 82 nF
common-mode capacitors of Figure 15 correspondingly. Again,
this solution is a suggestion; it has not been tested.
20
10
0
–10
–20
Finally, the pulsed nature of the load means there will be a
substantial ripple in the input current at the fundamental pulse
rate and its harmonics. This ripple can be calculated once the
power is known as a function of time by dividing by the input
voltage. For instance, if the load switches between zero and
200 W (260 W at the input) at 1 kHz with a duty ratio of 50%,
the current drawn by the converter will have a 9.3 A on, 0 A off,
50% duty ratio input current waveform (260 W/28 V = 9.3 A).
This waveform has an average of 4.65 A and a square wave of
plus and minus 4.65 A around this average. This square wave of
current has a fundamental component as well as odd harmonics
(3rd, 5th, 7th, . . .). The peak of the fundamental component is
(4/π) 4.65. The rms value of this component is .707 times the
peak, or 4.2 A.
•
•
•
5
6
7
1 10
1 10
FREQUENCY – Hz
1 10
Figure 39. Change in ADDC02808PB Differential Emis-
sions vs. ADDC02805SA Emissions with the Same Test
Setup
dB, by which the differential emissions would change if either of
these approaches were followed. Notice that the inductor solu-
tion provides substantial attenuation in the 1 MHz and higher
frequency range, while the larger capacitor solution has a more
uniform effect. These proposed solutions are suggestions; they
have not been tested.
With the test setup in Figure 15, given the impedances of the
LISNs and the 100 µF capacitor with its 1 Ω series resistance, a
3.6 V rms waveform would result from this fundamental
component of the input current. The MIL-STD-461D limit
shown in Figure 11 calls for approximately 100 mV at the 1 kHz
frequency. If this limit is to be met, substantial filtering at the
lower frequencies will have to be added to the system.
30
20
10
0
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
–10
–20
•
•
•
5
6
7
1 10
1 10
FREQUENCY – Hz
1 10
MTBF = T/r
where
Figure 40. Change in ADDC02808PB Differential Emissions
vs. the ADDC02805SA Emissions with External 30 µF
Capacitor
T = total operating time
r = number of failures
In lieu of actual field data, MTBF can be predicted per
MIL-HDBK-217.
30
MTBF, Failure Rate, and Probability of Failure: A proper
understanding of MTBF begins with its relationship to lambda
(λ), which is the failure rate. If a constant failure rate is assumed,
then MTBF = 1/λ, or λ = 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.
20
10
0
–10
–20
What users should be interested in is the probability of a power
supply not failing prior to some time t. Given the assumption of
a constant failure rate, this probability is defined as
•
•
•
5
6
7
1 10
1 10
FREQUENCY – Hz
1 10
Figure 41. Change in ADDC02808PB Differential Emissions
vs. the ADDC02805SA Emissions with External 0.5 µH,
16 A Inductor
R(t) = e–λt
–18–
REV. A
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