Technical Article

Powering Defense Applications With MIL-COTS Front-End Filters

May 16, 2024 by Goday Lai

Military applications are the most demanding and require special filtering and protection circuits to ensure safe operation under extreme conditions. This article examines how these challenges can be addressed.

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

 

Most power converters use topologies switching at several hundred kHz frequencies, generating a certain amount of conducted and radiated high-frequency noise. A simple PI filter solution integrated into a power converter may be sufficient for very low power levels of a few watts. Designing an EMI filter for a system drawing several tens or hundreds of watts can become a real challenge and requires a sound knowledge of HF design and proper layout. Each converter topology generates a different spectrum of high-frequency noise, varying also by the amount of power drawn by the application, and the filter must be designed for all operation conditions. Before a filter solution finally meets the specifications, it can take several trials and tests in an expensive approval lab specialized in MIL standards. This time-consuming process adds a significant amount of money to the overall design cost of a new project, which must be amortized over the usually lower quantities in MIL applications.

Space and weight in military equipment are often limited as systems are packed with complex electronics and sensors. A highly sensitive measurement or communication device may work near a power converter, and no interference is allowed between different systems. Therefore, EMI specifications for MIL applications are more stringent than in other markets, making the filter design even more complex.

Another challenge is the extreme environmental conditions, including high mechanical stress caused by shock and vibration. The magnetics and capacitors needed for EMI filters are bulky components, and special means are necessary to avoid damage to components or solder connections caused by high shock and vibration.

 

Figure 1. Regulated 28 V/80 W supply for a MIL touch screen. Image used courtesy of Bodo’s Power Systems [PDF]

 

Saving Engineering Ressources

Why spend a lot of money and weeks of valuable engineering resources when off-the-shelf filter solutions are available, meet all these requirements, and are tailored to a wide range of DC/DC converters? The MCF-028 front-end filter family from P-Duke was designed to cover power ranges from 45 W to 250 W in typical 28 V defense applications. Their filter behavior is compliant with hundreds of DC/DC converters and by following the recommended PCB layouts, it is easy to design a system that passes EMI tests the first time without the need for further redesigns.

An example is the 80 W power solution for a touch screen, which requires a regulated 28 V supply voltage to be generated from the unregulated voltage of MIL vehicles. It uses the MCF-028010-001 filter rated for up to 10 A and a 100 W converter from the HAE100 family (Figure 1).

P-Duke lists all necessary external components and a recommended PCB layout in an application note. Following this guide, a design passing all tests can be made even without a deep knowledge of HF and EMI filter technology. Figure 2 shows the EMI plot of this solution.

 

Figure 2. MIL-STD 461G CE102 Conducted Emission MCF 028010 001 combined with HAE100 24S28W at Vin (nom) and full load. Image used courtesy of Bodo’s Power Systems [PDF]

 

Another challenge in MIL applications is the wide range of input voltages, including heavy transients and spikes. The following table summarizes different standards.

 

Table 1. MIL Standards
Standard Un (VDC) Permanent Operating Input Range (VDC) Transient Spike
MIL-STD-1275E 28 23-33

40 V/500 ms

100 V/50 ms 

±250 V/70 μs
MIL-STD-704F 28 22-29 50 V/50 ms NA
RTCA DO-160G Cat.A/Z 28 20.5-32.2 80 V/100 ms ±600 V/10 μs

 

According to MIL-STD-1275, the 28 V nominal voltage can drop to 16 V during an engine’s cranking and even 12 V for one second during a system’s initial engagement. On the other hand, the system must also be able to handle these high-energy transients of up to 100 V/50 ms or 80 V/100 ms and spikes up to ±250 V/70 µs or ±600 V/10 µs.

Technically, it would be possible to design a DC/DC converter working over this wide input range of 12 – 100 V, but the solution would be bigger and less efficient than a significantly smaller filter and converter combination tailored to these specific needs.

The converter must work at low input voltages. All converters P-Duke offers for these applications can handle input voltages down to 9 V and are designed to work with input voltages up to 36 V continuous and 50 V for one second.

An active circuit already integrated into P-Duke’s MCF-028 front ends clamps the 100 V/50 ms and 80 V/100 ms transients to only 40 or 46 V are no problem for the P-Duke converters, which have an input transient capability of 50 V for one second. Figure 3 shows the input transients applied to an MCF-028010 and the output voltage clamped at 46 V.

 

Image used courtesy of Bodo’s Power Systems [PDF]

Figure 3. Transient voltages at the input of an MCF-0280100 module (top) and clamped output voltage (down). Image used courtesy of Bodo’s Power Systems [PDF]

 

The very short ±250 V/70 µs or ±600 V/10 µs spikes are suppressed by additional components inside the MCF028, and in combination with a P-Duke converter, all these transient specs are met. The flexibility of P-Duke’s power component offering is shown when looking at some more application examples. Figure 4 shows the block diagram of a complete onboard computer system, including the previously mentioned touch screen and a typical computer using ruggedized CPU, memory, and interface components.

 

Figure 4. Power solution for an onboard computer system with touch screen. Image used courtesy of Bodo’s Power Systems [PDF]

 

When more power is needed, the MCF-028015 can handle currents up to 15 A and support downstream converters for loads up to 250 W. A good example is a vehicle-mounted SAT com system (Figure 5). When the vehicle drives in rough terrain, powerful motors are needed to continuously reposition the SAT dish.

 

Figure 5. Power solution for a vehicle-mounted mobile SAT com system. Image used courtesy of Bodo’s Power Systems [PDF]


MIL equipment not only has to work under very harsh environmental conditions, but it must be fault tolerant to many operating or installation errors. A reverse polarity connection to a battery can damage the complete system. Switching on many loads simultaneously can generate high inrush currents and trip system-relevant fuses. P-Duke considered these handling errors when designing the MCF Frontends and has included an active inrush current limiter and reverse polarity protection as well as under and overvoltage, short current, and over-temperature protection. A remote on/off function allows switching the outputs of each module on and off, enabling a controlled ramp-up or ramp-down of converters in a larger array.

Figure 6 describes a complex mobile network link solution for the communication between UAVs, ground vehicles, ground and portable stations, and a central HUB. Different combinations of P-Duke’s MCF028 front ends and DC/DC converter families made designs for the various power needs quick and easy. Even when switching on various systems connected to the same source in a ground node group, the inrush current limiting guarantees that no fuse will trip. The filters’ EMI performance ensures that noise from a converter will not be coupled into the communication network, disturbing mission-critical communication.

 

Figure 6. P-Duke’s extensive range of filters and DC/DC converters enabled quick and easy designs for various power needs. Image used courtesy of Bodo’s Power Systems [PDF]

 

P-Duke’s product range for the MIL market includes modules with single and dual output voltages and voltages going beyond the standard offering of other suppliers. For the redesign of a surveillance system, a customer wanted to use PTZ cameras with high-power LEDs for long-distance night sight, each requiring almost 50 W of power. Installation in the field must be quick and easy, with one cable carrying power and signal. A power over Ethernet solution was needed but the typical solutions available at that time in the MIL market did not allow bringing 50 W to each camera over the thin cable. By using the PoE++ (IEEE 802.3bt, 50 – 57V supply voltage) specification and a standard P-Duke 100 W module with its 48 V nominal output trimmed up by 10% to 52.8 V, the challenge was solved.

The surveillance control system needed a regulated 28 V/40 W supply, which was easy to achieve with a RED60 module as the 24 V nominal output can be trimmed up by +20%, unusual for a standard DC/DC converter module. One 250 W MCF028 can power up to 4 cameras, and there is still enough power for the surveillance system (Figure 7).

 

Figure 7. PoE camera solution using the MCF frontend filter and standard converters with the outputs trimmed up to achieve the 52.8 V and 28 V voltage requirements. Image used courtesy of Bodo’s Power Systems [PDF]

 

The harsh environmental conditions of MIL applications can become a major challenge when designing power solutions. The systems must work reliably over wide temperature ranges and withstand high shock and vibration stress.

Engineers designing their own power converters and filters face the challenge that standard magnetics and capacitors are large and bulky and therefore sensitive to shock and vibration. P-Duke modules use special flat and lightweight components and together with silicone potting mechanical stress is reduced to a minimum. Therefore, all modules meet MIL-STD-810F for shock and vibration.

 

Table 2
  Portable system Base station
Frontend filter MCF-028005-001 19.7 g MCF-028010-001 64.0 g
12 V supply RCD20-24S12W 16.0 g HAE150 105.0 g
5 V supply RCD20-24S05W 16.0 g RCD20-24S05W 16.0 g
  Total weight 51.7 g Total weight 185.0 g

 

Another big advantage of using a modular approach and these small components is a significant size and weight reduction compared to any discrete design. The complete weight of all modules used in the example in Figure 5 is only 52 g for the portable and 185 g for the powerful base station version.

Military electronics are quite often mounted into sealed housings to protect them against water, humidity, and aggressive atmospheres. Conduction cooling can be realized by attaching the baseplate or top cover of the modules to the hermetically sealed chassis of the system (Fiigure 8). This construction offers ideal heat management options, can withstand very high shock vibration stress, and does not need unreliable fans.

 

Figure 8. All P-Duke modules come in a robust silicone potted housing and cooling can be done either through the baseplate (>100 W) or on top of the module (10 – 60 W). Image used courtesy of Bodo’s Power Systems [PDF]

 

This robust construction, designs with the lowest losses and low heat dissipation, the selection of highly reliable components in combination with a production process meeting high-quality standards, and 16 hours burn-in of every product ensure the components will work reliably in these demanding MIL applications. This table shows the MTBF values (MIL-HDBK-217F, full load) for the MCF028 filter series:

 

Table 3. MTBP Values for MCF028 Filters
MCF-028005 2.718 x 106 hrs
MCF-028008 1.146 x 106 hrs
MCF-028010 1.307 x 106 hrs
MCF-028015 6.095 x 105 hrs

 

The DC/DC converters shown in the examples achieve similar MTBF values, and P-Duke has a proven record of meeting the extremely demanding reliability requirements in defense and railway applications working under extreme environmental conditions. When using this modular approach, a design can be completed in a few days or weeks and does not require months of valuable engineering resources.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.