Reliability and performance are absolute requirements for electronic systems in defence applications. Engineers in this field have traditionally relied on highly customised designs to meet these demands, which are assembled using military-grade discrete components, necessitating a lengthy and expensive process to produce custom power supplies.
However, governments in Europe and the United States are encouraging military users to prioritise cost-effectiveness in procurement and to use commercial off-the-shelf (COTS) products whenever possible.
For DC/DC converters used in military or avionics applications, the COTS path offers numerous advantages to end users, primarily because it provides standardised, ready-to-use products suitable for a wide range of applications. Using mass-produced COTS devices is cost-effective and provides versatile components that can be configured as either distributed or centralised power supplies. They are suitable for various applications and are certified, significantly reducing development time compared to customised methods.
However, simply using COTS converters for avionics and military power supplies requires careful analysis, as there are specific obstacles and requirements that must be addressed.
COTS issues
So, let's take a look at the special requirements for high-reliability military commercial off-the-shelf (COTS) products. These issues can be broadly categorised into four main areas: input and efficiency requirements; electromagnetic compatibility; environmental performance; and specific requirements for military and avionics systems. These issues can be addressed by adopting a modular design approach, such as the one used by GAIA Converter. This approach allows COTS technologies to be used in areas where they perform optimally. Additionally, it enables the use of other components and subsystems to meet the specific requirements of military-grade systems.
Input requirements
Military electronic equipment may have a variety of power input sources during operation, as batteries must be used as a backup power source if the onboard generator cannot provide sufficient power. The resulting system switching may cause significant fluctuations in input voltage, accompanied by brief transients. If the converter is not designed to handle input voltages significantly higher or lower than the nominal value, these changes can result in reduced efficiency.
Designing a power subsystem capable of handling voltage variations, such as 9V to 80V, while still providing stable or constant conversion efficiency to meet military requirements, is a challenge. Many COTS converters are designed to operate at maximum efficiency at the nominal voltage. These products sometimes employ variable-frequency topologies to support zero-voltage switching within the input voltage range.
While variable-frequency switching makes noise harder to predict and thus harder to reduce, the implementation of zero-voltage switching (ZVS) can significantly reduce noise levels, often lowering its interference with sensitive systems such as radar and radio receivers. However, it is worth noting that variable-frequency switching may also introduce stability issues, especially when the load is affected by changes such as pulse width modulation (PWM) or pulsed loads.
Additionally, the effectiveness of eliminating conducted electromagnetic interference (EMI) depends on whether the DC/DC converter has synchronisation functionality, which allows for fine-tuning of noise frequencies to avoid interfering with the bandwidth of radar or radio systems. This functionality is common in many GAIA Converter products. Regarding high-frequency soft switching, it should be clarified that while it can improve efficiency, it does not completely eliminate the need to manage efficiency across the entire input voltage range.
Transient and EMI protection
The wide input range eliminates the need for a front-end voltage transient limiter to manage transients. Otherwise, compared to the inherent transient conversion topology, the front-end voltage transient limiter would reduce the overall conversion efficiency. In military applications, scenarios such as engine start-up or transitions between different power sources may generate significant transients. According to the Mil-Std-1275 standard, the start-up voltage is 12V, and the GAIA converter MGDD series can easily support this voltage because they begin operating at a 9V threshold, providing a significant margin. Additionally, at the upper end of the input range, since the MGDD series can operate within a 9V to 80V input range, it can easily handle the 80V/100ms surge specified by the DO-160 standard.
A related issue is that under-voltage and voltage drops may occur during energy switching or due to intermittent faults. Interruptions may last from 10 milliseconds to 1 second. Military standards test the ability to withstand these power outages. COTS power supplies designed for the mainstream market are unlikely to have the required resilience. However, this can be incorporated into the power supply design by combining appropriate power conversion topologies and additional modules. For example, a common technique to increase hold time is to place a large capacitor in the circuit to provide sufficient charge to temporarily maintain power to the rest of the system. However, simple COTS designs using this method may generate large surge currents when power is restored. GAIA has pioneered a technique that applies a boost voltage across the capacitor. This can provide greater energy storage capacity for a given capacitor. This can maximise hold-up time while minimising surge current issues.
Modular design techniques (such as those employed in the GAIA power architecture) provide mechanisms to handle larger transients. These techniques can combine specialised military circuit designs with advanced technologies available in COTS implementations. Organisations worldwide have established numerous military standards that strictly control transients and EMI.
For example, the British DefStan-461 standard specifies that ground equipment with a nominal power supply voltage of 28V must be able to withstand surges of up to 202V (28V nominal voltage plus 176V surge voltage) for a duration of 300ms. This poses a challenge for any circuit topology. However, using dedicated front-end filters can clamp this voltage to an acceptable range, handle this energy, and protect the rest of the system.
The core circuitry employs a defence-focused design combined with the use of modular front-end filters to meet the requirements of relevant standards. Most radiation typically originates from input cables or load circuits, so careful system design is critical, taking into account circuit topology, grounding strategies, and shielding and routing.
By incorporating high-frequency fixed switches in the core design, power converter designers can ensure predictable frequencies, simplifying the task of filtering out low-frequency conducted emissions and facilitating compliance with stringent emission standards.
GAIA's approach involves enclosing the circuit on five sides with a metal housing. System designers can then choose to complete the shielding by using the PCB ground shield beneath the converter on the sixth side.
Environmental performance
Enclosure design is equally critical to the environmental performance of military power converters. Many commercial off-the-shelf (COTS) power supplies can be designed with an open-frame design, but due to shock and vibration stresses, these power supplies often perform poorly in defence applications. Another technology not widely used in COTS designs is the use of potting technology to encapsulate individual components within the enclosure. To ensure high reliability in vibration-prone environments, power converters designed for the defence sector widely utilise potting technology. If this potting material is selected, it offers both thermal conductivity and vibration resistance, while also enhancing internal heat conduction. For GAIA modules, this technology simplifies the design by allowing the thermal resistance between the module's housing and the environment to be specified for the entire brick within the modular power supply.
Keep pace with the times
Like environmental factors, military components also require long life and long-term reliability, but mainstream COTS products do not adequately meet these requirements. While component reliability is critical, a related issue is designing for long life. The production life of mainstream commercial off-the-shelf (COTS) products is typically much shorter than the life of defence contracts and their equipment. As a supplier to the defence sector, GAIA has maintained continuity even in its first DC/DC converter (launched in 1993).
While mainstream COTS hardware may have limitations in defence applications, these limitations can be mitigated through modular design approaches, such as those adopted by GAIA-Converter, which also facilitate the integration of emerging technologies. Wide-bandgap semiconductors like silicon carbide (SiC) and gallium nitride (GaN) are highly favoured in sectors like automotive and renewable energy due to their low switching losses and high conductivity. Especially SiC, which offers high stability and reliable operation at high temperatures. These characteristics enable higher operating frequencies, leading to high-density, high-power modules.
However, the GAIA Converter has not yet adopted SiC or GaN technology in its products designed for relatively low voltages (nominal 28V) and low power (less than 500W), as these wide-bandgap semiconductors currently cannot compete with the best MOSFETs for such applications. Nevertheless, it is anticipated that as these technologies continue to gain traction in high-voltage automotive applications, they will eventually become more prevalent in low-voltage and low-power domains, which aligns with GAIA's strategic direction. GAIA Converter is closely monitoring future design trends.
Conclusion
As we have seen, the trend toward using COTS components and subsystems in the defence sector faces challenges. However, by partnering with suppliers who understand the limitations of these products and have a basic understanding of military project requirements, we can leverage the learning curve that comes with technological advances and mass production. This combination brings innovation and forward thinking to applications that require high reliability and long service life. Suppliers like GAIA-CONVERTER play a key role in shaping the future of military and avionics power systems by applying these principles.
Figure 1: High reliability 40W multi-standard low noise power supply with 100ms hold-up capability.
Figure 2: Printed circuit board diagram of a 40W power supply.
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