In terms of design flexibility, cost and lead time, COTS components have clear advantages; consequently, demand for COTS converters in aerospace and other high-reliability sectors is growing. By contrast, the hybrid market is shrinking: let's consider why.

Basic differences
One of the main advantages of using COTS converters in a standard package is their ready availability contrary to hybrid sealed devices, or compared with purchasing custom silicon wafers and dies. Furthermore, when taking the COTS approach, module makers will often be able to find "Form, Fit and Function"-compatible components that allow for dual sourcing on all but a very few items. Also, the cost and lead-time advantages of purchasing components that are already in mass production are significant. Nearly all DC/DC modules are now built using a high percentage of surface-mount devices, which means that the final assembly can be automated. After final assembly, modules are normally encapsulated in order to secure all of the internal parts. A hybrid power converter 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, thus 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 usually 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.

Comparing requirements
As far as size is concerned, COTS devices lend themselves well to 3D packaging techniques that can minimise the occupied board area (Figure 1). For example, building control or signals functions onto daughter boards and knitting them together in 3D 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 in the X-Y dimension are lost by the inherent inability of this construction technique to work in 3D.
In terms of weight, 15W COTS device can weigh as little as 20g, whereas a metal-packaged hybrid (Figure 2) will typically weigh 30% more.
Concerning reliability, COTS converters use automated production, which means higher reliability. Hybrid-substrate layers can be used to create resistive components and interconnects, which results in fewer soldered connections. However, many of the other components have to be placed by hand, which is less reliable than automated assembly. COTS products 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 heatsink for effective heat removal. 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 is less effective for heat removal.
As to temperature, many hybrid designers will specify their devices to work at full power at +125°C. If the waste heat can be removed, then this offers an advantage over COTS devices, which will normally be limited to a maximum ambient temperature of +85°C.
As far as environmental concerns go, 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 ensuring both long-term reliability and continued operation should the system seal ever be damaged. COTS designs intended for high-reliability environments can be fully potted. By adding the potting material using a vacuum process, these parts pass the required tests. The potting materials remain soft at low temperatures, providing mechanical elasticity. Hybrid engineers use a glass or ceramic seal to insulate each pin from the case. This is expensive, plus the glass can crack during lead preparation, insertion into test sockets, manufacturing or installation, causing reliability problems. Hybrid designs do not lend themselves to full encapsulation, and specialist tests required in production increase costs and lead-time.
Concerning EMI/RFI, metal cases used for DC/DC converters prevent radiated noise, and both input and output filtering is added to the PCB. The space within hybrids is often too restrictive to allow filtering components to mount in the most reliable way.
Table 1 shows a brief summary comparing two real-life DC/DC converters of similar power levels in COTS and hybrid formats. COTS DC/DC power has significant advantages over hybrid construction in most cases, and XP Power has released a range of COTS DC/DC converters designed with the special requirements of aerospace and other high-reliability applications.

Figure 1: Principle build-up of a COTS DC/DC converter.

Figure 2: Metal packed hybrid converter.

Table 1: Comparison of COTS and hybrid DC/DC converters.