Programmability, connectivity requirements drive digital power supply design for military use
Flexibility and programmability along with improved efficiencies and reduced footprints flavor requirements for military power supplies today from unmanned aircraft to ground vehicles to Navy warships.
Regardless of platform – air, ground, or sea – designers of power suppliers for military electronics applications find commonality driving many of their designs as the end user wants devices that can be used across multiple platforms and meet multiple power requirements regardless of the end application.
Users want solutions that “comply with different military power requirements” so they apply them to multiple applications, says Jeremy Ferrell, manager of Standard Product Engineering for VPT Inc. in Roanoke, Virginia. This puts pressure on designers to design that flexibility from the ground up.
For example, VPT’s “VXR series was designed around being very flexible and meeting many different military and commercial aviation requirements, including MIL-STD-704, MIL-STD-1275 and DO-160,” Ferrell says. (Figure 1). This device enables the user “to attach the heatsink on either side, giving more flexibility to the end user’s system design,” he adds.
Programmability and cyber
“We are seeing more programmable features being requested for all power products, including power supplies, solid-state power controllers, and motor controllers,” says John Santini, vice president of power engineering at Data Device Corp. (DDC – Bohemia, New York). “The military recognizes the total cost of ownership, including the logistics of support, so we see a strong desire for reuse and a push for additional features and specifications to allow a part to be used in multiple applications.”
With better programmability also comes the request for more connectivity.
“Connectivity has rapidly become the norm for power products,” Santini says. “While simple serial data links such as RS-232 and RS-485 have been popular for years, we are seeing faster and more complex protocols being requested, such as CAN, 1553, Firewire, Ethernet, and EtherCAT.”
However, increasing connectivity also provides cyber risks. Power supplies are no different than other electronics in terms of cyber protection in today’s world. Nearly every piece of electronics – hardware and software – is considered from a cyber threat perspective in the DoD.
The reality is that “countering cybersecurity threats is an important element in any military platform,” adds Leo Carbonneau, field application engineer at Milpower Source in Belmont, New Hampshire. “The vast majority of power conversion products are not connected to a network, and therefore do not require specific cybersecurity tools.”
That does not mean that cybersecurity is not a huge issue right now. What it does mean is that companies are “addressing TEMPEST requirements to prevent against unintended electrical emissions can be an important communications security [COMSEC] step,” Carbonneau clarifies. “A handful of customers do address TEMPEST requirements.”
[Editor’s note: TEMPEST is a National Security Agency specification and NATO certification referring to spying on information systems through leaking emanations, including unintentional radio or electrical signals, sounds, and vibrations. The NSA’s TEMPEST spec covers the methods used both to spy upon others and how to shield equipment against this kind of spying. The protection efforts are also known as emission security (EMSEC), which is a subset of communications security (COMSEC).]
Reliability is everything
When it comes to power supplies, reliability overarches all requirements. The power supply is critically important, since without power, any system is unusable.
“Military customers are seeking a truly rugged military power conversion solution,” Carbonneau says. “MIL-STD-461 EMI-qualified solutions [for addressing] conducted emissions is a primary concern. It is not uncommon for customers, before bench testing, to verify meeting all output specifications to first run an EMI [electromagnetic interference] conducted scan as this has been the most problematic.”
The military is very specific about what it needs for mission-critical systems and users “should be aware that all power conversion solutions do not address EMI adequately for military environments, which may result in the need for an external filter, which may adversely impact the power supply operation,” Carbonneau adds.
“Every power conversion solution is not equal; companies sourcing a power supply for use in a military environment should pay close attention to product qualification and design attributes evident between a commercial and a true military grade power conversion solution,” he continues.
Commercial/industrial versus military reliability
The military power supply market encompasses multiple applications, so much so that “The military power supply market tends to track the military market in general. Military power supplies usually need to operate over more stringent environmental conditions,” Santini says.
Compared to their commercial off-the-shelf (COTS) counterparts, “[Military] specifications are usually more restrictive since their mission is typically critical,” Santini. “Commercial and industrial supplies are usually optimized for a given percentage of the market, while most military power supplies have to work – and be reliable – under all expected environmental conditions.”
For example, Carbonneau observes, “In the commercial market, operating temperature range is less strenuous, mechanical requirements are minimal, and low cost is the primary objective Most commercial power supplies are designed and manufactured in China, often using overstressed questionable components. In the military market, components need to be of a much higher quality, operating temperature is much wider, mechanical requirements are strenuous, weight is important, obsolescence is to be avoided, and component traceability is important.”
“The Senate Armed Services Committee delivered a clear message to the defense community regarding the selection of power supply products for use in military systems,” Carbonneau says, pointing to the National Defense Authorization Act For Fiscal Year 2018.
“In the ‘Commercial off-the-shelf power supplies’ section, on page 199 of the document, the committee communicated two key points pertaining to the selection of power supplies for military applications,” Carbonneau notes. “First, it noted that COTS parts (e.g., products not designed to MIL standards) introduce unnecessary risk and are a primary source of failures in military systems. The committee went on to request that program managers and acquisition professionals prioritize the design and qualification of the power supply products selected for use in military applications.”
So, while commercial users do not to need worry about many of these requirements, they like the military still have to contend with reduced size, weight, and power (SWaP) restrictions, which some may argue is the biggest trend across all military electronics applications, even more so than cyber.
SWaP then and now
Historically, changes to power supply technology have been evolutionary rather than revolutionary, Santini points out. “The major disruptions have been the power switches that drive them.”
The history of power supplies show that the “earliest solid-state switches were germanium,” Santini says. “Then germanium was displaced by silicon, bipolar transistors gave way to FETs [field-effect transistors], while most recently the introduction of silicon carbide (SiC) and gallium nitride (GaN) parts is proving to be the next step. These changes have pushed switching frequencies up, and as switching frequencies go up, size and weight go down. Lower saturation losses and faster switching times reduce power losses, pushing efficiency up. Dissipating less power helps make the power supply smaller.”
Cutting-edge technology has proven to help meet the constant SWaP challenge. For example, says Santini, “[SiC] and [GaN] nitride parts are enabling switching times a fraction of the parts of just a few years ago. Output rectifier diodes are being implemented as synchronous switches in more and more designs.”
Santini points to DDC’s 60 W DC-DC Converter, “a legacy triple output 60 W design, which will soon be upgraded to a GAN-driven design.” It is currently a 60 W DC-DC Converter (Figure 2), however, “raising the frequency to 450 kHz could increase the output power rating to over 200 W.”
Moreover, “Control methodology has also played a part in this evolution,” Santini adds. “The availability of small, fast microcontrollers has enabled overall control to improve. Early uses were typically for monitoring and housekeeping, and later for cycle-by-cycle control.”
The end result: Fully digital power supplies that enable the programmability and connectivity that users are asking for. “Today, many power supplies are fully under digital control,” Santini notes. “As topologies, magnetics, and other passive components are developed to take advantage of these new parts, you can expect size and weight to continue to shrink, as efficiency continues to rise.”
This evolution has moved the industry toward small-form-factor power supplies, primarily VPX, Carbonneau says: “The VPX standard makes available compatible cards from different manufacturers, all based on a standard connector and mechanical design. This minimizes the packaging design, including the backplane, and can take advantage of off-the-shelf modules.”
Milpower Source's ’s 4065 VPX VITA 62 3U power supply (Figure 3) “is wedgelock conduction-cooled, providing six outputs at 600 W with an input voltage range of 18Vdc to 48Vdc over the full operating temperature range of -55 °C to +85 °C,” as Carbonneau describes the product.
“While a standard is being sought, this should not be confused with COTS solutions. The ability of a VPX power conversion solution to perform consistently in a military environment is critical,” Carbonneau adds.