Aerospace & Defense Technology - October 2024 - 13

Military Vehicles
to minimize power dissipation. These
capacitors are generally built in the picofarads
(PF) range to add another layer of
protection against power losses.
Q1
Cr
DC Input
Q2
Lm
D2
Lr
T
D1
Co
DC Output
High-Reliability MLCCs
To comply with MIL standards, capacitors
are tested under harsh conditions
well beyond their normal operational
ranges and monitored for current leakage.
Testing reveals any defects or failure
points to better understand the limits of
the component.
Dielectric formulations and chip
Resonant Tank
Figure 3: Simplified LLC resonant DC/DC converter. (Image: Knowles Corporation)
and zero current switching (ZCS) across
the operating range, reduces EMI, and
accommodates higher switching frequencies
all in a smaller footprint.
Combined with a switching network,
a resonant inverter serves as the foundation
for a resonant converter. The
inverter converts DC input voltage into
a square wave that can be applied to
the resonant circuit. The resonant tank,
shown in Figure 3, includes a resonant
capacitor Cr, resonant inductor Lr, and
the transformer's magnetizing inductance
Lm connected in series.
LLC circuits filter out higher-order harmonics
by absorbing maximum power at
the resonant frequency of the square wave
and releasing a sinusoidal voltage. The AC
waveform can be scaled up or down by the
transformer, rectified, and filtered to produce
the converted DC output voltage.
Considering the key role of resonant
capacitors in frequency-selecting resonant
circuits, stable capacitance is paramount.
Variability causes the converter
to operate less efficiently. Capacitors
with high temperature stability offer
more stable capacitance overall.
These components tend to heat up
when AC ripple passes through, so power
dissipates inside the component in the
form of heat, causing energy waste. This
mechanism, known as self-heating,
impacts capacitance, converter efficiency,
and overall component reliability.
When selecting resonant capacitors
for converters, losses can be understood
Aerospace & Defense Technology, October 2024
in terms of equivalent series resistance
(ESR). ESR is a measurement that encompasses
all of those non-ideal electrical
resistances in series with a capacitor.
Quality factor, or Q factor, is another
valuable figure for comparing MLCCs.
Similar to ESR, Q factor is frequency
dependent.
Q = 1
Xc
Rs
=
2πfC
= DF.Xc
Where Rs is ESR in ohms, DF is dissipation
factor, and Xc is capacitance reactance
in ohms.
ESR also offers a means of determining
how much ripple current is converted
into heat. Since temperature can have
such an adverse impact on performance,
it's important to understand power dissipation
and properly manage it.
P=I2R
(Where P is power dissipation in watts, I
is root mean square (RMS) current in amps,
and R is ESR in ohms.)
ESR and Q factor measurements can be
challenging to directly compare between
different manufacturers, but it's worth
taking the time to understand what's in
front of you, so you can make informed
component selections. MLCCs designed
with high Q material will have a low
dielectric constant. Low dielectric constant
values are preferable in high-frequency
and high-power applications
mobilityengineeringtech.com
DF
1
capacitors undergo burn-in or voltage
conditioning with set specifications over
a predetermined period. For example,
the test voltage might be twice the working
voltage rating of the device. MILC-55681,
MIL-C-123, and MIL-C-49467
offer burn-in specifications for MLCCs.
In addition to burn-in, MIL-C-55681
involves additional performance tests,
including dielectric withstanding voltage
and insulation resistance (IR) at elevated
temperature, voltage-temperature
limits, thermal shock, and solderability.
Some manufacturers may require
additional physical examinations like
Destructive Physical Analysis (DPA).
As military and defense systems continue
to advance, pushing the boundaries
of voltage, temperature, and miniaturization,
traditional components
are challenged to meet these rigorous
demands. Military specifications like
MIL-PRF-55681, MIL-PRF-49467, and
MIL-PRF-123 set the bar high to ensure
capacitors achieve the performance and
durability requirements needed for military
vehicle electrification.
Overall, capacitors are tasked with
diverse and demanding roles within power
systems. MLCCs stand out for their superior
size, weight, and performance across the
board. Ultimately, high reliability is imperative.
Rigorous testing and screening based
on MIL-SPECS ensure that these components
can withstand the harshest environments
and provide long-lasting service in
electrified mission-critical vehicles.
This article was written by Peter Matthews,
Product Director, Knowles Precision
Devices (Itasca, IL). For more information,
visit: https://www.knowles.
com/markets/aerospace-defense
13
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Aerospace & Defense Technology - October 2024

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