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Facing the pressure to reduce costs and use commercial off-the-shelf (COTS) components, many designers accustomed to using traditional, sealed, MIL-SPEC, hybrid dc-dc converters are curious to understand the pros and cons of whether a hybrid or a non-hybrid power converter is best for their application. It is important to compare the two approaches to determine whether the widely promoted benefits of COTS have any downsides and whether the perceived advantages of hybridizing a circuit are what they seem.

From design flexibility, cost and lead-time perspectives, COTS components have clear advantages. The hybrid dc-dc converter market is shrinking in all applications except space electronics. At the same time, demand for COTS converters in the defense and aerospace sectors is growing. Several factors are driving this trend.

The basic electrical principles of electronic power conversion are well understood, so the differences between individual products usually relate to mechanical design or manufacturing methodology. These differences can be significant, as they are between COTS and hybrid converters.

A COTS converter (Figure 1) is built from commercial components sold to thousands of customers for varied applications. If it is to be used in a defense or aerospace market, these components and their manufacturers must be carefully evaluated for reliability and long-term availability. One of the main advantages of using COTS converters in a standard package is their ready availability compared with MIL-SPEC sealed devices or custom silicon wafers and dies. Furthermore, when taking the COTS approach, module makers will often be able to find components with the correct form, fit and function (FFF), which allows for dual sourcing on most items. The cost and lead-time advantages of purchasing components that are already in mass production are significant, particularly with respect to consistent performance and reliability. Nearly all dc-dc modules are now built using a high percentage of surface-mount devices, which means the final assembly can be mostly automated. These modules are normally encapsulated to secure all of the internal parts.

In contrast to a COTS converter, a hybrid power converter (Figure 2) is built using a substrate that has surface-mount components, inductors and bare semiconductor dies attached to it. Using bare dies instead of packaged semiconductors occupies less space on the substrate and improves the thermal performance, reducing the size of the converter. However, the manufacturing process is much more complex and difficult to automate than a COTS approach. Furthermore, hybrid devices are generally not encapsulated. This presents a thermal design challenge and leaves hybrids more susceptible to shock and vibration than their potted-module counterparts. Hybrid housings are typically welded and hermetically sealed.

Traditionally, the main considerations when specifying an off-the-shelf dc-dc converter for defense or avionics applications are:

• small size,
• low weight,
• high reliability,
• 125 °C operation,
• hermetic sealing,
• low electrical noise,
• military electrical specification, and
• military environmental specification.

These requirements can form the basis of a comparison to evaluate whether a COTS or hybrid converter is the best solution for a given application.

The critical factor is the overall size of the power solution, not just the size of the power converter. Some converter manufacturers' data sheets are misleading in this respect, omitting EMI filters or decoupling capacitors to suggest an artificially small power system. COTS devices lend themselves well to 3-D packaging techniques that can minimize occupied board area (Figure 3). For example, building control or signal functions onto daughter boards and knitting them together in 3-D can reduce the overall volume of a dc-dc converter, compared with using a single-board approach. Many of the apparent space-saving advantages of using bare dies that are confined to a plane are lost by the inherent inability of this construction technique to work in 3-D.

A 15 W COTS device can weigh as little as 20 grams, whereas a metal packaged hybrid will typically weigh 30% more. The potential weight advantages of commercially available hybrid converters are often negated by packaging them in a machined metal case typically made from Kovar, a nickel-cobalt ferrous alloy, or cold-rolled steel (CRS). Using a ceramic package produces a far lighter result but the trade-off is poor thermal conductivity. This can mean having to use larger heat sinks — defeating the original objective of size and weight reduction.

COTS components are usually built in higher volumes than their hybrid counterparts because they are designed to serve a wider range of applications. High-volume automated production leads to higher reliability. In theory, the hybrid technique has an advantage over COTS, as the substrate layer can be used to create resistive components and interconnects, resulting in fewer soldered connections. However, many of the other components within a hybrid assembly have to be placed and soldered by hand, which is precluded by the automated surface-mount assembly used by COTS module manufacturers. Of course, automated assembly is more consistent and, therefore, more reliable than hand assembly.

The main cause of failure of power converters is the heat build up. As a rule-of-thumb, it is accepted that every 10 8C increase in temperature will halve the mean time between failure (MTBF) of devices. COTS products — an example of which is XP Power's MTC series — can be designed with all of the power components attached to a cooling baseplate, which in turn, is easily mounted to an enclosure sidewall or heat sink for effective heat removal (Figure 4).

This can be hard to achieve using hybrid construction, and the majority of off-the-shelf hybrid power converters have a baseplate on the same side as the pins, which makes heat sinking challenging and less efficient. Often, the heat removal path is implemented as part of a PCB or a specially built thermal path (Figure 5). COTS products can also benefit from topologies and techniques developed for the communications industry. One example is synchronous rectification. The higher efficiency of this technique, combined with an efficient heat removal system, means the device will run cooler, improving reliability.

Many hybrid designers will specify their devices to work at full power at 125 °C, something made possible through the short thermal paths that characterize this style of construction. If the waste heat can be effectively removed, then this offers an advantage over COTS devices, which will normally be limited to a maximum ambient temperature of 85 °C.

In many systems, the entire equipment package is hermetically sealed, removing the need for sealing of individual components. However, many OEMs still require individual components to pass salt/fog or other corrosion tests as a measure of long-term reliability, or to ensure continued operation should the system seal ever be damaged.

COTS designs intended for high-reliability environments are almost always fully potted. With good manufacturing techniques, such as applying the potting material with a vacuum process, and good design, these parts can pass a MIL-STD 810 salt/fog test, ensuring long-term integrity.

Hybrid engineers use a glass or ceramic seal to insulate each pin from the case. This glass-to-metal seal, incorporated into steel or Kovar cases (known as headers), represents a significant cost in hybrid production. The seal also creates potential reliability issues as the glass can crack during lead preparation prior to soldering, insertion into test sockets, during manufacturing, or in installation. Once the hermetic seal is broken the internal parts of the hybrid are particularly vulnerable to corrosion because the bare dies and other exposed surfaces are not conformally coated or encapsulated.

Metal cases used for most dc-dc converters help prevent radiated noise but input and output filtering are required at the PCB level, generally requiring inductor and capacitor filter networks. Because these components are identical for either manufacturing approach, noise levels will ultimately be minimized through good design practice. Often, the space within a hybrid circuit limits the availability of common filtering components such as capacitors, meaning that they need to be stacked on top of each other in order to achieve the correct value. This stacking of capacitors in a non-encapsulated hybrid module is a shock and vibration weak point. COTS designs will often use new technologies like planar magnetics. In a hybrid design, availability of suitable magnetic components is limited. Furthermore, mounting such components is more difficult; they typically have flying leads that need to be fixed into place and the body of the inductor's core needs to be fixed to the case sidewall or glued to the substrate.

All power system designs will have a range of electrical specifications to meet. These include wide input voltage range, MIL-STD 461E EMI performance, frequency synchronization capabilities, remote sense connections, and removable protection features for use in “battle-mode” situations. With respect to the ability to provide these features, there are no differences between hybrid and COTS approaches. Other similarities between hybrid and COTs are reflected in Table 1.

Both COTS and hybrid dc-dc converters are used in environments that have high levels of shock and vibration. As mentioned earlier, COTS designs have the advantage of using potting material to secure all of the internals of the design making it resistant to shock and vibration. The potting compound in the COTS device is thermally conductive, which provides a good thermal path to the case. Modern potting materials remain soft at low temperatures. This provides a degree of mechanical elasticity, which is important to prevent damage in low temperature/high-vibration environments.

Hybrid designs do not lend themselves to full encapsulation (Figure 6), so manufacturers for these devices have to use adhesives to prevent bulky inductors and wiring from becoming loose and getting damaged. Many end-users perform particle impact noise detection testing, often at an external laboratory, on sealed parts to confirm that there are no loose particles within the case that could cause premature failure. Introducing specialized tests into a production process increases costs and extends the manufacturing lead time.

COTS devices have other advantages over hybrid constructions. For example, making a change to a hybrid device, such as a different output voltage or current limit, is complicated by the need to develop a new substrate. A COTS device based on a PC board, which can be quickly changed and manufactured within a few days, offers greater flexibility in this respect.

The COTS manufacturing process is inherently less complex and, therefore, more reliable from a delivery point of view than the process of manufacturing a hybrid. Surface-mount pick-and-place, reflow ovens and screen printers rarely break down, and spare parts and maintenance services are readily available. By contrast, the varied specialist capital equipment needed to assemble, seal and test hybrids can be much harder to maintain. Consequently, the temporary breakdown of a single machine can adversely affect factory lead times.

As engineers look to meet tighter cost and delivery schedules they need to consider the benefits of emerging technologies. In this respect, COTS dc-dc power has significant advantages over hybrid construction in the majority of applications. XP Power has released a range of COTS dc-dc converters designed with the special requirements of the defense and aerospace markets in mind. The rationale was to produce a product that is inherently COTS, meets the most common performance requirements within these applications, but drops non-essential performance and features. This range of converters, therefore, fills the gap between commercial and full MIL-SPEC parts.

Martin Brabham is an industry director with XP Power in Anaheim, Calif. He holds a BSc with honors in electrical engineering and has been involved in the sales and marketing of hybrid and COTS power converters to defense and other high reliability markets for 16 years in the United Kingdom, Southern Africa and the United States.