How High-Reliability Capacitors Support Electrical Power in Military Vehicles

October 1, 2024

As the U.S. military embraces vehicle electrification, high-reliability components are rising to the occasion to support their advanced electrical power systems. In recent years, electronic device designers have started using wide band-gap (WBG) materials like silicon carbide (SiC) and gallium nitride (GaN) to develop the semiconductors required for military device power supplies. These materials can operate at much higher voltages, perform switching at higher frequencies, and feature better thermal characteristics. Compared to silicon, SiC-based semiconductors provide superior performance. The growing availability of these materials, in terms of access and cost, continues to encourage electrification. With the ever-present pressure of size, weight, and power (SWaP) optimization in military applications, and a desire to keep up with the pace of innovation, there’s a need for capacitors that can deliver higher power efficiency, switching frequency, and temperature resistance under harsh conditions.

In this article, we explore the rise of electrification in military applications, the enabling role of high-reliability power supplies and converters, and how capacitors are meeting the rigorous requirements of modern electrified land systems.

Benefits of Electrical Power in Military Vehicles

The shift towards electrification presents numerous transformational advantages in military vehicles including enhanced tactical capabilities, energy efficiency, and futureproofing.

Electric vehicles operate quietly compared to traditional combustion engine vehicles, so they’re advantageous in tactical and stealth operations. With better torque and acceleration, electric vehicles offer enhanced mobility across a wide variety of environments and terrains too.

Critically, modern military technologies like communications gear, sensors, and weaponry require a significant amount of electric power. Electric vehicles are better equipped to get power from the source to the system that needs it in a compatible format.

The U.S. Army released its first-ever climate strategy in 2022. Electrification aligns with these broader efforts and reduces reliance on fossil fuels, which are challenging to source and supply in combat zones.

Compared to combustion engines, electric vehicles have fewer moving parts, which can improve the operational lifespan of equipment. Electric powertrains are also less susceptible to mechanical failure under harsh conditions, so they require less maintenance overall.

Futureproofing

As new technologies emerge, military vehicles must adapt to support them. Modern warfare technologies have higher power demands and increasingly benefit from integration with autonomous systems. Electrically powered vehicles can provide that support in the field with on-site power generation. Logistically, there are design opportunities to simplify the supply chain to reduce spare parts and fuel types required in remote field environments.

Power supplies and power converters supply electrical power to a load in the format it’s designed to receive (e.g., voltage, current, frequency). Rectifiers, inverters, and DC/DC converters are common across industries, including aerospace and defense. While they function the same way, power supplies and converters in military applications are subject to functionality specifications determined by MIL standards. For example, MIL-PRF-GCS600, MIL-STD-1275, MIL-STD-1399, MIL-STD-461, MIL-STD-704, and MIL-STD-810 detail key standards for factors like voltage level and electromagnetic interference (EMI) tolerance to maintain consistent, high quality and support size, weight, and power optimization.

Electric Architecture in Military Vehicles

The U.S. Army recently investigated a 600V electrical supply platform for all land systems to replace its current 28V system. This change would increase available power and reduce cabling losses. At a given power level, losses decrease at higher voltages. This is observable in commercial electric vehicles as well. Many manufacturers opted for 800V electrical supplies so they could leverage thinner, lighter cables.

The example of vehicle electrification architecture (VEA), shown in Figure 1, includes a battery pack, electric motor, power electronics, a vehicle control unit (VCU), and a charging system. Here, the integrated starter/generator controller (ISGC) converts AC power from the onboard generator to the 600V high-voltage direct current (HVDC) bus voltage. The import/export power converter (IEPC) covers bidirectional conversion between 600V DC and external AC source of power. The system also includes a high-voltage power controller (HVPC), bidirectional converter (BDC), and low-voltage power controller (LVPC) for DC/DC conversions.

In the power architecture of an F-35 Joint Strike Fighter, shown in Figure 2, the power architecture includes an inverter/converter/controlled (ICC), electrical distribution unit (EDU), electro-hydrostatic actuation system (EHAS), environmental control system (ECS), avionics supplies, and legacy AC supplies. While these architectures represent two different applications in the military domain, they share operations (e.g., AC/DC, DC/AC, DC/DC conversions) and characteristics, like high voltage. Efficient power conversion is the foundation for a high-efficiency system. By reducing losses in a DC/DC converter, the converter, and by extension, the vehicle, benefits from better energy efficiency and less heat generation.

Advancing Power Efficiency at the Circuit Level

For the sake of example, we’ll focus on resonant DC/DC converters. Resonant converters are switching-based and incorporate a resonant tank to help manage input-to-output power flow. They’re known for smoothing waveforms, improving power factor, and reducing switching losses exacerbated by high-frequency power switches like MOSFETs and IGBTs. These capabilities make resonant converters particularly common in high-voltage applications.

The LLC circuit is a common resonant converter implementation because it achieves zero voltage switching (ZVS) 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 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.

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 to minimize power dissipation. These capacitors are generally built in the picofarads (PF) range to add another layer of protection against power losses.

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 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. MIL-C-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 here  .

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