How to Choose SMT DC-DC Converters


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Until recently, most DC-DC converters were only available as through-hole devices. As more become available as surface mount devices, users need to know how to choose the most appropriate one.

By Dorrel Vernon and Peter Brune

Surface mount device (SMD) technology has many advantages over through-hole technology. Most importantly, it avoids the necessity for drilled holes and allows components to be attached to both sides of the printed circuit board (PCB) for maximum component density in the board layout.

Other important advantages of SMD technology include speed of assembly, greater reliability of components, elimination of pin trimming and the availability of large-scale soldering methods. Despite these advantages, the exclusive use of SMT at the PCB assembly level has been stymied by limited accessibility to components that are not available as SMDs.

Until recently, 10 to 30 W DC-DC converter power supplies were not available in surface mount packages. For the most part, DC-DC converters are through-hole devices that require different and more costly assembly methods than conventional SMD reflow. They are commonly used in telecommunication switches, network devices, mainframe computers and minicomputers. Lack of availability of surface mountable DC-DC converters has prevented manufacturers from realizing the full potential for cost savings resulting from automated surface mount processes.

Surface mount DC-DC converters (SMDC) are especially desirable where SMT is already widely used, e.g., telecommunications, computers, wireless cellular base stations, computer mainframes or workstations. And with the right information in hand, choosing and using SMDCs can be no more difficult than designing with through-hole equivalents.

69332-th_68291.gifFigure 1. Typical PCB layout (single output shown).

The first major concern in choosing the right SMDC is the availability of design information from the manufacturer. Critical data such as PCB layout recommendations (Figure 1), reflow solder profiles and information on soldering to different metal finishes should be readily available in a product documentation package for easy referral by the end user. Product documentation can often be provided electronically as a .pdf, AutoCAD, or .jpg file from the manufacturers' Web site. With this information, decisions can be made as to whether the SMDCs are compatible with existing types of manufacturing processes, as well as equipment and various board-level components already in use.

Another important consideration is the overall weight and thermal dissipation of the converter. Unlike through-hole DC-DC equivalents that rely on potting compounds to dissipate thermal energy, SMDCs are generally non-encapsulated open-frame designs. They benefit from this kind of construction by being lighter in weight (typically less than 25 g) so that devices can be easily picked and placed by many of today's placement machines. And where heavier encapsulated converters tend to dissipate thermal energy back into the PCB, the latest generation of SMDCs take an entirely different approach to thermal management.

One SMDC manufacturer in particular uses a patented thermal strategy that convects most of the heat away from the board to the exterior air with minimal airflow requirements. This can be of great benefit for the many system applications where excessive thermal energy needs to be kept to a minimum at the PCB level. However, to verify that temperatures are within the SMDC manufacturer's recommended operating range, system air temperatures will need to be taken at the device level under test to ensure the maximum allowable ambient temperature is not exceeded. Derating guidelines are often provided by SMDC manufactures to assist engineers in the design-in process (Figure 2).

69332-th_68292.gifFigure 2. Typical SMDC output derating.

To properly reflow surface mount converters to the PCB, reliable solder joints require accurate reflow solder profiles to be run so the solder pads reach required temperatures for adequate periods of time. With all of the different types of reflow solder furnaces available, it is not uncommon to see 30° or more temperature differences between furnace set points and actual PCB surface temperatures.

Proper profiles for SMDCs require several thermocouples be attached to various points on the leads of the device, as well as the PCB, to ensure that each portion of the board is reaching proper temperatures for the right amount of time. These points need to be monitored so the hottest portion of the PCB does not reach potentially harmful temperatures while the coolest portion of the PCB is hot enough to ensure proper reflow of the solder paste. A recommended reflow profile (Figure 3) along with detailed instructions should be provided by the manufacturer in the SMDC product documentation package.

69332-th_68293.gifFigure 3. Example of a recommended SMDC reflow profile.

When evaluating the pick-and-place capability of the surface mount converter, size of the SMDC may become an issue, depending on the specific machine type and vision system capability of the pick-and-place equipment used. It is suggested that pick-and-place machine manufactures be contacted to provide the maximum size capability and vision limitations to set the machine accordingly. Trial and error often works well here. Some SMDC manufacturers take advantage of a square aspect ratio outer package for optimal placement within most vision system view areas of 50 x 50 mm. Components that exceed this range will generally require special handling, such as odd-form placement or some form of mechanical grippers. In some cases, a simple program change to the pick-and-place machine is all that is required to accommodate larger SMDC devices.

It should be noted that although higher powered SMDCs may exceed the size requirements of some manufacturer's pick-and-place equipment, they are still able to take advantage of the reflow soldering process they simply require manual placement on the PCB. To make manual placement easier for the user, high-power SMDC manufacturers are incorporating locating pins into the design of these converters. This will ensure that a consistent manual placement method can be achieved while taking advantage of long-term reliability because of reflow.

An important consideration when choosing a SMDC has to do with the relative flatness, or coplanarity, of the device during the pick-and-place solder process. The converter manufacturer should specify coplanarity speci-fications so that the end user is not faced with any surprises when actually going through the reflow process. In general, the most effective approach for achieving consistent unit-to-unit coplanarity is one that utilizes a very stable lead-frame attachment technique. Such designs guarantee meeting a specification of 0.006" or better, which is a well established number for reliable reflow practices. Traditional designs using staked pins and the bending of through-hole pins to achieve a surface mount package do not always consistently meet the coplanarity requirements for surface mount component applications.

Most small SMD components are currently available on tape-and-reel. However, because of the larger size of DC-DC converters, matrix trays have become the container of choice for shipping. Industry-standard plastic vacuum-formed trays are designed to allow automatic placement of the modules. Matrix trays should be constructed for holding various quantities of devices, depending on the size and power level of the device. The trays should be recyclable, biodegradable and conform to applicable standards for electrostatic discharge protection.

Of course, no SMDC product will be beneficial if it does not meet the basic requirements of the market it is meant to serve. With typical applications in the telecommunication, computer and distributed power markets, SMDC product specifications must be a close match to actual requirements. These users require features such as 10 microsecond startup, remote on/off, low profile, wide range inputs, 1,500 VDC isolation (I/P to O/P), regulated outputs, adjustable outputs, safety agency approvals and Bellcore compliance. Providing all these key features in a standard model will greatly reduce design time and board space normally required for additional circuitry.

Because the manufacturers offer a wide variety of standard models, the need for modifications addressing the specifications of a particular on-board power application will be minimized. Without a complete line of standard surface mountable converters, concerns about the design time and expense needed to develop SMT power solutions for specific applications has deterred manufacturers from moving to total SMT assembly. A custom solution for this requirement can take several months to develop and cost hundreds of thousands of dollars. The many advantages of implementing surface mount converters must be weighed against the consequent risks of delays and cost overruns in bringing the product to market.

DORREL VERNON is the product manager for Lambda Electronics Inc., 515 Broadhollow Road, Melville, NY 11747; (800) 526-2324, ext. 311; Fax: (516) 752-2627; E-mail: dvernon@lambda.com. PETER BRUNE is the hardware design manager for Lambda Electronics Inc., 3055 Del Sol Blvd., San Diego, CA 92154; (800) 275-5224, ext. 803; Fax: (619) 575-2245; E-mail: pbrune@lambda.com.

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